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Geology: Soil and Rock

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  Stratigraphic Cross-Section Return to Top  

Figure of a stratigraphic column showing a fossil timeline. The soil and rock of the Permafrost Tunnel Research Facility represents one of the typical cross-sections of the Yukon-Tanana Upland, the regional geology. The Yukon-Tanana Upland is bounded by the Tanana River Valley and eastern Alaska Range foothills to the south and by the Yukon River Valley to the north. It stretches from western central Alaska into Yukon, Canada. The diagram is a stratigraphic column that represents of the frozen soils and rock of the Permafrost Tunnel area.

In the vicinity of the tunnel, the bedrock is schist and quartzite that has been referred to as Birch Creek Schist. The schist has been thought to have been deposited in the Precambrian era, 550 million years before present. The sediments underwent recrystallization in the Jurassic and Cretaceous to early Tertiary, 60 to 200 million years before present. Tectonic forces squeeze this region in a counter-clockwise direction producing northwest/southeast trending strike-slip faults.

The schist bedrock is overlain by six to eight feet layer of gravel, called the Fox Gravel. The gravel was derived from the bedrock in the area and deposited by solifluction and then reworked by streams. During the creation of the gravel layer, local lode gold deposits were eroded and carried downstream by creeks, where the gold was eventually laid down with the gravels in placer deposits. In the vicinity of the tunnel, the gravels and gold were deposited by Goldstream Creek across the width of the valley due to the meandering nature of the creek. Felix Pedro discovered gold in 1902 north of Fairbanks, Alaska, and since then to the early 1960s, a significant portion of the gravels have been reworked by miners.

The Fox Gravel is sub-angular to rounded and measures up to 6 inches in length with woody fragments. The Fox Gravel is thought to be deposited in late Pliocene (2.5 to 3 million years before present). However, woody remains in the top layer gravels within the tunnel are dated to between 43,000 to 46,000 before present. The time difference suggests reworking of the gravels prior to the silt deposition. The photo shows the gravels within the tunnel which are cemented together by ice.

Cross-section of the Permafrost Tunnel.
Diagram from U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (1981) CRREL permafrost tunnel. CRREL Benchnotes 17: 1-7. Hanover, NH: Cold Regions Research and Engineering Laboratory.

Overlying the Fox Gravel is a thick layer of silt called the Goldstream Formation which was mostly deposited during the Wisconsin Glaciation (30,000 to 10,000 years before present). The Goldstream Formation is overlain by a thin silt layer called Ready Bullion Formation and was deposited from the start of the Holocene at 10,000 years before present to about 3,500 years before present. Both silt layers are windblown silt or loess that has been retransported by stream action. The retransported silt contains a higher volume of ice and organic material than the wind-deposited silt, and upon thawing it becomes very wet mud with a rotting-like smell that led to the nickname of the muck.

Deposits of rocks.
Deposits of rocks within the tunnel.
Ice wedge in the winze.
Ice wedge in the winze.

During the past ice ages, the Fairbanks area was a periglacial environment, where it was surrounded by glaciers but not directly covered by ice. The glaciers provided the silt that was windblown into the area and the cold environment allowed syngenetic permafrost to grow. The windblown silt is mainly derived from the Tanana River floodplain which contains washout from the Alaska Range south of Fairbanks. There is evidence that some loess inclusions are windblown derivations from the Yukon and Nenana Rivers. The regional loess was deposited during times of glaciation and interglaciation, and still today loess is collecting in the Fairbanks area. The loess thicknesses in the area varies from 4 to 80 feet on the low slopes of hills and tapers off on the upper slopes at an elevation of about 1000 feet above sea level. Retransported loess has moved some higher elevation silt to lower elevations resulting in thicker layers in the valley bottoms up to 360 feet in thickness. In 1987, a dust storm from the Tanana River Floodplains was observed at the University of Alaska Fairbanks and air samples were collected. During the dust storm, the wind was about 30 mph and the dust concentration was 5,100 particles per cubic foot. Intriguingly, the measured dust concentration that is regularly within the air, not during a dust storm, is 60 to 1,700 particles per cubic foot. If the climate is cold enough to create permafrost as this dust deposits on the ground surface, it will eventually create syngenetic permafrost as seen within the two silt formations in the tunnel.

The frost actions associated with syngenetic permafrost create many cryostructures that are visible in the silt of the Tunnel, such as ice wedges, segregation ice, and thermal erosion ice known as thermokarst-cave ice. These structures are encountered in all regions of the world where permafrost exists, but they are most prevalent in frozen fine-grained soils. The photo to the left shows an example of one of the ice wedges within the Goldstream Formation.

