Petrology, depositional setting, and reservoir quality of the Torok-Nanushuk transition, central North Slope, Alaska

Abstract

Cretaceous sandstones of the upper Torok and lower Nanushuk formations were deposited in a storm-dominated, deltaic depositional system during filling of the Colville basin, North Slope, Alaska. This study focuses on distal shelf and slope sandstones of the Torok Formation and shoreface sandstones of the Nanushuk Formation from Arc Mountain, Slope Mountain, Gunsight Mountain, and the Grandstand Test Well. Ten lithofacies were identified at these locations. Facies associations define depositional environments ranging from deep marine debris flows and tubidites deposited in distal shelf and slope settings of the Torok Formation to deposition of hummocky cross-stratified sandstones deposited above storm wave base in the proximal shelf settings of the Torok/Nanushuk transition. Sandstones from these locations are fine grained and include abundant detrital grains of quartz, chert, various lithic fragments, and feldspar. Compositionally these sandstones plot as litharenites to sublitharenites and are derived from a quartzose to transitional recycled orogenic provenance. These sandstones record the uplift of the ancestral Brooks Range and exhumation of the Endicott Mountain allochthon. There is a decrease in lithic material from the deeper water outer shelf setting to the storm dominated shoreface setting that is attributed to increased reworking and winnowing by waves in the higher-energy, shallow water settings.