Geology of the Sierra Foothills Melange and Adjacent Areas, Amador County, California By WENDELL A. DUFFIELD and ROBERT V. SHARP GEOLOGICAL SURVEY PROFESSIONAL PAPER 827 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975 UNITED STATES DEPARTMENT OF THE INTERIOR STANLEY K. HATHAWAY, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 74—600083 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-02616-3 #__4 CONTENTS Page Page Abstract __________________________________________________ 1 Serpentinite and related rocks ______________________________ 19 Introductlon “““““““““““““““““““““““““ 1 Superjacent rocks __________________________________________ 20 General geology “““““““““““““““““““ 4 Valley Springs Formation ______________________________ 20 Nomenclature “““““““““““““““““““““ 5 Mehrten Formation ____________________________________ 20 Melange belt ______________________________________________ 5 General interrelations of rocks in the melange belt ______ 8 Structure ------------------------------------------------ 21 Problems of regional correlation ________________________ 8 Melange belt ----------------------------------------- 22 Eastern belt ______________________________________________ 9 Bear Mountains fault zone ____________________________ 23 Logtown Ridge—Mother Lode belt __________________________ 10 Evidence for faulting of Logtown Ridge strata in Logtown Ridge Formation ______________________________ 10 pre-Mariposa time ---------------------------------- 24 Mariposa Formation __________________________________ 13 Tilting and falding ———————————————————————————————————— 24 Rocks of the Melones fault zone ________________________ 15 Deformation after regional tilting ______________________ 25 Western belt ________________________________________ ‘ ______ 16 Melones fault zone ________________________________ 25 Intrusive rocks ____________________________________________ 17 Mother Lode fault system __________________________ 26 Melange belt __________________________________________ 17 Regional crustal flexure ____________________________ 26 Logtown Ridge—Mother Lode belt ______________________ 18 Structural cross sections __________________________________ 27 Eastern belt __________________________________________ 18 Regional implications ______________________________________ 27 Ages of intrusion __________________________________________ 18 References ________________________________________________ 30 ILLUSTRATIONS Page PLATE 1. Geologic map of the western Sierra foothills between the Cosumnes River and the Mokelumne River,Amador County, California ___________________________________________________________________________________________ In pocket FIGURE 1. Index maps of the west-central Sierra foothills ______________________________________________________________ 2 2. Sample of unedited field data from which the geologic map was compiled ____________________________________ 3 3. Map showing the belts of rocks mapped on plate 1 __________________________________________________________ 4 4. Diagrams of bedding, regional cleavage, and axes of minor folds for slate and graywacke of the Mariposa Formation between Plymouth and Amador City __________________________________________________________________ 25 5. Diagrams of bedding, regional cleavage, and axes of minor folds for slate and graywacke of the melangebeltsouth of Highway 16 __________________________________________________________________________________________ 25 6. Diagrams of bedding, regional cleavage, and axis of a minor fold for slate and graywacke of the melange belt north of Highway 16 __________________________________________________________________________________________ , 25 7 Cross sections showing the evolution of interpretations of geologic structures in the pre-Cenozoic rocks along the Cosumnes River ______________________________________________________________________________________ 28 TABLE Page TABLE 1. Chemical analyses of greenstone from the Logtown Ridge—Mother Lode belt __________________________________ 11 lll 28580 LGEOLOGY OF THE SIERRA FOOTHILLS MELANGE AND ADJACENT AREAS, AMADOR COUNTY, CALIFORNIA By WENDELL A. DUFFIELD and ROBERT V. SHARP ABSTRACT Detailed outcrop mapping in the western Sierra foothills of Amador County, Calif., has resulted in some major changes in the interpreta- tion of stratigraphy and structure. The Amador Group was originally defined at its type locality on the south bank of the Cosumnes River in Amador County to include the Cosumnes Formation and the conform- ably overlying Logtown Ridge Formation, but the new data indicate that the lower boundary of the type Logtown Ridge should be located 600 m farther west (downsection) than originally designated and that this boundary is a fault. The strata that were originally called the Cosumnes Formation are part of a lithologically diverse assemblage of tectonically intermixed rocks that constitute a newly recognized melange and thus are not a formational rock-stratigraphic unit as the earlier workers believed. Thus, the names Cosumnes Formation and Amador Group are both inappropriate in their type area and are abandoned. The Logtown Ridge Formation is here divided into four members, some of which cross what earlier was considered to be a formational boundary of the Logtown Ridge with overlapping pyroclastic strata. The outcrop mapping requires additional changes, although of lesser importance, in the identification and correlation of other Mesozoic rocks in Amador County. The newly recognized melange forms a 4-km-wide belt underlying the Logtown Ridge Formation. In addition to the type section of the abandoned Cosumnes Formation and scattered fault-bounded blocks of strata of Cosumnes lithology, the melange comprises rocks hereto- fore mapped as “western belt of Calaveras Formation,” considered to be of Paleozoic age. Single clasts of this huge tectonic breccia range from a few centimeters to a few kilometers in maximum dimension. Distinctive strata are generally disrupted, and pervasive shearing is common. In the absence of fossils, no age of original deposition can be assigned to any clast or matrix of the melange, but on the basis of indirect structural evidence, the intermixing that formed the melange probably took place during the Late Jurassic or before, and therefore the now sheared and faulted strata must originally have been at least this old. Available data are ambiguous but suggest that rocks were inter- mixed to form the melange when the strata were horizontal or nearly so. Similarly, the overlying Logtown Ridge and Mariposa Formations were faulted when these rocks were essentially horizontal. The entire section was subsequently tilted to its present, nearly vertical position. Traditional syntheses of the tectonic history of the Sierra foothills argue that the faults there have always been steeply dipping. Al- though this may be true for some faults, the new interpretation suggests that most faulting occurred before the section was steeply tilted. Neither suggestion can yet be proved, but we maintain that the highly deformed rocks mapped in Amador County represent primar- ily the effects of subduction at a continental margin, possibly aug- mented by gravity tectonics in a trough of sediment accumulation there. On the basis of the ages of affected strata, this period of subduc- tion was Late Jurassic but possibly began at an earlier time. If this interpretation of the melange in Amador County is correct, a belt of similarly deformed rocks should extend far beyond the limits of the study area. INTRODUCTION Since the gold deposits of the well-known Mother Lode country of California were first discovered and mined over a century ago, the geology of that area has been of considerable interest. Prompted by the gold rush, initial geologic fieldwork emphasized the relation of the general geology to gold-bearing quartz veins of the Mother Lode system. More recent field investiga- tions have concentrated on the structural and strati- graphic complexities of the pre-Tertiary metamorphic rocks to explain the origin and history of the entire Sierra Nevada province. Relatively recent work has resulted in detailed in- terpretations of the geologic history of some small areas (for example, Baird, 1962; Best, 1963) and also more comprehensive treatments of a much larger part of pre- batholithic rocks of the Sierra Nevada province (Clark, 1960, 1964; Kistler and others, 1971). Nonetheless, much fundamental information, such as the strati- graphic thickness of rock systems, is still unknown, and the number and nature of tectonic episodes that have affected these rocks are not generally agreed upon. Attempts to answer these and other fundamental questions in the western Sierra foothills traditionally have been frustrated by such conditions as (1) the low ratio of outcrop area to total field area, (2) marked lithologic changes over short horizontal distances, with the result that important geological contacts commonly are more closely spaced than outcrops, and (3) a con- spicuous absence of fossils to aid in dating and correlat- ing strata. These conditions are not likely to change, but methods of fieldwork are. Large-scale mapping of all available outcrops in western Amador County (fig. 1) has enabled us to make fundamental changes in some earlier defined stratigraphic units and to demonstrate the presence of a previously unrecognized melange there. We studied only a small part of the total foothills area; very detailed mapping elsewhere in the foothills is required for a general understanding of the geology. We recognize, however, that sufficient natural exposures of 1 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA 120°47'30" 120° 57'30" 38°35' CALAVERAS ‘ COUNTY B FIGURE 1.——Index maps of the west—central Sierra Foothills, showing approximate area of plate 1 (shaded area in B). INTRODUCTION 3 bedrock generally are not available, and we urge all interested workers to make detailed observations at manmade exposures as they occur. Nearly all the evidence from which the conclusions in this paper are drawn is summarized on the geologic map, plate 1. Since contacts are rarely exposed in this area, the lines on the geologic map represent conclu- sions about the nature of the contacts based almost entirely on indirect evidence. Thus, a special explana- tion of how the map was constructed is helpful to its interpretation. The working scale was 112,000, and for most map units, except those in the eastern belt, indi- vidual outcrops were plotted. Contacts were drawn to interpret the distribution of lithologies at the working scale, then the entire map was reduced to a 1:24,000 scale without showing the outcrops. Figure 2 is an ex- ample of the unedited large-scale field data that pro- vided the control for compiling plate 1. We have shown the distribution of lithologies by the simplest possible arrangement of contacts on the map. The trend and linearity of some contacts are closely bracketed by field data. Comparison of these contacts we Wu \ “ZN // ., ./ «Pi; \\\ \ FIGURE 2.—Sample of unedited field data from which the geologic map, plate 1, was compiled. This area is about 1.2 miles square; Drytown is located at the east-central border. A few initial interpretations of the data are shown by straight dashed lines. Most outcrops are so small that they can be shown only as a dot on the map. Large outcrops are outlined with dashed lines. Letter symbols are field shorthand and do not correspond directly to the symbols of rock units used on plate 1. 4 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA with others whose form is not so well controlled sug- gests that the less well known ones should also be drawn as approximately linear where outcrop data permit, rather than as curved, sinuous, or even more complex boundaries. Nevertheless, it should be remembered that many of the contacts could be redrawn with differ- ent shapes and consequently would have different im- plications. However, the basic conclusions about struc- ture, especially in the melange, would remain un- changed. The subjacent series is divided into four belts of rocks (fig. 3, pl. 1); in the text, each belt is described in a separate section to permit the reader to move from the text to the geologic map with ease. The authors shared equally in the fieldwork and manuscript preparation for this study. Donald Swanson MELONES FAULT ZONE ackson MoKeMflbEiV-ej FIGURE 3.—Belts of rocks mapped on plate 1. and Othmar Tobisch reviewed the manuscript and of- fered many helpful suggestions. GENERAL GEOLOGY Western Amador County, as all the western Sierra foothills area, is underlain by folded and faulted metasedimentary and metavolcanic rocks of Paleozoic and Mesozoic ages (the subjacent series) upon which isolated erosional remnants of a formerly extensive capping of Tertiary volcanic conglomerate and tuff (the superjacent series) lie with angular unconformity. Bed- ding and metamorphic planar structures in the subja- cent rocks dip steeply and generally trend parallel to one another and to the crest of the adjacent Sierra Nevada. Bedding in the Tertiary rocks dips gently to the west. Paleozoic and Mesozoic rock units form north- northwest-trending bands that have suggested a homoclinal structure to some geologists (for example, Clark, 1964, pl. 1), but our work shows that this in- terpretation is valid only locally where the internal stratigraphy and structure of a few formations are reasonably well documented. A more nearly correct in- terpretation of the structural geology includes abun- dant faulting and shearing, both within and between lithologic units. Accordingly, although the earlier rec- ognized arrangement of mappable rock units into north-northwest-trending bands is preserved, the ages and the nature of juxtaposition of certain map units must now be reevaluated. To date, the most thorough treatment of the structure of the area has been that of Clark (1960, 1964), who mapped a homoclinal sequence, with tops facing east, interrupted by two steeply dipping north-northwest- trending pre-Cenozoic fault zones, the Bear Mountains and Melones. The Melones fault zone juxtaposes Paleozoic rocks upon Mesozoic, whereas the Bear Moun- tains fault zone is characterized by discontinuous mas- ses of serpentinite. In the terminology of this paper, the Melones fault zone separates the Logtown Ridge- Mother Lode belt from the eastern belt, and the Bear Mountains fault zone separates the western belt from the melange belt. (See fig. 3.) On the basis of fossil occurrences, Clark believed that the Bear Mountains fault zone also marked a reversal of stratigraphic suc- cession, but new data indicate that this need not be so. Both fault zones undoubtedly mark zones of major tec- tonic dislocation and are discussed in more detail in later sections. . All the subjacent rocks have been metamorphosed t some degree, generally to the greenschist facies. The most abundant minerals of the various lithologies in- clude white mica, epidote, chlorite, actinolite, quartz, and sodic plagioclase. Small areas of higher grade rocks occur locally. In the bedded rocks and in many lava MELANGE BELT 5 flows, primary structures and textures, and even some original minerals, are excellently preserved in spite of the metamorphism. Because these primary structures often are the most useful identifying features of the rocks, we often use such terms as graywacke and vol- canic rocks, even though metagraywacke and metavol- canic rocks would be more precise. NOMENCLATURE Before the work of Clark (1964), almost all the subja- cent rocks of the Sierra foothills were assigned to one of three formally named rock-stratigraphic units: the Calaveras Formation, the Mariposa Formation, and the Amador Group. The Calaveras Formation was origi- nally defined to include all the metasedimentary and metavolcanic rocks of Paleozoic age (Turner, 1893a, p. 309; 1893b, p. 425). The Upper Jurassic Mariposa For- mation was named by Becker (1885, p. 18—19; 1900, p. 3) for fossiliferous and highly metamorphosed auriferous slates in Hell Hollow just southeast of Bagby (fig. 1), about 12 miles northwest of the town of Mariposa. The Upper Jurassic Amador Group was named by Taliaferro (1942, p. 89—90; 1943; p. 282—284) to include several formations; those of interest to this report are the Cosumnes Formation and the overlying Logtown Ridge Formation, Whose type locality is along the Cosumnes River (fig. 1) immediately west of Highway 49. Most workers in the foothills have correlated their map units with one of these three formally named rock- stratigraphic units wholly on the basis of lithologic similarity. Such correlations were made despite the patchy, discontinuous nature of outcrops, the structural complications, and the general absence of a fossil re- cord; they are often unsatisfactory because none of the formations has a unique lithologic makeup. Clark (1964) recognized this problem and generally restricted his use of formation names to rocks at their type localities and adjacent areas where stratigraphic con- tinuity can be documented. In general, we follow Clark’s nomenclature (1964). However, our mapping indicates that the name Amador Group must be abandoned (Sharp and Duffield, 1973). Briefly, although the Amador Group was originally defined as two conformable formations, the Cosumnes and Logtown Ridge, new mapping indicates that (1) these two lithologic units are in fault contact, (2) the single fossil locality originally used to date the Cosumnes Formation is Within the Logtown Ridge Formation, and (3) most rocks of the Cosumnes Forma- tion form scattered, fault-bounded blocks in a melange. Accordingly, we herein abandon the name Cosumnes Formation; only the Logtown Ridge Formation remains a demonstrable rock-stratigraphic unit in Amador County, and the use of the term Amador Group there is inappropriate. Our use of the name Calaveras Formation follows Turner’s (1893, a, b) original definition—all rocks of Paleozoic age of the Sierra Nevada—and rocks that are assigned to the Calaveras Formation in this report are so named for one or more of the following features: (1) A characteristic assemblage of lithologies, (2) a relatively complex set of structures, and (3) a degree of metamorphism higher than Jurassic rocks of the west- ern Sierra foothills. We found no fossils during our work, and no previously described fossil localities are in the study area. Thus, our assignment of rocks to the Calaveras Formation is not beyond question. In gen- eral, we simply follow the designations used by Clark (1964), although the formational assignment of rocks in the western belt of Calaveras of all earlier workers is now questioned in view of the newly recognized melange there. . The melange forms a 4-km-wide belt of chaotically intermixed rocks that previously have been mapped as a homoclinal sequence of the Calaveras Formation (Lindgren and Turner, 1894), Calaveras plus complexly infolded Cosumnes Formation (Taliaferro, 1943, and unpub. map), and a homoclinal sequence of Calaveras Formation conformably overlain by Cosumnes Forma- tion (Clark, 1964, pl. 8). One clast in the melange, a fragment of limestone a short distance south of the map area (pl. 1), contains Permian fossils, suggesting Calaveras Formation. But because no other fossils have been found and because the melange is a tectono- stratigraphic unit of the type described by Hsu' (1968), essentially all the rocks that compose the melange are of unknown age. In the absence of new age data, how— _ ever, we retain the original Calaveras and Cosumnes designations for some rocks of the melange. The reader should realize that these designations are mainly for convenience in describing generally distinctive lithologies, and the implied correlation to the type Calaveras and Cosumnes Formations may, in fact, be incorrect. We further emphasize this ambiguity and also stress the fundamental chaotic character of the melange by referring to the rocks there as showing “Calaveras affinities” or “Cosumnes affinities,” rather than calling them formations. We recognize that, in general, the