STRATIGRAPHIC VARIATION IN OXYGEN AND CARBON ISOTOPE COMPOSITION WITHIN ONE GLACIO-EUSTATIC PENNSYLVANIAN CYCLOTHEM IN MIDCONTINENT NORTH AMERICA

The typical midcontinent Iola cyclothem consists of the basal transgressive Paola limestone, overlain by the offshore dark Muncie Creek Shale, in turn overlain by the regressive Raytown Limestone. We analyzed outcrop and core samples from carbonate-mud matrix of the two limestones, in a shoreward to basin transect from Iowa to Oklahoma.

In basinward sections carbon isotope 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 ‰. In the shoreward sections carbon isotope values range 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. Oxygen isotope values in the basinward sections typically range from ~ -8 to -5 ‰, except for one section, where average values are consistently ~2 ‰ heavier. In the shoreward sections, oxygen isotope values range from -6 to -3 ‰.

The carbon isotope enrichment in basinward Paola indicates that carbon sequestration was initiated during Paola deposition, and continued through Raytown deposition. Three alternative scenarios could account for the dissimilarities between basinward and shoreward signals in this cyclothem. 1) Diagenetic overprinting in the shoreward sections, causing negative carbon towards the top, and masking the positive carbon excursion preserved in the basin; 2) More basinward Paola is younger than shoreward Paola, and Muncie Creek blankets Paola; 3) Continuity of d¹³C enrichment unperturbed from Paola into Raytown suggests either that significant time did not elapse during Muncie Creek shale deposition, or that the Muncie Creek is time transgressive. Detailed analysis continues in an attempt to resolve among these alternatives.