Glenn Highway

Alaska Highway

Taylor Highway

Alaska Highway

Yukon-Tanana Upland

Northway-Tanana

Lowland

12,515±40 (WW-3861)

41,880±470 (Beta-171226)

1,610±40 (WW-5158)

11,715±40 (WW-5139)

Br

Qca

Qor

Qor

Qor

Qor

Qor

Qor

Qte

Qor

Qor

Qor

Qor

Qor

Qor

Qor

Qor

Qor

Qca

Qca

Qca

Qca

Qfpt

Qfpt

Qtfo

Qtfo Qtfy

Qtfy

Qtfy Qtfo

af

af

Qfpt

Qfpt

Qfpt

Qfpt

Qfpt

Qfto

Qfto

Qes Qes

Qes

Qes

Qes

Qes

Qes

Qes

Qca

Qls

Qfa

Qfa

Qca

Qca

Qca

Qca

Qca

Qla

Qlo

Qls

Qlo

Qla

Qca

Qca

Qca

Qca

Qca

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

Br

af

af

af

af

af

af

INTRODUCTION

The Tanacross B-4 quadrangle, encompassing an area of about 720 km2 in east-central 

Alaska, is about 330 km southeast of Fairbanks and about 160 km west of the border of the 

Yukon Territory of Canada. The quadrangle contains parts of two physiographic provinces, 

the Yukon-Tanana Upland and the Northway-Tanana Lowland (Wahrhaftig, 1965). The 

gently rolling hills of the Yukon-Tanana Upland, in the northern and eastern map area, 

generally rise to between about 850 and 1,000 m asl (above sea level) (see topographic map 

for equivalent elevations in feet). The highest peak in the area, an unnamed summit near the 

eastern map border, rises to 1,068 m. The Northway-Tanana Lowland contains the westflowing Tanana River and the broad Tok alluvial fan of the north-flowing Tok River. 

Altitudes along the valley floor generally range between 475 and 520 m asl. 

The dominant surficial feature within the map area is the Tok fan (composed of map 

units Qtfo, Qtfy, and Qfto), which occupies the southwestern map area and much of the 

adjacent Tanacross B-5 quadrangle to the west (Carrara, 2004a). This large (about 450 

km2), nearly featureless fan is composed of a high percentage of volcanic clasts derived from 

outside the present-day drainage basin of the Tok River. Relative to the size of this fan, the 

present Tok River is an underfit stream. During the various Pleistocene glaciations, a series 

of glaciers filled the Copper River basin to the south of the map area and ice lobes spilled 

northward over Mentasta Pass (75 km south of the map area), near the northern end of the 

Mentasta Mountains into the Tok River drainage. For instance, during the late Wisconsin 

(Donnelly) glaciation, an ice lobe extended about 16 km north of Mentasta Pass into the 

Mineral Lake area (Richter, 1976) about 55 km south of the map area. This ice lobe 

undoubtedly carried clasts of volcanic rocks from the Copper River basin, and upon 

deglaciation these clasts were deposited as outwash throughout the Tok River valley and 

onto the Tok fan. Hence, the Tok fan was built by outwash from a succession of glacial 

lobes, which spilled over Mentasta Pass during the various glaciations of the Pleistocene. 

Further information is given in the unit description of this fan.

Permafrost (permanently frozen ground) is common throughout the map area, 

especially in the highly organic deposits (Qor) within the Tanana Valley and in the finegrained colluvium and alluvium (Qca) of the Yukon-Tanana Upland. In these areas a thick 

mat of vegetation covers the ground, and permafrost is common below a depth of 50 cm. 

At many localities the presence of permafrost is indicated by stunted black spruce (Picea 

mariana). Man-made structures can be disrupted and damaged by the melting of underlying 

permafrost. Hence, care should be taken when building in permafrost areas.

