Oxygen-carbon isotope stratigraphy of upper Carboniferous to lower Permian marine deposits in midcontinent USA (Kansas and NE Oklahoma): Implications for sea water chemistry and depositional cyclicity
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abstract
Late Pennsylvanian (upper Gzhelian = Virgilian) to early Permian (lower Artinskian = upper Wolfcampian) sea water chemistry and depositional cyclicity are evaluated based on oxygen and carbon isotopic compositions of unaltered shells of the brachiopod Composita collected from shallow-water deposits in the Council Grove and Chase Groups in Kansas and Oklahoma. Mean MgCO3 content in these samples is 0.2 0.2 mole%, and mean 18OPDB and 13CPDB values are -2.1 0.6 and 4.0 0.8, respectively. There is no significant difference in mean percentage MgCO3 or isotopic compositions between primary prismatic and secondary fibrous shell layers, 18O trends do not appear to coincide with inferred sea water temperature changes attending deposition of stratigraphic sequences as has been postulated for older midcontinent Pennsylvanian cyclic deposits. Rather, the data seemingly reflect variations in sea water 18O values coincident with ice-volume fluctuations during the time period examined. This contention is in accordance with inferred glacio-eustatic forcing of depositional cyclicity in the section. Estimated amplitudes of sea-level fluctuations increase from the upper Gzhelian into the lower Artinskian from 30-100 m to 110-150 m, which concurs with maximum Gondwanan glaciation during the early Permian. Although there is not a significant long-term trend in mean 13C values, there is an upward trend to less negative 18O values that is interpreted as a signal of long-term increase in midcontinent aridity and salinity.
