NEW INSIGHTS INTO THE ORIGIN AND DISPLACEMENT HISTORY OF THE YAKUTAT TERRANE, SOUTHEASTERN ALASKA

The Paleogene tectonic setting and stratigraphic history of the Yakutat terrane remains one of the unsolved geologic problems of the northern Cordilleran margin. Important questions include the original location of the Yakutat terrane as well as the amount of northward translation along margin bounding transform faults. New stratigraphic analysis, U-Pb detrital zircon data, and palynological analysis from the Eocene Kulthieth Formation of the Yakutat terrane provide insight into these questions. We chose the Kulthieth Formation for such a study because it was deposited prior to northward transport. Detailed measured stratigraphic sections show an east to west change in the Kulthieth Formation from coarse-medium grained cross-stratified sandstones and conglomerates to abundant carbonaceous mudstones, coal and intensely bioturbated sandstones. We interpret these lithologic changes to represent a change from proximal deltaic environments in the east to more distal deltaic environments in the west. U-Pb detrital zircon data from four samples from the Kulthieth Formation (n = 386) show a dominance of peaks ranging from 50-160 Ma. Most of these detrital zircon ages can be matched with Mesozoic and early Cenozoic plutonic sources that are common along the entire northwestern continental margin. The most provenance diagnostic zircon grains found in the Kulthieth Formation have Mesoproterozoic and Paleoproterozoic ages with peaks at 1350-1470 Ma and 1640-1770 Ma. These grains are interpreted to have been recycled from older strata and\or derived directly from the Belt Supergroup in southern British Columbia. Utilizing previous geologic studies and our U-Pb detrital zircon data, we interpret deposition of the Kulthieth Formation as having occurred most likely north of Vancouver Island during late Eocene time in a forearc or transtensional basinal setting. This interpretation is also supported by our preliminary detrital zircon data from younger units of the Yakutat terrane that have detrital zircon populations consistent with derivation from more northerly sources.