LITHOSTRATIGRAPHIC FRAMEWORK OF THE TANGLEWOOD BUILDUP, LEXINGTON LIMESTONE (UPPER ORDOVICIAN) OF THE BLUE GRASS REGION, CENTRAL KENTUCKY

The Lexington Limestone of central Kentucky is a complex facies mosaic, the origin of which is poorly understood. Across most of central Kentucky, the Lexington Limestone forms a thin relatively uniform transgressive sequence into deeper water shales and fine-grained limestones. The lower part of the Lexington Limestone represents this transgressive sequence, and except for the Lexington-Frankfort area, grades upward into interbedded shales and micrograined limestones of the Clays Ferry, Kope, and equivalent units. In the Lexington-Frankfort area the Lexington Limestone includes up to 200 ft of additional limestones in a roughly triangular body that intertongue laterally with Clays Ferry and Kope shales and micrograined limestones. The triangular body seems to be outlined and controlled by local fault zones and is composed largely of shallow-water, high-energy calcarenites and calcirudites of the Tanglewood Member and interbedded, nodular limestones and shale of the Millersburg Member. Section lines show that the triangular body of Tanglewood is composed of three separate calcarenite tongues separated by nodular limestones and shales of the Millersburg Member. The lower tongue of the Tanglewood is mostly associated with the underlying, transgressive, lower parts of the Lexington Limestone. The middle tongue is separated from the lower part by a major break, the sub-Sulphur Well unconformity, and in many places is capped by a prominent beach sequence, the Devils Hollow Member. The Devils Hollow was rapidly inundated by transgressive shales and limestones of the Millersburg and Clays Ferry and was succeeded by the final upper tongue of the Tanglewood Member. Overlying deeper water shales and micrograined limestones of the Clays Ferry and Kope formations finally ended shallow-water carbonate deposition on the triangular body. The triangular nature of this large body and its stratigraphic relationships suggests that the upper part of the Lexington Limestone was a structurally controlled carbonate buildup and that each tongue of the Tanglewood may reflect localized structural movement in the area. Widespread unconformity development and the subsequent regional nature of the buildup suggest regional control through structural reactivation due to Taconian far-field forces and how distal causes may control local lithostratigraphic framework.