Stratigraphy of a Cenomanian Estuarine Complex in the Upper Nanushuk Formation, Ninuluk Bluff, Alaska

By

D.L. LePain and R. Kirkham (Alaska Division of Geological and Geophysical Surveys)

The uppermost Nanushuk and basal Seabee formations are exposed at Ninuluk Bluff west of Umiat. Here the uppermost 135 m of Nanushuk strata are comprised of sandstone, mudstone, and coal. The basal 48 meters consists of mudstone with corbiculid bivalves, coal, and fine-grained sandstone with small current- and wave-generated structures. Lithologies are arranged in small-scale coarsening- and fining-upward successions recording deposition in the inner reaches of an estuary as bayhead deltas and crevasse channels, respectively. The basal unit is overlain by 18 meters of mudstone and argillaceous sandstone with few sedimentary structures and records deposition below fair-weather wave base in the central portion of an estuary. The uppermost 60 meters of Nanushuk strata consists of two coarsening-upward (CU) successions, each capped by fine- to medium-grained, well-sorted sandstone with thin pebble lags, hummocky and swaley cross stratification, plane-parallel lamination, and visible porosity. These sand bodies record deposition in shoreface settings on the seaward side of an estuarine complex. In both CU successions the transition from mudstones to sandstone is abrupt, suggesting the presence of regressive surfaces of erosion at the base of each sandstone body. The upper CU succession is truncated by an erosion surface that marks the Nanushuk-Seabee contact. This surface, overlain by a 5 to 10 centimeter-thick pebble lag, is interpreted as a combined sequence boundary-ravinement surface. These shoreface sandstones represent attractive reservoir targets and the associated unconformities suggest the possibility that significant volumes of sand were transported seaward to form reservoir sand bodies in deeper water settings.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.