fBtaa wHSa WmmmtA ■NSW iSllHi ■HP B1IH1 HHw ■RHBHissi BHilBBIlM 118 'WlBBlmHl M IllKi URBANA STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON. Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON. Chief REPORT OF INVESTIGATIONS -NO. 14 PRESENT STATUS OF CORRELATION OF ILLINOIS COALS BY HAROLD E. CULVER PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA. ILLINOIS 1927 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON. Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON. Chief Committee of the Board of Natural Resources and Conservation A. M. Shelton, Chairman Director of Registration and Education Charles M. Thompson Representing the President of the Uni- versity of Illinois Edson S. Bastin Geologist Jeffersons Printing & Stationery Co. Springfield, III. 1927 PREFACE The paper herein published on the "Present Status of Correlation of Illinois Coals" is a valuable summary of the concepts gained by the author during- his three years' study of this phase of the geology of the Illinois coal field for this Survey. Because his work has stimulated a more careful study of coal correlation in Illinois and because he has accepted a position in another part of the country which prevents his continuing this work, it seems advisable that this summary should be published and made available for the benefit of further workers. M. M. Leightox, Chief, State Geological Survey Division. Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/presentstatusofc55714culv PRESENT STATUS OF CORRELATION OF ILLINOIS COALS By Harold E. Culver INTRODUCTION Stratigraphic studies of the past two years have revealed certain apparent discrepancies in the correlation of the Pennsylvanian beds of the Illinois field. These had been suggested in part by previous work in northwestern Illinois, where, as brought out in an earlier paper, 1 the available evidence strongly suggests that the rocks formerly classed as early Pennsylvanian are really late Pennsylvanian in age. The evidence as to stratigraphic relations throughout the Illinois field is as yet not sufficiently complete to establish a new or even a revised correlation, but in view of the writer's departure from the field it seems desirable to indicate the nature and trend of the evidence and to suggest the general conceptions which have been formed and which should probably be considered in any correlation statement. GENERAL RELATIONS OF THE ILLINOIS PENNSYLVANIAN The great mass of rocks comprising the Pennsylvanian system in the Illinois basin fills a very broad and shallow depression south of the latitude of Rock Island. Although the upper surface of the Pennsyl- vanian rocks is nearly flat, the lower surface slopes from the margin toward the deepest part of the basin. The mass as a whole has, there- fore, a plano-convex lens shape with the greatest thickness of the beds where the basin is deepest. The deepest part is not in the center of the field, but is well to the southeast in Wayne County, for the basin is dis- tinctly un symmetrical. The Pennsylvanian system is unevenly divided into three parts, a thick Pottsville formation below, a thinner Carbondale in the middle, and a thick McLeansboro at the top. The important key horizons in the section as a whole are the Murphysboro or No. 2 coal and the Herrin or No. 6 coal, which lie at the base and top respectively of the Carbon- dale formation. This division of the Pennsylvanian system was sub- stituted for earlier divisions on the basis of paleobotanical studies by David White. 2 Enough evidence was found to permit certain general correlations between the Illinois sections and the type localities in lCulver, H. ]•;.. Pennsvlvanian correlation in northwestern Illinois: Geol. Soc. America Bull., vol. 35, pp. 321-328, 1924. -White. David. Report on field work done in 1907: Illinois State Geol. Survey Bull. 8, PP. 268-272. 1907. Paleobotanical work in Illinois in 190S: Illinois State Geol. Survey Bull. 14, pp. 293- 295, 1909 5 6 CORRELATION OF ILLINOIS COAI.S Pennsylvania. White was satisfied as to the essential identity of the I coal horizons of the Northern, the Colchester, and the Murphysboro fields. He was also convinced that the so-called Rock Island coal was older than these and belonged in the I'ottsville division, although dis- tinctly younger than the I'ottsville of Battery Rock. Hardin County. Plants from above the horizon of llerrin. No. 6, coal appeared referable to a zone distinctly higher, possibly Upper Freeport in age. It has always been more or less definitely recognized that the first accepted limits of these divisions of the Pennsylvanian were temporary, and that certain features of the division were not strictly justifiable. Such a one is the separation of the Herrin coal from its overlying lime- stone cap by the formation boundary. No widespread unconformity between the coal and its cap rock was recognized, but the coal itself was thought to he one of the readily recognizable beds and therefore