TURINIAN AND CHATFIELDIAN (ORDOVICIAN) CARBON ISOTOPE EXCURSIONS AND THEIR COEVAL SUBMARINE DISCONFORMITIES ON THE GALENA CARBONATE SHELF: WHAT WAS THEIR PALEOCEANOGRAPHIC SIGNIFICANCE?

A newly-recognized Turinian positive d¹³C excursion in organic-rich carbonate strata of the Quimbys Mill Member of the Platteville Formation, and the previously-recognized Chatfieldian positive d¹³C excursion in organic-rich carbonate strata of the Guttenberg Member of the Decorah Formation, both occur in shoreward units that downlap to widespread complex hardground surfaces in more offshore areas of the Upper Mississippi Valley region. The Turinian excursion occurs below the position of the widespread Deicke K-bentonite, and downlaps to submarine disconformity DS1 of Kolata et al. (2001; GSA Bull. 113:8:1067-1078). The Chatfieldian excursion occurs above the position of the Elkport K-bentonite, and downlaps to submarine disconformity DS2 of Kolata et al. (2001). Kolata, Huff, and Bergström (ibid.) equated these surfaces to the M3-M4 and M4-M5 sequence boundaries of Holland and Patzkowsky (1997; JSR 67:250-263), and suggested that they originated from upwelling of oceanic-source waters from below a pycnocline in the Sebree trough onto the epeiric sea floor. Proposed counterclockwise circulation of these cold, nutrient-rich bottom waters accords well with the regional pattern of condensation of Turinian and Chatfieldian strata in the Upper Mississippi Valley. Given that all these processes are tied to the long-term greenhouse-icehouse transition in the Late Ordovician, do the coincident (1) positive carbon isotope excursions, (2) burial of marine organic matter in more shoreward locations, and (3) complete shut-off of carbonate accumulation in deeper epeiric sea settings all relate to oscillations in thermohaline circulation in the Iapetus Ocean? Moreover, high resolution d¹³C chemostratigraphic profiling shows that abrupt negative shifts are embedded within the positive excursions, and the origins of these events are not yet known.