The Nanushuk Group of Albian to Cenomanian age is a passive-margin deltaic deposit 3,500 m thick that underlies much of the western and central North Slope. Correlation of about 30 test wells, integrated with seismic control, indicates that the Nanushuk delta prograded from west-southwest to east-northeast across the subsiding Colville basin. The primary source area was in the distant southwest, probably in the area of the present Chukchi Sea or beyond. The ancestral Brooks Range, which bounds the south side of the basin, was a secondary but important source area.

Seismic data indicate the Nanushuk to be part of a sequence of topset beds that are laterally equivalent to and underlain by outer shelf topsets, slope foresets, and basinal bottomsets of the Torok Formation. Distal bottomset beds, which consist of shales and minor sandstones, were deposited in water depths of 450 to 900 m. The original continental slope angle generally steepened

Figure

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from west to east to as much as 6°.

The total incremental stratigraphic rise of the base of the Nanushuk is about 2,100 m from the westernmost well to the pinch-out on the east, a distance of 370 km. However, subsidence was not uniform throughout the basin, as indicated by less subsidence of the passive Barrow arch on the north side of the Colville basin.

Subcommercial amounts of oil and gas occur in shallow anticlinal and truncation traps in the Nanushuk Group, one of many objectives being evaluated by the current National Petroleum Reserve in Alaska (NPRA) drilling program.

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