Elsevier Publishing Company, Amsterdam

Marine Geology - ElsevierPublishingCompany,Amsterdam- Printed in The Netherlands
GEOLOGIC HISTORY OF THE CONTINENTAL MARGIN OF NORTH
AMERICA IN THE BERING SEA
DAVIDW. SCHOLL~EDWINC. BUFFINGTONAND DAVIDM. HOPKINS
Office of Marine Geology and Hydrology, U.S. Geological Survey, Menlo Park, Calif. (U.S.A.)
Marine Environment Division, U.S.N.U. W.C., San Diego, Calif. (U.S.A.)
Office of Marine Geology and Hydrology, U.S. Geological Survey, Menlo Park, Calif. (U.S.A.)
(Received September 18, 1967)
(Resubmitted November7, 1967)
SUMMARY
The North American continental margin beneath the Bering Sea is nearly
1,300 km long and extends from Alaska to eastern Siberia. The margin is a canyonscarred 3,200-3,400-m high escarpment separating one of the world's largest epicontinental seas (the shallow Bering Sea) and the Aleutian Basin (the deep-water Bering
Sea), a marginal oceanic basin distinguished by having its southern boundary formed
by the Aleutian Ridge.
Three geomorphic provinces can be recognized: a southeastern province
characterized by a gentle continental slope (lacking V-shaped canyons) and an
outlying continental borderland (formed by Umnak Plateau); a central province
distinguished by a steep canyon-scarred slope, and a northwestern province having
a gentler and, apparently, less eroded continental slope.
Continuous seismic reflection profiles show that the margin is constructed
of three major structural-stratigraphic units: (1) an acoustic basement underlying
the outer shelf and upper slope; (2) an overlying main layered sequence; and (3) a
stratified rise unit underlying and forming the continental rise at the base of the slope.
The existing margin evolved with downbowing and faulting of the acoustic
basement, an older margin probably of Late Mesozoic age, consisting in part of
well-indurated siltstone and mudstone, in Early Tertiary time. Concomitant with
subsidence as much as 1,500 m of main-layered-sequence strata were draped over
the basement. Intense canyon cutting, presumed to have been caused by the rapid
deposition of unstable masses of riverborn sediment over the outer shelf and upper
slope, is thought to have begun in Late Tertiary and Quaternary time. Concurrent
with canyon cutting, submarine fans, consisting of turbidites forming the rise unit,
accrued at the base of the continental slope.
Subsidence of the continental margin during the Tertiary may be related to
foundering ("oceanization") of a continental block to form the Aleutian Basin, or
Marine Geol., 6 (1968) 297-330
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pp.299-302
THE AMERICANCONTINENTALMARGININ THE BERINGSEA
303
to simple isostatic depression of a former segment of the North Pacific oceanic
floor in response to sediment infilling north of the Aleutian Ridge.
INTRODUCTION
The continental margin of North America in the Bering Sea stretches nearly
1,300 km northwestward from the tip of the Alaska Peninsula to Cape Navarin, a
promontory of coastal Siberia (Fig.l).
During the early summer of 1965, and for a short period in July 1967, the
authors conducted a reconnaissance continuous seismic reflection profiling investigation along 2,500 km of track over the northern continental margin of the Bering
Sea (Fig.l). The results of this investigation have led to a firmer understanding of
the geologic history of the Bering Sea and also have afforded an insight into the Late
Mesozoic and Tertiary history of the adjacent continental areas of Alaska and Siberia.
Measurements
The bulk of the seismic reflection work was done from the USNS "Charles
H. Davis" (AGOR-5). A high-energy spark, periodically generated across an array
of submerged electrodes, provided the acoustic source. A maximum of 38,000 Joules
of stored energy was discharged every 4 sec. Samples of bottom sediment or rock were
collected by gravity coring and dredging; deep-sea photographs were also taken.
Additional samples and seismic reflection profiles were obtained in July, 1967, during
a cruise of the R/V "Thomas G. Thompson", University of Washington. The track
of the "Davis" and the locations of all sampling and camera stations are shown
on Fig. 1, 4, and 11.
Throughout this paper sediment-section thickness is given in meters. Thicknesses were computed by using the average sediment sound velocities determined by
SHOR (1964) from refraction studies in the Bering Sea, or by considering values of
acoustic velocity measured in sediment cores from the Bering Sea in conjunction
with reasonable assumptions for the vertical acoustic velocity gradient (HouTz and
EWING, 1963).
