The middle upper Eocene Sitkalidak Formation is a 4,000-m thick sequence of interbedded sandstone, shale, and conglomerate. The formation crops out along the southeast side of the Kodiak Island archipelago and is believed to underlie the continental shelf for several hundred kilometers along a northeast structural grain. The Sitkalidak Formation, as used here, includes marine sandy flysch previously mapped as Sitkinak Formation.

The Sitkalidak Formation is a complex of submarine fans deposited at mid- to upper-bathyal depths in a northeast-trending structural trough. This trough was probably an accretionary basin located on an oceanic trench inner slope. The submarine fan deposits were subsequently faulted and uplifted during late Eocene subduction along an ancestral Aleutian trench. Emplacement was complete by early Oligocene time as the Sitkalidak Formation is overlain with angular discordance by the nonmarine Sitkinak Formation of Oligocene age.

Sitkalidak Formation sandstones were deposited as quartz-poor and mineralogically unstable feldspathic lithic arenites. Diagenesis has greatly reduced primary porosity and permeability to an average 2.5% ^phgr and 0.01 md k (101 samples - ^phgr = 1.0 to 7.5%; k = 0.0 to 1.3 md).

Sitkalidak Formation shales are gray to black silty claystones which contain dominantly terrestrially derived organic matter. Total organic content averages 0.44% (217 samples - TOC = 0.01 to 2.38%), suggesting lean source-rock potential for liquid hydrocarbons. Vitrinite reflectance values average 0.80% (53 samples - R_o = 0.54 to 0.92%), suggesting thermal maturity levels sufficient for oil generation.

Despite the presence of thick sandstone sequences and potential structural and stratigraphic traps, the Sitkalidak Formation has a low potential for hydrocarbon production because of the poor reservoir character of the sandstones and poor source potential of the shales.

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