Comparative geochemical and mineralogical studies of two cyclic transgressive pelagic limestone units, Cretaceous Western Interior Basin, US.

Pelagic limestone units were deposited in the North American Western Interior seaway during two major Cretaceous transgressive episodes. The Bridge Creek Limestone Member of the Greenhorn Formation, deposited during the Late Cenomanian-Early Turonian transgression, and the Smoky Hill Member of the Niobrara Formation, deposited during the overall Early Coniacian-Early Campanian transgression, are both enriched in organic-carbon and exhibit small-scale carbonate cycles representing periodicities in the range 20 to 40 Ka. The distinct periodicity and overall unusual depositional milieu of both units are reflected in their sedimentary structures, mineralogy, and geochemistry. -from Authors