EVALUATION OF TECTONIC AND OCEANOGRAPHIC FORCING FACTORS ON THE FORMATION OF EXCEPTIONALLY CARBONATE-RICH HEMIPELAGIC FACIES, CRETACEOUS WESTERN INTERIOR BASIN

The Upper Cretaceous (Turonian-Coniacian) Fort Hays Limestone Member of the Niobrara Formation is a unique lithofacies within the fill of the Western Interior basin of the United States. The Fort Hays Mbr. is the most carbonate-rich, mud-poor unit in the entire Cretaceous section of the Western Interior basin, which contains a very thick section of mudrocks. The Sevier Highlands to the west were the primary source of clays deposited in the basin fill. Peak Cretaceous highstand in the Western Interior occurred earlier in the Greenhorn Formation, which, compared to the Fort Hays Mbr., is relatively mud-rich. The paucity of siliciclastic mud in the Fort Hays Mbr. warrants explanation.

Bedding in the Fort Hays Mbr. is characterized by rhythmic oscillations of decimeter-thick limestones and centimeter-thick calcareous shales. These bedding alternations have been interpreted to reflect depositional sensitivity to orbital-scale climate change. Limestones are often composed of greater than 90 weight percent calcium carbonate. Regional studies have documented diachroneity of both the upper and lower contacts of this lithostratigraphic unit. The stratigraphic record is an important archive for addressing potential dynamics relating to both paleotectonics and paleoclimate. However, depositional systems may be sensitive to both sets of forcings and, therefore, deconvolution of these factors is often difficult.

This study utilizes an existing biostratigraphic framework, recently developed orbital timescales from 3 distal sections in the basin, lithostratigraphic correlations, and geochemical data sets to evaluate, at high resolution, spatial and temporal depositional trends. Integration of spectral analytical techniques with more traditional stratigraphic methods facilitates generation of highly resolved isopach maps, facies maps, sedimentary accumulation rate maps, and estimates of the relative timing of net deposition following the Upper Turonian lacuna. Results indicate general trends of earlier onset of deposition to the southwest and greater siliciclastic flux to the southwest. The timing, regional extent, and rates of mud delivery ultimately aid in discrimination of tectonic and oceanographic forcing factors responsible for transport pathways of fine-grained siliciclastics.