--> --> The Upper Cretaceous (Turonian) Carlile Shale of eastern South Dakota and adjacent areas with correlations to the Black Hills of South Dakota and Wyoming

AAPG Rocky Mountain Section Meeting

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The Upper Cretaceous (Turonian) Carlile Shale of eastern South Dakota and adjacent areas with correlations to the Black Hills of South Dakota and Wyoming

Abstract

The Late Cretaceous Carlile Shale (middle and upper Turonian) and its equivalents range from the Rocky Mountains to the Black Hills and eastward into Kansas, Minnesota, Iowa and northeast Nebraska and give a glimpse of the extent of the eastern margins of the Carlile Seaway. In eastern South Dakota, the Carlile Shale is exposed in at least two granite quarries near Milbank in Grant County, along Firesteel Creek near Mitchell in Davison County and at several small exposures near Yankton. These exposures preserve a few ammonites along with an abundant and diverse shark fauna. To date, Merewether and Cobban (1981, 1983) have been the only geologists and paleontologists to study and describe the Turonian cephalopod fauna and lithology from the eastern portion of the Western Interior Seaway. Only a limited amount of ammonite material has been found along the eastern margin so it has been difficult to correlate much of the eastern portion of the seaway with that of the west. The only way to accurately date the limited exposures and isolated occurrences from the eastern margin of the seaway is to identify the ammonites and compare them with those found around the Black Hills. Because Carlile Shale exposures on the flanks of the Black Hills comprise a nearly complete succession of Turonian aged rocks within the Western Interior, they constitute the best opportunity for comparison and accurate dating. Herein we describe the ammonite fauna from exposures in the eastern South Dakota and adjacent areas and compare them with those from the Black Hills to determine the biostratigraphic ammonite zones and ages present in the eastern margin of the Carlile Seaway.