Surficial Geologic Map of the Anchorage B-7 SE Quadrangle, Alaska

DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY Surficial geologic map of the Anchorage B-7 SE quadrangle, Alaska By Lynn A. Yehle1 and Henry R. Schmoll 1 Open-File Report 88-381 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. U.S.Geological Survey Denver, Colorado 1988 SURFICIAL GEOLOGIC MAP OF THE ANCHORAGE B-7 SE QUADRANGLE, ALASKA By Lynn A. Yehle and Henry R. Schmoll INTRODUCTION The Anchorage B-7 SE quadrangle is located in south-central Alaska within the Municipality of Anchorage, about 30 km northeast of downtown (fig. 1). The quadrangle is one of a group in the Knik Arm and Anchorage regions for which surficial geologic maps have been completed at 1:25,000 scale (Daniels, 1981a,b; Reger, 1981a,b,c,d; Yehle and Schmoll, 1987a,b). The map area is part of the Kenai-Chugach Mountains physiographic province (Wahrhaftig, 1965; fig. 1). The westernmost part of the Chugach Mountains can be subdivided into two parts: (1) a central core of very steep mountains with peaks in excess of 2,000 m, here called the Bashful region, after its highest mountain, Bashful Peak (2,440 m), and (2), a lower surrounding region where peaks and ridges are generally 500-1,000 m less in altitude. Except for its southeastern corner, the map area is entirely within this lower region. The Chugach Mountains are bounded to the west of the map area by a steep front that descends abruptly to the Anchorage lowland. The Chugach Mountains are composed of structurally complex and variably metamorphosed sedimentary and igneous rocks (Clark and Bartsch, 1971; R.G. Updike and C.A. Ulery, Alaska Division of Geological and Geophysical Surveys, written communs., 1986, 1987; G.R. Winkler, written commun., 1986); in this quadrangle the rocks are entirely within the Chugach terrane (Coney and Jones, 1985; Jones and others, 1987). The highest mountain peak within the map area is in the southeastern part and rises to an altitude of 1,663 m. Mount Magnificent, 1,302 m, the only peak named on the topographic base map, is located in the central-western part of the area. Two major subparallel valleys, those of the Eagle River and of Peters Creek, transect the mountains in a generally southeast to northwest direction, as do the smaller valleys of Meadow Creek and an unnamed stream in the northwest part of the map area and the valley of Four Mile Creek in the northeast extremity of the map area. Other small tributary valleys to both the Eagle River and Peters Creek trend mostly in a north-northeasterly direction. All of the valleys are cut deeply below the level of the mountain crests. Maximum relief is about 1550 m. The Eagle River and Peters Creek, the principal streams within the map area, both drain from glaciers to the southeast. Peters Creek and its tributaries to the east of the map area emerge from small glaciers that terminate at an altitude of about 800 m in the Bashful region (fig. 1). Within the map area, Peters Creek has a gradient of about 20 m/km. It is deeply entrenched to the north of the map area where it descends the Chugach Mountain front. The Eagle River on the other hand emerges from a lake at an altitude of 267 m that formed in historical time at the foot of the receding Eagle Glacier which is part of an interconnected complex of glaciers that dominates the mountains south of the Bashful region. Just east of the map area the Eagle River crosses the Bashful region in a short, narrow canyon; within the map area, the valley broadens and the river has a gradient of only 2.4 m/km. The valley is entrenched only moderately west of the map area where it emerges from the mountains and enters the Anchorage lowland. 149*00' Figure 1.--Location of Anchorage B-7 SE quadrangle and major geographic features. Double-weight dashed line indicates boundary between major physiographic provinces of Wahrhaftig (1965); single-weight dashed line and water bodies separate physiographic subprovinces of informal usage. Heavy pattern indicates glaciers. SURFICIAL DEPOSITS Surficial deposits within the map area consist of glacial drift that is entirely Pleistocene in age and alluvial, colluvial, rock-glacier, and peat deposits that are mainly Holocene in age. Glacial deposits occur mainly on the flanks of the major valleys, and at small, isolated areas higher on mountain ridges. Alluvial deposits occupy the floors of most valleys but are extensive only in the Eagle River valley where they overlie lacustrine deposits. Variably thick colluvial deposits cover bedrock on most mountain slopes throughout the area. Rock-glacier deposits occupy major areas within the several north-draining valleys tributary to Peters Creek. Peat and pond deposits locally overlie valley-bottom alluvium, mainly in the Eagle River valley. A ubiquitous mantle of organic and windblown materials, including minor amounts