Figure 1: Study area near Tok, Alaska

Figure 2: General location map of the project mapping area showing selected physiographic features and location of radiocarbon ages.

Figure 3: Helicopter view from above the Alaska Highway and the town of Tok looking east across the low gradient Tok fan.

The Tanacross B-6, B-5, and B-4 quadrangles contain parts of three physiographic provinces, the Alaska Range, the Yukon-Tanana Upland, and the Northway-Tanana Lowland (Wahrhaftig, 1965) (fig. 2). The high, rugged, glaciated landscape of the eastern Alaska Range dominates the southwestern map area (fig. 4). Within the map area, about a dozen peaks rise to more than 2,000 m asl (above sea level); the highest peak, an unnamed summit in the south-central part of the Tanacross B-6 quadrangle, rises to 2,166 m asl. In contrast, the gently rolling hills of the Yukon-Tanana Upland (fig. 5), in the northern map area, rise to only about 1,000 m asl. Between the Alaska Range and the Yukon-Tanana Upland lies the Northway-Tanana Lowland (hereafter referred to as the Tanana River valley) that contains the westerly flowing Tanana River. Altitudes along the floor of the valley generally range between 450 to 490 m asl.

Figure 4: The Alaska Range after an early season snowfall. The Alaska Highway bridge over the Robertson River is in the foreground. View is to the south-southwest.

Figure 5: Photo showing the gently rolling topography of the Yukon-Tanana Upland in the distance as seen from the Tanana River Valley.

Mapping of the surficial deposits in the study area was being accomplished by a variety of methods including: (1) compilation from existing geologic maps, (2) stereoscopic analysis of 1:40,000-scale 1954 black and white and 1:60.000-scale 1978 color-infrared aerial photographs, and (3) fieldwork, including limited helicopter use (fig. 6). Where accessed in the field, detailed information was obtained on the surficial deposits. Data from these easily accessed areas was then extrapolated to the less accessible areas. Unit boundaries were then plotted on a stable mylar topographic base with the use of a photogrammetric stereo plotting instrument. Lines were then digitized on screen from the scanned mylar.

Figure 6: Helicopter at the head of a valley in the Alaska Range. The use of a helicopter allowed access to the more remote locations in the study area.

Highlights and Key Findings

During the first three field seasons (mid-August to mid-September 2001, 2002, and 2003) approximately 25 surficial map units were recognized in the study area (Tanacross B-6, B-5, and B-4 Qds). These surficial units included artificial fill, floodplain alluvium along the Tanana and Robertson Rivers, alluvium on terraces along the Tanana River, active channel and floodplain alluvium of streams emerging from the Alaska Range, alluvial fans of the Alaska Range, large coalescing alluvial fans along the northern front of the Alaska Range, the large Tok alluvial fan (figs. 2 and 3), hillside colluvium, talus, landslides, organic deposits, eolian sand, glacial outwash, till of two ages, rock glaciers, felsenmeer (a flat area covered by large angular blocks derived from underlying bedrock by frost action, usually above timberline) and solufluction deposits (formed by the slow, viscous, flow of saturated surficial material, commmonoly occuring on slopes in areas underlain with permafrost).

On the Tanacross B-6 quadrangle a number of alluvial fans formed by streams emerging from the Alaska Range have spread out onto the floor of the Tanana River valley. These alluvial fans are in part formed by debris flows and/or flash floods as evidenced by the large boulders in their stream channels. Hence, areas bordered by these stream channels are prone to these hazards that may present a serious threat to the proposed high-pressure natural gas pipeline route.

Other studies

1. Porcupine Creek, a small drainage on the Tanacross B-4 quadrangle (fig. 2) was studied. The area around Porcupine Creek is covered in eolian (wind-blown) sand that is well-exposed in road cuts along the Taylor Highway. At one location along the highway, lacustrine (lake) deposits indicate the sand dammed Porcupine Creek (fig. 7). A radiocarbon age of 12,515 BP on freshwater clam shells near the upper limit of the lake beds provides an age for the end of eolian activity and the draining of the small lake. During September 2003, a joint study of the Porcupine Creek eolian sand (Tanacross B-4 Qd) was undertaken with Dan Muhs (USGS - Denver - Earth Surface Dynamics Program) to determine the origin, paleo-wind directions, and age of the eolian sand.

