68-2 Sedimentological and Geochemical Investigation of the Origin of Upper Pennsylvanian Black Shales in the East-Central Appalachian Basin

Session: Climate Transitions in the Paleozoic (Posters)

Presenting Author:

Authors:

Abstract:

Climate and sea-level changes during glacial-interglacial cycles of the Late Paleozoic Ice Age (LPIA) caused significant changes in coal-swamp vegetation patterns across the equatorial latitudes of Pangea and likely contributed to extinction of lycopsids near the Desmoinesian-Missourian boundary (~306 Ma) in a major paleobotanical evolutionary event known as the Carboniferous Coal Forest Collapse (CCFC). Previous investigations of the CCFC commonly implicate an episode of aridification that stressed the lycopsids beyond their ecological tipping point; however, it is unclear if this aridification was associated with intensification of glacial conditions or interglacial conditions. In the east-central Allegheny foreland basin, coal seams in the lower Conemaugh Group (Glenshaw Fm.) span the CCFC event interval, preserving a record of environmental changes in mainly terrestrial cyclothemic successions. Here, we present a sedimentological and geochemical investigation of laminated black shales overlying Glenshaw Fm. coal seams that record termination of peat swamp deposition in order to constrain the paleoenvironmental and paleoclimate conditions that terminated peat deposition before, during, and after the CCFC, as these deposits can reflect development of either marine environments developed during intensified interglacial climate or anoxic aquatic terrestrial environments developed during intensified glacial climate. Sedimentological analysis of shales at 5 Glenshaw Formation outcrops across the Allegheny basin in West Virginia reveals changes in color and physical properties, ranging from dark gray, moderately fissile shales to black, highly fissile shales. Geochemical analysis of forty samples from these locations indicate changes in clastic input, organic carbon content, salinity, and redox-sensitive elements, which reflect basin-wide responses to glacial-interglacial dynamics. Redox-sensitive trace elements show different correlation patterns with S/TOC contents, with shales above the Harlem coal, the youngest of the sampled intervals, exhibiting the highest S/TOC (0.16-1.70 wt. %) and trace element enrichment (0.45-1.11 U/Al, 11.03-27.18 V/Al, and 0.15-1.58 Mo/Al ppm/wt. %), which is consistent with marine deposition. Furthermore, these shales reflect low detrital influx proxies consistent with a shelfal environment. In contrast, shales overlying coal seams that pre-date the CCFC, including the Upper Freeport, along with coals that post-date the CCFC, including Brush Creek, Wilgus, and Bakerstown coal, all show higher detrital influx values and ambiguous salinity indicators. This may indicate orbital forcing of more intense glacial cycles influenced terrestrial ecosystems and possibly contributed to the CCFC.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11040

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.