Abstract: Analysis of Cretaceous Bedding Cycles in an Onshore-Offshore Transect of the Western Interior Basin

B. B. Sageman

For decades cyclostratigraphers have speculated about the origin of rhythmically bedded limestone-shale units in Cenomanian-Turonian strata of the Western Interior basin. Although climate forcing has been a common assumption, Milankovitch periodicity has lacked quantitative documentation. Furthermore, evidence has been presented in support of alternate models (siliciclastic dilution vs. carbonate productivity) as the primary depositional mechanism for the limestone-shale beds. Two recent developments have allowed for a new approach to these problems: (1) spectral analysis of the bedding cycles from a core drilled in the center of the basin; and (2) chronostratigraphic correlation of bedding cycles from the basin center to its western margin.

Carbonate bedding cycles show significant power in the precession and eccentricity bands. Lithostratigraphy and biostratigraphy confirm the one-to-one correlation of limestone beds in the basin to transgressive lag deposits overlying upward-coarsening progradational shoreface units (parasequences) along the western margin. These data suggest that northern hemisphere climate, modulated mainly by orbital precession, caused short-term changes in base level, either through changes in sediment supply to the western strand, or through changes in sea level, resulting in a consistent sedimentary response throughout the basin. The purpose of this study is to test the hypothesis of climatically forced change in sediment supply by analyzing onshore-offshore characteristics of bedding cycles. Ana yses include power spectra of stacked shoreface sequences corresponding to carbonate bedding cycles, and assessment of variations in sediment volume and accumulation rates from the shoreface to the basin center.

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