Paleoclimate and geochemical variation of the Stark Shale Member, Dennis Formation (Missourian), Mid-continent North America



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The Upper Pennsylvanian Stark Shale is the core shale of the Dennis cyclothem. Bottom-water oxygenation is an important control on the preservation and quality of sedimentary organic matter and may influence the enrichments of trace elements (TEs). Detailed sample intervals were collected from four cores of the Stark Shale from Nemaha Uplift in Kansas (uplift cores) to the Forest City Basin in Missouri (basin cores). The samples were analyzed for major and trace elements, organic carbon and clay mineralogy. X-ray diffraction showed that illite, quartz and pyrite are the predominant minerals. Detrital elements (Si, Al, Zr and Ti) showed higher abundances of clay in the basin compared to the uplift. The weathering index and chemical index of alteration both indicate that source minerals of the shale were highly weathered. The basin core showed a more mature source compared to the uplift cores.

Core shales were deposited during maximum transgression and may have high or low TOC depending on bottom-water oxygen levels. In high TOC cores (TOC > 10 %), abundances of V, Zn, Cr, Ni and Mo showed a moderate to strong correlation with TOC in the basin cores. The Heinen core (uplift) showed no significant correlation with the above elements at high TOCs. Low TOC shale (Emery core) showed no significant correlation with all trace elements (TEs). TEs enrichments were classified into TEs of euxinic affinity (V, Mo) and TEs associated with organic matter (Cu, Ni, Zn). Covariation of TEs of strong euxinic affinity with TOC were observed to be stronger in the basin core. Redox geochemical ratios V/Cr, TEs, V/ (V+Ni) and V-Mo covariation indicate euxinic conditions in the basin. Anoxic ¡V oxic conditions seem dominant in the uplift cores. Rock eval data showed a mixture of type I and II kerogen indicating terrestrial and marine organic matter.

These data suggest that the controlling factors in the distribution of elements regionally and stratigraphically within the Stark Shale are: -Degree of weathering before deposition -Paleoredox conditions in the depositional environment
-The composition of the organic -Settling time of detrital influx -Paleoclimate and paleogeographic conditions during deposition.