law of superposition and the concept of rock- and time-stratigraphic units should not be'applied to the chaotic mass of rocks that forms the melange, and we use the terms Calaveras and Cosumnes there only for convenience of discussion. ' MELANGE BELT The melange crops out in a north-northwest-trending belt about 4 km wide, and we have mapped it for about 15 km across Amador County. The melange includes chaotically intermixed clasts of both Calaveras and Cosumnes affinities. Common lithologies of Calaveras 6 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA affinities include quartzose slate-phyllite, chert,quartz- - ose sandstone, limestone, quartz-muscovite-albite . phyllonite, and massive to blocky mudstone. Those of Cosumnes affinities are black clay slate, graywacke, and slate-clast conglomerate. Intergradation between rock types precludes unambiguous assignment of all rocks of the'melange to one of these two groups, but where possible, a distinction seems valuable and may in fact reflect two or more sources of fundamentally differ- ent character. Virtually none of the map units in the melange con- sists of a single lithology. Each unit contains assem- blages of lithologies that generally are not unique, but the assemblages can be distinguished by the propor— tions of different lithologies and, locally, by the exclu- sion of one or more lithologies. The most widespread assemblage is quartzose slate-phyllite and intercalated thin-bedded chert, with or without more massive sili- ceous rocks (thick-bedded chert?), quartzose sandstone, and limestone. The most common association in a single outcrop is quartzose slate with interbedded chert and lesser amounts of quartzite. The slate generally is tan and gives a characteristic ring when hammered; cleav- age surfaces commonly exhibit a phyllitic sheen. Inter- calated gray chert beds range in thickness from about 2 to 20 em, but most do not exceed 6 cm. Cleavage surfaces in chert are less closely spaced than in the associated slate, and they are refracted at the boundary between the two lithologies. Thin beds of poorly sorted impure quartzose sandstone are locally intercalated with the slate to the exclusion of chert. Also, local rhythmically bedded chert with thin slaty partings crops out to the exclusion of both slate and sandstone. A metamorphic rock cleavage is apparent at nearly every outcrop in the melange, but bedding often is not evident. Rocks at many outcrops are pervasively sheared; the original beds of chert and quartzite are so thoroughly disrupted that they form isolated pods in a matrix of sheared pelitic material. Quartzite and poorly sorted impure quartzose sand- stone form many outcrops both within and adjacent to the slate-chert terranes. These quartzites are bedded, invariably with slaty partings, and are at least partly the lateral equivalent of associated slate. Typically, the brown to tan quartzose sandstones are characterized by sparse plates of mica and by scattered well-rounded medium-grained clasts of quartz in a brown matrix. Locally, this matrix is displaced completely to form beds of quartzite. . Scattered highly siliceous outcrops are common 10— cally. Some of this material is nearly pure massive mi— crocrystalline quartz of light- to dark-gray color. How- ever, much ofit (for example, in sec. 21, T. 7 N., R. 10 E) displays a distinct color lamination that trends gener- ally parallel to elongate outcrops or a group of closely spaced outcrops, suggesting that the color bands reflect original stratification. Limestone occurs as scattered outcrops within terrain underlain by quartzose slate, chert, quartzose sand- stone, quartzite, and clay slate. Exposures range from 1 to about 40 m in maximum dimension and often are clustered in relatively small north-northwest-trending elongated areas. Typically, the limestone is light gray and nearly pure carbonate; grain size is relatively uni- form for outcrops at one locality, but it varies from very fine (lithographic limestone) to coarse throughout the area. At one outcrop(SW1A sec. 9, T. 6 N., R. 10 E.), thin beds of chert are intercalated with limestone, and a distinctive calcarenite in sec. 22, T. 7 N., R. 10 E., contains about 10 percent sand-size quartz, plagioclase, and lithic fragments. Some limestone outcrops are massive, whereas others show a color banding, which, like that in the laminated highly siliceous rocks, is believed to parallel original stratification. By projecting color bands, some outcrops of limestone (for example, in secs. 4, 5, 8, 9, T. 7 N., R. 10 E., and sec. 16, T. 7 N., R. 10 E.) can be fitted into complex folds; scattered outcrops of laminated chert can be correlated in the same manner, and at one area (SE14 sec. 21, T. 7 N., R. 10 E.) similar folds are so outlined in both chert and limestone. These folds, together with the other structural characteristics of the melange belt, suggest that linear bands of limestone outcrops repre- sent limbs of sheared, disrupted folds. Irregularly shaped masses of gneissose to schistose phyllonite crop out about 0.8 km southwest of Drytown and locally elsewhere. These rocks are composed primarily of quartz, muscovite, and albite, commonly in that order of abundance, although muscovite is locally predominant. The plane of schistosity is locally folded and commonly diverges from the north-northwest trend of the regional metamorphic rock cleavage that per- vades nearly the entire map area. Also, the plane of schistosity is locally crosscut by a fracture cleavage with a north-northwest trend that may have formed at the same time as the regional foliation. Schistosity ap- parently developed before regional cleavage and occurs only in a few clasts of the melange. Medium-grained phyllonite, which closely resembles the schistose variety and probably differs from it only in degree of recrystallization, is associated with both the schistose phyllonite and phyllitic slate. At outcrops it is characterized by about 10 percent subrounded quartz grains 1 mm in diameter, which are embedded in a tan to gray matrix. Parallel surfaces of rock cleavage are evenly spaced at about 3-mm intervals and are lined with plates of muscovite that impart a phyllitic sheen to the rock. These surfaces appear to have resulted from MELANGE BELT 7 concentrated shearing on evenly spaced planes. Locally, they are folded, and some fracture cleavage is developed parallel to the axial planes of such folds. In thin section, these rocks are obviously sheared and crushed. About 80 volume percent of the rock is com- posed of quartz and feldspar, both as porphyroclasts and ‘finely crushed matrix. The rest of the matrix is primar- ily muscovite, chlorite, and accessory sphene, apatite, and an opaque mineral. Tan to dark-olive mudstone crops out along the west margin of the melange belt and generally underlies relatively small, northwest-trending elongate areas within the belt of serpentinite exposures. Chert, lime- stone, immature conglomerate, and graywacke crop out within areas that are underlain principally by the mud- stone, but they are scattered and volumetrically very small. The mudstone is unique among the epiclastic rocks of clay- and silt-size material, for it lacks the penetrative cleavage that is so widespread elsewhere. A weakly developed rock cleavage occurs locally, but rocks in most outcrops are either massive or fractured into blocks. Bedding generally is not evident, but where graywacke and immature conglomerate are interstratified, bedding strikes north-northwest and dips steeply eastward. Moderately well sorted mature quartzose conglomer- ate and sandstone crop out along the east and northeast margins and locally elsewhere in the melange belt. These rocks appear to grade both laterally and verti- cally from one to another. Conglomerate is most abun- dant south of Highway 16 near the latitude of Drytown (pl. 1). The clasts are well—rounded cobbles and pebbles of quartz and chert, which are cemented by silica to form very resistant outcrops. Rocks in most exposures are light colored, commonly a pale-pinkish tint. Where out- crops can be reasonably related to one another, they form north-northwest-trending bands. The quartz sandstone occurs primarily north of Highway 16 and extends in discontinuous north- northwest-trending fingers for about 8 km toward the north boundary of the map (pl. 1). The clasts are well- sorted medium-grained rounded to subrounded parti- cles of quartz, minor chert, and rare feldspar (albite?) and slate, all of which show some granulation at their edges and, commonly, undulatory extinction in thin section. Outcrops are light gray and, where weathered, exhibit a few volume percent of evenly distributed pore spaces of the same general diameter as the clasts. Gen- erally massive beds range from a few centimeters to a few meters thick. Tan-weathering slate is associated and interbedded with both the quartzose conglomerate and sandstone. It forms outcrops in roadcuts at the intersection of High- ways 49 and 16 about 2 km north of Drytown; the ridge to the south is underlain by the more resistant rocks. About 6 km to the north-northwest, the slate and inter- bedded quartz sandstone are well exposed in cuts along a dirt road (not shown on pl. 1) that follows the west flank of the conspicuous north-trending ridge in the west half of sections 28 and 33, T. 8 N., _R. 10 E. Typi- cally, bedding and cleavage are nearly parallel and trend north-northwest, but they are greatly divergent in the narrow axial regions of rare minor isoclinal folds. Poorly sorted unbedded conglomerate, the largest clasts of which are subangular to subrounded pebbles and cobbles, is abundant within the melange belt, espe- cially along the west margin. This conglomerate and associated clay slate and graywacke closely resemble slate and graywacke in the type Cosumnes Formation on the Cosumnes River. The conglomerate in some out- crops contains numerous clasts of black slate and an argillaceous matrix; in others, it is characterized by abundant volcanic clasts with no fragments of slate. These types form two end members of which most of the conglomerates are mixtures. The argillaceous conglom- erate is relatively concentrated in the eastern half of the melange belt, and the volcanic conglomerate in the western half, although there are many exceptions. In decreasing abundance, the lithologies represented by the clasts of a “typical” conglomerate are (1) fine-grained and porphyritic intermediate to basic vol- canic rocks, (2) black clay slate, chert, quartzite, lime- stone, and milky quartz, and (3) granitic rocks, mica schist, and serpentine. The only known serpentine clast occurs in the conglomerate that underlies the conspicu- ous hill 0.6 km southwest of Drytown. In the volcanic conglomerate, nearly all the clasts are fine-grained or porphyritic volcanic rocks of inter- mediate to basic composition. These clasts are firmly cemented by tuffaceous material to form very resistant massive dark-colored outcrops. Locally, outcrops of structureless augite-feldspar porphyry several meters in maximum dimension are included with these vol- canic conglomerates. Such bodies probably represent exceptionally large clasts in a fragmental rock, al- though they may be parts of intrusive bodies or flows. Some of the conglomerate appears structureless; however, deep weathering commonly etches the resis- tant varieties in a preferred north-northwest-trending direction, and those rocks with abundant clasts of slate and an argillaceous matrix often show a well-developed planar structure unaided by weathering. This structure is reflected both in the fine-grained matrix (as slaty cleavage) and in the general parallelism of the larger clasts, many of which are discoidal. Locally, deformed clasts also define a lineation that plunges steeply in the plane of foliation. The total strain that affected the 8 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA original rock, as seen in this flattening and elongation, is difficult to assess because the slaty matrix likely has undergone considerably more deformation than the coarser material. Bedding is generally absent, but the original surface of deposition is probably parallel to the metamorphic foliation because the regional trends of the conglomerate, the metamorphic foliation, and the bedding in associated graywacke are all parallel, or nearly so. . Available outcrops indicate that the entire spectrum of epiclastic rocks from cobble conglomerate to clay slate is represented in the conglomerate map units of plate 1. Slate and intercalated graywacke possibly un- derlie a major part of the areas mapped as conglomer- ate, but since the conglomerates are relatively resis- tant, they are preferentially exposed. Slate and graywacke form many outcrops in the easternmost zone of conglomerate perhaps because there they are shielded by the resistant volcanic rocks of the adjacent Logtown Ridge Formation. The slate is black and con- tains interbeds of graywacke ranging in thickness from about 3 to 20 cm. The clasts of the graywacke are suban- gular to subrounded grains of plagioclase and quartz, together with fragments of aphanitic and porphyritic intermediate to basic volcanic rocks, slate, and chert. Bedding and slaty cleavage are apparent at most out- crops, and with few exceptions, they diverge by 15° or less in either strike or dip. Graywacke beds commonly are graded with nearly all stratigraphic tops facing eastward. Known west-facing beds are everywhere as- sociated with small tight to isoclinal folds. A relatively thin (230—500 In in outcrop width) yet quite persistent band' of black slate with thinly inter- bedded graywacke and minor immature slate-clast con- glomerate roughly bisects the melange belt in a north- northwest direction. These rocks are indistinguishable from the slate and graywacke of much less volume, which are associated with similar conglomerate as de- scribed in the preceding paragraph. , Beds of graywacke in this narrow band range in thickness from 3 to 20 cm and provide abundant oppor- tunities for direct measurements of bedding, cleavage, and stratigraphic top. Nearly all tops face eastward; they are most commonly indicated by noticeable grad- ing, but a few examples of refracted cleavage were noted. Bedding planes are even and regular and seldom diverge from the attitude of associated cleavage by more than 20° in either strike or dip. Known folds in bedding are few; they are very tight to isoclinal with axial plane cleavage and measure only 1—2 m in wavelength. The continuity of this map unit is interrupted for a short interval near its intersection with Highway 16 (pl. 1). The exact nature of this break is not well documented, although it almost certainly is structural rather than stratigraphic. Regardless of the nature of the break, this sequence of rocks provides the most nearly throughgoing marker unit of the entire melange belt and thereby represents the largest known clast in the melange. GENERAL INTERRELATIONS 0F ROCKS IN THE MELANGE BELT The melange belt of this report was originally mapped as the “western belt of Calaveras Formation” on the US. Geological Survey folios. The Cosumnes Formation was later separated from these rocks by Taliaferro (1943, p. 306, fig. 2), and still later Clark (1964, p. 17) refined Taliaferro’s original designation of the Cosumnes Formation. Although these earlier work- ers differed on the number of formations and the struc- tural relations of one to the other, they all believed that the formations formed normal stratigraphic successions which had been tilted or folded or both. Our work shows that such simplicity does not exist. As plate 1 indicates, individual marker beds or se- quences of beds do not extend far, normally no more than a few kilometers. Lithologies of distinctive ap- pearance, such as limestone and laminated chert, gen- erally persist for far shorter distances and locally are greatly contorted. Many outcrops are pervasively sheared. Although the exact nature of contacts is un- known because they are not exposed, all this evidence suggests the presence of a chaotic tectonic intermixture of rocks. Thus, a normal stratigraphic succession,cannot be deciphered from rocks within the melange. In fact, the age of rocks that compose the melange, except for clasts which actually contain diagnostic fossils, must now be considered unknown. Indirect structural evidence dis- cussed in a later section suggests that the melange formed in Late Jurassic or older time; thus the rocks of the melange presumably also are Late Jurassic or older. PROBLEMS OF REGIONAL CORRELATION The retention of the Calaveras designation for some rocks of the melange is supported by lithologic similar- ity of these rocks to generally accepted Calaveras rocks described by others elsewhere and by the discovery of Permian fossils from the Allen marble quarry (Clark, 1964, p. 14) within an area directly on strike with what is now recognized as the melange. Clark (1964, pl. 1) indicated that this band of Paleozoic rocks (the “western belt of Calaveras Formation”) pinches out southward from Amador County between fault slices of Jurassic rocks. However, inspection of other maps south of Amador County and comparison of detailed rock de- scriptions for river profiles by Clark (1964, pls. 6, 7) suggest a southeastward continuation of the melange because of similarity in rock types, such as shown in the Jackson folio (Turner, 1894a). A brief excursion we EASTERN BELT 9 made along highways near the Calaveras River sub- stantiates the similarity of the rocks there to the “Calaveras” rocks exposed in the melange of Amador County. Although south of northwestern Calaveras County, Clark (1964) and Clark, Stromquist, and Tat- lock (1963) uniformly assigned these rocks to the J uras- sic Mariposa Formation, isolated blocks of limestone there locally have yielded Permian fusulinids (Doug- lass, 1967). Clark (1964, p. 14) interpreted these as blocks of Permian limestone that probably slumped into massive beds of the Mariposa Formation during Juras- sic time, but in view of the new data, the possible south- ward extension of Paleozoic rocks within the melange in Amador County must be considered. It now seems prob- able that sufficiently detailed mapping will show inti- mate tectonic intermixture within all the rocks labeled “western belt of Calaveras Formation” by Turner (1894a) and considerable extension of the melange belt southward from Amador County. A comparable problem exists for recognition of the melange northward from Amador County. Lindgren and Turner (1894) showed northward continuity of the Calaveras Formation into El Dorado County to the latitude of Placerville in their Placerville folio, but sub- sequent regional compilation by Clark (1964, pl. 1) re- placed much of this map unit in El Dorado County with the designation “epiclastic rocks of uncertain strati- graphic position.” Unfortunately, newer and more de- tailed maps than the original folio sheet have not been made in the critical area to solve this problem of con- tinuity, and structural intermixing of the type found in the melange belt in Amador County cannot yet be dem- onstrated there. Possible lateral extension of the melange beyond the western boundary as shown on plate 1 is suggested by aeromagnetic surveys. Within the map area, magnetic anomalies form north-northwest-trending patterns with especially strong positive anomalies directly over serpentinite bodies (Henderson and others, 1966; US Geological Survey, 1969). The most pronounced bands of magnetic highs are concentrated along the Melones fault zone and the west boundary of the melange belt, where serpentinite is most abundant. Similar bands of magnetic anomalies are located along the east edge of the Great Valley of California, over the relatively flat lying Cenozoic sedimentary rocks that floor the valley. These data suggest that north-northwest-trending belts of serpentinite lie beneath the Cenozoic rocks of the valley and imply the existence of major zones of faulting there. Thus the zone of chaotically intermixed rocks in the foothills region may have a far greater width than we suggest. EASTERN BELT Most of the metamorphic rocks in the eastern belt are phyllite, phyllonite, and chert, which occur separately or in various proportions in single outcrops. Lithologic similarity precludes delineation of separate map units among these rocks by the methods of this study, al- though known bedding attitudes generally trend north-northwest and dip steeply to the east, in accord with the regional structural grain of the foothills. As- signment of these rocks to the Calaveras Formation follows the usage of earlier workers. The phyllitic and phyllonitic rocks appear very simi- lar in the field. Both exhibit a dark-gray to silver-gray color, a sheen from oriented plates of muscovite, a rock cleavage parallel to this mica, and a steeply plunging color streaking in the plane of the cleavage. In thin section, however, the rocks show clear evidence of dif- ferent histories. Phyllite is composed principally of granoblastic quartz, muscovite, albite, chlorite, and ac- tinolite and probably represents recrystallized sedimentary rocks of appropriate composition. Phyllon- ite shows grain granulation textures. One phyllonite sample consists of quartz, microcline, perthite, and minor muscovite as strained and fractured grains that are in turn surrounded or partly bounded by more finely crushed material. Phyllite is more common than phyllonite or chert, and locally it exhibits two foliations. The older foliation trends north-northwest, dips steeply to the east, and is paralleled by plates of mica and the dominant direction of rock cleavage. The younger trends about north-south, dips steeply to the west, and appears either as crenula- tions on the older foliation or as distinct planar frac- tures. Steeply plunging color-streaking lies in the plane of the older foliation and generally parallels the axes of associated minor folds. The chert is indistinguishable from that in the melange belt. It is locally massive but more commonly occurs in beds from 3 to 20 cm thick, separated by thin phyllitic partings. Minor isoclinal folds as much as 1 or 2 m in wavelength occur in the chert; their axes plunge from 20° to 80° to the southeast, averaging about 60°. Two relatively large masses of coarsely porphyroblas— tic white-mica hornfels lie within and adjacent to a granodioritic pluton near the Cosumnes River. The textural and mineralogical character of the hornfels indicate a higher grade of metamorphism than that found in any rocks around the eastern periphery of the pluton. In addition to white mica, the hornfels contains medium-grained granoblastic biotite, quartz, and calcic oligoclase, with weak development of planar fabric defined by biotite grains. White mica forms relatively large (6 mm) poikiloblastic grains without preferred orientation, through which the earlier biotite foliation extends without interruption. Tourmaline and opaque minerals form small accessory grains. 