of tephra, covers all but the most recent or still actively accumulating deposits. This mantle has not been mapped separately. GLACIAL DEPOSITS Glacial deposits may be subdivided, in order of abundance within the map area, as follows: (1) deposits that occur in moraines and associated kame fans; (2) deposits of former glacial lakes and related deltas; and (3) outwash and channel deposits related to former glaciers. These deposits are the result of several glacial advances and retreats that have successively modified the terrain in the area (Miller and Dobrovolny, 1959; Karlstrom, 1964; Schmoll and Yehle, 1986). Here, as in most mountain regions where evidence for past glaciations is based largely on relict landforms, the oldest glacier advance for which there is evidence seemingly was the most extensive, and successively younger advances were successively less extensive; evidence for intervening advances of lesser extent than a following advance is lacking, although such ice advances probably occurred. The most widespread glaciers to reach this area probably advanced from distant sources in mountains to the north and east and merged with ice of local source within the Chugach Mountains to fill the Cook Inlet-Susitna lowland with glacier ice. Only relict deposits at high altitude remain as evidence for these early events. When glaciers from the distant sources were less extensive, local glaciers in the Peters Creek and Eagle River valleys had separate identities. At these times they joined ice of distant source in the lowland and flowed southwestward toward and beyond Anchorage. At times of least extensive ice advance, local valley glaciers did not reach far enough downvalley to join with the lowland ice, and thus parts of the valleys within the map area were free of ice. When drainage in these valleys was still blocked by a glacier covering the lowland, however, lakes formed between the lowland glacier and the local glaciers. The latter either terminated directly in lake water or were separated from lake water by outwash plains that were deltaic to the lakes. (Similar relationships between major glaciers and glacier-free tributary valleys elsewhere are described by Sturm and Benson, 1985; Booth, 1986; and Saunders and others, 1987). Beginning about 12,000 yr ago, climatic warming resulted in the final stagnation and retreat of glacier ice that ended glacier domination of the map area. However, the presence of glaciers upvalley from the map area continues to influence stream discharge and alluvial deposition today. The glacial deposits are named according to the principal named lateral and end moraines with which they are correlated, as discussed by Schmoll and Yehle (1986). Correlations of the lateral moraines within the map area to named moraines along the mountain front to the west were made largely on the basis of graphically determined gradients of lateral moraines within each major valley and their concordance with moraines along the mountain front; likewise the glaciolacustrine and associated deposits are correlated on the basis of relative altitudes with mountain-front moraines of glaciers that most likely dammed the valleys. Deposits that occur in moraines dominate the flanks of the Eagle River valley, and occur as scattered remnants in Peters Creek valley and in a few tributary valleys. The most continuous lateral moraines preserved along the flanks of the Eagle River valley descend northwestward in the map area from about 600 m to a probable but obscurely evidenced and unmapped end position at 100 m in altitude. These lateral moraines are correlated with the deposits of the Elmendorf Moraine near Anchorage (Miller and Dobrovolny, 1959) which are now regarded as having formed about 12,000 to 14,000 yr ago (Schmoll and others, 1972) and are the only morainal deposits that are dated closely by radiometric means. Older glacial deposits occur locally at higher altitudes bordering the Eagle River valley and are related with varying degrees of confidence to the successively older Dishno Pond and Fort Richardson moraines, which have informal type localities along the Chugach Mountain front southwest of the map area. (In a previous map of part of the area (Schmoll and others, 1980) deposits of the Dishno Pond moraines were not mapped separately from those of the Fort Richardson moraines). Deposits of the successively older Rabbit Creek and Ski Bowl moraines are tentatively identified at isolated localities within the map area on the basis of their position and altitude. A small area of drift on a topographic saddle at an altitude of about 950 m west of Mount Magnificent provides the highest conclusive evidence for this glaciation within the map area, and has been termed the Mount Magnificent ground moraine (Schmoll and Yehle, 1986) on the basis of this locality.

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