Physical Stratigraphy of the Post-Beekmantown-Pre-Liberty Hall Limestones, Central Rockbridge County, Virginia

Abstract

In central Rockbridge County, Virginia, lower Middle Ordovician limestones crop out in three northeast striking belts. Structurally, the area of study is a thrust plate bounded by the Staunton-Pulaski and Little North Mountain faults. The Middle Ordovician limestones are separated from the underlying Lower Ordovician Beek1nantown Dolomite by a major unconformity. The Blackford Formation and the New Market Limestone vary greatly in thickness because they were deposited on the post-Beekmantown erosional surface. The pre-Whistle Creek disconformity separates the Beekmantown and New Market from the overlying Lincolnshire Limestone, divided into the Whistle Creek Member, the Murat facies, and the Rockbridge facies. The New Market and Whistle Creek contain platy chert; chert nodules are found in the Rockbridge facies. The light-colored Murat facies is composed of coarse-grained detrital limestone. The Whistle Creek Member, Rockbridge facies, and Botetourt Limestone contain finer-grained detrital limestone mixed with a minor amount of terrigenous material. After the deposition of the rusty-weathering Botetourt Limestone, an increase in terrigenous material and a decrease in grain size resulted in the Liberty Hall facies of the Edinburg Formation.