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The Fox Permafrost Tunnel: a Late Quaternary Geologic Record in Central Alaska

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The Fox Permafrost Tunnel: a Late Quaternary Geologic Record in Central Alaska The Fox permafrost tunnel: A late Quaternary geologic record in central Alaska THOMAS D. HAMILTON U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508 JOHN L. CRAIG* U.S. Army CRREL, Building 4070, Fort Wainwright, Alaska 99703 PAUL V. SELLMANN U.S. Army CRREL, 72 Lyme Road, Hanover, New Hampshire 03755 ABSTRACT ice wedges beneath small frozen ponds or INTRODUCTION streamlets that occupied ice-wedge troughs. The Fox permafrost tunnel, which pene- A later episode of rapid loess influx under The Fox permafrost tunnel is situated 16 km trates 110 m into frozen sediments of Gold- drier conditions began after 30 ka and coin- north of Fairbanks near the southern margin of stream valley, provides a continuous expo- cided with glacial advances of late Wisconsin the Yukon-Tanana Upland near the community sure of fossiliferous silt and alluvium above age in the adjoining Alaska Range. Large ice of Fox (Fig. 1). It extends -110 m into the schistose bedrock. Deposition of fluvial grav- wedges also formed in the upper loess unit, eastern side of Goldstream valley (Fig. 2) and el was followed by a long interval of loess but only their bases are exposed in the tunnel, provides a long, continuous exposure of undis- accretion and permafrost aggradation that and their history of development is uncertain. turbed, perennially frozen, ice-rich, fossil-bear- was punctuated by episodes of thaw and of Fanlike deposits of poorly sorted debris ing silt and alluvium that overlie schistose gullying and redeposition of silt. near the tunnel portal formed between about bedrock (Sellmann, 1967, 1972). Excavation Imbricated sandy gravel above the bedrock 12.5 and 11 ka during deep erosion of loess was carried out in several stages between 1963 contains lenses of finer alluvium that contain slopes under moister conditions. The deposits and 1969, primarily by the U.S. Army Corps of wood fragments and some rooted stumps. locally form two subunits: the younger over- Engineers Cold Regions Research and Engineer- Radiocarbon dates indicate that the gravel is whelmed a stand of tall willows on the floor ing Laboratory (CRREL), with some additional older than 40 ka, but absence of mature soil of Goldstream valley between about 11.3 and excavation by the U.S. Bureau of Mines. The and weathering profiles at its upper contact 11.1 ka; the older may have formed about facility is now jointly supported by CRREL, the indicates that fluvial activity must have con- 1,000 yr earlier. University of Alaska, and the Bureau of Mines. tinued until shortly before loess accretion Stratigraphic records elsewhere in central began at the tunnel site. Alaska indicate variable middle Wisconsin Climate and Permafrost Silt is the most widespread depositional environments followed by colder and drier unit in the tunnel. This deposit is of eolian conditions that began between 30 and 25 ka The Fairbanks area has a subarctic continen- origin (loess), but some has been redeposited and persisted until perhaps 12.5 ka. Wide- tal climate with long, cold winters; short, warm by slope processes. The silt units contain spread loess erosion and redeposition subse- summers; and severe winter temperature inver- abundant ground ice as pore fillings, lenses, quently occurred under moister and probably sions (Streten, 1969, 1974; Holmgren and oth- wedges, and buried pond ice. Loess accretion warmer conditions. Renewed early Holocene ers, 1975). Summer temperatures are as high as was interrupted by a period when little loess loess deposition may have been widespread, 35 °C, winter temperatures are as low as -53 accumulated and when large ice wedges but its exact environmental controls are °C, and the mean annual temperature is -3.3 °C formed in the lower loess unit and subse- uncertain. (Fig. 3). During severe winter temperature in- quently were truncated by thaw. Loess began Our data challenge three generally ac- versions, the uplands near Fox can average >5 forming sometime before 40 ka and was rap- cepted concepts of late Quaternary periglacial °C warmer than the floor of the Tanana River idly accreting by 39 ka under xeric conditions processes in central Alaska. We contend that valley, where Fairbanks is situated (Haugen and with open vegetation. A sharply decreased (1) many ice-wedge systems may have formed others, 1982). Precipitation is generally light rate of loess accretion associated with local under interstadial conditions rather than full- throughout the year (Fig. 3), but snow typically erosion and thaw between about 36 and 30 glacial conditions, (2) episodes of rapid loess persists from October through April. ka is marked by anomalous cation concentra- influx may have been partly out of phase with The permafrost tunnel is situated within the tion values, lenses of buried sod, fossils indic- episodes of glacier expansion, and (3) redep- zone of discontinuous permafrost (Ferrians, ative of moist to wet substrates, and truncated osition of loess by solifluction, sheetwash, 1965), where the distribution of perennially fro- and gully formation may have been episodic zen ground is controlled by such natural pa- •Present address: 562d Engineer Company (Com- and required conditions moister than those rameters as (1) slope angle and orientation, bat), Ft. Richardson, Alaska 99505. under which the loess initially accreted. (2) vegetation, (3) snow cover, (4) presence of Additional material for this article (Appendices A and B) may be secured free of charge by requesting Supplementary Data 8811 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 100, p. 948-969, 18 figs., 3 tables, June 1988. 