Quaternary glacial, lacustrine, and fluvial interactions in the western Noatak basin, Northwest Alaska

Quaternary Science Reviews 20 (2001) 371}391

Quaternary glacial, lacustrine, and #uvial interactions in the western Noatak basin, Northwest Alaska Thomas D. Hamilton* U.S. Geological Survey (Emeritus), 4200 University Drive, Anchorage, AK 99508-4667, USA

Abstract The 130 km long Noatak basin is surrounded by mountains of the western Brooks Range. Middle and late Pleistocene glaciers #owing southeast into the basin dammed a succession of proglacial lakes de"ned by shorelines, outlet channels, and upper limits of wave erosion. More than 60 blu!s along the Noatak River and its principal tributaries expose glacial and glaciolacustrine sediments that exhibit cut-and-"ll relationships with interglacial and interstadial river-channel and #oodplain deposits. This report focuses on the western Noatak basin, where high blu!s created by deep postglacial erosion record four major glacial advances. During the Cutler advance, a #oating ice tongue terminated in a large proglacial lake that "lled the Noatak basin. The retreating glacier abandoned a trough along the valley center that subsequently "lled with about 40 m of sediment during several younger glaciations and probably two major interglacial episodes. Alluvium that formed near the beginning of the younger interglaciation contains the 140,000 yr old Old Crow tephra. The subsequent closely spaced Okak and Makpik advances are clearly younger than the maximum of the last interglaciation, but they preceded a middle Wisconsin (36}30 ka) nonglacial interval in the Noatak basin. The Okak advance terminated in an extensive lake, whereas glaciers of the Makpik and the subsequent Anisak advances #owed into much narrower lakes that "lled only the basin center. The Anisak advance, bracketed by radiocarbon ages of about 35 and 13.6 ka, represents the Last Glacial Maximum (LGM) in the western Noatak basin. Correlations with the oldest and youngest glacial deposits of the central Brooks Range are clear, but relationships to events of intermediate age are more tenuous. Early Pleistocene and older glacial advances from the central Brooks Range must have "lled the Noatak basin and over#owed northward through Howard Pass. A younger glacial advance, of inferred middle Pleistocene (Sagavanirktok River) age, extended down the Noatak valley into the basin center, but its deposits are deeply buried beneath the basin #oor and must be older than the Cutler moraine. The Cutler advance may have been synchronous with the older of two advances of Itkillik I age in the Atongarak Creek area, but other evidence indicates that the Okak-Makpik moraine succession more likely was synchronous with the two Atongarak Creek moraines. Radiocarbon ages, surface morphology, soil and weathering pro"les, and lake-level history all support correlation of the last (Anisak) major glacial advance in the western basin with the Douglas Creek moraine farther east and with Itkillik II (late Wisconsin) glaciation of the central Brooks Range.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction The Noatak basin of northwest Alaska contains an unusually rich depositional record of middle and late Quaternary age. Numerous blu!s along the Noatak River and its principal tributaries expose glacial and glaciolacustrine deposits that are interstrati"ed with #uvial sediments. The glacial-age deposits are assignable to at least "ve glacial advances of the middle and late Pleistocene; the #uvial sediments represent at least two major interglaciations as well as the Holocene interval. This extensive stratigraphic record provides a rare op* Corresponding author. Tel.: #1-907-786-7451; fax: #1-907-786-7401. E-mail address: [email protected] (T.D. Hamilton).

portunity to reconstruct glacial and lacustrine facies variations along and across the basin axis, and to relate them to moraines, former shorelines, and other sur"cial features. Because the succession of proglacial lakes over#owed at various times to the north, south, and west, the lacustrine record should be relevant to drainage history across much of northwest Alaska. The Noatak River originates in rugged granitic highlands in the central Brooks Range, and #ows westward to enter Kotzebue Sound (Fig. 1). For most of its course, the river is con"ned between the DeLong Mountains to the north and the Baird Mountains to the south. About 110 km below its headwaters, the river's valley broadens into a basin of low relief with a maximum width of about 80 km; this broad lowland extends downvalley about

0277-3791/01/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 1 1 0 - 4

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Fig. 1. Noatak River region, northwest Alaska. Dark shading shows approximate extent of glacial Lake Noatak sediments. Tree symbols designate arctic treeline.

130 km from near Douglas Creek to below the Anisak River (Fig. 2). In addition to the Noatak and Anisak Rivers, the basin is traversed also by the Aniuk River, which #ows southwest from Howard Pass, and the extensive north-#owing drainage network of the Cutler River. The Noatak basin has an arctic climate, with long cold winters and short cool summers. The basin #oor is underlain by permafrost, which generally is continuous also beneath surrounding uplands (Ferrians, 1965; Brown et al., 1997). Tundra vegetation predominates, with willow and alder shrubs present along stream courses. The riparian shrubs grow higher (2}3 m) west of the Cutler River, and scattered stands of small cottonwood (Populus balsamifera) trees also are present on this sector of the valley #oor. Boreal forest extends nearly to the Noatak basin from the south and west (Fig. 1) but no evidence has been found for its presence within the basin during Holocene time. During middle and late Pleistocene time, glaciers extended repeatedly down the Noatak River valley, terminating within the basin (Fig. 3). Other glaciers that originated farther to the northwest in the DeLong Mountains #owed eastward into the basin; they dammed the Noatak River and formed a succession of lakes with surface areas as large as 4400 km. These lakes have been collectively termed glacial lake Noatak (Hamilton and Van Etten, 1984). The more extensive glacial advances "lled most of the basin with glacier ice and with proglacial lakes, which over#owed at various times northward through Howard Pass into the Etivluk River,

southward though the upper Hunt River into the Kobuk valley, and westward around the ice margin into the lower Noatak River valley (Fig. 3A). Less extensive advances formed narrower water bodies that were centered on modern drainage courses (Fig. 3B). Blu! exposures up to 86 m high along the Noatak River and its principal tributaries (see Fig. 2) provide a basin-wide stratigraphic record of alternating lake stages and alluvial episodes and of glacial expansions and wastage. The presence of large glacial lakes in the Noatak basin was "rst suggested by Stephen C. Porter (University of Washington, pers. comm., 1973), who reasoned that glaciers expanding out of the DeLong Mountains (as mapped by Coulter et al., 1965) must have dammed an extensive lake along the Noatak River. Subsequent geologic mapping in the Noatak basin (Hamilton, 1984a, b) documented beach and deltaic deposits of multiple lake stages that were associated with end moraines assignable to at least three middle and late Pleistocene glaciations (Hamilton and Van Etten, 1984). Study of the blu! exposures shown in Fig. 2 commenced during the 1979}1982 mapping project and was continued by traverses of the Noatak and lower Cutler Rivers in 1983, 1984, and 1993}1995. This report focuses on the western sector of the Noatak basin, which extends downvalley (westward) from the Cutler River con#uence. This sector has been intensely dissected by the Noatak River, forming high blu!s in which glacial and glaciolacustrine sediments are exposed in erosional contact with multiple sequences of

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Fig. 2. Noatak basin, showing drainage features and locations of measured sections. Uplands above 600 m asl are shaded.

#uvial channel and #oodplain deposits. The discharge of the Noatak River is signi"cantly larger below the Cutler owing to the water contributed by that large tributary. Meanders consequently have much larger wavelengths than those farther upvalley, and some freshly eroded blu! faces extend a kilometer or more along river cutbanks. Erosive ability of the Noatak is much greater through this stretch, and the river commonly has cut down through late Pleistocene deposits to expose older sediments. However, deep erosion followed by alluviation has resulted in large-scale, #uvial cut-and-"ll structures that greatly complicate blu! stratigraphy. In some places, downcutting by the Noatak River has intersected plastic clay-rich sediments that normally occur below river level. These commonly appear as diapiric extrusions near the ends of river blu!s, where con"ning overburden has been removed by Holocene river erosion. As an additional complication, striking changes have occurred in the blu!s along this stretch of the Noatak River since their initial study in 1983. New exposures have been cut by the river or exposed in #ow-slides, and some former expo-

sures have been obscured by #owage, vegetation growth, and other surface processes. To facilitate discussion of its numerous high blu!s and complex stratigraphic successions, I have subdivided the western Noatak basin into sectors above and below Okak bend (Fig. 4). Glaciolacustrine and #uvial sediments older than the last interglacial dominate the stretch above Okak Bend, whereas blu!s at and below the bend expose diamictons related to a series of moraines of late Pleistocene age that cross the basin #oor.

2. Regional glacial succession The central Brooks Range glacial sequence was de"ned by Detterman et al. (1958) and later modi"ed by Porter (1964), Hamilton and Porter (1975), and Hamilton (1986, 1994). Five major regional events have been distinguished (Table 1), all of which have been mapped within the eastern Noatak basin or in adjoining drainages (Hamilton, 1984a, b).

