REEVALUATION OF PRE-LATE WISCONSIN GLACIAL DEPOSITS, LOWER NAKNEK RIVER VALLEY, SOUTHWESTERN ALASKA, USA

The lower Naknek River exposes thick (similar to 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 mine ralogical properties of diamicton beds, together with recently reporte d geochronological evidence, prompt a reevaluation of the pre-late Wis consin glacial history of the region. Three sites expose diamicton bed s that can be separated from overlying units on the basis of geochrono logic or sedimentologic criteria. These older diamicton beds predate f ormation of a last-interglacial marine-lag gravel at South Naknek benc h. Data on younger diamicton units lack stratigraphic or geographic tr ends 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 mout h of Naknek River) was deposited during a single glaciation. Lumping t he drift into one unit is consistent with geochronological evidence in dicating 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 s heet, was formed by ice thrusting of proglacial sediment during the la te Pleistocene. Previously it was interpreted as a conventional morain e and ascribed to a separate, climatically significant and regionally correlative glaciation of pre-late Pleistocene age.