Cambrian System

Type locality: The Davis Formation was named by Buehler (1907, p. 231) and Buckley (1908, p. 33-44) for exposures along Davis Creek, St. Francois County, Mo., where it underlies what were then called the Derby and Doe Run Dolomite. The Davis was traced eastward from Missouri into central Illinois by Workman and Bell (1948) and thence into Indiana by Becker, Hreha, and Dawson (1978).

Description: In Indiana the Davis Formation consists of the gradational rocks between the Potosi Dolomite and the sequence made up of the Galesville and Ironton Sandstones and the Franconia Formation. These rocks are composed of siltstone, shale, limestone, and dolomite that are conformable with the subjacent Eau Claire Formation and superjacent Potosi Dolomite. The Davis thins away from its area of maximum thickness of more than 200 feet (61 m) in western central Indiana but is not recognized north and west of an arbitrary nomenclatural cutoff in central northern and western Indiana. It is also absent from southwestern Indiana rough lateral gradation into the Potosi Dolomite, which rests, therefore, directly and conformably on the Eau Claire Formation in at part of the state.

Correlation: The whole of the Davis Formation of Indiana correlates with the Franconia Formation, including the Davis Member, of Illinois and with the Davis Formation of eastern Missouri; with the Galesville and Franconia rocks of the Lower Peninsula of Michigan; with the Elvins Formation of western Kentucky; and with the Conasauga into lower Knox sequence as recognized by Janssens (1973). (See Droste and Shaver, 1983, and Shaver and others, 1985.) Trilobites of the Elvinia Zone recovered from a core taken from a deep well in Vermilion County, Ind., help to establish the Franconian age of part of the Davis Formation (Palmer, 1982).