BATHYMETRY
A generalized bathymetric chart of the Aleutian Basin and the continental
margin beneath the Bering Sea is presented in Fig.2. This chart has a contour
interval of 400 m and is based on data assembled from published Russian charts
(GERSHANOVICH, 1962b; UDINTSEVet al., 1964; LISITZIN, 1966) and from both unpublished and published American sources (U.S. Coast and Geodetic Survey, U.S. Navy
Oceanographic Office, U.S. Navy Electronics Laboratory, and U.S. Navy Ordnance
Marine Geol., 6 (1968) 297-330
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Test Station). Isobaths drawn on, and adjacent to, the Aleutian Ridge werc extr~polated from the excellent charts of this region prepared by NICHOLSand PI,:RRV( 1966!.
Continental marght
The Bering Sea continental margin is a rugged, 3,200-3,400 m high canyonscarred declivity. Topographically it separates two exceptionally fiat areas of the
earth's surface, the Bering Sea shelf and the abyssal floor of the Aleutian Basin
(Fig.2). The margin comprises three morphologic elements: (1) the flat outer Bering
shelf; (2) the steep Bering continental slope; (3) the deeper and more gently seaward
sloping continental rise. KOTr~NEV(1965) has invoked a similar division of' the margin:
upper, central, and lower. The steepness of the slope averages about 5 °, which approximates the world-wide average for continental slopes (SHE~'ARO, 1963, p.298). In
some areas the slope averages between 10° and 15° for the first 2,000 m below the
shelf-break. The continental rise, which aprons the base of the slope, has a steeper
upper segment in some areas. Relief associated with leveed channels interrupts
the seaward slope of the rise. The lower rise is characteristically smooth and its seaward slope is O' 01' (1:350) or less. Intersection of the rise with the abyssal plain
of the central Aleutian Basin is generally abrupt at a depth close to 3,600 m; slopes
on the plain are less than 1 : 1000. This broad, flat expanse of sea floor is herein
named the Bering abyssal plain.
In accordance with general geomorphic differences, the continental margin
under the Bering Sea can be divided into three bathymetric provinces, southeastern,
central, and northwestern. These provinces are briefly described below.
Southeastern pro vince
The southeastern province extends from Unimak Island (Alaska Peninsula)
to Pribilof Canyon (Fig.2). This province is characterized by a gradual shelf-slope
transition at depths between 150-190 m, a smooth, gentle (1-2 °) slope profile, and
an outlying continental borderland herein named Umnak Plateau. This marginal
plateau occupies the triangle formed by the intersection of the Bering Sea continental
slope and the Aleutian Ridge; its surface lies at a depth between 1,800 and 1,900 m
(Fig.2).
Central province
The central bathymetric province of the Bering Slope is approx. 720 km in length
and occupies the region between Pribilof and Pervenets Canyons (Fig,2). In contrast
to the southeastern province, the slope of the central province is steep (5-6°), rugged,
and deeply scarred by canyons (Fig.2, 4, and 11). The top of the slope has an abrupt
junction with the outer edge of the shelf at a characteristic depth of 170 m, which
is deeper than both SHE]?ARD'S (1963) world-wide average of 132m and
Marine GeoL, 6
(1968)
297-330
THE AMERICANCONTINENTALMARGININ THE BERINGSEA
305
GERSHANOVICH'S (1962b) average shelf-break depth of 150 m for the Bering Sea in
general.
Most of the canyons start just below the shelf break. Their axes commonly
trend directly down-slope; however, in some areas, the trend is oblique or even
parallel, to the regional strike of the slope. The canyons merge with the continental
rise at depths between 3,200 and 3,300 m. Two canyons, Pribilof and Zhemchug
(KOTENEV, 1965), are exceptionally large; Bering Canyon may be the longest slope
canyon in the world (SHEVARD and DrLL, 1966, p.198), and Zhemchug may well be
the world's largest.
North western pro vinee
The northwestern bathymetric province lies south of Cape Navarin and includes
the region of Navarin submarine canyon (Fig.2). The configuration of this segment
of the margin is poorly known (GERSHANOVrCH,1963; UDINTSEVet al., 1964; KOTENEV,
1965; LISITZIN, 1966, fig.7). The continental slope here is apparently not as steep,
2-2.5 °, nor as rugged as the central sector. A sharply-defined shelf break is also absent.
Steep-walled submarine canyons appear to be few in number, but this may simply
be a consequence of sparse bathymetric information. A rather large, ill-defined
canyon, Navarin Canyon, marks the juncture of the Bering Sea continental slope
with the southwest-trending continental slope seaward of the Koryak Mountains
(KoTENEV, 1965).
