1

ABSTRACT

The surface morphology of the northern slope of the

Brooks Range (North Slope) from the Canning River, Alaska,

eastward is dominated by a series of large alluvial fans and

braided streams floored by coarse alluvium. On the basis of

the absence of sediment on the seasonal sea ice after spring

flooding, the measured stability of fan fronts during a 30-

year period, and the sparsity or absence of Holocene marine

sedimentary deposits seaward of the fans, we conclude that

the fans are not prograding now nor have they been

prograding at any time during the Holocene. In numerous

areas, surficial fan alluvium terminates on land at or below

2-m-thick glaciomarine deposits. These deposits, the

Flaxman Member of the Gubik Formation, formed during

the latest major interglacial transgression (oxygen-isotope

stage 5a), about 85–80 ka. We therefore believe that the fans

are still older. Our observations suggest that during the latest transgression and the following sea-level highstand, the

North Slope depositional environment and climate differed

greatly from the present ones.

INTRODUCTION

The surface morphology of the northern slope of

Alaska’s Brooks Range, called the North Slope, is dominated

by a series of large alluvial fans and braided streams containing coarse alluvium east of the Canning River (fig. 1).

We present evidence that these fans are not prograding today, probably were not fully active during Holocene time,

and may have acquired their regional, overall characteristics

before 80 ka, during the latest interglacial transgression. They

record the latest major sediment influx from the Brooks

Range northward to the Beaufort Sea and Amerasian Basin.

Knowing the timing of and conditions for fan construction

would be valuable for understanding both the major sedimentation pulses recorded in cores of the Canada Basin, and

the climatic conditions that triggered these pulses, as well as

possibly for predicting future changes in the sedimentary

regime. Understanding the modern hydrologic and depositional conditions of the fans would also be beneficial for any

future petroleum development and gravel mining in the Arctic National Wildlife Refuge.

Acknowledgments.—We thank Gita Dunhill and Susan

Vath for drafting the illustrations, and Ken Bird, Dave

Hopkins, John Andrews, and Kelin Whipple for reviewing

the manuscript.

BACKGROUND INFORMATION

The North Slope has a desert climate, with low precipitation (avg 16 cm/yr at Barter Island), which falls mainly in

the form of snow (Brewer, 1987). The snow melts during a

10- to 14-day period from late May to early June (Brewer,

1987), thus releasing most of the annual precipitation in a

short burst. Much of the meltwater is retained temporarily

on land until river breakup around June 1, after which peak

discharge lasts only 2 weeks. The largest North Slope river,

the Colville River, supplies more than 60 percent of its annual suspended sediment load to the sea during this 2-week

period, beginning with river breakup (Arnborg and others,

1967), when rivers flood the fast ice, which is relatively

smooth seasonal sea ice, as far as 10 km or more from their

mouths (Reimnitz and Bruder, 1972). Surface sediment on

the North Slope, which at that time is still frozen, begins to

thaw but again refreezes in late September. In 15 years of

records at Barter Island (fig. 2), daily precipitation exceeded

1.3 cm only three times, commonly as a gentle drizzle during the summer (Brewer, 1987). Thus, floods at breakup are

the major yearly sedimentation events. North Slope denudation rates are about 10 t/km2

 per year west of the study area

(Reimnitz and others, 1988), increasing eastward to 42 t/

km2 per year in the Babbage River just east of Herschel Island (fig. 2; Forbes, 1981). Such denudation rates are among

the lowest on the globe (Milliman and Meade, 1983).

Along the wide west half of the North Slope coastal

plain, average coastal retreat of 2.5 m/yr is contributing at

least 7 times more sediment to the sea than rivers supply

from upland sources (Reimnitz and others, 1988). In the study

area, characterized by bluff-retreat rates of ~0.6 m/yr (Barnes

and others, 1992) and probably higher denudation rates, the

ratio might be somewhat lower. Today’s general coastal reAre North Slope Surface Alluvial Fans

Pre-Holocene Relicts?

By Erk Reimnitz and Stephen C. Wolf

1
2 ARE NORTH SLOPE SURFACE ALLUVIAL FANS PRE–HOLOCENE RELICTS?

cession is accompanied by shoreface and shelf erosion seaward to at least 30-m water depth, as shown by data from

numerous boreholes correlated with shallow seismic stratigraphy (Wolf and others, 1985, 1987), as well as other observations.

