High Resolution Stratigraphy of Ordovician Carbonates, Kentucky: Evidence for a Greenhouse to Glacial Transition
Pope, Michael C
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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 (- 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.
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