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Fig. 3. Extent of glaciers and glacial lake Noatak. (A) Middle Pleistocene glaciation. Glaciers shown at Cutler moraine position in western Noatak basin, and possibly correlative outer Atongarak position in eastern basin. (B) Relatively restricted ice advances of LGM (last glacial maximum). Glacier at Anisak moraine in western basin and at correlative Douglas Creek ice-front position farther east.

Drifts of the Gunsight Mountain and Anktuvuk River glaciations, where distinguished along the Etivluk and Nigu rivers north and northeast of Howard Pass (Fig. 2), contain erratic clasts that are derived from granitic highlands in the Noatak River's headwaters. Dispersal routes

of erratics during Gunsight Mountain time are uncertain owing to widespread subsequent erosion, but the much younger drift assigned to the Anaktuvuk River glaciation is traceable southward into an impressive U-shaped trough wide that transects northern highlands of the

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Fig. 4. Geologic map, Noatak basin below Cutler River mouth. Modi"ed from Hamilton (1984a, b, and unpublished mapping).

Table 1 Central Brooks Range glacial events as expressed in eastern Noatak basin and neighboring drainages (based on Hamilton, 1984a, b, 1986, and unpublished "eld mapping) Glaciation

Inferred age

Comments

Itkillik II

Late Wisconsin

Ice #owed west-northwest down upper Noatak valley to Douglas Creek moraine. Bracketed between C ages of 30 and 15 ka (Hamilton et al., 1987; Hamilton, 1996)

Itkillik I

Late Pleistocene

Ice #owed down upper Noatak valley into eastern Noatak basin. End-moraine belt is divisible into Itkillik IA (older) and IB (younger) components that are comparable to Itkillik I end-moraine subdivisions in many valleys of central Brooks Range (Hamilton, 1986)

Sagavanirktok River

Middle Pleistocene

Mapped along several valleys of Cutler River drainage system (Hamilton, 1984b). Not expressed at surface in eastern Noatak basin owing to deep burial

Anaktuvuk River

Early Pleistocene

Ice-"lled upper Noatak valley and #owed north through Howard Pass. It extended down Etivluk and Nigu valleys and terminated 24 km south of Colville River

Gunsight Mountain

Late Tertiary

Forms highly eroded drift along Nigu River that extends to within 18 km of Colville River

Brooks Range at Howard Pass. Glacier ice must have entirely "lled the Noatak basin during Anaktuvuk River time, over#owing northward through the pass.

Mappable deposits assigned to two separate glacial advances of Sagavanirktok River age are widespread in the Cutler River drainage system (Hamilton, 1984b) and

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in upper valleys of the Etivluk and Nigu Rivers (Hamilton, 1984a); but none are exposed on the surface of the eastern Noatak basin. However, a broad topographic divide that crosses the basin #oor near the mouth of Aniuk River may represent an end moraine deeply buried beneath younger glaciolacustrine deposits. The arcuate lower course of Aniuk River may have been established marginal to the former ice tongue. During the subsequent Itkillik glaciation, ice descended the upper Noatak valley and formed a double moraine belt centered on Atongarak Creek. The older and younger components of that drift complex were assigned to the Itkillik IA and IB advances of the central Brooks Range succession (Hamilton, 1984b). A later ice advance formed a prominent moraine near Douglas Creek that morphologicallly is identical to moraines of Itkillik II age elsewhere in the central Brooks Range (Hamilton, 1986). Bracketing radiocarbon ages of 30 and 15 ka on the Douglas Creek moraine (Hamilton et al., 1987; Hamilton, 1996) con"rm that it is of Itkillik II age and correlative with the last glacial maximum throughout Alaska (Hamilton, 1982, 1994).

3. Lithofacies To simplify blu! descriptions, I utilize a lithofacies code that distinguishes "ve glacial, six nonglacial (interstadial or interglacial), and two additional lithofacies within the western Noatak basin (Table 2). 3.1. Glacial deposits Till (t). Rounded to subrounded, striated stones in highly compact matrix of dark gray sandy clayey silt. Platy stones aligned parallel to near-horizontal "ssility.

Table 2 Lithofacies units in the Noatak basin, Alaska Glacial deposits Till (t) Ice-contact deposits (ic) Glaciolacustrine deposits (gl) Distal glaciolacustrine deposits (gl-d) Loess (l) Nonglacial deposits Fluvial deposits, undi!erentiated (f ) Fluvial channel deposits (fc) Floodplain deposits (!) Pond and lake deposits (pl) Old Crow tephra (Oct) Peat (pt) Deposits of composite or uncertain a$nity Sandy capping deposits (sa) Deltaic deposits (ds)

Generally cut by near-vertical sets of joint planes. Commonly grades upward into glaciolacustrine sediments (see below). Ice-contact gravel (ic). Water-washed gravel and sandy gravel, moderately well sorted and commonly bedded. Generally contains collapse structures from melt of adjoining glacier ice. May contain poorly sorted debris-#ow deposits with muddy matrix. Commonly caps till deposits. Glaciolacustrine deposits (gl). Dark gray to dark olive gray (5Y3/2) clayey silt to slightly clayey silt; generally unstrati"ed and lacks fabric. Contains abundant to dispersed rounded to subrounded pebbles, small cobbles, and some larger cobbles, mixed with subangular clasts up to boulder size. Some stones faceted and striated. Interstrati"ed with beds of stony mud and muddy gravel in some exposures. Commonly grade into distal glaciolacustrine deposits (see below) near upper and lower contacts. Distal glaciolacustrine deposits (gl-d). Range from sticky gray clay and silty clay with thin silt beds to rhythmically laminated clay and silt with sparse dropstones. Common subfacies at base or top of glaciolacustrine deposits. Loess (l). Structureless silt and organic silt. Dark gray to black, ranging to yellowish brown where weathered. Commonly contains ice wedges. 3.2. Nonglacial deposits Fluvial channel deposits ( fc). Subangular to subrounded pebbles, small cobbles, and sparse larger stones in matrix of poorly sorted medium to coarse sand. Finer and coarser clasts commonly alternate in beds 4}10 cm thick. May also contain beds of strati"ed sand and granules. Floodplain deposits ( w ). Generally occur directly above #uvial channel deposits. Include sandy "ne gravel, sand, and interbedded sand and silt, generally horizontally bedded, but with some local ripple bedding or other cross-bedding. Commonly capped by organic-rich paleosols (Histosols) of humic silt, silty humus, or peat, which thicken within depressions. May include pond and lake deposits (see below). Generally strati"ed, but upper parts of unit may be frost-stirred, with admixed small stones and loss of strati"cation. Ice wedges or ice-wedge casts commonly penetrate downward from top of unit. Detrital wood fragments commonly present. Fluvial deposits, undiwerentiated ( f ). Channel and #oodplain deposits, as described above, that are too thin to designate individually. Also includes some #uvial deposits, such as gravelly sand, that di!er from typical channel and #oodplain deposits. Pond and lake deposits (pl). Faintly laminated to rhythmically laminated clayey silt, silt, and "ne sand. Commonly contains plant fragments and other organic

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detritus. Generally forms part of #oodplain unit, and is commonly capped by peaty Histosol. Old Crow tephra (Oct). Pinkish, silt-sized, angular glass shards, forming discontinuous lenses up to 4 cm thick. Identi"ed only at Nk-26 (Elias et al., 1999). Peat (pt). Compacted plant detritus, ranging from nearly unaltered plant remains to highly decayed fragments in dense black humic matrix. Rootlets common. May contain interbedded silt lenses. Commonly part of #oodplain complex (!), but mapped separately where unusually thick or where blu! sections are plotted at large vertical scale.

377

the #oor of the trough is inset within a planar surface that stands about 40 m above river level. This terracelike surface is nearly level near the valley center, but it rises at gentle gradients as it approaches the lateral moraines and wedges out against their inner #anks. The surface appears continuous with the sandy deposits that surround and overlap the Cutler moraine. Five exposures (Nk25}29A) intersect the planar surface and sediments of dominantly post-Cutler age that underlie it (Fig. 5). A higher exposure (Nk-30), at the northwest end of the section, may intersect the north arm of the Cutler moraine. 4.1. Exposure Nk-24

3.3. Deposits of composite or uncertain aznity Sandy capping deposits (sa). Largely vegetated "negrained deposits above steep blu! faces that extend upward to terrace or upland surfaces. Excavations and limited natural exposures exhibit mainly coarse sand to silty very "ne sand. Deltaic deposits (ds). Finely laminated, well sorted "ne sand with climbing ripples. May include some beds of peat or "ne sandy gravel and detrital plant remains along some bedding planes.

