STRATIGRAPHIC AND PROVENANCE RECORD OF NEOGENE FORELAND BASIN DEVELOMENT, TANANA BASIN, CENTRAL ALASKA RANGE

RIDGWAY, Kenneth D., Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, [email protected], THOMS, Evan E., U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, LAYER, Paul W., Geophysical Institute, Univ of Alaska, Natural Sciences Building, 900 Yukon Drive, P.O. Box 755780, Fairbanks, AK 99775-5780, and WHITE, James M., Geological Survey of Canada, 3303 - 33 Street North West, Room 2203, Calgary, AB T2L 2A7, Canada

Neogene strata of the Tanana basin provide a long term record of a northward-propagating foreland basin system related to regional shortening of the central Alaska Range. These strata form an ~ 2 km thick stratigraphic package that has been deformed and uplifted in thrust faults that form the foothills on the north side of the Alaska Range. The lower part of the sedimentary package, the Usibelli Group, consists of 800 m of mainly Miocene strata that were deposited in fluvial, lacustrine, and peat bog environments of the foredeep depozone of the foreland basin system. Compositional data from conglomerate and sandstone, as well as recycled Upper Cretaceous palynomorphs, indicate that the Miocene foreland basin system was supplied increasing amounts of sediment from lithologies currently exposed in thrust sheets located south of the basin. The upper part of the sedimentary package, the Nenana Gravel, consists of 1200 m of mainly Pliocene strata that were deposited in alluvial-fan and braidplain environments in the wedge-top depozone of the foreland basin system. Compositional data from conglomerate and sandstone, as well as Ar-Ar dating of detrital feldspars in sandstone and from granitic clasts in conglomerate, indicate that northward propagating thrust sheets provided detritus to the Pliocene foreland basin system. Ar-Ar dates from detrital grains in the Nenana Gravel also show that deeper levels of plutons were progressively exposed during deposition of the Nenana Gravel. Extensive Pleistocene glaciation and ongoing deformation reorganized the modern fluvial systems of the wedge-top depozone; these rivers now incise through deformed strata of the Nenana Gravel and are transporting detritus from the core of the central Alaska Range, as well as from uplifted strata of the Neogene foreland basin, northward into an actively deforming foreland basin.