Forty-two samples of probable outcrop dredged along the 250-km-long northwest-trending continental slope between long. 138°00^primeW and 142°30^primeW from water depths of 3,150 to 200 m provide control on the geology of the seaward margin of the Tertiary basin underlying the outer continental shelf (OCS) Lease Sale 55 on the Yakutat segment of the shelf.

Six rock units apparently in approximately normal stratigraphic position are tentatively recognized. (1) Hard graywacke, argillite, and possible intrusive rocks of inferred Late Cretaceous age crop out from Cross Sound to 139°W and probably underlie much of the Fairweather Ground. (2) 800+ m of shallow-marine basaltic flow and pyroclastic rocks, at least in part of early Eocene age, makes up much of the rugged lower slope from 139°W to about 141°30^primeW, and rocks of this type are sporadically present west of Yakutat Sea Valley. (3) 500+ m of shallow-marine fine to coarse clastics including organic-rich shale of early and middle(?) Eocene age overlies and intertongues with the volcanic unit from 139°30^primeW to 142°30^primeW. (4) 1,500+ m of upper to middle bathyal organic-rich shale, tuffaceous shale, and sandstone of late Eocene and early(?) Oligocene age, apparently overlies the older units between about 140°W to 141°30^primeW. (5) 450+ m of siltstone with sandstone of probable late Oligocene age underlies the upper slope between 139°W and 140°W. (6) Neogene marine glacial deposits and mud occur on the OCS as slope-basin fills and slope-rise prisms, and as a mantle over the slope at the mouths of the Alsek Canyon and Yakutut Sea Valley.

The lower Tertiary sequence sampled on the continental slope differs significantly from coeval rocks onshore or penetrated by wells on the Yakataga segment of the OCS to the west. The Eocene and Oligocene units sampled include abundant thermally mature source rocks. Most dredged sandstones are poor reservoirs, but some upper Eocene to lower Oligocene sandstones have fair to good porosity and permeability. Seismic reflection data indicate that the Eocene and Oligocene sequence dips beneath Lease Sale 55 where it could be a favorable exploratory target.

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