SEDIMENTOLOGIC AND GEOCHRONOLOGIC EVIDENCE FROM THE EOCENE TO OLIGOCENE KULTHIETH FORMATION FOR CENOZOIC TRANSPORT OF THE YAKUTAT TERRANE (Invited Presentation)

The Cenozoic translation history of the Yakutat terrane along the North American continental margin remains controversial. One hypothesis is that it originated as part of an Eocene spreading-ridge-centered oceanic plateau offshore of the Pacific Northwest. Part of this plateau accreted in present day Oregon and Washington and is known as Siletzia, whereas the conjugate pair was translated northward as the Yakutat terrane. Within this framework, we present new U-Pb zircon geochronology from strata exposed within the Yakutat terrane that demonstrate its basaltic basement formed coeval with Siletzia and sedimentologic data and detrital zircon geochronology that help constrain the location of its initial collision along the continental margin in British Columbia. These data allow us to better quantify the magnitude of margin-parallel translation that the Yakutat terrane experienced following initial collision with North America.

The Eocene—Oligocene Kulthieth Fm. represents the initial syn-collisional sedimentary sequence and unconformably overlies pre-accretion volcanic and sedimentary strata of the Yakutat terrane. Three lithofacies associations in the Kulthieth Fm. represent deposition in nearshore marine environments, tidally-influenced deltas, and terrestrial coastal plains. U-Pb detrital zircon geochronology from the Kulthieth Formation (N=10; n=2087) yields major peak age populations of 50-65 Ma and 75-100 Ma with minor 100-120 Ma peak ages.

These data are consistent with sediment derivation from sources near the Bella Coola and Terrace regions of the Coast Mountains Batholith. The Cretaceous—Paleocene accretionary prism strata of the Yakutat Group served as the backstop to this initial collision and were deformed, exhumed, and provided sediment to the lowermost Kulthieth Fm. This oldest part of the Kulthieth Fm. is informally referred to as the Marvitz Creek Unit and is separated from the Yakutat Group by a regional angular unconformity. The youngest zircon in a sandstone ~500 m above the regional unconformity are ~44.7 Ma, indicating the regional deltaic systems of the Kulthieth Fm. were well-established by this time. Subsequent translation of the Yakutat terrane from the Bella Coola-Terrace area allows for up to 1000 km of offset along major right-lateral strike-slip faults following deposition of the Kulthieth Formation.