A LARGE, SHALLOW INLAND SEA

During most of the Cretaceous, the western interior of the North American continent was covered by an epicontinental sea. During the Late Cretaceous, at the peak of the transgression, this shallow sea extended from the Rocky Mountains to close to the Mississippi Valley and from northwestern Canada into the Gulf of Mexico, flooding one-third of North America and covering an area of about 1,500 by 6,000 km (Fig. 224). The Western Interior Seaway was eventually filled with sediments from the rising western highlands, ancestors of the Rocky Mountains. From west to east, the sediments include non-marine lowland sands and silts, littoral marine sands and silts and offshore marine muds. The Cretaceous transgression reached western Kansas by the Late Cretaceous, about 85 million years before present. The sequence, with a total thickness of 450 m, starts here with deltaic sediments, represented by the sands and silts of the Dakota Formation. This interfingers laterally with the finer-grained Graneros Shale, deposited seaward of the delta. With continuing transgression, the eastern margin of the sea moved farther to the east. In western Kansas, shallow-water limestones began to accumulate, followed by laminated chalk (Lower and Upper Greenhorn Limestone). Partial regression of the sea is reflected in the deposition of the Carlile Shale; renewed transgression led to the deposition of the Niobrara Chalk, a fine-grained, rather pure limestone derived from shells, planktonic foraminifera and coccoliths. During this period, the bottom waters became stagnant at times, limiting life on the sea floor.