High resolution stratigraphy of Ordovician carbonates, Kentucky: Evidence for a greenhouse to glacial transition

Early Ordovician Knox Group carbonates consist of meter-scale dolomite cycles deposited on a passive margin. The meter-scale cycles were deposited under warm, semi-arid conditions, under low amplitude sea level fluctuations on a relatively ice-free earth. The overlying Knox unconformity ({dollar}sim{dollar}10 m.y. duration), formed by global eustatic sea level fall and tectonic uplift of the subducting margin at the onset of Taconic orogenesis.; Middle Ordovician carbonates were deposited on a ramp peripheral to a foredeep in Tennessee. The Middle Ordovician High Bridge Group consists of a 2nd-order supersequence with three 3rd-order sequences (each 30 to 100 m thick), which record a long term change from humid conditions in TST's into more arid conditions during HST's. Peritidal cycles formed under low-amplitude eustatic fluctuations.; The late Middle to Late Ordovician carbonates and clastics comprise a 2nd-order supersequence deposited on a ramp peripheral to the Taconic foredeep. The supersequence is composed of four 3rd-order sequences (each 40 to 80 m thick), which contain 11 parasequence sets (2 to 20 m thick). The small sequences are composed of stacked, predominantly subtidal meter-scale cycles (parasequences). Meter-scale subtidal cycles contain facies that suggest they formed under moderate amplitude (20 to 40 m) sea level fluctuations, likely produced by glacio-eustasy. However, peritidal cycles in the HST's indicate they formed under low-amplitude sea level fluctuations, thus suggesting the amplitude of sea level fluctuations decreased during 3rd-order sea level falls. The facies in the supersequence indicate they formed under cool, humid conditions during the 2nd-order TST and became more arid during the 2nd-order HST. The unconformity at the top of the supersequence formed during eustatic sea level drawdown associated with extensive Latest Ordovician glaciation.; The long term climatic shifts from semi-arid to more humid climates during the Ordovician probably reflects the uplift of orogenic highlands to the east during orogenesis, which altered the regional climatic conditions as well as plate motion to cooler latitudes. The change from humid to arid climate during the 3rd-order sequences may be due to marine inundation inducing humid conditions, whereas more arid climates during the HST's may reflect emergence of the ramp. The shift from low amplitude fluctuations in the Early Ordovician to moderate sea level fluctuations during the Late Ordovician suggests that glaciation began during this time.