The two White River Ash layers are widespread over eastern Alaska and the southern 

Yukon region (Lerbekmo and Campbell, 1969; Clague and others, 1995). The ash layers, 

erupted from a vent in the St. Elias Range of eastern Alaska about 230 km south-southeast 

of the map area, were deposited by two separate eruptions. The younger ash layer, which is 

limited to the southern Yukon Territory, was deposited about 1,200 14C yrs ago (Clague 

and others, 1995). The older ash layer, which is widespread in eastern Alaska including the 

map area, was deposited between 1,500 and 1,900 14C yrs ago (Fernald, 1962; Clague 

and others, 1995). This layer (about 2–10 cm thick) consists of a white (10YR 8/1), lightgray (10YR 7/2), or very pale brown (10YR 7/3) fine sand or silty fine sand, composed of 

volcanic glass fragments, is commonly found within the upper 20 cm of the modern soil. 

Based on stratigraphic position, the majority of volcanic ash sites shown on the map are 

probably the older of the White River Ash layers. However one ash layer, at a site along the 

north side of the Tanana River about 0.8 km west of the mouth of Porcupine Creek, was 

found in a deep gully in a thick loess deposit. Major element analyses of the glass fraction of 

this ash layer (E. Wan and A. Sarna-Wojcicki, written commun., 2005) indicate it is the Old 

Crow tephra, which was deposited about 140 ka (Westgate and others, 1990).

The climate of the map area is typical of that of the Alaskan interior. Winters are long 

and cold, summers are short and generally mild, and precipitation is light. Climate records 

for the town of Tok, along the Alaska Highway near the western map border, indicate a 

mean January temperature of –26.4°C, and a mean July temperature of 14.5°C. Mean 

annual precipitation at Tok is 23.5 cm with almost 60 percent occurring during the summer 

months (June, July, and August) (Western Regional Climate Center, unpublished data 

accessed Dec. 1, 2003, on the World Wide Web at URL 

http://www.wrcc.dri.edu/index.html).

Boreal forest and muskeg, which in many areas reflect the underlying geology, colonize 

much of the map area. The boreal forest consists primarily of black spruce, white spruce 

(Picea glauca), balsam poplar (Populus balsamifera), and quaking aspen (Populus 

tremuloides). Black spruce, characteristic of cold, poorly drained, nutrient-poor sites, 

commonly grows as small, stunted trees 3–5 m high (Johnson and others, 1995). Welldrained sites are commonly inhabited by white spruce, 10–20 m in height, and by balsam 

poplar, quaking aspen, and some black spruce, which can reach a height of about 10 m in 

these more favorable locations. Muskegs, which commonly contain Sphagnum mosses and 

heath shrubs, are characterized by areas of poor drainage and high water table, and may 

contain permafrost at shallow depths. Black spruce may also be present in the muskegs and 

will commonly form open-canopied stands of low, stunted trees (Johnson and others, 1995). 

The Alaska Highway (originally called the “Alcan” Highway) generally crosses east to 

west through the Tanacross B-4 quadrangle for about 28 km. The highway stretches for 

2,290 km from Dawson Creek, British Columbia, to Delta Junction, Alaska, and traverses 

rugged mountains, wild rivers, and large expanses of forest and muskeg. The U.S. Army 

Corps of Engineers built the original highway in just eight months and 12 days in 1942 

(Cohen, 2001) as an overland route to relieve Alaska from the wartime hazards of shipping; 

the highway was later turned over to civilian contractors for improvements (widening, 

graveling, and rerouting in many areas). Paving was completed in about 1984. Today, the 

Alaska Highway is one of the primary transportation corridors in Alaska, carrying both 

freight and passengers. Other roads in the map area include the Glenn Highway at the 

western edge of the map, connecting Anchorage to Tok, and the Taylor Highway near the 

eastern edge of the map, connecting the Alaska Highway to Dawson in the Yukon Territory.

The town of Tok (population about 1,400) is located near the western edge of the map 

at the intersection of the Alaska and Glenn Highways. Tok had its beginning as a construction camp on the Alcan Highway and was named after the mascot, a Husky pup, of the 

U.S. Army 97th Engineers Corps when they arrived at the present-day site of Tok on 

August 15, 1942 (data accessed June 30, 2006, on the World Wide Web at URL 

www.alaska-wintercabin.com/about_tok.htm). The town is the major overland point of entry 

to Alaska and is primarily a trade and service center for all types of transportation, especially 

for summer travelers along the Alaska Highway. 