Drainage patterns and topographic contours on maps,

as well as Landsat images, reveal a series of large, fanlike

features extending from the Brooks Range northward toward

the Beaufort Sea coast. A summer Landsat image of two of

these fans just west of the United States-Canadian border is

0 5 10 KILOMETERS

Pack Ice

Naled

Figure 1. Landsat image of two North Slope fans and braided

streams near the United States-Canadian border. The Kongakut Fan

is characterized by a perennial naled (river icing) and is protected

from pack-ice erosion by Icy Reef, a 26-km-long barrier island.

Configuration of <1-m-deep, mud-floored lagoon and reef is remarkably stable over a 30-year period. Dotted line shows landward

limit of overlying deposits of the Flaxman Member of the Gubik

Formation.
Figure 2. Orthographic map of study area in northern Alaska, showing fans schematically, locations of geographic features referred to in text, and areas o

and Gerin, 1980).

Mount Sedawick North Slope

Mount Michelson 

Brooks Range

Mount Weller

Herschel

Island

Malcolm River

Clarence River Brownlow Po

Fig. 5 Demarcation Bay

Canada Alaska

Fig. 1

Barter Island Well

Camden Bay

Fig. 4

Area of

figure 2 AK diagram 

70°

Malcolm River

BEAUFORT SEA
4 ARE NORTH SLOPE SURFACE ALLUVIAL FANS PRE–HOLOCENE RELICTS?

shown in figure 1, and the major fans on the North Slope are

shown schematically in figure 2. Gradients of fan surfaces

increase from about 3 m/km on the Canning Fan in the west

to about 40 m/km on the two eastern fans, where the foothills of the Brooks Range converge with the coast. These

fans consist largely of fluvial gravel to sandy gravel, decreasing in clast size toward the coast, and are locally overlain by

1 to 2 m of eolian silt to fine sand and peat (Carter and others, 1986). Other drainage systems between the Canning and

Kongakut Rivers are braided (fig. 2) and are also marked by

coarse clast sizes (Carter and others, 1986).

Where the fans meet the Beaufort Sea, the terminus

generally is a low, crenulated scarp, suggesting modern erosion (fig. 3). This scarp is most striking along the Canning

Fan (fig. 4), which in the subsurface extends offshore as a

mainly gravelly unit, approximately 100 m thick (Lindsay,

1992). A several-meter-thick surficial unit of glaciomarine

deposits of oxygen-isotope stage 5a (Carter and Whelan,

1991) constrains the age of the alluvial fans, as shown below. This surficial unit is exposed near sea level in numerous places along the coast (figs. 1, 4; Carter and others, 1988).

The unit contains pebbles, cobbles, and boulders of rock types

foreign to the Brooks Range that were ice rafted to the area

from the Amundsen Gulf region, Canada. The Canning River

today is deflected eastward into Camden Bay by relatively

high standing (>4 m high) exposures of the Flaxman Member of the Gubik Formation (fig. 4), where it is constructing

a small delta into the open sea (Wolf and others, 1987). In

contrast, the crenulated shoreline of the Kongakut Fan (fig.

5) is bordered by a 500-m-wide, less than 1-m-deep muddy

lagoon and an unbroken, 26-km-long, arcuate, 4-m-high

gravel barrier (Barnes and Reimnitz, 1988). The Clarence

Fan (fig. 1) largely terminates on the coastal plain even before reaching the sea.

EVIDENCE FOR MODERN INACTIVITY

RELATIVE TO THE PERIOD OF FAN

CONSTRUCTION

Evidence that the alluvial fans may be relict features is

based mainly on four factors: (1) the absence of sediment on

fast ice fringing the fans shortly after spring river breakup,

(2) the stability or retreat of fan shores during the past 30

Figure 3. Oblique aerial photograph of evidently eroding coast of the Canning Fan directly south of Flaxman Island (see fig. 4).
5

years, (3) the absence of significant Holocene marine deposits in the shallow coastal zone bordering the fans, and (4)

the stratigraphic relation of fans and subsurface fan deposits

to dated deposits of the Flaxman Member. These four factors show that our view of the modern fan setting is based

mainly on knowledge gained in marine geologic studies, and

on satellite images: we made no direct studies of fan-surface

morphology, stream terraces, and alluvial deposits.

