Marked fluctuations in the population structure of planktonic foraminifera, particularly in assemblages from epicontinental seas, are believed to recrod changes in water-mass properties. The Cretaceous Western Interior Seaway represented a dynamic interface between widely varying water masses responding to eustatic sealevel fluctuations, episodic tectonism and subsidence, and climatic variability. The Cretaceous marine sequences are rich in foraminifera and represent a rare opportunity to rigorously evaluate foraminiferal paleoecology. With the assumption that well-preserved foraminiferal assemblages recrod shifts in water mass boundaries and/or changes in water mass structure of properties through time, PIs will test the hypothesis that episodic shoreline migrations of the western margin influenced the paleoceanography of the seaway, and can be observed as such in offshore mudrock facies. Detailed analyses of planktonic and benthic foraminiferal population structure, coupled with clay mineralogy and other sedimentologic and geochemical techniques, will be used to develop robust paleoecologic models for Cretaceous foraminifera of the Western Interior Seaway. Such models will be useful for evaluating the resource potential of unexplored areas.