The Frasnian-Famennian (Late Devonian) boundary within the Foreknobs Formation, Maryland and West Virginia GEORGE R. McGHEE, JR. Department of Geological Sciences, University of Rochester, Rochester, New York 14627 ABSTRACT The approximate position of the Frasnian-Famennian (Late De- vonian) boundary is determined within the Foreknobs Formation along the Allegheny Front in Maryland and West Virginia by utiliz- ing the time ranges of the articulate brachiopods Athryis angelica Hall, Cyrtospirifer sulcifer (Hall), and members of the Atrypidae. INTRODUCTION The age of strata previously called the "Chemung Formation" along the Allegheny Front in Maryland and West Virginia (Fig. 1) has been of interest to Devonian wokers for some time. Recent at- tempts to resolve this problem include the works of Dennison (1970, 1971) and Curry (1975). New paleontological contribu- tions to the resolution of time relations within the Greenland Gap Group ("Chemung Formation") are the object of this paper, which is an outgrowth of a much larger ecological analysis of Late Devo- nian benthic marine fauna as preserved in the central Appalachians (McGhee, 1975, 1976). STRATIGRAPHIC SETTING The following is a condensation and summary of the evolution of Upper Devonian stratigraphic nomenclatural usage in the study Figure 1. Location map of study area, showing positions of the mea- area; for a more complete and thorough discussion, the reader is sured sections used in this study (after Dennison, 1970). referred to Dennison (1970) and Kirchgessner (1973). The Chemung Formation was originally designated by James lower Cohocton Stage." Elsewhere, concerning the upper limit of Hall (1839) from Chemung Narrows in south-central New York. the Greenland Gap Group, he stated: "Along the Allegheny Front The name "Chemung," though in reference to the Chemung Series, in the area studied no fossils in the uppermost member of the was later introduced into West Virginia by I. C. White (1881), Foreknobs Formation appear younger than Cohocton Stage" based on similarities of the fauna and stratigraphic sequence with (Dennison, 1970, p. 71). However, in view of the thickness of the the New York section. Marine shales and sandstones beneath the Red Lick Member of the Foreknobs Formation at the Virginia nonmarine Hampshire Formation were called the Jennings Forma- Route 250 section, Dennison (1971, p. 1186) later considered that tion by Darton (1892), from Jenning's Gap and Jenning's Branch the Foreknobs Formation may extend upward into the post- in western Augusta County, Virginia, and included both the Cohocton toward the southern end of the study area. Chemung and present-day Brallier Formations. The Chemung was Palynologic zonation of the Greeland Gap Group was attempted considered to be a member of the Jennings Formation by Swartz by Curry (1975), though without marked success due to the largely (1913a). Later workers referred to the strata as the "Chemung unfossiliferous nature of the outcrops with respect to miospores. Formation" (Butts, 1918, 1940; Woodward, 1943). Curry (1975, p. 129) tentatively assigned a Middle to Late Frasnian With continued work in New York State and more recent in- age to the Greeland Gap Group, after noting that almost all of his terpretations of facies relationships there, New York stratigraphy described samples came from the Mallow Member of the Fore- was considerably reorganized, and the term "Chemung Forma- knobs Formation at the Corriganville section. He further noted tion" was dropped (Rickard, 1964). With the abandonment of the that microfossils from the Mallow Member displayed affinities term "Chemung Formation" as a formal stratigraphic unit in the with both Givetian and Frasnian microfloral assemblages described area where it was first proposed, its usage was rendered meaning- from other regions, while exhibiting a complete dissimilarity with less elsewhere. Exposures that were once called "Chemung" along Famennian assemblages. the Allegheny Front were renamed the Greenland Gap Group by Thus, a Late Frasnian age was generally accepted for most of the Dennison (1970). He also subdivided the Greenland Gap Group Greenland Gap Group. McAlester (1962) criticized the use of into two formations, the Scherr and the Foreknobs. The younger "Cornellites" chemungensis (Conrad) as a guide for the Chemung Foreknobs Formation, which ranges in thickness from 403 to 690 (Cohocton) Stage, noting that its present usage is based solely on its m was further subdivided into five members (Fig. 2) by Dennison empirically determined distribution in the New York Devonian to (1970) and McGhee and Dennison (1977). date, and not upon its evolutionary position, which remains uncer- tain. He suggested that the presently observed distribution of EARLIER NOTES ON THE AGE OF FOREKNOBS "Cornellites" chemungensis is strongly influenced by facies, as the FORMATION species preferred the sandier depositional environments, and that non-Chemung (Cohocton) Stage occurrences are to be expected Concerning the geologic age of the Greenland Gap Group, Den- with further exploration. However, the occurrence of the nison (1970, p. 64) stated: "The basal beds of the Scherr Formation brachiopod Nervostrophia nervosa (Hall) in the Mallow Member contain Cyrtospirifer chemungensis (Conrad) and Cornellites of the Foreknobs Formation (McGhee, 1976) confirms that the chemungensis (Conrad), which indicated age assignment to the Scherr-Foreknobs contact lies within the Middle Cohocton Stage, Geological Society of America Bulletin, v. 88, p. 806-808, 3 figs., June 1977, Doc. no. 70607. 