10 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA- Although no similar rock is known elsewhere in the Calaveras terrane of the eastern belt, the hornfels is included with the Calaveras Formation because one mass is continuous with typical Calaveras rocks in El Dorado County to the north, according to relations shown on the Placerville folio (Lindgren and Turner, 1894). Probably because of their geometry, the bodies of hornfels are mOre extensively recrystallized than other weakly hornfelsed rocks found along the eastern con- tact of the pluton. One mass is surrounded by granodio- rite, and the other forms a deeply penetrating append— age of wallrock. Minor bodies of serpentinite, gabbro, diorite, and limestone (for example, in sec. 6, T. 7 N., R. 11 E.) also crop out in the eastern belt, but most are too small to be shown on plate 1. LOGTOWN RIDGE—MOTHER LODE BELT Most of the Logtown Ridge—Mother Lode belt is un- derlain by marine stratified rocks of Jurassic age. These rocks are divided into tWo formations, the Logtown Ridge and overlying Mariposa, which trend north- northwest and dip steeply to the east. Detailed lithologic descriptions of outcrops along the Cosumnes River have been reported by Clark (1964); the present discussion emphasizes significant new stratigraphic subdivisions, refinements, and changes relating to Clark’s work. LOGTOWN RIDGE FORMATION The Logtown Ridge Formation, a sequence of Late Jurassic mafic volcanic sedimentary rocks and inter- layered flows and sills, has been substantially revised by our detailed mapping in Amador County. The forma- tion has been removed from the Amador Group, a term no longer retained, and its base at the type locality has been redefined (Sharp and Duffield, 1973); the forma- tion is herein divided into four distinctive members, most of which are traceable on a regional scale. The distribution of the various members sheds much light on original depositional features within the formation, as well as the character of subsequent deformation. Strata of the Logtown Ridge Formation form a nearly vertical homocline with its top toward the east (pl. 1). The thickness of the formation varies greatly within Amador County, ranging from a maximum of about 3,000 111 near Sutter Hill to a minimum of about 600 In at Drytown. At the type locality, along the Cosumnes River, the thickness is about 2,000 In. As will be discus- sed later, much of the variation in thickness may have resulted from complex tectonic truncation along the base of the formation, rather than original strati- graphic irregularity. The basal unit of the Logtown Ridge Formation in Amador County is here designated the Rabbit Flat Member for good exposures on the north bank of the Mokelumne River, sec. 17, T. 5 N., R. 11 E., about 3 km south of Rabbit Flat. It consists of coarsely porphyritic augite basalt breccia and massive flows or sills, as well as minor bedded pyroclastic rocks. The base of the member is in fault contact with the melange (pl. 1). The Rabbit Flat Member reaches a maximum thickness of about 2,000 m west-southwest of Martell and thins less abruptly to the south than to the north. As shown on plate 1, the thickness at the Mokelumne River is about 640 m. At that location, the Rabbit Flat Member was called unit 29 of the Brower Creek Member of the Mariposa Formation by Clark (1964, pl. 7). Clasts in the breccia range from subangular to sub- rounded pebbles and boulders of coarsely porphyritic augite basalt. The clasts and matrix are generally simi- lar in lithology, having phenocrysts as much as 1 cm in diameter, but are distinguished chiefly by abrupt though slight contrasts in abundance or size of the phenocrysts. The breccia appears unstratified in single outcrops but probably is coarsely bedded at a scale larger than most outcrops. A few well-bedded fine-grained pyroclastic rocks similar to those of the overlying member crop out near the base of the member at Sutter Creek. The Rabbit Flat Member is basaltic in composition, as shown by chemical analyses of two representative sam- ples of massive augite porphyry (table 1). The composi- tion of these samples is nearly the same as those for all of the overlying members of the formation; it is espe- cially similar to that of the Pokerville Member, which consists chiefly of the same rock types. The age of the Rabbit Flat Member is unknown, in- sofar as reported discoveries of fossils have not been definitely located within it. The only possible fossil oc- currence from this unit is a specimen of the Callovian ammonite Pseudocadoceras cited by Imlay (1961, tables 2, 3,loc. 15). However, the exact site of the fossil collec- tion is unknown. Nonetheless, since the Rabbit Flat Member is conformably overlain by the Goat Hill Member of documented Callovian age, both are consid- ered to be of Late Jurassic age. The Goat Hill Member is here named for exposures on the north bank of the Mokelumne River, secs. 16 and 17, T. 5 N., R. 11 E., about 1 km south of Goat Hill. This member is composed chiefly of well-bedded marine pyroclastic rocks, locally forming as much as half the thickness of the Logtown Ridge Formation. In Amador County the member is exposed along a continuous belt ranging in width from 335 to 1,465 In. Because the underlying Rabbit Flat Member is structurally trun- cated at Drytown, the Goat Hill Member from that point northward to beyond the Cosumnes River is the basal unit of the formation. Southward stratigraphic thin- LOGTOWN RIDGE—MOTHER LODE BELT 11 TABLE 1.—Chemical analyses of greenstone from the Logtown Ridge—Mother Lode belt [Analyses by rapid-rock method. Analysts: Lowell Artis, G. W. Chloe, P. L. D. Elmore, J. L. Glenn, James Kelsey, Hezekiah Smith. Analyses recalculated with PhD—omitted] Logtown Ridge Formation Unknowg as: may PM? “if?“ .. Member 1 2 3 4 5 6 7 8 9 49.2 48.7 51.2 51.1 50.0 49.1 49.6 49.2 55.5 15.2 15.5 16.6 17.0 17.9 14.6 10.6 14.8 16.3 .9 1.7 .5 1.9 3.2 1.0 1.7 1.5 3.0 7.9 6.4 9.9 7.9 5.2 8.0 6.8 8.0 6.1 7.1 6.3 4.8 2.9 3.3 8.5 11.8 6.9 3.4 11.0 11.4 5.7 7.7 8.2 8.9 11.6 10.7 9.5 2.1 3.2 2.8 3.3 3.8 3.6 2.4 2.8 1.7 1.1 1.1 1.3 2.8 2.7 .9 1.2 .9 1.1 3.2 2.7 4.5 3.4 2.6 3.8 2.3 3.0 2.1 1.0 .6 1.7 1.2 1.3 1.1 .7 1.0 .8 .2 .2 .5 .2 .5 .3 .4 .2 .3 .1 .2 .2 .2 .2 .2 .2 .2 .2 1.0 1.9 .4 .3 1.0 .2 .8 .9 .1 Total ________________ 100.0 99.9 100.1 99.9 99.9 100.2 100.2 100.1 100.1 augite porphyry 9mg mmpwpu . Augite orphyry dike(?) west of Jackson (precise location unknown). Sample . East side Plymouth Ditch, 340 it north of south boundry of sec. 26, T 8 ning of the overlying Pokerville Member leaves the Goat Hill Member as the uppermost unit of the forma- tion for the 3-km interval immediately north of the Mokelumne River (pl. 1) and perhaps for a considerable distance farther south in Calaveras County. Some of the rocks of the Goat Hill Member are here incorporated into the Logtown Ridge Formation for the first time. At the Mokelumne River, the member cor- responds to Clark’s (1964, pl. 7) units 30, 31, and 32, all designated by him as the Brower Creek Volcanic Member of the Mariposa Formation. Furthermore, the Goat Hill Member at the Cosumnes River is equivalent to most of Clark’s (1964, pl. 8) unit 47 of the type Cosumnes Formation, combined with units 48 and 49 assigned by him to the Logtown Ridge Formation. The structural and lithologic evidence supporting the latter reassignment has been discussed in detail by Sharp and Duffield (1973). Three principal rock types constitute the Goat Hill Member: (1) thin- to thick-bedded very fine to medium- gr ined tuff, (2) coarse pumice lapilli tuff in commonly thi k locally graded beds that are interlayered with the fin r tuff, and (3) thick-bedded fine to coarse volcanic br ccia that grades upward into medium- and -grained tuff. Additional but relatively minor types of ock found within the Goat Hill Member and mapped se arately on plate 1 include massive and pillow- str ctured very coarse plagioclase porphyry and au ite-plagioclase porphyry (Duffield, 1969). These roc 5 occur in thin sheetlike flows and sills north of Su ter Creek but have not been found farther south in . 900 ft east of west boundary and 300 it south of north boundary of sec. 2, T. 6 N., R. 10 E. Massive augite porphyry. 1,600 ft east of west boundary and 1,000 ft south of north boundary of sec. 2, T. 6 N., R. 10 E. Massive augite porphyry. . South bank Cosumnes River, 50 ft west of east boundary of sec. 21, T. 8 N. R. 10 E. Very fine grained tuff. . Roadcut, Old Sacramento Road, 750 it east of west boundary of sec. 10, T. 7 N., R. 10 E. Nonporphyritic volcanic breccia. . North side of Tonzi Road, 90 ft west of east boundary of sec. 2, T 6 N. R. 10 E. Coarse plagioclase porphyry pillow lava. Roadcut, west side of State Highway 49, 100—150 ft south of south end of Huse Bridge across Cosumnes River. Sample location and analysis from Clark (1970, table 2). Coarse Roadcut, west side State Highway 49, 1, 390 R north of south boundary sec 15, T. 7 N. R. 10 E. Coarse augite porp rpyrh location and analysis from Turner (1894b,yr p. y473). Coarse augite porphyry. R. 10 E. Phyllonitic greenstone. Amador County (pl. 1). Known outcrops are either within or at the base of the Goat Hill Member. Similar rocks that form sill-like and other irregular masses intrude the melange. The interbedding of the various lithologies of the Goat Hill Member, as well as other sedimentary fea- tures including cut-and-fill structures and soft- sediment deformation, is dramatically exposed along the banks of the Cosumnes River. Most of these features have been described and depicted in detail by Clark (1964, p. 19—22). The common occurrences of unde- formed highly vesicular fragments of pumice, graded beds, abundant examples of soft-sediment deformation, and known associated marine fossils indicate that the bedded sequence of this member is of subaqueous pyro- clastic origin of the type described by Fiske (1963) and Fiske and Matsuda (1964). Chemical analysis of typical specimens of bedded Goat Hill rocks suggests that the predominant composi- tion of the member is basaltic (table 1). Samples of very fine grained tuff and volcanic breccia with granule- to pebble-size clasts in very fine grained matrix that is similar in lithology to the clasts strongly resemble one another in composition. However, these rocks, as well as the coarse plagioclase porphyry separately mapped within the member, contain slightly more Si02 and A1203 and less MgO and CaO than samples from other members of the formation. The age of the Goat Hill Member is well established by fossils along the Cosumnes River. Specimens of the ammonite Pseudocadoceras, of Callovian age, have 12 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA been collected from strata about 395 m above the base and within 275 m of the top of the member (compare Eric and others, 1955, p. 10; Imlay, 1961, p. 3, 6; Clark, 1964, p. 18, 21; pl. 1, this report). Although a slightly older age than Callovian is possible for the basal part of the member, the uppermost part probably is also Callo- vian in age, on the basis of fossil occurrences in the overlying Pokerville Member. The Pokerville Member is here named for exposures on the south bank of the Cosumnes River in secs. 14 and 15, T. 8 N ., R. 10 E. Pokerville is an early name of the nearby town of Plymouth (Gudde, 1969, p. 252). Nearly all the Pokerville Member is coarsely porphyritic augite basalt; it forms massive flows, pillow lavas, and coarse- bedded pyroclastic deposits, but volcanic breccia with virtually monolithologic clasts and matrix is dominant. Even the few clasts of different lithology observed in the breccia were mostly mafic volcanic rocks. The clasts and matrix in the breccia are essentially identical to those found in the Rabbit Flat Member. Chemical analyses of samples of the Pokerville Member are nearly identical with those made on the lithologically similar Rabbit Flat Member. (See table 1.) Analyzed samples from Widely scattered localities within the Pokerville, from the Cosumnes River on the north to the latitude of Jackson on the south, are all basaltic and vary little in bulk chemistry. Throughout most of Amador County, the Pokerville Member is the uppermost map unit of the Logtown Ridge Formation, but east and southeast of Drytown it is OVerlain by the New Chicago Member. West of Plymouth, the Pokerville forms nearly three-fourths of the formation, but its proportion and thickness di- minish stepwise southward through a series of three tectonic blocks. In the southernmost block, south of Drytown, the member commonly forms less than 10 percent of the formation, and it gradually thins south- ward and pinches out about 3 km north of the Mokelumne River. Its maximum thickness in the county is approximately 1,370 m 0.8 km north of Plymouth. The Pokerville Member overlies the Goat Hill Member with apparent conformity south of Drytown. North of Drytown, however, the irregularity of the con- tact between the members suggests disconformity. At two localities northwest and southwest of Plymouth, the members seem to intertongue, suggesting that the relief on the contact may be a depositional rather than an erosional feature and that the contact is everywhere conformable. Strong evidence, to be discussed more fully elsewhere in this report, suggests that faulting occurred in the Rabbit Flat and Goat Hill Members of the Logtown Ridge Formation when sediments of the Pokerville Member were accumulating. The age of the Pokerville Member is probably Callo- vian, but it may extend upward into latest Oxfordian or early Kimmeridgian, depending on the actual locations of two previously recovered and identified ammonites described by Imlay (1961, table 3). Although the loca- tional data for one latest Oxfordian or early Kimme- ridgian ammonite, Idoceras, led Imlay to believe that it came from the upper part of the Logtown Ridge Forma- tion near the Cosumnes River (Imlay, 1961, table 3, 10c. 10), the description of the enclosing rock as “metamorphosed andesitic tuff” makes it difficult to assign it to any part of the Logtown Ridge except the Goat Hill Member in the lower half of the formation. It is more likely that the andesitic tuff is one of the lenticu- lar tuffaceous greenstone bodies that we assign to the Mariposa Formation, immediately overlying the Pokerville Member at that location. (See pl. 1.) This interpretation resolves an additional dilemma regard- ing the other ammonite, Peltoceras, recovered from the nearby Mariposa Formation, according to original rec- ords (Imlay, 1961, p. 6-; compare to his table 3, 10c. 13). Despite the original notes referring this specimen of Peltoceras to a location near Big Indian Creek, which parallels Highway 49 north of Plymouth, Imlay argued that it probably came from a position near the middle of the Logtown Ridge Formation, in order to avoid placing the Callovian specimen above the Oxfordian or early Kimmeridgian one. No inversion of the time sequence results if this specimen of Peltoceras is assigned to the Mariposa Formation but at a stratigraphically lower position than the Idoceras-bearing tuff unit within the Mariposa. Such a reconstruction is consistent with all the published data and suggests that the entire type Logtown Ridge Formation on the Cosumnes River is Callovian in age. The uppermost unit recognized in the Logtown Ridge Formation is here named the New Chicago Member, for exposures in and near the roadcut of Highway 49, sec. 36, T. 7 N., R. 10 E. This member is named after the settlement of New Chicago, about 1.6 km north of the type locality. The member is composed chiefly of fine volcanic breccia with vesicular fine-grained to aphani- tic clasts in a similar matrix. Vesicular fine-grained massive greenstone, sparsely and finely porphyritic au- gite porphyry, and coarse volcanic breccia containing clasts of both fine-grained to aphanitic greenstone and minor amounts of vesicular coarse augite porphyry are also intermixed in this member. Volcanic breccia of the Pokerville Member that im- mediately underlies the New Chicago Member also pos- sesses a distinctive vesicular character. Although scat— tered clasts of such vesicular augite porphyry can be found through much of the Pokerville breccia, such clasts are especially abundant throughout a thickness LOGTOWN RIDGE—MOTHER LODE BELT 13 of several meters where overlain by the New Chicago. This close spatial association of unusually vesicular rocks assigned to both members suggests that a single source or vent which produced abundant vesiculated debris possibly existed locally from Pokerville into New Chicago time. The New Chicago Member is restricted to the south half of the Logtown Ridge Formation (pl. 1). It forms thin, discontinuous, elongate bodies; the thickest is 150 m, and the longest is about 3 km. Fossils have not been found in the New Chicago Member, but its conformable relationship with the un- derlying Poke Ville Member indicates that it is only slightly younge . If the basal beds of the directly over- lying Mariposa Fo mation are Callovian, as those near the Cosumnes Riv may be, then the New Chicago Member would be ad itionally restricted to a Callovian age along with the re ainder of the formation; how- ever, the actual age ma e somewhat