Bennington, J Bret, 2002. Eustacy in cyclothems is masked by loss of marine biofacies with increasing proximity to a detrital source: An example from the Central Appalachian Basin, U.S.A. In Carboniferous and Permian of the World, L.V. Hills, C.M. Henderson, and E.W. Bamber, eds., Canadian Society of Petroleum Geologists, Memoir 19, p.12-21. (Full-text PDF 852 K)


In cyclothems of the Midcontinent basin in the central United States, eustatic sea level changes are recorded in a distinctive biofacies sequence developed through each transgressive-regressive cycle. Cyclothems of the Appalachian basin have been found to contain few marine biofacies, so this line of evidence has not been used to support glacial eustacy in the eastern basin. Recent paleontological and stratigraphic analyses of one eastern cyclothem, the Magoffin Member of the Four Corners Formation, reveal a stratigraphic sequence of biofacies analogous to that seen in midcontinent cyclothems. The complete transgressive to regressive package of biofacies in the Magoffin Member is developed over a thin interval in the northern part of the basin, indicating regression independent of sediment aggradation, a characteristic of eustatic sea level fall. Moving across the axis of the Appalachian basin toward the Alleghenian orogenic belt to the southeast, the Magoffin becomes increasingly dominated by a thick, coarsening-upward sequence, which begins progressively lower in the cyclothem approaching the detrital source. Only the lowermost biofacies, developed above the flooding surface, is present at all localities throughout the basin. Late transgressive and early regressive biofacies that otherwise would have developed through the T-R cycle were overwhelmed by the earlier arrival of prodeltaic sediments and are lost toward the southeast. This demonstrates that rapid progradation of deltas can inhibit the development of all but early transgressive biofacies, suggesting that eustatic sea level fall in marine cyclothems in the Appalachian basin may commonly be masked by thick coarsening-upward sequences of detrital sediments.