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

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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 unknown, 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 permafrost. Cold-air drainage and temperature in- versions may cause locally colder soil temperatures 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 resulting from (1) periodic fires (Lutz, 1956) and (2) differences in slope exposure, parent material, 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 poplar (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 permafrost 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 crystalline 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.

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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). The only tunnel supports, lo- thick in its type locality near the permafrost tun- tion, and running water (Wu, 1984). Bones of cated at the entrance, were installed the same nel and is inferred to be of early or middle Pleis- large, extinct vertebrates, such as bison and year; no others were required because of the tocene age. According to Pewe, the Fox Gravel mammoth, are common in the valley-bottom strength and rigidity of the frozen ground. is a poorly to fairly well stratified, coarse, angu- deposits, which generally contain abundant In 1969, U.S. Bureau of Mines personnel ex- lar, sandy gravel that contains lenses of silt and plant and animal remains. cavated a 61-m-long winze (inclined passage) sand and is stained to a tan color by iron oxides. The perennially frozen, valley-bottom muck from the main tunnel near the portal to the un- Clasts consist mainly of schist and quartz, with deposits of the Fairbanks area have been of par- derlying gravel ~6 m below the tunnel floor. lesser phyllite, gneiss, quartzite, and igneous ticular scientific interest because their well- Several chambers were excavated in the gravel rocks. The unit is as much as 30 m thick along preserved flora and fauna provide rich environ- at that time. A major objective of this phase of larger streams and contains a sparse Pleistocene mental and climatic records. the excavation was to evaluate underground- fauna that includes mammoth and bison. mining techniques for developing placer-gold The silt that covers the gravel and is exposed History of Investigations deposits in permafrost areas (Chester and Frank, in the valley sections is of eolian origin. This 1969; Dick, 1970). fine-grained sediment was blown from the flood The permafrost tunnel was excavated in a The permafrost tunnel has proven to be an plains of sediment-laden meltwater streams that steep face of perennially frozen silt that had been excellent natural laboratory for both geological

[ | Snowfall Figure 3. Average monthly values for snowfall, total precipitation Total precipitation (water equivalent), and mean temperature at Fairbanks, Alaska. Data Mean monthly temperature from National Oceanic and Atmospheric Administration (1985).

35_ Mean annual temperature have been studied by Johansen and others (1981). The present report is based on studies of stratigraphy and sedimentology carried out in the permafrost tunnel during 1982-1984 and on 18 radiocarbon dates obtained during that interval. In addition, one of us (Craig) made a systematic attempt to collect and identify organic remains from the silt and gravel (Craig and others, 1983). Because the earlier reports of Sellmann (1967, 1972) are out of print and not easily accessible, we include some data from those papers in order to provide a single comprehen- sive account of the geology and paleoecology of the Fox permafrost tunnel.

STRATIGRAPHY

The permafrost tunnel provides continuous and undisturbed exposures of ice-rich silt that FMAMJ JASOND overlies gravel and bedrock (Fig. 4). Fanlike deposits of poorly sorted debris unconformably overlie the silt near the tunnel portal. All of the and engineering studies of frozen silt and allu- and modulus of deformation. Slow, plastic de- units are perennially frozen, and large ice vium and associated ground ice. The general formation tests and dynamic loading tests were wedges are common within the silt. geology of the tunnel was discussed by Sellmann also run on the frozen, fine-grained sediment (1967, 1972), who based his studies on strati- (Smith, 1970; Thompson and Sayles, 1972; Bedrock graphic relationships of sediments and ground Sayles and Carbee, 1981). Long-term measure- ice, pore-water chemistry, and 14 radiocarbon ments in the tunnel have yielded rates of defor- Bedrock is exposed only in the lowest part of dates. Sellmann also discussed the engineering mation of ice-rich permafrost and their controls the tunnel system: the floor and lower 0.5-1 m properties and distribution of materials exposed by time, temperature, widths of openings, and of the walls of the chamber at the bottom of the in the tunnel, summarizing his own work and other variables (Johansen and Ryer, 1982; winze. The bedrock is weathered schist of the that of others. Petrographic studies of the ground Weerdenburg and Morgenstern, 1983; Huang, Yukon crystalline terrane (Templeman-Kluit, ice in the tunnel were carried out by Watanabe 1985). Acoustic and electrical properties have 1976). It is frozen but has little visible ice; water (1969). been determined for the various deposits ex- content ranges from 6.5% to 19.9% by dry Determinations of engineering properties of posed in the tunnel (Arcone, 1984; Arcone and weight, averaging 11.7%. X-ray diffraction anal- tunnel materials included standard index proper- Delaney, 1984; Delaney and Arcone, 1984), ysis shows the presence of montmorillonite ties as well as compressive and tensile strength and sublimation processes in the ice-rich silt (Sellmann, 1972), a clay mineral that is

DISTANCE FROM TUNNEL PORTAL (m)

Figure 4. Generalized geologic section of Fox permafrost tunnel showing horizontal tunnel (H), ventilation shaft (V), winze (W), and chamber within Fox Gravel (C).

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A B Figure 5. Typical occurrences of gravel in the Fox permafrost tunnel (scale in centimetres). Reprinted from Sellmann (1972). A. Near- horizontal contact between gravel and bedrock in lower chamber (center of photo). Sand lens in imbricated gravel is evident near top of photo. B. Sand lens containing wood fragments in lower part of gravel unit.

characteristic of (but not confined to) poorly lenses are underlain by oxidized zones ~0.3 m with ice. Ice is visible in voids, but gravel parti- drained soils above permafrost in interior Alaska thick. The lenses become more abundant and cles are still in direct contact with each other. (Allen, 1969). more continuous upward in the gravel, and Moisture content generally is 8.9% to 10.3% those near the top of this unit commonly con- by dry weight. Other physical properties of Gravel tain small willow logs up to 10 cm in diameter. the gravel are discussed by Sellmann (1972, Some logs have intact bark and matted root p. 10-14) and by Chester and Frank (1969). Imbricated sandy gravel interstratified with masses; they lack abrasion and must have origi- layers and lenses of gravelly sand forms a de- nated at or close to the site of deposition. The Silt posit 3 to 4 m thick that overlies the bedrock contact between the gravel and the overlying silt (Fig. 5A). Clasts are subangular to subrounded is sharp but irregular, with channel-like depres- Silt is the most widespread lithologic type and are mostly platy; pebbles and cobbles pre- sions as deep as 1.5 m (Fig. 6). Silt at, and just in the permafrost tunnel and contains the most dominate, with some very small boulders up to above, the contact contains abundant, large abundant and varied types of ground ice ~36 cm in length also present. Quartz-mica wood fragments that may have been derived (Fig. 7). The silt section is 14 to 17 m thick; it is schist is dominant (44% of clasts in cobble from willows growing on the gravel unit. exposed from the base of the surface sod above range), with lesser quartzose schist (24%), quartz The gravel is perennially frozen and bonded the tunnel through nearly all of the vertical shaft, (18%), gneissic rocks (10%), and granitic rocks (4%). Angular to subangular blocks of quartz as much as 44 cm in diameter are scattered as lag deposits along the upper surface of the underlying bedrock. Matrix is light brown (7.5YR 6/4), poorly sorted, subangular, medium to coarse sand with granules. Matrix grains are dominantly quartz (50%-60%), but abundant rock chips (30%-40%) and mica particles (10%) attest to the immaturity of the sediment. The gravel contains near-horizontal lenses of organic silt, silty fine sand, and sandy fine gravel that commonly are 1-2 m long and 20-30 cm thick. Many lenses contain willow wood (Fig. 5B), and several have rooted stumps that appear to be in growth position. Many

Figure 6. Contact between gravel and overlying silt in lower part of winze (scale in centimetres). From Sellmann (1972).