younger. Published maps indicate e continuity of the Log- town Ridge Formation northward from Amador County; southward continuity into Calaveras County is not agreed upon. Clark (1964, pl. 1) believed that the Logtown Ridge Formation thinned southward strati- graphically and disappeared in Amador County under an overlapping tongue of lithologically similar rocks, the Brower Creek Volcanic Member of the Mariposa Formation. Our mapping, however, shows that the Log- town Ridge Formation extends southward across this boundary to the Mokelumne River and probably beyond. Examination of relatively detailed maps of Calaveras County by Eric, Stromquist, and Swinney (1955), Clark, Stromquist, and Tatlock (1963), and Clark (1964), as well as the early folio maps of the US. Geological Survey, strongly suggests that the Logtown Ridge rocks in Amador County are equivalent to at least some of the metavolcanic strata to the south. Indeed, all existing data suggest that the Logtown Ridge designa- tion by Eric, Stromquist, and Swinney (1955, pl. 1) for rocks exposed northwest from Brower Creek along Hogback Mountain in Calaveras County and depicted on many maps (for example, Clark and others, 1963; Clark, 1964, pl. 1) along a belt connecting with what is now definitely Logtown Ridge Formation at the Mokelumne River is valid by reason of stratal con- tinuity. Proof of this correlation, as well as the possible inclusion of other rocks deemed Logtown Ridge by Eric, Stromquist, and Swinney (1955) in Calaveras County, must necessarily await more detailed subdivision and mapping. However, regional stratigraphic correlation of these rocks is now open to multiple interpretation, and future detailedwork may invalidate some of the cur- rently favored stratigraphic nomenclature of the Upper Jurassic metavolcanic rocks south of Amador County. MARIPOSA FORMATION Rocks immediately overlying the Logtown Ridge Formation west of the Melones fault zone are here des- ignated the Mariposa Formation. These rocks include black clay slate interbedded with graywacke, conglom- erate, and fine-grained mafic tuffaceous rocks and are intruded by porphyritic volcanic rocks or hypabyssal sills. Although these rocks were originally termed the Mariposa Slate by Turner (1894a) on the Jackson folio, Clark (1964, p. 27) later assigned them an unknown but post-Logtown Ridge stratigraphic position because they could not be clearly connected with the well- documented Mariposa Formation in Calaveras County to the south. However, lateral continuity with rocks termed Mariposa Formation at the Mokelumne River by Clark (1964, pls. 1, 7) is now established by detailed mapping. (See pl. 1.) Following the usage established by Clark (1964, p. 23), volcanic rocks within the Mariposa Formation are here designated the Brower Creek Volcanic Member. These volcanic rocks apparently dominate the Mariposa section in the southern half of Amador County, al- though the eastern extent of the Mariposa Formation has not been defined in plate 1. Northward from Amador City, bodies of Brower Creek rocks diminish in abundance and are virtually absent midway between Drytown and Plymouth, but north of Plymouth small bodies of Brower Creek crop out as numerous thin bands in the Mariposa Formation. Although the greatest width of the belt of Mariposa Formation where both of its edges have been mapped is about 2,100 m measured east-northeastward from near Drytown, it is not possible to establish the true strati- graphic thickness there owing to isoclinal folding and possible repetition of section by faulting. Northward from that location, the formation thins by tectonic trun- cation to about 800 m at the Cosumnes River. Uni- .formly eastward stratigraphic top determinations on nearly vertical bedding near the Cosumnes River, com- bined with a distinctive stratigraphic succession, sug- gest that the 800-m width of the Mariposa belt there approximates a true stratigraphic thickness. The Mariposa Formation along the Mokelumne River possi- bly forms a nearly 915-m-wide belt (Clark, 1964, pl. 8), but abundant small-scale folds there suggest that the section may be considerably repeated. (See pl. 1.) Field evidence indicates that there was little, if any, hiatus in deposition across the Logtown Ridge- Mariposa boundary. This is supported, first, by appar- ent stratigraphic conformity at the only clear exposure of the contact in the roadcut of Highway 49 immediately south of the Cosumnes River (see also Clark, 1964, p. 26), second, by the general conformity of mapped marker beds within the Mariposa Formation to the con- 14 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA tact, and third, by the continuity of a very thin strip of Mariposa slate between the Logtown Ridge and Brower Creek rocks from Martell southward to the Mokelumne River (pl. 1). Such a conformable relationship is consis- tent with the ages of fossils discussed in the section “Logtown Ridge Formation.” The dominant nonvolcanic rock type in the Mariposa Formation is black clay slate, generally with sparse thin interbeds of fine- to medium-grained graywacke. At most locations along the contact, these rocks rest directly on the Logtown Ridge Formation. Between Plymouth and Drytown, these interbedded rocks consti- tute nearly the entire thickness of the Mariposa Forma- tion, but to the north and south other lithologies, prin- cipally greenstones, dominate the section. Detailed descriptions of the lithologic character and sedimentary features found in Mariposa slate and its interbedded graywacke were given by Clark (1964, p. 24, 39—41). This slate and graywacke are virtually in- distinguishable in hand specimen and outcrop from some rocks of Cosumnes affinity in the melange belt. However, Mariposa slate contrasts strongly with both the siliceous and nonsiliceous slates of Calaveras affinity found in the melange. The chief differences are the total absence of chert beds and silicified lentils or other irregular masses that are nearly ubiquitous in Calaveras siliceous slate and the presence of graywacke interbeds that have not been observed in the Calaveras nonsiliceous slate. Although we think that these con- trasts are noteworthy and consistent with other major differences between the Mariposa and Calaveras For- mations, Clark, Stromquist, and Tatlock (1963) and Clark (1964) have not made such distinctions south of Amador County; there, much of the slate shown on their maps as Mariposa Formation is lithologically equiva- lent to slate of Calaveras affinity in the melange. Two types of conglomerate have been mapped in the Mariposa Formation in Amador County. Conglomerate with abundant well-rounded clasts of quartzite and similarly resistant rock types form locally very thick beds near the base of the Mariposa Formation between Amador City and Enterprise (near the Cosumnes River). Thick beds of immature conglomerate, made chiefly of granule-sized clasts of shale chips and subor- dinate amounts of cherty and quartzose clasts, are in- terbedded with coarse graywacke and slate for 3.2 km south and an unknown distance north from the Cosumnes River. The quartzite-cobble conglomerate intertongues and locally grades laterally into finer conglomerate, coarse graywacke, and slate. An unusually thick bed of such conglomerate with intercalated coarse graywacke part- ings forms a section about 275 m thick along Dry Creek northeast of Drytown, but this unit abruptly thins to the north and south and intertongues with slate and graywacke. The lateral extent of individual thin beds of quartzite conglomerate is remarkable, more than 3 km for some beds. Well-rounded quartzitic clasts every- where predominate, but there is an appreciable fraction of more angular clasts of volcanic rocks as well. The matrix is poorly sorted coarse graywacke containing substantial amounts of relatively well rounded quartz grains. The quartzite clasts are fine to very coarse grained and contain some relatively coarse white mica that defines foliation surfaces randomly oriented from clast to clast. These clasts were derived from a metamorphic terrane possible of relatively high grade, judging by the coarseness of the white mica and appar- ently annealed textures. The matrix and interbedded slaty rocks show only slight evidence of metamorphism, and so the foliation in the clasts is definitely pre- Mariposa in age. The distribution of the quartzitic conglomerate sug- gests that the main deposits are fillings of channels that followed depresssions on the upper surface of the Log- town Ridge Formation. The principal channel was east and north of Drytown, where the conglomerate is thick- est. The fact that this channel lies directly above a zone of structural dislocations within the Logtown Ridge Formation (pl. 1) suggests that faulting affected the area at or just before the time of Mariposa deposition. This conclusion is critical to our interpretation of struc- ture in Amador County, and its implications are more fully discussed under the section “Structure.” The second kind of conglomerate in the Mariposa Formation, the immature shale-clast granule to pebble conglomerate, is mostly restricted to a small area within 3 km of the Cosumnes River in Amador County. Elsewhere, similar rocks have been recognized only in a small lens near Sutter Creek. These rocks are inter- layered with thick-bedded fine- to coarse-grained graywacke and black clay slate, both of which resemble similar rocks from the main slate zone in the Mariposa belt but lack the common thinly interbedded relation- ship. Although most of the clasts in the conglomerate appear to be angular very fine grained argillaceous fragments which are similar to the matrix of the rock, except for slight variations in color, an appreciable frac- tion of the clasts consists of chert, quartzite, and fine-grained volcanic rocks. Metamorphic rock cleavage generally is almost as well developed within the clasts as in the matrix, rendering them markedly fissile. The shale-clast conglomerate is virtually identical litholog- ically to some rocks of Cosumnes affinity in the melange. p Volcanic greenstones of chiefly pyroclastic origin in- terbedded with epiclastic rocks constitute the Brower Creek Volcanic Member, which forms a major part of LOGTOWN RIDGE—MOTHER LODE BELT 15 the Mariposa Formation in Amador County, particu- larly south of Sutter Creek. The member also includes ‘ bodies of intrusive and extrusive greenstone. Although much of the Brower Creek appears to be small lenticu- lar masses entirely enclosed in sedimentary contact within Mariposa slate, the largest bodies as depicted on plate 1 probably are separated from the remainder of the formation by faults, and thus their assignment to the Brower Creek Volcanic Member is tentative. Lo- cally, however, epiclastic rocks similar to those of the Mariposa Formation are widely intercalated within the larger bodies mapped as the Brower Creek. The justification for including many of these rocks in the formation therefore is chiefly based on these interbed- ding relations and cannot yet be substantiated by age dates from fossil occurrences. The most prevalent rock in the Brower Creek Vol- canic Member in Amador County is fine- to medium- grained thin- to thick-bedded tuffaceous greenstone that is intercalated with black clay slate and graywacke similar to that found in other parts of the Mariposa Formation. The largest body of this rock extends from about midway between Plymouth and Drytown south- ward to the Mokelumne River, where it corresponds to Clark’s (1964, pl. 7) units 34 through 37. Many smaller lenses of tuffaceous greenstone are scattered through- out the Mariposa section north of Plymouth. The tuf- faceous rocks form beds from less than 1 cm to more than 1 m thick. Grading within these beds has only rarely been detected. No chemical analyses are avail- able, but on the basis of lithologic resemblance to the Copper Hill Volcanics of the western belt, these rocks probably are andesitic to basaltic in composition. The proportion of graywacke and slate interbedded with the tuffaceous greenstone varies greatly, and no attempt has been made to separate these rocks on plate 1 be- cause of the complexity and scale of interbedding. Volcanic rocks of basaltic composition with coarse phenocrystic augite form relatively thick massive flows and flow breccia within the large body of bedded pyro- clastic rocks of the Brower Creek Volcanic Member at least as far south as Sutter Creek. Other lenticular bodies of augite porphyritic basalt probably represent- ing flows or shallow sills occur locally in the Mariposa Formation north of Plymouth. These rocks are litholog- ically identical to the augite porphyry greenstone of the Rabbit Flat and Pokerville Members of the Logtown Ridge Formation. An analysis of one specimen of augite porphyry from the Brower Creek Volcanic Member shows that the composition of these rocks is basaltic and nearly equivalent to the lithologically similar Logtown Ridge Formation (table 1). volcanic breccia containing blocks of fine-grained volcanic rocks is a minor rock type inthe Brower Creek Volcanic Member. A single lenticular mass of such rock occurs along the base of a bedded tuffaceous greenstone section about halfway between Plymouth and the Cosumnes River. It is superfically indistinguishable from similar breccia in the Goat Hill Member of the Logtown Ridge Formation. Molluscan and ammonite faunas from scattered loca- tions throughout the Mariposa Formation indicate the late Oxfordian and early Kimmeridgian Stages of Late Jurassic age (Imlay, 1961, p. 7—8; Clark, 1964, p. 25—26). Fossils previously discovered in Amador County gener- ally corroborate this age assignment, but as discussed earlier, the lower part of the Mariposa might possibly be as old as Callovian. Recovery of the pelecypod Buchia concentrica at a position probably within 152 m strati- graphically above the base of the Mariposa Formation about 1.2 km north-northwest of Plymouth (Imlay, 1961, table 2, 10c. 14) demonstrates a late Oxfordian and early Kimmeridgian age there. The question remains, however, as to whether basal Mariposa strata laterally transgress time and whether beds somewhat older than late Oxfordian might also exist, or, alternatively, whether the lower range of B uchia concentrica is some- what older than presently recognized. ROCKS OF THE MELONES FAULT ZONE A narrow band (about 250—800 In) of distinctive sheared rocks separates the Mariposa Formation on the west from the large granitic pluton and the Calaveras Formation of the eastern belt to the east. Widespread evidence of pervasive shearing in this band of rocks, as well as regional stratigraphic relations, suggests that large fault offset occurs across the zone. This. zone in- cludes the Melones fault as defined by Clark (1960, 1964). However, Clark believed that the width of the fault zone in Amador County was considerably less than we show on plate 1, and he (1964, p. 27) referred many of the rocks that compose the fault zone of plate 1 to a volcanic unit of unknown stratigraphic position. (See Clark, 1964, pl. 1.) In general, we assign to the fault zone rocks that show noticeably greater shearing and recrystallization than adjacent rocks, on the assumption that this band of relatively highly metamorphosed rocks is coextensive with a zone of faulting. Within Amador County most of the rock types found in this band are unique to it; they include phyllitic and phyllonitic greenstone as the dom- inant types, hornblende-bearing rocks of metaigneous aspect ranging from leucogabbro to hornblendite, mylonitized rocks probably derived from all the previ- ous types of rock, and medium-grained alaskitic rocks that commonly show mylonitic effects. Bodies of rock similar to material of Calaveras affinities in the melange are also incorporated within this band; they are described with the rocks of the melange, although 16 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA the setting of their occurrence here is markedly differ- ent. The phyllitic and phyllonitic greenstone generally appear nearly massive on the scale of outcrops. In hand specimen, they are distinctly more coarse grained than greenstone found in all other formations in the area except some rocks assigned to the Copper Hill Volcanics of the western belt. Typical specimens contain abun- dant plates of relatively coarse white mica (easily Visi- ble to the unaided eye) that give a pronounced sheen to cleavage surfaces. Some of the rocks have relict hornblende phenocrysts, but most phenocrysts have been completely or partially replaced by very fine grained aggregates of white mica, epidote-chlorite, and quartz. Bedding has not been observed in any outcrops of this rock, but in thin section, compositional layering in some specimens is evident. Most of this layering results, hoWever from late vein formation or possibly from metamorphic segregation developed during shear- ing of the rock. The field distinction between phyllite and phyllonite is made on the basis of whether obvious fluxion shown by milled-down hornblende augen is evi- dent in the rock. However, on the microscopic scale, all samples studied show generally pervasive shearing and thus possess a phyllonitic aspect. The parent from which the phyllite and phyllonite formed probably consisted primarily of intermediate to basic porphyritic volcanic rocks. Analysis of a typical specimen (see table 1) shows an andesitic composition, but the high proportion of dark minerals in some of these rocks, generally exceeding 50 percent and reach- ing locally as much as 90 percent, suggests that basaltic compositions also occur. Inasmuch as hornblende por- phyritic rocks are unknown as large-scale masses else- where in the immediate region, this belt of rocks ap- pears to be lithologically unique. Other rocks associated with the phyllite and phyllon- ite distinguish the fault zone, too. Irregularly shaped masses of fine- to coarse-grained metaplutonic rocks of generally gabbroic affinity are widely scattered in the zone. In detail, these rocks apparently grade from rela— tively leucocratic rocks (leucogabbro) to very dark rocks approaching hornblendite. Many of these rocks display abundant irregular greenish veins consisting almost entirely of very fine grained aggregates of epidote; the veins are generally small and follow irregular fractures in the host rock. Greenschist-facies metamorphic over- print, as well as some cataclastic shearing and post- shearing recrystallization, gives these rocks an excep- tionally complex aspect both in hand specimen and in thin section. All the rocks consist of the same minerals as the associated phyllite and phyllonite: hornblende, oligdclase, epidote, white mica, chlorite, quartz, and some pyrite. A few specimens contain small remnants of pyroxene in the cores of large hornblende crystals. White mica and epidote grains, as in the phyllite and phyllonite, form felted masses generally without pre- ferred orientation, even though a pronounced planar fabric is defined by the other minerals. This relation suggests that at least one stage of recrystallization postdated much of the shearing evident in these rocks. Some bodies of strongly mylonitized rocks of the fault zone with abundant large hornblende augen have been mapped separately on plate 1 because of their great size and apparent internal homogeneity. These rocks re- semble the phyllitic and phyllonitic rocks in mineralogy and probable bulk composition and also possess the cataclastic texture that is commonly but less strikingly developed in the other rocks. Moreover, the mylonitic rocks appear to be gradational with many of the por- phyritic rocks included in the phyllite-phyllonite unit in terms of the abundance of the hornblende pheno- crysts. A few dikelike or sill-like bodies of light-colored foliated alaskite lie withinthe phyllite-phyllonite belt. These rocks are granitic in composition and are rela— tively coarse grained. Nearly all outcrops exhibit folia- tion to some degree, and thin sections show that it is related to cataclastic effects of widely ranging intensity. In some mylonitized phases, compositional layering