4. Cutler moraine to Okak Bend Near its con#uence with the Cutler River, the Noatak River intersects a massive arcuate end moraine 3}4 km wide that was deposited by a glacier that #owed southeast into the Noatak basin (Fig. 4). The glacier terminated in a proglacial lake (Hamilton, 1984a, b), and its end moraine is overlapped by glaciolacustrine sediments. Five very similar blu! exposures occur near the Cutler River con#uence. Only Nk-24, which is centrally located and typical in composition, is described below. Downriver from the Cutler River con#uence, the Noatak #ows north-northwest for 18 km within a trough about 7}10 km wide that is con"ned within lateral arms of the Cutler moraine. The Holocene #oodplain along

Exposure Nk-24 intersects a gently undulating upland surface that probably is underlain by the Cutler moraine. The basal unit, 46 m of clayey silt with abundant striated stones up to small boulder size (Fig. 5), is plastic when moistened and #ows readily. It di!ers from normal basal tills, which generally are compact and form resistant units in river blu!s. Because of its noncompacted nature and its association with a probable water-laid end moraine, it is considered to be an ice-proximal glaciolacustrine deposit. The stony silt contains abundant chert pebbles and gabbro or pyroxene clasts that were derived from sources to the north and northwest in the DeLong Mountains (Nelson and Nelson, 1982). The glaciolacustrine unit is overlain by poorly exposed, vegetated sandy sediments in which stones are rare or absent. Test pits expose well sorted, strati"ed, "ne to medium sand. The sandy deposit appears to be largely alluvium that may have formed in a slackwater setting when the Noatak basin was blocked by a later, less extensive glacial advance. 4.2. Exposure Nk-25 Exposure Nk-25 contains glaciolacustrine sediments overlain by a poorly exposed sandy deposit. The glaciolacustrine unit is generally unstable along the blu! face, but is generally less susceptible to #owage than the

Fig. 5. Blu! exposures along Noatak River from Cutler River con#uence to Okak Bend, showing exposed lithofacies. Triangles refer to locations of Pinus wood (Pn), Old Crow tephra (OCt), and bone dated by uranium-series method (152 ka age). Circles mark radiocarbon-dated samples (ages in thousands of yr BP).

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glaciolacustrine deposits at Nk-24 and elsewhere around the outer limit of the Cutler moraine. It is strongly jointed, with joint planes passing through stones. Along one part of the blu! face, #uvial deposits about 5 m thick are exposed above the glaciolacustrine unit. Alluvial gravel is overlain by stony silt, which is capped by layers of black humic silt and silty humus. Beds of sand and "ne gravel overlie the organic deposits.

water-washed muddy sand and granules. The sandy deposit that caps the section is generally eroded back from the blu! face and consequently is poorly exposed. However, a single small exposure of cross-bedded sand with sparse "ne gravel and thin lenses of organic detritus indicates a #uvial origin for at least part of the deposit. Bedforms indicate paleocurrent directions parallel to the modern valley, and detrital wood dates '47 ka (Table 3).

4.3. Exposure Nk-26

4.4. Exposure Nk-27

Exposure Nk-26 has been described in detail by Elias et al. (1999). This blu! extends for 0.5 km along the Noatak River, forming a steep face 30}35 m high that rounds back to the planar 40-m surface. Two glaciolacustrine units are separated by multiple alluvial deposits (Fig. 5). The older glaciolacustrine unit forms an erosion remnant about 10 m high near the middle of the blu!, but is absent elsewhere. Several beds of "ne sand and laminated silty clay within the erosion remnant have apparent northward (downriver) dips of 2}33. Cobble lithologies are similar to those in the Cutler moraine, but also include a rounded clast of conifer wood identi"ed as Pinus (white pine group) by the U.S. Department of Interior's Forest Products Laboratory (USDA-FPL) (written commun., 11/83). The basal unit along much of the blu! is "nely laminated "ne sand and organic "ne sand that probably formed as deltaic foreset beds. A deformed peat bed occurs near the top of the deltaic deposits. Overlying #uvial gravel is oxidized to a depth of 3 m, and clasts in the most permeable beds have oxide coatings to 2 m additional depth. In the southern part of the blu!, the gravel contains several `stone linesa of cobbles and small boulders that probably resulted from downcutting and lateral erosion by the Noatak River into the lower glaciolacustrine unit. Pond, bog, and marsh sediments "ll deep channel-like depressions in the gravel's upper surface. Twigs and #attened wood fragments near the top of both channel "llings include Picea or Larix (USDA-FPL, written commun., 11/83 and 10/93). Floodplain deposits that typically cap the alluvial gravel and channel "llings contain wood identi"ed as Salix (USDAFPL, 11/93), and dated as '49,500 yr BP (Table 3). An upper gravel unit is visible only near the north end of the blu!. It probably has been eroded farther south, but widespread debris cover at this level on the blu! face makes exact relations unclear. It is less compact than the underlying gravel, and is oxidized only near its upper contact. The upper gravel locally is capped by up to a meter of silt and humic silt, which "lls depressions on its surface. A lens of pinkish tephra up to 4 cm thick near the base of the silt cap was identi"ed as Old Crow tephra (Elias et al., 1999). The upper glaciolacustrine unit, which extends entirely across the blu! face, includes beds 1-3 m thick in which stone content remains fairly constant but matrix alternates between unsorted clayey silt and

Exposure Nk-27 stands 44 m high. Rhythmically laminated silt and clay above a paleosol developed on stony silt occurs at river level near the center of the blu!. The paleosol surface strikes north and dips 10}123E; it bears polygons 40}50 cm in diameter. The dip may in part be due to undercutting by the river, but part also appears to follow the downvalley dip of the unit's upper contact. Overlying diamicton, interpreted as till, is highly compact and maintains vertical faces. It has a general "ssile structure, with near-horizontal zones about 0.5 m thick of intense "ssility associated with oriented platy stones. Oxides are concentrated along near-vertical joint planes and for 0.7 m below the upper contact, which rises in height from 5 m above the river near the south end of the blu! to nearly 11 m near the center. At two localities near the center of the blu!, lenses of probable #uvial overbank deposits which lack visible organic remains overlie the till. A probable glaciolacustrine deposit occurs higher in the section. This unit is faintly bedded, with stony mud alternating with water-washed layers of stony muddy sand and some beds of matrix-free pebbles. The upper one-third of the blu! is covered, but sand is exposed locally at the base of the covered section. A gravel bar along the Noatak River opposite Nk-27 contains detrital fragments up to 18 cm in width of Pinus (USDA-FPL, 10/93). The source of this detritus is unknown. 4.5. Exposure Nk-28 Exposure Nk-28 has an exposed face only 16 m high, capped by vegetation-covered sandy deposits about 10 m thick. Alluvium overlies and underlies a thin (2.6 m) tilllike deposit, and a poorly exposed diamicton of similar thickness occurs higher in the section. The diamictons could be #ow deposits derived from erosion of older glacial units, and consequently this blu! is not illustrated in Fig. 5. 4.6. Exposure Nk-29A Exposure Nk-29A, on the west side of the narrow neck of Okak Bend, stands 26 m high, rounding back to a nearly level upland surface 37 m above the river. (NK29B, on the opposite side of the neck, is poorly exposed

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Table 3 Radiocarbon age determinations (uncalibrated), western Noatak basin Blu!

Age (yr BP)

Laboratory no.

Material dated

Stratigraphic position (1)

Comments

Nk-26

'47,000 '49,000 '57,000

USGS-1840 USGS-1457 USGS-1841

Wood fragments Wood fragments Organic slit

(2) (2) (3)

Nk-29A

'46,400 '51,000 52,500> \

AA-20752 USGS-1842 USGS-1843

Peat with wood Peat Humic silt

Nk-31 Nk-32 Nk-32C (14.5-m terrace)

'49,000 35,000$2300 9655$75 10,085$95 10,205$80 11,310$85

USGS-3402 I-13,466 AA20756 AA20754 AA20755 AA20758

Wood (Picea) Peat Peat Woody peat Peat Peat

12,890$190

I-12,542

Peat

12,970$280

I-12,517

Peat

13,620$95

AA20770

Detrital twigs

9565$90

AA20762

Wood (Salix)

9830$75

AA20771

Wood (Salix)

10,560$80

AA20786

Peat

10,720$85 13,640$80

AA20761 USGS-3406

Wood (Salix) Peat

40,180$750 '44,800 '45,200

USGS-3405 AA20759 AA20760

Peat Wood fragments Wood fragments

33,690$300

USGS-1846

Wood (Picea/¸arix)

36,610$400

USGS-3287

Peat

44,800> \

USGS-3408

Silty peat

'42,000

USGS-3407

Wood (Salix)

'46,000

USGS-1845

Wood (Picea/¸arix)

Within capping sand at 32 m ht. Floodplain deposit at 9.5 m ht. Floodplain deposit at base of lower diamicton Floodplain deposit at 17.5 m ht. Ice-wedge cast in alluvium at 16 m ht. Channel "ll in #oodplain deposit at 17 m ht. Floodplain deposit near river level Paleosol within alluvium at 12.5 m ht. Peaty channel "ll: 3 m depth Peaty channel "ll: 6 m depth Base of peaty channel "ll Within overbank sand#silt; 5.8 m depth Base of sandy #oodplain deposit: 8.5 m ht. Base of depression "ll in #oodplain deposit: 10.5 m ht. Base of sandy #oodplain deposit: 8.5 m ht. Near top of silt#peat depression "ll; 31.8 m ht. Log ca 8 cm diam. near top of silt#peat unit; 2.2 m depth Base of silt#peat unit; 3.9 m below surface Base of silt#peat unit; 28.1 m ht. Bryophyte mat in eolian sand at 26 m ht. Floodplain deposit near river level Floodplain deposit near river level Peaty layer in postglacial kettle "ll at 23 m ht. Distal part of detrital wedge, near blu! center Floodplain deposit below upper diamict; E sector of blu! Distal part of debris wedge near blu! center Distal part of debris wedge near blu! center Detrital wood in imbricated gravel unit; E sector of blu!

