Devonian System
Type section and use of name: The name Blocher Formation was first used by Campbell (1946, p. 840) for the brownish-black, slightly calcareous shale in the basal part of the New Albany Shale in the outcrop area in southeastern Indiana. At some localities beds of sandstone were included in the top of the Blocker. The unit was redefined by Lineback (1968, 1970) to include the section up to the base of the Spathiocaris Zone of Campbell and was reduced in rank to member status. The established type section is near Blocher in the SE¼SW¼SW¼ sec. 9, T. 3 N., R. 8 E., Jefferson County (Campbell, 1946; Lineback, 1968,1970) mapping of the Blocher in the subsurface of Indiana indicates that the member more closely corresponds to the Blocher delineated by Campbell in the crop area in southeastern Indiana (Hasenmueller and Bassett, 1981).
Description: The Blocher Member of the New Albany Shale is a brownish-black to grayish- black, slightly calcareous fissile pyritic shale, which is light gray and has brown and yellow stains on weathered surfaces. Beds of dolomite, dolomitic sandstone, and gray shale are present in places, and a brownish-black calcareous mudstone is present in the basal part of the member in central Indiana. In surface exposures the Blocher Member paraconformably overlies the North Vernon Limestone (Middle Devonian) and ranges from 3 to 15 feet (0.9 to 4.6 m) in thickness. It is overlain by the Selmier Member of the New Albany Shale. The Blocher is thinnest at Lexington, Scott County (Lineback, 1970), and reaches its maximum thickness of 67 feet (20 m) in Posey County (Hasenmueller and Bassett, 1981). It is widely recognizable in the southeastern Indiana outcrop area and in the subsurface.
Correlation: The Blocher Member of Indiana is correlative with the Blocher Shale mapped by North (1969, fig. 20) in Illinois, the Blocher Shale and the lower part of the Selmier Shale mapped by Cluff and others (1981) in Illinois, and the Blocher Member of the New Albany Shale mapped by Schwalb and Norris (1980a) in western Kentucky (Hasenmueller and Bassett, 1981). In Illinois evidence from geophysical logs and sample studies indicates that the lower part of the Blocher grades laterally into the upper part of the Lingle Formation in a westerly direction (North, 1969). The Blocher Member is equivalent to the lower part of the Antrim Shale of the Michigan Basin and to the lower part of the Dowelltown Member of the Chattanooga Shale of Tennessee. Conodonts from the Blocher are indicative of the doI division of the German Devonian standard. (See Lineback, 1970; Hasenmueller and Bassett, 1981.)