Figure 7. Ground ice exposed in main tunnel. Large ice wedge overlain by lens-shaped ice body that represents frozen ponded water in trough-like depression. Three depth zones are recogniz- able within the frozen pond: CW, clear-water ice; SS, ice with suspended organic sediment; BS, ice-rich bottom sediment. B. Small ice wedge (drill holes are 11.1 cm in diameter). C. Ice lenses in main tunnel (thicknesses vary from less than 1 mm to more than 1 cm). From Sellmann (1967).

the main tunnel, and the inclined drift down to the upper contact of the gravel. Further informa- tion on the silt and its associated ground ice is provided by a series of drill holes -45 m upslope from the tunnel ventilation shaft (Delaney, 1987). The silt generally is unstratified and well sorted. Coarse to medium silt (4-6 ) predomi- nates (Fig. 8), and grain-size distribution is similar to that of loess samples collected elsewhere in the Fairbanks area (Pewe, 1955). Sellmann (1967, p. 11-12) has shown that bulk density of silt in the permafrost tunnel generally ranges from 1.25 to 1.73 g cm-3. Moisture content generally is high (39% to 139% by dry weight), and ice content ranges from 53% to 80% by volume. The silt contains 2.7% to 6.8% organic matter by weight.

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100 n

JO O) 1 JO © 50 0 a) Q. a>) 1 3 E o3

Figure 8. Grain-size analyses of silt from the Fox permafrost tunnel. A. Lower silt. B. Silt associated with thaw unconformity. C. Silt above thaw unconformity. Sample locations are shown in Figure 12. 1001

The silt is divided into two major depositional units (Fig. 4). The lower unit, which generally consists of well-sorted coarse silt with some medium silt (Fig. 8A), is 4.5 m thick where measured along the winze. It contains large ice wedges whose flat tops have been truncated by deep thaw prior to deposition of the upper unit. ? The basal 1.0-1.5 m of the lower silt is organic rich, with total organic content averaging 6.3% n by weight and abundant visible peat lenses and wood fragments as much as 4 cm in diameter. ®50 Silt higher in this unit has a lower organic con- o A period of relatively deep thaw of permafrost coupled with little or no deposition of loess is recorded by the unconformity between the E two silt units, as reported by Sellmann (1967, p. 14-15). This unconformity is marked by o lenses of buried sod, by the distribution and geometry of massive ground ice near the top of the lower silt unit, and possibly by minor inflec- tions at ~ll-m depth in cation concentration values (Fig. 9). Deposits and features associated with degradation of the ice wedges can be found in a zone

Figure 9. Cation concentration with depth in meq/liter (modified from Sellmann, 1967).

~1 m thick above the wedges (Fig. 10). Deep- ening and infilling of troughs above the wedges is marked by small thaw ponds and channels and by organic-rich lenses and collapse blocks composed of grasses, sedges, and small twigs. Clumps, blocks, and deformed layers of organic silt indicate that the troughs acted as drainage courses which accumulated waterborne detritus as well as debris that slumped from their margins (Fig. 10). Where measured near the front of the tunnel, the thaw zone has an organic content of 4.1% to 6.8%, averaging 5.7%, and contains more fine silt and coarse clay than any other samples in the loess section (Fig. 8B). Two of the samples also contain small amounts of mi- caceous, sandy detritus derived from local bedrock. The upper silt, which is -8-11 m thick, is exposed in the upper walls of the tunnel and in the vertical shaft (Fig. 11), and it also was pene- trated in drill holes upslope from the tunnel (Delaney, 1987). The basal 6 m of the upper silt has an organic content of 4.7% to 5.6% by weight, averaging 5.1%; it consists of well-sorted, grayish brown to brownish gray, coarse to me- 0.5 0 40 dium silt with grain-size distribution interme- meq/ J meq/ì, meq/i, diate between those of the lower silt and the

5 METERS

NO VERTICAL EXAGGERATION

EXPLANATION

Younger debris-fan subunit Silty sod fSS? Willow log 11,300 116 0 1 N V 11,400 4 50

« ^ VN Older debris-fan subunit Ice lenses

I* } S | Silt with rootlets Cl§g> PonPondd icicee * (location approximate) I | Silt without rootlets ^'IV// Ice-wedge ice

Figure 10. Silt, ground ice, and debris-fan deposits exposed near tunnel portal. Solid rectangles mark locations of radiocarbon samples (dates in 14C yr B.P.).

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Figure 11. Diagrammatic sketch of vertical ventilation shaft, showing ice wedges, peat lay- Z [^Reddish brown organic sandy silt with ers, and locations of radiocarbon samples. Per- gray to red mottled oxidized zones. mafrost table is at 0.7 m depth. Modified from Sellmann (1967). Reddish brown to brownish gray silt. ]>Woody peat. these wedges taper smoothly with depth, indicat- Gray silt in thin lenses. ing epigenetic rather than syngenetic growth (that is, growth following rather than during ac- Gray silt with fibrous organic matter. cretion of the massive silt). All of the large ice wedges in the lower silt unit have flat to concave upper surfaces; they probably were truncated Numerous large logs. after growth by degradation of permafrost along the thaw unconformity, with some channeling

Fibrous organic gray silt. by running water. Wedges in the upper silt subunit are poorly exposed in the tunnel. Those that >Gray silt with mostly woody organic matter. are visible are up to 1 m wide and generally have exposed depths of only 1-2 m. The tops of large wedges were encountered 3.5 to 5 m below the surface -50 m uphill of the tunnel, »Grayish brown organic silt with rootlets. however, and one wedge was continuous to a depth of 12 m (Delaney, 1987). Buried ice also is present as frozen thaw ponds that overlie many of the truncated ice wedges (Fig. 7A). These features are horizontal, Brownish gray silt with large ice veinlets. saucer-shaped bodies 2-6 m wide and 0.5-2 m Woody zone at 10.4-10.7 m. deep that generally consist of 3 successive depth zones: (1) clear ice with vertical bubble trains, Fibrous peaty masses transitional downward into (2) ice containing and small twigs in silt. reddish brown, suspended organic matter that overlies (3) a lenticular body of unusually ice- P>Sandy silt. RADIOCARBON DATES rich silt. The ice-rich silt, which directly overlies (yr BP) truncated ice wedges, probably originated as Foliated Ice. sediment that slumped or fell into the wedge A 6,970 +135 troughs from their margins. The thaw ponds [Jciear ice. B 8,460 +250 were probably annual features that formed each C 2,510 +570 summer and froze the following winter. This D 30,700 +2100/-1600 cycle was terminated by rapid burial beneath sediment of sufficient thickness to prevent their thaw horizon (Fig 8C). At -5.5 to 6.5 m below tres to several centimetres in thickness (Wata- subsequent thaw. the ground surface within the vertical shaft, nabe, 1969). Most of the pore ice and segregated these sediments grade upward into silt with a ice formed during freezing of the silt and has Debris-Fan Deposits generally higher content of macroscopic organic been preserved since that time. remains that include peat layers and wood frag- The ice wedges formed as a result of repeated The fanlike deposits of debris near the tunnel ments and with organic content as much as 7.4% ice accumulations in contraction cracks that portal consist of subangular pebbles and cobbles by volume. Radiocarbon dates from the vertical opened repeatedly during a series of winters and of quartz and schist in a matrix of pale brown shaft indicate that most of the organic-rich silt were then filled with windblown snow and (10YR 6/3), poorly sorted silty sand to sandy accreted during Holocene time (Fig. 11). The snow meltwater (Lachenbruch, 1962). Large ice silt (Fig. 10). Platy stones generally are horizon- lower tips of several large ice wedges protrude wedges occur in both the upper and lower silt tal to gently dipping (as much as 15°) and tend through the roof of the tunnel, but only small units. Wedges within the lower silt are 2-4 m to lie parallel to bedding. Roll structures are wedges were observed in the vertical shaft (Sell- wide at the top and extend vertically 3-4 m. present in places, suggesting mass movement by mann, 1967). They are vertically foliated, with concentrations flowage. Grain shape and composition indicate Ground ice in the silt includes four of of silt and organic detritus along foliation planes. extreme immaturity and local derivation of the the major types described by Pewe (1975b, Some wedges contain blocks of silt that must sediment. Cobble-sized blocks are dominantly p. 48-49): pore ice, segregated ice, foliated have been incorporated when a new frost crack quartzose schist and quartz-mica schist, with wedge ice, and buried surface ice (Fig. 7). The propagated through the edge of a wedge and lesser quartz (42%, 40%, and 18%, respectively). pore ice fills the voids between silt particles and into the host sediment (D. M. Hopkins, Univer- Most of the blocks (90%) are subangular; the bonds them into a solid mass. The segregated ice sity of Alaska, 1987, written commun.). Ice others are angular. Sand- and granule-sized ma- occurs as small films, lenses, veinlets, dikes, and crystals and trains of bubbles also exhibit general trix grains are —40% quartz, 55% rock frag- irregular masses that range from several millime- vertical orientations (Watanabe, 1969). Most of ments, and 5% mica. Elongate platy slabs show a

