DETERMINING TIMING OF UPLIFT AND DRAINAGE REVERSAL IN THE ALASKA RANGE THROUGH BURIAL DATING

The Alaska Range is a transpressional orogen with modern exhumation initiating ~6 Ma. The stratigraphic record of exhumation, largely preserved along the northern flank of the Alaska Range, suggests that early growth of the Alaska Range led to a reversal of drainage direction for the region. Past work indicates that the Oligocene- and Miocene-aged Usibelli Group, a coal-bearing fluvial and lacustrine sequence, exhibits paleoflow indicators of generally southwestward flowing streams, while the overlying Pliocene-aged Nenana Gravel, an alluvial fan and braidplain deposit, contains northward paleoflow markers. Chronometric control on the Nenana Gravel is largely lacking, with the limited available age control based on underlying units and stratigraphic correlations. We present a suite of ²⁶Al/¹⁰Be cosmogenic nuclide burial ages collected from the lower, middle, and upper sections of the Nenana Gravel. Early results from the Usibelli Group-Nenana Gravel contact suggest drainage reversal occurred ~6.2 Ma. Multiple samples collected from the middle of the Nenana Gravel unit were dated to 2.3 Ma, with a single sample stratigraphically above dating to ~1.2 Ma. Samples have been collected from the upper-most portion of the Nenana Gravel to provide a baseline estimate of foreland basin uplift.