(y^^J no.. I ILLINOIS STATE GEOLOGICAL SURVEY M. M. Leighton. Chief Urbana GUIDEBOOK FIELD CONFERENCE ON NIAGARAN REEFS IN THE CHICAGO REGION Prepared by H. B. V/illman, H. A. Lov/enstam, and L. E. V.orkman Held in connection with the 35th Annua]. Convention of the American Association of Petroleum Geologists at Chicago April 28, 1950 ffiSSfil IffiHS fif ,? L0GICAL SURVEY 3 3051 00006 3192 - 1 INTRODUCTION This field conference is being carried out by the Illinois State Geologi- cal Survey at the request of the Executive Committee of the American Association of Petroleum Geologists • The committee in charge of arrangements is as follows: M» M, Leighton, Chairman, H, A. Lowenstam, G» 0. Raasch, H. B, Willman, and L. E. Workman, SOURCE OF DATA Information on the Niagaran strata is largely from unpublished manu- scripts by H» A. Lowenstam, H« B» Willman, and L» E. Workman in the files of the Illinois State Geological Survey. Descriptions of Pleistocene features are based largely on the Chicago Areal Geologic Maps by J Harlen Bretz, PUBLICATIONS As a convenience to those who may wish additional information on the Chi- cago region and Niagaran reefs in Illinois, copies of the following publications may be purchased at the registration desk: "Geology of the Chicago Region, Part I - General," by J Harlen Bretz, Illinois Geological Survey Bulletin 6 5, 50 cents, "Niagaran Reefs in Illinois and Their Relation to Oil Accumulation," by H. A» Lowenstam, Illinois Geological Survey Report of Investigations 145, 25 cents, Chicago Areal Geologic Maps (22 sheets), Illinois Geological Survey, $2,20 per set, A set of the Berwyn, Calumet City, and Hinsdale geologic maps, which cover areas where stops vail be made, may be purchased for 45 cents. ' . Figjure l r - Route map 3 showing major exposures of Niagaran reefs (klintar) in the Chicago region* Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/fieldconferenceo01will Figure 2, - Generalized stratigraphic sootion of Silurian strata in northeastern Illinois, showing the approximate stratigraphic position of the quarries to be visited. SERIES FORMATION NIAGARAN RACINE 300 ' ALEXANDRIAN WAUKESHA 25» JOLIET 75t KANKAKEE 30* EDGEHOOD 70 ' REEF REEF rz V REER^/- ~l - Continuing south on Alternate 30 (Stony Island Avenue), the Stony Island klintio reached at 92nd Street,, about 3 miles south of Jackson Park,, From 87th Street to 92nd Street, the klinb may be seen on the left (east) extending as an east-west ridge a little more than a mile long 5 about 1 l/4 miles wide, and rising about 25 feet above the lake plain (figc 1) c. The Stony Island klint is typical of several in the Chicago Region-. Bocause of the superior weather resistance of the reef-type dolomite., many of the reefs were prominent oval or nearly circular hills on the preglacial ercsional surface-, Before it was covered by glacial drift the Stony Island klint was a hill rising about 50 feet above the general level of the deeply dissected bedrock surface,. After burial by the glacial drift it was partially exhumed by erosion in the glacial lakes and was an island during the late stages of Lake Chicago Throughout the hill^ bedrock lies immediately beneath a thin cover of soil. Several road-cuts and quarries, now mostly filled, formerly showed the characteristic reef lithology> and the radial dip of the beds on the flanks of the hill. From Stony Island the route continues south on Alternate 30 for about 5 miles along the west side of Calumet Lake, a large shallow lo.ko at nearly the same level as Lake Michigan, to which it is connected by the navigablo channel of the Calumet Rivor c On the left (east), across the lake, are the plants of the Inter- national Harvester and other large concerns at South Dceringj on the right (west) are the plants of the Pullman Car Manufacturing Company at Pullman, as well as many other large industrial plants,, South of Lake Calumet turn right (west) on 130th Street (road to Blue Island) e At 1,5 miles turn left (south) on Indiana Avenue (County Road M)« Little Calumet River is crossed in .6 mile and about a block south the road crosses the Toloston beach and rises about 10 feet to the flat covered by the Calumet stage of Lake Chicago. Continue ahead on Indiana Avonue, Two miles farther south Little Calumet River is crossed again. Slightly over 2 miles farther south the road ascends the north slope of the Thornton klint (fig, 4) joining Vincennes Avenue on the slope. The klint has a relief of 25 to 30 feet, Calumet beach of Lake Chicago is on the slope of the klint, Continuo south on Vincennes road on top of the Thornton klint, Tho North quarry of the Material Servico Corporation, which wo shall visit, may be seen on the right (west) of the road (fig. 4), Tho North quarry is separated from the South quarry by a narrow ridge of rock on which is Ridge Road, The North quarry is en- tirely in a reef near the top of tho Niagaran of this region (fig, 2), At tho north end of tho North quarry, reef-flank bods dip steeply to the north. At the south end similar strata dip steeply to tho south. Between tho dipping beds is the core of the reef, about one-third mile across from north to south, AH tho rocks in the North quarry are pure reef -typo dolomito. Relatively impure interreof-type dolo- mite occurs only in the South quarry. Turn right (west) on Ridge Road at the 4-way stop in tho middle