HEAVY MINERAL PROVENANCE AND STRUCTURAL HISTORY OF THREE LATE JURASSIC TO CRETACEOUS SEDIMENTARY BASINS IN THE ALASKA RANGE SUTURE ZONE, SOUTH-CENTRAL ALASKA

Late Jurassic-Cretaceous sedimentary basins record the upper Mesozoic accretion of the Wrangellia composite terrane with inboard terranes associated with the North American continent. Of these basins, found in a discontinuous belt along the inboard margin of the composite terrane, the Alaska Range suture zone in south-central Alaska preserves the most complete record of accretion. Here, Late Jurassic to mid-Cretaceous marine sedimentary rocks of the Kahiltna assemblage are comparable to rocks in the Gravina-Nutzotin Basins in southeast Alaska, however, the Late Cretaceous nonmarine sedimentary rocks that unconformably overlie the Kahiltna assemblage are not preserved in the southeast.

Recent stratigraphic and detrital zircon data document changing sediment provenance in basins associated with the suture zone during accretion. Framework component data indicate that the southernmost suture zone rocks, proximal to the Wrangellia composite terrane, are arc-derived, while rocks further north have a significant continental signature. Existing data sets, however, do not necessarily provide unique provenance correlations with source rocks inboard or outboard of the suture zone. To complement existing data and address discrepancies in models of composite terrane accretion and the relationship between the basins, we investigate heavy mineral provenance and the structural history of three fault-bound Late Jurassic-Cretaceous sedimentary basins in the Alaska Range suture zone.

The heavy mineral assemblages from sedimentary sections in two basins separated by the modern Denali fault system have similar trends, showing a strong influence of mafic-ultramafic source rocks (indicated by spikes in detrital serpentine and clinopyroxene) near the marine-nonmarine transition. Source rocks for these samples, and samples containing both clino- and orthopyroxene have not been found in the immediate vicinity, however, one possible source is a Cretaceous gabbro containing two pyroxenes that is located in the eastern Alaska Range. We have also identified a glaucophane grain, though the source of this grain is uncertain, as no known subduction complex is currently exposed. We will also present preliminary results from the third sedimentary basin, located between the Broad Pass fault zone and the Denali fault system.