weak fabric approximately parallel to the direc- TABLE 1. RADIOCARBON DATES FROM THE FOX PERMAFROST TUNNEL tion of slope of the valley side. Date (yr B.P.) Material dated and Reference The debris deposits are divisible into two and lab no. stratigraphic position subunits. The lower subunit consists of silt that •2510 ± 570 Fine fibrous organic material; 7.9-m depth Sellmann, 1967 contains sparse gravel, abundant shrub wood, (1-2120) in vertical shaft

and thin lenses of unidentified organic matter. It 6970 ± 135 Fine fibrous organic material; 1.8-m depth Sellmann, 1967 fills channel-like depressions on the surface of (1-2118) in vertical shaft the underlying silt. The upper subunit contains 8460 ± 250 Fine fibrous organic material; 3.7-m depth Sellmann, 1967 (1-2119) in vertical shaft relatively abundant wood fragments, rock rub- 11,000 ±280 Wood. Log near tunnel portal Sellmann, 1967 ble, bones, and logs of willow up to ~ 15 cm in (1-1370)

diameter. One of the willow logs leans toward 11,300 ± 160 Wood (willow). Upper debris-fan deposit This paper (I-12,655) near tunnel portal. Check date on the valley center at -45° and has an accumula- following sample (1-1369)

tion of detrital organic debris on its upslope side. 11,400 ±450 Wood. Upper debris-fan deposit near Sellmann, 1967 Willow trees or large shrubs evidently acted as (1-1369) tunnel portal barriers that trapped some of the larger debris 11,910 ± 180 Wood (willow?). Lower debris-fan deposit This paper (1-12,657) near tunnel portal fragments streaming past them. The upper sub- 12,570 ± 390 Peat. Contact between upper and lower This paper unit overlies an erosion surface that cuts across (1-12,656) debris-fan deposits near tunnel portal

the lower subunit. 13,470 ± 420 Bone; debris-fan deposit near tunnel Sellmann, 1967 (1-2196) portal

14,280 ± 230 Bone; debris-fan deposit near tunnel Sellmann, 1967 RADIOCARBON DATING (1-2197) portal

•27,790 ± 560 Peaty silt a few centimetres above silt/gravel This paper The 33 radiocarbon dates from the Fox (Beta-4584) contact permafrost tunnel are listed in Table 1. Materials 30,160 ± 160 Small organic fragments (wood, grass, and This paper (USGS-1516) rootlets); silty peat about 1.8 m above dated were primarily large wood fragments, large ice wedges peat, and fibrous plant remains concentrated 30,700 + 2100 Fine fibrous material; silt 1.8 m above Sellmann, 1967 - 1600 wedge ice at 10.4-m depth in vertical from the silt by screening. Ice-wedge residues, (1-2121) shaft

bones, and organic detritus each provided two 31,200 + 3000 Wood, grass, and rootlets; base of frozen This paper samples. The large wood fragments were from -2200 thaw-pond deposit above ice wedge (USGS-1517) logs and stumps ~8 to 15 cm in diameter that 31,400 + 2900 Amorphous organic material (ice-wedge Sellmann, 1967 occur in the debris apron near the portal and in -2100 residue); wedge ice near rear of tunnel lenses of silt and fine sand associated with the (1-1842) 32,300 + 2000 Amorphous organic material (ice-wedge Sellmann, 1967 Fox Gravel. A few stumps in the Fox Gravel - 1600 residue); about 65 m from tunnel portal were still rooted at the time of sampling; other (1-1843) logs and stumps associated with the gravel re- 32,790 ± 560 Silty peat with abundant rootlets. Buried This paper (USGS-2553) sod 0.9 m above ice wedge tained twigs or bark and clearly had not been 33,200 ± 1900 Woody peat; 40 cm above silt/gravel contact Sellmann, 1972 transported far. Four samples of woody peat, (1-4493) taken within 1 m of the base of the lower silt 33,700 + 2500 Twig; sandy sediment above flat-topped ice Sellmann, 1967 - 1900 wedge near rear of tunnel unit, contained unabraded twigs that probably (1-1841)

were derived from shrubs that grew locally. 33,750 ± 2000 Wood about 1 m below silt/gravel contact Sellmann, 1972 Four other peat samples represent buried sod (1-4494) horizons higher on the section. The fibrous plant 34,180 ± 600 Wood at silt/gravel contact This paper (Beta-4582) remains, which occur throughout the silt, in- 35,500 ± 2400 Peat; sod mat 1 m above fiat-topped ice This paper clude abundant fine rootlets and the stems of (1-12,658) wedge

grasses and sedges. These organic materials were 35,970 ± 920 Silty peat; 1 m below tops of large ice This paper sampled mainly where concentrated in peaty (USGS-1518) wedges near tunnel portal layers or lenses. Two samples of organic residue «36,000 ± 1200 Wood from woody peat; silt/gravel contact This paper (USGS-1714) (later redated as >46,000 yr B.P.) and amorphous plant material were obtained by 36,200 ± 2500 Wood (willow) within channel filling 6 m This paper 3 melting large samples (-0.1-0.2 m ) collected (1-13,122) from back wall of tunnel from ice wedges in the tunnel walls (Sellmann, 38,470 ± 470 Peaty organic mat; 0.9 m below tops of This paper 1967, p. 16). Both samples consist of unidentifi- (USGS-1519) large ice wedges near tunnel portal 38,860 ± 930 Wood; peaty horizon 1 m above silt/gravel This paper able organic detritus that was washed into open (USGS-1520) contact

contraction cracks during spring, and our dates 41,000 ± 1400 Wood (stump) from gravel unit 1 m below its This paper demonstrate that they were derived from the (USGS-1713) upper contact ground surface or its sod layer at the time of 43,300 ± 1600 Wood (willow?) on channel floor at silt/ This paper (USGS-1521) gravel contact ice-wedge growth. Because the ice samples were >39,900 Small log; about 1 m above gravel/bedrock Sellmann, 1972 collected across the entire width of each wedge, (1-4588) contact the dates should provide mean ages for the ice >39,940 Peaty silt a few centimetres above silt/gravel contact This paper wedges. Two dates were obtained on stream- (Beta-4583) (duplicate of Beta-4584) >40,740 Wood at silt/gravel contact (duplicate of This paper worn and possibly redeposited bones (Sellmann, (Beta-4581) Beta-4581)

1967, p. 18) and may provide poor age control >4«,000 Wood from woody peat; silt/gravel contact This paper on their host sediments. The remaining samples (USGS-1714) (initially dated at 36,000 ± 1200 yr B.P.)

are detritus from above the tops of truncated ice "Date probably invalid (see text).

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wedges. One sample was composed of wood, TABLE 2. RADIOCARBON DATES NEAR SILT/GRAVEL CONTACT, FOX PERMAFROST TUNNEL grass, and rootlets from the base of a frozen Dale Laboratory no. Stratigraphic position thaw pond; the other was a small twig from