SEISMICREFLECTIONPROFILES
General results
Acoustic reflection records indicate that the upper 1-1.5 km of the continental
margin in the Bering Sea comprises two principal structural elements: (1) an acoustic
basement, composed of lithified rock, and (2) a 1-1.5 km thick overlying stratified
section of semi-consolidated sedimentary rock and unconsolidated sediments. The
stratified section can be subdivided into three units: a main layered sequence, a rise
unit and a surface-mantling unit (Fig.3).
Acoustic basement
The acoustic basement was detected only beneath the outer shelf and middle
and upper reaches of the continental slope (Fig.8, 9, 10, 12 and 13). Near the shelf
edge the basement lies beneath as much as 1,500 m of the main layered sequence;
the surface of the basement crops out intermittently along the axis of canyons dissecting the central bathymetric province (ScHOLLet al., 1966). SHOR's (1964) seismic
Marine Geol., 6 (1968) 297-330
306
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refraction data indicated that the basement has an acoustic velocity ranging l¥~,m
3.2-3.6 km/sec {Fig. 14).
Beneath the outer shell" and upper slope (down to a below-sea-level depfl~ o~
1,000-1,500 m) the surface of the acoustic basement is locally irregular. Although
this relief is in part attributable to faulting, regional considerations, whicla are discussed below, stress that the top of the basement may be an erosional unconformilv
cut across rocks of Mesozoic age. In contrast, the surface of the acoustic basemcm
exhibits little relief beneath the lower part of the slope and the rise. Below the base
of the continental margin the surface of the acoustic basement presumably represents
little geologic time and simply separates sedimentary units of contrasting litholog).
Judged from SHOk'S (1964) seismic refraction evidence, a surface that is probably
equivalent to the acoustic basement of the continental margin passes beneath the
rise as the upper surface of a rock or sedimentary unit having a seismic velocity of
3.2-4.0 km/sec (Fig. 14).
In an earlier paper we (ScuoLL et al., 1966) speculated that the acoustic basement beneath the outer shelf of the central bathymetric province was an erosional
surface cut across volcanic and lithified rocks of Mesozoic or older age. Contributing to this judgment were considerations of the regional geology of southwestern
Alaska (Duvko and PAVNE, 1956; GATES and Gkvc, 1963; BURK, 1965) and the
results of the seismic refraction studies by Srto~ (1964) over the southeastern area of
the Bering Sea. More recently the areally extensive unconformity separating rocks
of Mesozoic (and older) and Tertiary age along the west coast of Alaska was traced
from the mainland (via seismic reflection profiles) toward the continental margin
of the Bering Sea, where it appears to crop out as the surface of the acoustic basement.
A sample of the basement was also dredged from the inner gorge of Pribilof Canyon
at a depth of 1,600 m (Fig.4, station TT-1) during a recent (September, t967) cruise
of the R/V "Thomas G. Thompson" (University of Washington). The dredge-"haul"
consisted of angular pebbles, cobbles, and boulders of well-indurated siltstone and
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J~rarine Geol., 6 (1968) 297-330
307
THE AMERICAN CONTINENTAL MARGIN 1N THE BERING SEA
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mudstone, a lithologic assemblage common to the Cretaceous exposed over western
and southern Alaska 1.
Farther to the west the probable nature of the acoustic basement can be inferred
from the regional geology of the Koryak Mountains of eastern Siberia (Fig.l, 13).
The Koryak Mountains comprise broadly-folded neritic, volcanic, and continental
deposits of Cenozoic age unconformably overlying a basement of intricately-folded
and locally-metamorphosed Late Mesozoic rocks. The latter are composed of altered
alkalic basalts and volcanic, siliceous, and terrigenous flysch deposits of Early Cretaceous age, as well as younger neritic and coal-bearing deposits of Campanian to
Danian age (NALrVKIN, 1960; BLAKE and HOVKINS, 1962; BURK, 1965, pp.154-155;
CHIKOV, 1965; NAGOLSK1Y, 1965; ROTMAN and MARKOVSK1Y, 1965; GNIBIDENKO
and ROZENBLYUM, 1966; KOSTYLEVand BURLIN, 1966; LTStTZIN, 1966, p.59). Accordingly, south of Cape Navarin the acoustic basement may either represent: (1)
lithified Mesozoic deposits underlying relatively unlithified neritic and hemipelagic
deposits of Cenozoic age; or (2) the top of a section of relative dense rocks, i.e.,
1 A Late Cretaceous ( C a m p a n i a n ) foraminiferal f a u n a h a s been f o u n d in the Pribilof samples.
Marine Geol., 6 (1968) 297-330
308
IX W. S C t t O L L , E. C. B U F F I N G T O N A N D 1~. M. HOPKINS-
volcanic flows, within a thicker sequence of Cenozoic deposits. The regional continuity of the basement (profile 14:- W' X. Fig.13) suggests that it represents a wide-.
spread lithologic boundary of the type generally associated with a major regiona!
unconformity; thus, the first possibility is considered most likely. The structure oi
the Anadyr Depression, which extends beneath the Gulf of Anadyr irnmediateh.
north of Cape Navarin (Fig.l), supports this interpretation. The depression is
underlain by as much as 3,000 m of neritic deposits of Cenozoic age overlying a
basement of folded and volcanic-rich Mesozoic rocks (KoSTYLEVand BuRr.iN, 1966).
Main layered sequence
The main layered sequence forms the bulk of the stratified section underlying
the continental margin. The sequence is as much as 1,500 m thick; it festoons the
continental margin from the outer shelf to the base of the slope, where the sequence
passes beneath the rise unit. In most regions the main layered sequence is characterized by strong, coherently reflecting horizons that can be traced laterally for many
kilometers. The typical seismic velocity of the main layered sequence is 1.7 km/sec
(SHoR, 1964).
Over the southeastern bathymetric province the main layered sequence has had
a complicated depositional history and consists of two units, an upper succession
of internally reflective beds, and a lower sequence of more weakly reflecting strata
(Fig.6, 7 and 8). Except beneath the sloping flanks of Bering and Bristol Canyons,
the two units are generally conformable.
Strata of the main layered sequence also underlie the outlying Umnak Plateau.
The broad, nearly flat summit of the plateau is the result of sedimentary processes
as it is underlain by at least 1,400 m of horizontal to gently westward-dipping strata
(Fig.7). These beds can be projected eastward across Bering Canyon to the gentlydipping strata forming the base of the Bering slope. To the west, these same beds are
depositionally draped over the seaward or marginal slope of the plateau.
Along the central province of the continental margin much of the main layered
sequence has been removed by intensive slope erosion, i.e., canyon cutting. Consequently, it is difficult to reconstruct its original depositional geometry (Fig,10 and 12),
although the sequence is clearly a constructional unit deposited layer-by-layer over
an older continental margin consisting of rocks of the acoustic basement.
The outer shelf of the northwestern province (south of Cape Navarin) is
underlain by seaward-overlapping units within the main layered sequence (Fig.13).
Except for minor deformation, the strata of the main sequence dip smoothly down
the continental slope over the surface of the acoustic basement. As is true elsewhere,
the dip of the main sequence flattens at the base of the slope, and the sequence passes
beneath beds of the rise unit.
The geometry and stratigraphic position of the main layered sequence suggest
it is constructed of neritic to hemipelagic deposits of Cenozoic age. This is largely
substantiated by the lithology and age of semi-consolidated rocks dredged from the
Marhte Geol.. 6 (1968"b 297-~V~0
THE AMERICAN CONTINENTAL MARGIN IN THE BERING SEA
309
walls of Pribilof Canyon and the nearby continental slope (Fig.4). These samples
consist of diatomaceous siltstone and tuffaceous sandstone bearing a mid-Tertiary
marine flora (K. E. Lohman, U.S. Geological Survey, personal communication,
1967). The siltstone also contains an abundant and well-preserved pollen and spore
assemblage of middle Tertiary age (J. A. Wolfe, U.S. Geological Survey, personal
communication, 1967). In addition, tuffaceous sandstone bearing a Miocene macrofauna has been dredged from outcrops of the main layered sequence near Zhemchug
Canyon (D. E. GERSHANOVlCH, All-Union Scientific Institute of Fisheries and
Oceanography, Moscow, personal communication, 1966). Although samples of the
basal strata of the sequence have not been collected, we presume the base of the
sequence is of Early Tertiary age.
Rise unit
The rise unit forms the upper 500-800 m of the continental rise. This unit is
best developed near the mouths of large submarine canyons and extends up into the
canyons (Fig.8, 10, 12 and 13). It overlies the main layered sequence on the rise and
abuts the sequence at the base of the slope. Reflecting horizons within the rise unit
are commonly coherent, but in some areas they are discontinuous and can be followed
for only a short distance (1 km or less). The average acoustic velocity in the rise unit
may be as high as 2.1 km/sec (SnOR, 1964).
The rise unit is thought to be composed of turbidite sequences deposited in
conjunction with the formation of submarine fans at the base of the continental
slope (MENARD, 1955; HAMILTON, 1967). This interpretation is based on:
(1) The association of thick sections of rise sediments with the mouths of large
submarine canyons.
(2) The occurrence of leveed channels (Fig.8 and 12) on the surface of the rise
unit and identical structures buried within it.
(3) The generally undulating surface of the rise unit.
(4) The gentle but consistent seaward dip of the unit (except, in the vicinity of
the levees).
