Alexandrian and Niagaran Series

Silurian System

Type section and use of name in Indiana: The Brassfield Limestone was named by Foerste (1906, p. 18, 27) for exposures along the now-abandoned Louisville and Atlantic Railroad between Brassfield and Panola in Madison County, east-central Kentucky. The Brassfield of Indiana was first called the Clinton Limestone (Foerste, 1896, 1897), a now-abandoned term because the Brassfield of Indiana is continuous with the type Brassfield in east-central Kentucky. In southeastern Indiana and adjacent Kentucky the Lee Creek Member is now recognized at the top of the formation (Nicoll and Rexroad, 1968, p. 8-10). The Belfast Bed of Foerste (1896, p. 163-164), named for strata in Ohio on the eastern flank of the Cincinnati Arch, has been tentatively included as a basal part of the Brassfield at one locality in Indiana (Rexroad, 1967; Burger, 1970, p. 12) but is here considered to be absent from Indiana, possibly because of either nondeposition or facies change.

Description: In Indiana the Brassfield Limestone is generally a medium- to coarse-grained fossiliferous limestone having numerous irregular blebs and stringers of shale scattered throughout and in many places containing Ordovician pebbles in the lower part. Small amounts of fine-grained dolomite are present in most sections, and the Lee Creek Member is also a dolomite. Color is variable yellowish brown to salmon pink is common, but near Richmond the basal part is nearly white and the upper part is dark gray and contains scattered yellow grains.

In Indiana exposures of the Brassfield are found near Richmond and near Connersville and thence southwestward to the Ohio River near Charlestown. The formation is generally less than 4 feet (1.2 m) thick along the outcrop belt, but its maximum thickness on outcrop is 14 feet (4.3 m) and in the subsurface is 20 feet (6 m). The Brassfield is absent from parts of Decatur, Ripley, Jennings, and Jefferson Counties, so that the Osgood Member of the Salamonie Dolomite lies directly on rocks of Ordovician age (Rexroad, 1967).

Northward in the subsurface, in about northern Randolph County and Delaware and Madison Counties, the Brassfield is in a facies relationship with the Manitoulin Dolomite and Cabot Head Members of the Cataract Formation (Rexroad, 1980). To the west the Brassfield merges with the Sexton Creek Limestone that underlies approximately the western two-thirds of Indiana (Rexroad and Droste, 1982). The Brassfield unconformably overlies the Whitewater Formation of Ordovician age and in turn is unconformably overlain by the Osgood Member of the Salamonie Dolomite or by the undifferentiated Salamonie.

Correlation: The Brassfield Formation (Rexroad and others, 1965) of the Cincinnati Arch area is time transgressive, the Brassfield Limestone of Indiana being younger than the type Brass field. In Indiana Brassfield conodonts below the Lee Creek belong in the Distomodus kentuckyensis Assemblage Zone of middle and later Llandoverian age the Lee Creek Member is C5 in age in reference to the standard donation of the Llandoverian Series of Great Britain (Nicoll and Rexroad, 1968). In some localities evidence is lacking of an unconformity between the Lee Creek and the underlying part of the Brassfield, which suggests that part of the Brassfield proper is as young as the C4 interval in the upper part of the type Llandoverian. The Indiana Brassfield correlates with the Noland Formation of east-central Kentucky, and the main part of the formation also correlates approximately with the Elwood and Kankakee Formations of northeastern Illinois. It is continuous with the Sexton Creek Limestone of western Indiana, Illinois, and Missouri and with the Manitoulin and Cabot Head rocks of the Cataract Group (Formation in Indiana) of the Michigan Basin.