fluvial sand. 38,860 ± 930 USGS-1520 1 m above silt/gravel contact

Three of the dates (marked with an asterisk 33,200 ± 1900 1-4493 0.4 m above silt/gravel contact on Table 1) are clearly spurious. The date of 27,790 ± 560 Beta-4584 A few centimetres above silt/gravel contact 2510 ± 570 yr B.P. (1-2120), which is strati- >39,940 Beta-4583 33,180 ± 600 Beta-4582 graphically out of place (Fig. 11), was obtained 43,300 ± 1600 USGS-1521 At silt/gravel contact from a very small sample of fibrous organic >40.740 Beta-4581 >46,000 USGS-1714 matter that contained little datable carbon 33,750 ± 2000 1-4494 1 m below silt/gravel contact (Sellmann, 1967, p. 18). The date of 27,790 41,000 ± 1400 USGS-1713 ± 560 yr B.P. (Beta-4584) conflicts with other dates of 39,000 yr B.P. and older near the base of the eolian silt; a date of >39,940 yr B.P. (Beta-4583) on a duplicate sample by the same laboratory seems more likely to be correct. A sample at the contact between the silt and the underlying gravel was dated at 36,000 ± 1200 yr B.P. (USGS-1714), but a second age determina- tion on that sample yielded a date of >46,000 yr B.P. Similar samples from the same horizon or slightly above it are dated at 43,000 ± 1600 (USGS-1521), >39,940 (Beta-4583) and >40,740 yr B.P. (Beta-4581), confirming the laboratory's conclusion that the 36,000-year-old date probably is too young (Stephen R. Robin- son, U.S. Geol. Survey, 1985, written commun.). Ten samples were dated in an unsuccessful attempt to determine the age of the contact between the lower silt and the gravel, and hence the beginning of loess accumulation at the tunnel site. A listing of these dates in stratigraphic order (Table 2) clearly indicates that many of the dates must be too young and that the best age approximations are those provided by the U.S. Geological Survey's research laboratory at Menlo Park, California. One plausible interpre- tation of the USGS dates is that loess deposition may have started about 43,000 yr B.P. and that loess accumulated very slowly until about 38,860 yr B.P., accelerating thereafter (as discussed below). The highly organic character of the basal 1 m of the silt provides some support for the inference that loess accreted slowly at first and later was deposited much more rapidly. Nine younger dates between about 38,500 and 30,000 yr B.P. define an interval of rapid loess accretion followed by growth and truncation of large ice wedges (Figs. 10 and 12). Peaty silt ~ 1 m below the level of truncated upper surfaces of the ice wedges dates 35,970 ± 920 yr B.P. (USGS-1518), and a second sample from -0.5 m greater depth dates 38,470 ± 470 yr B.P. * Sampled 0.8-0.9 m below tops of 2 separate ice wedges (USGS-1519). A subsequent episode of sod ac- ** Exact stratigraphic placement uncertain cumulation is dated at 32,790 ± 560 yr B.P. (USGS-1553), and probably concurrent ice- Figure 12. Diagrammatic sketch of silt and upper part of gravel wedge formation is indicated by dates of 31,400 exposed in main tunnel and winze, showing typical ice wedge and +2900/-2100 yr B.P. (1-1842) and 32,300 overlying thaw pond. Thaw-pond facies same as in Figure 7A. Solid +2000/-1600 yr B.P. (1-1843) from organic rectangles, 14C samples; solid circles, granulometry samples.

bulk carbon technique that later was shown to TABLE 3. VERTEBRATE FOSSILS IDENTIFIED FROM FOX matter extracted from two ice wedges. A similar PERMAFROST TUNNEL, 1982-1984 date of 31,200 +3000/-2200 yr B.P. (USGS- yield erratic results (James Buckley, Teledyne

1517) was obtained from sediment beneath a Isotopes, 1987, oral commun.). Unit Taxa Elements) recovered thaw pond above an ice wedge (Fig. 10). Two The two youngest dates, on fine fibrous plant Debris-fan Equus sp. Teeth concordant younger dates (USGS-1516 and I- remains near the top of the ventilation shaft (Fig. deposits 2121) indicate that further loess accretion was 11), suggest that ~2 m of silt was deposited Bison sp. Vertebrae Long bones taking place by 30,160 ± 160 yr B.P. Much of between about 8460 and 6970 yr B.P. (1-2119 Horn core the interval between about 33,000 and 30,000 and 1-2118, respectively) and that an additional c.f. Mammuthus(7)Massiv e bone fragments yr B.P. must have been a time of reduced loess 2 m of silt was emplaced sometime after 6970 Spermophilus Teeth Vertebrae influx, with formation of surface sod, growth of yr B.P. These are maximum accumulation rates, Calcanae ice wedges, and later truncation of some wedges. because a volume increase of -20% to 30% oc- Various long bones Coprolites Two additional dates of about 36,200 ± 2500 curs in these sediments owing to ice-lens forma- Silt c.f. RangiferC1) Weathered incomplete rib yr B.P. (1-13,122) and 33,700 +2500/-1900 yr tion upon freezing. Uncertain (probably Weathered long bone B.P. (1-1841) were obtained from woody detri- from mammal of with condyles* caribou or elk size) tus in alluvium near the inner end of the perma- PLANT AND ANIMAL REMAINS

frost tunnel. The older date is from a log of •Removed by persons unknown and never identified. willow wood 8 cm in diameter that lies within a A diverse assemblage of animal macrofossils lens of sandy, fine gravel formed by stream and plant remains was found in the Fox perma- downcutting into the loess; the younger date is frost tunnel (Figs. 13 and 14). Disarticulated from an abraded detrital twig in sandy sediment insect remains include abundant and diverse 15). T. mannerheimi has been collected from the above an ice wedge. Their large counting errors beetles, as well as the remains of mites, flies, sandy slopes of a moraine with scarce, low vege- suggest that both dates could be minimum limit- moths or butterflies, true bugs, and sawflies tation in Denali National Park (Lindroth, 1961). ing ages, and the detrital character of the wood (Fig. 13 A); the shells of snails are also present in The rove beetles Tachinus apterus and brevi- indicates the possibility of reworking from much several horizons. Plant macrofossils are repre- pennis are presently restricted to tundra (Fig. older deposits. Alternatively, deposition of the sented by seeds and leaves of grasses, sedges, 16), and their presence during the middle Wis- alluvium possibly could have been contempo- shrubs, and trees; wood fragments from shrubs consin suggests cooler summers and perhaps raneous with the episode of ice-wedge growth and trees; mosses; and various herbs (Fig. 13B). warmer winters than the present-day Fairbanks and truncation elsewhere in the tunnel. Most of the samples contained little pollen, or area. The rove beetle Stenus and the ground Dates from seven samples taken near the tun- grains that were so poorly preserved that they beetle Pterostichus (Cryobius) ventricosus are nel portal are between 11,300 and 14,280 yr could not be identified, but the base of the lower presently restricted to tundra and the tundra- B.P. The five youngest dates are from the fanlike silt unit contained sufficient pollen for percen- forest transition zone but are usually found not debris deposits that formed near the valley mar- tage counts. Vertebrate fossils (Table 3) are rep- far from running water or moist ground (Lin- gin between about 12,500 and 11,000 yr B.P. resented by bones of bison, horse, mammoth(?), droth, 1966). P. (Cryobius) brevicomis inhabits (Fig. 10). Dates of about 11,000 and 11,400 yr and caribou(?), as well as by coprolites and dry, open areas of tundra and boreal forest with B.P. (1-1370 and 1-1369, respectively) were re- bones of arctic ground squirrel. a vegetative cover of grasses, low shrubs, and ported by Sellmann (1967, p. 18) on logs asso- Most of the species listed in Figure 13 are leaf litter (Lindroth, 1966). Tall shrubs or trees ciated with the debris; a nearly identical date of common members of the present-day boreal in the area are indicated by the presence of the 11,300 ± 160 yr B.P. (1-12,655) later confirmed forest and (or) tundra biomes in Alaska. Because fly Xylophagus, whose larvae occur under the the age of the older log. These three dates sug- several specimens have not been identified to bark of rotten trees (Chu, 1949). As there is an gest that a stand of tall willows was over- species, their distribution and ecology cannot be abundance of willow wood (Salix), this may whelmed by an accreting fan of detritus near the discussed in detail, and we cannot be certain that have been the host. A pollen sample from the base of the valley side, probably sometime be- every individual species in our assemblage is ex- basal metre of the silt shows a predominance of tween 11,300 and 11,150 yr ago. Two addi- tant. Notes on the ecology and distribution of grasses (61%), birch (Betula) (18%), and alder tional dates were obtained from an older fan selected taxa are summarized in Appendix A.1 (Alnus) (9%) (T. A. Ager, U.S. Geological Sur- deposit that underlies an erosional unconform- Abundant willow logs and stumps in the vey, 1983, written commun.), although macro- ity. A peat lens that lies along the unconformity gravel unit demonstrate that thickets of tall wil- fossils of these taxa have not been found. The is dated at 12,570 ± 390 yr B.P. (1-12,656), and lows were growing on the flood plain of Gold- relatively high percentages of alder suggest pos- wood from the lower debris fan is 11,910 ± 190 stream Creek sometime before about 43,000 yr sible interstadial climate, as most samples from yr old (1-12,657). The minor inversion in the B.P. (Fig. 12). Sparse pollen, spores, and macro- full-glacial deposits in interior Alaska either lack two dates is within the range of their combined fossils associated with this unit suggest a normal alder or have only trace amounts, and birch counting errors and is not considered to be sig- riparian ground cover that included alder, birch, pollen is usually rare in full-glacial samples as nificant. The two dates suggest that an earlier willow, grasses, sedges and other herbs, and well (T. A. Ager, U.S. Geological Survey, 1987, episode of gullying on the hillslope and fan mosses. written commun.). These assemblages represent a local environment of relatively dry, open deposition at its base may have occurred about In the lowest metre of the silt, the presence of meadow composed of shrubs and herbs (mainly 1,000 yr before emplacement of the younger the weevil Lepidophorus lineaticollis and sedges and grasses). fan. Bone fragments within the debris fan ground beetles Notiophilus, Pterostichus (Cry- yielded significantly older dates of about 13,470 obius) brevicomis, P. C. ventricosus, and Tricho- Higher in the lower silt unit, the continued and 14,280 yr B.P. (1-2196 and 1-2197, respec- cellus mannerheimi suggests a xeric, open, presence of dry, open habitat is evidenced by the tively), but the bones were dated in 1966 by a mainly sunny substrate (Lindroth, 1966) (Fig. weevil L. lineaticollis and by pollen of Artemisia

