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During the Cretaceous Period, the Western Interior Seaway (WIS) was influenced by both gradual, long-term environmental changes and catastrophic, short-term environmental changes. Vertebrate fossil-rich bioclastic accumulations are particularly well developed along the eastern margin of the WIS within Cenomanian aged strata. In addition to these deposits, the uppermost Cenomanian to Turonian record along the eastern margin is characterized by rich, fairly undiluted planktic foraminiferal assemblages. The major control for the accumulation of these beds is interpreted to be related to a long-term process, namely the timing and magnitude of sea-level fluctuations on the eastern margin of the WIS. Bioclastic elements are interpreted to be concentrated as lags along erosion surfaces during transgressions. Superimposed on this are the effects of short-term processes: the taphonomic character of each bioclastic deposit is interpreted to be linked to the frequency of episodic events that influenced the seaway. Proximal and distal planktic foraminiferal assemblages record a changing environment with frequent catastrophic environmental perturbations including ashfalls and expansion of the oxygen minimum zone into the photic zone. Frequent environmental perturbations affect both distal and proximal settings, though their effect is stronger nearer to the paleoshoreline.Looking at both the depositional and faunal response to these short-and long-term processes that affected the seaway improves our understanding of the dynamics of this epeiric sea during this enigmatic time.