Upper Jurassic to Lower Cretaceous sandstones of the western melange belt in the North Cascade Range of Washington contain three distinct petrofacies of arkosic, lithic, and chert-rich compositions. These petrofacies were most likely mixed together in an accretionary wedge within an active subduction zone. The arkosic and lithic petrofacies were probably derived from a differentially dissected volcano-plutonic arc system, and the chert-rich sandstones were in part derived from reworking of uplifted ocean-floor deposits in the developing accretionary wedge. Compositional similarity between the western melange belt and the contemporaneous Franciscan Complex of northern California suggests that similar sedimentary-tectonic processes operated in the two melange systems, or that similar polymodal compositions can result from more than one process, or that the two units, now widely separated, were once together.

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