Do Pennsylvanian Cyclothems Record Glacioeustacy?
Dyer, Blake C.; Maloof, Adam C.
During the Carboniferous and Early Permian, the super continent Gondwana was covered with dynamic ice sheets as it was positioned over the southern pole. The physical and chemical properties of tropical carbonates from this time period provide information about climate variability and sea level change. In the Paradox Basin of the western United States, shallow water carbonates are arranged in shallowing-upward meter scale parasequences that have been interpreted to reflect periodic orbitally forced sea level change. Here we present high resolution coupled physical and chemical stratigraphy of Hermosa Group (Pennsylvanian) 'cyclothems' from two sedimentary sections and 12 cores across Utah, Colorado, and New Mexico. Oxygen isotopes from conodont bioapatite provide partial constraints on the amount of ice volume change associated with individual parasequences, and carbon isotopes from carbonate provide diagenetic clues to understanding the nature of exposure surfaces at parasequence tops. Some exposure surfaces are marked by decreasing carbon isotopic values towards the parasequence tops, followed immediately (less than 10cm) above by a reset to heavier isotopes. This isotopic variation could represent secular changes in the burial of organic matter or seawater isotopic composition. In this interpretation, the sudden return to heavy values marks a time gap in the sedimentary record. On the other hand, these meter-scale negative excursions could be generated by meteoric diagenesis associated with the syn-glacial exposure of the carbonate platform. The scale of these excursions would imply that, if diagenetic, the amplitude of sea level change associated with the meter scale parasequences very small. In contrast, a singular and extensive karst surface exists near the Mississippian-Pennsylvanian boundary in Nevada, Utah, Wyoming, and Colorado. This exposure horizon is characterized by solution collapse breccia and a negative excursion in carbon isotopes that matches the Pliestocene record of the Bahamas in length scale and magnitude of isotopic change. We present data from three sedimentary sections and two cores through this regional exposure surface, and conclude that unlike the Paradox Basin meter scale parasequences, this interval represents a large drawdown in sea level, perhaps associated with the a large and sustained growth of ice sheets in Gondwana.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013