(5) The sharp abutment of the rise unit against the base of the continental
slope and more steeply dipping beds of the main layered sequence.
(6) The fact that cores taken on the rise (GERSHANOVICH, 1965; BOYCE, 1967)
penetrate 1-2 m of diatomaceous nonturbidite, silty clays and clayey silts overlying
clayey-to-sandy diatom-poor deposits of Late Pleistocene age. These latter, coarser
beds have many of the characteristic structures of turbidites (GERSHA~OVrCn, 1965).
As will be emphasized in subsequent paragraphs, the rise unit is thought to be
largely of Pleistocene age and to have formed in response to rapid outer-shelf sedimentation and concomitant slope erosion. Turbidites of the rise unit can be traced
seaward where they are overlapped by, or interfinger with, (Fig.10 and 13) turbidites
underlying the floor of the Bering abyssal plain (GERSHANOVlCH,1965).
Marine Geol., 6 (1968) 297-330
310
i). ~k,. s ( 1 t ( ) 1 1 . , li. c . t:IIo}::FINGTON A N D 1) M. H O P K I N ~
Related abyssal-plahz d~Tosils
EWIN(; et al. (11965) have determined that the southern region of the Berin~
abyssal plain (near Bowers Bank, Fig.2)is underlain by about 1 km of reflectivc
beds overlying an undetermined but probably thicker sequence of acoustically homogenous deposits. The reflective layer is thought by them to be constructed of turbiditcs
laid down during the Pleistocene. This is a reasonable assumption considering that
abyssal turbidite deposition would be expected during the Pleistocene in response to
massive outer-shelf sedimentation.
Over the central region of the abyssal plain our profiling records show that the
upper reflective layer is only about 470 m thick, considerably less than that found
by EWlNG et al. (1965) farther to the south. The maximum average thickness of
sediment that could have accumulated over the central floor of the deep Bering Sea
(Aleutian and K a m c h a t k a Basins) during the Pleistocene can be estimated by assuming the terrigenous debris shed by the surrounding continents accumulated only
here. According to LlSITZIN'S (1966, pp.77-80), the present annual sediment discharge
to the Bering Sea is approximately 115 • 106 metric tons. I f this figure is increased
by a factor of 1.5 for glacial ages 1, then, during a 3 • 106 year long Pleistocene (HoI'KINS
et al., 1965a; CURRY, 1966; MATHEWS and C U R T I S , 1966; OPDYKE et al., 1966) a
maximum of about 430 • l012 tons of terrigenous sediment could have reached the
abyssal floor of the Bering Sea. Assuming this mass of sediment would have an average
grain density of 2.65 g/cm a, and a mean compacted porosity of 65 ~ (based on measured surface values and the compaction curves of HAMILTON, 1959, 1964; MOORI~:,
1968), then it would form a column of sediment approximately 580 m thick over
the abyssal floor of the Bering Sea (8.3 • l0 s km 2) below a depth of 3,000 m. To this
thickness must be added at least 100 m of compacted diatomaceous ooze (GERSHANOwcH, 1962a, 1965, 1967). Thus, during a 3 • l06 year long Pleistocene, an average
thickness of about 700 m of sediment could have accumulated below a depth of
3,000 i'n, provided no terrigenous debris settled on the continental2slope above this
depth, on the shelf as terrestrial, neritic or deltaic accumulations, or was lost to the
Arctic Ocean via Bering Strait. It is also assumed that the narrow Aleutian Ridge
served as a local rather than basin-wide sediment source.
1 This estimate is based on a 14.1 m long core collected at a depth of 3,655 m in the abyssal plain of
Kamchatka Basin, which is the weStern deep-water region of the Bering Sea. SA~OVAand LtsrrziN
(1961) report that this core penetrated a generally complete stratigraphic section comprising all of
the Quaternary since late Illinoian time. Differential sedimentation rates for this core can be computed
by using the ages assigned by EMmrANI(t955, 1964) for the glacial-interglacial boundaries of the
Quaternary in consideration of the thickness of coeval sediments penetrated by the core. Accordingly,
the rate of sedimentation for the Holocene (last 9,000 years) is 8 cm/1,000 years, for the Wisconsin
14 era/l,000 years, and for the Sangamon 12 cm/1,000 years, which gives an average of 10 cm/1,000
years for interglacial periods and 14 cm/l,000 years for glacial. It should be emphasized that this
core shows evidence of a displaced benthonic fauna within glacial-stage turbidite sediments; thus,
in comparison with the rest of the core, the Wisconsin sedimentation rate may be slightly too high
in terms of estimating a possible glacial-age increase in continental sediment discharge.