Mapping of the surficial deposits in the Tanacross B-4 quadrangle was accomplished by 

a variety of methods, including (1) compilation from existing geologic maps—mainly Holmes 

(1965), Foster (1970), and Carter and Galloway (1978); (2) stereoscopic analysis of aerial 

photographs (1:46,000-scale 1954 black and white and 1:63,000-scale 1978 colorinfrared); and (3) fieldwork, including limited helicopter use. Where localities could be 

examined in the field, detailed information was obtained on the surficial deposits. Data from 

these easily accessible areas were then extrapolated to the less accessible areas. Map unit 

boundaries were plotted on a stable mylar topographic base with the use of a 

photogrammetric stereo plotting instrument (Kern PG2). The contacts were then digitized on 

screen from the scanned mylar.

Surficial deposits in the Tanacross B-4 quadrangle consist of artificial fill, alluvial, 

colluvial, eolian, lacustrine, and organic deposits. Deposits shown on this map are generally 

greater than 1 m thick; thinner discontinuous colluvial and eolian deposits, residual material 

on bedrock, and some artificial fill were not mapped and are incorporated with the 

underlying mapped unit. For example, many areas are mantled by a light-yellowish-brown 

(10YR 6/2) loess, usually less than 25 cm thick, but because it is thin and discontinuous and 

lacks geomorphic expression, it could not be accurately mapped. In addition, many contacts 

between map units are approximately located because of the lack of exposures or the 

gradational nature of the surficial deposits (for example, the contact between colluvium and 

alluvium (Qca) and bedrock (Br) on steep slopes in the Yukon-Tanana Upland). 

The durations of the various time divisions of the Quaternary Period are modified from 

Hansen (1991) and Richmond and Fullerton (1986) and are as follows: (1) Holocene, 

0–10,000 yrs ago; (2) late Pleistocene, 10,000–127,000 yrs ago; (3) middle Pleistocene, 

127,000–778,000 yrs ago; and (4) early Pleistocene, 778,000–1,806,000 yrs ago. Age 

assignments for the surficial deposits in the quadrangle are based chiefly on stratigraphic and 

depositional relations, the degree of erosional modification of the original-surface 

morphology, several radiocarbon ages, and stratigraphic position relative to the White River 

Ash layer. Grain-size terminology of the surficial deposits is based on visual identification and 

follows the modified Wentworth grain scale (American Geological Institute, 1982). In 

descriptions of surficial deposits, the term “clast” refers to a particle greater than 2 mm in 

diameter, whereas the term “matrix” refers to particles less than 2 mm in size. The dry 

matrix colors of the surficial deposits in the map area were determined by comparison with a 

Munsell Soil Color Chart (Munsell Color, 1973).

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

Artificial fill deposits

Artificial fill (latest Holocene)—Compacted and uncompacted fill composed 

mostly of silt, sand, gravel, and rock fragments beneath the Alaska, Glenn, 

and Taylor Highways. Beneath the Glenn Highway and Alaska Highway 

west of the Tanana River, the fill consists of subrounded and rounded, 

pebble- and cobble-size clasts derived mainly from local gravel pits on the 

Tok fan (Qtfo and Qtfy) and angular (crushed), pebble-size basaltic rock 

fragments, also derived from Tok fan deposits, all in a pale-brown (10YR 

6/3) to light-yellowish-brown (10YR 6/4) silty sand to sand matrix. 

Beneath the Taylor Highway and the Alaska Highway east of the Tanana 

River, the fill commonly consists of angular (crushed) pebble-size granitic 

fragments derived from the local bedrock. Matrix is mostly pale-brown 

(10YR 6/3), coarse sand. In places, where unit overlies areas with a high 

water table, such as along the floodplain of the Tanana River, unit may be 

susceptible to failure by liquefaction and slumping of underlying material 

during seismic events. Such liquefaction and slumping caused severe 

damage along the Glenn Highway between Slana and Mentasta Pass (about 

75 km south of map area) during the magnitude 7.9 earthquake of 

November 3, 2002 along the nearby Denali fault (Harp and others, 2003). 