is formed by thin epidote-rich and white mica-rich bands separating layers composed almost entirely of potas- sium feldspar and quartz. Contacts of alaskite with the surrounding materials are not exposed, and so the in- trusive nature shown on plate 1 is interpretive. Al- though the generally concordant geometry suggests that these bodies were sills prior to the cataclastic shearing, they conceivably could represent fault slivers of much larger intrusive masses not presently exposed. WESTERN BELT All rocks of the western belt are assigned to the Cop- per Hill Volcanics, as defined by Clark (1964, p. 30—31), and consist chiefly of fine-grained pyroclastic green- stones. Although the stratigraphic base of these rocks has not been mapped in this study, Clark (1964) stated that the Copper Hill Volcanics intertongue to the west with the Salt Spring Slate, which has been dated as late Oxfordian and early Kimmeridgian age on the basis of fossils found along the Cosumnes River in Sacramento County. Stratigraphic top determinations noted by Clark along the Cosumnes River thus show the upper Copper Hill Volcanics to be younger than this slate, but the range of ages represented by these volcanic rocks is unknown at present. It should be added that tectonic intermixing of strata in a manner equivalent to that found within the melange shown on plate 1 may con- ceivably have occurred much farther west than has been recognized in our study; if so, then the Copper Hill INTRUSIVE ROCKS 17 Volcanics may not rest in undisturbed stratigraphic position on the Salt Spring Slate. Detailed mapping in the Copper Hill Volcanics and farther west should an- swer this question. Pyroclastic rocks of the Copper Hill Volcanics consist typically of relatively fine grained and nearly massive phyllitic greenstone. Although some of the rocks along the east margin of the formation contain relatively large quartz grains, most are nearly equigranular. Lo- cally, on a microscopic scale, faint compositional band- ing is defined by white mica-chlorite-quartz-albite- epidote laminae interlayered with bands lacking white mica. Larger scale bedding, as well as other sedimen- tary structures, are apparently absent in the Copper Hill Volcanics. Presumably, these features, if once present, have been masked during the low-grade metamorphism that affected these rocks. Slaty argil- laceous rocks interbedded with the Copper Hill Vol- canics have been recognized at several locations lying west of the map area. Parallel bands of greenschist and amphibolite, possi- bly part of the Copper Hill Volcanics, crop out in the northwest section of plate 1. Field evidence suggests that the transition from greenschist to amphibolite is gradational, but tectonic juxtaposition cannot be ruled out. Although these rocks form north-northwest- trending bands, schistosity diverges greatly from this direction. The plane of schistosity has been deformed into chevron folds at some outcrops, broader open folds at others, and locally (NEM; sec. 18, T. 7 N., R. 10 E.) into isoclinal folds with superposed broad open folds. A few outcrops show interbeds of chert folded into a similar shape as the enclosing greenschist, suggesting that schistosity parallels original stratification of the parent volcanic rocks. Contacts within and surrounding the greenschist- amphibolite terrane have not been completely de- lineated by our mapping, but available evidence sug- gests that these rocks arefault bounded rather than in normal stratigraphic sequence with underlying Copper Hill strata. The structural relations shown by Clark (1964, pl. 8) suggest that the greenschist-amphibolite rocks form a fault-bounded block of the melange that has been transported from deeper levels of the crust than adjacent rocks of the Copper Hill Volcanics. INTRUSIVE ROCKS Plutonic masses of both relatively deep seated and hypabyssal origin intrude rocks of all but the western belt. Part of a large coarse-grained granitic intrusion crops out in the eastern belt, but nearly all the other plutons are relatively small, irregular in outline, mark- edly discordant to host rocks, and fine grained or por- phyritic. Compositions of the plutons range from ultramafic to granitic, although ultramafic rocks occur only in the melange. Original mineralogy has been al- tered in many plutons, but despite local overprinting of metamorphic texture or cataclastic structural fabric, the initial textures commonly are intact. Two masses of coarsely recrystallized hornfels lie within the large eastern pluton, but marked contact metamorphic ef- fects usually are lacking around intrusive boundaries; where present, such effects are restricted to apparent annealing, as shown by obliteration of rock schistosity without obvious recrystallization. MELANGE BELT Many types of intrusive rocks, ranging in composition from pyroxenite and serpentinite to granodiorite, have been recognized in the melange. Relative ages of the various intrusive rocks are generally impossible to de- termine because of isolation or lack of exposure along contacts. Outcrops of coarse-grained pyroxenite and hornblende pyroxenite are localized in a small area near the southwest corner of plate 1 (W1/2 sec. 15, T. 6 N., R. 10 E.); these ultramafic rocks are completely bounded by serpentinite or other intrusive rocks, including augite-hornblende gabbro and aphanitic fine-grained greenstone. Contact relations are not exposed, and the shape and distribution of these ultramafic bodies permit them to be interpreted either as tectonic blocks or inclu- sions in surrounding rocks, or as intrusions lying essen- tially in their original positions. Other equigranular rocks suggesting relatively deep seated crystallization are widely scattered in the melange. These rocks occur chiefly as small masses commonly associated with serpentinite and include pyroxene gabbro nearly completely altered to the greenschist facies, fine- to coarse-grained augite- hornblende gabbro (in a 1.6-km-long intrusive mass near the southwest corner of sec. 9, T. 7 N., R. 10 E.), brecciated medium-grained hornblende quartz diorite (known only from a single outcrop in southern sec. 9, T. 6 N., R. 10 E.), and relatively fine grained brecciated biotite granodiorite that forms very thin sill-like bodies (western sec. 27, T. 7 N., R. 10 E.). This last rock is extensively altered locally to dolomite, magnesite, and quartz. The bodies of ultramafic to mafic composition may be genetically related to the Pine Hill layered gabbro com- plex, located on strike with the melange belt about 13 km north of the area in El Dorado County. Springer (1969) and Emerson (1969) showed that the Pine Hill igneous complex is partly composed of sill-like intrusive masses that became gravitationally layered during crystallization. The conformable relation of this layer- ing to the steeply eastward dipping country rock there indicates that intrusion and crystallization took place 18 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA while the enclosing rocks were still subhorizontal, that is prior to development of the steep regional dip. Diverse types of aphanitic to porphyritic intrusive rocks are widely distributed in the melange. Lithologi- cally, some of these rocks are virtually indistinguish- able from flows and sills in the Logtown Ridge Forma- tion. Such rocks include coarse plagioclase porphyry greenstone and plagioclase-augite porphyry greenstone that are mineralogically and texturally equivalent to sills and pillow lavas in the Goat Hill Member of the Logtown Ridge Formation, as well as coarse augite por- phyry greenstone that is identical to flows and breccia clasts in the Rabbit Flat and Pokerville Members. Plagioclase porphyry greenstone similar to that re- . ported here was also recognized by Clark (1964, p. 31) in the Copper Hill Volcanics along the Cosumnes River a few miles west of the map area. The porphyritic rocks at that location are at least partly extrusive pillow lavas (Clark, 1964). Other porphyritic intrusive rocks have no counter- parts in the Logtown Ridge Formation. Plagioclase por- phyry, hornblende-plagioclase porphyry, and biotite- hornblende-plagioclase porphyry form relatively large but extremely irregular intrusive masses that are par- ticularly abundant in the eastern half of the melange belt. Quartz is sparse but ubiquitous in these rocks. The first two types of rock apparently grade into one another and into fine-grained aphanitic greenstone. All these rocks locally exhibit pervasive brecciation, metamor- phism to greenschist facies, and rock cleavage concor- dant with the adjacent metasedimentary rocks. The geometry of some plutons in the melange belt strongly suggests that their emplacement followed the main structural disturbances there. For example, the finely porphyritic and aphanitic rocks associated with the gabbroic body at the corner of secs. 8, 9, 16, and 17, T. 7 N., R. 10 E., appear to cut across boundaries of structural blocks within the melange. The brecciation of these intrusive rocks, moreover, does not appear to be related to brecciation in the wallrocks that the intrusive masses locally transect; brecciated intrusive rocks gen- erally are magmatically healed and tightly coherent, in strong contrast to the marked fissility of the apparently earlier sheared metasedimentary rocks. LOGTOWN RIDGE—MOTHER LODE BELT A single small intrusion is emplaced in this belt about 1.2 km east of Sutter Creek. This rock consists of medium-grained hornblende quartz monzonite; rocks of this composition have not been found elsewhere in the map area. Other intrusive rocks of generally mafic composition lie at the east edge of this belt along the Melones fault zone. They have been thoroughly metamorphosed and are described in the section “Melones Fault Zone.” EASTERN BELT The large coarse-grained granitic pluton underlying most of the eastern belt is part of a much larger mass that extends nearly 32 km to the northeast. It is sepa- rated from the main body of the Sierra Nevada batholith by intervening metamorphic rocks, but its composition, texture, and structural setting suggest that it is simply an outlying part of the batholithic complex. ‘ . The rock generally is fine- to medium-grained hornblende-biotite granodiorite, but at one locality near the Cosumnes River, it has a distinctly more mafic character, with a color index over 50. This variant, probably a quartz gabbro in composition, appears to grade laterally into the more typical and much lighter colored (color index 10—20) granodiorite. A north- to northwest-trending foliation of mafic minerals is com- mon through much of the pluton but is more strongly developed near the west margin. Prismatic crystals of hornblende form a steeply plunging lineation in the west marginal zone. Although there is abundant evidence of cataclasis along the west contact, other contacts appear to have formed by intrusion of a liquid or liquid-crystal mix- ture, with no subsequent crushing effects. Strongly dis- cordant contact relations in secs. 6, 7, and 18, T. 7 N., R. 11 E., and the weak to absent aureole of contact metamorphism in some parts of the adjacent Calaveras Formation suggest only a moderately deep level of em- placement. AGES OF INTRUSION Intrusive rocks in the west-central Sierra foothills generally have been assigned a Late Jurassic age by previous workers (Clark, 1964; Bateman and Wahrhaf- tig, 1966, p. 117; Evernden and Kistler, 1970). A Juras- sic age has been determined (R. W. Kistler, written commun., 1970) by the potassium-argon method for two rock types in western Amador County. A specimen of the granodioritic pluton in the eastern belt, collected near the center of sec. 22, T. 8 N., R. 11 E., about 4.8 km east of the area included on plate 1, contains biotite dated at 157.4:4.7 million years. A small quartz diorite mass in southern sec. 9, T. 6 N., R. 10 E., within the melange, gives a hornblende age of 172:4.3 million years. The latter date suggests an Early rather than Late Jurassic age, and owing to possible disturbance of the potassium-argon radiometric clock by faulting, this should be considered a minimum. Springer (1969) and Emerson (1969) described a com- plex of gabbroic sills near Pine Hill, about 13 km north of the map area and on strike with the melange belt, that has compositional layering of gravitational origin. The sills originally were nearly horizontal as indicated by the layering, but they now dip steeply eastward in SERPENTINITE AND RELATED ROCKS 19 conformable relation with the preintrusion metasedimentary rocks. Thus, at least some plutonism in the Sierra foothill belt had begun prior to regional tilting of strata to nearly vertical positions. Moreover, serpentinite at the same locality predates the gabbroic sills, and thus its emplacement also predates regional tilting. . Structural and mineralogical features of hypabyssal intrusive rocks in the melange belt suggest that these bodies may have been emplaced earlier than most of the coarse-grained rocks in this and other belts. All the porphyritic rock types at least locally are altered to greenschist facies, suggesting that the regional metamorphic overprint has affected them as well as the stratified rocks.- Rock cleavage locally present in the hypabyssal intrusive bodies further substantiates this interpretation. However, no radiometric ages have been obtained for these rocks. Most coarse-grained intrusive rocks apparently post- date regional metamorphism, as shown by the general absence of greenschist facies metamorphism. Moreover, some slight hornfelsing has occurred along the east margin of the large granodioritic body in the eastern belt; the zone of hornfelsic rocks is subparallel to the margin of the pluton, but it is discordant to the metamorphic foliation of the wallrocks and hence is younger. Foliation is evident in some coarse-grained rocks, but it probably is a primary igneous feature unre- lated to cleavage in the wallrocks, despite subparal- lelism of both with the regional structural grain. SERPENTINITE AND _RELATED ROCKS Serpentinite forms a 14.5-km-long sheared and faulted sill-like body that trends north-northwest along the west margin of the melange. It ranges in outcrop width from about 1.6 km to several tens of meters. Smaller satellitic bodies of serpentinite crop out both to the east and west of the main mass and are thought to define crudely the lateral limits of a zone of significant tectonic dislocation. Two approximately equant bodies of serpentinite are associated with the large plutons in secs. 8, 9, 16, and 17, T. 7 N., R. 10 E.; these are thought to be genetically related to the plutons or their em- placement. Most outcrops of serpentinite in the sill-like mass have many features in common. In plan, nearly all are elongate in a north-northwest direction, parallel to the regional structural grain. Some outcrops are massive, and others show a layered structure with scattered grains of bastite; however, most consist of slickensided lozenge-shaped fragments about 2 cm to 1 or 2 m in average diameter and generally flattened in one plane. The resulting foliation is steeply dipping but shows considerable variation in strike among outcrops, gener- ally ranging between N. 30° E. and N. 60° W. and ap- pearing to parallel the trend of the nearest wallrock contact. Locally, contacts are irregular in plan, but more commonly they are linear, or nearly so, and change trend abruptly, suggesting that they are steeply dipping, planar, and likely tectonic. However, they rarely are exposed for direct examination. The contact of one small serpentinite mass is exposed in a roadcut along Highway 124 (not shown on pl. 1) about 520 m north of the highway crossing over Dry Creek (El/z sec. 28, T. 7 N., R. 10 E.). Exposures there suggest that the contact between serpentinite and wall- rock conglomerate has an antiform shape that is dis- cordant to wallrock bedding and foliation in the axial region of this arched surface. Contact metamorphic ef- fects are clearly evident in a zone a few meters wide. The unmetamorphosed conglomerate has a black slate ma- trix that surrounds relatively unrecrystallized clasts of slate and porphyritic to fine-grained volcanic rocks of intermediate to mafic composition. The metamorphosed conglomerate has retained all original structures and textures, but it is pale green, more difficult to fracture, and almost completely replaced by chlorite and actinoli- tic amphibole. Similar metamorphic effects are not evi- dent at the contacts of other bodies of serpentinite. Rare inclusions of conglomerate and slate as much as 10 m in maximum dimension occur Within some bodies of serpentinite. Original structures and textures are preserved in these inclusions, but the rocks are har- dened and show a gray to gray-green color suggesting some recrystallization. Gneissic amphibolite locally oc- curs as both inclusions and narrow marginal bodies to serpentinite. Two occurrences of rodingite in serpentinite form steeply dipping dikes from 1 m to several centimeters thick; they have sharp contacts and generally are con- formable with foliation in the host serpentinite. At one locality, slickensided shear planes pass from serpentin- ite through the rodingite dike, but elsewhere unde- formed dikes are exposed for as much as 60 m along strike. Well-preserved relict textures in thin section suggest that the original rock was diabasic. Rare minor bodies of altered gabbro and related mafic rocks occur both marginally and internally to serpentinite, and large masses of brecciated red and gray carbonate with or without incrustations of chalcedonic silica show a similar association. This carbonate also has small scattered clots of chlorite, The presence of bastite and associated gabbroic rocks suggests that at least some of the serpentinite formed from preexisting ultramafic plutons, but evidence for the general origin of many bodies of serpentinite is lacking. If ultramafic rocks were the common precur- sors of serpentinite, the serpentinite cannot now be in the original intrusive position of the plutons because 20 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA contact metamorphic effects that would be expected in sedimentary rocks intruded by ultramafic melt are not evident. The spatial association of nonfissile mudstone of hornfelsic aspect near some serpentinite bodies sug- gests the possibility that the serpentinite or its precur- sor rocks were emplaced as at least moderately warm bodies later than the regional metamorphism. Alterna- tively, baking of the wallrocks bordering some of the serpentinite bodies could have preceded regional metamorphism if the resultant mechanical properties effectively prevented the development of slaty cleavage during later folding and metamorphism. Although the evidence in our study area is inconclusive and no definite assessment of the age of the now serpentinized bodies relative to metamorphism is possible, Springer (1969) and Emerson (1969) showed that ultramafic and mafic plutons, including serpentinite, are at least in part older than regional metamorphism in western El Dorado County, on strike with and within 13 km of the melange belt in Amador County. SUPERJACENT ROCKS Shallow-dipping sediments and sedimentary rocks of Tertiary age rest in angular unconformity on rocks of the subjacent series. Although such deposits once formed extensive aprons that may have completely covered the older rocks at the lower elevations of the Sierra foothills, only scattered and relatively small ero- sional remnants of this blanket are preserved in west- ern Amador County. Stratification in these deposits dips gently to the west, essentially parallel to the re- gional orientation of the surface of unconformity at their base. Detailed examination of the unconformity shows abundant evidence of marked local topographic irregularity and incision on the regional surface. Two units of Tertiary age have been differentiated on plate 1. Both are included with rocks called the inter- volcanic gravels by Lindgren (1911) and Bateman and Wahrhaftig (1966, p. 133). Other gravels called prevol- canic gravels (Bateman and Wahrhaftig, 1966, p. 