Paleoenvironment and geochemistry of the northern portion of the Western Interior Seaway during late Cretaceous time

Oxygen and carbon isotopic compositions of molluscan shells and planktonic foraminiferal tests from the Ashville and Favel formations indicate that the δ¹⁸O value of the Greenhorn sea normally was at least 4 per mil(‰) lower than that of the contemporaneous open ocean. Exceptions occurred as Cenomanian time gave way to Turonian time and the sea approached peak transgression. Although the δ¹⁸O values of the Greenhorn, Niobrara and Claggett seas were lower and the temperatures higher than those of the coeval open oceans, δ¹⁸O values of both the open oceans and the intracratonic seas of the Western Interior Basin increased and the temperatures decreased progressively throughout the Late Cretaceous Epoch. -from Authors