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The Upper Silurian Tonoloway Limestone at Pinto, Maryland, is divided into three informal members on the basis of field, paleontologic, and petrographic studies. The lower member is characterized by thin bedding, stromatolites, gypsum molds, 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. 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 outcrop from Pinto, Maryland, to Greenbrier County, West Virginia.

Under the eastern Appalachian Plateau, 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 is the Salina Formation, which consists of light to dark-gray dolomite and anhydrite with minor green and gray shale. Several salt beds generally are developed. Most of these are in the Salina F unit (equivalent to the upper Tonoloway member), but in Marshall County, West Virginia, salts as low in the section as the D evaporite (lower Tonoloway) are also well developed. Total salt thickness exceeds 200 ft (61 m) in several wells; however, excessive thicknesses may be due to salt flowage in anticlines.

The upper and lower members of the Tonoloway in the outcrop belt were deposited on inter-tidal-supratidal mud flats. In these two members beneath the eastern Plateau, salinity apparently increased toward the depocenter, and halite precipitated within the deeper central region of the evaporite basin. The evaporite basin was closed, with periodic influxes of normal sea water. The middle member of the Tonoloway was deposited in environments where water depth fluctuated from intertidal to shallow subtidal to deeper subtidal. A widespread transgression led to the more normal marine conditions during deposition of the middle member. Effects of this transgression can be traced into the basin, where the calcareous middle member supplanted evaporites.

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