Abstract: The Sag River Sandstone (Triassic), North Slope, Alaska: An Example of Shelf Sandstone Deposition under the Influence of Upwelling

Michael D. Wilson

The Sag River Sandstone is a thin marine sandstone unit of sheet-like geometry developed in a 160 km wide belt paralleling the coastal portion of the North Slope of Alaska. Though a minor reservoir in the Prudhoe Bay Field, the Sag River Sandstone may be one of the largest shelf sandstone reservoirs in the world.

The body of the Sag River Sandstone consists of bioturbated, slightly argillaceous and glauconitic, well-sorted, very fine-grained sandstones. Primary sedimentary structures are absent in all cores studied. Phosphate nodules are common near the base and top of the unit. Sponge spicules, preserved only as molds within phosphate nodules, are estimated to have originally formed between 10 to 20% of the detrital fraction.

The Sag River Sandstone represents a single progradational sequence. Grain size and sorting increase upward from the base of the unit. A Cruziana burrow assemblage dominates at the base of the unit, and a Skolithos assemblage occurs at the top of the unit.

The Sag River Sandstone is the product of a depositional regime in which upwelling played a significant role. Upwelling of cold oceanic waters was associated with coastal aridity that severely reduced fluvial sediment supply and promoted the development of sponge spicules, phosphate nodules, and glauconite. Moderate wave energies were required to maintain high levels of framework grain sorting and low detrital matrix content. Upwelling is a major control on the composition, texture, and bedding of shelf deposits and has largely been ignored in the formulation of previous shelf sandstone models.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994