AERIAL OBSERVATIONS OF SEASONAL FAST

ICE AFTER SPRING FLOODING

During an investigation of fast-ice flooding by North

Slope rivers, a coastal reconnaissance and photographic mission was made, covering most of the North Slope coast, notably where the large fans terminate (Reimnitz and Bruder,

1972). Several-kilometer-diameter patches of open water

already had formed off the major river mouths by June 21,

1970, similar to, but smaller than, the areas of open water

shown in figure 5. Regionally, however, the smooth and clean

fast-ice cover remained intact against the coast, revealing in

some places areas that previously had been flooded and then

drained.

The absence of sediment on the fast ice bordering the

fans at a time when the bulk of the annual river supply had

just been flushed to sea indicates that the fans were not

prograding that year. Satellite images and aerial photographs

available for subsequent years (fig. 5) show that the 1970

breakup was normal.

Figure 4. Landsat image of the Canning Fan, showing crenulated, bluffed, eroding shoreline. Extensive outcrops of 85–80-ka glaciomarine

sedimentary deposits (Flaxman Member of the Gubik Formation) are delineated. Main river distributary presently is deflected at nearly

right angle eastward by the Flaxman Member. Squares, boreholes.

AERIAL OBSERVATIONS OF SEASONAL FAST ICE AFTER SPRING FLOODING

Flaxman Island Tigvariak Island

0 10 20 KILOMETERS

Canning River
6 ARE NORTH SLOPE SURFACE ALLUVIAL FANS PRE–HOLOCENE RELICTS?

Figure 5. Presently active mouth of the Kongakut River, which has built a large fan, partly visible on right. During spring flood 4

weeks earlier, the river probably discharged onto ice through tidal inlets, but no surface sediment is visible on its remnants or on

drifting floes. From 1951 to 1981, fan and delta on left locally showed minor deposition and elsewhere erosion (Barnes and others,

1992), whereas barrier islands had minor net land loss. Aerial photograph by U.S. National Aeronautic and Space Administration,

Johnson Space Center, taken July 8, 1978.

Icy Reef

Kongakut River

Naled

0 1 2 KILOMETERS

N
7

STABILITY OR RETREAT OF THE COASTLINE

From the United States-Canadian border westward, the

bluffed fan shores shown in figure 3 have been either stable

or retreating during a 30-year period (Barnes and others,

1992). Although small deltas have accreted off river mouths

during that period, these fine-grained deposits, reaching to

less than 25 cm above sea level, can be ignored when considering modern fan accretion. On the Canadian side of the

border, fan shores also are stable (Harper, 1990). Along the

entire coast from the Kongakut River eastward (fig. 2), fans

typically are separated from the ocean by rather stable gravelly beaches or barrier islands backed by narrow, shallow

lagoons (Barnes and Reimnitz, 1988; Reimnitz and Barnes,

1988; Harper, 1990; Barnes and others, 1992). These stable

landforms are not nourished by sediment from rivers (Harper,

1990; Barnes and others, 1992) but from offshore by ice push

(Reimnitz and others, 1990). Where the crenulated outline

of the Canning Fan meets the lagoon (figs. 3, 4), it is erosional, with 1- to 2-m-high bluffs. The setting depicted in

figure 3, a low-level aerial photograph, is typical for the outline of the fan—its sloping surface nowhere grades into the

sea. An analysis of coastal landforms and deposits off the

fans therefore clearly indicates that they are not accreting

today.

SPARSITY OF HOLOCENE MARINE DEPOSITS

Interpretation of high-resolution seismic profiles in the

study area shows that the shelf at present is largely an erosional surface (Wolf and others, 1985, 1987; Reimnitz and

Barnes, 1987). The shelf truncates two units correlated with

the latest two interglacial intervals (Wolf and others, 1987).

Lithologic studies of pebbles on the island chain extending

westward from the Canning River and fan show only a weak

influence of Brooks Range sources (Reimnitz and others,

1990). Wolf and others (1987) constructed an isopach map

of a small Holocene delta off the mouth of the Canning River

in Camden Bay (fig. 2), and of shallow pockets of Holocene

sedimentary deposits in lagoons and bays. The sheltered confines of Demarcation Bay (fig. 1) between two conspicuous

alluvial fans hold only a small pocket containing no more

than 4 m of fine-grained Holocene sediment (fig. 6) lacking

evidence for internal deltaic structures. The fine-grained

Holocene deposits observed in small pockets seaward of the

fans, and the absence of modern gravel supply, also indicate

that the fans are not prograding today.

