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

A relatively unstudied incised valley in Aberdeen Member of the Upper Cretaceous Blackhawk Fm is identified in both outcrop and subsurface coal isopach maps. The incised valley, ranging from 1-3 km in width and exposed along a 25 km transect along depositional dip, provides information on an important lowstand event as well as information about the internal structure of incised valley fills, which can be overly simplified in existing conceptual models. While some outcrops of the Aberdeen Member are difficult to access, they provide expansive views of the internal architecture of the incised valley. Several easily accessible road-side outcrops provide excellent examples of this incised valley fill. Together, these exposures are useful teaching tools to understand the complexity of incised valley fills.

Measured sections taken at 8 locations in a west-east transect along the depositional dip of the valley, along with drone imagery taken of inaccessible cliff exposures, and LiDAR images from measured section localities, were used to characterize the transition between fluvially dominated strata updip, and tidally dominated strata downdip. In the updip extent, the incised valley erodes through a 12ft (3.7m) coal seam, and is identified in coal isopach maps. In the most updip exposures, the valley fill consists of thin (0.35-1m), amalgamated channel-fill sandstones with lateral accretion, thin, discontinuous coal seams, and rooted overbank intervals. Tidal indicators are absent in the updip exposures. When traced slightly down-dip, exposures contain thicker fluvial sandstones up to 2m, characterized by an upward decrease cross-bed thickness. Many of these channel-fill sandstones are laterally discontinuous and separated vertically by overbank fines. Exposures down depositional dip are tidally dominated, often containing tidal flat facies between 3-5m thick. Tidal indicators include flaser, wavy and lenticular bedding, interference ripples, and sigmoidal cross-stratifiied sandstone with tidal bundling. LiDAR data was collected and compared with drone imagery and outcrop data (measured sections) to evaluate the efficacy of each data collection method.