HIGH-RESOLUTION FORAMINIFERAL BIOSTRATIGRAPHY AND PALEOENVIRONMENTS AROUND THE C/T BOUNDARY IN THE EASTERN WESTERN INTERIOR SEA

The Cenomanian/Turonian boundary interval records one of the most prominent climatic perturbations in the Cretaceous greenhouse world. At times of sea level highstand, the Western Interior Sea (WIS) connected Boreal and Tethyan waters. Moreover, the unique tectonic and geographic orientation of the WIS created complex oceanographic circulation patterns resulting in temporal and lateral variations in the sedimentological and biological record. Foraminifera have been effectively utilized to interpret the paleoenvironments and paleoceanography of the most parts of the WIS. However, such studies from the eastern part of the seaway are few. Sixty-nine closely spaced samples around the C/T boundary from a site in Cuba, KS have been studied for their foraminiferal content. Surprisingly, planktic foraminifera dominate all the studied samples in spite of the relative proximity of the site to the paleo-shoreline, indicating inhospitable benthic environments. Benthic foraminifera are scarce in most samples with low species diversity index. Yet, diversity increases abruptly in the "benthonic zone" coincident with a δ¹³C_org positive excursion marking the onset of Oceanic Anoxic Event 2. This 50-cm-thick interval is associated with trochospiral planktic morphotypes dominating over biserial morphotypes, appearance of keeled planktic species, and lowest TOC and calcium carbon content values of 8-m-thick study interval. Such diverse planktic foraminiferal assemblages require a stratified water column thereby challenging the increase in benthic foraminiferal species abundance and diversity and low TOC content values. The favorable conditions ended nearly as abruptly as they began. As in the basin center, the “Heterohelix shift” follows a brief benthic recovery. Stressed environmental conditions throughout the water column allowed only dwarfed species of H. mormani and H. planispira to dominate the planktic assemblages along with scattered specimens of the benthic species N. albertensis. Moreover, TOC values significantly increased to 9.65 wt%. This coincides with what so called plateau of the δ¹³C curve. Subsequently, a brief environmental improvement allowed slight diversity increase in both benthic and planktic species which coincides with the gradual termination of δ¹³C positive excursion.