Sedimentary basin development within the Wrangellia composite terrane, Mesozoic Wrangell Mountains basin, southern Alaska: A long-term record of terrane migration and arc construction.

摘要

The Wrangell Mountains basin consists of ∼7,000 m of Upper Triassic-Upper Cretaceous sedimentary strata that depositionally overlie the allochthonous Wrangellia composite terrane in southern Alaska. The well-exposed sedimentary strata provide a valuable long-term record of tectonic processes and basin development associated with translation of the Wrangellia composite terrane from an intraoceanic setting at low paleolatitudes (∼12°N) to its present position along the continental margin of southern Alaska (∼61°N). New sedimentologic, geochronologic, compositional, and structural data document five stages of basin development: (1) Lithofacies associations indicate that Upper Triassic-Lower Jurassic sedimentation occurred on a northward-dipping carbonate platform that probably formed in response to thermal subsidence of an underlying volcanic platform. (2) Lithofacies associations, sandstone framework modes, and northward-directed paleocurrent indicators suggest that Middle to Late Jurassic sedimentation was largely a product of volcanic processes in a backarc basin located northward of a coeval intraoceanic arc. (3) Cross-cutting relationships, provenance data, and a new 40Ar/39Ar date (152.8 ± 1.1 Ma) from an granitic clast of an Upper Jurassic conglomerate demonstrate that Late Jurassic sedimentation occurred synchronous with thrust-fault deformation in a retroforeland basin. (4) Lithofacies distributions, southward-directed paleocurrent indicators, and southward-thickening of sedimentary strata across intrabasinal normal faults indicate that Early to Late Cretaceous deposition occurred by trenchward- (southward-) prograding marine deposystems in an extensional forearc basin. (5) The final stage of basin development was characterized by coarse-grained sedimentation (MacColl Ridge Formation) along the trenchward (southern) margin of a compressional forearc basin. New palynological data and Ar-Ar radiometric dates (79.4 ± 0.7 Ma and 77.9 ± 2.1 Ma) indicate that the MacColl Ridge Formation was deposited during the late middle to late Campanian. Sediment gravity flows deposited sediment on arcward- (northward-) prograding submarine fans. New palynologic and paleomagnetic data indicate that the Wrangellia composite terrane was positioned at northerly paleolatitudes (∼40°–65°N) during the Campanian (∼80 Ma). Regional integration of new geologic data from the Wrangell Mountains baste located along the outboard margin of the Wrangellia composite terrane, and the Nutzotin basin, located on the inboard margin of the Wrangellia composite terrace delineates distinct changes in the locations and tectonic setting of depocenters, the nature of intrabasinal structures, and the composition of detritus throw time. These variations are interpreted as recording discrete tectonic events that shy the northwestern Cordillera.