Shelf to Shallow Marine Deposition of Ivishak Formation, Arctic National Wildlife Refuge, Northeastern Alaska

N. T. Harun, R. Keith Crowder

The Lower Triassic Ivishak Formation in Alaska's Arctic National Wildlife Refuge is composed of a progradational-aggradational-retrogradational depositional sequence. The Permian Echooka Formation underlies the Ivishak Formation. The Ivishak is overlain by the Middle and Upper Triassic Shublik Formation, except in the northern Sadlerochit Mountains where the Lower Cretaceous unconformity cuts down section into the Ledge Sandstone Member of the Ivishak Formation. In ascending order, the Ivishak Formation consists of the Kavik, the Ledge Sandstone, and the Fire Creek Siltstone Members. The Kavik Member is composed of thin-bedded, nodular siltstone and silty shale up to 70 m thick. Beds in the Kavik gradually thicken and coarsen into the overlying Ledge Sandstone Member. The Ledge is the chief hydrocarbon reservoir at Prudhoe Bay. In the Arctic National Wildlife Refuge, the Ledge forms a thick (40-90 m) succession of regularly bedded very fine to middle-grained sandstone with conglomeratic intervals. It becomes progressively thinner bedded and finer grained toward the east from the Sadlerochit Mountains to Leffingwell Ridge. The Ledge is gradationally overlain by interbedded sandstone, siltstone, and shale of the Fire Creek Siltstone Member. The Fire Creek consists predominantly of highly bioturbated sandstone, slump structures, and graded beds.

The Kavik Member was deposited in open-marine waters beneath storm wave base. The transition into the overlying Ledge Sandstone Member represents a progradational sequence. The nearshore deposits of the Ledge Sandstone Member are aggradational. The Fire Creek Siltstone Member records deposition in an inner to middle shelf environment and represents a general retrogradational shift in depositional environments.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.