suited I for use as a marker for the top of the Carbondale. It was evident also that there was no prominent unconformity below the Murphysboro coal to mark the beginning of Carbondale deposition, hut that the ease of recognition made the use of the Murphysboro bed desirable. It should be emphasized, therefore, that the splitting of the Pennsylvanian was based upon expediency in practical application and was never intended to be final. Areally, the three subdivisions of the Pennsylvanian system have been considered to lie in roughly concentric bands, the lowest at the outside, and the highest in the central part of the basin. This arrange- ment leads to the structural conception that the beds occupy a broad, shallow trough. SOME APPARENT DISCREPANCIES With the foregoing statement of general relations in mind, it is now in order to inspect some of the features of the rocks of the region which appear to be inharmonious with the general conceptions outlined. In northwestern Illinois, as already noted, 3 the apparent continuity of a recognizable lithologic sequence from the Rock Island field eastward to the accepted McLeansboro area of Bureau County, strongly suggests that the marginal beds of the Rock Island held are McLeansboro rather than I'ottsville and indicates an overlap of considerable extent in late Pennsylvanian time. On this basis, it should he noted, the massive sandstone in the vicinity of Muscatine. Iowa, which has been considered as typical I'ottsville, is in reality late Pennsylvanian. \lon really a north extension for the Standard district for the Belleville coal, and the south- ern Illinois district for No. 6 and locally for No. 5 coal, and lastly, the Vermilion County district for the Grape Creek and Danville beds, com- monly called the No. 6 and No. 7. It will be noted that only a single coal zone is recognized in most ot these areas, and that even in the aggregate only a moderate part of the whole Pennsylvanian system is involved. Further, the known areas are separated by rather wide gaps in which the evidence for correlation is inadequate. It is obvious that any statement of discrepancy or of correlation must be made on an assumption of identity of certain strata of the gen- eral section throughout the State, or at least in the areas in which corre- lation is attempted. This is in itself one of the important points in ques- tion. In the main, state-wide correlation of the whole section has been on the assumed recognition of two prominent coal beds, the Murphvs- boro and the Herrin. The Murphysboro bed has been identified pri- marily bv fossil flora in the overlying shale, and the Herrin coal by its "blue-band" or shale parting and by its cap rock of fusulina-hearing limestone. It will be well to scrutinize these criteria carefully. The very careful studies carried on in many parts of the Illinois basin by Dr. A. C. Noe during the past few years have served to show that the similarities between the flora over the supposed No. 2 in differ- ent parts of the State are not as marked as had been thought, and that there is reason to doubt that the beds so correlated are in all cases con- temporaneous.''' Indeed the work on plants from other horizons, both 6 Personal communication, 1925. CORRELATION OF ILLINOIS COALS 9 ligher and lower than that of the Murphysboro coal, has fairly deraoii- •trated that much more extensive collecting and additional careful study ire needed before an adequate paleobotanical ladder can be made. The loral zones so far recognized are roughly placed in three distinct strati- jraphic groups by Dr. Noe, and it seems probable their relations are inderstood. Within the groups, however, the relations are not as yet letermined. Similar studies on the beds near the horizon of No. 6 coal have shown that it is not safe to correlate this coal on the basis heretofore Itilized, i. e., the median shale parting of the coal itself, and the fusulina, j'trtyi/ia ventricosa, in the cap limestone. Specificallv diamond drill ;ores from Madison County reveal coal beds other than No. 6 carrying i shale parting in places and overlain by thin limestone carrying fusu- inas indistinguishable on present data from Girtyina ventricosa. The recent finding of fusulinas in several distinct beds which appear o be widely separated stratigraphicallv has led to a recognition of the leed of a critical study of the means of identification of this interesting ;hell. The work of Walter Searight makes it appear probable that several types of fusulinas can be identified in the Illinois section, and lere, as in the case of paleobotanical work, the investigations give prom- be of great value in the determination of stratigraphic relations within he Pennsylvanian of the Illinois field. Other fauna! evidence may not be lacking for correlation purposes. Hie strata are usually fossiliferous, and frequently abundantly so. It s unreasonable to suppose that there were no distinctive faunal changes luring the whole Pennsvlvanian time, and so we may confidently look to laleontologv for important contributions to the problem. Some results )f this nature have recently been published. 