of Thornton, Buses will unload at the entrance to the Brown Derby Grove across tho Baltimore and Ohio Railroad on the west sido of the quarry, STOP l c - QUARRY OF THE MATERIAL SERVICE CORPORATION AT THORNTON, ILL, Assemble at the southwest corner of tho North quarry for a general view of the quarry and a preliminary talk on tho geology of Niagaran reefs and tho gen- eral setting of the Thornton reef. Procedure o - Because of the difficulty of transporting adequate sound equipmont about the quarry and of assembling a large party at various places in the i . ; :.i -,{: ■■' ; i i . t, ! MILE quarry, the route to be followed and the localities to be examined arc shown on the accompanying map (fig. 5). Descriptions of the features to be observed at each locality are given on the following pagos and thero will bo no talks at the locali- ties. Leaders (distinguished by red bandanas) will be stationed at most of the localities to point out features and answer questions. A further opportunity for questions and discussions will be given at the lunch stop. Members of the party may spend as much time as they desire in studying parts of the roof structure in which they aro particularly interested,, However, thoso who wish to oxamine all the localities described should boar in mind that, allowing an average of 5 minutes for walking between localities., only about 15 minutes can be spent at each locality. At 12 noon you are due at the Brown Derby Grove for lunch (fig. 5), Samples c - If you wish to collect samples but do not have transportation for them, the committee will arrange to have them shipped to you by express collect. To savo a walk from the distant incline, the descent to tho quarry will bo made over blasted rock. The grade is gentle but tho rock is loose. Take your timo. Walk north along the west quarry face until you come to good exposures of reef core, about 600 yards north of Ridge Road, Locality 1, - The rcef-corQ e The core of the reef is distinguished by its massive structure and excep- tional purity. In this part of the quarry, which is thought to be near tho center of the roof (fig. 4), the entire face is reof-coro. The top of the roof-core is eroded and the base is not exposed. It is not known how far the roof extends be- neath the quarry floor, but the baso of tho Ni agar an strata is about 300 feet below (fig. 3). The rock in the reef-core is a medium-grained dolomite, commonly medium gray but locally streaked or mottlod with light gray. Some areas aro vory dense Figure 5« - Materia.! Service Corporation Quarry, T.iorjit&n.. 111. BROWMDERBY GROVE (LUNCH STOP - 6 - but vesicular streaks and large vugs are common. The total porosity varies from 6 to 7 percent in the dense areas to about 20 percent in the vesicular parts. The high purity of the reef dolomite is shown by the chemical analysis in table 1, Insoluble residues are negligible except for occasional well-rounded grains of quartz sand, the local presence of a mere trace of clay, and variable amounts of asphaltum. The sand grains probably were dropped on the reef by animals. Table 1, - Comparison of Chemical Composition of Reef and Intcrroef Dolomite Reef Rock Intorrcef Rock from North Quarry from South Quarry (bed at base of south face) CaC0 3 54,57 36,47 MgC0 3 44.30 26,77 Si0 2 0.06 26,39 A1 2 3 0o25 4.66 Fe 2 3 0o02 1,12 FeO 0.07 MgO 21,54 14,70 CaO 30v57 20.43 Na 2 0.11 K 2 0.01 C0 2 47.12 30.01 The asphaltum which impregnates many porous areas throughout the reef, as well as the bordoring intcrroef rocks, is extremely viscous but on hot days it drips slowly from openings, especially those in frosh oxposuros. The occurrence of asphal- tum in reefs is common in the Chicago region but is not universal. Some exception- ally good reef structures are entirely froe from asphaltum and others have very little. It appoars to be more common in the higher Niagaran reefs which have strong- ly devolopod flank bods than in the reefs lower in the Niagaran, which generally are smallor, The reef-coro and flank beds arc characterized by the abundance and diver- sity of fossils, in contrast to the limited fauna of the intcrroef beds. Although crystallization and dolomitization have destroyed the fossils in parts of the reefs, - 7 - tho preservation of fossils as casts and molds is oxcollcnt in many places. The framework of tho roof appoars to have been a mesh of colonial corals and stromato- poroids . The reef assemblages are characterized by largo heavy-shollod robust forms in contrast with tho small fragile forms in the intorroof deposits . Among the most abundant fossils to bo found in tho rocf are tho corals FavositoSj Halysitos, Lyellia, Thecia, Holiolitos, and plasmoporaj tho crinoids Crotalocrinites and Eucalyptocrinitos; tho brachiopods Monomorclla, Conchidium, Eospirifor, and ^ilsonollaj and tho trilobitos Bumastus and Calymono, In some parts of the roof-core the dolomite is almost structureless, but in other parts, as at this locality, poorly dofinod bodding can bo obscrvod. It is usually inclinod, the amount and direction varying irregularly in short distancos. Much of tho inclination of tho bedding may be duo to slumping. At places the massivo structuro of the core is interrupted by a few prominent undulating but near- ly horizontal breaks or bedding-pianos. Although woakly devoloped hero, they can bo soon in the foro-roof at locality 