PROVENANCE EVOLUTION OF THE COOK INLET BASIN IN SOUTH-CENTRAL ALASKA DURING LATE CENOZOIC TIME BASED ON MUDSTONE GEOCHEMISTRY

South-central Alaska has a complex Cenozoic geologic history making it difficult to determine the sediment sources for forearc basin strata in the Cook Inlet basin. The southern margin of Alaska has undergone two different styles of flat-slab subduction. Late Paleocene to Early Eocene subduction of a spreading ridge and Late Eocene to present subduction of an oceanic plateau. Substantial amounts of detrital zircon U-Pb ages and Hf isotope analyses have been conducted and were previously collected from Miocene-Pliocene strata and modern rivers. These data provide insight into the provenance evolution of the basins from Miocene to recent time. After spreading ridge subduction and before insertion of the oceanic plateau, fluvial systems brought sediment from both margins of the basin and deposited primarily sandstone-rich strata. During the late Miocene, subduction of the Yakutat microplate caused the axis of main subsidence to rotate north which resulted in fluvial systems that traversed the width of the basin and originated primarily from the eastern margin of the basin above the flat-slab region. Around 6 Ma an increase in volcanogenic grains in Pliocene sandstone-dominated strata indicates the reactivation of the volcanic arc system. The goal of this research is to use mudstone geochemistry to test this model and determine the provenance of the Late Oligocene-Middle Miocene Tyonek Formation, Late Miocene Beluga Formation and the Pliocene Sterling Formation. Eighteen samples from the Tyonek Formation, thirteen samples from the Beluga Formation and fourteen samples from the Sterling Formation were analyzed for trace and rare earth elements. The geochemical signatures for the ancient strata are compared to geochemical signatures for various igneous and sedimentary sediment sources. Initial analysis reveals that all the forearc strata contain higher proportions of light rare earth elements compared to heavy rare earth elements. The Beluga Formation and Sterling Formation have overall higher concentrations of light and heavy rare earth elements compared to the Tyonek Formation. In the future, principal component analyses will be conducted to test this evolution model. This research is ongoing, and the results are expected to enhance previous provenance interpretations and provide a more detailed provenance history for the Cook Inlet Basin.