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BUTTERFLIES WASPS

Figure 13A. Invertebrate macrofauna recovered from the Fox permafrost tunnel.

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+ + + + + + + + + + + + O + + + + + + + + + + + + + o + + + + + + + + + o + + + + + + + + + + + o + + + + + + + o + + + + + + + + + + +0 + o +

+ + O +

O O O Or, u Pond x x x x Organic •+• Present n ¿rw Rooted w vvooWoood a o 0a cr Bones % fragments °A

Figure 13B. Macroflora and pollen recovered from the Fox permafrost tunnel.

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 Figure 14. Scanning electron micrographs of selected macrofossils from Fox permafrost tunnel (bar is 200 n). A. Pronotum of ground beetle Trichocellus mattnerheimi. B. Dorsal view of orbatid mite Parachipteria nivalis. C. Anterior view of head and pronotum of weevil Lepidophorus lineaticollis. D. Seed of sedge Carex. Ground beetle is from horizon dated at older than 39 ka; the three other fossils are from horizons dated between about 35 ka and 30 ka.

(sagebrush). The presence of the fly Xylophagus cottongrass (Eriophorum), and grasses (Hiero- sisted of low shrubs and herbs (mainly grasses suggests that some tall shrubs may have been chloe ordorata or H. pauciflora; both species are and sedges); associated mammal remains proba- growing locally. Trace amounts of spruce pollen restricted to tundra and do not occur in the bly include caribou (Table 3). possibly indicate that spruce stands were present Fairbanks area today). T. apertus and T. brevi- We have no detailed fossil record from higher in the lowlands (see Matthews, 1974a); alterna- pennis, presently restricted to tundra, are pres- levels in the silt because this part of the section is tively, they might indicate that some redeposi- ent. A marked increase in the seeds of sedge exposed primarily in the vertical shaft, which tion of older loess was taking place. Willow (Carex) may also indicate moister, more favor- presently is inaccessible. wood is smaller in diameter (generally <0.4 cm) able growth conditions. Shrub willow continues The lower debris fan subunit contains shells than in the basal silt, suggesting lower shrub to persist. Some fossils (particularly weevils, of amphibious gastropods (Succinea strigata), height. These assemblages represent an envi- chrysomelid beetles, and mites) are extremely which indicate moist to wet substrates. At least ronment of open, low shrub-herb meadow well preserved (Fig. 14). Some species, such as two well-preserved mosses are present: Hyloco- dominated by sedges. Fossils were scarce and ground beetles Amara and Harpalus and wee- mium splendens, reported from moist, acidic to poorly preserved, and many were impossible to vils L. lineaticollis, indicate dry, open substrate. neutral soils and humus, particularly in boreal identify. This mixture of taxa characteristic of wet and forest, and Rhytidium rugosum, reported from Silt near the thaw unconformity contains dry substrates is an expectable result of the di- moist or dry calcareous soil and gravel, some- more numerous taxa indicative of moist to wet verse microhabitats associated with active ice- times among shrubs near the overflow zone of substrates (Figs. 13 and 15). Moister conditions wedge polygons. Orbatid mites are numerous rivers (Steere, 1978). Shrub willows, horsetails are indicated by macrofossils of amphibious gas- and diverse, but their environmental implica- (Equisetum), sedges (Carex), and other herbs tropods (Columella sp. c.f. edentula), ground tions are uncertain. These assemblages represent are also present. beetles [Bembidion and Pterostichus (Cryobius) a more mesic local environment than those re- The upper debris fan subunit contains abun- ventricosus], hydrophilid beetles (Helophorus), corded lower in the section. Vegetation con- dant willow wood up to —15 cm in diameter and bones of extinct horse (Equus), Bison, and probably mammoth (Mammuthus), a late Pleis- Minimum number of individuals tocene vertebrate assemblage common to the Depth (m) per g silt (dry wt) Fairbanks area (Guthrie, 1968a, 1968b, 1972). 0 0.2 0 Fossils of arctic ground squirrel (Spermophilus) indicate that a tundra-like environment must have existed, because arctic ground squirrels are

Figure 16. Modern distribution of Tachinus apterus (solid squares) and T. brevipennis (solid triangles) and their fossil locations (open Figure 15. Abundance and ecologie affinities of fauna! macrofossils squares and triangles, respectively). After Ullrich and Campbell from lower silt unit and thaw unconformity in Fox permafrost tun- (1974). Inset shows maximum and minimum temperature envelopes nel. A, arboreal; M, mesic and aquatic; X, xeric. Depth scale same as for summer and winter for these two species as compared to present- in Figure 12. day Fairbanks area.

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B D BIRCH AND Years PERMAFROST ISABELLA BASIN CANYON CREEK HARDING x 103 m TUNNEL LAKES

1 10B C 10 A • 3510 ±215 PICEA- 2 ^ ALNUS- I 9200 ±160 BETULA • 12,570 ±390 ZONE • 11,910 ±180

3 • 7840 ±280 • 8680 ±110 PICEA-BETULA B > 9460 ±155 9860 ±120 10 • 11,500 ±190 10 POPULUS- »30,160 ±160 SAUX ZONE I 11,300 ±150

BETULA 32,300 IfOOO ZONE • 13,690 ±500 V"35,970 ±920 39,360 ±1740(?) • 14,730 ±830 • 38,860 ±930 71,000 ±7000 15 ^J"43,300 ±1600 ° P:b°„•'46,C,00 0 •¿.•o'i o °'.o 4 HERB • >31,900 ZONE Ab 2 0 20

> > ^

a. << • 34,900 25 ±2950 PICEA- • 26,500 ±400 ERICACE AE SPHAGNUM EXPLANATION ZONE

3 0 L 30 Colluvial or alluvial slope deposit y Twigs

Silt/sandy silt (loess) Organic layers

ggg Thaw pond ° " û\ Arkosic rubble Qi\ «o O o

lltf Ice wedge Sand/gravel (alluvium)

ESS Bedrock www paleosol

Figure 17. Stratigraphy and radiocarbon dates from the Fox permafrost tunnel compared with stratigraphy and dates from the Isabella basin and Canyon Creek sites and with regional pollen zones of Ager.

