BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 18, PP. 449-476, PLS. 49-52 DECEMBER 17, 1907 ORIGIN AND SIGNIFICANCE OF THE MAUCH CHUNK SHALE* BY JOSEPH BAURELL (Presented in abstract before the Society December 26, 1906) CONTENTS Page Part I. Observations bearing on the problems of origin................................. 449 Introduction .......................................................................................... 449 General description of the Mauch Chunk formation................ 450 The Mauch Chunk of the anthracite coal basins......................... 453 Stratigraphie characters ........................................................... 453 Inorganic evidences of subaerial exposure........................... 456 Organic evidences of subaerial exposure............................... 460 Part II. Inferred conditions of origin.................................................................... 462 Previous opinions ................................................................................462 Inferences as to geographic conditions of origin....................... 463 Floodplain origin of the Mauch Chunk................................. 463 Relations of land and sea.......................................................... 463 Relations of the delta plain to regions of erosion.............. 468 Character of the delta surface..................................................468 Inferences as to climatic conditions................................................ 469 Introductory statement ............................................................. 469 Chemical and structural evidences as to climate.............. 469 Organic evidences as to climate................................................ 472 Conclusions and comparisons.................................................... 474 P a r t I. Observations bearing on th e Problems of Origin INTRODUCTION A discussion of the nature of geographic and climatic control of sedi­ mentation has been previously made by the writer in order that the origin of certain specific formations might be dealt with, and the present paper on the interpretation of the Mauch Chunk follows these previous articles in logical sequence, f * Manuscript received by Secretary of the Society from Censor October 28, 1907. f Relative geological importance of continental, littoral, and marine sedimentation. Journal of Geology, vol. xiv, 1906, pp. 316-356, 430-457, 524-568. Relations between climate and terrestrial deposits. Presented before the Geol. Soc. Am. December 26, 1906. Printed in abstract in the Proc. Geol. Soc. Am. XXXIX—Bull. Geol. Soc. Am., Vol. 18, 1906 (449) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/18/1/449/3412352/BUL18-0449.pdf by guest on 28 September 2021 4 5 0 J . BARRELL----ORIGIN OP THE MAUCH CHUNK SHALE The first part of the present paper consists of a presentation of the significant facts of the Mauch Chunk shales, gathered both from the literature and from personal observation. The second part deals with the interpretation of these facts. Where a new view is to be proved the observational and inferential portions should be strictly separated, but at the same time the purpose for which the facts are presented should be evident when they are given. It is therefore desirable to state in advance the conclusion finally reached, which is that in the anthracite region, more surely in the southeastern and eastern portions, the whole forma­ tion, from top to bottom, was a subaerial delta deposit laid down under a semiarid climate. Besides merely the essential facts, there are included in the descriptive portion many details which are not used in drawing the conclusions, but which, if the hypothesis of terrestrial deposition be accepted, are interest­ ing as showing in a definite case what the minor characteristics of such a formation may be. They are doubtless also valuable as indicating specific features of climate or geography, even though their significance may not at present be understood. GENERAL DESCRIPTION OF THE MAUCH CHUNK FORMATION The Mauch Chunk shale is the name of a strongly individualized for­ mation of the Lower Carboniferous of Pennsylvania, chiefly consisting, in the region of its typical development, of alternating red shales and red sandstones, the ratio of the two varying through different horizons of the formation and in different districts. Not a little of the distinctive character of the Mauch Chunk is due to its sharp contrast with the preceding and succeeding formations. The inferior, the Pocono sandstone, consists of a great thickness of grayish green to light gray, cross-bedded sandstones, the whole weathering white and constituting a massive and resistant formation, topographically