R. A. Smosna, S. M. Warshauer, D. G. Patchen, W. J. Perry
The Upper Silurian Tonoloway Limestone at Pinto, Maryland, is divided into 3 informal members on the basis of field, paleontologic, and petrographic studies. The lower member is characterized by thin bedding, stromatolites, gypsum pseudomorphs, intraclasts, and mud cracks. The rocks typically are laminated micrite and pelmicrite. In the field and in thin section, this member is similar to the upper one. The faunal diversity is extremely low in both. The middle member, composed of biopelsparite and biomicrite, is more fossiliferous than either of the other members and shows an extreme variability in the development of communities. This member can be traced along the Silurian outcrop belt from Pinto, Maryland, to Pendleton County, West Virginia. Under the eastern Plateau in West Virginia, silty, argillaceous beds in the Keyser Formation above the Tonoloway and in the upper Wills Creek Formation below provide useful correlation zones. There dolomite and anhydrite occur in the upper and lower parts of the Tonoloway, whereas the middle member is chiefly limestone. Farther west, the subsurface equivalent of the Tonoloway of the eastern outcrop is the Salina Formation which consists of light- to dark-gray dolomite and anhydrite with minor green and gray shale. Several salt beds commonly are developed. Most of these are equivalent to the F evaporite of Ohio, but in Marshall County, West Virginia, salts as low as the D evaporite also are well developed. Total salt thickness exceeds 250 ft in several wells; however, excessive thicknesses may be due to tectonic thic ening within anticlines.
The upper and lower members of the Tonoloway in the outcrop areas were deposited on intertidal flats. The middle member was deposited in environments where water depths fluctuated from intertidal to shallow subtidal to deeper subtidal. A parallel sequence of environments is represented under the eastern Plateau where salinities apparently increased toward the depocenter of the basin. In general, a widespread transgression was responsible for the more normal marine conditions in the middle member.
AAPG Search and Discovery Article #90978©1975 GCAGS-GC Section SEPM Annual Meeting, Jackson, Mississippi