Nk-28

Nk-32B (12.5-m terrace) Nk-33 (14.5-m terrace)

Nk-35A

Nk-37

(2) (2) (2)

(4)

(5,6) (5) (7)

Notes: (1) Heights are above river level; depths are below ground surface. (2) Elias et al. (1999). (3) Not illustrated in Fig. 5. See explanation in text. (4) Nk-32B and Nk-32C terrace records are combined in Fig. 8. (5) Diapiric extrusion at downstream end of blu!. (6) Later redated as '44,800 yr BP (AA20759). (7) C age is too old. Wood may be redeposited from older sediments.

and will not be discussed here). Two diamicts alternate with alluvium in the exposed blu! face. A #oodplain assemblage consisting of "ne gravel, sand, silt, and peat, is exposed intermittently near river level beneath the lower diamict. Willow wood (Salix; USDA-FPL, 12/13/83) occurs within the peat, and detrital fragments

of Populus wood (USDA-FPL, 10/93) occur within sand beds. Pollen and insect remains indicate tundra vegetation similar to that of the present day, with summers perhaps slightly cooler (Elias et al., 1999). The overlying diamicton is a poorly exposed glaciolacustrine deposit that probably extends close to river level, where a trench

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uncovered stone-free, sticky, plastic, "ssile, silty clay, with oxide concentrations along the "ssile partings. Channel gravel above the glaciolacustrine complex is oxidized yellowish red through its upper 75 cm, and ice-wedge casts up to 1.5 m wide originate at its upper surface. Deformed peat within one ice-wedge cast has a non-"nite radiocarbon age of '51,000 yr BP The gravel generally is capped by thin (0.5 m) silt and peat, but channel-like depressions in its surface are "lled by up to about 1.5 m of sand and sandy "ne gravel that grades upward into peaty and silty #oodplain deposits. Humic silt from a buried organic soil (Histosol) in one channel "lling has a radiocarbon age of 52,000> yr BP, which should be \ considered a minimum age limit. A fragment of caribou antler from the same unit has a uranium-series age of 152,000$5,000 yr BP (B.J. Szabo, U.S. Geological Survey, written commun., 1995). Pollen and insect remains from a channel "lling near the center of the blu! indicate tundra conditions and summers cooler than present (Elias et al., 1999). The #uvial deposits are overlain by a probable till that contains clasts up to slightly more than 1 m diameter in a matrix of extremely compact sandy silt. This unit maintains nearly vertical, strongly jointed faces, with yellowish red oxidation along all joint planes. Sparse near-horizontal shear planes, with aligned platy stones, also are present. The till grades upward into a glaciolacustrine deposit 2}4 m thick in which pebbles and cobbles are dispersed in a matrix of clayey silt. This unit #ows readily, and generally is poorly exposed. When initially investigated, Nk-29A was capped by a vegetated slope underlain by "ne-grained sandy sediment, similar to the upper slopes of Nk-24}27. However, a retrogressive thaw #ow-slide (Harry et al., 1988; Burn and Friele, 1989) in the summer of 1996 formed an ampitheater-like thermocirque (Czudek and Demek, 1970) that exposed lacustrine silty clay, oxidized pebbly sand, and loess containing ice wedges. 4.7. Exposure Nk-30 Exposure Nk-30 at the apex of Okak Bend, is situated close to the inner #ank of the Cutler River moraine. Its face stands 41 m high, rounding back to a gently inclined soli#uction slope that rises northward toward the moraine. Deposits at the base of the blu! are poorly exposed. Dark gray silty clay is extruding plastically near the river's edge, and a trench near the downstream end of the blu! exposed 2.6 m of deformed silt and pebbly sandy silt. No organic matter was evident in any of these deposits. The overlying till is unusually thick (21 m), compact, and strongly jointed, with oxide concentrations along some joint planes. Sets of subhorizontal shear planes, with platy stones in parallel alignment, are concentrated in zones 10}12 cm thick. Alluvium above the till contains local channel "llings of cross-bedded sand and "ne gravel, and is capped by 75 cm of ripple cross-

bedded, slightly oxidized, well-sorted "ne sand. The uppermost diamict in the blu! has been entirely disturbed by #owage, and no primary structures are present. However, its glacial origin is demonstrated by striated and faceted stones that include sparse boulders up to 1.5 m diameter. The top of this unit is not exposed, but limited exposures into the overlying vegetated slope appear to consist of stone-free, loosely packed, yellowish brown loess. 4.8. Discussion Subglacial tills must be di!erentiated from ice-proximal glaciolacustrine deposits to reconstruct paleoenvironments and correlate blu! exposures within the Noatak basin. These two deposits can be super"cially similar, both containing abundant rounded to subrounded pebbles and small cobbles, with sparse and more angular larger cobbles and boulders. However, they di!er in many other characteristics (Table 4), and also have generally di!erent vertical successions (Fig. 6). Till deposits typically are massive, strongly jointed; and highly compact, forming vertical faces on river blu!s. They commonly contain near-horizontal shear planes, with platy stones aligned parallel to nearby planes. Lower contacts generally are sharp and erosional, with underlying sediments commonly deformed by the overriding glacier. Proximal glaciolacustrine deposits, in contrast, have a more clayey matrix; they tend to have crude strati"cation and basal stone-free facies. Water-washed beds of muddy gravel, which are interstrati"ed with unsorted deposits or "ll erosional channels within them, may represent intervals of sudden drainage of the lake due to breaching of its glacial dam. The stone-free basal facies, which generally are about 1 m thick and commonly include rhythmites, may have formed during an initial shallow-lake stage or when glacier ice was too distant to contribute dropstones to the site. In contrast to the stable and near-vertical faces of till deposits, glaciolacustrine deposits tend to #ow readily where exposed along river blu!s. The till must have been compacted and partly dewatered by overriding glacier ice, becoming `drya permafrost (permafrost with negligible water content) when later frozen. The glaciolacustrine sediments evidently were water-saturated during deposition, later becoming ice-rich permafrost subject to thaw, collapse, and #owage on blu! faces. Correlations between depositional units in Nk-24}30 are suggested in Fig. 7, which was redrawn from Fig. 5 to show correct altitudes and spacing of the exposures. The glaciolacustrine sediments at Nk-24, which extend to about 46 m height, are clearly part of the Cutler moraine. Similar glaciolacustrine deposits at Nk-25, which are strongly jointed and overlain by unusually thick capping deposits, are probably also of Cutler age. They were truncated by postglacial river erosion at 24 m height, but

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Table 4 Characteristics of tills vs. glaciolacustrine deposits, western Noatak basin

1. 2. 3.

4. 5. 6. 7.

Till

Glaciolacustrine deposits

Underlying deposits commonly truncated by erosion and (or) glacially tectonized Lower contact sharp; generally erosional Generally massive, with no vertical trends in clasts or matrix except at top of unit

Underlying deposits generally undisturbed; may be transitional upward to marsh or pond deposits Lower contact sharp to gradational; no erosion of underlying beds Basal sediments commonly are rhythmites. Main body of unit may be crudely bedded, with cut-and-"ll structures and interbeds of waterwashed muddy gravel Matrix generally clayey silt to gritty clayey silt Generally uncompacted and ice-rich; #ows readily upon thawing No jointing; dark gray color without oxides Fabric generally absent; compressional deformation common beneath dropstones

Matrix commonly sandy silt to silty sand Highly compact; stands in vertical faces Strongly jointed, with oxides along joint planes Commonly "ssile, with platy stones parallel to "ssility

Fig. 6. Typical vertical successions in till contrasted with glaciolacustrine deposits, western Noatak basin.

