CHARACTERIZING FACIES VARIABILITY IN AN INCISED VALLEY FILL: ABERDEEN MEMBER, UPPER CRETACEOUS BLACKHAWK FORMATION, BOOK CLIFFS, UTAH

Incised valleys are important stratigraphic elements and act as hydrocarbon reservoirs and aquifers. This study seeks to expand upon the current model by describing the facies variability within a single incised valley fill (IVF) along depositional dip within Upper Cretaceous strata. This will help to document and quantify the internal heterogeneities that occur below seismic resolution which are crucial to the utilization of IVFs in the subsurface.

The Aberdeen Member contains highstand parasequences of nonmarine to shoreface strata of 2 depositional sequences (Kamola and Huntoon, 1992). The younger sequence is truncated by a sequence boundary and overlain by an IVF, the focus of this study. Each of these parasequences includes a shoreface succession which records the progradation of a shoreline into the basin. Overall, the observed segment of the IVF is divided into three categories: fluvially dominated updip, tide dominated downdip, and mixed in the medial portion of the IVF.

IVF outcrops updip are composed of aggradational channel-fill sandstones overlain by thin (cm to <1m thick) low quality coals. These coals indicate wetted, subaerial exposure surfaces on a fluvial floodplain. The valley fill contains 14 flooding surfaces indicating an aggradational nature to the valley fill, consisting of 14 steps, resulting in a complicated parasequence succession.

Down depositional dip, tide dominated facies directly overlie a sequence boundary which juxtaposes tidal strata over lower shoreface hummocky cross strata of the youngest shoreface in the Aberdeen Member. Four parasequences are identified within the valley fill at the tidal type locality, each averaging 2m in thickness. Each parasequence contains a variety of tidal indicators and complex association of sedimentary structures. Parasequences generally coarsen upwards, can show aggradational fill, and often initiate with estuarine-floor mud at the base of the succession. Parasequence thickness of 2 m indicates periodic base-level rises of around 2m, which is interpreted to reflect eustatic rise.

It is crucial to characterize the depositional environments and partitioning within an IVF as these details are not resolvable in traditional seismic data, These complexities, however, can have profound impacts on reservoir and aquifer quality in the IVF.