Lithostratigraphy of the western Blue Ridge cover rocks in Maryland
1992, Brezinski, D.K.
Report of Investigations 55
Lower Cambrian Blue Ridge cover rocks of Maryland have traditionally been considered to be in stratigraphic continuation with rocks of the Blue Ridge core. Geologic field mapping at scales of 1 :24,000 and larger, as well as detailed stratigraphic studies of the cover rocks, allows subdivision into members of several formations in the Chilhowee Group and overlying carbonate rocks.
Three members are named and described within the Weverton Formation. These Members, from base to top, are: Buzzard Knob, medium-bedded, light-gray, moderately well-sorted quartzite, which is the major ridge-forining unit of the Maryland Blue Ridge; Maryland Heights Member, the medial interval consisting of interbedded, gray quartzite, graywacke, and olive siltstone; and the Owens Creek, consisting of medium- to dark-gray quartzite, conglomerate, and graywacke.
The overlying Harpers Formation is not formally divided, yet numerous ferruginous sandstone units may be locally traced. In northern Washington and Frederick Counties numerous coarse-grained sandstone and quartzite tongues are present in the lower and middle parts of the Harpers. These sandstone and quartzite tongues are interpreted to extend from the much thicker Montalto Member of the Harpers Formation of Pennsylvania. Overlying the Harpers Formation is white to very light-gray, medium-bedded, blanket sandstone known as the Antietam Formation. The Antietam Formation is gradational with the underlying Harpers Formation and grades upward into a well-sorted, Skolithos-burrowed, quartzarenite at the top. Above the Antietam Formation is the lowest carbonate unit of the Maryland Great Valley, the Tomstown Formation. Four new members are identified and named within Tomstown Formation. The lowest member, named Bolivar Heights, consists of basal, tan, sandy dolomite, and laminated marble, informally called the Keedysville marble, with the remainder composed of burrowed, dark-gray limestone. Overlying the Bolivar Heights Member is a sequence of thick-bedded, dark-gray, burrowmottled dolomite, herein named the Fort Duncan Member. Massive, very light-gray, saccharoidal dolomite, named the Benevola Member, overlies the Fort Duncan Member. The uppermost part of the Tomstown consists of interbedded, darkgray, bioturbated and algal laminated dolomite and limestone and is named the Dargan Member.
Stratigraphically overlying the Tomstown Formation is the Waynesboro Formation. Three members are recognized and named. The basal member, named Red Run, consists of interbedded, tan, fine-grained sandstone, olive shale, and tan, shaly dolomite. The middle member, named Cavetown, is composed of thi ck-bedded , bioturbated dolomite, laminated limestone, and a few thin shale and sandstone layers. The uppermost member of the Waynesboro, named Chewsville, consists of interbedded, red-brown shale and siltstone, light-gray and brown sandstone, and tan, sandy dolomite. Overlying the Waynesboro Formation is a thick sequence of interbedded, gray limestone, tan dolomite and olive, dolomitic shale, known as the Elbrook Formation. The Elbrook has not been subdivided.
Utilizing the above-described stratigraphic nomenclature, several discontinuities, attributable to faulting, are recognized . The Chilhowee units of South Mountain are disrupted by the High Rock , the Black Rock , and the Monument Knob faults. Along the western base of South Mountain, discontinuities suggest the existence of a major fault system, called South Mountain fault. Truncation of the members of the Waynesboro Formation di stinguish the Beaver Creek fault, which has been subsequently folded.