806 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/806/3429373/i0016-7606-88-6-806.pdf by guest on 26 September 2021 FOREKNOBS FORMATION, MARYLAND AND WEST VIRGINIA 807 SOUTHWEST NORTHEAST VA.-W.VA w. va'-md. I <r A» ,0 LOWEST CONGLOMERATE YELLOWISH GRAY SANDSTONE GRAY SILTSTONE MBR IN BRALLIER FM. 30 KILOMETERS Figure 2. Stratigraphie cross section of the Foreknobs Formation along the Allegheny Front (after McGhee and Dennison, 1977; modified from Denni- son, 1970). based on the range of that species in New York and as documented in the Pound Sandstone at the Hopeville Gap section and at La by Rickard (1964). Vale. Cyrtospirifer sulcifer (Hall) is now considered to extend no lower than the Middle Cassadaga Stage in New York (Fig. 3), as NEW EVIDENCE ON THE POSITION OF THE Greiner (1957) reported the lowest occurrence of the species to FRASNIAN-FAMENNIAN BOUNDARY have been in the upper Rushford Sandstone. My observations, con- cerning the distribution of Cyrtospirifer sulcifer in the central Questions concerning age assignments of the Hampshire- Appalachian Allegheny Front region, extend the range of the Foreknobs contact are resolved by observations made in the species down to the beginning of the Cassadaga Stage. present study. These suggest that the upper part of the Pound Highest observed occurrences of members of the Atrypidae along Sandstone and the Red Lick Members (Fig. 2) of the Foreknobs the Allegheny Front, usually the species Spinatrypa hystrix (Hall), Formation are of Cassadaga Age (Famennian Epoch) at the are in the lower Pound Sandstone. This entire family of articulate Hopeville Gap and La Vale sections, and that the Frasnian- brachiopods are believed to have become extinct by the close of the Famennian boundary lies within the Pound Sandstone Member, at Frasnian Epoch (Boucot and others, 1965; Copper, 1973, Text- least at the two previously mentioned sections. This hypothesis is fig. 1). based on the first occurrence of two brachiopods, Atbyris angelica In summary, the evidence strongly indicates that the Red Lick Hall and Crytospirifer sulcifer (Hall), and the last-observed occur- Member and Pound Sandstone Member (upper part) of the Fore- rence of members of the Atrypidae (Fig. 3). Two specimens of knobs Formation at the Hopeville Gap and La Vale sections are of Athyris angelica Hall were observed; one in the Pound Sandstone Cassadaga Age (Famennian Epoch), and not Cohocton Age (Fras- Member at the Hopeville Gap section, and one in the Pound nian Epoch) as previously thought. Sandstone at the La Vale section. Interestingly enough, Atbyris It is interesting to note that Johnson (1974) attributed end-of- angelica was also reported in the "Upper Shale and Sandstone Frasnian extinctions in the shallow benthos to a sudden world- Beds" of the "Chemung" Formation at the La Vale section (then wide marine regression-transgression event (also see discussion in called the "Section near Allegheny Grove") by Swartz (1913b, Copper, 1973, p. 491). An abrupt and widespread regression at the p. 524). Earlier Devonian workers are in agreement that the first ap- end of the Frasnian should produce an equally widespread uncon- pearance of Atbyris angelica Hall marks the beginning of the formity, which Johnson (1974, p. 480) documented for western Canadaway (Cassadaga) Stage (Chadwick, 1935, p. 323; Willard, Canada and the United States. The fact that no widespread 1939, p. 251; Cooper and others, 1942; McAlester, 1962, p. 6). lithologic break is known in mid—Late Devonian strata of the Fifty-seven specimens of Cyrtospirifer sulcifer (Hall) were found Appalachian Basin was attributed by Johnson (1974) to "orogenic Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/806/3429373/i0016-7606-88-6-806.pdf by guest on 26 September 2021 808 G. R. McGHEE, JR. REFERENCES CITED European North American Range of Selected Series Series Stage Species Boucot, A. J., Johnson, J. G., Pitrat, C. W., and Staton, R. D., 1965, Spiriferida, in Moore, R. C., ed., Treatise on invertebrate paleontol- ogy, Part H, Brachiopoda: Lawrence, Kansas., Geol. Soc. America and Univ. Kansas Press, p. H632-H728. Butts, C., 1918, Geologic section of Blair and Huntingdon Counties, Pennsylvania: Am. Jour. Sci. (4th ser.), v. 46, p. 523-536. z £ ; z < < Ö 1940, Geology of the Appalachian Valley in Virginia: Virginia Geol. < o > Survey Bull., v. 52 (1), 568 p. a •5 <0 Chadwick, G. H., 1935, Faunal differentiation in the Upper Devonian: z < <3 a v. Geol. Soc. America Bull., v.