133) are exposed in Amador County, but they lie entirely west and downslope of the area covered by plate 1. The lowermost intervolcanic deposit consists of two lithologies: quartzitic gravel, and conglomerate under- lain by rhyolitic tuff; it is here assigned to the Valley Springs Formation of Piper, Gale, Thomas, and Robin- son (1939). Unconformably overlying this formation at many locations, but resting directly on the subjacent rocks at most places, are volcanic gravels and conglom- erate correlated with the type Mehrten Formation of Piper, Gale, Thomas, and Robinson (1939). VALLEY SPRINGS FORMATION The Valley Springs Formation was defined by Piper, Gale, Thomas, and Robinson (1939) to include welded rhyolite tuffs and associated sedimentary rocks exposed in northern Calaveras County near the Mokelumne River. Geochronologic age determination of correlative rocks higher in the Sierra Nevada by Dalrymple (1964, fig. 3) showed that the Valley Springs Formation proba- bly ranges in age from 30 to 20 million years (late Oligocene to middle Miocene). A unit of Tertiary quartzitic gravel and conglomerate mapped on plate 1 is considered part of the Valley Springs Formation because it overlies at several places rhyolitic tuff that is unknown in the prevolcanic series (Slemmons, 1966, p. 201). Although we cannot demon- strate that all the quartzitic gravel is younger than the tuff, the close spatial distribution and lithologic uni- formity justify its inclusion with the Valley Springs. Quartzitic conglomerate and equivalent unconsoli- dated gravel form most of the Valley Springs Forma- tion. These strata are poorly bedded and contain well- rounded pebbles and cobbles of quartzite and bull quartz, with only very minor amounts of other types of clasts. Although the conglomerate is known to be as much as 12 In thick in the map area, only a thin veneer generally less than 2 m thick is preserved at most loca- tions. Many of the unconsolidated deposits have been extensively worked for gold by placer operations. At a few locations, Valley Springs conglomerate is underlain by massive white fine-grained tuff. Else- where, deeply weathered clay deposits have been found under the Valley Springs conglomerate, suggesting that tuff was once considerably more common than now and that it has weathered to clay. Tuff exposed at the top of the hill 3 km east-northeast of Plymouth is strongly indurated, suggesting that at least some of the tuff may be welded. MEHRTEN FORMATION The name Mehrten Formation was applied by Piper, Gale, Thomas, and Robinson (1939) and Curtis (1951; 1965) to andesite-clast mudflows and conglomerate at widely separated points on the west slope of the Sierra Nevada. Dalrymple (1964) radiometrically dated these rocks at 19—5 million years. Conglomerate and poorly consolidated gravel containing nearly monolithologic andesitic clasts within the map area are referred to this formation, although they could be equivalent to the Relief Peak Formation of Slemmons (1966, p. 203). The Relief Peak Formation includes rocks previously as- signed to the lower part of the Mehrten Formation in the foothills south of Amador County (Dalrymple, 1964; Slemmons, 1966). The maximum thickness of these de- posits is about 60 m, but presumably much thicker accumulations, perhaps as much as 300 m, once covered the area. (See Slemmons, 1966, fig. 4.) Andesitic gravels and conglomerate in the map area STRUCTURE 21 are generally poorly bedded and sorted where exposed. Clasts range in size from pebbles to very large boulders and are subangular to subrounded. The matrix is gray- colored finer volcanic debris, presumably also of andesi- tic composition. The andesitic rock that forms nearly all the clasts contains phenocrysts of plagioclase, clinopyroxene, hornblende, and olivine. Although the clasts are very nearly monolithologic, scattered frag- ments of granitic and metamorphic rocks are present locally' STRUCTURE Traditional reconstruction of geologic history in the western Sierra foothills involves the following succes- sion of principal events: (1) Deposition of strata from Paleozoic through at least Late Jurassic time (including possible periods of nondeposition or erosion, local metamorphism, and early folding episodes), (2) low- grade regional metamorphism with steep tilting as- sociated with complex major isoclinal folding and ac- companied by intrusion of plutonic and related hypabyssal rocks of the Sierra Nevada batholith, and (3) major faulting episodes along well-defined north- northwest-trending belts that have substantially al- tered the spatial distribution of the stratal units. Field relations and structural data obtained in our detailed study of the Sierra foothills in Amador County suggest an appreciably different scheme of events that may be applicable to a much larger area, even perhaps to most of the foothills west of the Melones fault zone. These events include (1) accumulation of volcanic and epiclastic material on the sea floor, accompanied by large- and small-scale differential movement on numerous gently dipping faults, (2) regional steep tilt- ing associated with small—scale very tight to isoclinal folding, greenschist—facies metamorphism, and intru- sion by plutonic and hypabyssal rocks, and (3) large- and small-scale local faulting and minor refolding of strata. These two solutions to the complex structural rela— tions differ mainly in the timing and nature of the large-scale movements that caused the gross redis- tribution of the principal rock types into their present patterns. This paper demonstrates that many of the large displacements probably occurred while the sedi- ments were essentially flat lying and that sediments continued to accumulate while this disruption of strata was in progress. Thus the mechanism of the major dislo- cations is thought to have operated in a relatively shal- low marine environment essentially within the basin of accumulation (near the continental margin?), perhaps resulting strictly from gravitational and subductional stresses rather than from compressional stresses and attendant‘ shearing associated solely with the em- placement of the Sierra Nevada batholith. Abundant literature has been devoted to melanges in the Franciscan terrane of coastal California (for exam- ple, Hsu, 1968; Page, 1970) and the probable relations to lithospheric plates subducting at the continental mar- gin there. In this paper, we suggest, specifically on the basis of newly obtained field data, the possibility that such events took place east of the Great Valley of California in Mesozoic time. Others ('for example, Hamilton, 1969; Davis, 1969) attempted to explain the tectonics of this area using these processes, but such attempts have simply interpreted grand-scale rock dis- tribution on the basis of the geological literature of this region. While we agree with this general type of in- terpretation, we believe that data published from pre- vious field studies in the Sierra foothills are not ade- quate to support such interpretations. For many geologists melange means literally a tec- tonic mixture of rocks, without connotations of genesis. For others, however, it carries a definite implication of how the mixture originated. We propose a specifically subduction related genesis for the melange of this re- port, but regardless of whether our proposal is correct or not, the existence of the melange is demonstrated by the outcrop mapping and must be accounted for in any com- prehensive explanation of the tectonic history of the foothills. In general, the Sierra foothills melange is indeed a chaotic tectonic mixture of rocks, but a limited degree of internal order is apparent from the geologic map (pl. 1) and gives some clues. to the extent of mixing that the originally undisturbed rocks have undergone. To some geologists, the degree of order shown by the map may not justify using the term melange; we believe that this viewpoint is incorrect, for if we have erred in accurately portraying structure within the melange, the map sug- gests a lesser amount of tectonic chaos than is actually present. Despite the apparent simplicity of some of the map units in plate 1, as well as the relatively undisturbed bedding visible in many outcrops within such map units, exposures in roadcuts and along stream courses reveal a marked degree of internal complexity in most units. In fact, these exposures suggest that many, and perhaps most, of the natural outcrops between roadcuts and streambeds are individual “clasts” in a great megabreccia. Their unfragmented state makes them relatively resistant to erosion, whereas the sheared matrix surrounding them is easily eroded. Thus, it is important to recognize that simplicity and continuity as recognized on older geologic maps may in many places mask faulting and shearing that are nearly pervasive between outcrops but cannot be proven generally be- cause of incomplete exposure. To interpret the geology of the melange strictly on the basis of natural outcrops 22 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA could lead to important misconceptions because the key evidence is largely concealed between the exposures. MELANGE BELT Inspection of plate 1 reveals that individual map units possess both extreme complexity and a limited degree of order and continuity on a local basis within the melange. By mapping essentially all outcrops, we have determined that most of the map units probably have abrupt, rather than gradational, contacts with one another and that in many places the possible loca- tions of the contacts essentially rule out sedimentary intertonguing as a common relationship. It is important to reemphasize here that most contacts have not been observed directly but rather are inferred from the dis- tribution of outcrops. Almost all those few contacts that have been observed are exposed in stream gullies and in relatively new roadcuts along Highways 16 and 124 (the latter is not shown on pl. 1). Certain associations of rock types have been critical in establishing some of the map units within the melange. Some lithologic assemblages contain rock types that are not found in other assemblages. Still other assemblages that consist chiefly of widely distri- buted lithologies are themselves distinctive because of the absence of certain rock types that are relatively resistant to erosion and hence should be exposed if pres- ent. Although the definitions of several of the units in the map explanation might imply virtually complete overlap, we think that each of the divisions is real and justifiable. Such divisions demonstrate the existence and the general geometry of each assemblage in the melange. To explain the distribution of lithologies and the sheared nature of many outcrops by lateral facies changes or folding does not seem tenable to us. The exact geographic extent of pervasive shearing in rocks of the melange is difficult to establish, although all indications suggest that it is widespread and gen— eral. Relatively few sheared outcrops are known in the western part of the melange, but they are quite common to the east, where good examples can be found on the banks ofAmador Creek (sec. 34, T. 7 N., R. 10 E.), in the banks ofthe Cosumnes River (secs. 20, 21, 28, T. 8 N., R. 10 E.), and in roadcuts along Highways 16 and 124 (sec. 21, T. 7 N., R. 10 E.). Elsewhere, we believe, exposures of sheared rock are less common simply because such rocks are not resistant enough to form outcrops. The pervasive shearing is characterized in pelitic material by countless steeply dipping subparallel shear surfaces along which considerable displacement ap- pears to have occurred. The trend of shear surfaces generally is constant at an outcrop but ranges from about N. 30° E. to N. 60° W. over the entire melange. This style of deformation is best developed in quartzose slate with associated chert or quartzose sandstone. Chert and sandstone typically form subrounded frag- ments or pods—remnants of once continuous beds, now floating in a mass of thoroughly sheared pelitic matrix—and these fragmented lithologies apparently remained relatively competent as the pelitic material flowed plastically to fill spaces left by disruptions in bedding. Several broad generalizations may be made concern- ing the distribution of rock types and distinctive associl ations of rocks Within the melange. South of the latitude of Drytown and along the west margin of the melange belt, irregularly sized and shaped blocks of Calaveras affinity, Cosumnes affinity, serpentinite, and intrusive rocks of various kinds are intermixed in a zone 1 km wide. (See pl. 1.) North of the latitude of Drytown, how- ever, this zone appears to be missing; its absence is perhaps partly reflected by the shorter distance be- tween the western serpentinite' belt and the base of the Logtown Ridge Formation. East of this zone and south of the latitude of Drytown, another distinctive band consisting chiefly of conglomerate of Cosumnes affinity is intermixed with a small proportion of rocks of Calaveras affinity and a few small serpentinite bddies. The continuity of this band is broken and displaced at the latitude of Highway 16, but a similar band of rocks continues northward, in contact with the massive block of serpentinite, to the Cosumnes River. Both north and south of Drytown, this zone gives way eastward to a regionally distinctive map unit consisting of thinly and rhythmically interbedded graywacke and clay slate of Cosumnes affinity. These strata apparently rest in dep- ositional contact on the underlying conglomerate, al- though the geometry of the contact is quite probably complicated by cross-faulting at low angles to the strike of the unit. Very locally, thin slices of silicic rocks of Calaveras affinity lie between the two bands, such as in sec. 3, T. 6 N., R. 10 E., and in sec. 5, T. 7 N., R. 10 E., suggesting that further tectonic complication is possi- ble along the contact. Graded bedding in the graywacke layers is prevalent throughout the unit and almost uni- formly shows the top of the section to be eastward; where the top directions are not eastward, small-scale folding is evident. . Another distinctive zone of rocks south of the latitude of Drytown lies in essentially continuous fault contact with the graywacke-slate band on the west and consists principally of rocks of Calaveras affinity that give way to rocks of Cosumnes affinity near the base of the Log- town Ridge Formation. Rocks with similar characteris- tics also extend northward from the latitude of Drytown, although the zone there has a considerably greater east—west dimension. This zone is further characterized, both north and south of Drytown, by many irregularly shaped small to relatively large STRUCTURE . . 23 bodies of hypabyssal igneous rocks that are lacking in the more western zones. Contacts between and often within these zones of the melange are tectonic. None of these faults, however, transect the contact between the melange and the over- lying Logtown Ridge Formation, suggesting that in- termixing within the melange predates the Logtown Ridge For ation. It might be argued that intermixing occurred a ter eruption and deposition of the Logtown Ridge For ation but did not affect them because of . their grea er strength or because they were effectively separated rom underlying rocks by the fault at the base of the L0 own Ridge. In view of the nature of faulting within th Logtown Ridge, however, it seems more likely to u that the melange formed either before the Logtown idge was deposited (before Callovian time) or at a some hat later time but before the melange and the Logto n Ridge were juxtaposed. If this la ter explanation is correct, then the two units must hav been subsequently juxtaposed from very dif- ferent geo ogic environments. The two are indeed in fault cont ct, but since both show greenschist-facies metamorp ism, only huge lateral tectonic dislocation would pe it this explanation; if one unit had been buried uch deeper than the other, grade of metamorp ism should reflect the difference. Large lat- eral motio would fit our scheme if faulting were along subhorizo tal surfaces, but we find no evidence for huge lateral di placement on steeply dipping faults, as the traditiona interpretation would require. Large dis- placemen along a subhorizontal thrust fault (subduc- tion surfa e?), however, is a distinct possibility. BEAR MOUNTAINS FAULT ZONE The we t boundary of the melange belt corresponds to part of th Bear Mountains fault zone of Clark (1960, 1964). Cl rk inferred the presence of this fault zone througho t the Sierra foothills from the gross strati- graphic r lations and distribution patterns of Mesozoic ably Paleozoic rocks, the localization of ser- pentinite in a relatively narrow linear band, and the occurrenc of sheared rocks at appropriate locations. In particula , the homoclinal sandwiching of what Clark and other before him believed to be a belt of Paleozoic rocks bet een Mesozoic rocks required major faulting to accoun for the repetition of Mesozoic strata; thus thousand .of meters of reverse-slip displacement on a group of teeply dipping faults, the Bear Mountains fault zon , was inferred. The ne evidence obtained in the present study, how- ever, allo s a very different interpretation of the field relations. In fact, with recognition of the melange, no evidence ow prohibits the accumulation and intermix- ing of a l rocks of the melange (“western belt of Calaveras” of earlier workers) after the accumulation of the underlying Mesozoic Copper Hill Volcanics, and thus no compelling evidence remains for a reversal of stratigraphic succession across the Bear Mountains fault zone in Amador County. Regardless of the presence or absence of this strati- graphic reversal, however, shearing and fault displace- ment occur at the west edge of the melange belt. The serpentinite bodies that are localized there provide much evidence of faulting. But there is no evidence of net fault displacement across a serpentinite mass. If the serpentinite bodies and their associated deep-seated igneous rocks were solidified before they intruded over- lying rocks, the abundance of shearing within and along their margins is explained. The driving force for such a process need have been no more than gravitational in- stability of low-density serpentinite, but other forces, perhaps reflecting horizontal compressive stress, could have been involved as well. The boundaries of the ser- pentinite masses may have been controlled by preexist- ing faults, as Clark (1964, p. 42) argued, but these faults may have formed during melange development, shortly after sedimentary accumulation rather than later, as initially steeply dipping “rooted” faults. In the sense that a fault should have a demonstrable net displacement across it, we conclude that the exis- tence of the Bear Mountains fault zone cannot be ascer- tained at present. The dissimilarity of stratigraphic units on opposing sides of the zone makes it difficult to prove net displacement. Nevertheless, the possibility that such displacement has occurred and that serpen- tinite masses subsequently have invaded these pre- existing shear zones, as suggested by Clark (1964), can- not be discounted. Ultimately, the distinction between “conventional” faulting as suggested by Clark and melange movements proposed by us probably must be based on whether the fault surfaces were rooted within the crust or were essentially parallel to the earth’s sur- face at the time of their activity. We have already of- fered some evidence favoring the second interpretation; additional support is provided in following sections. Even though the existence of large net displacement across the Bear Mountains fault zone is now question- able, an important reversal of stratigraphic succession apparently does exist in the foothills of Amador County between the Salt Spring Slate of Clark (1964), west of our map area, and the Mariposa Formation. Both of these rock units are late Oxfordian and early Kim- meridgian in age (Clark, 1964, p. 25, 29), and the some- what older Logtown Ridge Formation lies between them. All the'rocks dip steeply to the