5, Those appoar to mark interruptions in tho growth of the roof and arc interpreted as growth lines. They show, thorofore, that tho core is the zone of vertical roof growth. Tho gonorrl lack of well-dofinod bodding suggests that reof-cores developed in a protected lagoon. Roofs are distinguished from tho intorroef strata in electric logs by thoir consistently highor resistivity and particularly by consistently higher nega- tive solf-potontials. This docs not apply to tho northwestern part of tho Stato whoro the intcrreof bods havo nearly the same lithology as the roofs and aro differ- entiated by structure and fauna. Follow tho west faco southward to observe the transition from tho reef- core to the rcof-flank deposits. The transition zone is marked by an increase in dogroe of bodding, with a persistent dip to tho south which rapidly increases in steepness until it reachos a maximum angle of 30 to 40 degrees which persists across the zone of roof-flank deposits. » '•■-*• - 8 - Locality g, - Roof- flank deposits* The roof-flank bods aro typically developed and may bo examined noar tho tunnel, especially on tho south side* Tho roof -flank bods aro distinguished by high purity, excellent bodding, and a stoop dip away from tho rocf-coro. Most of tho bods arc 2 to 6 inches thick* In parts of tho sequence the bods havo a remarkably uniform thickness and individual beds maintain a uniform thickness throughout their exposure. In chemical and minor- alogical composition the dolomite is similar to that in the recf-coro. The total thickness of the individual beds exposed in the roof-flank zone oxcoeds 1,000 foot. Before the roof origin was recognized the strong dip was inter- preted as a fold in the strata, and tho formation was assumed to have vory great thickness. However, a well in Thornton, on tho outer margin of tho dipping bods, penotratod only 465 foot of both Niagaran and Alexandrian strata overlying tho Maquokota shalo, a normal figure far this part of the region (fig* 3), As previously noted, the roof -flank beds at tho oxtrcmo north ond of tho North quarry dip north and are idontioal in character with those observed at this locality. The radial dip of the roof-flank beds is also shown by a quarry on tho west side of tho roof (fig, 4) whore similar bods dip steeply to the west* The reef-flank beds havo tho relation and general appearance of foro-e^t bods in deltas and have boon interpreted as representing reef-detritus broken by waves from the core zone and deposited on tho stoop sides of the reef* Dolomitiza- tion has dostroyod the original granular character of the matrix material, but tho prosence of broken and irrogularly orientod fossils and tho local presence of lenticular bods and breccias supports tho dotrital origin for at least part of tho material. However, many of the coral growths are oriented parallel to the sloping surface of tho bods and appear to have grown in place. In addition, tho uniform thicknoss of individual bods for many foot down tho steep slopes offers serious •.-../' AI-W - 9 - difficulties to an interpretation of detrital origin,, Also tho tendency for many of tho bods to havo about tho samo thickness suggests a cyclical control of growths It appears, thoroforo, that tho steeply dipping flank beds may havo grown largely in place, representing a lateral growth on tho slope of the reef not greatly differ- ent from that on tho roof-core The upper margin of these bods may havo boon high onough to form a rim which gavo some protection to tho zono of vertical growth which formed the rocf-corc a Continuo southeastward to tho incline for trucks (fig. 5), Locality 3, - Transition to intcrroef deposits.. Along the incline the gradual introduction of argillaceous bods into tho sequence can be observed. This marks the outer limit of the steeply dipping reef- flank deposits. The rapid flattening of tho dip can bo soon in the faco near tho hoisting incline. These bods appear to mark tho final stages of growth of the major reef e Devonian sharks 1 teeth are found above tho incline in the clay-filled joints, Devonian strata were penetrated in a well a short distance to tho southeast (fig. 3). From tho baso of the incline proceed wost to the road f ork p then south on tho road lending to tho hoisting incline At tho point whore this road turns oast to the baso of tho hoisting incline continuo south across the tracks. Many routes may bo followed in crossing tho quarry floor to tho next locality, which is on a line towards the 3 cylindrical storage bins beyond tho quarry and noar tho middle of the south face (fig* 5) » Locality 4 3 -> Intcrroef strata* The interreef sediments consist largely of still-water deposits which accumulated in the relatively doep water surrounding tho reefs. Those rocks aro characteristically dense, very fine grained, contain argillaceous and siliceous im- purities 9 and are well bedded^ The most common fossils aro sponges and crinoids, t ;: ■ l - 10 - less commonly bryozoa. Locally a gastropod-trilobito-cephalopod assomblago is found, Tho most common crinoids aro Pisocrinus , Gisocrinus , and Locanoorinus , The trilobitos aro mostly E ncrinurus and Calymono, Somo rough-water sodimonts aro found in tho intorroof habitat. Thcso con- tain only a slight amount of silicoous elastics and aro relatively coarse-grained and porous , In physical appoaranco they arc not