Thickness of unit ranges from about 1–1.5 m on the Tok fan to as much as 

5 m in areas of permafrost and muskeg

Alluvial deposits

Floodplain alluvium of Tok River (Holocene)—Floodplain deposits of Tok 

River, including those in recently abandoned channels and low terraces. 

Upper 1 m of unit, commonly an overbank deposit, consists of lightbrownish-gray (10YR 6/2), well-sorted, massive, micaeous, silty fine to 

medium quartz sand which locally contains minor amounts of pebbles. The 

sand contrasts markedly with that of the Tanana River floodplain (Qfpt) and 

Tok fan (Qtfo and Qfty) in that it is much lighter in color and contains 

more quartz and mica derived from the Alaska Range to the south. Poorly 

exposed lower part of unit consists of light-brownish-gray (10YR 6/2), 

unstratified to stratified, well-sorted, fine to medium sand, pebbly sand, and 

sandy pebble gravel. Clasts are chiefly subrounded to rounded cobbles and 

pebbles and include various quartz-rich gneisses and schists, and quartzites 

from the Alaska Range as well as basalt and andesite probably reworked 

from the older parts of the Tok fan and originally derived from the 

Wrangell Mountains to the south. Unit locally includes organic deposits 

(Qor). Areas underlain by this unit are subject to periodic flooding during 

times of high discharge and, because of the high water table, it may also be 

prone to liquefaction during seismic events. Exposed thickness about 2 m; 

estimated maximum thickness 10 m 

Floodplain alluvium of Tanana River (Holocene)—Floodplain deposits of 

Tanana River, including those in recently abandoned channels and low 

terraces. Upper 1 m of unit, commonly an overbank deposit, consists of 

dark-olive-gray (5Y 3/2), massive, well-sorted, fine to medium sand, which 

locally contains minor amounts of pebbles. Poorly exposed lower part of 

unit consists of unstratified to stratified, well-sorted sand, pebbly sand, and 

sandy-cobbly pebble gravel. Clasts are chiefly subrounded to rounded and 

include a high percentage of basaltic and andesitic cobbles and pebbles 

derived from the Wrangell Mountains to the south, as well as various 

quartz-rich gneisses and schists, and quartzites from the Alaska Range. Unit 

locally includes organic (Qor) deposits and alluvium underlying low terraces 

that are marked by meanders and channel scars (Holmes, 1965) along 

river. A backhoe pit, in the east-central map area just east of the Tanana 

River, exposed 30 cm of dark-brown (7.5YR 3/2) peat overlying 90 cm of 

dark-brown (10YR 3/3) silt. Below the silt is a black (10YR 2/1) peaty unit 

containing numerous wood fragments. An accelerator mass spectrometer 

(AMS) radiocarbon age of 1,610±40 yr B.P. (WW-5158) was obtained 

from a Salix fragment at a depth of 120 cm in lower peat unit, which was 

also at permafrost depth. Areas underlain by this unit are subject to 

periodic flooding during times of high discharge and, because of unit’s high 

water table, are also prone to liquefaction during seismic events. Harp and 

others (2003) observed that in the Tok area nearly every bar along the 

Tanana River showed extensive liquefaction effects from the November 3, 

2002 earthquake. Exposed thickness about 2 m; estimated maximum 

thickness 20 m

Younger Tok fan deposit (Holocene and late Pleistocene)—Large, lowgradient, poorly drained, younger part of Tok fan deposited by Tok River. 

Unit occupies large part of southwestern map area. Unit consists mainly of 

well-stratified, well-sorted, unconsolidated pebble, cobbly pebble, and 

pebbly cobble gravel. Matrix is light-brownish-gray (10YR 6/2), well-sorted, 

micaeous, silty fine to medium quartz sand, similar to that of Tok River, and 

dark-olive-gray (5Y 3/2) medium sand, similar to that of Tanana River. 