Marine Geol., 6 (1968) 297-330
31 1
THE AMERICAN CONTINENTAL MARGIN IN THE BERING SEA
In comparison to the computed 700-m thick Pleistocene section, which is based
on the unsupportable assumption of total sediment pooling in abyssal areas, the 470 m
of reflective sediments found by us over the central area of the Bering abyssal plain
could well represent a mixed turbidite-pelagic sequence largely of Pleistocene age--a
conclusion in substantial agreement with that reached by EWING et al. (1965) based
on a much thicker section. In contrast to the measured and computed sediment
thicknesses over the central region of the Bering abyssal plain, the nearly 1 km
of reflective sediment found by EWING et al. (1965) beneath its southern edge may
indicate contribution of turbidites from Bowers and the Aleutian Ridges (Fig.2).
There is also no compelling reason to reject the possibility that a portion of the thick
reflective sequence includes turbidites of mid to Late Tertiary age, which corresponds
to a period of intense erosion along the length of the Aleutian Ridge that preceded
the outbreak of Late Tertiary and Quaternary volcanism (GraTES and GIBSON. 1956:
COATS, 1962).
Surface mantling unit
The surface-mantling unit is a slope-conforming deposit that covers the outer
shelf and the slope. It is found only in the southeastern bathymetric province (Fig.5,
6 and 7). On the outer shelf the contact of this unit with the main layered sequence
is comformable; however, on the continental slope the two units are separated by an
angular unconformity. Surface-mantling sediments fill in or smooth over former,
and possibly erosional, relief on the Bering slope.
Data published by GERSHANOVlCH (1965, 1967) indicate that the surfacemantling unit is accumulating rapidly and is substantially comFosed of diatemaceous
and terrigenous components. Measured on partially compacted sediments, medern
sedimentation rates over this region average as high as 15 cm/1,000 years 1. If an
average Pleistocene rate of 25 cm/1,000 years (see footnote, p.310), and a maximum
uncompacted thickness of 500 m (based on compaction curves for sandy silts;
HAMILTON, 1959; MOORE, 1968), are taken, then initial deposition of the surfacemantling unit would have begun approximately 2.0. l06 years ago, which is close
to the Plio-Pleistocene boundary. The implication of the figure is that the surfacemantling unit is largely of Pleistocene age and that it is simply the uppermost depositional unit of the main layered sequence.
STRUCTURAL AND MORPHOLOGICAL EVOLUTION OF THE MARGIN
Regional geologic considerations, samples of bedrock from the continental
shelf and slope, and seismically resolved structures, indicate that the continental
1 About 35% of the mass of this sediment is contributed by pelagic diatoms, which amounts to a
volume contribution of nearly 40% considering the low specific gravity of the opaline diatoms
(BYRNEand EMERY,1960; CALVERT,1966).
Marine Geol., 6 (1968) 297-330
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77/
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Fig.5. Profiles A - B and B - C , with added geologic interpretations. Symbols read as follows for this and all other figures: A B
acoustic
basement; M L S = main layered sequence; S M U - - s u r f a c e - m a n t l i n g unit; R U -- rise unit; f ~ fault with displacement indicated; dashes separate
unconformable units; dots separate conformable units that are acoustica]ly mappable. Reflection time is given in seconds; depth is in meters based
on an acoustic velocity of 1,500 m/see.
2~
B
M
3000 J
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T H E A M E R I C A N C O N T I N E N T A L M A R G I N IN T H E B E R I N G SEA
313
margin passed through two major developmental or evolutionary stages during the
Cenozoic:
(1) An initial constructional or outbuilding phase associated with subsidence
and faulting of an older margin (acoustic basement) and the accumulation of an
overlying stratified section (main layered sequence).
(2) A subsequent destructional-constructional phase associated, respectively,
with intense canyon cutting, and a contemporaneous formation of submarine fans
at the base of the slope and surface-mantling sediments higher up on the continental
slope.
Initial constructional phase
Subsidence and sedimentation
Downbowing of the acoustic basement evidently began in Early Tertiary
time. Because the surface of the basement is generally parallel to that of the present
margin, it is reasonable to assume the basement is simply an older, now deeply
downwarped, continental margin consisting of rock units more acoustically reflective
than those of the overlying main layered sequence. Beneath the outer shelf and upper
part of the continental slope the basement is thought to be an erosional surface cut
across volcanic and sedimentary rocks largely of Mesozoic age (SCnOLL et al., 1966).
