Map of Western Copper River Basin, Alaska, Showing Lake Sediments and Shorelines, Glacial Moraines, and Location of Stratigraphic Sections and Radiocarbon-Dated Samples

Abstract

The purpose of this report is to make available basic data on radiocarbon dating of 61 organic samples from 40 locations in the western Copper River Basin and adjacent uplands and in the uppermost Matanuska River Valley. The former distribution of late Quaternary glacial lakes and of glaciers as mapped from field work and photo interpretation is provided as background for interpretation of the radiocarbon dates and are the basic data needed for construction of the late Quaternary chronology. The glacial boundaries, formed and expressed by moraines, ice-contact margins, marginal channels, deltas, and other features, are obscured by a drape of glaciolacustrine deposits in a series of glacial lakes. The highest lake, represented by bottom sediments as high as 914 m to 975 m above sea level, extends from Fog Lakes lowland on Susitna River upstream into the northwestern part of the Copper River Basin (the part now draining to Susitna River) where it apparently was held in by an ice border. It was apparently dammed by ice from the Mt. McKinley area, by Talkeetna G1acier, and may have had a temporary drainage threshold at the headwaters of Chunilna Creek. No shorelines have been noted within the map area, although Nichols and Yehle (1961) reported shorelines within the 914-975 m range in the Denali area to the north of that mapped. Recent work by geologic consultants for the Susitna Hydroelectric Project has confirmed the early inferences (Karlstrom, 1964) about the existence of a lake in the Susitna canyon, based originally on drilling by the Bureau of Reclamation about 35 years ago. According to dating of deposits at Tyone Bluff (map locations 0, P), Thorson and others (1981) concluded that a late Wisconsin advance of the glaciers between 11,535 and 21,730 years ago was followed by a brief interval of lacustrine sedimentation, and was preceded by a long period of lake deposition broken by a lowering of the lake between 32,000 and about 25,000 years ago. An alternate interpretation of the late Wisconsin till at Tyone Bluff is that it is a glaciolacustrine diamicton of the 914-975 m lake into which the ice advanced to the Hatchet Lake and to the Old Man moraines. The level of this regional lake in the Susitna drainage and on Heartland Ridge then dropped from over 914 m to about 777 m, to uncover the Tyone Spillway. An intermediate lake level in the Susitna-Tyone-Louise lake region was lowered rapidly by erosion of the spillway to 747 m. The drainage of the 747 m lake was concentrated in the spillway leading west from the West Fork Gulkana River. This spillway or a rock threshold downstream apparently was stable enough to permit formation of basin-wide, apparently undeformed, shoreline systems at 747 m, and, on recession, local shorelines at 717 m and 700 m and lower levels. The level of the 747 m lake that was confined to about 9000 km2 of the present Copper River Basin fluctuated for one or more reasons such as: the volume of ice added to or withdrawn from the system, because of changes in water budget (assuming no outflow), and/or because of temporary releases through the only outlets, perhaps Mentasta Pass, but importantly, the Copper River canyon. The 747 m lake persisted until glaciers had withdrawn to well within the Chugach Mountains, perhaps 10 to 20 km from the present glaciers.