POPE, MIKE C., and J. FRED READ, Geology Department, Virginia Polytechnic Institute and State University, Blacksburg, VA

ABSTRACT: Ordovician Eustasy: Effects on Depositional Sequences and Carbonate Cycles

Ordovician carbonates developed during a transition from Early Ordovician global greenhouse to Late Ordovician glacial conditions. In Kentucky, Early Ordovician carbonates are dominated by meter scale dolomitized tidal flat cycles. The overlying early Middle Ordovician sequence (0-350 m thick) consists of a transgressive dolomitic sandstone on the Knox unconformity, overlain by burrowed muddy limestones with 1-10 m shallowing upward subtidal cycles, passing up into peritidal highstand carbonates with 1-3 m dolomitized tidal flat cycles capped by a regional disconformity; these cycles are possibly related to low amplitude sea level oscillations.

The late Middle to Late Ordovician carbonates consist of at least three (30-150 m thick) depositional sequences. Basinal facies are thin, evenly bedded shales, silts and limestones that may record very high frequency 1-3 k.y. humid/arid climate fluctuations. Deep ramp skeletal and pelletal packstone with thin shale interbeds contain 1-5 m thick deeper subtidal cycles. Shoal water ramp cross-bedded skeletal grainstone is localized over the Cincinnati Arch. They locally contain 1-10 m thick cycles with basal units of deep ramp carbonate; these likely formed under moderate (20 m or more) amplitude sea level oscillations, possibly related to the transition into Late Ordovician glaciation. Tidal flat, laminated to burrow mottled dolomitic mudstones or fenestral lime mudstone, occur at seve al levels near the tops of depositional sequences, pinching out to the northwest. The Ordovician package is capped by a regional tidal flat dolomite and overlying unconformity. Preliminary data suggests that although the Early Ordovician carbonates developed under low amplitude sea level fluctuations, the later Ordovician sections do not show unequivocable characteristics of high amplitude glacio-eustasy.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.