High-Frequency Relative Sea Level Fluctuations: Evidence from Skeletal Limestone Deposits, Western Interior Cretaceous

B. B. Sageman, A. E. Murphy, B. Van Mooy, E. G. Kauffman

High-resolution stratigraphic analyses have demonstrated correlation of basinal limestone-marlstone units thought to represent Milankovitch cycles to shoreface parasequences in Cenomanian-Turonian strata of the Greenhorn Formation, Western Interior basin. Although high-frequency sea level fluctuations would provide an effective mechanism for linkage of these depositional systems, the sea level hypothesis is difficult to verify. In addition to problems in documenting short-term eustatic changes in actively subsiding forelands, the widely held view of a relatively ice-free mid-Cretaceous world removes the most effective driver of high-frequency sea level fluctuation (although some climate modellers suggest alternate views). Recently, detailed analyses of fine-grained basina facies in the Western Interior has resulted in development of new depositional models for interpreting high-frequency sea level fluctuations. Skeletal limestones are an important component of these depositional models because they preserve abundant primary sedimentologic data. Cenomanian-Turonian skeletal limestones can be grouped into 3 main categories: 1) thinly bedded to amalgamated, planar to cross-stratified units representing wave-winnowed accumulations; 2) massive to graded beds with rippled tops representing tempestite or turbidite-type deposits; and 3) thin, homogenous to planarbedded units representing starvation surfaces. Analysis of sedimentary structures, grain size variations and lateral thickness trends, and onshore-offshore correlation, suggests that deposition is relate to high-frequency sea level fluctuation. The type of deposit varies based on position within long-term sea level cycles (i.e., 2nd-order Greenhorn Cycle) and location within an onshore-offshore transect.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995