Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley

Abstract

The Decorah Formation of eastern Iowa was deposited on an open-marine subtidal shelf near storm wave base. Maximum transgression is represented in organicrich strata of the Guttenberg Member. Brachiopod-derived marine carbonate isotopic compositions from the Decorah are approximately δ13C = +1‰ and δ18O = –3.5‰; these data are consistent with a recently recognized long-term secular increase in the δ18O of Middle Ordovician marine carbonates. Decorah carbonates lithified in early diagenetic modified-marine phreatic environments. Isotopic data from diagenetic components show that the Spechts Ferry Member lithified in a more fluid-dominated diagenetic system than the immediately overlying Guttenberg Member, which was characterized by a rock-dominated system. Whole-rock carbonate δ13C shifts in the Decorah Formation are carried by micritic components. A positive shift in the Guttenberg to micrites with δ13C values up to +2.5‰ indicates that some of the micrite isotopic signal is primary and not of benthic origin and/or that early diagenetic marine phreatic fluids in the organic-rich Guttenberg were affected by bacterial methanogenesis. Subtidal shallowing-upward depositional cycles in the Decorah are internally characterized by micrites with stratigraphically upward trends toward 13C depletion as a consequence of increasing diagenetic water/rock ratios. Decoupling between coeval carbon isotopic signals carried by open-marine brachiopod carbonate and those of organic carbon and micrite indicates that the positive carbon isotopic excursion in the Guttenberg resulted from an episode of increased photosynthetic productivity near the sea surface. This event was a consequence of quasiestuarine circulation associated with marine transgression during deposition of the Guttenberg Member. Results from this study suggest that the extinct organic-walled microfossil Gloeocapsomorpha prisca, the principal source of organic carbon in the Decorah Formation, was a phytoplanktic organism. Ludvigson, G. A., Jacobson, S. R., Witzke, B. J., and Gonzalez, L. A., Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley, in Witzke, B. J., Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy: Views from the North American Craton: Boulder, Colorado, Geological Society of America Special Paper 306. 67 SP306-06.QXD 6/27/96 12:39 AM Page 67