Western Interior Early Cretaceous Hiatus Likely to Be Much Shorter Than Previously Reported—New Biostratigraphic Results Derived from Nonmarine Ostracod Correlations

The timespan represented by the unconformity separating nonmarine Upper Jurassic and Lower Cretaceous rocks in the North American Western Interior foreland basin is controversial and has been under discussion for decades. Despite an integration of different geological, geophysical, and paleontological methods, there has been no major breakthrough regarding the age determination during the last fifteen years—particularly the maximum age—of Lower Cretaceous nonmarine formations in the U.S. Western Interior. Time intervals ranging from at least 15–20 up to 35 Ma have been given, according to assumed ages of middle Albian to Barremian for these. In particular, the usage of the term Neocomian and the difficulty in determining exactly the age of the unconformity surface of the Morrison Formation may have intensified inconsistencies.

As part of a joint research project, Lower Cretaceous ostracods (microcrustaceans with a calcified shell) of the Lakota (Black Hills, SD) and Cedar Mountain formations (San Rafael Swell, UT) were taxonomically revised and tested for their potential for biostratigraphical application. Despite some remaining taxonomical problems, a supraregional correlation with well dated contemporary European strata strongly suggests that the most probable maximum age of the lower part of both these formations is upper Berriasian to lowermost Valanginian, and therefore considerably older than other published lines of evidence have indicated. Thereby, the Upper Jurassic-Lower Cretaceous hiatus would be reduced to a time interval less than 10 Ma, if not less than 5 Ma. However, that is not the end of the matter. Ostracods are considered to have the potential to improve both intra- and interformational correlations, and along with an integration with other stratigraphical methods and an application of ostracod biostratigraphy to other formations, it is possible to further improve the Lower Cretaceous stratigraphy in the Western Interior.