SEQUENCE AND EVENT STRATIGRAPHY ON A LATE ORDOVICIAN CARBONATE BUILDUP:PARASEQUENCES AND SEISMITES IN THE UPPER PART OF THE LEXINGTON LIMESTONE (EDENIAN), CENTRAL KENTUCKY, U.S.A

The Lexington Limestone, and Trenton equivalents in the subsurface, are typically about 50-m thick, but in central Kentucky, the Lexington is more than 100-m thick as part of the structurally controlled Tanglewood carbonate buildup. Although the Lexington reflects upward transgression into deeper-water units in Kentucky, Ohio and Indiana, in central Kentucky, the lower Lexington is overlain by about 40 m of coarse calcarenites and calcirudites that form the Tanglewood buildup and reflect locally regressive conditions, related to local structural uplift. The upper tongue of the Tanglewood Member is the uppermost part of the Lexington Limestone and of the buildup in central Kentucky; it occurs nowhere else. It consists of about 12 m of fossiliferous calcarenite and calcirudite, which is separated from lower parts of the Lexington by a thin transgressive interval of deeper-water Clays Ferry shales or nodular, shaly Millersburg limestones. Laterally and vertically, the upper tongue passes into deeper-water Clays Ferry, Point Pleasant and Kope formations in all directions from central Kentucky.

The upper tongue is divided into five, cyclic, fining-upward parasequences that were used for correlation. Facies analyses show that the thickest and coarsest part of each parasequence corresponds to specific basement fault blocks, which were involved in uplift during Lexington deposition. Occurrence of thick, coarse facies in each sequence on the same blocks suggests that the blocks continued uplift into shallow waters, where waves and tides redistributed sediments. Moreover, four of five sequences show horizons of penecontemporaneous soft-sediment deformation, correlative across the region, suggesting coeval episodes of deformation that may correspond to seismic events accompanying fault-block uplift. In this regard, the upper tongue sequence may be unique in that it provides a means for relating possible seismic events to episodes of fault-block uplift and stratigraphic thickening.