Physiographic Region

Interior Alaska

Contained within the Northern Plateaus Physiographic Province (Wahrhaftig 1965), interior Alaska is bounded on the south by the Alaska Range and on the north by the Brooks Range, and includes a gross area of approximately 136,799,600 ha (Van Cleve et al. 1993). The Alaska Range is glacially sculptured, and trends west and southwest 1,000 km from the Canadian border to the Aleutian Range. It contains numerous peaks over 3,000 m in elevation and culminates in Mt. McKinley at 6,195 m (Pewe and Reger 1983). This mountain wall is an effective barrier to coastal air masses, and is responsible for the continental climate experienced at BNZ. The Brooks Range is the northern extension of the Rocky Mountain System which turns west as it reaches the Arctic Ocean near the Alaska-Canada border. These extremely rugged mountains reach to over 2700 m in the east, but are lower to the west. Glaciation during the Pleistocene carved the rugged landform which persists at present with little change (Johnson and Hartman 1969). Although central Alaska has not been glaciated, small cirque glaciers occurred in local mountainous highlands. Glaciers from the Alaska Range approached to within 80 km of Fairbanks during extensive glacial expansions.

The Bonanza Creek Experimental Forest (BCEF) is located in the southern portion of the Yukon-Tanana uplands and adjacent Tanana River valley to the south. The Caribou-Poker Creeks Research Watershed (CPCRW) is located in the Yukon-Tanana Uplands near the town of Chatanika in interior Alaska. Part of a larger area of rolling country in central Alaska between the Brooks and Alaska Ranges (Pewe 1958), the Yukon-Tanana Uplands are a region of westward-trending highland between the Yukon and Tanana Rivers. The elevations of these ridges range from 450 to 900 meters with rises of 150 to 500 meters above the adjacent valley bottoms. The alluvial-covered valley floors are generally flat (Wahrhaftig 1965), and current tree line is at about 900 m. The physiography and geology of the Yukon-Tanana uplands include loess-mantled bedrock hills (the strongly weathered, Precambrian quartz-mica and quartzite schist of the Birch Creek formation), lower hill slopes and creek-valley bottoms, organic-rich lowlands at the base of hills, and the Tanana River floodplain.

General physiographic setting of the Taiga and the BNZ region (rectangle). From Van Cleve et al. (1993) Figure 1. Click here or on figure to enlarge.

Rivers

The major river systems draining interior Alaska are the Yukon River and its largest tributary, the Tanana River. The Tanana River valley is a large structural basin with much of its bedrock floor below sea level. Fluvial and glaciofluvial sediments, largely from the rising Alaska Range, have accumulated in deposits 91 to 230 m thick. These deposits have pushed the Tanana River northward, near the Yukon-Tanana Upland (Pewe and Reger 1983).

Geologic History (from Pewe 1958)

The oldest exposed formation in the BNZ region is the Birch Creek schist of Precambrian age. This formation consists of water-laid sediments, and underlies a large portion of the area. There was an intrusion of ignous granitic rocks in Mesozoic time that may have been accompanied by the gold mineralization of the area. It is believed that continental Tertiary sediments with local lava flows once covered much of the Yukon-Tanana uplands, with a marine embayment extending into central Alaska as far as Fairbanks and depositing estuarine sediments.

Quaternary deposits show a record of alternating deposition and erosion of silt and gravel, the formation and destruction of permafrost, and climatic fluctuations ranging from a climate warmer than that which exists now to one colder than the present. In early Quaternary time gold placers were formed in creek valleys of the upland. This was followed by the occurrence of gravel alluviation in these valleys. This early period of gravel deposition was followed by erosion and removal of most of this coarse angular local gravel. Streams reconcentrated much of the gold in the earlier placers and deposited additional gold placers.

In later Quaternary time the hills were blanketed with loess derived from the river floodplains and the glacial outwash plains. Much of this windblown silt was retransported to valley bottoms, incorporated with much organic debris and became perennially frozen. Much of the loess and most of the valley bottom silt was removed during a later erosional period that immediately preceded Wisconsin events. Much of the permafrost may have thawed and disappeared during this warm interval. In late Quaternary time more loess was deposited on the hills and valleys of the upland. Organic silts accumulated in valley bottoms, became frozen and large ground-ice masses formed. About 5,000 to 6,000 years ago a slight warming of the climate caused a small amount of the permafrost to thaw. Since then additional loess and organic silt have been deposited and cooling of the climate has caused the organic silt in valley bottoms to become perennially frozen. During Quaternary time the sediment fill of the Tanana Valley probably was also modified by alternating periods of erosion and deposition and the formation and destruction of permafrost.

References Cited

Pewe, T.L. 1958. Geology of the Fairbanks (D-2) Quadrangle, Alaska. Department of the Interior, U.S. Geological Survey.

Pewe, T.L. and R.D. Reger. 1983. Middle Tanana River Valley. In Guidebook 1: Richardson and Glenn Highways, Alaska: Fourth International Conference on Permafrost. T.L. Pewe and R.D. Reger, Editors. Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK. p. 5-45.

Van Cleve, K., Viereck, L.A., and Marion, G.M. 1993 . Introduction and overview of a study dealing with the role of
salt-affected soils in primary succession on the Tanana River floodplain of interior Alaska. Canadian Journal of Forest
Research, 23 (5): 879-888.

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