greatly different from the reof lithology, Crinoidal fragments aro generally common with lessor numbers of colonial corals and stromatoporoids. Lenticular bodies of reef-rock or "baby" roofs aro common in the intorroof strata. Tho sequence of intorroof bods oxposod near tho middlo of tho south face of the quarry consists of tho following major units* Unit A - Argillaceous dolomito. This dolomite contains 20 to 45 percent insoluble residue* A chemical analysis is given in tablo 1. It is locally at least 12 foot thick but its base is not oxposod, Tho cross-section (fig, 3) indicates that it oontinuos downward for about 95 foot below tho floor of the quarry. Unit B - Reef-detritus bed. Above the argillaceous dolomito is a distinc- tive massive breccia 4 to 10 foot thick, consisting of blocks of porous reef-type dolomito in a matrix of argillacoous dolomite, similar to but not so impure as that in Unit A» This is a continuous massive unit, set off from adjacent units by strong bedding-planes. It is easily traced because of the presenco at tho top of a dis- tinctive 6-inch to 1-foot bed of brown finely porous relatively pure dolomite over- lain by 1 to 4 inches of argillacoous dolomite. The unit is described as a recf- dotritus bod bocauso the reef blocks appear to have boon eroded from tho major reof to the north. As will bo seen in tracing it in the west faoo, the roof-dotritus bod rises to the north, up the slope of the roof. Some of tho reef-typo massos in this unit arc very largo and may have grown in place, but in many blocks tho corals are invertod^ E £' ;/ 7..V *C » . , •.-..'... .. ) - 11 - Unit C - Nodular chorty dolomite. Overlying tho reof-dotritus bod is 7 to 10 foot of nodular impuro chorty dolomito with thin wavy lenticular argillaooous partings. In sovoral places it grades laterally, within a foot or two, to a n&xturo of nodules in groon shale* Even in tho more dolomitic facias this unit contains about 20 porcont insoluble residue consisting of micaooous silty clay. In general the nodular character increases toward tho reef. At the contact with tho fore-roof, to bo examined at locality 5, it locally changes to shale containing nodules. Tho origin of tho nodular structure is not clear, but it may havo resulted from deposi- tion undor tho particular typo of agitation characteristic of wator noar tho odgo of tho reof , This nodular character is not woll developed in many inter reef soo- tions which aro presumed to be farther from the roofs, and it thorcforo may bo an indication of reef proximity. Unit D - Upper variable bods* The uppormost unit (Unit D) is 15 to 30 foot thick and its top is everywhere eroded. At this locality it contains some bods of nodular dolomito similar to that in Unit G but mostly not as impuro and with chert loss common. Most of the strata aro slightly porous and havo genorally loss than 10 percent of insolublo residua* Some beds arc pure dolomite of the reof typcj a distinctivo lens of pure dolomite crowded with silicifiod horn corals is present near tho top of tho section. The unit becomes more impure \vhon tracod westward to- ward tho margin of the fore-roof and nearer to the main reef. Close to the foro- reof the unit is all nodular and in places is differentiated from Unit C below only by tracing tho prominent bedding-plane between them. Continue westward along tho south faco of tho quarry noting tho continuity of the reof-dotritus bed to tho exposure of tho fore-reef in the west faco just north of tho southwest corner of tho quarry* Locality 5„ - The foro-reof, Tho fore-roof exposed in tho west faco near the corner of tho quarry is only one of several which wore oncountored and quarried in oxprnsion of the quarry ■• c .,;-•' i ' } '■'.'• < :•,..., ,, . . , .■';■'■<,.■■: ■■; 'in \. ■J I ■ I <■-" 7: '.';<*! ,- f * '• >'■'=.;_ i : ; "' ■ '•-.• '|. 0':*i. ■ :' J '^ ' • j v ,'; ■:''.:. "I'i: I ;< } ■ 'i'w' C = ,- "J; ; >■ . . f ■. ... • i ■.' ■ - 12 - southward from the main roof. Although tho oxtcnt of this foro-roof is not known, it appears to havo boon a linoar feature parallel to the edge of the main roof* Numerous exposures of massive reef-rook in tho quarry floor on a line slightly north of erst from tho exposure in the wost face apparently show tho continuation of the roof. Exposures of argillaceous dolomite in tho floor south of the roof-rock indi- cato relations similar to those in the wost face* Examination of the foro-rcef gives much information on tho growth of the roof and its relation to interrocf sedimentation* It emphasizes the complexity of conditions around tho margin of a largo reef and the abruptness of facies variations. Observations on tho growth of tho fore-reef and othor "baby" roofs are not entirely applicable to growth of tho main reef* Surrounded as it is by interroof-type sedi- ments, it is apparent that the foro-recf grew in relatively quiet water. There is no evidence that it contributed detrital material to the adjacent sediments. Its growth was essentially vertical and no roof-flank bods wore formed* A preliminary view of tho fore-rocf from a short distance out in the quarry (fig, 6) will shov; tho major relations - the sharpness of tho margins of the reof, the overlapping intcrroef beds, tho absence of roof-flank bods, and tho mas- sivo structure of the rocf-core, broken only by growth lines which arc not conspicu- ous and arc not easily traced through tho roof* A closo examination of tho south side of tho reof (fig, 7) shows that tho stratigraphic units differentiated in tho south face (Locality 4) continr.o to tho roof. The roef-dotritus bed (Unit B) ends about 10 foot from tho roof r.nd is sopa- ratod from it by a mass of very argillaceous dolomite like that in Unit A, Unit C laps onto the roof and Unit D overlaps it, Tho sequence of events which produced theso relations is interpreted ten- tatively as follows; 1 Deposition of part of tho rrgillacoous dolomite (Unit A)« 2. Vertical growth of tho fore-roof* . i -:>.