Facies analysis and sedimentology of the Cretaceous Nanushuk Formation at Slope Mountain, Alaska

An analysis of sedimentary lithologies and structures in selected outcrop sites at Slope Mountain, Alaska identified thirteen distinct facies within a subsample of the Cretaceous (Albian-Cenomanian) Nanushuk Formation. Based on the distribution of the thirteen facies, this study defines five unique facies associations at Slope Mountain, each representing a specific depositional environment. The facies associations are as follows: 1) distributary channel bar, 2) tidal channel, 3) upper shoreface, 4) distributary mouth bar, and 5) fluvial channel. The relative stratigraphic positions of the facies associations in outcrop have implications for fluctuations in shifting shoreline positions within the Nanushuk Formation over time. The progression of facies associations within several hundred meters of section analyzed in this study depict a shift from a regressive deltaic system (sedimentation>accommodation) to an alluvial system affected by local transgression (sedimentation＜accommodation), followed by a return to rapid and potentially forced regression (sedimentation>accommodation). Despite significant overall accommodation in the Colville basin, the dominant pattern of Nanushuk Formation sedimentation is highly regressive, with significant progradation and aggradation of deltaic and alluvial deposits throughout the Albian-Cenomanian.