east in an upright position. This stratigraphic reversal and repetition of time units may have resulted from displacement on the fault at the base of the Logtown Ridge Formation, on the 24 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA Bear Mountains fault zone, or on presently unknown faults west of the map area. EVIDENCE FOR F AULTING OF LOGTOWN RIDGE STRATA IN PRE-MARIPOSA TIME Near Drytown, all the distinctive members in the Logtown Ridge Formation are displaced abruptly along a linear east-southeast trend (pl. 1). Because four mem- bers of the formation are involved, it is unlikely that this offset resulted from primary differences in strati- graphic thickness. A fold in the strata seems unlikely because contacts between members are nearly linear up to the discontinuity. A fault offset seems to be the only reasonable interpretation. Unfortunately, no direct evidence of faulting has been observed because of poor exposures, but the offsets of the easily mapped units are clear and, in our opinion, constitute sufficient evidence for the existence of the fault. The top of the Logtown Ridge Formation north of this transecting fault is downdropped as much as 250 m. Distinctive quartzitic conglomerate at the base of the Mariposa Formation abuts this positive topographic ir- regularity on the top of the Logtown Ridge, but higher Mariposa beds are essentially flat and continuous across the irregularity. The continuity of these higher beds indicates that the fault does not extend from the Logtown Ridge into the Mariposa Formation. The top of the Logtown Ridge Formation most probably is not a fault surface, in view of its irregularity and direct evi- dence from a single exposure where Highway 49 crosses theCosumnes River; thus the fault transecting the Log- town Ridge terminates at the top of the section and must be pre-Mariposa in age. A similar though less well documented pre-Mariposa fault in the Logtown Ridge is located about 3 km north of Drytown (pl. 1). The age of faulting along the base of the Logtown Ridge Formation is difficult to determine. A11 field data indicate that the fault crossing westward through Drytown terminates at the base of the formation, sug- gesting that the basal fault is relatively younger. How- ever, the cross-fault and the basal fault may have been simultaneously active, with the cross-fault splaying from the principal plane of dislocation as a minor finger. Some support for coeval activity of this sort is seen in field relations at and near the base of the Logtown Ridge Formation west and southwest of Jackson. (See pl. 1.) For example, about 3 km west of J ackson, the base of the Logtown Ridge Formation is displaced about 550 m by a northeast-trending cross-fault. The distribution of nearby outcrops in the melange belt suggests that the cross-fault does not extend west of the basal fault, and the undisturbed contact between the Rabbit Flat and Goat Hill Members of the Logtown Ridge Formation indicates that the fault does not extend far on trend to the northeast. Several reconstructions of the time rela- tions involved with this faulting are possible, but con- sidering the characteristics of faulting in the Logtown Ridge Formation as a whole, we believe that this tec- tonic dislocation occurred simultaneously on the basal fault and cross-fault in pre-Goat Hill time. Other fault relations show that some movement at the base of the Logtown Ridge Formation probably postdated the Mariposa Formation. At the latitude of Plymouth, a northeast-trending fault transects the Logtown Ridge Formation and continues through the Mariposa Formation before converging with the Melones fault zone. On the southwest this structure apparently terminates at the base of the Logtown Ridge, as suggested by the distribution of rock types nearby in the melange. (See pl. 1.) This implies that at least part of the fault at the base of the Logtown Ridge was active either coevally with deposition of the Mariposa Formation or at a later time. Episodes of faulting involving the Logtown Ridge Formation apparently began while Logtown Ridge dep- osition was underway and continued locally during and after accumulation of the Mariposa sediments. Be- cause the initial slopes on which deposition occurred probably were relatively flat, the faults that are now essentially vertical must have been much less steeply dipping when they were active. TILTING AND FOLDING After the formation of the melange and the Logtown Ridge and Mariposa faulting, all the rocks were tilted to their present steeply dipping positions, a process that was probably contemporaneous with late Mesozoic magmatism of the Sierra Nevada province. As the rocks were tilted, some were very tightly to isoclinally folded, and a slaty cleavage developed, which is locally axial planar and pervades rocks of the entire foothills area. Detailed statistical studies of these and associated structures elsewhere in the foothills (for example, see Best, 1963; Baird, 1962) indicate multiple episodes of folding, and a more recent analysis of regional deforma- tion extends the multiple episodes to the entire Sierra Nevada province (Kistler and others, 1971). Structural data from our map area generally suggest only two widespread periods of deformation—subhorizontal dis- locations and later tilting and folding—although the subhorizontal displacements continued for a long period of time, judging from the complex relations within the melange belt and Logtown Ridge and Mariposa Forma- tions. Metamorphic rock cleavage that parallels the domi- nant structural grain of the foothills is evident at nearly all outcrops, and its general orientation is north trend- ing (pl. 1). Paired measurements on bedding and cleav- age and on axes of folds are restricted to a few sequences of interbedded slate and graywacke in the melange belt ‘ STRUCTURE ( 25 and to Mar'posa slate and graywacke; elsewhere, either most roc s are not visibly bedded, or original stratificati n has been thoroughly disrupted. The paired me surements summarized in figures 4, 5, and 6 show the e sential parallelism of bedding and cleavage. Contoured stereographic projections of poles to bedding reflect the very tight to isoclinal nature of known folds, whereas measurements of fold axes suggest one episode of noncylindroidal folding. Clearly, the data are too few to develop strong statistical arguments against multi- ple episodes of folding, but the simplest explanation of available data is that axial plane cleavage and isoclinal folds formed simultaneously; the plunge of resulting fold axes varies greatly along a uniform strike. Such variation might have resulted from locally anomalous B FIGURE 4.—Summary of bedding, regional cleavage, and axes of minor fold for slate and graywacke of the Mariposa Formation between 'lymouth and Amador City. Projection on the lower hemispher- of an equal-area net. A, One hundred sixty-six poles to bedding co toured at 0.6, 3.0, 5.4, and 7.8 percent per 1 percent total area. Fold axes are marked by X. B, Fifty-seven poles to cleavage contoured t 1.8, 5.3, and 19.3 percent per 1 percent total area. N N B FIGURE 5. minor fol s for slate and graywacke of the melange belt south of Highway 6. A, Fifty-five poles to bedding contoured at 1.8 and 12.7 percent p.r 1 percent total area. Fold axes are marked by X. B, Fifty-seve poles to cleavage contoured at 1.8, 12.3, and 29.8 per- cent per 1 percent total area. ummary of bedding, regional cleavage, and axes of FIGURE 6.—Summary of bedding, regional cleavage, and axis of a minor fold for slate and graywacke of the melange belt north of Highway 16. A, Sixty-two poles to bedding contoured at 1.6, 4.8, 14.5, and 21.0 percent per 1 percent total area. Fold axis is marked by X . B, Sixty-five poles to cleavage contoured at 1.5, 4.6, 13.8, and 35.4 percent per 1 percent total area. fluid pore pressure and the uneven vertical or horizon- tal strain of material during deWatering of what proba- bly was predominantly a sequence of water-saturated sedimentary rocks. . A word of caution is offered here for structural studies of cleaved slate and graywacke sequences elsewhere in the Sierra foothills. Even though cleavage generally parallels axial planes of known folds, and, statistically, zones of cleavage-bedding intersections are parallel to zones of fold axes, the relation of bedding to cleavage at an outcrop is not a dependable indication of strati- graphic top. This is evident from our study at numerous outcrops where stratigraphic top was determined inde- pendently from multiple graded beds. A similar situa- tion probably exists throughout the foothills. DEFORMATION AFTER REGIONAL TILTING The map area contains evidence for both local and more widespread deformation subsequent to the episode of tilting and folding. Fracture cleavage in phyllite was described in the section “Eastern Belt.” The areal ex- tent of this cleavage is unknown, but it appears to be restricted to rocks east of the Melones fault zone in the map area. Other posttilting structures are better documented. MELONES FAULT ZONE Clark (1960, p. 493—494; 1964, pl. 1) described the Melones fault zone as a major Late Jurassic zone of dislocation that juxtaposes Paleozoic rocks upon Mesozoic ones and truncates major folds in rocks of both ages. Rocks within the zone of faulting are also gener- ally characterized by relatively great shearing. Lacking any substantial new evidence to the con- trary, we follow Clark’s suggestion that the Melones fault formed in Late Jurassic time after the rocks of the foothills had been tilted to their presently steep at- 26 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA titudes. Future studies, however, may indicate a sub- horizontal origin, similar to that proposed by us for the faulting in the melange and in the Logtown Ridge and Mariposa Formations. Davis (1969) has already pro- posed such an origin with broad correlations between major structures and map units in the Klamath Moun- tains and Sierra Nevada. 7 In Amador County, the Melones fault zone ranges from about 250 to 800 m in outcrop width and generally separates the Mariposa Formation on the west from a granodiorite pluton and Calaveras Formation on the east. Most rocks within the fault zone are pervasively sheared and recrystallized phyllitic greenstone of un- known age. The phyllitic fabric and the fault zone itself are parallel. Minor amounts of other lithologies are also contained within the fault zone, especially in the south- eastern part of plate 1; these are described elsewhere in the text. Contacts of the fault zone with adjacent rocks gener- ally are gradational over a width on the order of tens of meters. However, the eastern contact, along the granodioritic pluton of the eastern belt, is difficult to determine because cataclastic texture of the pluton is structurally conformable to phyllitic greenstone of the fault zone there. Moreover, these cataclastic structures closely resemble shear structures in rocks of the fault zone, suggesting that the age of the pluton is prefault- ing and that marginal cataclasis is due to Melones fault- ing. This interpretation is consistent with radiometric and fossil-derived ages. Near Indian Gulch east of Amador City, however, the granodiorite transects the boundary between Calaveras rocks of the eastern belt and the phyllonitic greenstone belt, indicating that the intrusion is younger than the juxtaposition of Calaveras and the phyllonitic greenstone. If that jux- taposition resulted from movement on the Melones fault zone, then the marginal cataclasis in the pluton must postdate at least some of this movement. However, it is conceivable that this Calaveras-phyllonitic green-l stone contact is essentially an original irregular strati- graphic boundary and that subsequent shearing has oc- curred but has not substantially altered the relative positions of the two rock types. If this were true, then shearing of all the rocks, including the granodiorite, could be coeval with or younger than the emplacement of the pluton. We arbitrarily chose to exclude the mar- gin of the pluton from the fault zone. The direction and amount of slip on the Melones fault zone are not well known. Clark (1964, p. 51) concluded that a vertical component of 900—4,500 m is indicated by the juxtaposition of Paleozoic strata upon Mesozoic rocks; he also believed that steeply plunging lineations within the fault zone owe their origin to a major compo- nent of strike-slip displacement. Similarly, our work suggests vertical displacement on the order of hundreds and possibly thousands of meters, but no substantive support for strike-slip movement was discovered. MOTHER LODE FAULT SYSTEM The term Mother Lode fault system is used here for the eastward-dipping faults that are locally occupied by gold-bearing quartz veins of the Sierra foothills. This system of faults is minor in terms of offset but is of considerable importance for the great wealth that came from its ore bodies. Little new information on the nature of this system of faults was found during this study. Therefore, most of the pertinent information comes from previously published reports, primarily from the study within mine shafts by Knopf (1929). The Mother Lode fault system traverses the Sierra foothills in a north-northwest direction for at least 190 km, from Mariposa to Georgetown (Knopf, 1929, p. 2, fig. 1). For most of its length, the Mother Lode fault system is in or very near the Melones fault zone (Clark, 1964, p. 52). However, locally the two structures di- verge, and there is evidence that formation of the Mother Lode veins postdates movements on the Melones fault zone. (See Knopf, 1929, p. 24—25.) Move- ment on faults of the Mother Lode system apparently was neither simple nor great, but Knopf (1929, p. 5) believed that offset was primarily in ,a reverse sense, and he cited a maximum displacement of 115 m along dip. In the southern half of the area of plate 1, most of the Mother Lode faults lie about 1.6 km west of the Melones fault zone, but from about 1.6 km south of Plymouth to the north boundary of the map, the two structures over- lap extensively. Surface evidence of the Mother Lode faults includes scattered massive veins of milky quartz, thin bands of altered wallrocks, and folded cleavage originally formed during regional tilting. The veins trend roughly north-northwest and dip steeply to the east, and the chief evidence that deformation accom- panied their formation is seen locally where nearby slaty cleavage has been folded and relatively thorough shearing is apparent in massive greenstones. Such ef- fects rarely extend more than several meters away from known veins. In the southern part of the area, most of the major veins are located at or very near the contact between slate and greenstone of the Mariposa Formation. How- ever, Knopf (1929, p. 54, 55, 57, 58) showed that these veins dip less steeply to the east than the lithologic contact, and so at depth they are enclosed entirely within greenstone. REGIONAL CRUSTAL FLEXURE The map area includes the axial zone of a large-scale flexure of possibly fundamental significance to under- standing late Jurassic or post-Jurassic crustal move- ments fo the Western United States. This flexure is evident o . geologic maps of the Sierra Nevada province as a change in trend of prebatholithic rock systems and the batho ithic axis itself. South of the general latitude of High ay 16, the trend is approximately north- Geologic stratigra why and structure by N. L. Taliaferro, and finally th- revisions offered by L. C. Clark. These prin- cipal inv -stigators shaped the present understanding of pre-Ceno oic foothills geology, and fortunately, each of these wo kers summarized his ideas of the fundamental nature 0: the geology in Amador County with a cross section d awn along the Cosumnes River, the north boundar of plate 1. For co parison, each of the earlier cross sections to- gether w'th our interpretation are reproduced in figure 7. Altho gh greatly generalized, these diagrams sum- marize t e evolution of thought pertaining to the geol- ogy of th: central foothills region. They do not indicate the impOI tance of subhorizontal tectonic dislocations inferred rom the present study; this point has been emphasied adequately in earlier parts of the text. The of such divergent interpretations is graphic during ative subduction at a continental margin before the strat were tilted to their present steeply dipping attitude , we have tried to avoid overinterpreting the new dat.. However, some degree of overinterpretation is necess .: ry, for neither our reconstruction of structural history n r the more traditional synthesis can be proved from th presently available data. We believe, REGIONAL IMPLICATIONS 27 nevertheless, that the “subhorizontal” explanation is at least as reasonable as the traditional one, and implica- tions of both must be entertained equally. Ultimately, the choice between this and the traditional type of synthesis can probably be made only after many more data are available. Recent advances in understanding motions of the earth’s lithospheric plates and processes at plate boundaries as embodied in the rapidly expand- ing literature of global plate tectonics provide some indirect support for the foothills structural history we propose. The general foothills setting—belts of greatly deformed rocks that face parallel belts of time-related igneous rocks—is appropriate for the generally agreed upon features and related processes of subduction zones, and the melange is just the sort of structural chaos that might be expected to form at or near the surface of such a zone. This line of reasoning is, of course, largely circu- lar but is at least partly satisfying in that it relates observations to known processes of tectonism and mountain building. The first modern comprehensive synthesis of the geologic history and origin of rocks of the Sierra Nevada province was attempted by Bateman and his coworkers (Bateman and others, 1963; Bateman and Eaton, 1967). These investigators believed that Paleozoic and Mesozoic prebatholithic rocks collected in a subsiding trench, a complex synclinorium, whose root eventually melted in part to form magma; the relatively less dense magma subsequently pierced the overlying strata, forming a batholithic spine along the major axis of the trench. This is essentially a specific application of the traditional tectogene model of Griggs (1939). The recent popularity of global plate tectonics, how- ever, has given rise to several new and entirely different interpretations for the origin of structures and plutons in the Sierran province (for example, Davis, 1969; Hamilton, 1969; Dickinson, 1970; Kistler and others, 1971; Shaw and others, 1971). In general, these new syntheses relate known data to the presumed dynamics at plate boundaries with a tantalizing degree of success. In our opinion, neither type of genetic model can be proved or disproved with the present state of knowledge of rocks of the province. We hope that this paper demon- strates the need for many more detailed field observa- tions before any well-founded choice between the two models (or a compromise or a completely new model) can reasonably be made. We have now shown, for example, that fundamental stratigraphy and structure of the least deformed subjacent rocks of the foothills are not yet accurately established, which encourages the suspi- cion that even larger errors have been made in the presently depicted geology of the more deformed units. If additional detailed mapping demonstrates the per- sistence of the melange belt throughout the foothills, 28 Latrobe Bridge 5 m 2 llllll IHIII C alaveras Formation (Carboniferous) m —— Latrobe Bridge ~ 2 E SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA Huse Bridge APPROXIMATE SCALE Age of Mariposa Slate and older k l u < vial"! . . . Manposa Slate Amphibolite Serpentine Diabase (Jurassic — Derived from diabare, Cretaceous) gabbra, and other mafic racks _I\\l\//// /\/ \\/\ Granodiorite Calaveras Formation, undifferentiated (Paleozoic) Middle and Upper Jurassic Amador Group ‘ " I / i ' I i - I - i Cosumnes Formation Well-bedded ruffr, Chen‘s, slates, sandstone, and black limestone Serpentine and gabbro (Upper Jurassic) Marlposa Formation SIa res, sandstone, and conglam era 22: Logtown Ridge Formation chiefly nugite anderite, agglomerate, flawx. and ruff: Basal conglomerates and sandstones FIGURE 7.