Tertiary sedimentary rocks in the Alaska Pacific-margin Tertiary basin extend onto the continental shelf, which constitutes approximately 85 percent of the 40,000-sq mi (103,600 sq km) basin. Faunal and lithologic data from the shelf are insufficient to determine age or to describe the lithology of stratified sedimentary sequences identifiable on continuous seismic-reflection records.

Structures in the Kodiak Tertiary province follow N45°E trends of the Aleutian structural system. The major feature is the Kodiak basin, bounded on the northwest by a zone of discontinuous faults extending from Hinchinbrook Island to Trinity Islands, and on the southeast by an arch at the shelf break exposing in its core upper Miocene and Pliocene strata. As much as 4 km (2.5 mi) of late Tertiary sedimentary beds fills the basin. Additional areas having potential for petroleum lie at greater depths on the upper continental slope.

Major structures offshore in the Gulf of Alaska Tertiary province east of Kayak Island predominantly follow trends of the Alaska Mainland structural system. They are west-trending anticlines 10-20 km (6-12 mi) wide and sediment-filled depressions up to 100km (62 mi) wide. Strata commonly dip 30° or less in the depressions; however, dips steeper than 30° may have been filtered out by the seismic technique. Crestal areas of folds have been truncated by erosion. An arch is believed to form the shelf edge at most localities. Seismic records in this region are limited to 1 second or less of penetration because of the instrument used.

In the St. Elias transition, west of Kayak Island, structures following both Mainland and Aleutian structural trends are intermixed. The nature of intersection of individual structures is unknown. Structures favorable for petroleum are present, but the geology is more complex than in the area east of Kayak Island or in the Kodiak Tertiary basin, and exploration will be more difficult.