Ordovician metre-scale cycles: implications for climate and eustatic fluctuations in the central Appalachians during a global greenhouse, non-glacial to glacial transition

Metre-scale shallowing-upward cycles in Ordovician carbonates of the central Appalachian Basin record climate and eustatic fluctuations during a transition from Early Ordovician global greenhouse to Late Ordovician icehouse conditions. Peritidal facies and shale abundance suggest a long-term trend in this area from semi-arid (Early Ordovician) to more humid (Middle to early Late Ordovician) conditions, with a return to semi-arid conditions during the Late Ordovician. The climatic fluctuations were most likely produced by tectonics (uplift and erosion) related to Taconic orogenesis, plate motion of North America and the areal extent of water covering the shelf. Peritidal cyclic facies indicate that high-frequency relative sea level fluctuations were of small amplitude (< 10 m) throughout the Early and Middle Ordovician. Late Middle to early Late Ordovician subtidal cycles, and increased compartmentalization of buildups, suggest higher amplitude (> 20 m) relative sea-level fluctuations that decreased into the later Ordovician. If these sea-level changes are eustatic, then the increased amplitude may mark the early initiation of continental glaciation on Gondwana in the late Middle Ordovician, followed by waning of ice sheets prior to the latest Ordovician glaciation.