Paleobathymetry of Cretaceous Greenhorn Sea in Eastern Colorado

Abstract

The Cenomanian and Turonian (Cretaceous) Greenhorn sea reached maximum extent and presumably maximum depth at about the time of upper Greenhorn (Bridge Creek Member) and lower Carlile (Fairport Member) deposition. This depth is estimated in two ways--from the percentage of planktonic specimens in the very diverse normal marine foraminiferal fauna of the lower Bridge Creek, and from inference of the paleoslope that the sea transgressed. Chiefly because calcareous benthonic specimens are missing from most of the Graneros-Greenhorn-Carlile sequence, the foraminiferal profile does not correspond to one expected from simple transgression and regression on a modern continental shelf. Therefore planktonic percentages that prevail on continental shelves and slopes today cannot be used to infer depth for most of the foraminiferal profile. However, planktonic percentages might indicate approximate depth for the time of lower Bridge Creek deposition, when conditions temporarily become favorable for diverse calcareous benthonic species. Planktonic percentages for the Bridge Creek, everywhere 80 percent or more, suggest that the water depth was at least 1,640 feet (500 m). If the slope of the western margin of the marine basin remained about the same between deposition of the sandstone of the Lytle Formation (lower Dakota) and deposition of the Greenhorn, the stream slopes necessary to transport Lytle pebbles 400 mi from a western Utah source to the Colorado Front Range foothills area were the same as the slope which the sea transgressed later. When it was deepest, the Greenhorn sea reached western Utah, advancing as far as the source area for the Lytle. If the upper 50 mi of the Lytle stream courses is eliminated from consideration (here slopes were steepest and tectonics least stable), total fall of the streams in the remaining 350 mi was probably of the order of 2,000 ft or more. This figure, modified by sedimentation and isostatic depression of the ea floor, provides an estimate for the maximum depth of the sea in eastern Colorado.