The late Paleocene-early Eocene Orca Group records much of the depositional and deformational history of the Tertiary accretionary prism found in the subduction zone of the Pacific plate in south Alaska. The thick (?5,000+ m) quartzo-feldspathic sandstones and interbedded shales were deposited as a submarine fan complex. Paleocurrents show derivation of detritus from a northeastern source. Three facies belts are distinguished. A northwestern belt characterized by massive sandstones and strong deformation is thrust southeasterly over a central melange belt containing originally interbedded basalt and turbidites deposited on an ophiolite basement. To the southeast, the third belt comprises a classic turbidite sequence which may have been deposited upon imbricated offscrapin s of pelagic Upper Cretaceous sedimentary rocks now exposed only in one faulted outcrop on southeast Montague Island.

The predominant structures of the Orca Group are steeply northwest-dipping thrusts and folds overturned to the southeast in harmony with northwesterly subduction of the Pacific plate. However, on Montague Island the neotectonic southeasterly overturned structures overprint early major northwesterly overturned folds. Along strike to the northeast, northwest-trending cross structures overprint the early folds. Structural analysis indicates that the structure of the late Tertiary offshore basin is complex.

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