LATEST CRETACEOUS SEDIMENTARY RECORD OF FOREARC-ARC-RETROARC RELATIONSHIPS IN SOUTH-CENTRAL ALASKA

New stratigraphic and geochronologic data from Campanian-Maastrichtian strata in south-central Alaska document synorogenic sedimentation in forearc and retroarc basins related to exhumation of a continental arc. Campanian-Maastrichtian sedimentary deposits up to 3,000 m thick are exposed outboard (south) and inboard (north) of a >400-km-long belt of latest Cretaceous arc plutons and volcanic rocks. South of the arc, forearc deposition was dominated by sediment gravity flows on submarine fans in the Matanuska Valley (upper Matanuska Fm.) and Wrangell Mountains (MacColl Ridge Fm.). Crystal-vitric tuffs interbedded with these strata yield 77-79 Ma Ar-Ar ages. Matanuska Fm. sandstones yield detrital zircons with U-Pb ages of 70-86 (n=36), 95-185 (n=37), and 213-441 Ma (n=9). Matanuska Fm. conglomerates contain felsic plutonic clasts with U-Pb ages of 77.14±0.97, 79.2±1.1, 146.7±0.82, 151.8±1.2, 161.2± 2.3, and 173.9±0.98 Ma. Within the arc, newly recognized conglomerate >1,000 m thick is exposed in the northern Talkeetna Mountains near Pass Creek. These unnamed strata are interpreted as intra-arc or proximal retroarc basin deposits based on their proximity to coeval arc plutons. Sandstones from these strata yield detrital zircon ages of 75-89 (n=22), 95-192 (n=37), 197-343 (n=10), and 1181-2670 Ma (n=4). Collectively, the abundant 70-90 Ma zircon grains in sandstone, 77-79 Ma plutonic clasts, and 77-79 Ma tuffs document contemporaneous retroarc-forearc sedimentation, volcanism, and pluton exhumation. Detritus was also derived from remnant arcs based on diagnostic clasts, 146-174 Ma plutonic clasts, and 95-195 Ma detrital grains, which overlap the age range of Jurassic-Cretaceous arc rocks (Chisana and Talkeetna arcs). Grains >900 Ma record a sedimentary link to older continental margin source terranes (Yukon-Tanana terrane). Inboard of the arc and Pass Creek strata, retroarc sedimentation is recorded by >3000 m of Campanian-Maastrichtian alluvial-fluvial strata (lower Cantwell Fm.). Collectively, these stratigraphic relationships provide a regional sedimentary record of continental arc construction related to northward subduction of oceanic crust beneath southern Alaska.