Ostracod Evidence for a Berriasian to Valanginian Maximum Age of Some Nonmarine Formations In the U.S. Western Interior Foreland Basin—Early Cretaceous Hiatus Likely to Be Shorter Than Previously Reported

The taxonomic revision of nonmarine Lower Cretaceous ostracods (microcrustaceans with a calcified shell) of the Cedar Mountain (San Rafael Swell, Utah) and Lakota (Black Hills, South Dakota) formations led to their successful application to biostratigraphy and an improvement of the age determination of these units. It can be demonstrated that these ostracods are not as endemic as formerly presumed. Therefore, based on intercontinental correlations of representatives of the genera Theriosynoecum and Cypridea, two important ostracod index taxa in Lower Cretaceous nonmarine sediments worldwide, many North American taxa can be tied to well dated contemporary European strata ('Purbeck/Wealden' of England, UK, NW Germany, and Spain). Therefore, a Berriasian to lowermost Valanginian maximum age of the lower part of both the Cedar Mountain and Lakota formations is most probable. Regarding the Cedar Mountain Formation, the results are also supported by palynomorphs. This new evidence leads to the conclusion that the Upper Jurassic-Early Cretaceous hiatus in the Western Interior foreland basin is much likely to be considerably shorter than most published results (15-35 Ma, upper Tithonian to Barremian-middle Albian) have indicated, i.e., not longer than 10 Ma and perhaps below 5 Ma. Many other Lower Cretaceous formations in the Western Interior basin are likely to have a much higher maximum age as well. Furthermore, the ostracods also have the potential to improve intra- and interformational correlations and a further biostratigraphical subdivision of the particular units. An integration with other chronostratigraphic methods and an application of ostracod biostratigraphy to other Lower Cretaceous formations are hoped to further contribute to the controversial discussion about the age of these formations and to improve the nonmarine Lower Cretaceous stratigraphy in the Western Interior.