--> Integrating Outcrops and Cores to Differentiate Meter-Scale Cycles and Depositional Sequences in Ordovician of Kentucky During the Transition from Greenhouse to Glacial Conditions, by M. C. Pope and J. F. Read; #90986 (1994).

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Abstract: Integrating Outcrops and Cores to Differentiate Meter-Scale Cycles and Depositional Sequences in Ordovician of Kentucky During the Transition from Greenhouse to Glacial Conditions

Mike C. Pope, J. F. Read

An ongoing study using outcrops, cores, and gamma ray logs focuses on determining whether the Ordovician rock record in Kentucky preserves a recognizable record of the transition from Early Ordovician greenhouse to Late Ordovician glacial conditions.

Lower Ordovician carbonates are dominated by meter-scale dolomitized tidal flat cycles formed by small, high-frequency eustatic fluctuations possibly with some autocyclic component. The overlying Middle Ordovician sequence consists of a transgressive dolomitic sandstone on the Knox unconformity, overlain by burrowed muddy limestones with poorly defined 1-10 m thick shallowing upward subtidal cycles, passing up into peritidal highstand carbonates showing well-defined 0.5-3 m thick tidal flat cycles. These cycles were formed by small sea level oscillations.

Upper Middle to Upper Ordovician rocks contain two depositional sequences. The lowermost sequence is composed of 1-10 m thick shoaling upward subtidal cycles with nodular mid-ramp or basinal facies capped by crossbedded skeletal grainstones. The uppermost sequence consists of transgressive shale and limestone storm generated cycles, 1-5 m thick, overlain by thicker (5-15 m) cycles of deep ramp nodular carbonates capped by northward prograding tidal flat dolomites. The individual cycles within these upper two sequences show evidence for large sea level fluctuations possibly related to the transition into Late Ordovician glaciation.

The gamma ray logs show numerous kicks in the Lower Ordovician that may relate to wind blown feldspathic silt/clay in laminite cycle caps. Conversely, the abundant gamma ray kicks in the Upper Ordovician are related to the high shale content in deeper water bases of meter-scale cycles that are recording large sea level fluctuations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994