Importance of Geochemical/Isotopic Events as Stratigraphic Markers in Cyclic Cretaceous Strata: ABSTRACT

Abstract

Five major episodes of marine transgression and regression are recorded in Upper Cretaceous rocks of the US Western Interior by cyclic sequences with periodicities of about 5 to 15 m.y. In the lower four marine cycles, organic-rich shales accumulated during peak transgressive conditions. These deep-water shales include two calcareous pelagic deposits (Greenhorn Limestone and Smoky Hill member, Niobrara Formation) and two noncalcareous pelagic to hemipelagic deposits (Mowry Shale and Sharon Springs Member, Pierre Shale). Shorter term paleo-oceanographic cycles can be recognized in the Greenhorn and Smoky Hill because pelagic sedimentation in these units was not interrupted by current-induced depositional and erosional events (such as characterize the Mowry) and sedimentation rates were not as low as for the Sharon Springs. Rhythmic interbedding of limestones and calcareous shales resulted from climatic oscillations with a periodicity of about 40,000 years for the Greenhorn and about 20,000 for the Smoky Hill. Geochemical/isotopic events are superimposed on the rhythmic bedding and are expressed as excursion in organic carbon contents, whole-rock manganese contents, and stable carbon isotopic values of carbonate and organic matter. The most pronounced geochemical/isotopic event is about 0.5 m.y. in duration and spans the Cenomanian-Turonian boundary. This event is recognized in many epicontinentalmore » sections by a positive shift in the carbon isotopic composition of organic matter and carbonate and by enrichment of whole-rock manganese contents. The narrow chronostratigraphic interval defined by the carbon isotopic excursion can be used to directly compare sediment accumulation rates and bedding periodicities in a variety of paleoenvironments worldwide.« less