• x -A Figure 6 # - Sketch showing general rolations of fore-reef (Locality 5) near the southwost cornor of the quarry; viewed from a short distance out in the quarry* TOP OF ROCK QUARRY FLOOR ". I ■'.■.'. I I -I :v, V! «* G'tH! .■••i't; - J ,,..:- • \ ■ •<,. ■ \s..j •■ C : -:.' i Figure 7, - Sketch showing contact of the fore-reef with interroof strata on the south sido of tho fore-reef (Locality 5). TOP OF ROCK D . _ REEF 5HLAR| NG A-* FLOW LINES T GROWTH LINES TALUS QUARRY FLOOR Figure 8, - Sketch showing relations on tho north sido of the fore-reef (Locality G) , CHERT GROW ULJ i-iz $ \N CLAY a 1 Letters mark the units described at Locality 4, Unit B is the roef-detritus bed with its distinctive top 9 - 13 - 3, Doposi tion of tho roof -detritus (Unit b) in tho matrix of argillacoous dolomite. At this stago tho foro-roef had an uneven top and stood only slightly abovo tho bottom so that dotritus moving down tho slope from tho main roof on the north overrodo or by-passed tho fore-reef, 4, Tho roof then attained such size that the argillaceous mud benoath could not support tho weight of tho reef, Tho roof settled and tho mud flowed up along the sido of tho roof breaking through tho detritus bod. Flaring flow lines may be observed in the argillaceous dolomito. As will bo seen on the north sido of the roof tho sottling appears to havo been almost entirely on the south sido. The top of tho vertical side of tho roof is about 10 foot lowor on tho south side than on tho north, which excoeds the regional dip, 5, Tho fore-reef continued to grow upward. All the roof upward from the tonguo at about the middle of the south sido grow during this stago. There was al- most no deposition of intorroof bods during this growth* 6, Tho argillaceous bods of Unit C were deposited after roof growth ended and they built up the sea floor to tho level of tho top of tho roof, Tho off-roof dip of tho upper surface of the unit may have resulted entirely from compaction of the argillaceous muds by the woight of the ovorlying sediments, Tho rigid framo of tho roof would prevent its compaction, but concentration of tho load on tho roof re- sulted in tho devolopmont of vortical shears, particularly on the south sido whore settling had occurrod, li Unit D was deposited after the roof had ceased to bo a topographic feature on tho soa floor, Tho growth of 10 to 15 foet of the roef without appreciable deposition of intorreof bods, as woll as tho apparent lack of consolidation of the muds beneath the reef, attests tho rapid reef growth. Furthermore, tho lack of sedimentation marginal to tho roof during its growth suggests that waters relatively freo from dotrital siliceous impurities prevailed during growth of the roef. The increase in I :-; : . - ■■ .'. ■ ' \i [;■■ ft<"•' - 14 - muddinoss of tho wator perhaps accounts for tho decline and ond of roof activity as shown by tho rotroating upper slope of tho roof. The presence of dense chert in Unit C noar the reef is an interesting problem. The presence of a similar zone in the same unit at the same position on tho north sido of tho reef suggests that tho chort is onvironmontally controllod, Bodding-planes pass through tho chort so tho.t the silicification apparently is sec- ondary or at least diagenotic. Continue northward to the north sido of the foro-reof, whero tho relations aro as shown in figuro 8, Noto that there is no evidence of settling and flowing of mud on this sido of the roof. The reef-detritus bed (Unit b) was deposited against the roof. It may havo settled slightly during compaction. The top of tho fore-roof at the end of doposition of the roof-detritus bod is shown by the bcdding-planc or growth lino indicated in the sketch (fig* 8), The bodding-planc dips steeply into tho faoo showing tho undulation of tho surface at that stago of growth. The surface of tho reef may well havo boon low enough on tho west for detritus from the main roof to by-pass tho fore-rcof. The oxtremo argillaceousnoss of Unit C close to tho roof is well shown. As Unit C apparontly was not deposited until tho roof had reached full growth, tho stoep faco of tho roof may havo provided a protected place favorable for tho set- tling of suspended clay. Continue northward to tho next exposure, which is beyond tho first waste pilo. Locality 6, - Roef-detritus on a foro-recf. In this exposure (fig, 9) tho roof-dotritus bod (Unit b) rests directly on a roof. Although not continuously exposed, this reef may be continuous with tho foro-rcof just doscribod. If so, it confirms partial growth of tho fore-roof boforo deposition of tho reef-detritus bod. H:. :.;•! './ » '■■'' » , » Figure 9, - Sketch showing relations at Locality 6, TOP OF ROCK TALUS QUARRY FLOOR L 0' - 15 - Continue northward passing a long interval of face