NEW GEOCHRONOLOGIC AND SEDIMENTOLOGICAL CONSTRAINTS ON THE PALEOGENE RECORD OF FOREARC BASIN DEVELOPMENT, MATANUSKA VALLEY-TALKEETNA MOUNTAINS, SOUTHERN ALASKA

Paleogene sedimentary and volcanic strata in the Talkeetna Mountains and Matanuska Valley of southern Alaska record forearc basin development in the northern North American Cordillera. We combine new and published geochronologic, compositional, and sedimentologic data to provide an improved chronostratigraphic framework and reconstruct the evolution of depositional environments and sediment routing. Along the arcward (northern) basin margin, strata rich in plutonic/volcanic lithics and ca. 75–57 Ma detrital zircon age peaks record arc erosion and deposition in transverse fluvial-lacustrine environments. Sedimentation spanned ca. 60–52 Ma judging from palynology and detrital zircon U-Pb maximum depositional ages from previously undated strata in the eastern Talkeetna Mountains (Nowhere Ck. Fm.) and lava/tuff ages in the southern Talkeetna Mountains (Arkose Ridge Fm.). Basin axis strata in the Matanuska Valley (Wishbone and northern Chickaloon Fms.) rich in plutonic/volcanic lithics and ca. 72–58 Ma detrital zircon age peaks reflect transport chiefly from arc sources, minor input from the accretionary prism, and deposition in diverse fluvial-lacustrine depositional environments. New zircon U-Pb ages of northern Chickaloon Fm. tuffs (~56.7 Ma, ~55.0 Ma, ~54.3 Ma) and detrital zircon U-Pb maximum depositional ages indicate ca. 57 to <54 Ma sedimentation. Strata along the trenchward (southern) basin margin (southern Chickaloon Fm.) contain abundant metasedimentary lithics and ca. 170–190 Ma detrital zircon age peaks that reflect accretionary prism erosion and deposition in transverse alluvial-fluvial drainages. Trenchward margin sandstones and bentonites yield ca. 55–54 Ma zircon U–Pb maximum depositional ages. Overlying the fluvial strata along an angular unconformity are ca. 49–36 Ma lavas attributed to slab-window magmatism. In summary, the Paleogene stratigraphy records alluvial-fluvial-lacustrine deposition ca. 60–52 Ma, intra-basinal deformation/erosion ca. 52–49 Ma, and slab-window magmatism ca. 49–36 Ma. Ongoing mapping of fluvial lithofacies changes across the Castle Mountain fault supports Pavlis and Roeske’s (2007) proposal that >130 km of dextral separation was transferred from the Border Ranges fault to the Castle Mountain-Capps Glacier fault system during Cenozoic time.