The upper Cenozoic White River "tillites" of southern Alaska: Subaerial slope and fan-delta deposits in a strike-slip setting

Abstract

The White River tillites crop out over 8 km 2 either side of the Totschunda fault in the northern foothills of the Wrangell Mountains in southern Alaska and record glacially influenced slope and fan-delta deposition in a high-relief basin close to an active volcano between 10 and 2.7 Ma. Two measured sections, 1,200 and 450 m thick, show diamictite and conglomerates interbedded with augite andesite lavas. Facies range from massive diamictites, deposited subaerially as debris flows, to crudely stratified sandstones and conglomerates formed by sheet-flow processes on alluvial fans, and well-stratified and normally graded sandstones and conglomerates that accumulated subaqueously as turbidites; large-scale southward-dipping foresets as much as 17 m high identify a fan delta that prograded into a temporary lake. The White River succession is identified as a valley infill complex consisting of subaerial slope and lacustrine fan-delta deposits; there are no in situ primary deposits present in the succession. The component of sedimentation was limited to the supply of coarse-grained debris identified by the presence of glacially striated and faceted clasts. The White River deposits may be typical of glacial sequences preserved as isolated local basin fills in high-relief areas such as occur in convergent or strike-slip plate-margin settings. They provide only a limited and fragmentary record of paleoenvironmental changes compared to coeval glacially influenced marine rocks preserved in the Yakataga Formation (5 km thick) along the Gulf of Alaska coast.