Lithostratigraphy of the Silurian rocks exposed on the west side of the Cincinnati Arch in Kentucky

This report discusses the Silurian rocks that crop out in Kentucky west of the axis of the Cincinnati arch. The Silurian System has a maximum thickness in this outcrop area of about 190 feet. It is in most places separated from the underlying Ordovician System by a minor erosional unconformity. It is separated from the overlying Devonian System by a major erosional unconformity along which the entire Silurian section has been removed in some areas. The Silurian is divided into five formations, which are, from lowest to highest: (1) Brassfield Dolomite (or Formation), composed of dolomite in the south and both limestone and dolomite in the north; (2) Osgood Formation, shale and minor dolomite; (3) Laurel Dolomite, dolomite; (4) Waldron Shale, shale and minor dolomite; and (5) Louisville Limestone, calcitic dolomite and dolomitic limestone. The Brassfield is of Early Silurian age; the other formations are Middle Silurian.

The main outcrop area of the rocks considered in this re- port extends about 85 miles south from the Ohio River and has a maximum width of about 20 miles. The rocks in this belt generally dip westward about 30 feet per mile. Part of this dip was imparted to the rocks prior to the cutting of the overlying unconformity, but most of it came later. Minor tectonic movement along west-northwest and east-northeast trends probably took place during Silurian time. Thickening of the Brassfield and Osgood southward across the westnorthwest-trending Bardstown monocline and thickening of the Brassfield in the axial zone of the east-northeast-trending Lyndon syncline indicate that movement took place along these structures during or immediately before the deposition of these formations.

Several remarkably persistent thin units of dolomite and shale that make up the Osgood, the Laurel, and the Waldron indicate a depositional strike about north. The dolomite was originally mostly fossiliferous limestone deposited in shallow marginal seas. The source of the shale was to the east.