now restricted to arctic and alpine tundra and do DISCUSSION but is poorly recorded within the tunnel itself. not occur in the Fairbanks area today. The exact chronology of the younger ice wedges The debris fan provides the poorest pollen The Permafrost Tunnel is therefore uncertain. record, with most grains severely corroded and The imbricated gravel at the base of the Pleis- unidentifiable. The poor pollen preservation The unconsolidated deposits within the Fox tocene section contains lenses of silt, sand, and may be a result of oxidation (T. A. Ager, U.S. permafrost tunnel record deposition of fluvial sandy fine gravel that may represent channel Geol. Survey, 1983, oral commun.). Gastropod gravel followed by a long interval of loess accre- fillings of a braided stream system. These fine- and moss taxa, together with large willows, sug- tion and permafrost aggradation (Fig. 17A). The grained deposits become more abundant up- gest that a warmer, more mesic environment loess-forming interval was punctuated by a pe- ward and, at the top of the unit, form a laterally followed the late Wisconsin interval of loess riod when little loess accumulated at the site and extensive overbank deposit that supported a deposition and preceded the spread of boreal when large ice wedges developed on the floor of thicket of large willows. The absence of soils and forest during the Holocene. Low-growing, open Goldstream valley and were truncated subse- weathered clasts suggests that the vegetated vegetation must still have been present to ac- quently during an interval of thaw. A younger flood plain existed only a short time before it commodate the large herbivores whose remains episode of ice-wedge formation is known from was buried by loess. Pollen and macrofossil as- are found in the debris fan. drill holes upslope from the permafrost tunnel semblages indicate that boreal forest was absent

at this time, and the sediments may therefore (Carex), willows, and various herbs are present, B.P. on organic material 7 m higher in the core record an interstadial interval rather than a true although they are not well represented in the provides some support for this inference. Fossil interglacial. pollen assemblage. Grain-size distribution shows taxa at the permafrost tunnel and at Isabella The gravel unit is physically identical to the that loess from this interval (Fig. 8B) contains basin are compared in Appendix B.2 Most Fox Gravel of Pewe (1975a) and occupies the more fine silt and coarse clay than most other noteworthy is the number of aquatic taxa pres- same stratigraphic position at the base of a thick loess samples from the permafrost tunnel. This ent at Isabella basin but absent from the tunnel loess section (Pewe's Goldstream Formation). fine-grained component represents either a site during the interval from 35 to 30 ka. Pollen Radiocarbon dates and stratigraphic relations somewhat finer loess rain at the tunnel site or a analysis of the Isabella basin core by Matthews suggest, however, that it is younger than the separate influx of fine, windblown dust that be- (1974a) shows a basal interstadial environment early or middle Pleistocene age inferred by Pewe came mixed with older and coarser loess in a (subzone Aa) with scattered spruce that was (1975a) or at least that its deposition continued frost-churned active layer above permafrost. succeeded by an abrupt change to milder condi- into the late Pleistocene. The absence of soil or The upper loess unit, which began to accumu- tions more like those of today (subzone Ab). weathering profiles supports our radiocarbon late sometime after about 30 ka, is poorly ex- Subzone Ab, marked by alders and arboreal dates, which indicate that gravel deposition at posed in the permafrost tunnel. It may have birches, may correspond to the period of slow the tunnel site may have continued until shortly accreted to ~6 m in thickness by 11.5 ka if the loess accretion and ice-wedge growth and thaw before 40,000 yr B.P. zone of abundant large logs in the vertical shaft that occurred in the permafrost tunnel between The silt that overlies the gravel was trans- is of the same age as the radiocarbon-dated wil- about 35 and 30 ka. The succeeding pollen zone ported to Goldstream valley as loess, and our low logs near the tunnel portal. If loess accumu- B is marked by extremely low spruce, alder, and observations in the tunnel suggest that much of lation were continuous during the time of Sphagnum and very high percentages of Artemi- the silt formed in place as eolian sediment and maximum glaciation in the Alaska Range, dated sia, indicating a cold, treeless "steppe tundra" was not displaced significantly by running water at about 25 to 11.8 ka by Ten Brink and Way- environment that probably was more arid as or by mass-wastage processes. The silt typically thomas (1985), then rates of accretion would well as colder than the present environment. contains undisturbed peaty layers and lenses and have been slightly lower than those of the lower Wood fragments higher in the core are dated at in situ rootlets, and it lacks either dispersed indi- loess unit. The uppermost organic-rich silt sub- 11,500 ± 190 yr B.P., and this woody zone vidual fragments or lenticular concentrations of unit is inaccessible at present because of heavy marks a major change in soil characteristics that exotic detritus. It also lacks sedimentary struc- ice accumulation in the ventilation shaft, but Allen (1969) interpreted as indicative of milder tures indicative of running water or mass- Sellmann's earlier (1967) observations suggest climatic conditions. The radiocarbon dates indi- wastage processes. In addition, our observations surprisingly rapid deposition in early Holocene cate an acceleration in the rate of silt deposition coupled with radiocarbon dates, granulometry, time (2 m in 1,500 yr) followed by a much at this time, with a maximum of ~6 m accumu- and paleontological data show that, through slower rate during the middle and late Holocene. lating during the following 2,300 yr. Pollen zone much of the tunnel, slow deposition of highly The episode of rapid accumulation may reflect C, representing the past 8,500 yr, is marked by organic loess alternated with rapid accretion of active redeposition of sediment from surround- high spruce and birch, reflecting taiga conditions less organic loess beneath a more open plant ing uplands at that time. similar to those of the present. Major incon- cover. An episode of deep-seated erosion of the loess sistencies in the Isabella basin core occur be- The basal 1.0 to 1.5 m of peaty loess evidently slope that overlooks the site is marked by debris- tween depths of 4 and 10 m, an interval accreted slowly during an interval of perhaps fan deposits that accumulated at the tunnel por- radiocarbon dated at 11,500 to 9,200 yr B.P. 4,000 yr or more. Pollen data suggest the proba- tal between about 12.5 and 11 ka. These This interval includes the only major date rever- ble presence of shrub tundra or herb-shrub tun- deposits have been inferred by Hopkins (1982, sal in the core (a spurious date of 7840 ± 280 yr dra, with birch, alder, and grasses dominating p. 10-11) to be part of a widespread system of B.P.), major discrepancies between climatic in- the pollen assemblage; plant macrofossils are inactive gullies and coalesce« fans in the Fair- ferences from soils and pollen, a marked in- dominated by sedge (Carex). The succeeding banks area. Rollover structures and tilted stumps crease in soil conductivity (Brown and others, 3 m of loess has less organic material, and its few indicate that mass flowage accounted for part of 1969), and an apparent rate of loess accretion fossils are poorly preserved. This part of the the transport of loess to valley bottoms at this that is far higher than any that we have docu- loess section was deposited between perhaps 38 time, but abundant schist rubble in the upper-fan mented in the permafrost tunnel. All of these and 34 ka; it must have accreted at least twice as subunit suggests that deep gullies must have anomalies could be explained by an episode of rapidly as the basal loess, and both plant and eroded through the thick loess on the slope to slope movement, concentrated within the inter- animal fossils indicate a more xeric depositional "unroof' the underlying bedrock during the val of about 11,500 to 9200 yr B.P., that caused environment. same interval. mixing and redeposition of older sediments. A subsequent interval of slow loess accretion The Canyon Creek vertebrate-fossil locality, between about 36 and 30 ka is marked by an Comparisons with Other Late Pleistocene described by Weber and others (1981), lies 85 episode of vigorous ice-wedge growth. At the Records km southeast of Fairbanks along the north side end of this period of wedge growth, the active of the Tanana River (Fig. 1). River gravel asso- layer probably thickened, and thermokarst de- The stratigraphic record of a 27-m core from ciated with a 15-m outwash terrace of the Ta- veloped; troughs above the ice wedges deepened the basin of Isabella Creek, 4.3 km southwest of nana River (unit 1 in Fig. 17C) is succeeded and were occupied by small, shallow ponds or the permafrost tunnel (Fig. 1), has been de- upward by alluvium and colluvium derived discontinuous stream courses as described else- scribed by Allen (1969), Brown and others from the basin of Canyon Creek (units 2-6) and where in areas of present-day permafrost degra- (1969), and Matthews (1974a). The core pene- then by sandy loess (units 7-9). The outwash dation (for example, see Czudek and Demek, trates frozen silt and organic silt which has been gravel is correlated with the Delta glaciation of 1970). Well-preserved and abundant macrofos- radiocarbon dated at 34,900 ± 2950 to 4510 ± the Alaska Range, which was inferred by Pewe sils suggest moister conditions than those of the 120 yr B.P. (Fig. 17B). The large counting error (1975b) to be of Illinoian age, but which Weber previous episode of loess accretion (Fig. 15). suggests that the older date may actually be a Remains of grasses (Hierochloe), sedges minimum age limit, and a date of >31,900 yr 2See footnote 1.

