Abstract: Cyclostratigraphy of the Cenomanian-Turonian Bridge Creek Limestone Member of the Greenhorn Formation in the U.S. Western Interior

M. A. Arthur, B. B. Sageman, J. Rich, W. E. Dean, C. Savrda, T. J. Bralower, M. Leckie

Cyclic alternation of bioturbated, light-colored limestone and dark colored, laminated to somewhat bioturbated marlstone beds typifies the Bridge Creek Limestone Member of the Greenhorn Formation over much of the central Cretaceous Western Interior Seaway. Integrated sedimentological, geochemical and paleobiological studies of continuously cored sections from Kansas, Colorado and Utah, obtained as part of a drilling program funded by the Department of Energy and the U.S. Geological Survey, provide data for determining the timing and depositional mechanisms for the cyclicity over a period of about 1 million years centered on the Cenomanian-Turonian boundary.

The data provide evidence for significant short- and long-term changes in surface-water biologic productivity superimposed on variations in terrigenous clastic input and bottom-water oxygenation. Carbonate in the Bridge Creek Limestone Member was produced primarily by calcareous nannofossils and planktonic foraminifers, so that the limestone-marlstone cycles may have been produced by variations in carbonate production. On the other hand, geochemical

and mineralogical data show that there is systematic variation in the amount and composition of clastic materials across cycles so that dilution of carbonate by influx of clastic material brought into the basin from streams may have produced the cycles. Fluvial runoff to the basin also might have exercised some control over inferred variations in surface salinity, water-column stability, and biotic productivity.

Spectral analysis of high resolution carbonate data show that the carbonate cycles have periodicities similar to those of the Earth's orbital cycles of precession (ca. 21 ky) and eccentricity (ca. 100 ky), whereas color variations and organic carbon contents appear to have been modulated by the Earth's obliquity cycle (ca. 44 ky periodicity). Benthic oxygenation, based on a bioturbation index, appears to follow precession rather than obliquity. Although precessionally forced dilution explains much of the variability in carbonate and sediment fabric, this signal may have been overprinted by water mass mixing and upwelling caused by climatic variations farther north in the seaway that were forced by obliquity variations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994