—See facing page caption for explanation. Granodiorite (Upper Jurassic) REGIONAL IMPLICATIONS 29 WESTERN FAULT BLOCK CENTRAL FAULT BLOCK AND FAULT ZONES B O a «as! a E 32 3 £6 :5 MELONES FAULT BEAR MOUNTAINS FAULT ZONE C .‘ ' l l - 0 ° 0 0 ° ‘0 ' ‘ - . o o 1 o a o o H I'I' I I I ' Ii Ni": «I - ' o ’ o - n ' n o O 0 ' l i - 0: 1° 2 '1“ l ‘ Z . I . I . o o - o o o- o . . °o I o o "I ° Volcanic: . ' I - J °o . o o o ' o ‘I. I - I. °°l30nlol Epiclanic Calaveras Cosurnnes Logtown Ridge Epiclastic Volcanic rocks Formation Formation Formation rock: rock: 0 1 2 3 4 5 KM I I I | I 4 APPROXIMATE SCALE Amador Group ~ - I not}, -?:o' o: 0’ ’// Mili' gIM’ 1020:? 5955,!“ l///\/\ .; _ . o a _ . ' . , . IaII- Oiao. 01?:90 5M 7~((uli l\\\ Calaveras Cosumnes Formation Logtown Ridge Epiclastic rocks Volcanic rocks Greenschist Ultramafic rocks Plutonic rocks Formation (Upper Jurassic) Formation of uncertain of uncertain (Upper Jurassic) (Jurassic and (Permian) (Upper Jurassic) stratigraphic stratigraphic position Cretaceous) position (Upper (Upper Jurassic) Jurassic) 0 WESTERN LOGTOWN mocs— EASTERN n a: BELT MELANGE BELT MOTHER LODE BELT BELT .. u m ‘: 3‘3; .1 m 3: MELONES BEAR MOUNTAINS “3 FAULT FAULT ZONE ZONE W§W A Sedimentary and voIcnnic rocks 2 I l I 3 I I APPROXIMATE SCALE . o ' 9 i ‘ I n g I I I ‘ a \/ / > w I . m /\ 011 (1610 1lo 15> , (U 1\<\\ Amphibolite Serpentinite Sedimentary and Logtown Ridge Mariposa Formation Greenschist Granodiorite (Jurassic) volcanic rocks Formation (Upper Jurassic) (Jurassic) (Upper Jurassic) FIGURE 7.—Four cross sections illustrating the evolution of interpre- g tations of geologic structures in pre-Cenozoic rocks along the Cosumnes River. Sources of information: A, Lindgren and Turner (1894); B, Taliaferro (1943, fig. 2); C, Clark (1964, p1. 8);D, Present study. The lithologic symbols do not correspond from one section to another, but each author generally uses them to depict the orientation of bedding. Light lines represent normal strati— this will constitute important new data for selecting the most probable among various models of rock deforma- tion, mountain building, and plutonism for the Sierra Nevada province. The melange may well mark the locus of convergence between an oceanic plate that abuts and plunges beneath a continental margin, but it only sug- gests this conclusion and must be weighed together with all other data to select the most probable solution. graphic contacts, and heavy lines represent faults. Light broken sinu- ous lines in sections C and D depict shearing of certain zones, and unbroken sinuous lines in A, C, and D depict folding in amphibolite and shearing in the melange (D only). With the exception of the newly discovered melange, the section from the present study is most nearly like section A, which was the first to be published. Considered alone, however, the melange does require that those who adhere to the ideas of a subsiding trough and anatexis explain the apparent absence of similarly deformed rocks on the east limb of the trough. The presence of only a western melange gives a pronounced asymmetry to the trough that is not easily explained by a simple subsiding basin in which there is sediment . accumulation. 30 SIERRA FOOTHILLS MELANGE, AMADOR COUNTY, CALIFORNIA REFERENCES :» Baird, A. A., 1962, Superposed deformation in the central Sierra Nevada foothills east of the Mother Lode: California Univ. Pubs. Geol. Sci., v. 42, p. 1—70. Bateman, P.C., Clark, L. D., Huber, N. K., Moore, J. G., and Rinehart, C. D., 1963, The Sierra Nevada batholith—a synthesis of recent work across the central part: U.S. Geol. Survey Prof. Paper 414—D, 46 p. Bateman, P. C., and Eaton, J. P., 1967, Sierra Nevada batholith: Science, v. 158, p. 1407-4417. Bateman, P. C., and Wahrhaftig, Clyde, 1966, Geology of the Sierra Nevada: California Div. Mines and Geology Bull. 190, p. 107—172. Becker, G. F., 1885, Notes on the stratigraphy of California: U.S. Geol. Survey Bull. 19, 28 p. 1900, Mother Lode District folio: U.S. Geol. Survey Folio 63. Best, M.G., 1963, Petrology and structural analysis of metamorphic rocks in the southwestern Sierra Nevada foothills, California: California Univ. Pubs. Geol. Sci., v. 42, no. 3, p. 111—158. Clark, L. D., 1960, The foothills fault system, western Sierra Nevada, California: Geol. Soc. America Bull., v. 71, p. 483—496. 1964, Stratigraphy and structure of part of the western Sierra Nevada metamorphic belt, California: U.S. Geol. Survey Prof. Paper 410, 70 p. 1970, Geology of the San Andreas 15-minute quadrangle, Calaveras County, California: California Div. Mines and Geology Bull. 195, 23 p. Clark, L. D., Stromquist, A. A., and Tatlock, D. B., 1963, Geologic map of San Andreas quadrangle, Calaveras County, California: U.S. Geol. Survey Geol. Quad. Map GQ—222, scale 1:62,500. Curtis, G. H., 1951, The geology of the Topaz Lake quadrangle and the eastern half of Ebbetts Pass quadrangle: California Univ., Ber- keley, Ph. D. thesis, 310 p. 1965, Hope Valley to Coleville, in Guidebook for Field Confer- ence 1, Northern Great Basin and California: Internat, Assoc. Quaternary Research, 7th Cong, USA, 1965, p. 63—7 1. Dalrymple, G. B., 1964, Cenozoic chronology of the Sierra Nevada, California: California Univ. Pubs. Geol. Sci., v. 47, 41 p. Davis, G. A., 1969, Tectonic correlations, Klamath Mountains and western Sierra Nevada, California: Geol. Soc. America Bull., v. 80, no. 6, p. 1095—1108. Dickinson, W. R., 1970, Relations of andesites, granites, and deriva- tive sandstones to arc-trench tectonics: Rev. Geophys. and Space Phys., v. 8, no. 4, p. 813—860. Douglass, R. C., 1967, Permian Tethyan fusulinids from California: U.S. Geol. Survey Prof. Paper 593—A, 13 p. Duffield, W. A., 1969, Concentric structure in elongate pillows, Amador County, California, in Geological Survey research 1969: U.S. Geol. Survey Prof. Paper 650—D, p. D19—D25. Emerson, D. 0., 1969, Petrology of the Pine Hill layered gabbro complex, Sierra Nevada foothills, California [abs]: Geol. Soc. America Spec. Paper 121, p. 503—504. Eric, J. H., Stromquist, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angels Camp and Sonora quadrangles, Calaveras and Tuolumne Counties, California: California Div. Mines and Geology Spec. Rept. 41, 55 p. Evernden, J. F., and Kistler, R. W., 1970, Chronology of emplacement of Mesozoic batholithic complexes in California and western Nevada: U.S. Geol. Survey Prof. Paper 623, 42p. Fiske, R. S., 1963, Subaqueous pyroclastic flows in the Ohanapecosh Formation, Washington: Geol. Soc. America Bull., v. 74, no. 4, p. 391—406. Fiske, R. S., and Matsuda, Tokihiko, 1964, Submarine equivalents of ash flows in the Tokiwa Formation, Japan: Am. Jour. Sci., v. 262, p. 76—106. Griggs, D. T., 1939, A theory of mountain-building: Am. Jour. Sci., v. 237, p. 611—650. Gudde, E. G., 1969, California place names: California Univ. Press, 416 p. Hamilton, Warren, 1969, Mesozoic California and the underflow of Pacific mantle: Geol. Soc. America Bull., v. 80, no. 12, p. 2409—2430. Henderson, J. R., Jr., Stromquist, A. A., and Jespersen, Anna, 1966, Aeromagnetic map of parts of the Mother Lode gold and Sierra Foothills copper mining districts, California, and its geologic interpretation: U.S. Geol. Survey Geophys. Inv. Map GP—561, scale 1:62,500. Hsii, K. J., 1968, Principles of mélanges and their bearing on the Franciscan-Knoxville paradox: Geol. Soc. America Bull., v. 79, no. 8, p. 1063—1074. Imlay, R. W., 1961, Late Jurassic ammonites from the western Sierra Nevada, California: U.S. Geol. Survey Prof. Paper 374—D, 30p. Kistler, R. W., Evemden, J. F., and Shaw, H. R., 1971, Sierra Nevada plutonic cycle; Part 1, Origin of composite granitic batholiths: Geol. soc. America Bull., v. 82, no. 4, p. 853—868. Knopf, Adolph, 1929, The Mother Lode system of California: U.S. Geol. Survey Prof. Paper 157, 88 p. Lindgren, Waldemar, 19 1 1, The Tertiary gravels of the Sierra Nevada of California: U.S. Geol. Survey Prof. Paper 73, 226 p. Lindgren, Waldemar, and Turner, H. W., 1894, Placerville, Calif: U.S. Geol. Survey Geol. Atlas, Folio 3, 3 p. Page, B. M., 1970, Sur-Nacimiento fault zone of California: Continen- tal margin tectonics: Geol. Soc. America Bull., v. 81, no. 3, p. 667—690. Piper, A. M., Gale, H. S., Thomas, H. E., and Robinson, T. W., 1939, Geology and ground-water hydrology of the Mokelumne Hill area, California: U.S. Geol. Survey Water-Supply Paper 780, 230 p. Sharp, R. V., and Duffield, W. A., 1973, Reinterpretation of the boun- dary between the Cosumnes and Logtown Ridge Formations, Amador County, California: Geol. Soc. America Bull., v. 84, p. 3969—3976. Shaw, H. R., Kistler, R. W., and Evemden, J. F., 1971, Sierra Nevada plutonic cycle; Part 2, Tidal energy and a hypothesis for orogenic-epeirogenic periodicities: Geol. Soc. America Bull., v. 82, no. 4, p. 869—896. Slemmons, D. B., 1966, Cenozoic volcanism of the central Sierra Nevada, California: California Div. Mines and Geology Bull. 190, p. 199—208. Springer, R. K., 1969, Structure of the Pine Hill layered gabbro com- plex, Sierra Nevada foothills, California [abs]: Geol. Soc. America Spec. Paper 121, p. 563. Taliaferro, N. L., 1942, Geologic history and correlation of the Juras- sic of southwestern Oregon and California: Geol. Soc. America Bull., v. 53, no. 1, p. 71—112. 1943, Manganese deposits of the Sierra Nevada, their genesis and metamorphism: California Div. Mines Bull. 125, p. 27 7—332. Turner, H. W., 1893a, Some recent contributions to the geology of California: Am. Geologist, v. 11, p. 307—324. 1893b, Mesozoic granite in Plumas County, California, and the Calaveras formation: Am. Geologist, v. 11, p. 425—426. 1894a, Jackson, California: U.S. Geol. Survey Geol. Atlas, Folio 11, 6 p. 1894b, The rocks of the Sierra Nevada: U.S. Geol. Survey 14th Ann. Rept., pt. 2, 1892—1893, p. 435—495. U.S. Geological Survey, 1969, Aeromagnetic map of the northern Mother Lode area, California: U.S. Geol. Survey Geophys. Inv. Map GP—671, scale 1162,500. \‘ PLATE 1 CRETACEOUS EASTERN BELT Cretaceous PROFESSIONAL PAPER 827 MELONES FAULT ZONE m C i s C m RM Ho Z OR S O A E U .J R L .. w M a. .m w m ) r n’. 0“ Wu W P P n. U U . I} LL 4. . m. K + C N E _ W A E GN R Lm AK w AO N PZ U Upper Jurassic LOGTOWN RIDGE ~ MOTHER LODE BELT } QUATERNARY UNCONFORMITY } TERTIARY iii ozoN :53 3:032 05 ‘8 830% th i Medium grained locally grada- ine grained pumiceous- equigranular to 1 -c1ast conglomerate. , breccia, and minor fine to coarse grained; Jurassic) — Medium Coarse grained ized equivalent of units ' locally intersheared w J ) , micaceous quartzitic sand— fine grained nonporphyritic (Upper? medium grained ins abundant white mica; inter- i Thickly interbedded. Equivalent to VOLCANIC-CLAST CONGLOMERATE shale-clast conglomerate, and sparse ted; locally gradational into phyllitic , quartzitic 7- Thinly and rhythmically interbedded. locally brecciated 7 conta ina Dark gray to black, > altered, medium grained FAULT Holocene Pliocene and Miocene Miocene and Oligocene U NN AM ED } } ANGULAR SUPERJ ACEN T SERIES SUBJACEN T SERIES Qa Qf CORRELATION OF MAP UNITS REGIONAL MELANGE BELT .820 He 338: _. 33 romcfiua t8 EoEQBuSU mo ow< antigoritic; associated with serpentinite abundant chert nodules , (Paleozoic?) ~ Siliceous clay slate with abun- dant interbedded chert. Equivalent to unit ssh in the melange belt - MOTHER LODE BELT GREENSTONE AND AMPHIBOLITE # Phyllitic and phyllonitic greenstone grained quartzite stone, and micaceous quartzite CLAY SLATE — Siliceous. Contains distinctive interbedded red chert and minor ) another same as ssh, but also interbedded with scattered gradational into limestone (unit 152) WESTERN BELT MELANGE BELT : abundant interbedded chert Brecciated, altered, EASTERN BELT 7 - to coarse Augite porphyry greenstone flows (locally pillow structured), breccia, and minor bedded tuffaceous greenstone Goat Hill Member. Thinly to thickly interbedded massive volcanic-breccia DESCRIPTION OF MAP UNITS plagioclase porphyry greenstone locally including white-mica amphibolite a minor metaleucogabbro and hornblendite with veins of epidote BIOTITE GRANODIORITE layers of unit mep Volcanic-breccia greenstone with chiefly — Siliceous a to medium-grained bedded to massive tuffaceous greenstone; locally interbedded with clay slate and graywacke. Locally includes hite-mica hornfels, locally schistose LOGTOWN RIDGE grained schistose equivalent of Jchl, with interbedded chert and other grained siliceous rocks grained schist equivalent to Jchz; tightly crenulated and locally con- verted to amphibolite -structured sills or flowsof coarse plagioclase porphyry greenstone grades locally into quartz gabbro. Western margin is strongly foliated HORNBLENDE GABBRO — Fine grained SERIENTINITE (Upper? Jurassic) te, and tremolitic rocks RELATIVELY UNSHEARED SERPENTINITE minor e a clasts gradational(?) into augite porphyry greenstone (unit mep) tional(?) into nonporphyritic greenstone (unit meb) yritiC' grainedphyllitic-greenstone, minor clay slate and tuffaceous slate mgr Massive or brecciated coarse augite porphyry greenstone; clasts, but includes sparse clasts of coarse augite porphyry ER HILL VOLCANICS (Upper Jurassic) Pillow Fine- Massive sills or flows of coarse plagioclase (—augite) porphyry greenstone ‘ONGLY SHEARED SERPENTINITE- Locally contains talcose rocks, greenstone (with fine-grained clasts), tuffaceous greenstone, artzitic gravel and conglomerate lapilli—tuff greenstone, and minor slate e n 0 t S n e e r g S u 0 e C a f f u t. d e d d e b N m T A M R O F S A R E V A L A C intrusive contacts Medium-grained limestone P Fine- Locally include minor shale—clast granule to pebble conglomerate conglomerate. Locally gradational into quartzite (unit Q2) MUDSTONE — Nonfissile to weakly fissile; sparsely interbedded chert; blocky usually associated with serpentinite bodies t r e h c d n a e n o t S d n a S C .1 t .1 Z t r a u 0.. f 0 S d e b s, u 0 e .w H S _ E T A L S Y A L C dolomite-magnesite-quartz rock HORNBLFNDE QUARTZ DIORITE i Brecciated, medium grained e, k C a w y a «1 mg um ww wa new 5.0 66 db mm C mm WI. ME C 0T LA L S locally gradational into one Aphanitic greenstone lenses of medium glomerate VALLEY SPRINGS FORMATION (Miocene and Oligocene) fine rod red limestone QUARTZITE i Calcareous weathering LIMESTONE — Fine to coarse grained riceous w intrusive, and fault contacts, as well as possible gradational boundaries of shear zones at large angles to shear surfaces, and coarse recrystallization boundaries surrounding some phyllitic rocks. Approximately located Strike and dip of beds Strike and dip of inclined axial plane, showing bearing and plunge of fold axis lnterbedded graywacke, granule shale-clast conglomerate, and clay slate Pebble to cobble conglomerate with well-rounded siliceous clasts New Chicago Member. Volcanic-breccia greenstone with chiefly f Siliceous phyllite, chert, and phyllonite, with local hornfelsic texture near Bearing and plunge of fold axis. Orientation of axial plane not determined Tightly crenulated medium—grained amphibolite and minor chert LOGTOWN RIDGE FORMATION (Upper Jurassic) Black clay slate; locally thinly interbedded with graywacke Strike of vertical axial plane, showing plunge of fold axis 6 SI. S km w WS 0 IS fl rum m H n ts .md m an g e w. nm V. i h )2 p mt r. r o t8 p .Iu nq m (u\ .1 h g M.” A mw . T. S dw .poe u“ m me 6 mm M .me t mm m F WE t mE m OT. w DA R L S rocks, and mineral layering in amphibolite Overturned, top unknown but suggested Inclined E T. A R E M O L G N 0 cc in wm we tE n L mA pm mE mL mm aE mm WT mm mU mm R G White welded rhyolitic tuff Plagioclase porphyry greenstone bedded with clay slate QUARTZITE MARKER BEDS A Lam quartzite (unit lq1 ) Brower Creek Volcanic Member Pokerville Member. white milky quartz HORNBLEN DE Top unknown but suggested Overturned, top known Vertical, top known Vertical, top unknown WHITE-MICA PHYLLONITE — Laminated Hornblende crystals and amphibolite HORNBLENDE METAGABBRO—Fine to coarse grained Medium Medium gnined located Contact ~ No evidence of tectonic movement.Probably includes depositional, Top known Vertical Inclined Vertical Inclined Vertical porph HORNBLENDE MYLONITE v Porphyroclastic; mylonit Qu S MEHRTEN FORMATION (Pliocene and Miocene) — Andesitic gravel and con- VERY COARSELY PORPHYRITIC INTRUSIVE ROCKS FILL, MINE DUMPS, AND TAILINGS Plagioclase—augite porphyry greenstone Augite porphyry greenstone APHANITIC AND FINELY PORPHYRITIC INTRUSIVE ROCKS — Variants are Plagioclase porphyry greenstone Hornblende-plagioclase porphyry greenstone Biotite-hornblende ALLUVIUM AUGITE COP ALASKITE 7 Fine to medium grained; locally foliated and showing cataclastic Strike and dip of rock cleavage in metamorphic rocks, foliation in intrusive TECTONIC BRECCIA — Strongly sheared; locally contains monolithologic QUARTZ VEINS (Upper Jurassic or Cretaceous) _ Large-scale veins of massive CONGLOMERATE — Siliceous granule to cobble conglomerate and quartzitic LIMESTONE — Quartzose; gradational into calcareous quartzite (unit q] ) o t .m 4.4 n 0 .U a d a r g N. m C O L e. k c a w Y a r g d e d d e b r e t .m t n a d n U .0 a .W. .m C o L L E T A L S BIOTITE GRANODIORITE 7 Fine grained brecciated and locally altered to QUARTZITE — Locally gradational into siliceous conglomerate (unit sc) PEBBLE T0 COBBLE POLYMICTIC CONGLOMERATE HORNFELSIC SILTSTONE INCLUSIONS IN SERPENTINITE HORNBLENDE QUARTZ MONZONITE (Upper Jurassic) — MARIPOSA FORMATION (Upper Jurassic) PYROXENITE AND HORNBLENDE PYROXENITE CHERT MARKER BEDS ~ Laminated to thinly bedded AUGITE-HORNBLENDE GABBRO — Medium grained m 0 Z 0 e .m mu N m ,A M R O F. S A R E v a L A C PYROXENE GABBRO 7 Massive, PYROXENE GABBRO v TALCOSE ROCKS CARBONATE ROCK — Reddish HORNBLENDE AMPHIBOLITE — GRAYWACKE AND CLAY SLATE GRAYWACKE AND CLAY SLATE PEBBLE T0 BOULDER OLIGOMICTIC CLAY SLATE LIMESTONE 7 Very fine grained” LIMESTONE — Banded (bedded) QUARTZITE — Phyllitic, laminated STR Contact ~ Direct or suggestive evidence of tectonic movement. Approximately Strike and dip of bedding and cleavage observed at same point Strike and dip of fracture cleavage ~ eastern fault block only Strike and dip of shear foliation in pervasively sheared rocks Orientation of minor folds Bearing and plunge of lineation a 4., I «a Qf Qa a r I a I V“ 5 45 IO 80 I IA..| 3x8 83 EEC? $55500 Co 380% ‘rl 1 Ear: 60 70 .._u_ + _A_ + I 6 80 :755 -9—> 45 LL. 11p 3% r l ‘i .. 385m magflmo m0 330% BEAR MOUNTAINS FAULT ZONE OF CLARK (1960) JURASV SIC Upper Jurassic } WESTERN BELT 38° 34' 550.5. Ho: sermon oEn—Emzmbm o>zflom .30.. Color streakings or crinkling in surface of cleavage or foliation ——-> 120°45' ” .) .21’05\\«~ 47'30" 2...: R. 11 E. .Efimr Kafka... R. 10 E. if; at. .. . . _ 3...?!er «V ) . i a, “Xx J?- a s . Imd..!§i#§i$%t»l\t£~fi E; 31,. 9:13»: . ,, r a”). 52’30" 55' R. 10 E. R.9E. UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Te 32 \w uh. p. “in; , «tide. e , , lutqsb, misery}. v t a. I sp~!\miflu f, Qatari. tartan?» « My»... Bumvamn set. . . ~ , . as a analgesics.» , , «taakwtr , . .2. i a simian” , a if: A at‘ aft? a a :4 m '5’ 4r . p Swansea‘s 33.2541 J , A, A. .a r 1 J1 mesmfsf\\!ami\p§31fln \W. a a... .. .. aliawmfliefiaewf i «I. :5. «mttwkun‘fia wafii (25 K1“: v r Ix; c in, i , 4,. ,1 v“ 14* \Il‘ axiaéirsall (tin... xsflmthcéviu unteiiflsmwxmh: 23213::pr tshil.bhr— is? ’ , leKr! \r «r! x/imcd I/ ..... 120Q 56’15" -69 -1975 A. Duffield and R. V. Sharp, 1967 —Geological Survey, Reston, Va. Geology by W. GEOLOGIC MAP OF THE WESTERN SIERRA FOOTHILLS BETWEEN THE COSUMNES RIVER AND THE MOKELUMNE RIVER, AMADOR COUNTY, CALIFORNIA 101' Inter .30.. 47 R.11 E. R. 10 E. 50' 120O 52'30" 17'30" — .. 38° 17' 1 MILE 1 KILOMETER APPROXIMATE MEAN 'VECLINATION, 1975 24 000 SCALE 1 CONTOUR INTERVAL 10,20,AND 40 FEET DATUM IS MEAN SEA LEVEL Amador City, , '24,000 Latrobe 9 Base from US. Geological Survey 1 Irish Hi , Jackson, and lone, 1962 and Fiddletown, 1949 120° 56'15" 38° 20’