The Ivishak Formation forms a regressive-transgressive deposit. The stratigraphic divisions are (1) a lower prograding deltaic unit of massive sandstone; (2) a middle fluvial unit of sandstone, shale, and minor conglomerate; and (3) an upper destructive deltaic unit of thin-bedded to massive sandstone. These Ivishak units defined in ANWR are recognized in the subsurface and traced over much of the North Slope.

Basin analysis consisted of isopach and percent-sandstone mapping and paleocurrent measurement of 15 outcrops. Formation thickness averages 400 ft (120 m) with a northeast-trending depocenter axis through the Romanzof Mountains. Paleocurrent data define two main provenances of quartz-chert sands: northwest and east. Paleocurrents are oriented normal to, and dip toward, the basin axis. Outcrops located within the axis record bidirectional transport.

A Lower Cretaceous unconformity (LCU) truncates the Ivishak in the Sadlerochit Mountains. Here, Neocomian "pebble shale" rests atop the Ivishak, with Shublik through Kingak formations missing. The LCU truncation is part of a regional unconformity that occurs along the north side of the North Slope.

Ivishak units thin near the unconformity, suggesting an older high, which we term the Nularvik high. This high is part of a regional trend extending through ANWR from the Point Thomson area to Bathtub syncline.

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