STRATIGRAPHY OF THE BIG SPRING STATION MEMBER OF THE CONOCOCHEAGUE LIMESTONE, SOUTHCENTRAL PENNSYLVANIA

The Conococheague Limestone (Upper Cambrian) is a part of the Lower Paleozoic passive continental margin sedimentary succession of the central Appalachians. It represents a 750-meter thick shallow water carbonate platform succession that overlies the Middle to Upper Cambrian Elbrook Limestone, and underlies the Ordovician Beekmantown Group carbonates. The lower part of the Conococheague is characterized by partially to extensively dolomitized carbonate deposits with common sand-size siliciclastic detritus. These deposits comprise the Big Spring Station Member of the Conococheague Formation.

This study focuses on an outcrop of the Big Spring Station Member located along Hagerstown road, approximately two miles south of Waynesboro in the Great Valley of southcentral Pennsylvania. In this roadcut, south of the bridge over the Antietam Creek, a 53-meter thick, well exposed section of the Big Spring Station Member was measured, described and sampled. The base of the section is approximately 40 meters above the contact with the Elbrook Limestone, and the upper boundary of the Big Spring Station Member is near the top of the measured section.

Previous biostratigraphic studies interpreted the upper Big Spring Station Member strata at this locality as deposited "above top Dresbachian fauna," judging from the occurrence of rare Dresbachian fossils in the lower part of the measured section (Wilson, 1952). This interpretation suggests that the presence of detrital quartz in these strata indicates increased siliciclastic input onto the carbonate platform in response to a sea level fall at the Dresbachian/Franconian (or the Sauk II-Sauk III) boundary in the Late Cambrian (Steptoean). This time interval is also characterized by a large, global, positive carbon-isotope excursion (d¹³C=4-5 ‰ VPDB). Future studies will examine the relationship between lithostratigraphy, biostratigraphy, and carbon-isotope stratigraphy of the Big Spring Station Member with the goal of improving stratigraphic interpretations of this poorly fossiliferous and dolomitized interval.