Facies architecture, petrofacies, and ⁴⁰Ar/³⁹Ar analyses of sediments from key measured sections in the Nenana Gravel reveal a history of uplift and erosion of the central Alaska Range during the first half of the late Cenozoic orogeny. Nenana Gravel deposits within the Cantwell trough are very coarse, contain unconformities marked by thin coal beds, and were deposited within drainages that either were tectonically disrupted and became stagnant or became antecedent to subsequent structures. These sections were deposited in a wedge-top depozone on top of the deforming orogenic wedge. Sections just north of the Mt. Healy Anticline (our term for the regional anticlinal upland of schist forming the northern margin of the Cantwell trough) characterize deposition in a foredeep to wedge-top transitional depozone. Gravel aggradation began here shortly after deposition of the Grubstake tephra, recently re-dated at 6.4 ± 0.6 Ma, in the form of a distal to proximal transitional alluvial braidplain dominated by sandy and gravelly stream-flow deposits. This timing is in close agreement with estimates on the onset of rapid uplift and denudation of the McKinley pluton provided independently by fission track thermochronology (Fitzgerald et al., 1995). Two coarsening upward megasequences are next. Each records a transition from stream-flow to sheetflood deposits. We interpret this facies transition as indicative of increasing sedimentation rate, probably due to the tectonically induced stripping of unconsolidated Nenana Gravel and Usibelli Group deposits upstream.

Statistical analysis of a pebble count dataset collected by Clyde Wahrhaftig nearly 50 years ago reveals successive erosion of rocks from three south to north source areas. They are 1), rocks south of the Denali fault (DF) and the upper Cantwell Formation, 2), the lower Cantwell Formation and terranes within the Cantwell trough uplifted along steep re-activated thrust faults, and 3), Nenana Gravel and Usibelli Group deposits in the Cantwell trough and schist of the Mt. Healy Anticline. Pebble counts from distal sections also indicate that growing schist uplands emergent in the mid to late Nenana Gravel time diverted braidplain deposits from the south and were themselves a local source of alluvial deposits. We take this as evidence for the basinward progression of the underlying orogenic wedge. ⁴⁰Ar/³⁹Ar analysis of potassium feldspar grains from sands and granitic clasts collected from the Suntrana Creek section independently supports this model by indicating that granite plutons south of the DF were eroded early in the orogeny and that plutons north of the DF were not eroded until during stage 2.

Nenana Gravel deposition ended shortly after the emergence of the Healy Anticline which served as a barrier to north flowing streams and marked the beginning of the inversion of the Nenana Basin. This time is constrained by the intrusion of Jumbo Dome at 2.79 ± 0.3 Ma, which deformed the post-Nenana Gravel surface. Very poorly exposed Late Pliocene glacial deposits apparently mantle the Nenana Gravel, but we examined no outcrops that exhibit unequivocal evidence for glaciogenic sediments within the formation.