INSIGHTS ON PROVENANCE OF FOREARC BASIN STRATA: U-PB GEOCHRONOLOGY FROM CENOZOIC STRATA IN THE COOK INLET BASIN, ALASKA

Most provenance studies of forearc basins assume that the chief sources of sediment were from the nearby magmatic arc or accretionary prism. New detrital zircon U-Pb geochronologic data from Cenozoic strata within the Cook Inlet forearc basin point to the importance of far inboard (>300 km) sediment sources. One major sediment conduit was located on the western margin of the basin and drained source terranes to the north, mainly the central Alaska Range and Yukon-Tanana terrane. Another key conduit was situated along the eastern margin of the basin and derived sediment from along strike to the east, including accreted Mesozoic terranes and distal parts of the accretionary prism.

The detrital record from the western margin shows an upsection decrease in Precambrian and Paleozoic grains interpreted to be related to surface exhumation of the central Alaska Range >225 km to the north. A Middle Eocene sample yields 37% Precambrian and 16% Paleozoic grains, inferred to be derived from the Yukon-Tanana terrane located north of the present-day central Alaska Range. Mesozoic (36%) and Cenozoic (11%) grains are interpreted to be derived from the Alaska Range plutons that stretch from the western margin of the basin north to the central Alaska Range. A late Miocene sample yields only 10% Precambrian and 5% Paleozoic grains, with larger populations of Mesozoic (55%) and Cenozoic (30%) grains. A modern sample from the Susitna River contains only 4% Precambrian and 1% Paleozoic grains, whereas Mesozoic and Cenozoic grains account for 35% and 60%. This upsection trend is interpreted to be a result of the generation of topography in the central Alaska Range region during late Miocene time, which created barriers that gradually sequestered sources farther to the north.

The detrital record from the east side of the basin contains distinct populations compared to the west. Eastern samples also generally show unvarying populations from middle Miocene through Early Pliocene time. Each sample consists of 1-3% Precambrian, 3-4% Paleozoic, 58-69% Mesozoic, and 25-35% Cenozoic grains.

In summary, detrital geochronologic data from the Cook Inlet basin shows that in addition to the adjacent magmatic arc and accretionary prism, principal sources of sediment in forearc systems may be located far inboard from the basin or along strike within the basin.