SYNOROGENIC SEDIMENTATION ALONG THE ARCWARD MARGIN OF A MESOZOIC FOREARC BASIN, NAKNEK FORMATION, TALKEETNA MOUNTAINS, ALASKA

The 1400-m-thick Upper Jurassic Naknek Formation provides an excellent opportunity to evaluate arc-proximal sedimentation within an ancient forearc basin. The Matanuska Valley-Talkeetna Mountains basin formed within the allochthonous Wrangellia composite terrane (WCT) coeval with northeastward subduction beneath the terrane and accretion of the terrane to the western margin of North America. New measured stratigraphic sections, paleocurrent data, and petrofacies data document the sedimentology, deposystems, and provenance of the well-exposed arcward margin of the basin. Northernmost strata depositionally onlap remnant volcanic arcs and include poorly sorted, boulder conglomerates interpreted as debris flow deposits on fan-deltas. Proximal conglomerates merge basinward into mudstone and normally graded sandstone interpreted as turbidity flow deposits on a steep submarine slope. Slope deposits are overlain by horizontally stratified sandstone and coquina interpreted as shelf/shoreface deposits that infilled the basin following rapid initial subsidence. Paleocurrent data (n=190) indicate southeastward to southwestward sediment transport and conglomerate clast counts document >95% volcanic, metavolcanic, and plutonic clasts. Collectively, these data indicate uplift and erosion of the volcanic edifice and plutonic roots of remnant Early to Middle Jurassic arcs coeval with construction of a contemporaneous Late Jurassic arc inboard (northward) of the remnant arcs. Arc uplift and coarse-grained forearc basin sedimentation was synchronous with retroarc contractile deformation and synorogenic sedimentation inboard (northward) of the arcs and forearc basin (Kahiltna, Nutzotin, and Wrangell Mountains basins). Regional orogenesis is interpreted as recording initial collision of the WCT with the western margin of North America.