The Microfauna of the Sully Member of the Pierre

The upper Cretaceous of the Great Plains, from Kansas northward, is divided, from the base up, into the Dakota, Graneros, Greenhorn, Carlile, Niobrara, Pierre, Fox Hills, and Lance. The Pierre formation has recently been divided along Missouri River in South Dakota into five members on the basis of lithology. These subdivisions, beginning with the lowermost were named the Gregory, Sully, Virgin Creek, Mobridge and Elk Butte.1 In the type locality of the Sully member, in and opposite Sully county, South Dakota, it has been divided into three zones and these zones have been observed along Missouri River in South Dakota and also in northern Nebraska. Microfossils have been found in each member of the Pierre and some of these faunas are characteristic of the member in which they occur. The microfauna of the Sully consists of representatives of the forarninifera, radiolaria, silisispongia, echinoidea, and fishes. It is the middle zone, the Oacoma, which contains the most abundant fauna, which makes possible the recognition of the member from isolated doubtful outcrops and from sub-surface samples. The forms which occur most commonly may be listed as follows