Concomitant with downbowing of the Bering Sea continental margin, as much
as 1,200-1,500 m of main-layered-sequence strata accumulated over the outer shelf
and continental slope. Dredged samples of siltstone and sandstone of mid Tertiary
age from the walls of canyons and from the continental slope, and highly-indurated
siltstone and mudstone of probable Cretaceous age from the acoustic basement,
attest that the main layered sequence probably represents much of the Tertiary. Prior
to intense canyon cutting, the structure of the margin was essentially that of MOORE'S
and CURRAY'S (1963) type "C" continental framework, a structural class that is
common to many other continental margins (CURRAY et al., 1966; UCHtJ~I and
EMERY, 1967).
Over the southeastern bathymetric province downwarping of the shelf edge, and
accumulation of the main layered sequence, took place in conjunction with even greater
subsidence and sediment infilling of the adjacent shelf underlying Bristol Bay. Strata
of the main sequence at the top of the southeastern margin (above Bering Canyon)
dip landward, or back, beneath the shelf as well as seaward down the continental
slope' (Fig.5). These opposing dips form an outer-shelf, anticlinal tlexure or structural
high that probably marks the seaward limit of Bristol Basin. This basin underlies
Bristol Bay and contains at least 3,000 m of marine and nonmarine deposits of
Cenozoic age,(BuRK, 1965, pp.90, 127, 129). The northeastward-dipping flank of the
high is constructed of bayward-thickening wedges of the main sequence; the opposite
or seaward-dipping flank is constructed of beds inclined essentially parallel to the
present depositional surface of the Bering continental slope. This structure suggests
depositional draping and thinning of stratigraphic units over a basement high sepaMarine Geol., 6 (1968) 297-330
,,.v
--..
c~
Sec
4.0 ~
2.0-
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Fig.6. Profiles D-E and E-H, with added geologic interpretations. See Fig.5 for symbol explanations, Fig.4 for lincsofproliles.
]~.
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Fig.7. Profiles E-F-G and E-I, with added geologic interpretations. See Fig.5 for symbol explanations, Fig.4 for lines of profiles.
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317
THE AMERICAN CONTINENTAL MARGIN IN THE BERING SEA
rating two subsiding regions, Bristol Basin to the northeast and the continental slope
and Aleutian Basin to the south.
Umnak Plateau is an erosionaUy isolated segment of a broad, flat, sedimentary
prism extending seaward from the base of the continental slope of the southeastern
province. Prior to the cutting of Bering and Umnak Canyons, this apron banked
against the Aleutian[Ridge (Fig.2 and 4). The plateau is thought to have formed by
the accumulation of at least 1,400 m of main-sequence strata over a subsiding but
relatively flat basement platform, or structural terrace. In many respects Umnak
Plateau is structurally similar to the Blake Plateau off the southeast coast of the
United States (EwING et al., 1966; UcHuI,~ and EMERY, 1967).
Faulting andfoMing
Marginal warping also involved outer-shelf basement faulting and slight to
moderate faulting and folding of overlying main-sequence strata (Fig.6, 7, 8 and 9).
In many areas the location, shape, trend, and part of the relief of submarine canyons
reflects this deformation. This is also implicit in the observation of GERSHANOVICH
L
K
0
_
Pribilof Canyon
L
--_ _
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Fig.9. Profiles K - L - M and P-Q, with added geologic interpretations. See Fig.5 for symbol
explanations, Fig.4 for lines of profiling.
Marine Geol., 6 (1968) 297-330
318
l). \ v . s l H o I . I . l!. ( ' . f l U F F I N ( ] - I O N
ANI)
1~. M. I t O P K I ! - . >
(1962a, 1962b, 1963) and KOTt!Nt..V (1965} that the larger canyons, e.g.. Navarin.
Zhemchug, Pribilof, and Bering (Fig.2) are situated at junctures or breaks in the
regional trend of the margin. In the vicinity of Zhemchug Canyon, ~hich must be
one of the world's largest slope canyons, the outermost edge of the shell: is underlain
by a basement high capped by only a fe~ hundred meters of main-sequence strata
(Fig.12). The peculiar outer-shelf depression formed by the headward bifurcation
of Zhemchug Canyon is almost certainly partially due to lateral removal of the mare
layered sequence from behind the inner, or landward-facing, scarp of this high. The
southern side of the depression is straight, steeply sloping, and probably reflects
normal faulting along the shelf edge prior to, or contemporaneous with, deposition
of the main sequence (Fig.12). Lack o f significant bathymetric relief on the shelf
along the strike of the fault suggests that there has been little recent faulting. Outershelf basement highs impounding, or in part overlapped by, younger shelf deposits
are common on other continental margins (CURRAY, 1965). Projected to the southeast,
the strike of this fault, and the basement high, intersect the shelf edge along profile
N - O (Fig.10) approximately midway between Zhemchug and Pribilof Canyons.