lack of well-developed #oodplain deposits at this level suggests that truncation could have taken place during river downcutting to a more stable level. The lower glaciolacustrine deposits at Nk-26 contain several beds of laminated sediment with apparent downvalley dips of 23}33. These imply tectonic or isostatic deformation that is not present in any of the younger horizontally bedded glaciolacustrine deposits, and suggest a probable Cutler age. The basal diamict at Nk-27 has been truncated at a height comparable to the erosion surface on the basal glaciolacustrine unit in Nk-26; suggesting that these two units may be of comparable age. It might also be signi"cant that redeposited Pinus wood is associated with basal units in both blu!s. Because the basal diamict at Nk-27 is

considered to be a till, a grounding line (Eyles et al., 1985) of the Cutler glacier may have been situated between Nk-26 and Nk-27. The strongly jointed and partly oxidized basal till at NK-30, which was truncated by river erosion at 24 m height, occurs near the inner #ank of the Cutler moraine and is believed to also be part of that deposit. Fluvial erosion of the glacial deposits at Nk-25 and Nk-30 was at comparable heights, which could indicate widespread beveling of glacial deposits by the Noatak River at that level. However, the alluvial sequences above the two erosion surfaces are very di!erent from each other, and the corresponding heights may be fortuitous. The channel gravel above the till at Nk-30, which occurs to a height of 36 m, re#ects a Noatak River level higher than that associated with any of the blu!s immediately upriver but is close to the upper limits (38}46 m) of the glaciolacustrine deposits around the Cutler River con#uence. The gravel probably represents late-Cutler or early interglacial conditions when the Noatak valley may have been still partly blocked by stagnating glacier ice or glacial detritus. Two alluvial sequences (each consisting of #oodplain above channel deposits) at Nk-26 postdate the Cutler advance. The lower sequence, which is oxidized to 3}5 m depth, must be signi"cantly older than the weakly oxidized and less compact overlying alluvium; it perhaps was deposited during a long interglacial that followed the Cutler advance. The presence of Old Crow tephra at the top of the younger gravel suggests that it may have formed near the beginning of the last interglaciation (i.e., near the marine-isotope-stage 6/5 transition; Elias et al., 1999). This alluvial succession suggests that two separate interglacial intervals may have intervened between the Cutler advance and deposition of the younger glaciolacustrine unit, which must be of late Pleistocene age. The two upper depositional units in Nk-27 are similar to those at Nk-26, and are developed at comparable heights. Both glaciolacustrine units contain waterwashed muddy gravel, indicating that deposition was

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Fig. 7. Blu! exposures and their inferred correlations, Cutler River con#uence to Okak Bend. Exposures are plotted at true altitudes and correct relative spacing.

interrupted periodically by breaching of a glacier dam farther downvalley. Glaciolacustrine deposits at Nk-30 are developed at a comparable altitude, and may be correlative, but #owage has obliterated all primary structures. Similar poorly exposed sandy deposits that may in part be alluvial or deltaic cap all three blu!s. Nk-28, which intersects a terracelike surface 26 m high, shows little similarity to the higher neighboring river blu!s. However, its basal alluvial sequence resembles the interglacial #oodplain deposits at the base of Nk-29A, and these sediments dip at a few degrees like the lower glaciolacustrine deposits at Nk-26. The basal deposit at Nk-28 could represent interglacial alluvium that formed shortly after the Cutler advance. The lower glaciolacustrine deposit at Nk-29A has no analogs in the other river blu!s. It could represent a less extensive glacial advance farther downvalley * one that perhaps took place between deposition of the two alluvial units at Nk-26. Alluvium above the glaciolacustrine unit terminates upward in #oodplain deposits that are considered correlative with the upper tephra-bearing #oodplain unit at Nk-26 on the basis of similar stratigraphic position, comparable age determinations, and a similar juniper-rich pollen assemblage (Elias et al., 1999). The overlying till has no equivalent upvalley, and must be part of the Okak moraine (see next section). It grades upward into glaciolacustrine deposits that may once have been continuous with the upper glaciolacus-

trine units at Nk-26, Nk-27, and Nk-30. The fresh exposure into the `sandya deposits that cap Nk-29A, reveals a complex succession of lake, stream, and marsh deposits, overlain by loess. The vegetated `sandya deposits above the other blu!s must conceal similar or even more complex records.

5. Okak Bend to west margin of basin Near Okak Bend, the Noatak River #ows through a pair of unusually involuted meanders; it then straightens into a broadly meandering westward course (see Fig. 4). The involuted meanders near Okak Bend appear to be controlled by bouldery erosion remnants of two moraines. The older feature, the Okak moraine, lacks surface expression. It trends north-northwest between the two arms of Okak Bend and forms a boulder-choked ri%e where intersected by the river at the apex of the bend. The younger, more conspicuous moraine, the Makpik moraine, intersects the valley center just west of Makpik Creek. About 16 km farther downvalley, near the mouth of Anisak River, the Noatak is intersected by the Anisak moraine, which forms the upvalley limit of rugged glacial deposits with negligible loess cover and little soil development. River blu!s farther west expose deposits that formed during several readvances or stillstands of the Anisak glacier. The appearance of bedrock in the

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383

Fig. 8. Blu! exposures along Noatak River from Okak Bend to Nk-36. Circles mark radiocarbon samples (ages in thousands of C years BP).

valley center 16.5 km below the mouth of Anisak River marks the western limit of the Noatak basin. 5.1. Exposure Nk-31 Exposure Nk-31 is located near Makpik Creek at about the position where the Makpik moraine must formerly have intersected the Noatak valley center (Fig. 5). The exposed face of the blu! is 27 m high, and meets at a sharp angle the planar upper surface of a river-terrace remnant (Fig. 8). The basal deposit, strongly oxidized pebble gravel capped by oxidized silt and "ne sand, is exposed only as a possible diapiric structure at the upstream end of the blu!. Detrital wood from this unit has been identi"ed as spruce (Picea, USDA-FPL, 10/93) and dated at '49,000 yr BP (Table 3). Overlying clayey glaciolacustrine sediments grade upward into compact till. Gravel above the till has higher chert and quartz content than is normal for the Noatak River, and may have been deposited by Makpik Creek. Limited exposures into the capping unit of the blu! reveal well sorted "ne sand, with some beds of medium sand and organic silt, that is cut by ice wedges. Peat and humic sand above oxidized, frost-churned sand cap the deposit.

5.2. Exposure Nk-32 Exposure Nk-32 downriver from Makpik Creek, is largely obscured by vegetation and debris, which generally cover #ow deposits. The blu! is highest (30 m) at its east end, where it intersects the inner #ank of the Makpik moraine. An exposure in the east-central part of the blu!, which stands 23 m high, contains severely deformed glaciolacustrine deposits that are overlain by slumped alluvial gravel (`1995a column in Fig. 8). When initially mapped in 1983, part of the blu! face exposed two units of alluvial gravel that were separated by #oodplain sediments which thickened to about 3.5 m near the central part of the blu! (`1983a column in Fig. 8). A 1-m-thick paleosol, consisting of peat and humic silt above oxidized sand, capped the #oodplain section at that locality. A radiocarbon age of 35,000$2300 yr BP was obtained on shrub roots in the paleosol, but its large counting error makes this age determination suspect. Attempts to relocate the #oodplain deposit on the severely deformed face of the blu! have been unsuccessful. A 14-m river terrace near the downstream end of the blu! contains a deep channel incised into alluvial gravel. The channel is "lled with sand that is interbedded with and capped by silty peat. Detrital twigs near the base of

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the channel date 11,310$85 yr BP, and the peat is dated at 10,205$80 yr BP and younger. 5.3. Exposure Nk-33 Exposure Nk-33, midway between Makpik Creek and Anisak River, intersects an alluvial terrace of similar height that also is inset within the inner #ank of the Makpik moraine. Most of the terrace face exposes only alluvial gravel overlain by #oodplain deposits. However, a plastic clay containing abundant straited stones is locally extruded from below river level at the base of the blu!. Overlying alluvial gravel is capped by #oodplain deposits consisting of well-sorted very "ne sand, with some beds of medium sand and pebbly sand near its base and beds of organic silty "ne sand near its top. Peat lenses 50 cm above the base of the unit have a radiocarbon age of 12,890$190 yr BP, and detrital twigs near this level are dated at 13,620$95 yr BP. The blu! is capped by about 1 m of sod, peat, silt, and very "ne sand that thickens over depressions in the underlying sand. Peat from the base of a 3-m depression "lling dates 12,970$280 yr BP. 5.4. Exposure Nk-34 Exposure Nk-34, a short distance below the mouth of Anisak River, is a prominent blu! about 55 m high that marks the outer limit of the Anisak moraine. The modern river bed here contains abundant boulders up to 1 m length. The blu! face is heavily vegetated, and exposures are limited to recently active #ows in which striated stones up to small boulder size occur in a matrix of slightly clayey, gritty silt. The #owing debris probably was derived from glaciolacustrine sediment deposited near or beneath a #oating ice tongue. The upper surface of the blu! rises inland at a gentler angle into a complex of gravel knolls and terracelike benches as much as 77 m above the river that probably were formed by meltwater #owing along the margin of the glacier tongue.

