QUATERNARY GEOLOGIC MAPPING IN WRANGELL-SAINT ELIAS NATIONAL PARK AND PRESERVE; A PERSPECTIVE FROM THE COPPER RIVER BASIN, ALASKA

The Copper River Basin (CRB) is surrounded by glacier-clad mountains; during each major Quaternary glacial event, glaciers advanced into it, blocking drainage to the south and resulting in the formation of glacial Lake Ahtna, whose size and location varied over time. At times, the lake may not have had an outlet; at other times, it drained either northeastward or northwestward. Evidence for the lake includes deposits on the CRB floor (some not obviously lacustrine), deposits exposed in bluffs along the Copper and tributary rivers (some of controversially lacustrine or glacial origin (which were termed “diamicton” by 1961), and by shorelines. Clear but scattered evidence for the highest shorelines imply the lake achieved elevations at least a few hundred meters above the basin floor; shoreline features at lower elevations are numerous but poorly documented. Prominent lateral moraines, kame complexes, and hanging deltas are present, particularly along the flanks of the Wrangell Mountains, but end moraines are conspicuously absent except in the northern and western parts of the CRB at elevations above highest lake levels. Previous studies suggested that the entire basin was filled with glacier ice during the most recent glaciation. More likely, ice of the most recent glaciation terminated in lake water. Map data hints at the possible positions of some glacial termini and shows that small high-elevation (> 900 m) lakes formed along ice margins. During older more extensive glaciations, ice did occupy the entire CRB, as evidenced by high-level glacial erratic boulders and fragmentary moraines.

The most accessible part of Wrangell-Saint Elias National Park and Preserve (WRST) lies in the Copper River Basin. Although studies there U.S. Geological Survey began in the late 19^th century, surficial geologic mapping was first done under five two-person field projects between 1952 and 1962. Intermittent work with material from these projects has continued to the present, mainly in response to both national and international needs, but no regional mapping at a suitably large (such as 1:250,000) scale has been attempted. Support from the National Park Service has enabled us to begin work on such a compilation. We are using both published and unpublished sources from the original projects as well as from subsequent studies.