7 The point of difficulty, ust as in the case of plants, is that as vet sufficient work has not been "eported, either in collecting or in identification, to make possible the iccurate correlation of Pennsylvanian beds on that basis alone. The careful student cannot but conclude, in view of this situation, hat at the present time the commonly accepted bases for correlation )f the Illinois section are fallible and must be used with great caution f at all. ' Savage, T. E., Marine invertebrate fossils as horizon markers in the Pennsylvanian OCks of Illinois: Jour, of Oeol., vol. 32, pp. 575-582, 1!)24. 10 CORRELATION' OF ILLINOIS COALS GEX E R.\ LI ZATIONS R E( WARDING PENNSYLVA X I A N STRATIGRAPHY AND CONDITIONS OF SEDIMENTATION Present data are insufficient to permit any revision of the existing correlation, but there has been recent accumulation of evidence pointing definitely toward certain generalizations which may well be considered. It should be emphasized here that the following statements are by no means considered final, but are offered in the hope that they will provl helpful in future work by suggesting possible relations which may bl upheld or discarded as new and better data appear. In its larger features the Pennsylvanian of the Illinois field is per- haps to he considered as a filling of a periodically subsiding basin. Making use of the present names for the subdivisions, we may say that the early Pottsville appears to he confined to that part of the State lying south of Sangamon County. Here are the well-consolidated shales and sandstones such as have not been found in the northern part. Included with these beds, and perhaps typical of them, are the "gun-metal"* shales which outcrop in Gallatin and Hardin counties and have been re- ported in Lawrence and Clark counties. Later Pottsville and early Carbondale strata, including the recog- nized horizons of No. 1 and No. 2 coals, with some beds formerly called No. 3 or Xo. 4. probably cover a somewhat wider area. They appear to be rather well distributed over the State, reaching from the southerr boundary northward to Illinois River and westward into Knox Count} at the north and Jersev County at the south. On the east side relation? are not clear, but there is some evidence that beds of this age underlie the later deposits along some of the margin. The later Carbondale and basal McLeansboro beds, including hori zons of No. 5, Xo. 6 and perhaps No. 7 coals, appear to have been mor widespread originally than any of the earlier deposits. They seem t< overlap the earlier Pennsylvanian locally along much of the wester- margin of the basin. A still later group of beds, the uppermost McLeansboro, includin all beds above the horizon of No. 7 coal, represent the last Pennsylvania deposition. Although some coal was deposited at this time, the onl commercially important beds are found in the Danville district, and th main deposition was of clastic and marine calcareous types. The pron inence of the limestone beds in this portion of the section is an inte estine feature. Considered in connection with the successive overla] BA term applied by early workers in southern Illinois to hard, shiny, dark gray sha: CORRELATION OF ILLINOIS COALS 11 during the Pennsylvanian, the widespread marine deposition suggests that in late Pennsylvanian the gap separating the Iowan and Illinoian fields was bridged. So much for the general relations between the larger stratigraphic units of the Pennsylvanian. The attempt to distinguish and to correlate the smaller units and individual members of the formations involves the identification and recognition of key beds. The selection of a datum horizon is of the greatest importance since the problem involves consid- eration of separated areas in which the geologic columns have little in common except their lithlogic monotony. The basal stratum of what is now called the McLeansboro formation, the cap rock of No. 6 coal, is a stratigraphic unit which gives promise of rather extensive areal dis- tribution. As will be pointed out later, greater reliance for purposes oi correlation can be placed on the zone of beds including this basal lime- stone than on the single stratum. It is commercially the most important bed in the Illinois section and is accessible in hundreds of mines in a score of counties. Another feature is that this limestone is near the middle of a thick Pennsylvanian column, the relations to which can be determined readily for limited areas. Still another point for the selection of this stratum is that if any changes were to be made in the correlation. the retention of the old number for this coal would greatly reduce, in the industrial world, the disturbance caused by a shift in names of the coals. The general conditions of sedimentation as revealed in the