covered by waste mate- rial. Keep to the right and descend to the lowest quarry floor. Locality 7-, - Contact of reef-flank beds of the main reef with a fore-reef. This exposure is in the edge of the bench close to the water. It is directly east and lower than an exposure in the main west face (Locality 8), and the two sections are combined in figure 10 to show their relation. Observation of the face north of this looality will show that the steeply dipping reef-flank beds of the main reef continue from this point northward to the north face of the quarry. They were examined at the tunnel (Locality 2), At this locality they end against a fore-reef. Most of the actual contact is covered, but projection of the dip of the reef-flank beds shows that they would cut the reef. It appears, therefore, that the fore-reef was present when lateral growth of the main reef reached this position. Light-colored slightly argillaceous dolomite was deposited later on top of the reef -flank beds and between them and the upper part of the fore-reef» Climb over this exposure or pass around it on the south in order to reaoh the main face direotly west from Locality 7, Local ity 8, - Contact of reef-flank of th e main reef v:ith interreef strata. This exposure is shown in the upper part of figure 10, The continuity of the argillaceous dolomite (Unit A) and the reef-detritus bed (Unit b) up the slope of the reef is well shown. As suggested by the projection of the surface of the reef-flank beds from the lower bench (fig, 10) f the deposition of the interreef strata (Units A, B, and C) occurred during a recession of the main reef face, and they appear to mark an interval of reduced reef activity. This may be related to the advent of the detrital siliceous muds. Later, under conditions more favorable for reef growth, the reef expanded laterally over the interreef beds and over a "baby" reef which had started on the surface of Unit C, Erosion of the overlying strata ends the record of succeeding Figure 10 ■> - Sketch shoeing relations at Localities 7 and 8 # The dashed lino shows the projected margin of the roof-flank beds and their interfingering with the intorreof bods. REEF- < CORE i ALUS FLOOR OF B'c >iCH_ _ LOC 8 f LOC.7 * REEF- FLANK Talus o' L LOWEST QUARRY FLOOR * 16 - events in the history of the Thornton reef. Either ascend the quarry face along the exposure or use the inclino just to the north to reach the top of the quarry. Continue north along Ridge Road to the Brown Derby Grove where the buses are parked, LUNCH STOP •» We will eat lunch in the park. Rest rooms are available on the west side of the dance pavillionj a drinking fountain is located northwest of the pavillior* We will leave the lunch stop promptly at 1 o'clock. Continue south on Ridge Road along the quarry for a quarter mile, then west along the south side of the Thornton klint, here bordered by sand dunes, for another quarter mile (fig. 4), We leave the klint at the cemetery. Halstead Street is reached in a quarter mile* Turn left (south) on Halstead Street (Chicago Heights Road), At ,4 mile turn right (west) on 183rd Street, The edge of the Lake Chicago bottom is reached in half a mile and the road ascends to the gently undulatory surface of the Tinley ground-moraine on which the town of Homewood is located. About one mile west of the underpass of the Illinois Central Railroad in Homewood, the more hilly surface of the Tinley moraine is reached and is crossed for 2 l/2 miles. The front of the Tinley moraine, facing west, is marked by a descent of about 30 feet to a flat which is underlain by the sediments of Lake Tinley, a glacial lake formed when drainage eastward down the back slope of the Valparaiso moraine was dammed by the Tinley ice front. The edge of the lake flat is prominent north of the road. The highway is on the lake bottom for about 2 l/2 miles and then rises slightly to the undulatory topography of the Valparaiso ground-moraine, a half mile before reaching 80th Avenue, •:c ■ ;■ < ■ , :... ■") .7 •'•;-' ,„ ' ■; - 17 - Turn right (north) on 80th Avenue, The route continues on the Valparaiso ground-moraine crossing two branches of the Lake Tinley flat. At 3 miles turn left (west) on U. S. Highway 6 (159th Street), At 2 miles turn right (north) on U« 3. Highway 45 (96th Avenue), One mile north of the turn the highway ascends the Valparaiso moraine. Characteristic mo- rainal topography may be observed in the 4 miles to Sag Valley, "The Sag" was a former outlet of Lake Chicago, About 4 miles to the west it enters DesPlaines Valley, which also served as an outlet to Lake Chicago,? Sag Valley was a swampy area without a stream until the Calumet«Sag Channel^ a feeder to the main Chicago Sanitary and Ship Canal in the Desplaines Valley, was excavated to supply an addi- tional flow of water from Little Calumet River near Blue Island,, On the north side of Sag Valley the highway cresses a terrace underlain by grovel outwash from the Tinley ice-frcnt The highway then ascends onto the Val- paraiso moraine. The Tinley moraine which rises onto the Valparaiso moraine in this area is a half mile to the right (east). About 1 l/2 miles north, just north of the intersection with 95th Street, the highway rises onto the Tinley moraine* About 2 miles north of 95th Street, Desplaines Valley is reached, and the highway in succession crossos Illinois Highway 