DETRITAL ZIRCON GEOCHRONOLOGY OF THE UPPER JURASSIC-CRETACEOUS KAHILTNA ASSEMBLAGE IN THE CLEARWATER MOUNTAINS, SOUTH-CENTRAL ALASKA

The southern margin of Alaska was formed by the accretion of both major and minor terranes along convergent boundaries with the North American continent. The stratigraphic record of terrane accretion is preserved in the Upper Jurassic-Cretaceous Kahiltna assemblage that is exposed inboard of the allochthonous Wrangellia composite terrane in south-central Alaska. A detrital zircon study was conducted on 5 samples from the Kahiltna assemblage in the Clearwater Mountains, located just north of the Talkeetna fault, a structure that has been described as the southernmost boundary of the suture zone between the Wrangellia composite terrane and the North American continent.

LA-ICPMS analysis of detrital zircons from 5 sedimentary samples (n=526 grains) was performed at the Arizona Laserchron Center. Four medium-fine grained sandstone samples contained exclusively Mesozoic grains with strong single peaks at 150-155 Ma. Maximum depositional ages calculated for these samples indicate that the unit was deposited after 133-141 Ma. One cobble conglomerate sample contained Mesozoic (n=76 grains), Paleozoic (n=15 grains), and Precambrian (n=16 grains) zircons. The youngest zircons in this sample indicate deposition after 70-80 Ma.

The presence of exclusively Mesozoic age zircons in the Kahiltna assemblage suggests that the sediment was derived from source areas outboard of the continental margin with no sedimentary input derived from the older, more inboard Yukon-Tanana. The detrital zircon profiles are strikingly different from the profiles of younger sediments elsewhere in the Kahiltna basin, suggesting that the source area was changing with time. The sandstone samples dated are volcanic-lithic rich, but no volcanic units of this age are found immediately adjacent to the Clearwater Mountains, suggesting translation of an oceanic arc basin along the margin subsequent to deposition. In contrast, the conglomerate has both local (Wrangellia) sources and continental sources, indicating proximal basin development in an area with active uplift. A profound structural unconformity between the two units also indicates most of the deformation of the Kahiltna happened prior to deposition of the conglomerate.