DETRITAL ZIRCON AGES FROM THE VALDEZ GROUP INDICATE RAPID LATEST CRETACEOUS DEPOSITION IN THE CHUGACH ACCRETIONARY COMPLEX, SOUTHERN ALASKA

Four volcanic litharenite (greywacke) samples from the Valdez Group were dated in a study of the Late Cretaceous-Paleocene flysch facies of the Chugach terrane accretionary complex. Petrography supports previous interpretation of an arc provenance for these marine turbidite deposits. U-Pb detrital zircon ages were obtained using LA-MC-ICPMS; total number of analyses per sample ranged from 50 to 100. Three samples were collected on a ~50 km transect of the Valdez Group along Turnagain Arm southeast of Anchorage, Alaska. AnJ06 has a youngest population of three grains with an age of 67±3 Ma (2s), a dominant Late Cretaceous population, a very minor Triassic-Jurassic population, two Paleozoic and two Proterzoic grains. AnJ13 has a population of six grains with an age of 68±2 Ma, a dominant Late Cretaceous population, a significant Triassic-Jurassic population, two Paleozoic, and three Proterozoic grains. Ked24 has a population of seven grains with an age of 68±2 Ma, a dominant Late Cretaceous population, a significant Triassic-Jurassic population, eight Paleozoic and nine Proterozoic grains. AnJ40 was collected from near Mt. Magnificent which is mapped as part of the Valdez Group ~30 km northeast of Anchorage and has an older population of seven grains with an age of 82±3 Ma, a large (30% of grains) Late Cretaceous population but a more significant (46% of grains) Triassic-Jurassic population, one Paleozoic, and nine Proterozoic grains. We propose that based on petrology, and the detrital zircon ages, the Coast orogen in British Columbia was the main source of Late Cretaceous zircons in the Valdez Group flysch. These similar youngest ages constrain the maximum depositional age on the ~50 km transect could indicate rapid, large volume deposition. These data indicate a gap of 15-20 m.y. between the McHugh complex mélange facies and the Valdez Group may indicate a period of tectonic erosion between accretion of the McHugh Complex and the Valdez Group.