ORDOVICIAN METER-SCALE CYCLES - IMPLICATIONS FOR CLIMATE AND EUSTATICFLUCTUATIONS IN THE CENTRAL APPALACHIANS DURING A GLOBAL GREENHOUSE, NON-GLACIAL TO GLACIAL TRANSITION

Metre-scale shallowing-upward cycles in Ordovician carbonates of the c entral Appalachian Basin record climate and eustatic fluctuations duri ng a transition from Early Ordovician global greenhouse to Late Ordovi cian icehouse conditions. Peritidal facies and shale abundance suggest a long-term trend in this area from semi-arid (Early Ordovician) to m ore humid (Middle to early Late Ordovician) conditions, with a return to semi-arid conditions during the Late Ordovician. The climatic fluct uations were most likely produced by tectonics (uplift and erosion) re lated to Taconic orogenesis, plate motion of North America and the are al extent of water covering the shelf. Peritidal cyclic facies indicat e that high-frequency relative sea level fluctuations were of small am plitude (<10 m) throughout the Early and Middle Ordovician. Late Middl e to early Late Ordovician subtidal cycles, and increased compartmenta lization of buildups, suggest higher amplitude (>20 m) relative sea-le vel fluctuations that decreased into the later Ordovician. If these se a-level changes are eustatic, then the increased amplitude may mark th e early initiation of continental glaciation on Gondwana in the late M iddle Ordovician: followed by waning of ice sheets prior to the latest Ordovician glaciation. (C) 1998 Elsevier Science B.V.