GEOLOGIC MAP OF BROXSON GULCH, EASTERN ALASKA RANGE, ALASKA

We present new geologic mapping and geochronology from Broxson Gulch, an ~100 km² area in the Mount Hayes B5 1:63,360 quadrangle at the southern margin of the eastern Alaska Range, Alaska. In addition to a new 1:24,000 scale geologic map, we provide two structural cross sections, detrital zircon U/Pb age spectra from eight samples, and one new ⁴⁰Ar/³⁹Ar whole rock age from the map area.

The correlation of metamorphic units, location of major faults, and interpretation of structural style presented here differ significantly from published bedrock geologic maps of the area. While previous mappers emphasize textural features to distinguish metamorphic units in the field area, we rely primarily on the composition and age of protoliths. Protolith ages for the metasedimentary units are based on U/Pb detrital zircon data, which provide a maximum depositional age. Our mapping and re-interpretation of structural style in the area implies large scale shortening accommodated by Oligocene S-vergent nappe-style structures, which are overprinted by later reverse faults with less slip. Large scale shortening had previously been proposed in the area, but the key structures remained enigmatic. The new map, cross sections, and geochronology illuminate key structures responsible for Cenozoic shortening in the map area.

The Quaternary geology in the field area records multiple phases of glaciation. A paleolandscape containing a pro-glacial braided alluvial system is preserved as perched terrace deposits. Sufficiently large terrace gravel deposits contain terrace risers and paleochannels belonging to the paleolandscape. The modern glacial landscape and related deposits are inset into the older braided alluvial deposits. The difference in elevation between the two suites of glacial deposits requires a significant drop in local base level, likely in the late Pleistocene. Though not recorded in the field area, regional correlation of paleoshorelines and glacial deposits would suggest that draining of glacial Lake Atna in the late Pleistocene is a mechanism to drop base level between glaciation events. Landslide and colluvial deposits onlap glacial deposits and record active erosion of the steep glacial landscape within the map area. Geomorphic evidence precludes Quaternary surface rupture of any faults in the map area.