Late Wisconsin Glacial History of the Northern Alaska Peninsula, Southwestern Alaska, U.S.A

The late Wisconsin Brooks Lake glaciation of the Alaska Peninsula includes five previously recognized stades, indicating considerable climate variability during the last glaciation. During the early phase of the Brooks Lake glaciation, an outlet lobe of the Cordilleran Ice Sheet advanced westward across a low topographic divide into the Iliamna valley where it merged with ice from local mountain sources. During the earliest (Kvichak) stade, which commenced sometime after 26,150 ±300 IJC yr BP, ice filled the Iliamna valley and deposited the type Kvichak moraine. Following retreat into the Iliamna Lake basin, the outlet glacier readvanced to the Iliamna-stade moraine. Subsequently, and sometime prior to 12,600 14C yr BP, ice retreated from Iliamna valley. This two-fold sequence of moraines deposited during the early phase of the Brooks Lake glaciation is not present in the Naknek valley, 80 km south of Iliamna valley, where high mountains diverted the ice sheet while supporting a confluent system of local-mountain glaciers. These glaciers deposited the moraine enclosing Naknek Lake, which marks the outer limit of late Wisconsin ice in the valley. Apparently, differing ice sources and glacier configurations caused the two glacial systems to respond differently to climate change or glacier-bed dynamics, or to be influenced by different climate changes. During the late phase of the Brooks Lake glaciation, valley glaciers derived from local-mountain sources terminated well behind the mountain fronts and deposited the Newhalen, Iliuk, and Ukak moraines. Equilibrium-line altitude (ELA) over the Naknek valley was lowered 250-650 m below the present during the Iliuk stade. Full-glacial ELA was probably just a few tens of meters lower, but generated extensive advance over broad, relatively flat troughs. Slope angles measured on the Kvichak and Iliamna moraines, and the correlative Naknek moraine are less steep (-11-15°) than those of younger moraines (-18-20°), suggesting that a substantial length of time separated the Iliamna and Newhalen stades. This interstadial may correlate with an interval of restricted ice extent separating early and late phases of the late Wisconsin recognized elsewhere in Alaska around 14,000 yr BP.