The contact between the Upper Cretaceous Valdez Group and the Paleocene and Eocene Orca Group has been inferred to be the boundary between the Chugach and the Prince William tectonostratigraphic terranes. Sandstone petrographic data from the Prince William Sound area show no compositional discontinuity across this contact. These data are best explained by considering the Valdez and Orca Groups to be part of a single terrane—a thick flysch sequence derived primarily from a progressively unroofing magmatic arc with increasing input from subduction-complex sources through time.

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