DEPOSITIONAL ENVIRONMENTS IN THE UPPER TONGUE OF THE TANGLEWOOD MEMBER, UPPER ORDOVICIAN LEXINGTON LIMESTONE, BLUEGRASS REGION, CENTRAL KENTUCKY, U.S.A

The subsurface Lexington or Trenton Limestone is typically about 50-m thick, but in central Kentucky the unit increases to more than 100-m thick as part of the structurally controlled Tanglewood carbonate buildup. Although the Lexington and Trenton generally reflect upward deepening into the Clays Ferry, Point Pleasant, and Kope formations, in central Kentucky equivalent parts of the Lexington are overlain by about 40 m of coarse calcarenites and calcirudites that form the Tanglewood buildup and reflect locally regressive conditions, apparently related to local structural uplift. Most of the coarse carbonates are included in the Tanglewood Member of Lexington Limestone, but even in the buildup, episodes of deeper water flooding are reflected in intervening shaly tongues of the Clays Ferry and Millersburg. In fact, the upper tongue of the Tanglewood is the last major body of coarse skeletal sands in the central Kentucky Lexington sequence, and it is separated from lower parts of the Tanglewood buildup by a tongue of the Clays Ferry Formation or the Millersburg member, both of which reflect a brief transgression across the buildup. Overall, the upper tongue of the Tanglewood is a 12-m thick succession of fossiliferous calcarenite and calcirudite with occasional shale interbeds. The unit reflects a localized area of regressive shoals before final inundation by regional transgression. In all directions, the unit grades into deeper water shales of the Clays Ferry, Point Pleasant and Kope formations.

Lateral and vertical facies in the unit represent depositional conditions ranging from platform lagoonal to shoal environments characterized by herring-bone cross-beds and low-angle, planar cross-beds with ripple lamination, suggesting deposition under both tide- and wave-dominated conditions. The unit is also divisible into five fining-upward cycles which contain prominent deformed horizons. Cyclicity and deformation may be related to local movements on basement structures reactivated by Taconian far-field forces and, hence, may have correlative value.