MAJOR MIOCENE PALEODRAINAGE IN SOUTH-CENTRAL ALASKA: SEDIMENTOLOGY, DEPOSITIONAL AGE, AND PROVENANCE OF STRATA EXPOSED IN THE SOUTHWESTERN TALKEETNA MOUNTAINS

Miocene sedimentary strata exposed in the southwestern Talkeetna Mountains provide insight on regional paleodrainage and paleotopography along the southern Alaska convergent margin. Lithofacies mapping along the Little Susitna River document an upward fining succession of siliciclastic strata within a footwall syncline south of the Castle Mountain oblique strike-slip fault. Lower strata consist of several hundred meters of lenticular clast-supported pebble-conglomerate and cross-stratified lithic sandstone with plant fragments. Overlying these strata are carbonaceous mudstone and cross-stratified lithic sandstone with abundant organic debris. The strata record an upsection shift from gravelly to sandy braided channel-bar complexes to muddy channel-point bar-floodplain environments. Paleocurrent measurements record southwestward migration of channel-bar complexes. Diverse pollen and spores indicate deposition of upper strata during late Miocene time (<12 Ma), consistent with the youngest population of U-Pb detrital zircon ages (~27-22 Ma) from sandstones spanning the section. Sandstone samples yield a broad distribution of U-Pb detrital zircon ages (n=508) and ⁴⁰Ar/³⁹Ar detrital muscovite ages (n=155), indicative of bedrock sources located >350 km northeast of the study area. The youngest population of detrital zircons and volcanic clasts (~23 Ma) match the oldest phases of the Wrangell volcanic arc along the north flank of the Wrangell-St. Elias Range south of the Denali fault. A broad distribution of Jurassic-Cretaceous detrital zircon and muscovite ages, including abundant early-middle Cretaceous (140-90 Ma) ages, and sparse Devonian-Mississippian and Precambrian ages reflect erosion of plutons and metamorphic rocks in the eastern Alaska Range and Yukon-Tanana Uplands north of the Denali fault. Proximal sources in the Talkeetna Mountains may have contributed Jurassic and Late Cretaceous ages. Collectively, our datasets indicate a major trunk river transported detritus southwestward across interior Alaska during Miocene time, eroding topography northeast of the actively colliding Yakutat terrane. This drainage system was likely reorganized and ultimately reversed by latest Miocene shortening along the Castle Mountain and Denali faults and expansion of the Wrangell arc.