CHEMO-STRATIGRAPHY OF THREE PENNSYLVANIAN MIDCONTINENTAL CYLOTHEMS: PRELIMINARY REPORT

North American Midcontinent Pennsylvanian cycles are recognizable by their distinct alternation of shales and carbonates and are produced by temporal shifts in depositional environments from non-marine to marine. Because of the distinct cyclicity, these cycles have been intensely studied. However, there have been no comparative basin wide studies of complete cyclothems, and temporal and spatial variability of the geochemistry of the Midcontinent Pennsylvanian oceans is poorly constrained. This research is aimed at understanding the effects of environmental change on the carbon cycling in three cyclothems, the Pawnee (Desmoinesian), Iola (Missourian), and Oread (Virgilian). These are representative of the Mid-Late Pennsylvanian, are among the most extensive cyclothems within the Midcontinent, and thus have the potential for capturing a major portion of a transgressive-regressive cycle. Three cores from NE Kansas were sampled at cm-scale intervals to create high-frequency organic matter (OM) and carbonate δ¹³C profiles from lower to upper cycle boundaries. These profiles are compared to ascertain if the three time slices recorded the same environmental shifts, response rates, and magnitude of change. The chemo-stratigraphic profile exhibit a cyclic hierarchy, with fifth order δ¹³C excursions (20-40 ky) superimposed upon fourth order (400 ky) oscillations. Each of the three cyclothems exhibit slightly differing isotopic profile, indicating deposition in differing environmental and sequence-stratigraphic settings, despite the apparent similarity in litho-stratigraphy. Timing, magnitude, and amplitude of change are not the same for the OM and carbonate δ¹³C. The apparent decoupling of these signals would suggest that multiple processes affect isotopic compositions and factors such as relative contribution of marine versus terrestrial OM to the sediment OM pool, differing _PCO₂, or the riverine dissolved inorganic carbon (DIC) flux was contributing to epeiric sea DIC. The high-resolution chemo-stratigraphic profiles for the Pawnee, Iola, and Oread cyclothems will serve as isotopic reference for other Pennsylvanian cycles worldwide. The technique utilized for sampling OM δ¹³C and understanding the processes controlling this isotopic signature can be directly applied to other deposits where there is sufficient OM.