4A, the Chicago and Alton Railroad, the Illinois and Michigan Canals the Chicago Sanitary and Ship Canal, which connects Lake Michigan with the Illinois Waterway near Joliot a the Diversion Channel of Des- plaines River, the Atchison, Topeka and Santa Fe Railroad, and a major power line. On the north side of Desplaines Valley the highway rises onto the Tinley ground-moraine Three miles north of Des plainos Valley turn right (east) on 47th Street (road to Lyons) at the couth edge of LaGrangc., At half a mile the road descends to the Lake Chicago flat, crossing the Glenwood (highest stage) beach The Material Service Corporation quarries are on w 18 - the right. Continue ahead (oast) beyond the quarx-y and then turn back to the right (southwest) to tho plant, STOP 2 - MATERIAL SERVICE) CORPORATION LAGRANGE QUARRY, T * T alk down the incline leading northeast from the east sido of the crushing plant. To point out features and discuss problems, leaders will be located at the base of the incline ., at tho north face, at tho north end of the tunnel loading to tho South quarry, and in the South quarry. It is anticipated that about 1 hour will bo available for this stop* The strata exposed in this quarry are in the lower part of the Racine for- mation (fig, 2), There aro three quarries at this location and the deopest one, on the oast and now filled with water 9 reached to tho Waukesha formation, A diamond- drill core obtained by drilling in tho floor of tho east quarry reached tho top of the Edgowood formation, as shown in the cross-section (fig» 3). From the incline it will be noted that tho section in the quarry consists of a lower light-colored well-bedded sequence of intorroef deposits and an upper zone 30 to 40 feet thick which consists of dark-colored irregularly bedded roof-rock. The reefs in this klint, which extends south for about 2 miles, havo a complex structure and differ in many respects from the large reef at Thornton, In this area reef growth started from many centers and lenticular bodies of massive roof-core developed* In places they overlap each other in a complicated pattern. Apparently none of the reefs attained sufficient size to havo reef— flank beds, or they grew entirely in water too deep to favor lateral growth and development of roof-flank beds, Most of the massive lenticular reef-cores occur along the base of the upper reef zone. They aro overlain by pure reef-type dolomite which is well-bedded. It conforms to the surface of the reef-cores and consequently rises and falls in great waves , It has many of the aspects of reef-flank deposits but lacks the - 19 - uniform dip away from a wcll-dofincd core. No coro or cores have been found to which those deposits may bo related. They obviously are younger than the reef cores on which they lie. It eppoars that they represent a roof-typo of growth undor con- ditions favoring the development of bedding like in the reef-flank beds, but lacking the detrital characteristics of those bods Under this condition they represent essentially a vertical growth rather than the lateral growth of the reef-flank beds, Near the base of the inclino the section in the oast quarry face, sketched in figure 11, can bo readily examined. Some may prefer to examine other sections first, if the party is large. This section shows a complex intergrowth of roof masses, many showing lateral variations to woll-beddod interroef-typo dolomite. The intorreef rocks in this section are only slightly argillaceous and are recognized largely by their bod- ding, finer grain size, and lower visible porosity. By tracing the break between the reef masses, the surface at various stages during growth of the roofs can be re- stored, A considerable roliof on tho sea floor resulted from the thick roof growth on tho loft (north) sido of tho exposure Continue down the inclino and cross on the narrow fill extending to the north face. At this place the chorty argillaceous intcrreof dolomite is well ox- posed near water level t An abundant fauna has boon described from the chert nodules* In the intorreef rocks a typical "baby" reef may be studied* Some thin- ning of the beds beneath, the reef probably results from squeezing. Laterally the reef is represented only by a thin bed of relatively pure dolomite, which suggests that the reef grow during an interval whon the water was comparatively free from siliceous muds. The contact of the upper zone of reefs on tho intorreef sediments below can be seen well,, although above reach 9 The occurrenco of lenticular masses of reef cores along the base of the reef zone is well shown. Figure 11, •• Skotch of oast faco of Material Service Corporation LaGrango Quarry noar. baso of incline a ^_QE RUCK BEDDED REEF -ROCK A : ^r\ RE | M C L 1 N c A - Argillaceous interreef type dolomite. 