Abstract: Age of Lakota Formation (Lower Cretaceous) in South Dakota and Wyoming as Determined by Nonmarine Ostracods

I. G. Sohn

The age of the Lakota Formation in South Dakota and Wyoming may be Barremian or older rather than early Aptian as previously considered. This age is based on the study of certain ostracods in 45 collections from the Chilson Member of the Lakota Formation, and on a revision of previously described taxa in North America.

The species present in the Lakota Formation are similar to but not identical with described forms in Eurasia; this fact suggests an age for the rocks of Barremian or older. For example, one new species is related to the Cypridea valdensis group of Wolburg recorded from the Berriasian of Germany, another new species is related to the C. alta group recorded by Wolburg from the late Purbeckian through the early Valanginian of Germany, and Cypridea (Pseudocypridina) piedmonti Roth, 1933, is related closely to Cypridea unicostata Galeeva, 1955, described from the Barremian of Mongolia.

At least two species from the Black Hills are present in a collection from Nye County, Nevada, 650 mi (1,040 km) southwest of the Black Hills. Their presence indicates that the nonmarine ostracods of Early Cretaceous age are widely distributed and can be used in age determination.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC