ASYMMETRY OF LATE PENNSYLVANIAN GLACIO-EUSTATIC FLUCTUATIONS IN THE NORTH AMERICAN MIDCONTINENT

The growth and decay of continental icesheets are highly asymmetric processes. During the Pleistocene, growth proceeded slowly (for up to 100 kyr) as ice mass built up over large areas, whereas decay proceeded quickly (~8-12 kyr) in response to global warming and icesheet disintegration. Similar patterns have been inferred for icesheets during pre-Pleistocene ice ages, but the evidence for differential rates of growth and decay is mainly circumstantial, e.g., the greater thickness of regressive versus transgressive facies in Upper Pennsylvanian cyclothems of the North American Midcontinent region. In this study, we quantified relative rates of eustatic regression (= icesheet growth) and transgression (= icesheet decay) in Midcontinent cyclothems on the basis of fine-scale (5-10-cm) cyclicity. Although the exact origin of this cyclicity is not known with certainty, it is unquestionably (quasi-)periodic and record the ~21-kyr orbital precession cycle. At any given locale on the Midcontinent Shelf, the onset and termination of black shale deposition marked, respectively, the transgression and regression of the chemocline. Relative rates of chemocline transgression and regression can be assessed based on the number of small-cycles below and above the maximum flooding surface of each cyclothem. We analyzed two Upper Pennsylvanian cyclothemic black shales, the Hushpuckney and Heebner, along transects from SW Iowa to central Oklahoma in order to reconstruct lateral changes in transgression-regression patterns. Although the number of cycles with each study unit varies somewhat as a function of position on the shelf, the transgressive and regressive portions of the black shale facies typically contain ~4±1 and ~8±2 cycles, respectively. On this basis, we infer that transgressions proceeded roughly twice as quickly as regressions. It should be noted that our conclusions are limited to only the late transgressive and early regressive stages of glacio-eustatic cycles recorded by the black shale facies of cyclothems, and that rates of glacio-eustatic change during peak intervals of transgression and regression may have been different. High-resolution stratigraphic analysis of cyclothems, as undertaken in this study, has the potential to provide insights into contemporaneous icesheet dynamics.