RELATION OF THE FANS

TO GLACIOMARINE DEPOSITS

Numerous exposures of glaciomarine deposits of the

Flaxman Member of the Gubik Formation, commonly blanketed by a several-meter-thick veneer of eolian sand and silt,

were mapped by Carter and others (1986, 1988), particularly along the fringes of the Canning Fan (fig. 4), as well as

on other fans reaching eastward as far as the United StatesCanadian border (fig. 2). The exact relation of the fans to

deposits of the Flaxman Member and the nature of their contact is unknown because of a thin surface blanket of loess

(L.D. Carter and J.P. Galloway, oral communs., 1995); however, analysis of figure 1 suggests that major surface-modifying processes on the Clarence Fan ended before deposition of the Flaxman Member. Furthermore, computer modeling by Lindsay (1992) of Canning Fan sediment penetrated

in 1,224 boreholes, reaching as deep as 104 m, indicate that

the Flaxman Member, as described earlier (fig. 4), overlies a

series of offshore-dipping gravelly strata extending seaward

as far as the island chain (fig. 2). Sediment of the Flaxman

Member was deposited during the latest sea-level highstand

(Carter and others, 1988, 1991), about 85–80 ka, suggesting

that main fan construction ended before that time.

DISCUSSION AND CONCLUSIONS

Our hypothesis that since before Flaxman time North

Slope drainage systems have provided insufficient sediment

for further fan growth is controversial but consistent with

the available data. It requires that the next-to-latest

Figure 6. Geophysical tracklines in Demarcation Bay (fig. 1). Contours show thickness of Holocene(?) marine sediment (in meters)

(Wolf and others, 1987). Absence of foreset beds in seismic records

adjacent to fans shows nonoccurrence of fan progradation during

the Holocene in this sheltered embayment.

141°15'

69°

15'

Barrier

 Island

Sand

spit

0

0 3 KILOMETERS

3 MILES 

0

0

1

2

0 1

1

2

3

3

3

4

4

0

2

4

2

DISCUSSION AND CONCLUSIONS
8 ARE NORTH SLOPE SURFACE ALLUVIAL FANS PRE–HOLOCENE RELICTS?

deglaciation produced much more coarse alluvium than did

the latest one. That previous period differed in other respects

as well. Thus, Arctic coastal waters reached a higher level

and were warmer and more turbid, and the winter sea-ice

limit was 800 km farther north, than today (Carter and others, 1988; Brigham-Grette and others, 1994; Carter and

Whelan, 1991, 1994). The interpretation that muddy deposits of the Flaxman Member accumulated at depths less than

20 m (Carter and others, 1988), an environment marked by

erosion today (Barnes and others, 1987; Reimnitz and

Barnes, 1987; Reimnitz and others, 1988), is convincing

evidence that conditions indeed differed drastically. Further

evidence for a drastically different depositional environment

during that period and even the previous interglacial stage

is visible in two inner-shelf marine units identified in numerous boreholes and traced by seismic-reflection profiles

over wide areas (Wolf and others, 1985, 1987). Here, as

well, the region marked by deposition during the latest two

interglacial stages is presently eroding.

We speculate that the coarse glacial-outwash fans were

most active during rapid glacial retreat coupled with high

meltwater discharge. For understanding land-sea interaction

during the latest glacial-interglacial cycles, knowledge that

fan deposition extended several times well beyond the

present shoreline is important. A pre-Flaxman fan surface

had been traced by way of seismic reflectors offshore to a

125,000-yr-old erosional unconformity penetrated by five

boreholes shown in figure 4 (Smith, 1985; Wolf and others,

1987, fig. 6.19). This position suggests that the fan formed

before 125 ka, and the absence of coarse alluvium above

the unconformity (Smith, 1985) indicates that fan deposition had ended in this region. A drill hole near the 20-m

isobath east of Barter Island (fig. 2) penetrated 30 m of

gravel, indicating that in this area coarse alluvium spilled

seaward as far as the midshelf.

During peak times for fan construction, streams of turbid meltwater flowing across the partly exposed shelf should

have caused increased sedimentation rates and, probably, a

coarsening of the sedimentary record in the Canada Basin

to the north. Such coarsening is evident during the early

part of the latest interglacial stage in cores from Northwind

Ridge (R.L. Phillips, oral commun., 1995).

In summary, the presence of large alluvial fans of northern Alaska, never acknowledged in the literature as not

prograding today, raises significant unanswered questions

about the North Slope environment. Learning about the period of fan construction and environmental conditions during that time would be valuable for understanding late Pleistocene global change and, particularly, for understanding

differences between the latest several glacial/interglacial

transitions.

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