Investigating Core Shale Depositional Environments of Late Pennsylvanian Cyclothems Utilizing Geochemical Proxies to Test the Superestuarine Model
Master of Science (MS)
Geology and Geophysics
The Late Paleozoic Midcontinent Sea (LPMS) inundated vast areas of the North American interior during glacio-eustatic transgressions, depositing widespread black shales facies within the core shale intervals of major cyclothems. These black shale deposits are unique because no modern analogs can adequately explain the depositional environments and model for organic matter preservation across such vast ancient epicontinental settings. One possible explanation is that a superestuarine circulation system developed across the LPMS during humid interglacial phases, which promoted strong water column stratification and benthic anoxia.
The goal of this research was to test the validity of the superestuarine estuarine model and its key features. Specifically, whether preconditioned (i.e. oxygen-poor and intermittently denitrified) waters from the eastern tropical Panthalassic Ocean (etPan) laterally advected into epicontinental settings in tandem with large-scale, quasi-estuarine circulation and pycnocline formation, resulting in stronger reducing conditions in locations proximal to the paleoshoreline. Samples were collected from one Late Pennsylvanian cyclothemic core shale, the Hushpuckney Shale Member of the Swope Formation, and its lateral equivalents across five locations. This research utilized δ15N values, elemental, and TOC-S concentrations as spatio-temporal proxies for evaluating redox gradients and paleocirculation patterns across the study sites.
Analysis revealed that samples from Eastern Shelf of Texas are organic-poor, maintain background δ15N values, and express no enrichments in paleoredox proxies. In contrast, samples from the Midcontinent Shelf and Illinois Basin of the LPMS show an increasing gradient in TOC, δ15N, U, and Mo approaching the paleoshoreline. These results support the hypothesis that superestuarine circulation produced stronger reducing conditions in shallow, proximal locations of the LPMS. However, this model does not appear to be applicable for core shale deposits on the Eastern Shelf of the Permian Basin Seaway, where sediment dilution and depth-stratification may have influenced organic preservation and redox conditions. The data also does not support the idea that preconditioned seawater advected from the etPan to epicontinental environments during core shale deposition. Instead, it appears that widespread benthic anoxia across the LPMS was a function of strong water column stratification associated with continental runoff, combined with large settling fluxes of both terrestrial and marine organic detritus.
Mathis, Bryce A., "Investigating Core Shale Depositional Environments of Late Pennsylvanian Cyclothems Utilizing Geochemical Proxies to Test the Superestuarine Model" (2017). LSU Master's Theses. 4309.