948 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/6/948/3380380/i0016-7606-100-6-948.pdf by guest on 23 September 2021 FOX PERMAFROST TUNNEL: A QUATERNARY GEOLOGIC RECORD, ALASKA 949 water bodies, (5) thermal properties of soil and rock, and (6) history of site disturbance by fire or human activities (Brown and Pewe, 1973). Permafrost is widely present in the Fairbanks area but is absent beneath ridge crests, moderate to steep south-facing slopes, lakes and large streams, and recently abandoned river channels (Pew6,1982). Permafrost thickness at the tunnel site is un- known, but permafrost >40 m thick occurs ~2 km south of the tunnel beneath silt deposits comparable to those which the tunnel penetrates (Pew6 and Bell, 1975). Ground temperatures of -2.2 to -0.6 °C, measured along several profiles adjacent to the tunnel during and immediately after its construction (Sellmann, 1967), are within the normal range for discontinuous per- mafrost. Cold-air drainage and temperature in- versions may cause locally colder soil tempera- tures in marshy surface depressions, where ice wedges are forming at present (Hamilton and others, 1983). The depth of seasonal thaw above permafrost ranges from 0.4 to nearly 2 m, de- pending on surface cover and local disturbance. Vegetation Forests are widespread below about 1,000 m altitude in interior Alaska (Matthews, 1970). Local forest vegetation is a complex mosaic re- sulting from (1) periodic fires (Lutz, 1956) and (2) differences in slope exposure, parent mate- rial, and permafrost (Pewe and Reger, 1983). Paper birch (Betula papyrifera) and quaking aspen (Populus tremuloides) develop following forest fires on well-drained upland areas and are succeeded by stands of white spruce (Picea glauca). Upland areas underlain by shallow permafrost usually bear stands of black spruce (Picea mariana) with associated shrubs such as Betula glandulosa (shrub birch), Vaccinium spp. (lingonberry and alpine blueberry), and sedges (Cyperaceae). Willow (Salix, spp.), balsam pop- lar (Populus balsamifera), larch (Larix lari- cina), and white spruce grow on recent alluvial surfaces where permafrost is absent; those are succeeded by black spruce and/or by sedge and Sphagnum bogs on older surfaces where perma- frost has aggraded to shallower depths. Regional Geology The Yukon-Tanana Upland is underlain by a complex assemblage of metamorphic rocks that was formerly termed the "Birch Creek Schist" but is now considered part of the Yukon crystal- line terrane of Templeman-Kluit (1976). Mi- caceous quartzites and pelitic schists are domi- Figure 1. Fairbanks area, showing location of nant, and parent rocks are of Precambrian to Fox permafrost tunnel and other localities discussed in text. Pattern on inset map designates late Paleozoic age (Forbes and Weber, 1985). dredge tailings on floor of Coldstream valley; heavy black lines are road network. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/6/948/3380380/i0016-7606-100-6-948.pdf by guest on 23 September 2021 950 HAMILTON AND OTHERS Figure 2. South flank of Goldstream valley, showing entrance to Fox permafrost tunnel (white arrow) during early phase of construction. (U.S. Army photograph) Drill records, excavations, and seismic observa- issued from the glaciers of the Alaska Range cut during placer mining operations when over- tions near the permafrost tunnel indicate that during major glacial advances of the middle and burden was removed from the valley center to bedrock commonly lies at 20- to 25-m depth late Pleistocene (Pewe, 1975b). These deposits prepare the underlying gravel for dredging. The (Fig. 4) and that the upper surface of the bed- can be divided into two groups based on their 110-m-long horizontal tunnel (Fig. 4) was exca- rock is deeply decomposed. Depth to bedrock is subsequent history: (1) primary eolian silt (loess) vated by CRREL personnel during 1963-1966 less on the ridges that flank Goldstream valley, that mantles the hilltops along the southern (Swinzow, 1970). Late in 1965, a 1.22-m- and rock crops out on some ridge crests. margin of the Yukon-Tanana Upland (Pewe, diameter vertical shaft for winter air circulation In the valleys, metamorphic assemblages are 1958) and (2) organic-rich silt of the valley bot- was augered from the ground surface to inter- overlain by the perennially frozen, gold-bearing toms (locally termed "muck"), that was in part cept the tunnel near its inner end (Linell and Fox Gravel of Pewe (1975a), which is —15 m transported downslope by frost creep, solifluc- Lobacz, 1978).
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