JURASSIC-CRETACEOUS BASIN DEVELOPMENT AND STRUCTURAL CONFIGURATION, ALASKA RANGE SUTURE ZONE

The Upper Jurassic to Upper Cretaceous Kahiltna assemblage is located in the suture zone between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America. The Kahiltna assemblage is well exposed in the southern Alaska Range and northern Talkeetna Range. Much of the Kahiltna assemblage in the Talkeetna Range consists of upward-fining packages of siltstone and shale that we interpret as distal submarine fan deposits. Conglomerates and coarse sandstones in the Talkeetna Range are limited to outcrops adjacent to the Talkeetna fault. Clast composition from conglomerates and paleocurrent indicators in this area are interpreted to represent the unroofing of the Wrangellia composite terrane exposed in the hanging wall of the Talkeetna fault. Previous studies have interpreted the Talkeetna fault as a major terrane boundary. Our data, however, indicate that the Talkeetna fault represents a relatively thin-skinned thrust. The Kahiltna assemblage in the southern Alaska Range, in contrast to exposures in the northern Talkeetna Range, is characterized by boulder to cobble conglomerate, thick packages of coarse sandstone, and upward-fining packages of siltstone and shale. We interpret the Kahiltna assemblage of the Alaska Range to represent proximal submarine fan and marine shelf deposits. The composition of conglomerates and paleocurrent indicators in the Alaska Range are indicative of a Paleozoic sedimentary and igneous source terrane that may have been located inboard of the suture zone.

The Kahiltna assemblage of the Talkeetna Range is separated from the Kahiltna assemblage of the southern Alaska Range by a major topographic lineament known as Broad Pass. Compilation of previous mapping and geophysical studies indicates that Broad Pass consists of ophiolitic and mélange deposits underlain by serpentinite. These previous studies suggest to us that Broad Pass may represent a former subduction zone and may be a major terrane boundary. Collectively, our new data suggest that what has been mapped as the Kahiltna assemblage in south-central Alaska may represent two unrelated basins that have been juxtaposed within the "Broad Pass" subduction zone.