Abstract

Glacial deposits and related sediments in moraines and in bluff exposures in upper Cook Inlet basin are mainly of Pleistocene age. They are interpreted as the products of seven glacioevents that are defined as single or multiple glacial events recorded by a substantial body of evidence. Each glacioevent may be equivalent to one, part of one, or more than one glaciation.

The two oldest glacioevents are represented by erratics on Mount Susitna and some other high peaks, and by the Mount Magnificent ground moraine, a relict drift at high altitudes. Younger Pleistocene deposits in conspicuous moraines and in bluff exposures are grouped into five units called glacioestuarine associations (GEAs) that are each the product of a single glacioevent; taken together, the five glacioevents may represent one or two glaciations, principally the last major glaciation (stages 2-4 of the oxygen-isotope record). Each GEA includes units deposited both as moraines and in an ancestral Cook Inlet, as well as deposits of the interface between glaciers and the inlet water in which many of the glaciers terminated.

The glacier that produced the East Foreland GEA, the oldest association, extended south to that locality as a trunk glacier from the Alaska Range and had major tributaries from the east and west sides of the basin. For the next younger association, the Point Possession GEA, the trunk glacier had a similar configuration but terminated farther upbasin. In producing the succeeding Fire Island GEA, the glaciers from western sources probably terminated separately, but glaciers derived from the north and east joined and terminated just south and west of Anchorage.

For the two youngest GEAs, the Chickaloon Bay and Cairn Point, the trunk glacier from the north did not extend into the area of present-day Cook Inlet, and glaciers from western sources formed the Denslow Lake moraine on the Beluga plateau. Glaciers flowing west from the Knik and Matanuska valleys terminated near Anchorage, initially south of and later at the Elmendorf Moraine; glaciers in Turnagain Arm terminated first at Chickaloon Bay and later farther east near Bird Creek. The two youngest GEAs are clearly of late Pleistocene age, one somewhat older than 14,000 years and the other just younger.