redated at '44,800 yr BP. The lower part of the blu! is generally obscured by debris #ows that contain striated and faceted stones in a matrix of gritty clayey silt. A thin (0.8 m) horizon of rhythmically laminated clay and silt occurs at the base of the probable glaciolacustrine deposit. Ice-contact alluvium above the debris #ows has a poorly sorted sand and gravel matrix. Collapse structures re#ect meltout of ice masses, and an undisturbed capping #oodplain deposit of faintly laminated "ne sand indicates that local meltout was completed while the river remained at a high level. A wood fragment within the collapsed sediments is dated at '45,000 yr BP, but a bryophyte mat in "nely laminated "ne sand near the top of the deposit has a "nite age of 13,640$80 yr BP. Peat and organic silt, which cap the blu!, are typically 4}5 m thick. The lower part of this deposit contains lenses of silt, sand, and "ne gravel, and probably accumulated on a #oodplain. Salix (willow) wood near the base and top of this unit are dated at 10,720$85 and 9565$90 yr BP, respectively. The southeast part of the blu! complex (Section B) is an alluvial terrace 24 m high that consists of pebble} cobble gravel in a poorly sorted matrix of silty sand and granules. Steeply dipping beds of "ner alluvium "ll a steep-sided depression in the gravel surface. The steepness and poor sorting of these beds suggest that deposition could have been contemporaneous with meltout of underlying glacier ice. The terrace alluvium is highly deformed at its east end, where beds drape over and become nearly vertical. The gravel also may have been collapsing above melting glacier ice at this locality. 5.6. Exposure Nk-36 Exposure Nk-36 is situated where a steep-sided (183), narrow-crested (2}5 m) moraine intersects the north edge of the Noatak River's #oodplain. The blu! face is almost entirely vegetated, but a debris #ow exposes striated clasts up to 1.4 m diameter in a matrix of gritty, slightly clayey silt. Based on its position, form, and composition, the poorly exposed diamict must be dominantly till.

5.5. Exposure Nk-35 5.7. Exposure Nk-37 Exposure Nk-35, which is within the Anisak end moraine, is separated into two components (A and B on Figs. 5 and 8) by a large kettle depression that has been breached by the river. Section A is higher and more complex, but is partly obscured by slumps and debris #ows. Section B is lower and is dominantly alluvium. Section A is as high as 32 m. A basal #oodplain section that includes organic silt, silty "ne sand, and peat is exposed in diapiric extrusions up to 3.5 m above river level at the extreme northwest (downstream) end of the blu!. Alluvial sand and gravel beneath the organic deposits extend below river level. Peat from the #oodplain deposit was dated at 40,180$750 yr BP, and later

Exposure Nk-37 is an unusually large blu! that stands as high as 57 m and extends for more than a kilometer along the Noatak River. An apparent recessional moraine of the Anisak drift complex crosses the valley center here (see Fig. 4), and the Noatak is de#ected northward into the glacial deposits that underlie the northern #ank of its valley. The blu! exposes a complex glacial, #uvial, and lacustrine record that may span three separate glacial advances (Fig. 9). Its eastern part contains two probable tills overlain by a largely concealed, thick sequence of glaciolacustrine and lacustrine sediments capped by loess; its western section contains a higher-standing pair

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385

Fig. 9. Exposure Nk-37, north side Noatak River 12 km west-northwest of Anisak River. Stratigraphic relations across blu! face (from a "eld sketch). Radiocarbon ages in thousands of C yr BP.

of diamicts overlain by thinner silt and peat. The central part of the blu!, which is mostly concealed by massive slumps and #ows, contains a wedgelike body of sediment that tapers westward and probably formed as a debris apron at the face of an ancient river blu!. Gravel at the base of the eastern section is divisible into two units that di!er markedly in oxidation and probably are separated by a signi"cant time interval. The lowest unit is strongly cemented by iron oxides and stands in vertical faces. Rounded pebbles and some small cobbles occur in a brownish yellow (10YR 6/8) sandy matrix. Near the central part of the blu!, the strongly oxidized gravel contains detrital logs up to about 18 cm diameter of spruce or larch (Picea or Larix, USDA-FPL, 10/93). The basal contact of the higher gravel locally is a stone line of large cobbles and very small boulders. Above the stone line, strongly imbricated pebbles, small cobbles, and some larger cobbles occur in a mature (high quartz) sand matrix. Sparse detrital wood fragments include Picea or Larix (USDA-FPL, 11/83), and are dated at '46,000 yr BP. The two tills that overlie the gravel have sharp basal contacts and are highly compact, standing in vertical faces. The lower till is strongly jointed, with oxides concentrated along joint planes and on stone surfaces. The upper till lacks jointing, and is oxidized only locally by ground-water seepage. Channel gravel between the two tills commonly is oxidized to a depth of 20 cm; it locally is capped by 20 cm of black, highly compact stony peat that contains sparse #attened fragments of Picea or Larix wood (USDA-FPL, 11/83) dated

at 36,610$400 yr BP. Sediments above the upper till are generally displaced by #owage and slumping, but rare exposures at the headwalls of active thaw ampitheaters show 10}14 m of stony silt overlain by a nearly equal thickness of stone-free laminated silt and clay. A thin (0.7 m) deposit of "ne alluvium separates the two glaciolacustrine units, and loesslike silt that contains abundant ice wedges caps the section. Much of the western part of the blu! is obscured by slumps and #ows, but limited exposures exhibit two thick diamicts separated by glaciolacustrine deposits. The lower diamict is strongly jointed, with slight oxidation along joint planes. Overlying silt and "ne sand is rhythmically laminated in places, but elsewhere contains probable #owtill deposits and turbidite layers. It appears to be entirely inorganic, and could represent a brief glacial #uctuation with the ice front nearby. The upper diamict, which is disturbed by #owage in all exposures, consists of striated clasts in a matrix varying from clayey silt to very sandy silt. Slumped and vegetated deposits above the upper diamict appear to be largely stone-free silt and peat. Their upper surface is accordant with the crests of thaw ampitheaters along the eastern part of the blu!, suggesting that the loess and lacustrine sediments exposed in those basins may extend across the entire blu! face. The upper part of the central segment of the blu! has been entirely destroyed by #owage, but a westwardthinning wedge of highly indurated gravel provides near-vertical basal exposures about 5}10 m high. The

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Fig. 10. Blu! exposures plotted at true altitudes and approximately correct spacing, Okak Bend to west margin of Noatak basin. Pc designates Picea (spruce) wood samples; C represents covered intervals.

thick eastern part of the wedge consists of coarse gravel in beds up to 1 m thick that are separated by thinner beds of stony sand, pebbly silty sand, and organic silt; its upper 4.5 m is oxidized reddish yellow. This unit appears to have been redeposited from the imbricated gravel which occurs through the eastern part of the blu!. The redeposited gravel inter"ngers westward with "ner oxidized gravel, which farther west inter"ngers with, and is overlapped by, peat and organic silt. The peat contains logs up to 10 cm diameter of Picea or Larix (USDA-FPL, 11/83) that yield a probably incorrect radiocarbon age of 33,690$300 yr BP. and two other ages greater than 42,000 yr BP. Sediments overlying the colluvial wedge are generally obscured by #owage, but alluvial gravel with a stone line at its base appears to overlap the eastern part of the wedge. 5.8. Exposure Nk-38 Exposure Nk-38 stands about 23 m high and rounds back to an undulating upland surface at about 40 m height. Schist and quartzite bedrock at the base of the exposure is overlain by alluvial gravel, which has been oxidized to 6 cm depth (Fig. 10). Overlying diamict, a probable glaciolacustrine deposit, has been severely disturbed by slumping and #owage. Striated and faceted clasts up to 65 cm long are visible in a matrix of sandy, slightly clayey silt. The upper part of this unit is entirely vegetated, and any deposits that overlie it are obscured.

