THE CONODONT BIOSTRATIGRAPHY AND LITHOSTRATIGRAPHIC FRAMEWORK OF THE DEVONIAN-MISSISSIPPIAN ENGLEWOOD FORMATION, BLACK HILLS, SOUTH DAKOTA, USA

The Englewood Formation comprises two depositional sequences that contain four distinct lithostratigraphic members named here including, in ascending stratigraphic order: the Crook City, Little Crow, Griffis Canyon, and Mickelson members. The Crook City Member is a yellowish gray, silty, dolomitic shale. The Little Crow Member is a pink-red-tan, silty to argillaceous to fossiliferous dolomitic limestone. The Griffis Canyon Member is a dull yellow, stromatolitic, argillaceous, dolomitic microbolite. The Mickelson Member is a yellow, slabby, thin bedded, argillaceous, silty, fossiliferous, dolomitic, boundstone to wackestone. The Crook City, Little Crow, and Griffis Canyon members have gradational contacts with each other, possess conodonts of the Bispathodus aculeatus aculeatus Zone, and comprise the upper Famennian (Devonian) sequence of the Englewood. A regional disconformity surface lies at the base of the Mickelson Member, which yielded conodonts of the Siphonodella sandbergi Zone and comprises the lower Tournaisian (Mississippian) sequence of the Englewood. The basal beds of the Pahasapa limestone rest on a regional disconformity on the top of the Mickelson Member. The Pahasapa beds comprise gray, massive to thickly bedded, crinoidal, vuggy, dolomitic limestone that possess conodonts of the Siphonodella crenulata Zone, which is early Tournaisian, (Mississippian) in age. The Englewood Formation in the Black Hills region represents a shallow marine carbonate setting with mixtures of bioturbated siltstones, argillaceous and microbolitic limestones, fossiliferous pack-wackestones, and coral boundstones. These carbonate deposits are the age equivalents of the organic-rich black shales in the Williston Basin of North Dakota, and this work has helped improve our understanding of the local paleogeography for the region and controls on mineralogical content of source and reservoir rocks in the region.