0' - 20 - From this point return to tho road end continue along it to the tunnel leading to tho South wuarry. Exposures near tho tunnel show tho lenticular roof- cores and the steeply inclined bedding of tho roof-type strata which mantle the reefW cores. Continue through the tunnol and examine tho quarry faco on the left (east), Tho extremely wavy surface of tho reef is shown by the many variations in dip. The contact of the roofs with the underlying intorreof bjds may bo observed on the lov/cr level farther oast. Films of green clay which will be noted on fresh bedding sur- faces of tho intorreof beds are characteristic of those strata, Roturn to the buses. Leaving tho Material Service Corporation quarrios turn left (west) on 47th Street and roturn to U, S. Highway 45 at LaGrango, Turn right (north) on U. S. Highway 45 and follow it through LaGrango, About half a mile north of tho C. B. & Q» Railroad crossing in LaGrango the highway is on the Glonwood. boach and follows it northwest for 1 l/2 miles to Salt Creek at 22nd Street, About 1 l/2 miles farther north the Hillside klint is reached. The top of this klint is about 50 foot abovo the Lake Chicago flat. Turn left (west) on Harrison Street (south edge of Bollwood) to tho Con- sumers Company plant, STOF 5„ - CONSUMERS COMPANY QUARRY AT HILLSIDE. The section will be studied from the base up. Do not delay tho party by stopping to examine the section when going down the stairs. It is anticipated that about 1 hour will be available at this stop. This is one of the most complete Niagaran sections in the Chicago region (fig. 2), oxtonding from a Racine reef at tho top down through Racino intorreof bods, - 21 - tho Waukesha formation, and nearly through tho Jolict formation, Tho basal beds aro only about 15 foot above tho contact with tho Kankakee formation of tho Alexandrian series, Joliot format ion a - Tho lower 60 feet of the section oxposed belong to tho Joliot formation. Three members aro prcsont. The lower 11 feet oxposed in the southwest part of the quarry consist of greonish-gray locally pinkish argillaceous dolomite which contains green shale partings and fucoidrl bodding-planos. This is a distinctive horizon characterized by an abundant fauna of siliceous f oraminifcra. The top is marked by a persistent 4- to 5-inch bed sot off by relatively strong bodding-planos, a characteristic widely recognized in outcrops in the Joliot region, 40 miles to the southwost c Rook with the same lithology extends dov/n to the top of tho Kankakee formation below. Because of the east dip, only the upper 5 feet of this unit aro oxposed in the deepest part of the quarry, which is near the southeast corner and bolow the crushing planto Above the dolomite with strong shale partings is a member 19 feet thick consisting of fine-grained dense light brownish-gray dolomito with persistent bands of chert nodules and in thicker bods than below. It has a high silt residue which decreases upward* It has been found widely distributed in northeastern Illinois, This member is overlain by 29 feet of relatively pure porous dolomite which is light gray, locally mottled dark gray or pink, and occurs in 6- to 12-inch bods c This unit has a lithology closely approaohing reef dolomito but it contains slightly more impurities, does not havo reef structure or a reef fauna. The upper 7 foot is transitional to tho Waukesha formation abovo. The Joliot formation has b-on punotratod in only a few of the deeper quarries in the Chicago region,, but its distinctive sequenco of strata can be tracod widely in the subsurface (fig, 3) and in outcrops in the Joliot region, W-ukosha formation- ~ Overlying tho Jolict formation is 23 feet of groon- ish-gray very silty and argillaceous dolomite comprising tho Waukesha formation* - 22 • It occurs in massivo lodges but is laminated with dark groon wavy clay partings* It contains oval silicifiod nodules showing all gradations from soft incipient chort to dense hard ohort. This distinctive unit is traced throughout most of the region but in at least ono locality is replaced by a reef. For many years all the argillaceous cherty beds overlying the Joliot for- mation and beneath tho Racine roofs woro included in the Waukesha formation, When subsurface and outcrop studios showod that a considerable part of tho strata included in the Waukesha is contemporaneous with reefs which woro classified as Racino, the namo Waukesha was restricted to the distinctivo basal unit. In this quarry, for example, all tho strata from tho Joliot formation to the roof at the top of tho quarry wero formerly included in tho Waukesha formation, Racine formation^ As usod at prosont the Racine formation includos all tho Niagaran strata abovo tho Waukesha formation. It thorofore includos all the roofs in this region. In this quarry the basal 80 foot of tho Racine formation consists of intor- rocf dolomite which is slightly argillacoous and silty, cherty, dense, fino grained, and contains 15 to 20 percont insolublo rosiduo f largely silt and clay, A few bods of relatively puro reof-typo dolomito are intcrbeddod with the impuro strata. Those may bo formod largoly of calcareous wash from roofs which are known to occur at this horizon. In somo quarries small "baby" roofs arc common in those bods. The interreof strata are overlain by 15 foot of high-purity porous massivo dolomito which is tho coro of tho roof which forms the klint, Woll-bcddod roof- flank bods dipping down tho slope of the klint are exposed in a railroad cut ono- fourth mile southwest of the quarry. Return to tho busos* Leaving tho Consumors Company Quarry at Hillside, return to U, S* Highway 45 and turn right (south). 1 ■ ■ ' • t ~ 23 « At «5 milo turn loft (oast) on U. S. Highway Altornatc 50 (Roosevelt Road) and follow it oast for 13 miles to Wabash Avenue, entirely on the Lake Chicago plain. Turn loft (north) on Wabash Avonuo, In 4 blocks turn right (cast) on Balbo Drivo to the Stevens Hotel •