Miocene−Pliocene Nenana Gravel deposition in response to tectonic denudation of the Alaska Range (USA)

Disentangling the effects of tectonics and Pliocene−Pleistocene climate forcing on rates of erosion remains key to understanding hypothesized feedbacks among these processes. In central Alaska (United States), deposition of the Nenana Gravel has long been attributed to foreland basin sedimentation coupled to tectonically controlled Miocene−Pliocene Alaska Range exhumation. However, recent cosmogenic burial dating of the lower Nenana Gravel implies that deposition did not begin until ca. 2.3 Ma, consistent with Alaska Range erosion driven by concurrent intensified glaciation during Pleistocene global climate changes. Our new 3.67 ± 0.75 Ma cosmogenic 26Al/10Be isochron burial age at the top of the Nenana Gravel exposed at the Suntrana section indicates that deposition concluded ∼1 m.y. before the previously reported onset. We show that the prior data do not support early Pleistocene burial, but instead yield a 5.5 ± 1.5 Ma simple 26Al/10Be burial age of initial Nenana Gravel deposition, consistent with tephrochronologically constrained 6.7 ± 0.1 Ma strata that conformably underlie the gravel. Deposition of the ∼800-m-thick Nenana Gravel exposed at the Suntrana section thus started between 6.7 ± 0.1 Ma and 5.5 ± 1.5 Ma and concluded by 3.67 ± 0.75 Ma, concurrent with increased Alaska Range exhumation rates associated with tectonic forcing at ca. 7−6 Ma. The Nenana Gravel therefore records Alaska Range denudation induced by Miocene−Pliocene tectonics, and not early Pleistocene climate.