69-5 Depositional Environment of the Late Devonian-Early Carboniferous North American Seaway Inferred from Organic Petrology and Geochemical Analysis

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change (Posters)

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Extensive deposits of organic-rich shales, including the New Albany Shale (NAS), accumulated in deepwater anoxic environments within the North American Seaway during the late Devonian-Carboniferous (D/C). Yet the role of restricted circulation within the seaway on preservation of sedimentary organic matter (OM) is poorly understood, as is its influence on the composition and geochemical characteristics of OM. Here, we integrate evidence from organic petrology and geochemical proxies to reconstruct depositional environments of the NAS in the Illinois Basin. Our studies include evaluation of the composition of OM (terrigenous versus marine macerals) and its microbial sources (sterane/hopane ratios, isotopic profiles), coupled with assessment of the redox properties (tricyclic terpane ratios, U enrichment) and salinity (B/Ga ratio) of the depositional setting. Stratigraphic variations in d¹³C_org (-27.0 ‰ to -30.3 ‰) for the Blocher and Selmier Members of the NAS indicate fluctuations in chemocline depth and intensity accompanying changes in bottom-water oxygenation. The subsequent combination of sea level decline and uplift of the Cumberland Sill associated with the Frasnian-Famennian boundary event restricted water circulation between the seaway and the global ocean. This change in depositional setting is reflected by the Morgan Trail and Camp-Run Members of NAS that are characterized by euxinic conditions, brackish waters, contributions of OM from green algae, and d¹³C_org averaging -29.66 ‰. Gradual sea level rise and subsidence of the Cumberland Sill reestablished deepwater circulation during the Upper Clegg Creek Member as reflected by an increase in vitrinite relative to alginite and a shift in OM sources from aquatic to terrestrial dominance. d¹³C signatures for n-heptacosane and pristane, coupled with elevated sterane/hopane ratios (1.6), suggest that increased runoff transported more terrestrial OM into a brackish environment, where efficient burial under anoxic conditions preserved it. Rapid sea level regression accompanied by substantially higher sedimentation rates during the NAS section corresponding to the D/C boundary resulted in OM dilution with intermittent euxinia and d¹³C_org values averaging -28.29 ‰. The ensuing transgressive episode of the Henryville Beds records increased salinity to near-marine levels, reestablishing environmental conditions analogous to the Frasnian. Thus, results from this study further recognition of connections between the petrological, molecular, and isotopic characteristics of OM and temporal changes in the depositional dynamics of a restricted marine anoxic system.

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

doi: 10.1130/abs/2025AM-10187

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