9 Navigating Ancient Midwestern Seas: The Geologic Record of Flooding and Emergence on a Large Paleozoic Platform

Monday, 23 April 2012
1:00 PM - 3:00 PM
Room 203

FINE-SCALE LITHOLOGIC VARIATIONS IN LATE ORDOVICIAN (KATIAN) PERITIDAL DEPOSITS OF THE KENTUCKY BLUEGRASS REGION SUGGEST SEA-LEVEL FLUCTUATIONS AS THE PRIMARY MECHANISM FOR TYPE CINCINNATIAN METER-SCALE CYCLES

Paper number: 9-1
Program: Theme Sessions
Time: 1:00 PM – 1:20 PM

Authors

Abstract

Late Ordovician peritidal facies of central Kentucky are laterally equivalent to cyclic subtidal facies of the Cincinnati region but correlation details, and causes of cyclicity are poorly understood. If type Cincinnatian shale-limestone (meter scale) cycles were driven by sea-level fluctuations then equivalent peritidal facies should be cyclic. Likewise, the same magnitude of base level change should result in greater environmental variability in these shallow facies. If cycles are of Milankovitch origin, it should be reflected in cycle duration.

We attempt to test these predictions by examining litho, sequence, and macro-biostratigraphic evidence at Point Leavell, KY and other localities. Exposures at Point Leavell display the upper Calloway Creek Fm. (containing Orthorhynchula), and the Tate and Gilbert Mbrs. (Ashlock Fm.); they are capped by a bedding-plane rich in Dystactospongia. These units are lateral equivalents to the Maysvillian Stage, upper Fairview Fm., Miamitown Shale, Bellevue, and lower Corryville Mbrs., respectively, based on faunal epiboles and sequence stratigraphic correlation. The Tate Mbr. at Point Leavell is a resistant greenish dolomitic, muddy siltstone about ten times thicker than its counterpart on the Cincinnati Arch. The Tate Mbr. displays cyclicity, at multiple orders, as decimeter scale variations in sedimentary fabric, and as bedding variations from finely laminated to massive intervals, and broader cycles which record changes from mudcracked and hardground bearing strata to marine fossiliferous argillaceous carbonates. Micritic strata of the Gilbert Mbr. show a deepening upward trend from a fenestral fabric to marine fossil assemblages.

This first documentation of cyclicity in Cincinnatian peritidal deposits suggests that sea-level fluctuations are the mechanism of cyclic variation in the coeval subtidal facies of the Cincinnati region. Multiple orders of cyclicity at Point Leavell may reflect Milankovitch-band orbital forcing and associated climatic effects including weathering, sedimentation, and sea-level changes. We will use high resolution magnetic susceptibility and gamma ray analysis to quantify high-frequency cycles and attempt their correlation throughout the outcrop belt and into the subsurface.