PROVENANCE OF JURASSIC TO MIOCENE STRATA ON THE WESTERN ALASKA PENINSULA

Late Jurassic to Miocene strata along the southwestern Alaska Peninsula contain a record of deposition during and after Late Cretaceous accretion of the Wrangellia composite terrane, subsequent establishment of continental magmatism, and Late Paleocene spreading ridge subduction beneath southern Alaska. The detrital zircon signature of the Late Jurassic Naknek Formation, deposited during the accretion stage, contains two dominant age groups (159-164 Ma and 185-201 Ma) that are interpreted to represent sediment derivation solely from the adjacent Talkeetna arc. The Late Cretaceous Chignik Formation was deposited after accretion and has a similar Talkeetna arc signature (157-203 Ma), as well as Late Cretaceous (86-98 Ma), Late Triassic (219-228 Ma), and Precambrian (1143-1646 Ma) populations. These three populations may represent sediment input from mainland Alaska prior to the Late Cretaceous dextral displacement of the Wrangellia composite terrane. After Paleocene passage of a subducting spreading ridge, the detrital zircon signature of the Early-Middle Eocene Tolstoi Formation has Mesozoic age groups interpreted to be recycled from the older strata, as well Late Cretaceous peaks (65, 69, 88 Ma) that could indicate exhumation of the Cretaceous arc, and a subsidiary peak at 50 Ma that may represent a nascent stage of the modern Alaska Peninsula-Aleutians volcanic arc. The Oligocene Stepovak Formation contains similar age peaks as the Tolstoi Formation, but also has a younger Late Eocene peak (38 Ma) that is interpreted to represent sediment input from the Meshik volcanic arc. Two samples from the Miocene Bear Lake Formation have very different detrital zircon signatures. One sample has diverse and abundant age peaks ranging from Mississippian (342 Ma) to Early Miocene (22 Ma) and a small group of Precambrian grains, consistent with recycling from older strata as well as direct input from younger arc rocks. The other sample is dominated by two age groups composed of Jurassic and Late Cretaceous grains, respectively, that represent recycling of older strata and no input from Cenozoic arc rocks. Overall, the detrital zircon signatures of these strata record a long history of magmatism and sediment recycling associated with terrane accretion, continental magmatism, and spreading ridge subduction.