NANNOFOSSILS AND ENVIRONMENT OF THE UPPER UNIT OF THE CROW CREEK MEMBER, PIERRE SHALE (UPPER CRETACEOUS), CROW CREEK SIOUX INDIAN RESERVATION, CENTRAL SOUTH DAKOTA

The Crow Creek Member of central South Dakota has received notoriety as having perhaps been caused by a tsunami associated with a meteor impact in the adjacent state of Iowa. This event, known as the Manson Impact, has been cited as the source of this light-colored calcareous unit interbedded within otherwise thick members of black shale. Interestingly, however, two light-colored units, informally termed the lower and upper Crow Creek units, respectively, occur in the Big Bend area of Crow Creek Indian Reservation near the type area. The lowermost unit is that normally considered the Crow Creek Member by most authors elsewhere where only a single unit occurs and is characterized by a coarse, ferruginous basal unit overlain by calcareous yellow-brown marl. The upper non-calcareous marl is separated from the lower Crow Creek by a thick interval of bentonitic shale identical to that of the DeGrey Member. Samples were taken at 20 cm superposed intervals through both Crow Creek units, and nannofossils were found in both units but are particularly abundant and definitive in the upper Crow Creek unit. Stratified nannofossils that elsewhere have been found superposed over great intervals of time suggest that the upper unit was not deposited during a single depositional event. The nannofossils indicate a Campanian age of the upper Crow Creek, and the occurrence of Braarudosphaera bigelowi from the upper part of the unit would normally suggest shallow water, probably associated with a marginal marine setting or restricted marine environment. Overall, two yellow-brown marl units exist in the Big Bend area of central South Dakota, rather than the single unit found elsewhere. The upper unit is not calcareous, was deposited in shallow marine waters, and superposed assemblages argue against a single-event origin.