Sandstones of the Upper Jurassic to mid-Cretaceous Seymour Canal Formation (northern Gravina belt, southeastern Alaska) are volcaniclastic wackes. They exhibit a variably developed solution cleavage and zeolite- to low-greenschist-grade metamorphic assemblages. Point counts of 37 lithofeldspathic sandstones, spanning the unit's stratigraphic range, yield framework modes typical of detritus shed from undissected arc sources (Q4F40L56, Qm3F40Lt57, Qm8P91K1, Qp3Lvm91Lsm6) with only minor detritus characteristic of older terranes or continental-margin sources. Sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous Gravina arc. In contrast, sandstones of channelized conglomeratic facies are richer in quartz, sedimentary, metasedimentary, and plutonic detritus derived from basement terrane(s). The compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas, and we interpret them to reflect minor unroofing of the Alexander terrane and the superposed Gravina arc, mostly during Early Cretaceous time. A distinctive, although volumetrically minor, clinopyroxene-rich sandstone petrofacies signals a change in modal expression of source-area volcanism during the Cretaceous, and it probably records initial eruption of pyroxene-rich basalts of the mid-Cretaceous Douglas Island Volcanics. Supracrustal records preserved in the Nutzotin Mountains sequence, Dezadeash flysch, and Gravina sequence are also dominated by shallow-level arc detritus, although coeval supracrustal units on the Insular terrane of British Columbia reflect deeper incision of arc basement, possibly due to along-strike variation in arc geometries or tectonics of the Jura- Cretaceous Insular terrane.