Re-evaluation of pre-late wisconsin glacial deposits, lower Naknek River valley, Southwestern Alaska, U.S.A.

The lower Naknek River exposes thick (~20 m) successions of glaciogenic sediment of pre-late Wisconsin age on the northern Alaska Peninsula. The stratigraphy of the deposits, and the physical and mineralogical properties of diamicton beds, together with recently reported geochronological evidence, prompt a reevaluation of the pre-late Wisconsin glacial history of the region. Three sites expose diamicton beds that can be separated from overlying units on the basis of geochronologic or sedimentologic criteria. These older diamicton beds predate formation of a last-interglacial marine-lag gravel at South Naknek beach. Data on younger diamicton units lack stratigraphic or geographic trends that indicate distinct tills related to separate ice advances. We therefore suggest that the regionally extensive surficial drift (from Pauls Creek, at the outer margin of the Mak Hill moraine, to the mouth of Naknek River) was deposited during a single glaciation. Lumping the drift into one unit is consistent with geochronological evidence indicating that ice from the Alaska Peninsula reached its maximum limit relatively recently, since the last interglaciation. We speculate that Johnston Hill, a prominent landform within the limits of this drift sheet, was formed by ice thrusting of proglacial sediment during the late Pleistocene. Previously it was interpreted as a conventional moraine and ascribed to a separate, climatically significant and regionally correlative glaciation of pre-late Pleistocene age.