character of the deposits are of great importance in the problem of correlation. In the Illinois field these can be fairly well outlined for the larger features. The marine limestone became increasingly prominent during Pennsyl- vanian time, but its formation began in early Pottsville if not in the earliest Pottsville. Limestones referable to this time are found in the central portion of the field as at Assumption, Christian County, and in the marginal portions as at Alton and Frederick, Schuyler County. It is : probable that further study of the sections in the southern Pottsville belt will bring out additional occurrences. Limestone of marine oriein is more common in later Carbondale and seems to have reached its climax- in late McLeansboro. Clastic deposition preceded and followed that of limestone, and although in some parts of the field, as at Morris, the hthology suggests continental origin, elsewhere the origin is perhaps largely marine. The deposition of coal was apparently restricted to small separated -wamps in Pottsville time, but these areas were scattered over much of the southern half of the State. In Carbondale time the relief of the basin was probably less and the swamps of coal deposition were proba- 12 CORRELATION OF ILLINOIS COALS My correspondingly greater in size. It is not likely, however, that the separate swamps were of great extent. Detailed studies in the La Salle and Murphysboro fields indicate that the thicker portions of the coal beds grade laterally in short distances to carbonaceous shales or to splits ii clastic beds. In late Carbondale and earlv McLeansboro time, emialh widespread coal swamps appear to have existed, but here again, although the area of thicker coal is much greater than in the preceding epoch, the main portion of the held seems to have been the site of elastic deposi tion, for the coal beds <^rade laterally into such beds relatively near tin main mining districts. An important exception is probably to be found in the area near the Duquoin fold. Here, during the later Carbondale, conditions appeal to have been exceptionally favorable to coal deposition, and in the deepei part of the downfold north of Franklin County we may confidently lool for extensions of the thicker portions of the llerrin coal. To the east or west, farther north, however, it is likely that the coal swamps wen limited by land or by the open sea. In either case the formation o commercially important bodies of coal was impossible. This epoch wa- followed by one in which coal swamps were of little importance. Thii coal beds demonstrate the presence of swamps which may have beei widespread in the nearly flat basin of late McLeansboro time. but. if so: they were short lived and did not produce any important accumulation of vegetal matter, except on the east margin where in limited zones wen deposited the Grape Creek and Danville beds. Study of the stratigraphic details of the Illinois sections emphasize two points. (1 ) Although several rather extensive areas appear to hav< been swamps at different times, there are few sections which show mor than one important coal bed. (2) A single coal bed in one area ma- be represented in adjacent territory by several beds separated by clastic- For correlation purposes, it is probably necessary to consider these zone of coal rather than the individual beds as stratigraphic units. In sue' zones may be combined separate beds of approximate contemporaneiti without assumption of definite relations between them. From the general statements of depositional conditions during tin period, it is clear that the most widespread recognizable beds are th marine limestones. The close similarity of all the beds in section stratigraphicallv adjacent makes it impossible to correlate separate roc masses on the basis of limestones alone. By taking into consideratio the conditions of sedimentation as revealed by the lithohgic sequenci which include the limestones, it has been possible to make satisfactor correlation over short distances. There seems to be no obstacle t o-reater extension of this mode of correlation. By this is not meant an CORRELATION OF ILLINOIS COALS 13 [eparture from standard geologic practice. The careful observer will lot fail to note that correlation by fossils alone, either plant or animal orms. is only feasible after the correlation ladder has been prepared rom a study of the fossils in a series of beds whose relations are known. Phis has not as yet been done, nor will the mere presence of distinguish- ible forms serve in this instance to prove a correlation. Not only the listinctions but also the relationships between the forms recognized as ypical of the several stratigraphic zones must be determined before the evidence can be accepted as final. It is probable that the final and :orrect correlation will be accomplished only by the active cooperation if workers in all three lines. lER'S" IBRARY BINDERS 507 3. Goodwin Urbane. N. Umm Hnfis BSBMISe Iw B]