The Goldstream Formation has long thought to have undergone climate change periods and reworking, one example being the two sets of ice wedges have been observed in the Permafrost Tunnel. The first and lower set ranges in age of about 25,000 to 33,000 years before present and are larger than the second set. The second and upper set ranges in age of about 10,000 to 14,000 years before present. These two separate ice wedge units suggest that a warming period thawed the permafrost before deposition of the overlying second set. The warming period has been termed the Fox Thermal Event. Near the tunnel entrance exists a large amount of colluvium that was deposited after a thermal erosion event removed the silt. The debris consists of gravel, sand, and silt along with bones, grasses, and wood fragments. The flora and fauna remains are abundant in this section, especially with large vertebrates or megafauna (mammoths and bisons) with Bison bone being dated from approximately 10,000 to 14,000 before present. In addition, viable living bacteria was collected from the first set of ice wedges and dated to be 25,000 years before present.

The depth of the tops of the second and upper ice wedge set, with their corresponding ages, are not really known. The only encounter with the tops of these ice wedges was a small ice wedge that was briefly seen within the ventilation shaft shortly after the tunnel was constructed. The diagram showing geology of ventilation shaft is drawn by Paul Sellmann in 1967, where the asterisks mark carbon dating samples. The significance of the tops of ice wedges is that it is known through the region as the boundary between the Goldstream Formation and the Ready Bullion Formation at about 10,000 years before present. The Ready Bullion Formation differs from the Goldstream Formation by having large wood pieces and no large ice wedges. In the diagram, the second asterisk from the top was dated to about 8,500 years before present, and was assumed to be the top of the ice wedges at the time. However, the assumption is probably incorrect with drawing depicting more of an ice vein than a full ice wedge, and therefore not the Goldstream and Ready Bullion Formation boundary. In addition, below the ice vein are numerous large logs that are commonly identified with Ready Bullion Formation. The boundary is more likely just above the large ice wedge drawn at 25 feet at the third asterisk below the top, however the carbon dating at that point was bad with a date of about 2,500 years before present. The top of this ice wedge is the only documented unaltered ice wedge top at the tunnel, therefore it may not mark the exact depth of the boundary.

Diagram of the geology of the Permafrost Tunnel.
Diagram from Sellmann, P. V. (1967) Geology of the USA CRREL Permafrost Tunnel Fairbanks, Alaska, CRREL Technical Report 199. Hanover, NH: U.S. Army Cold Regions Research and Engineering Laboratory.

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  Gold Mining Operations Return to Top  

In 1902, Felix Pedro discovered gold north of Fairbanks, Alaska, and since that time gold has been mined in the area. The large scale placer deposits in Interior Alaska were mined with gold dredges, which required much preparation work. The diagram shows the process used to mine the placer deposits, where the silt was removed, the gravels were thawed, and finally dredged. The stripping of the silt was accomplished with hydraulic and mechanical removal methods. Hydraulic excavation was the most common method of silt removal using large volumes of high pressure cold water directed through cannons called hydraulic giants. The photo shows one of the hydraulic cannons at work at an unknown location in Interior Alaska.

Hydraulic canons at work.
Photo from Boswell, John C. (1979) History of Alaskan Operations of United States Smelting, Mining, and Refining Company. Fairbanks, Alaska: Mineral Industries Research Laboratory, University of Alaska Fairbanks. (Used with permission.)
Geological survey diagram.
Diagram adapted from Péwé, T.L, J.W. Bell, R.B. Forbes, and F.R. Weber (1976) Geologic Map of the Fairbanks D-2 NE Quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations 950, 1 sheet, scale 1:24,000.

The diagram (above) shows the approximate height the original silt within the valley before the area was mined. The exact dates for the stripping in Goldstream Valley are unknown. However, the water source was most likely from the Davidson Ditch, which did not have water flowing to the Goldstream Valley until 1929. Borehole exploration was used to determine where to cease stripping operations. The stripping produced near vertical silt escarpments along valley margins, and the portal to the tunnel was built in one of these left escarpments. The photo shows the dredging progress in 1938 in the vicinity of the tunnel with the arrow showing the approximate location of the portal entrance today. Unfortunately, the edge of the escarpment was removed later for another row of dredging, but the photo does give an idea of what the escarpment looked like shortly after stripping.

In the Fairbanks area, the gold dredging lasted until the early 1960s, therefore significant portion and significant amounts of silt were removed and gravels reworked by the miners. The exact dates for the mining of placer gold deposits in Goldstream Valley are not clear. The mining claim in 1925 states that some of the property was "stoped and worked" meaning that these gold deposits were mined with underground drift mining. The vicinity was then reworked by Gold Dredge No. 8 from 1928 to 1959, the area directly in front of the tunnel was dredge in 1942. The dredging was done by the Fairbanks Exploration Company (F.E. Co.), part of the United States Smelting, Mining, and Refining Company (U.S.S.R.&M Co.).

Diagram of the operations of the U.S. Smelting, Refining, and Mining Company.
Diagram from Boswell, John C. (1979) History of Alaskan Operations of United States Smelting, Mining, and Refining Company. Fairbanks, Alaska: Mineral Industries Research Laboratory, University of Alaska Fairbanks. (Used with permission.)
Geological photo.
Altered Photo (to 72 dpi) from Bradford Washburn Collection, Photo 2417, Archives, Alaska and Polar Regions Collections, Rasmuson Library, University of Alaska Fairbanks. (Used with permission.)

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