Lithologies and Correlation of the Basement Complex (Devonian and Older) in the Subsurface of Northern Alaska

By

J.A. Dumoulin (U.S. Geological Survey)

Rocks of the basement complex (Devonian and older) were studied from 30 exploratory wells in the NPRA and 34 wells in the central North Slopel¸ located on the Barrow arch. Fine-grained, variably deformed sedimentary rocks deposited in a slope or basinal setting predominate; lithologies include varicolored argillite in the Simpson area, dark argillite and chert near Barrow and Prudhoe Bay, and widespread gray argillite. Chitinozoans and graptolites of Middle Ordovician through Silurian age occur in the dark argillite and chert unit. Sponge spicules and radiolarians establish a Phanerozoic age for the varicolored and gray argillite units, both of which contain interbeds of chert-rich sandstone and siltstone. These argillite units have similarities to the Ordovician-Silurian Iviagik Group on the Lisburne Peninsula. Chert-rich conglomerate and sandstone interbedded with mudstone and coal, of Early-Middle Devonian age and formed in a fluvial environment, occur in the Topagoruk area. At East Teshekpuk, granite of probable Devonian age was penetrated. The Topagoruk conglomerate and the Teshekpuk granite resemble the Ulungarat formation and the Okpilak batholith, respectively, in the northeastern Brooks Range. Basement rocks in wells near ANWR consist chiefly of siliciclastic strata, including feldspathic sandstone, interbedded with 5- to 50-m-thick intervals of limestone and dolostone that contain ooids, peloids, and rounded detrital quartz. These strata resemble, and may correlate with, Cambrian-Late Proterozoic(?) rocks in the northeastern Brooks Range. Fossils from the North Slope basement complex are mainly cosmopolitan; forms with Siberian affinities, common in coeval carbonate strata in the Brooks Range, have not been found.

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.