INTERPRETING SEDIMENTARY FACIES AND FACIES ASSOCIATIONS FROM A 1500-FT LONG, BEHIND OUTCROP CORE THROUGH UPPER CRETACEOUS STRATA IN SE UTAH

The Book Cliffs in SE Utah have been a playground for many generations of geologists interested in clastic sedimentary rocks. Many of the exposures have been studied extensively because they offer good analogs for a range of nearshore, coastal, and terrestrial reservoirs in the subsurface. Most outcrops, however, only provide a 2D view and fail to fully characterize stratigraphic architectures and facies heterogeneity.

In this study we present data from a new, behind outcrop core through the upper Cretaceous Price Canyon, Castlegate, and Blackhawk Formations. This study is part of a collaboration between Southern Utah University and the University of Montana that focuses on characterizing the 3D subsurface architecture and facies heterogeneity of these formations from a series of cores.

The core described here contains a wealth of sedimentary facies associated with coastal and nearshore settings. In the depth interval from 1340 to 497 feet, packages correlative to rocks of the Blackhawk Fm contain prominent 10- to 35-ft thick packages of white, fine-grained, laminated and cross bedded sandstones, with thin mud partings, separated by 1- to 7-ft packages of contorted and laminated gray to black mudstone and gray siltstone and by coal seams from 1 to 8 feet thick. Bioturbation (1- to 5-ft thick packages) occurs in some mudstones and in the upper part of sandstone packages.

Sediments associated with the Castlegate SS in the depth interval from 497 to 25 feet exhibit a change to light tan and gray, medium to fine-grained massive and laminar sandstone packages ranging in thickness from 16 feet to 40 feet. Separating the sandstone packages are mudstones and siltstones similar to those in the Blackhawk Fm. Coal beds typically are less than one foot thick in the Castlegate intervals, and bioturbation is absent.

We interpret these packages to represent a transition from a coastal/delta plane environment dominated by fluvial processes to a fully fluvial environment during Castlegate time. Sandbody thickness (channel dimensions) change through time, and amalgamated, multi-story channels are more frequent in the Castlegate interval. Research will continue, beginning with a systematic analysis of the remaining cores through a series of undergraduate projects focusing on facies analysis and three dimensional facies models.