Also contains lenses and layers of sand and pebbly sand, usually less than 

25 cm thick. Clasts are chiefly subrounded to rounded and include 

abundant basaltic and andesitic cobbles and pebbles derived from Copper 

River drainage to south, as well as various quartz-rich gneisses and schists, 

and quartzites from the Alaska Range. Unit assigned a Donnelly age by 

Foster (1970). Unit locally overlain by 10–20 cm of loess consisting of 

light-yellowish-brown (10YR 6/4) silt and fine sandy silt. Exposed thickness 

about 3–5 m; estimated maximum thickness 20 m

Terrace alluvium of Tanana River (late Pleistocene)—Poorly exposed, darkgrayish-brown (2.5Y 4/2), well-sorted, massive medium sand, deposited by 

Tanana River. Approximately 30 cm below top of unit, a thin (2 cm) layer 

of light-brownish-gray silt contains small gastropod shells. An AMS 

radiocarbon age of 11,715±40 yr B.P. (WW-5139) was obtained from 

these shells indicating a late Pleistocene age for the terrace. Unit is 

commonly overlain by 50 cm of loess consisting of massive, yellowishbrown (10YR 5/4) fine sandy silt. Unit underlies a terrace remnant about 

5–6 m above Tanana River floodplain in east-central map area. Thickness 

5–6 m

Older Tok fan deposit (middle Pleistocene)—Large, low-gradient, welldrained, nearly featureless fan deposited by Tok River, which occupies 

southwestern map area and large part of adjacent Tanacross B-5 

quadrangle to west (Holmes, 1965; Foster, 1970; Carrara, 2004a). 

Surface is generally 5–10 m higher than younger Tok fan deposit (Qtfy). 

Unit consists mainly of well-stratified, well-sorted, unconsolidated pebble, 

cobbly pebble, and pebbly cobble gravel with a matrix of dark-olive-gray 

(5Y 3/2) medium sand. Also contains lenses and layers of sand and pebbly 

sand, usually less than 25 cm thick. In places, upper meter or more of the 

unit is iron stained (Holmes, 1965; Foster, 1970), and in some areas clasts 

have a thin (< 2 mm) coating of calcium carbonate. Clasts are chiefly 

subrounded to rounded and include high percentage (commonly 60–70 

percent) of basaltic and andesitic (Holmes, 1965) cobbles and pebbles 

derived from Copper River drainage to south, as well as various quartz-rich 

gneisses and schists, and quartzites from Alaska Range. Unit locally overlain 

by 10–20 cm of loess consisting of a light-yellowish-brown (10YR 6/4) silt 

and fine sandy silt. Unit assigned a Delta age by Foster (1970); subsequent 

work indicates that Delta-age deposits are equivalent in age to marine 

oxygen isotope stage 6 (Begét and Keskinen, 2003), thought to have 

occurred between about 130 and 188 ky ago (Martinson and others, 

1987).

Unit is an aquifer and supplies water to homes and public structures on 

it. At head of Tok fan, near southern map boundary, water table is at a 

depth of 40–50 m; in vicinity of town of Tok, water table is about 15–20 

m deep; at Tanacross (about 20 km west of Tok) near distal margin of Tok 

fan, water table is at a depth of 2–4 m (T. Holohan, Holohan Drilling Co., 

oral commun., 2001; G. Burnham, Burnham Construction Inc., oral 

commun., 2003). Exposed thickness about 15 m; estimated maximum 

thickness 50–100 m

Alluvial and colluvial deposits

Colluvium and alluvium (Holocene and late Pleistocene)—Poorly exposed; 

appears to consist mainly of poorly sorted, poorly stratified, locally organicrich, silt, silty sand, sand, and pebbly sand deposited by both colluvial and 

alluvial (including sheetwash) processes. Foster (1970) referred to these 

deposits as “alluvium and colluvium in small stream valleys” of Holocene 

age and described unit as consisting of “primarily silt and sand.” Unit 

occupies valleys and adjacent slopes primarily in the northern map area 

where it forms broad, gently sloping areas. Permafrost common at depths 

below 50 cm. Maximum estimated thickness of unit 10 m

Fan deposits (Holocene and late Pleistocene)—Fans deposited mainly by 

flowing water and debris flows. Poorly exposed; fans on the adjacent 

Tanacross B-5 quadrangle consist of unstratified to poorly stratified, poorly 

sorted gravel with pale-brown (10YR 6/3) silty sand matrix (Carrara, 

2004a). Clasts are angular to subangular cobbles and boulders mainly of 

fine-grained biotite and biotite-hornblende granodiorite, although a small 

fan along western map border north of Tanana River is composed of darkgray basalt clasts. Unit’s surface may contain bouldery debris flow levees 

about 1 m high. Locally, may include colluvium and sheetwash alluvium. 

