CARBON STABLE ISOTOPE PATTERNS IN THE PAWNEE CYCLOTHEM, MIDDLE PENNSYLVANIAN OF MIDCONTENENT NORTH AMERICA

The Pawnee cyclothem includes the transgressive Childers School Limestone overlain by the offshore dark phosphatic Anna Shale, in turn overlain by regressive Myrick Station Limestone, and clastic Mine Creek Shale. The regressive sequence is interrupted by a lesser transgression producing the Frog Cemetery Limestone, offshore Joe Shale and regressive Laberdie/Coal City Limestone. The major Lower Pawnee and intermediate Coal City cyclothems merge basinward into Oklahoma as the Joe Shale joins the top of the Anna where the Frog Cemetery Limestone disappears. We analyzed carbonate-mud matrix of the limestone members from outcrop and core samples, in a shelf to basin (north-south) transect from Iowa to Oklahoma.

All units are most depleted in the north and most enriched basinward in the south. The transgressive Childers School is depleted in ¹³C at ~ -1 to 0.8 ‰ on the shelf and enriched to ~ 3 to 4 ‰ in the south. The early regressive Myrick Station ¹³C values range from ~ -2 to 0 ‰ shoreward, and from ~ -1.5 to 1.2 ‰ basinward. The Frog Cemetery increases from ~ -1 to 0 ‰ at its north end southward to ~ 0 to 1 ‰. The regressive Coal City/Laberdie, like the other units, has its most enriched compositions basinward. However, it is characterized by a distinct upward depletion, both in the north where it ranges from ~ -1 at the base to ~ -3 ‰ at the top, and in the south where it ranges from ~ 4.5 at the base to ~ 1 ‰ at the top.

The general ¹³C enrichment trend basinward indicates that the overall relative contribution of ¹³C-enriched marine dissolved inorganic carbon (DIC) was greater basinward than the contributions of ¹³C-depleted DIC derived from oxidation of terrestrial and/or marine organic matter. The vertical depletion in the regressive units suggests that as the shoreline migrated basinward, the relative nearshore contribution of terrestrial DIC progressively increased. Importantly, minor isotopic excursions appear to be preserved and might provide a means of evaluating potential causal mechanisms for minor changes in the various DIC fluxes into the basin, such as minor sea-level oscillations or changes in riverine fluxes.