Silurian System

Type area, reference sections, and use of name: The term Osgood Beds was applied by Foerste (1896, p. 191) to the fossiliferous lower part of what was then called the Laurel Formation near Osgood in Ripley County, Ind. A type section was not designated, and the member is not now well exposed in the Osgood area. Good reference sections, however, include: (1) the New Point Stone Co. New Point Quarry 1 mile (1.6 km) north of New Point in the SW¼SW¼ sec. 8, T. 10 N., R. 11 E., Decatur County; (2) the New Point Stone Co. Napoleon Quarry on the east edge of Napoleon in the SW¼SW¼ sec. 21, T. 9 N., R. 11 E., Ripley County; and (3) the road cut on the south bank of Graham Creek half a mile (0.8 km) north of New Marion in the W2NE¼ sec. 36, T. 7 N., R. 10 E., Ripley County. The Osgood beds attained formational status when Foerste (1897, p. 217, 230) restricted the Laurel to the beds above the Osgood. The Osgood Formation was later reduced to member rank by French (1967), who assigned it to the lower part of the Salamonie Dolomite.

Description: As recognized by Foerste (1897), the Osgood consisted of lower and upper shaly units, an intervening middle carbonate unit, and in many places a basal dolomitic limestone. These lithologies are gradational, and the units have limited lateral extent in southeastern Indiana. The carbonate content of the Osgood increases to the north and the west, so that it cannot be separated from the overlying Laurel Member of the Salamonie Dolomite, and the undifferentiated equivalents of the Osgood and the Laurel are referred to the Salamonie Dolomite. Osgood thickness ranges from 10 to 30 feet (3 to 9 m) and averages about 15 feet (4.6 m). The Osgood unconformably overlies the Brassfield Limestone or rocks of Ordovician age where the Brassfield is absent it is overlain conformably by the Laurel Member.

Correlation: Conodonts of the Pterospathodus amorphognathoides-Kockelella ranuliformis Assemblage Zone are present in the lower part of the Osgood (Nicoll and Rexroad, 1968), which suggests that most of the member is of early Wenlockian age (early Niagaran in the North American standard). The Osgood is the thinning edge of a tongue of the Estill Shale of east-central Kentucky, a unit that contains the same zonal conodonts (Rexroad and Nicoll, 1972; Rexroad and Kleffner, 1984). In west-central Kentucky where the upper shaly unit thins and disappears, the boundary between the Osgood Formation and the Laurel Dolomite is dropped to the base of the limestone that in Indiana is the middle carbonate unit of the Osgood. The upper Osgood of Indiana, therefore, has a lower Laurel equivalent in west-central Kentucky. The Osgood of west-central Ohio is limited to a thin shaly interval between the Dayton Formation and the Laurel Limestone, so that the Osgood of Indiana equates with the Osgood of Ohio and with rocks above and possibly below. The Osgood correlates approximately with the Stroh Member of the Cataract Formation of northeastern Indiana (Rexroad, 1980) and approximately with the upper part of the Brandon Bridge Member of the Joliet Formation of northeastern Illinois (Liebe and Rexroad, 1977).