Unit is subject to both flood and debris-flow hazards from adjacent uplands. 

Estimated maximum thickness 20 m

Colluvial deposits

Landslide deposits (Holocene and late Pleistocene)—Two landslide deposits 

were identified within map area. Variations in size and lithology of clasts 

and matrix depend on various bedrock and surficial deposits involved in the 

landslide. One of the deposits, near confluence of Tanana River and 

Porcupine Creek, was inspected in August 2003. Trunks of many large 

white spruces on this small landslide were tilted and split by recent 

movement, and ground contained “pull-apart” trenches 1–2 m deep and 

3–5 m across. This deposit may have been induced by magnitude 7.9 

earthquake of November 3, 2002, which triggered thousands of landslides 

in nearby Alaska Range and surrounding areas (Harp and others, 2003). 

Maximum thickness of unit estimated to be about 10 m

Eolian deposits

Loess (Holocene)—Mainly light-grayish-brown (10YR 6/2) and light-yellowishbrown (10YR 6/4) to brown (10YR 5/3), massive silt and fine sandy silt 

deposited by wind. Mapped at only two localities in map area, immediately 

east of Tanana River and north of Alaska Highway and in area west of 

mouth of Porcupine Creek. Loess was noted at many locations in map 

area, but was not mapped in most cases as it is generally less than 25 cm 

thick and has no geomorphic expression. Unit locally contains a White 

River Ash layer, usually 3–5 cm thick, in its upper 20 cm indicating a late 

Holocene age. At several locations unit overlies eolian sand (Qes). 

Maximum thickness 3–5 m

Eolian sand (late Pleistocene)—Sand deposited by wind at several locations 

north and east of Tanana River. Unit consists of massive to poorly bedded, 

olive-gray (5Y 4/2), fine- to medium-grained sand in sand sheets and 

dunes. The largest dune field in map area, extending from Tetlin Junction 

north along Taylor Highway for a distance of about 10 km, forms a field of 

parabolic dunes with as much as 30 m of local relief. Sand within these 

dunes was found to consist of “about 2 percent magnetite and 35 percent 

dark-gray to black fine-grained rock fragments which give it the dark color. 

About 50 percent of the sand is quartz, feldspar, and small fragments of 

granitic and metamorphic rocks” (Foster and Keith, 1969, p. 6). Many 

sand grains are rounded and frosted by wind action (Foster and Keith, 

1969). Orientation of dunes indicates they were formed by strong winds 

from the southwest. Although regional late Pleistocene winds were from 

north (Lea and Waythomas, 1990), this large dune field was probably 

deposited by katabatic winds that drained an ice field in the Copper River 

basin and funneled a high-velocity wind down the Tok River Valley. The 

wind scoured sand from the Tanana River floodplain and deposited it north 

and east of river forming this large dune field. Unit is well drained and 

presently stabilized by a forest of mainly quaking aspen and white spruce. 

Unit is locally overlain by 10–30 cm of loess, which locally contains White 

River ash. Stippled pattern indicates areas of dunes. Maximum thickness 

about 30 m 

Lacustrine deposits

Lacustrine deposits (late Pleistocene)—Mainly dark-grayish-brown (10YR 

4/2) and grayish-brown (10YR 5/2), well-sorted, well-bedded, fine to 

medium sand eroded from nearby deposits of eolian sand (Qes) with which 

it is thought to be contemporaneous, and deposited in former lakes. Unit 

locally contains thin (< 1 cm) clay and silty layers, and a few pebbly sand 

layers less than 10 cm thick. Mapped at two localities, both along 

Porcupine Creek in east-central map area. At eastern location an exposure 

along Taylor Highway, first described by Foster and Keith (1969), consists 

of about 12 m of well-bedded, lacustrine sand overlain by 5 m of darkgrayish-brown (10YR 4/2), fine to medium eolian sand. An AMS 

