Nuka Formation of Late Mississippian to Late Permian Age, New Formation in Northern Alaska

Abstract

An unusual succession of fine clastic, feldspathic coarse clastic and cherty strata, and correlative coarse clastic rocks in a 170-mile-long band along the northern front of the Brooks Range in northern Alaska is named the Nuka Formation. The succession is more than 6,500 feet thick. It is composed of 13 members that are characterized by light-colored arkosic to calcarenitic layers, by dark-colored layers of mud shale to calcilutite, or by varicolored chert and cherty shale. Rhythmic repetition of the three main rock types may represent depositional cyclicity. The succession is overlain by Jurassic (?) graywacke-type strata; the base, though not exposed, may be gradational with the Lisburne Group (Mississippian). Fossils in the upper half are Permian in age and those in t e lower third are Late Mississippian; no diagnostic Pennsylvanian fossils have been recognized. Away from the type area, the Nuka consists predominantly of arkosic and calcarenitic rocks that contain fossils of Permian age. The feldspathic rocks indicate that a granitic provenance in northern Alaska spread over a considerable area during Permian time and was at least periodically exposed locally for the estimated 80 million years of Late Mississippian to Permian time. Granitic detritus and rocks deposited under varied and active conditions, rather than in a stable shelf environment, differentiate the Nuka Formation from other upper Paleozoic units in the region. These contrasting facies are now juxtaposed, probably as a result of large-scale structural dislocations. The Nuka may extend into the subsurface toward the north and have characteristics favorable to the accumulation of petroleum.