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and others (1981) assigned to the early Wiscon- and drier climate associated with increased fre- northwestern Canada. Plant assemblages were sin. Overlying the gravel is 1-4 m of inter- quency of wildfires. similar to present during Boutellier time but bedded sand and arkosic gravel. The arkosic Additional radiocarbon-dated records of late were displaced downslope from their present gravel is of local origin, originating from granitic Wisconsin loess accumulation and ice-wedge limits. plutons near the head of Canyon Creek; the sand growth in interior Alaska have been published Radiocarbon dates from organic sediments is in part an overbank deposit of the Tanana by Matthews (1968), Pewe (1975a, 1975b), associated with till and outwash indicate that River, but higher in the section it was predomi- and Ten Brink and Waythomas (1985). These late Wisconsin glaciation in the Alaska Range nantly deposited by Canyon Creek. Fossil bones records show ice wedges penetrating deposits began about 25 ka and that glaciers were under- and redeposited tephra in the upper part of the dated at 24 ka (Matthews, 1968) to >35 ka going rapid retreat by about 13.5 or 12.5 ka unit 2-6 sequence have an apparent radiocarbon (Ten Brink and Waythomas, 1985). Basal dates (Hamilton, 1982; Porter and others, 1983; Ten age of 39,300 ± 1740 yr B.P., but the tephra is of peat that caps the wedges range from about Brink and Waythomas, 1985). Deposition of the correlated with the Sheep Creek tephra of the 10 ka (Pewe, 1975a, 1975b) to 7320 ± 140 yr upper silt subunit in the permafrost tunnel oc- Fairbanks area, dated by thermoluminescence at B.P. (Ten Brink and Waythomas, 1985). Pub- curred during this general time period; it began 76,000 ± 7,000 yr B.P. (J. A. Westgate, Univer- lished data are inadequate to determine whether about 30 ka or shortly thereafter and terminated sity of Toronto, 1986, oral commun.), and the the ice wedges formed during or after accretion at or slightly before 12 ka, when the debris fan bones have yielded U-series determinations that of the loess. began to accumulate at the base of the valley suggest an age of about 80 ka (Hamilton and side. The Isabella basin soil record shows that Bischoff, 1984). A pronounced unconformity at Regional Synthesis cold conditions persisted until about 11,500 yr the top of the colluvium is marked by venti- The environmental records reviewed above B.P., but the pollen record seems to show that facted quartz pebbles and the upper limits of can be integrated into a regional history of en- full-glacial conditions continued past that time sand-filled wedge casts and rodent burrows. A vironmental fluctuations during the past 40,000 (Matthews, 1974a). Ager's pollen data from thick deposit of moderately well sorted sandy yr or more in the Fairbanks area. Strong argu- Birch Lake and Harding Lake define the late silt that overlies the colluvium contains a paleo- ments for extensive glacier advances of early Wisconsin interval more narrowly, with a sol dated at 9460 ± 155 yr B.P. The loess was Wisconsin age in the Alaska Range have been change to full-glacial pollen spectra beginning later incised by small streams whose channels made by Weber and others (1981), Kline and about 26,500 ± 400 yr B.P. at Harding Lake became partly filled with peat after 3510 ± 215 Bundtzen (1986), Thorson (1986), and Weber (Ager and Brubaker, 1985) and ending with in- yr B.P. Eolian fine sand interfingers with the (1986). These advances are represented by the flux of birch shrubs about 14 ka (Fig. 18). At peat at the east end of the exposure. 15-m outwash terrace at Canyon Creek, but Canyon Creek, this long-lasting, full-glacial interval probably is represented only by the Pollen records from Birch Lake, Harding they evidently predate the beginning of the ventifact-littered surface from which sand Lake, and other lakes in the Tanana River valley known records from the permafrost tunnel, Isa- wedges originate. No loess accreted at Canyon have been described by Ager (1975,1983) and bella basin, Birch Lake, and Harding Lake. Creek during this interval, probably because of Ager and Brubaker (1985). Middle Wisconsin Fluctuating middle Wisconsin interstadial strong katabatic winds from the Alaska Range interstadial pollen spectra from Harding Lake conditions are recorded in the Fox permafrost (Thorson and Bender, 1985). are dominated by Picea, Betula, Alnus, and Eri- tunnel and at Isabella basin. The permafrost caceae, Cyperaceae, and spores of Sphagnum. tunnel records an episode of stream deposition The permafrost tunnel record shows an inter- Spruce muskeg evidently rimmed the lake until and growth of large willow thickets, followed by val of debris-fan accumulation that began per- about 26,500 yr B.P. (Fig. 17D). Full-glacial increasingly rapid loess deposition until some- haps 12,500 yr B.P. and lasted until at least pollen spectra from both Birch Lake and Hard- time after 35,000 yr ago. Slower loess accretion 11,000 yr B.P. This appears to have been a ing Lake are dominated by grasses, sedges, tun- and ice-wedge growth prevailed between about widespread interval of loess redeposition in the dra forbs, and Artemisia-, they reflect a herba- 33 and 30 ka at the tunnel, and associated flora Fairbanks area, as advocated by Hopkins (1982, ceous tundra with some willow present. Climate and fauna are those expected for the varied mi- p. 11-12) and as confirmed by D. E. Lawson was significantly colder and drier than that of crohabitats of an active ice-wedge polygon sys- (U.S. Army CRREL, 1987, written commun.). today; it probably was more intensely continen- tem. The Isabella basin record likewise shows We believe that the anomalies in the Isabella tal, with short, warm, dry summers alternating fluctuating environmental conditions, with pol- basin record in the interval between 11.5 and with long, severe winters. Herbaceous tundra len substage Aa (scattered spruce) succeeded by 9.2 ka can probably be explained by widespread was replaced by birch-shrub tundra about subzone Ab that records conditions more like erosion and redeposition during this interval. 14,000 yr B.P., probably because of an abrupt those of the present day and associated with These slope processes probably were initiated by change to warmer and moister summers through- aquatic plant and animal macrofossils that are abrupt change to moister conditions near the out the Tanana valley. Poplar expanded into the absent in the Fox permafrost tunnel. The Hard- close of the full-glacial interval: either increased region about 11,300 yr ago, and open poplar ing Lake pollen record (Fig. 17D) also shows winter snow cover, as advocated by Hopkins woodland with birch-shrub tundra may have middle Wisconsin spruce, birch, alder, and (1982), or warmer and moister summers (Ager, been the most common plant assemblage during Sphagnum (Ager and Brubaker, 1985). This is 1983), or both (T. A. Ager, U.S. Geol. Survey, the next 2,000 yr. Spruce invaded about 9500 consistent with the altitudinal vegetation se- 1987, written commun.). They may have ceased yr B.P., and alder appeared about 1,000 yr later. quence reconstructed by Schweger and Janssens abruptly with the stabilization of slopes by A marked decrease in spruce abundance about (1980) for the mid-Wisconsin Boutellier non- spruce forest that invaded the Tanana valley 8400-7000 yr B.P. is perhaps due to a warmer glacial interval, which is dated at 38-30 ka in about 9500 yr B.P. (Ager and Brubaker, 1985).

Figure 18. Composite pollen percentage dia- gram Crom lacustrine sediment cores, Tanana Val- ley, Alaska. Reprinted from Ager and Brubaker (1985), with permission of the authors and pub- lisher (American Associa- tion of Stratigraphie Paly- nologists Foundation).