5.9. Discussion Most of the blu! exposures west of Okak Bend contain short or fragmentary depositional records that are controlled by their locations relative to former glacier termini (Fig. 10). Correlations of depositional units are particularly tenuous through this stretch because of (1) abrupt downvalley changes across terminal positions of former glaciers, (2) lateral changes between valleycenter and valley-margin deposits intersected in river blu!s, (3) incision by the Noatak River during each interval of glacier recession, (4) localized scour by advancing ice tongues, and (5) recycling of older glacial, alluvial, and lacustrine sediments by advancing glaciers and the Noatak River. Several of the blu!s exhibit diapiric extrusions of glaciolacustrine or organic-rich #oodplain sediments, implying that deformable sediments are present below river level. These basal sediments are older than about 50 ka, according to radiocarbon ages from Nk-29A and Nk-31, and presence of Picea wood in highly oxidized sediments at Nk-31 suggests an interglacial environment. However, diapiric sediments at the base of Nk-33 are unweathered and probably of glaciolacustrine origin; and basal organic #oodplain deposits at Nk-35A, although older than 44.8 ka, are little weathered and contain pollen indicative of shrub tundra (M.E. Edwards, written commun., 8/14/94). Perhaps areas downstream from Nk-31 were scoured by the advancing Makpik glacier to

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depths below modern river level, eradicating all older deposits. Till and glaciolacustrine sediments assigned to the Okak advance are prominent at Nk-29A, although end moraines associated with that advance are not expressed on the valley #oor. The Okak glacier probably terminated in a lake that "lled much of the western Noatak basin. The subsequent Makpik advance formed wellde"ned end moraines on the north and south sides of the Noatak River that extend to within 4 km of each other near Nk-31. The Makpik ice front drained southward through Nanielik Creek (see Fig. 4), and this drainage apparently was e!ective in preventing formation of a large deep-water lake in the Noatak basin during Makpik time. Nk-32 exposed interstadial deposits with an apparent age of about 35 ka when examined in 1983, but those deposits subsequently have been either eroded or covered. Some support for the occurrence of organicbearing #oodplain deposits of this age was found at Nk-37, where similar sediments dating to about 36.6 ka occur at a comparable height. The subsequent advance of the Anisak glacier is recorded at Nk-34 by a thick glaciolacustrine section capped by ice-contact gravel. The expanding glacier dammed the westernmost part of the Noatak basin and must have advanced as a #oating ice tongue. An outlet channel along Aklumayuak Creek (see Fig. 4) drained the ice front at about 300 m asl. The Anisak water plane must have attained this altitude in the western part of the Noatak basin (see Figs. 7 and 10), but it may have risen eastward owing to isostatic deformation (Hamilton, 1984a, b). The till-like morainal deposit farther downvalley at Nk-36 indicates that glacial lake Noatak had drained or become greatly diminished in size when the #uctuating Anisak ice front occupied this position. Late-glacial and early postglacial terraces occur at multiple levels. Nk-31, which stands 27 m high, is the only terrace below Okak Bend that bears a thick cap of sandy sediments. This terrace may correlate with Nk-28 upvalley from Okak Bend, which is similar in height and has a comparable sandy cap. The 23-m terrace at Nk-32, although nearly as high, is capped by alluvial gravel, and therefore may be younger. Nk-35 contains a complete late-glacial to early postglacial terrace record. Alluvial gravel deposited above wasting glacier ice of Anisak age accreted to 26 m above modern river level by 13.6 ka or shortly thereafter. Thick peat subsequently accumulated on the terrace surface beginning about 10.7 ka after channel incision had started. Nk-33, upvalley from the Anisak moraine, stands only 14 m high but exhibits a comparable history of alluviation. Nk-32C, a similar 14-m terrace a short distance upvalley (see Fig. 8) shows a longer duration of channel incision and "lling (ca 11.3}10.0 ka) before downcutting. Perhaps a local #oodplain stream occupied

387

the channel during this interval. The westernmost exposure in the Noatak basin, Nk-38, is comparable in height to Nk-35, but its glaciolacustrine deposits lack a gravel cap. A lake dammed by glacier ice or ice-cored glacial deposits might still have been present in the extreme western part of the Noatak basin around 13 ka. That water body would have provided a base level for the prominent late-glacial terraces farther up the Noatak River. Nk-37 intersects signi"cantly older sediments than are exposed in any other blu!s along the Noatak River west of Okak Bend. It yields a complex depositional record that probably extends back in time through the last interglacial and includes older glacial and interglacial deposits as well. The colluvial wedge in the central part of the exposure links the depositional records of the eastern and western segments of the blu!. The eastern part of the wedge is dominated by clasts that probably were redeposited from a former blu! face that was eroded into the imbricated gravel, and it is capped by boulders that probably were derived from the diamict that overlies that gravel. The western part of the wedge incorporates colluvial and alluvial deposits that include probable interglacial #oodplain peat beds that contain Picea or Larix wood. To the east, the wedge is overlapped by younger gravel, which is overlain by diamict. To the west, it probably is overlain by proglacial lacustrine sediments and by the lower of two diamicts. If correlations between the eastern, central, and western parts of the blu! are correct, the overall sediment record probably represents a sequence of 10 major depositional events that include three distinct glacial advances (Table 5). The strongly oxidized lower till in the eastern part of the blu! predates the interglacial colluvial wedge. This till resembles highly weathered till associated with the Cutler advance farther up the Noatak valley, and it may be of Cutler age. The upper till of the eastern and central parts of the blu! overlies channel gravel that is oxidized to only shallow depth and bears #oodplain deposits dated at about 37 ka. If that age determination is correct, the upper till probably represents the Anisak advance of late Wisconsin age. In the western part of the blu!, only unweathered, inorganic, silt beds separate the lower and upper diamicts, suggesting that a brief interstade separated those two units. Because the upper diamict must represent the readvance of the Anisak glacier that terminated at Nk-37, the lower diamict must also be of Anisak age. This would con"rm an Anisak age for the upper till farther east in the blu! if correlation between those two units is valid.

6. Western basin synthesis The glacial record for the western Noatak basin, together with water planes associated with the four principal outlets of glacial lake Noatak (Table 6), are shown in

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T.D. Hamilton / Quaternary Science Reviews 20 (2001) 371}391 Table 5 Correlation of depositional units in Exposure Nk-37.( - - -, absent or obscured)

Table 6 Major water planes of glacial lake Noatak. Based on Hamilton (1984a, b, and unpublished "eld mapping) Altitude (m) Age

Western Central

Eastern

Outlet

Anisak Makpik Okak Cutler

300 300? 335 425

* * 365 460

* * * 520

Older

475

505

520

Aklumayuak Creek Aklumayuak Creek(?) Hunt R. Howard Pass(?) #Hunt R.(?) Howard Press

Fig. 11. The Cutler moraine is traceable eastward to Nk-23, about 5 km above the mouth of Cutler River. It is shown close to its true height west of Okak Bend (where the fold line on Fig. 11 is located), but farther east the moraine probably has lost much of its original height owing to meltout of incorporated glacier ice and to loss of #otation in its former terminal zone. The water plane controlled by Howard Pass (520 m asl) would seem to be too high for Cutler time because the beds of small proglacial lakes along the south edge of the Cutler moraine (shown in Fig. 4) occur to altitudes as low as 430 m. However, the lake beds could have been lowered by meltout of glacial ice within the moraine, and isostatic de#ection resulting from extensive glaciation in the central Brooks Range might also have lowered the Howard Pass outlet su$ciently to have drained the lake during part of the Cutler stand. The pass at the head of the Nanielik outlet (490 m) probably served as a local drainage route from the south margin of the Cutler glacier, but

outlets as low as Aklumayuak Creek could not have drained glacial lake Noatak at this time. In the absence of signi"cant isostasy, the water plane associated with the Hunt River outlet (390 m asl) would be at an appropriate outlet level for glacial lake Noatak during the Cutler glaciation. The extreme river de#ection and boulder-"lled ri%es at Okak Bend indicate that a separate glacial advance to this location is more likely than a recessional stand or a grounding-line position of the Cutler glacier. Correlations between Nk-29A (at the Okak ice front) and Nk-26 (which contains the Old Crow tephra), together with pollen data from both exposures (Elias et al., 1999), also indicate that the Okak and Cutler advances were separated by one or more major interglaciations. The uranium-series age estimate from Nk-29A supports tephrochronology in showing that the younger interglacial episode probably is correlative with marine-isotope substage 5e. Because end moraines are not evident in its terminal zone, the Okak glacier must have formed a #oating ice tongue. The massive lateral moraine that extends east to Lake Kangilipak may be a composite feature of both Okak and Makpik age, as mapped in Fig. 4. If this is the case, the Okak glacier may have developed a #oating tongue at a level appropriate for the Hunt River water plane. A higher-level abandoned channel of Aklumayuak Creek at about 425 m asl may have served as a local conduit for ice-marginal drainage. The subsequent Makpik moraine has clearly de"ned lateral segments that extend close to the valley center, indicating that it terminated in a lake shallower than that of Okak age that probably did not extend far up the Noatak valley. An outlet into Aklumayuak Creek may have been e!ective in draining the Noatak basin at this

T.D. Hamilton / Quaternary Science Reviews 20 (2001) 371}391

389

Fig. 11. Western Noatak basin, showing former ice-tongue positions, lateral moraine pro"les, and reconstructed water planes of glacial lake Noatak.

time, but its level has been obscured by subsequent downcutting along the creek, and the height of the Makpik water plane is uncertain. Bracketing radiocarbon ages support surface morphology in demonstrating a Last Glacial Maximum (LGM) age for the Anisak advance. The present-day Aklumayuak Creek outlet was active at this time, causing the proglacial water plane to stand at only about 300 m asl and #ooding the Noatak basin to only to a short distance upvalley from Nk-26. Terraces formed during deglaciation stand as high as 26 m above modern river level, probably because the Noatak valley farther west was still blocked by glacier ice or by ice-cored drift. Comparable relationships occurred during earlier interglaciations, because #oodplain deposits at high levels in the Noatak blu!s typically have pollen pro"les of earlyinterglacial character whereas alluvial deposits at or below present modern river level contain full-interglacial pollen (Elias et al., 1999).

