Linking anoxia, biotic events, and basin evolution in the Late Devonian Illinois Basin, North America: A geochemical approach

Abstract

Epeiric seas and their sedimentary deposits are vital for understanding the history of Earth's oceans, but the lack of modern analogues and the frequent dominance of uniform mudrocks in these environments hinder our ability to identify key markers and interpret sea-level changes and past climatic/biotic events in epeiric sea successions. This study focuses on the Late Devonian Illinois Basin, where fine-grained mudrocks, occasionally organic-rich, were deposited in one of many variably interconnected epeiric seas that flooded much of the North American craton. Through the correlation of gamma logs from 50 wells across the basin and the generation of high-resolution chemostratigraphic profiles for four cores, a basin evolution model for the Upper Devonian New Albany Shale (NAS) in the southeastern depocenter of the Illinois Basin (USA) is established. Results indicate that NAS deposition occurred in three phases, characterized as one aggradational-progradational, one progradational, and one retrogradational package. The basin experienced an initial flooding event at the base of the NAS, followed by a second transgression immediately after the Frasnian-Famennian boundary (FFB). The mid- to upper Famennian interval witnessed progressive sea-level rise (a third transgression) and full basin inundation, extending nearly to the top of the NAS. Based on redox proxy data, four discrete anoxic pulses were identified during NAS deposition. By incorporating published biostratigraphic and geochemical datasets, we then correlate these signals observed in the Illinois Basin across the Appalachian, Michigan, Permian, and Anadarko basins of the broader North American Seaway. Additionally, some of the anoxic pulses are linked to key biotic crises of the Late Devonian, such as the Rhinestreet, Upper Kellwasser, and Enkeberg events, which caused substantial turnover in the marine biosphere. This study contributes an integrated basin evolution framework for the Late Devonian North American Seaway and serves as an exemplar of utilizing geochemical signals to correlate sea-level changes, anoxic pulses, and key biotic events in ancient mud-dominated epeiric seas.