Figure 7: Exposure of lake sediments along the Taylor Highway resulting from the damming of Porcupine Creek by very active eolian activity. A radiocarbon age of 12,515 BP on shells near the upper limit of the lake beds provides an age for the end of the eolian activity and the draining of the small lake.

2. A stratigraphic section near the confluence of Porcupine Creek and the Tanana River (Tanacross B-4 Qd) was studied during the 2002 field season (fig. 2). The upper part of the section mainly consists of reworked wind-blown sand and scattered thin peats containing pollen deposited in a shallow pond. The lower part of the section, separated from the upper part by an unconformity, consists of a poorly-laminated, clayey-silt that contains numerous willow fragments. One of these fragments yielded a radiocarbon age of 41,880 BP. Pollen samples were collected from the thin peats and the lower unit. Analysis by Tom Ager (USGS - Denver - Earth Surface Dynamics Program) indicates a boreal forest (similar to the present day) in the upper part of the section and a shrub-herb tundra (glacial conditions) in the lower part of the section. Hence, the stratigraphic section, although discontinuous, spans from the Holocene to the last glaciation and provides important insights into the vegetation and climate history of the Upper Tanana valley.

3. A stratigraphic section near the Alaska Highway on the Tanacross A-3 quadrangle, first described by Fernald (1965) and later by Carter and Galloway (1981), has been revisited. The lower unit in this section, which contains willow fragments (fig. 8), had previously yielded a radiocarbon age of greater than 40,000 BP and possibly much older (Fernald, 1965). Pollen samples were collected from the lower unit. Analysis by Tom Ager indicates a herb tundra (glacial conditions) during the deposition of the lower unit.

Figure 8: The lower section of the stratigraphic unit near the Alaska Highway on the Tanacross A-3 quadrangle that was found to contain fossil pollen. The section, which contains willow fragments, visible in the photo, had previously yielded a radiocarbon age of greater than 40,000 BP (Fernald, 1965).

Funding Sources

The project is funded by the National Cooperative Geologic Mapping Program.

Staff

The project staff consisted of Paul Carrara (USGS - Earth Surfaces Processes Team - Denver) who was assigned to this project on a half-time basis.

Collaborators and clients

The project was originally set up by Allison Till (USGS - Anchorage) in September 2000 in collaboration with Milt Wiltse (Alaska State Geologist - at the time), who interacted with individuals within their own organizations, state and federal agencies, Native Corporations and Tribal governments, universities, industry, environmental groups, engineering and geotechnical communities, and others. Existing groups with roles in identifying priority needs for scientific information have also been involved, including the State Geologic Mapping Board, which sets priorities for the Alaska Division of Geological and Geophysical Surveys (ADGGS), and the Interagency Minerals Coordinating Group (IMCG), which identifies priority areas of cooperation amongst USGS, ADGGS, U.S. Department of Agriculture-Forest Service, and the Bureau of Land Management. At the end of this project (September 30, 2006) the surficial geologic maps will be available to these agencies.

Publications (as of December 2006)

Carrara, P.E., 2004, Surficial geologic map of the Tanacross B-6 quadrangle, east-central Alaska: U.S. Geological Survey Scientific Investigations Map 2850, scale 1:63,360.

Carrara, P.E., 2004, Surficial geologic map of the Tanacross B-5 quadrangle, east-central Alaska: U.S. Geological Survey Scientific Investigations Map 2856, scale 1:63,360.

Carrara, P.E., 2006, Surficial geologic map of the Tanacross B-4 quadrangle, east-central Alaska: U.S. Geological Survey Scientific Investigations Map 2935, scale 1:63,360.

Carrara, P.E., 2007, Geologic map and engineering properties of the surficial deposits of the Tok area, east-central Alaska: U.S. Geological Survey Scientific Investigations Map 2986, scale 1:100,000.

References Cited

Carter, L.D. and Galloway, J.P., 1978, Preliminary engineering geologic maps of the proposed natural gas pipeline route in the Tanana River valley, Alaska: U.S. Geological Survey Open-File Report 78-794.

Fernald, A.T., 1965, Glaciation in the Nebesna River area, Upper Tanana River Valley, Alaska: U.S. Geological Survey Professional Paper 525-C, p. C120-C123.

Wahrhaftig, C., 1965, Physiographic divisions of Alaska: U.S. Geological Survey Professional Paper 482, 52 p.

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