BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 46, PP. 1275-1290. 2 FIGS. AUGUST 31. 1935 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY HAMILTON GROUP ALONG THE ALLEGHENY FRONT, PENNSYLVANIA* f BY BRADFORD WILLARD (Read before the Paleontological Society, December 28, 1934) CONTENTS Page Introduction................................................................................................................ 1275 Hamilton group along the Allegheny Front........................................................... 1277 Distribution................................... ..................................................................... 1277 Stratigraphy........................................................................................................ 1278 Divisions, thickness, character................................................................... 1278 Description by counties............................................................................. 1279 Interpretation.............................................................................................................. 1286 Correlation........................................................................................................... 1286 Geologic History................................................................................................. 1289 Conclusions.................................................................................................................. 1289 INTRODUCTION This paper is the concluding number of three articles on the Hamilton group of Pennsylvania, which the author has read at successive meet­ ings of the Paleontological Society. In 1932 the first paper, prepared jointly with Arthur B. Cleaves,1 was given. It covered the eastern part of the State from the Delaware Valley to the Susquehanna Valley and also included observations on, and correlations of, the Hamilton of the Green Pond Mountain region, New Jersey. In 1933 the author read the second article of the series,2 which described the group in cen­ tral Pennsylvania from the Susquehanna Valley west into Fulton and eastern Huntingdon counties. The present article covers the remain­ ing portion of the State where rocks of Hamilton age are known at the surface. It will be necessary first to review previous observations be­ fore taking up the new region. * Manuscript received by the Secretary of the Society, February 7, 1935. t Published with the permission of the State Geologist of Pennsylvania. * Bradford Willard and Arthur B. Cleaves: Hamilton group of eastern Pennsylvania, Geol. Soc. Am., Bull., vol. 44 (1933) p. 757-782. a Bradford Willard: Hamilton group of central Pennsylvania, Geol. Soc. Am., Bull., vol. 46 (1935) p. 195-224. (1275) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/8/1275/3430542/BUL46_8-1275.pdf by guest on 30 September 2021 1276 BRADFORD WILLARD----HAMILTON GROUP ALONG ALLEGHENY FRONT The four formations of the Hamilton group of eastern Pennsylvania are lithologically and paleontologically comparable to those of central New York. Several subdivisions occur, but only the formation names need be repeated: (1) Moscow, (2) Ludlowville, (3) Skaneateles, and (4) Marcellus. These four maintain their identity between the Sus- TrmnrrA . ,i-"-.-r-*T. , r,,--“. if,..-..11* " F igure 1.—Sketch map showing distribution of the Hamilton group in Pennsylvania quehanna and Delaware valleys. However, in the most easterly sec­ tions characters are obscured except for the Marcellus, which is the one formation of the group recognizable throughout the Hamilton in Penn­ sylvania. In northern New Jersey most of the Hamilton passes over into the red Catskill continental facies. Along the Susquehanna Val­ ley, local, lithologic changes largely obliterate formation boundaries. In central Pennsylvania the group is divided as follows: Mahantango formation Moscow faunal facies Ludlowville faunal facies } Plus or minus Montebello sandstone mem- Skaneateles faunal facies \ ber and Knobsville red beds. Marcellus formation Mahanoy black shale member Mexico sandstone member Turkey Ridge sandstone member Shamokin black shale member Note: A thin limestone found in the midst of the Shamokin shale near Mexico, Juniata County, is not to be directly correlated with Marcellus lime­ stones of the Allegheny Front region; they are widely separated and of quite dissimilar nature. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/8/1275/3430542/BUL46_8-1275.pdf by guest on 30 September 2021 INTRODUCTION 1277 Thinning northward from a maximum thickness in Perry and Dauphin counties into the Susquehanna-Juniata valleys region, a thick sand­ stone, the Montebello, and the thinner Mexico and Turkey Ridge sand­ stones dominate the group. These appear to be the marine portions of a delta spreading fan-wise from a shore line to the south. Another deltaic deposit, the Knobsville continental beds of northeastern Fulton County, was forming during most of Hamilton time. The lithologic identity of the four eastern formations, excepting the Marcellus and perhaps the Moscow, is obscured in the Susquehanna Valley in and beyond the region of the coarser clastic sediments. In their stead, there is, beyond the limits of these sandstones, a succession of dark gray, more or less finely arenaceous shales, the Mahantango formation, which overlies typical, black, fissile Marcellus shale. The only prominent lithologic variation of the Mahantango in central Pennsylvania is an occasional sandstone lens. At first thought to represent definite divi­ sions of the formation, detailed studies show them to come in at ran­ dom. These sandstones may carry a facies fauna characterized by Spirifer granulosus (Conrad), Adolfia (Spirifev) audacula (Conrad) ,s and Pleurotomarias. Compilation of lists of fossils shows that there are in central Pennsylvania faunal divisions corresponding to the forma­ tions in the eastern part of the State. Characteristic Marcellus and Mahantango formations and newly discovered variants of these two are present in the Allegheny Front region. The faunal divisions are less definite than in central Pennsylvania. HAMILTON GROUP ALONG THE ALLEGHENY FRONT DISTRIBUTION The Allegheny Front escarpment * enters the State of Pennsylvania from Maryland by way of Bedford and Somerset counties and trends north, then northeast to, and across, Lycoming County. In front (east and southeast) of this escarpment and parallel to it throughout, Devo­ nian formations are exposed. The Hamilton group is mostly valley- forming shales, but lenticular sandstones produce local ridges. From the Maryland line northward a continuous band of Hamilton forma­ tions crosses Bedford, Blair, Huntington, Centre, Clinton, and Lycom­ 8 The generic name Adolfia is used in conformity with previous reports, but G. A. Cooper informs the writer that recent researches indicate that such usage probably is not strictly correct. * Paul H. Price: The Appalachian structural front, Jour. Geol., vol. 39 (1931) p. 24*44. R. E. Sherrill: Symmetry of northern Appalachian foreland folds, Jour. Geol., vol. 42 (1934) p. 225-247. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/46/8/1275/3430542/BUL46_8-1275.pdf by guest on 30 September 2021 1278 BRADFORD WILLARD— HAMILTON GROUP ALONG ALLEGHENY FRONT ing counties (Fig. 1). It joins the Hamilton exposed in central Penn­ sylvania, by way of the Susquehanna Valley. Scattered areas of Hamilton rocks in Bedford, Mifflin, and Huntingdon counties are in­ cluded in the present paper. From eastern Centre and Clinton, south­ western Lycoming, and most of Union counties the Devonian has been eroded, widely separating the Allegheny Front exposures from those of the central region. Despite the extent of the area underlain by the Hamilton group along the Allegheny Front, exposures are often poor. The dominance of soft shale is one cause; outwash from the escarpment and, in the north, glacial débris are other factors. STRATIGRAPHY Divisions, thickness, character.—The Hamilton group along the Alle­ gheny Front consists of an upper formation and a lower one, the Mahan­ tango and the Marcellus, respectively. The Mahantango contains faunas suggestive of the Moscow and the Ludlowville, doubtfully of the Skaneateles, farther east. These faunal separations may, however, be very obscure. In the south the Mahantango formation is subdivided on lithologic data. For the Marcellus no important members are known. The thickness of the Hamilton group along the Allegheny Front is fairly uniform, the average being not far from 1200 feet, but conceal­ ment usually prevents exact measurement. The best data were secured in southern Bedford County, where the thickness is 1575 feet. This checks closely with Stevenson’s figure of 1587 feet.5 Along the “Front” the base of the Hamilton group is transitional with dark gray to black Onondaga shale, which weathers deeply to olive- drab and may include thin, impure limestone beds. This Onondaga- Marcellus transition is seldom thick. The top of the Onondaga is drawn above the highest occurrence of Anoplotheca acutvplicata (Conrad). Were it not for a Marcellus-Onondaga diastem in central Pennsylvania, one would group the Onondaga with the Hamilton in concurrence with the Maryland usage.6 The top of the group along the Allegheny Front is clearly defined. The Mahantango shale makes sharp contact with any one of three members which may occupy the basal Portage,7 the Tully limestone, the Burket black shale,