A STRUCTURAL AND HEAVY MINERAL PROVENANCE STUDY OF LATE JURASSIC TO CRETACEOUS SEDIMENTARY SECTIONS IN THE ALASKA RANGE SUTURE ZONE, SOUTH-CENTRAL ALASKA: PRELIMINARY RESULTS

The late Jurassic to mid-Cretaceous Kahiltna assemblage and overlying Late Cretaceous nonmarine rocks in the Alaska Range suture zone record the upper Mesozoic accretion of a major island-arc assemblage, the Wrangellia composite terrane, with inboard terranes with a continental affinity. The Gravina-Nutzotin Basins in southeast Alaska contain rocks comparable to the Kahiltna but do not have a Late Cretaceous equivalent. Recent stratigraphic and detrital zircon data from the suture zone document the changing sources of the basins during accretion. To complement existing data sets and address discrepancies in estimates of the timing of accretion, magnitude of post-accretion displacement, and relationship between basins now separated by major fault zones, we investigate the structural history and heavy mineral provenance of Late Jurassic-Cretaceous sedimentary sections in the Alaska Range suture zone. We will present preliminary results from heavy mineral and metamorphic fabric analyses.

Detrital zircon age clusters do not necessarily provide unique provenance correlations as sources of the appropriate age may be found inboard or outboard of the suture. Point count data improve provenance correlations indicating that the southernmost suture zone rocks in the Talkeetna Mountains, proximal to the Wrangellia composite terrane, are clearly arc-derived, while rocks further north have a significant continental signature. The heavy mineral assemblage makes clear ties to igneous or metamorphic source rocks and may be used to more specifically link sediment provenance. We compare heavy mineral assemblages in the northwest Talkeetna Mountains to those in the Cantwell Basin, north of the modern Denali fault, to constrain the relationship between these basins.

Field observations in the northwest Talkeetna Mountains suggest that the contact between Late Jurassic to mid-Cretaceous marine rocks of the Kahiltna assemblage and Late Cretaceous nonmarine sedimentary rocks may be a profound angular unconformity or a local disconformity. Both sections contain mudstones and fine sandstones, thus the boundary between these two sections can be difficult to identify in the field. To constrain the significance of this contact, we compare metamorphic fabric development and heavy mineral assemblages across the boundary.