radiocarbon age of 12,515±40 yr B.P. (WW-3861) was obtained from 

small freshwater bivalves near top of lake beds. Radiocarbon age provides a 

close minimum age for unit. Additional evidence of a late Pleistocene age 

for these lacustrine deposits was found at larger western deposit. In a 

stream cut along Porcupine Creek, lacustrine sand contains several thin 

(0.5–1 cm) peaty layers. Pollen analysis of these peaty layers indicates 

presence of shrub-herb tundra whereas present pollen spectra at this site is 

that of boreal forest. Shrub-herb tundra suggests harsh periglacial climate 

present during the late Pleistocene (T.A. Ager, U.S. Geological Survey, 

written commun., 2003). In addition, at this site lacustrine sand is underlain 

by a dark-green to dark-brown, poorly laminated, organic-rich clayey silt. 

Salix fragments near top of this clayey silt unit yielded an AMS radiocarbon 

age of 41,880±470 yr B.P. (Beta-171226), providing a maximum age for 

lacustrine sand. Maximum thickness about 15 m

Organic deposits

Organic-rich deposits (Holocene and late Pleistocene)—Mainly black (10YR 

2/1) to brown (10YR 4/3) peat, woody peat, muck, and organic-rich sand, 

silt, and clay. Unit occurs mainly in low-lying areas mostly on floodplain of 

Tanana River. As limits of this unit were hard to identify in the field and 

on aerial photographs, in most cases unit boundaries were taken directly 

from those areas mapped as marsh on topographic base. Areas underlain 

by this unit have poor drainage and a high water table, are subject to 

periodic flooding, and may contain permafrost at shallow depths. 

Thickness 1–10 m 

BEDROCK

Granitic, metamorphic, and volcanic rocks (Quaternary(?) to Paleozoic)— 

Mountain near western map border north of Tanana River is composed of 

dark-gray or dark-greenish-gray basalt believed to be Quaternary or Tertiary 

in age (Foster, 1970). In northeastern map area, Foster (1970) identified 

felsic volcanic rocks consisting of light-colored lava, tuff, tuff breccia, 

pumice breccia, volcanic conglomerate, and tuffaceous sediments of 

Tertiary age. Much of remaining bedrock in map area is primarily granitic 

rocks of Mesozoic age, consisting of mainly fine-grained biotite and biotitehornblende granodiorite but ranging in composition from diorite to granite 

or biotite gneiss and schist of Paleozoic or Precambrian age (Foster, 1970)

Contact

Terrace scarp—Hachures on downside

Escarpment in sand dune field—Hachures on downside

Abandoned river meander

Pingo

Volcanic ash site

Radiocarbon age site 

Gravel pit 

 

Depositional wind direction, as indicated by dune form

REFERENCES CITED

American Geological Institute, 1982, Grain-size scales used by American geologists, 

modified Wentworth scale, in Data sheets (2nd ed.): Falls Church, Va., American 

Geological Institute, sheet 17.1.

Begét, J.E., and Keskinen, M.J., 2003, Trace-element geochemistry of individual glass 

shards of the Old Crow tephra and the age of the Delta glaciation, central Alaska: 

Quaternary Research, v. 60, p. 63–69.

Carrara, P.E., 2004a, Surficial geologic map of the Tanacross B-5 quadrangle, east-central 

Alaska: U.S. Geological Survey Scientific Investigations Map 2856, scale 1:63,360.

Carrara, P.E., 2004b, Surficial geologic map of the Tanacross B-6 quadrangle, east-central 

Alaska: U.S. Geological Survey Scientific Investigations Map 2850, scale 1:63,360.

Carter, L.D., and Galloway, J.P., 1978, Preliminary engineering geologic maps of the 

proposed natural gas pipeline route in the Tanana River valley, Alaska: U.S. Geological 

Survey Open-File Report 78–794, scale 1:125,000.

Clague, J.J., Evans, S.G., Rampton, V.N., and Woodsworth, G.J., 1995, Improved age 

estimates for the White River and Bridge River tephras, western Canada: Canadian 

Journal of Earth Sciences, v. 32, p. 1172–1179.

