Determination of the Dominant Surface Water Source for the Eastern Margin of the Maastrichtian Western Interior Seaway Using Calcareous Nannofossils as Paleoecologic Indicators

John A. Sarao and David K. Watkins
Geosciences, University of Nebraska, Lincoln, NE

The relationship of water masses in the Western Interior epeiric sea has been the subject of considerable research for the early part of its Late Cretaceous history, when Greenhouse conditions led to equable global conditions. Relatively little work has focused on examining water mass relationships in the later part of Late Cretaceous, when increased global temperature gradients should have enhanced the contrast between southern (Gulf of Mexico) and northern (Arctic Ocean) ends of the basin. The objective of this project is to determine the dominant source of surface water on the eastern margin of the Maastrichtian Western Interior Seaway as a means to reconstruct the circulation in the basin. Samples were collected at high-resolution from the Mobridge Member (mid-Maastrichtian) of the Pierre Shale at Verdigre, Nebraska. Assemblage census counts of smear slides prepared from the Mobridge samples revealed the presence of abundant, well preserved nannofossil assemblages. Although the Maastrichtian paleolatitude of the Mobridge is not significantly different from the modern day location of the site, comparison of the nannofossil assemblages with previously published studies of coeval assemblages indicates that the Mobridge assemblages had strong affinities to those from polar latitudes. This suggests that the dominant paleocurrent along the eastern flank of the Western Interior Seaway was coming from the north (Arctic Ocean) and that perhaps the Seaway was cut off from the warmer waters of the Gulf Coast.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas