MIOCENE AND PLIOCENE SEDIMENTARY FOOTPRINT OF FLAT-SLAB SUBDUCTION OF THE YAKUTAT TERRANE, SOUTHERN ALASKA

The Middle Miocene to Pliocene sedimentary record in southern Alaska is characterized by extensive packages of coarse-grained strata and thick coal seams. These sedimentary packages require two conditions: adequate accommodation (regional subsidence) and high sediment supply. We propose that during Middle Miocene time, regional basin subsidence was at a maximum around the perimeter of an area of crustal exhumation driven by flat-slab subduction of the Yakutat terrane beneath south-central Alaska. This exhumed area is ~160,000 square km and is represented by the modern day Chugach, Wrangell-St. Elias, and Talkeetna Mountains and the central and eastern Alaska Range. The northernmost basin is the Tanana basin, located north of the central Alaska Range. The Usibelli Group and Nenana Gravel in this basin consist of ~2 km of coal-rich nonmarine sandstone and conglomerate that are mostly Middle Miocene through Pliocene in age. To the southwest, the Bristol Bay basin of the Alaska Peninsula contains ~3.3 km of marginal marine strata, the Miocene Bear Lake and Pliocene Milky River formations. New biostratigraphic and sedimentological analysis of the Bear Lake Formation document a change from fluvial and paludal environments to intertidal and subtidal deposits within a regional estuarine system. The Cook Inlet basin in south-central Alaska contains ~5.2 km of Middle Miocene to Pliocene nonmarine strata, the Beluga and Sterling formations. These formations contain abundant and thick coal seams indicative of substantial amounts of subsidence during Neogene time. In the Yakutat basin of southeastern Alaska, the Yakataga Formation consists of up to ~5 km of Middle Miocene and younger glacial marine strata. Newly published geophysical data suggest that the crust subducted in front of the continental Yakutat terrane is up to 15-20 km thick, and is composed of buoyant yet subductable oceanic crust possibly representing an oceanic plateau. Other geophysical studies have recognized a region of high topography on the upper plate that approximates the area of the subducting Yakutat slab. Our sedimentary data suggest that by 10 Ma, flat-slab subduction of this more buoyant oceanic crust may have initiated regional basin subsidence and also provided substantial sediment sources that contributed to filling these basins.