ARCHITECTURAL ANALYSIS AND FOLD GEOMETRY OF SYNTECTONIC FLUVIAL CONGLOMERATE IN THE NANUSHUK FORMATION, BROOKS RANGE FOOTHILLS, ALASKA

The fluvial style and syndeformational character of conglomerate in the upper part of the Nanushuk Formation is resolved using facies architectural analysis and structural geometry. The Nanushuk Formation filled the foreland basin north of the Brooks Range from south to north and west to east during the mid-Cretaceous. Along the Kanayut River in the northern foothills of the Brooks Range, fluvial conglomerate is exposed as benches on the north and south flanks of the Arc Mountain anticline. Photo mosaics of each bench on the north side along with thirty-three detailed measured sections were used to evaluate facies architecture. Eight lithofacies were described that characterize six facies associations including longitudinal and transverse gravel bars, diffuse gravel sheets, sediment gravity flow deposits, crevasse splay and floodplain deposits, and scour fills of a gravel-bed-braided river. Petrographic analysis was used to determine provenance for the conglomerate. Strata on the south limb of the anticline show characteristics of syndeformational deposition with the growth of the south limb of the Arc Mountain anticline. These rocks both increase in thickness away from the anticline and decrease in dip upsection. This structural data provides new evidence for syndepositional contractional deformation in the northern foothills of the Brooks Range at this locality during the mid-Cretaceous. This study also provides evidence that coarse-grained rocks of the fluvial system were trapped close to the mountain front during deposition. This provides clues to the character of potential reservoir rocks from the same stratigraphic unit that are present in the subsurface of the National Petroleum Reserve in Alaska, which is important for continued recovery of natural resources on the North Slope.