prominent wherever it occurs. Above the Mauch Chunk lies the Potts- ville conglomerate, interstratified with gray sandstones. The Pottsville, like the Pocono, is a resistant formation and one to whose protecting rim many of the smaller synclinal coal basins owe the preservation of their coal during the Tertiary cycle of erosion. In contrast with these adjacent formations the Mauch Chunk as a whole is weak and conse­ quently readily reduced to local base level. It yields a red loamy soil, and where the formation is thickest, as around the southern and middle anthracite coal basins, gives rise to comparatively level and broadly open valleys. These are utilized for agriculture, and the checker-work of fields contrasts strikingly with the forest-covered and level-topped slopes Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/18/1/449/3412352/BUL18-0449.pdf by guest on 28 September 2021 GENERAL DESCRIPTION OP THE MAUCH CHUNK FORMATION 451 which rise on each side to a height in the neighborhood of a thousand feet. The broader characters and relations of the Mauch Chunk shales of Pennsylvania and the synchronous deposits of other portions of the Ap­ palachian basin have been recently so admirably summed up by J. J. Stevenson* from the previous studies of himself and others that only sufficient need be here outlined to bring the chief features to mind. The formation in question was laid down with a character, thickness, and distribution much more variable than that of the underlying Pocono. Its maximum development occurs in the anthracite coal regions and it there possesses its typical character. Beyond the limits of the state of Pennsylvania the equivalent formations are so different that other names must be applied. In eastern Ohio the strata pass into the upper part of the Waverly group, while to the southwest in Maryland and Virginia the Greenbrier limestone occupies the horizon of the lower portion of the shales. In contrast to the considerable thickness to the south, the shales are thin along all the northern outcrops in Pennsylvania and often missing from between the Pocono below and the Pottsville above. The Mauch Chunk reaches its greatest thickness in the region of the southern anthracite coal field, according to Rogers measuring at least 3,000 feet at Pottsville. At Mauch Chunk, Winslow gives figures of 2,168 feet on the south side of the syncline and 3,342 on the north. From these maximum thicknesses it diminishes northward to 1,000 feet at Solomons gap, near Wilkes Barre, and 150 feet of green shales and flaggy sandstones at Pittston. Passing westward from Pottsville, first to the Broad Top coal field, Ashburner and Billin give it a thickness of 1,100 feet, divided into: 1, Upper shales and sandstones, 910 feet; 2, Mountain (Greenbrier) limestone, 49 feet; 3, Lower shales and sand­ stones, 141 feet.f Northwest of the Broad Top field and back of the Alleghany Mountain front the thickness is given by Butts, in the Ebens- burg, Pennsylvania, folio, as but 180 feet, of which the lower 80 feet consist of thick bedded gray to greenish sandstone, corresponding per­ haps to the Greenbrier limestone, and underlaid by 5 to 6 feet of inter­ bedded red shale and sandstone. At the most southwestern outcrop in Pennsylvania, that of Laurel ridge (Masontown-Uniontown quadrangle), M. R. Campbell gives the entire thickness as 250 feet, consisting of three members. At the base are 50 feet of typical Mauch Chunk red shale, followed by 30 feet of Gieenbrier limestone, which is in turn succeeded by 170 feet of red and green shales, with occasional beds of greenish sandstone. * Lower Carboniferous of the Appalachian basin. Bull. Geol. Soc. Am., vol. 14, 1902, pp. 16-96. t Second Geological Survey of Pennsylvania. Summary, final report, vol. 3, part 1, p. 1833. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/18/1/449/3412352/BUL18-0449.pdf by guest on 28 September 2021 452 J. BARBELL---- ORIGIN OP THE MAUCH CHUNK SHALE It is seen from these details that the edge of the great Greenbrier limestone of Virginia and farther west reaches into southwestern Penn­ sylvania only, thinning away to a knife-edge, and its place being appar­ ently taken near its margin by thick bedded gray to greenish sandstones, noted as 80 feet thick in the Ebensburg folio. Recent observations tend to show that over the western half of Penn­ sylvania, as well as over the states of the central west, uplift and erosion intervened to a greater or less extent between the latest deposits of the Mississippian period and the earliest of the Pennsylvania,* the adjacent formations being sharply distinct and different members of the Mauch Chunk or equivalent formations coming into contact with the Pottsville sandstone. The scantiness