Percent of Pollen Sum

An interval of rapid silt accumulation in the interval that lasted until about 6 ka. Although CONCLUSIONS permafrost tunnel is bracketed by radiocarbon the early Holocene climatic signal is complex dates of about 8460 and 6970 yr B.P. This and somewhat contradictory, the general pattern The Fox permafrost tunnel contains a late interval correlates closely with the weathering seems to be an episode of increased loess deposi- Quaternary environmental record that spans data that indicate a reversal to colder conditions tion resulting from increased dryness. more than 40,000 yr. This record indicates vary- about 9200 to 6900 yr B.P. at Isabella basin, Cooler and moister Neoglacial conditions ing environmental conditions during middle with a decrease in spruce cover in the Tanana may have caused a widespread peat-forming in- Wisconsin time: fluvial activity on the floor of River valley about 8000 to 6500 yr B.P. (Fig. terval that began at Canyon Creek and else- Goldstream valley followed by rapid loess accre- 18) that may have resulted from warmer and where in interior Alaska about 3500 yr B.P. tion and further followed by an interval of re- drier conditions (Ager, 1983) and with renewed (Hamilton and others, 1983). A change of this duced loess accumulation, more vigorous plant loess deposition after about 9400 yr B.P. at nature does not appear in the local pollen re- growth, and formation of large ice wedges. Re- Canyon Creek. A very late glacial readvance in cord, which shows no significant change in the newed loess accretion during the late Wisconsin the Alaska Range has been dated at 10.5 to 9.5 past 6,500 yr (Ager, 1983; Ager and Brubaker, was followed by an episode of gully erosion and ka by Ten Brink and Waythomas (1985), who 1985), and its record was not noticeable in the by deposition of debris fans during the time of maintain that it was followed by a warm, arid permafrost tunnel. the birch zone of Ager (1975, 1983; Ager and

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Brubaker, 1985) and Hopkins (1982, p. 10-11). sweeping morphology, but conditions may have Arcone, S. A., 1984, Pulse transmission through frozen silt: Hanover, New Hampshire, U.S. Army CRREL Report 84-17,9 p. A younger generation of ice wedges formed been too dry for major redeposition by solifluc- Arcone, S. A., and Delaney, A. J., 1984, Field dielectric measurements of within the upper silt, but their time of initiation tion or gullying at times when loess deposition frozen silt using VHF pulses: Cold Regions Science and Technology, v. 9, p. 29-37. is uncertain. The local vegetation cover proba- was most active. Those slope processes may Brown, J., Gray, S., and Allen, R., 1969, Late Quaternary evolution of a valley-fill, Fairbanks, Alaska, Pt I: Geochemistry and stratigraphy of bly varied between tundra and shrub tundra. It have been only episodically effective and were the permafrost: Hanover, New Hampshire, U.S. Army CRREL Techni- formed an open landscape dominated by grasses, most recently active between about 14 and 9 ka, cal Note, 18 p. Brown, R.J.E., and Pcwe, T. L., 1973, Distribution of permafrost in North sedges, other herbs, and at least intermittently by after precipitation had increased from full- America and its relationship to the environment—A review, 1963-1973: International Permafrost Conference, 2nd, Yakutsk, willows. glacial values but before boreal forest had ex- 1973—North American Contribution: Washington, D.C., National panded into the Fairbanks area. Academy of Sciences, p. 71-100. Ice-wedge formation in the lower silt unit ap- Chester, J. W., and Frank, J. N., 1969, Fairbanks placers fragmentation re- pears to have been favored by interstadial condi- search, final report: U.S. Department of Interior, Bureau of Mines, Metals Program, Auth. No. 9-1115-33,52 p. tions during which loess accreted slowly, and ACKNOWLEDGMENTS Chu, H. F., 1949, The immature insects: Dubuque, Iowa, William C. Brown Co., Publishers, 234 p. continuous cover of vegetation, sod, and plant Craig, J. L., Hamilton, T. D., and Sellmann, P. V., 1983, Paleoenvironmental litter insulated the ground. These conditions We are particularly grateful to John V. Mat- studies in the CRREL permafrost tunnel: International Permafrost Con- ference, 4th, Fairbanks, Alaska, 1983, Abstracts and Program, p. 92. probably promoted shallow, ice-rich permafrost thews, Jr., of the Canadian Geological Survey Czudek, Tad&, and Demek, Jaromir, 1970, Thermokarst in Siberia and its influence on the development of lowland relief: Quaternary Research, at the tunnel site. We speculate that many ice- and Thomas A. Ager, U.S. Geological Survey, v. 1, p. 103-120. wedge systems in areas of late Quaternary loess for their assistance with identification of fossils Delaney, A. J., 1987, Preparation and description of a research geophysical borehole site containing massive ground ice near Fairbanks, Alaska: accretion in central Alaska may have formed from the Fox permafrost tunnel and for help in Hanover, New Hampshire, US. Army CRREL Special Report 87-7, 15 p. under interstadial conditions rather than full- interpretations of paleoecology. We also wish to Delaney, A. J., and Arcone, S. A., 1984, Dielectric measurements of frozen silt glacial conditions. thank the following individuals for identifica- usiog time domain reflectometry: Cold Regions Science and Technol- ogy, v. 9, p. 36-49. The gravel deposit beneath the loess in the tions of specific taxa: Valerie Behan, National Dick, R. A., 1970, Effects of type of cut, delay, and explosive on underground blasting in frozen gravel: U.S. Bureau of Mines Report of Investigations permafrost tunnel may have formed shortly be- Museum of Canada (orbatid mites); J. M. 7356,17 p. Campbell, National Museum of Canada (rove Ferrians, O. J., 1965, Permafrost map of Alaska: U.S. Geological Survey fore 43,000 yr B.P.; it therefore indicates that Miscellaneous Geologic Investigations Map 1-445, scale 1:2,500,000. streams in the Fairbanks area were geologically beetles); R. D. Guthrie, University of Alaska Forbes, R. B., and Weber, F. R., 1985, Regional geology and tectonic history, in Weber, F. R., Smith, T. E, Hall, M. H., and Forbes, R. B., eds., active during at least part of Wisconsin time. (vertebrates); B. M. Murray, University of Geologic guide to the Fairbanks-Livengood area, east-central Alaska: Coarse alluvium in valleys of the Yukon-Tanana Alaska Museum (mosses); Raymond Rye, U.S. Anchorage, Alaska, Alaska Geological Society, p. 2-7. Guthrie, R. D., 1968a, Paleoecology of the large-mammal community in inte- Upland may in part be relics of moister condi- National Museum (gastropods); and H. R. rior Alaska during the late Pleistocene: American Midland Naturalist, v. 79, p. 346-363. tions during early or middle Pleistocene time, Wong, Canadian Forestry Service (saw flies). 1968b, Paleoecology of a late Pleistocene small-mammal community but at least parts of these deposits have been Wood samples were identified at the U.S. De- from interior Alaska: Arctic, v. 21, p. 223-244. 1972, Recreating a vanished world: National Geographic, v. 141, actively reworked by streams right up to the partment of Agriculture's Forest Products Lab- p. 294-301. Hamilton, T. D., 1982, A late Pleistocene glacial chronology for the southern present day. oratory in Madison, Wisconsin. Brooks Range: Stratigraphic record and regional significance: Geologi- cal Society of America Bulletin, v. 93, p. 700-716. Correlations with other late Quaternary en- Radiocarbon dates were provided by Stephen Hamilton, T. D., and Bischoff, J. L„ 1984, Uranium-series dating of fossil bones vironmental records in the Fairbanks region R. Robinson, U.S. Geological Survey, and from the Canyon Creek vertebrate locality in central Alaska, in Reed, K. M., and Bartsch-Winkler, Susan, eds., The United States Geological suggest that loess accumulation was variable in James Buckley, Teledyne Isotopes. George A. Survey in Alaska—Accomplishments during 1982: U.S. Geological Survey Circular 939, p. 26-29. space and in time. Loess is generated primarily Lancaster, U.S. Geological Survey, carried out Hamilton, T. D., Ager, T. A., and Robinson, S. W., 1983, Late Holocene ice when unvegetated glacial silt is available for granulometric analyses of the silt units. One of wedges near Fairbanks, Alaska, U.S.A.—Environmental setting and history of growth: Arctic and Alpine Research, v. 15, p. 157-168. wind erosion and transport, and it may not al- us (Craig) was supported by the U.S.A.E. Wa- Haugen, R. JC., Slaughter, C. W., Howe, K. E., and Dingman, S. L, 1982, Hydrology and climatology of the Caribou-Poker Creeks Research Wa- ways be directly in phase with fluctuations of terways Experiment Station, Vicksburg, Missis- tershed, Alaska: Hanover, New Hampshire, U.S. Army CRREL Report glaciers in the Alaska Range. Additionally, kat- sippi, during most of the preparation of this 82-26, 34 p. Holmgren, B., Spears, L., Wilson, C., and Benson, C., 1975, Acoustic soundings abatic winds can limit full-glacial loess deposi- paper. of the Fairbanks temperature inversion, in Weller, Gunter, and Bowling, S. A., eds., Climate of the Arctic: Alaska Science Conference, 24th, tion to sheltered sites, as demonstrated by We are indebted to David M. Hopkins (Uni- 1973, Proceedings: Fairbanks, University of Alaska Geophysical Insti- Thorson and Bender (1985), and such variables tute, p. 293-306. versity of Alaska), Daniel E. Lawson (CRREL), Hopkins, D. 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