ALLOGENIC SEA LEVEL CYCLES FROM THE CENOMANIAN-TURONIAN GREENHORN MARINE CYCLE: NEW BULK CARBONATE AND FORAMINIFERAL DATA FROM THE DAKOTA, GRANEROS, AND GREENHORN FORMATIONS IN WESTERN IOWA

Fourth order marine cycles that punctuate the latest Cenomanian and early Turonian Greenhorn Marine Cycle in the Western Interior Seaway have been the focus of recent intense study. The origin of these cycles has been attributed to allogenic processes such as eustasy and/or climate. We present new data from the uppermost Dakota Fm. (Woodbury Mbr.) and Graneros Shale, sampled at two locations in western Iowa. At these sites, the Dakota consists of sandstone and thinly laminated silty shale or mudstone, commonly with ripple cross-laminae, root casts, burrows, and thin lignite beds, while the Graneros consists primarily of calcareous silty mudstones. Samples were analyzed at 10 cm intervals for bulk CaCO₃ content using a gravimetric method involving treatment of crushed sediment with 3N HCl. Our results indicate low CaCO₃ (< 6 wt. %) that corresponds to relative abundances of agglutinated foraminifera in both the Dakota Fm. and the lowermost Graneros Shale. Significantly higher CaCO₃ content (< 42 wt. %; ave.: 18 wt. %) associated with relative abundances of planktonic foraminifera characterize the overlying Graneros Shale and Greenhorn Formation. The low carbonate marginal marine sediments we attribute to flooding during early-to-late transgression. The calcareous shales we attribute to deeper basinal deposition at or near highstand. Our results demonstrate comparable temporal flooding to that documented for the coeval Dakota Formation and lowermost Tropic Shale in western Utah. This bolsters the conclusion that fourth order Greenhorn Sea flooding events were basin wide and allogenic.