INVESTIGATING ANOXIC DEPOSITIONAL SETTINGS OF LATE PENNSYLVANIAN CYCLOTHEMS UTILIZING GEOCHEMICAL PROXY ANALYSIS OF THE HUSHPUCKNEY SHALE

Previous analyses of cyclothem core shales in the Late Pennsylvanian Midcontinent Sea (LPMS) have identified some key issues with earlier depositional models that require revision. Firstly, black shale strata overlie coal bed deposits within Illinois-type cyclothems creating a conundrum of having what is considered deep, offshore sediments superimposed on terrestrial sediments. Secondly, no modern analogs of anoxic facies deposition can be adequately applied to the paleoceanographic conditions of the LPMS. This study tests the idea that anoxic, black shale deposition was facilitated by “superestuarine circulation” of preconditioned (O₂-poor, intermittently denitrified) bottom-waters advecting across the LPMS beneath a strong pycnocline during times of sea level highstand. If correct, there should be observable spatial and temporal shifts in trace element concentrations, isotopic abundance, and sediment provenance.

Samples of the Hushpuckney Member Shale of the Swope Formation (Kansas City Group, Missourian Stage) are analyzed across a north-northeast transect of the North American midcontinent. Measurements of TE concentrations, δ¹⁵N and δ¹³C abundance, and TOC-TIC-S concentrations will be used to track nutrient availability, determine the onset of hypoxia, and speculate sediment provenances and paleo-circulation patterns. Preliminary δ¹⁵N results from Texas outcrop samples exhibit normal oceanic values around +4‰, whereas, samples from Kansas and Illinois show a sharp δ¹⁵N excursion to +14 to +15‰ in conjunction with the onset of anoxia, before gradually returning to background values of +4‰. In the Illinois sample, this sharp rise in δ¹⁵N occurs immediately after the transition from coal to black shale. The relaxation phase of each isotopic excursion was completed before reaching the maximum flooding surface.

These preliminary results suggest that the preconditioned bottomwaters advecting from Panthalassa into Texas were not as strongly anoxic or denitrified as those found in Kansas and Illinois. This is likely due to differences in pycnoclinal strength, and it also suggests that upwelling in the Anadarko and Arkoma Basins contributed to further enhancing bottomwater anoxia and denitrification before circulating around the LPMS during times of marine transgression.