CARBON ISOTOPE CHEMOSTRATIGRAPHY OF TWO NORTH AMERICAN PENNSYLVANIAN CYCLOTHEMS: THE DESMOINESIAN PAWNEE AND MISSOURIAN IOLA

We analyzed carbonate-mud matrix of the limestones of the Desmoinesian Pawnee and Missourian Iola cyclothems, in a shoreward to basin transect from Iowa to Oklahoma. The Pawnee 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 Pawnee regressive sequence is interrupted by a lesser transgression producing the Frog Cemetery Limestone, offshore Joe Shale and regressive Laberdie/Coal City Limestone. The Iola consists of the basal transgressive Paola limestone, overlain by the offshore dark Muncie Creek Shale, in turn overlain by the regressive Raytown Limestone.

Iola basinward sections δ¹³C values start at ~0 ‰ at the base of the Paola, increase to ~2 ‰ immediately below and above the offshore shale, and continue to increase in the Raytown to values ranging from 3.5 to 5 ‰. Shoreward δ¹³C values ranges from -5 to 0 ‰ in the Paola and in the Raytown immediately above the offshore shale, and from 0 to 2 ‰ higher in the Raytown. The most shoreward Raytown ranges from ~0.4 ‰ near the base and gradually depletes to ~ -2 ‰ at the top.

Pawnee transgressive Childers School has δ¹³C values of ~ -1 to 0.8 ‰ on the shelf and ~ 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 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 ¹³C shoreward depletion and basinward enrichments indicates that relative contribution of marine dissolved inorganic carbon (DIC) was greater during lowstands while DIC from oxidized terrestrial or marine organic matter was greater during highstands. Pawnee isotopic variations are all easily explained by glacio-eustatic transgression-regressions. In the Iola trends are easily interpreted for the Raytown, while interpretation of Paola limestone trends pose difficulties. Minor isotopic excursions are preserved in many sections and provide a means of evaluating causal mechanisms for minor changes in the various DIC fluxes into the basin