FORAMINIFERAL PALEOCEANOGRAPHY OF THE GREENHORN CYCLE - CRETACEOUS WESTERN INTERIOR SEAWAY

Benthic and planktic foraminifera of the Cenomanian-Turonian Greenhorn Cycle are excellent proxies for sea-level change in the Late Cretaceous Western Interior Seaway. By examining the foraminiferal assemblages, one can track the transgression and regression of the third-order (106 yr) Greenhorn Cycle and gain detailed information on salinity, benthic oxygenation, productivity, and stratification of ancient water masses. Foraminiferal assemblages and their response to sea-level change were documented for three sites: a USGS drill core near Escalante, UT (Tropic Shale), and two outcrop sections from Billings, MT (Belle Fourche Shale-Greenhorn Formation-Carlile Shale) and Mesa Verde, CO (Mancos Shale). Assemblage analyses are based on split counts of the >150 micron size fraction. Chronostratigraphy and correlation of the sections are based on molluscan and foraminiferal biostratigraphy, percent carbonate, and widely dispersed bentonites within each site. These sections allow for a regional comparison of foraminiferal assemblage composition, providing a reliable spatial and temporal comparison of biotic trends within the seaway.

Changes in sea-level directly affect benthic foraminifera assemblages providing insight into community response to rapidly changing paleoenvironments. For example, assemblages of benthic foraminifera corresponding with transgression of the Greenhorn Sea are dominated by the species Gavelinella dakotensis. Pulses of Gavelinella abundance indicate fourth-order (105 yr) changes in sedimentation rate, benthic oxygenation, and organic matter deposition. Abrupt influxes of planktic foraminifera and high planktic:benthic ratios (p:b), coupled with an acme of the agglutinated species Haplophragmium arenatum, mark late transgression and peak flooding at all sites. Many factors including transgression of warm marine waters, coupled with the incursion of oxygen-poor intermediate waters or stratification-induced dysoxia are reflected in assemblage changes favoring planktics. High p:b ratios with mixed benthic assemblages dominated by Gavelinella and/or nodosariids characterize early highstand deposits. Finally, highly variable p:b ratios and waning numbers of foraminifera indicate increasingly variable salinities and high sedimentation rates with regression.