Importance of geochemical/isotopic events as stratigraphic markers in cyclic Cretaceous strata

Conference · Fri Jan 01 04:00:00 EST 1988

OSTI ID:5923859

Pratt, L M

Five major episodes of marine transgression and regression are recorded in Upper Cretaceous rocks of the U.S. 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.

• OSTI ID: 5923859
• Report Number(s): CONF-880301-
• Country of Publication: United States
• Language: English

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