7. Age estimates and regional correlations Although the sequence of glaciations and related events within the western Noatak basin appears to be clearly de"ned, the ages of some of these events are uncertain and some correlations with the central Brooks range glacial record are tenuous. Glacial source areas in the DeLong Mountains are relatively low (1300}1500 m asl) compared to the rugged highlands that rise to 1800}2300 m asl in nearby parts of the central Brooks Range. Much of the DeLong Mountain glacial source area also is within 100 km of the Chukchi Sea coast, and therefore subject to much greater maritime in#uence during times of high sea level than is the central Brooks Range. For these reasons, the glacial record of the western Noatak basin could be o!set from that of the central

Brooks Range, and simple correlations based on positions within glacial sequences may be invalid. If glacier advances were not in phase, either in timing or in relative extent, then isostatic de#ection of water planes may be much more complex than the simple warping envisioned in Table 6. Until these planes are dated independently and traced by careful surface and subsurface mapping, their use for basinwide correlations is suspect. The Anisak moraine, which represents the last major glacial maximum (LGM) in the western Noatak basin, is "rmly correlated with the Douglas Creek moraine farther east and with the Itkillik II glacial phase of the central Brooks Range. Glacial deposits of Anisak and Douglas Creek age are identical in character to Itkillik II deposits (Hamilton, 1986). They are little modi"ed morphologically, have minimal weathering and soil pro"les, and exhibit only minor weathering of surface clasts; each represents the LGM in its area, and deglaciation was followed immediately by Holocene downcutting. They also have comparable bracketing radiocarbon ages*35 and 13.6 ka for the Anisak advance; 30 and 15 ka for the ice advance to Douglas Creek. In addition, the Anisak water plane did not extend upvalley much beyond the mouth of Cutler River (see Fig. 7), and prior mapping (Hamilton, 1984b) has shown that standing water in the Noatak basin had negligible e!ect on the Douglas Creek moraine and its proximal outwash. However, the glaciers that formed the Anisak and Douglas Creek glaciers experienced di!erent histories of ice wastage. The Douglas Creek glacier stagnated while it still occupied its terminal zone, and no younger moraines have been identi"ed farther up the Noatak valley (Hamilton, 1984b; Hamilton et al., 1987). In contrast, #uctuating ice-front retreat from the Anisak moraine was marked by prominent recessional moraines. The next-older events in the western Noatak basin were glacial advances to the closely spaced Okak and

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T.D. Hamilton / Quaternary Science Reviews 20 (2001) 371}391

Makpik ice-front positions. These events occurred after the maximum of the last interglaciation (marine isotope substage 5e) according to stratigraphic and paleoecologic data from Nk-26 and Nk-29A (see also Elias et al., 1999). They were followed by deposition of interstadial #oodplain sediments with radiocarbon ages of about 35}37 ka. A similar age bracket has been inferred for the double moraine belt centered on Atongarak Creek, which was correlated with Itkillik IA and IB advances elsewhere in the central Brooks Range (Hamilton, 1984b, 1986, 1994). Alluvial and outwash deposits "ll a broad topographic depression that separates the older and younger components of the Atongarak drift sequence; however, neither those deposits nor those between the Atongarak complex and the Douglas Creek moraines are graded to modern river level, contain abundant organic sediments, or contain wood from trees or large shrubs. This suggests that all three advances were separated by signi"cant nonglacial intervals, but that neither of those intervals were of interglacial character. However, in opposition to these arguments, the front of the outermost Atongarak Creek moraine shows evidence for wave erosion to altitudes as high as 520 m, indicating that glacial lake Noatak may have risen to the Howard Pass outlet level at that time. This outlet level may have been last occupied during the Cutler glacial advance, which therefore could be correlative with the Itkillik IA glaciation. The inner Atongarak Creek moraine is truncated close to the valley center at altitudes of about 425 m, where the ice tongue may have encountered standing water. A narrow glacial lake that extended upvalley to intersect this glacier terminus could correlate with the Hunt River water plane that may have been associated with the Okak ice tongue. Although the Cutler glacier formed a #oating ice tongue in the basin center, much of its southern lateral moraine was grounded on bedrock-cored uplands (see Fig. 4). The moraine is little dissected, and therefore must be signi"cantly younger than the heavily eroded drift of Anaktuvuk River (early Pleistocene) age in adjoining parts of the central Brooks Range. The depositional complex that "lled the #oor of the western basin after it was abandoned by the Cutler ice tongue includes alluvial deposits that formed near the isotope stage 5/6 transition and a heavily oxidized #oodplain complex that probably formed during a preceding interglaciation. The Cutler moraine therefore formed sometime during the middle Pleistocene, possibly during isotope stage 8. The Cutler advance may have been synchronous with formation of the outer Atongarak Creek moraine, as suggested by an apparently contemporaneous water plane, but this line of evidence is suspect and other data suggest that correlation between the Okak-Makpik and Atongarak moraine sequences may be more likely. If that were the case, then the Cutler moraine must be older than the Atongarak deposits. On the other hand, correlation of the Cutler moraine with next-older, deeply buried

Aniuk moraine also is unlikely. Exposures of till and glaciolacustrine sediments correlated with the Aniuk advance are capped by #uvial deposits that rise to heights as great as 75 m above the Noatak River and 50 m above the Aniuk (T.D. Hamilton, and G.M. Ashley, unpublished "eld data). Because there is no evidence for the Noatak River #owing more than about 25 m above present-day level during or after the Cutler advance, the Cutler glaciation must postdate the Aniuk.

8. Conclusions Surface morphology and blu! exposures record four major ice advances from the DeLong Mountains into the western Noatak basin. Each advance was associated with glaciolacustrine sedimentation at and beyond the ice front, and commonly with alluviation of sandy deltaic and marsh deposits farther up the Noatak River and its tributaries. Deposition of high-level channel and #oodplain deposits took place during interstadials and early phases of major interglaciations; downcutting to positions at or below modern river level occurred during interglacial maxima. The Cutler glaciation, of middle Pleistocene age, was marked by a #oating ice tongue that extended southeast into the Noatak basin and by an extensive proglacial lake that "lled unglaciated parts of the basin. The Cutler advance is clearly younger than deeply buried glacial deposits and overlying coarse alluvium in the eastern sector of the basin. It may correlate with the outer Atongarak moraine farther upvalley, but other evidence indicates that it may be older than that moraine. If that were the case, then the Cutler advance may have been out of phase with glaciation in the central Brooks Range. The Okak and Makpik advances reached closely spaced terminal positions near Okak Bend. Drift of the Okak advance is poorly represented in the valley center, suggesting that the Okak glacier may have terminated in a proglacial lake that was deeper and more extensive than any lake associated with the Makpik advance. Downcutting of outlets may have prevented deep-lake formation during Makpik time. The Okak and Makpik advances are younger than the deposition of the 140,000 yr old Old Crow tephra, but both advances predate a middle Wisconsin nonglacial interval that took place about 35}30 ka in the Noatak basin and elsewhere in the western Brooks Range (Hamilton and Ashley, 1993). The two moraines of the Atongarak drift complex in the eastern basin seem to have formed during the same interval, but the earlier of the two Atongarak advances could alternatively be older than the Okak-Makpik complex. The Anisak glaciation, of LGM age, is clearly correlative with the Douglas Creek advance and with the Itkillik II glacial phase of the central Brooks Range. However, #uctuating retreat of the Anisak glacier contrasts with

T.D. Hamilton / Quaternary Science Reviews 20 (2001) 371}391

the general stagnation and downwastage of the glacier that built the Douglas Creek moraine. Note added in proof During "eld work in July 2000, a deposit of presumed Old Crow tephra was found near the base of the #oodplain deposits with ice-wedge casts that cap the oxidized gravel unit at Nk-29A. If this deposit proves to be the old Crow tephra, it would con"rm the correlations beetween exposures Nk-26 and Nk-29A suggested in this paper.

Acknowledgements My geologic studies in the Noatak basin have been carried out intermittently since 1979, and consequently have involved several organizations and many individuals. Field work initially was supported by the US Geological Survey's Arctic Environmental Studies and Climate History programs, and later was funded under National Science Foundation Grant OPP-9424279. Logistical support also has been provided by the US National Park Service through Robert Gal. Field associates and assistants have included Gail M. Ashley, Kim Ashley, Daniel P. Bauer, Scott A. Elias, Mary E. Edwards, Andrea P. Krumhardt, George A. Lancaster, Charles Moore, and Douglas P. Van Etten. Maps and stratigraphic sections used in this report were prepared by Charles Moore, Bo Wilmer, and Linda-Lee Harris. I am grateful also to Alejandra Duk-Rodkin and an anonymous reviewer, whose thoughtful and constructive comments greatly improved an earlier draft of this paper.

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