Cohen, S., 2001, The trail of '42; A pictorial history of the Alaska Highway: Altona, 

Manitoba, Friesen Printers, 15th revised edition, 112 p.

Fernald, A.T., 1962, Radiocarbon dates relating to a widespread volcanic ash deposit, 

eastern Alaska: U.S. Geological Survey Professional Paper 450-B, p. B29–B30.

Foster, H.L., 1970, Reconnaissance geologic map of the Tanacross quadrangle, Alaska: 

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1:250,000. 

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Qfpt

Qte

Qtfo

Qca

Qfa

Qls

Qlo

Qes

Qla

Qtfy

Qor

Br

Qfto

170° 160° 150° 140° 130°

70°

65°

60°

GULF OF ALASKA

ARCTIC OCEAN

CANADA

UNITED STATES

63°15'

63°00'

63°30'

63°45'

144°30' 143°00' 144°00' 142°00' 143°30' 142°30'

Tanacross

Tok Tetlin

Jct.

Tok River

TANANA

RIVE R

 ALASKA RANGE

Tanacross

B-6

Tanacross

B-5

Tanacross

B-4

SIM 2850 SIM 2856

GLENN

HIGHWAY

ALASKA HIGHWAY

TAYLOR

HIGHWAY

Figure 1. Location map of Tanacross B-4 quadrangle within east-central Alaska. 

Other U.S. Geological Survey geologic maps in area (Carrara, 2004a, b) are listed by 

publication number. Regional map illustrates selected physiographic and cultural 

features.

ALASKA

Barrow

Tok

Fairbanks

Bethel

Anchorage

Yukon

River

Tanana Riv er

YUKON-TANANA UPLAND

MENTASTA MOUNTAINS

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS BEDROCK

Organic

deposits

Lacustrine

deposits

Eolian

deposits

Alluvial and colluvial

deposits

Alluvial

deposits

Artificial fill

deposits

Holocene

Pleistocene

QUATERNARY

TERTIARY

Colluvial

deposits

CENOZOIC

MESOZOIC

TO

PRECAMBRIAN

Qfto Qfpt

Qtfy Qte

Qtfo

Qca Qfa Qls

Qlo

Qes Qla

Qor

Br

?

af

SURFICIAL GEOLOGIC MAP OF THE TANACROSS B-4 QUADRANGLE, EAST-CENTRAL ALASKA

By

Paul E. Carrara

2006

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 2935

Version 1.0

Any use of trade names is for descriptive purposes only 

and does not imply endorsement by the U.S. Government

This map was produced on request, directly from 

digital files, on an electronic plotter

For sale by U.S. Geological Survey Information Services

Box 25286, Federal Center, Denver, CO 80225

1-888-ASK-USGS

ArcInfo coverages and a PDF for this map are available at 

http://pubs.usgs.gov

CONTOUR INTERVAL 100 FEET

SUPPLEMENTARY CONTOUR INTERVAL 50 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

Geology mapped by P.E. Carrara in 2001–2004;

assisted in field by J.P. Beann, J.S. Honke, and 

F.R. Weber

Digital database prepared by Kenzie J. Turner

Editing and digital cartography by 

Gayle M. Dumonceaux,

Denver Publishing Service Center

Manuscript approved for publication

July 13, 2006

Base from U.S. Geological Survey, 1949;

minor revision 1994

Universal Transverse Mercator projection

10,000-foot grid based on Alaska coodinate 

system, zone 2

1,000-meter Universal Transverse Mercator

grid ticks, zone 7

1927 North American Datum

APPROXIMATE MEAN

DECLINATION, 1994 TRUE NORTH

MAGNETIC NORTH

28° 1 .5 0 1 2 3 4 5 6 7 KILOMETERS

SCALE 1:63 360

1 1/2 0 234 1 5 MILES

ALASKA

QUADRANGLE LOCATION

CONVERSION FACTORS

Multiply By To obtain

centimeters (cm) 0.394 inches (in.)

meters (m) 3.281 feet (ft)

kilometers (km)

To convert Celsius (°C) to Fahrenheit (°F), use formula (°C x 1.8) + 32

0.621 miles (mi)