Northwest of Zhemchug Canyon this fault aligns with major northwest-trending
fractures offsetting Cretaceous rocks on Cape Navarin (PuSHCHAROVSKY, 1963:
BURK, 1965, p.153; YAYSHIN,1966).
The headward bifurcation of Pribilof Canyon appears to be related to a faulted
synclinal depression trending roughly parallel to that of the margin (Fig.9; unpublished reflection profiles). Deformation of subshelf strata in the vicinity of the canyon
head may extend northward to include the Pribilof Islands, which are constructed of
faulted basaltic flows, cinder cones, and volcanogenetic sediments of Quaternary
age resting on a crystalline basement (BARTH, 1956; Cox et al., 1966; D. M. HOPKINS,
unpublished field work).
The origin of the relief of Bristol Canyon is uncertain. All that presently can
be said is that the Canyon is fundamentally a synclinal trough formed in conjunction
with marginal sedimentation, faulting and interrupting episodes of sediment removal,
or possibly lack of accumulation, over the axial region of the canyon.
The upper reaches of Bering Canyon are aligned with the axis of Bristol Basin,
the base of the northern slope of the Aleutian Ridge, and the folded and deformed
zone adjacent to the eastern rim of Umnak Plateau. Bering Canyon follows a zig-zag
path along the base of the continental slope, has relatively few major tributaries,
and its axis is deflected to the north by the base of Bogoslof Island, an active volcano
lying north of the Aleutian Ridge (BYERS, 1959). These facts, some of which were
previously pointed out by SHEJ'AR~) and DIl.L (1966, pp.198-202), strongly imply
that the course, location, and some of the relief of Bering Canyon is structurally
determined. Bering Canyon is also presumed to mark the inner or landward edge of
the basement platform underlying Umnak Plateau. This may indicate that the canyon
follows a zone of deep basement faulting (Fig.7). If the fault is present, its sense of
motion may be dominantly strike-slip, as little evidence of major cross-canyon vertical
offset can be detected in the seismic reflection records. The zone of folding may just
Marine Geol.. 6 ([968) 297-330
t,.)
~D
~D
C~
Sec
4.0.
2.0-
S
o.
.... 6 ~"
z.o,,,,
,o
VRUE SLOG
2o ,=
o
.... ~?o..
zp,~
-
i
L
ML
AB
AB
:"
o
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km
tO00
M
-2000
-I000
N o
-10OO
ll o
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6.0Sec
4.0
2.0-
o
ta,.)
>
Z
Z
Z
>
Z
©
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Z
>
320
D.w. s( HOLL,E. C. BUFFINGTONAND D, M, ttOPKIN~,
as well indicate: (1) deformation of sedimentary units overlying a hingeline, or zone
of inflection, of a regionally subsided basement; (2) depositional draping over irregular
basement relief; and (3) gravity' sliding of main sequence strata down the continentai
slope against the nearly horizontal beds underlying Umnak Plateau.
The seaward edge of the basement platform presumed to underlie the plateau
is evidently faulted or downwarped below the western or marginal slope of the
plateau; main sequence strata appear to be depositionally draped over this offset
(Fig.7). It is interesting to note the general alignment of the seaward slope of the
plateau with the axis of Amukta Canyon, which obliquely cuts the northern flank
of the Aleutian Ridge (Fig.4). With less certainty, this alignment can be extended
to the north to fall along the axis of Pribilof Canyon, or to intersect the Pribilof Islands
(Quaternary volcanic structures). This suggests a deep-basement fault extending
northeastward across the Bering Sea from the Aleutian Ridge to the Bering shell'.
Erosional and secondary constructional phase
Canyon cutting
Although faulting and folding are in part responsible for canyon relief, the seismic reflection records indicate that much of the relief has been produced by erosion.
The most likely process would appear to be erosion in conjunction with downslope
movement, via gravity sliding or turbidity currents, of unstable masses of sediment
(MENARD, 1964, pp.218-220).
The youthful geomorphic ruggedness (Fig.2, 4 and 11) of the canyon-scarred
continental slope, and the exposure of mid-Tertiary units along the walls of the
canyons, are indicative that erosion began in Late Tertiary or Quaternary time. Slope
erosion may have been initiated by gravity slumping of the main layered sequence
in response to steepening of the slope. Deformational oversteepening can be linked
to the strong Pliocene uplift of the Alaska Peninsula (BuRK, 1965, pp.145-146),
downwarping of the Bering-Chuckchi platform that probably began in Late Miocene
time (HOPKINS, 1967), and the Late Neogene folding and uplift of the northern Koryak
Mountains and Anadyr Basin of northeastern Siberia (NALIVKIN,1960; KOSTYLE¥
and BURLIN, 1966). It is far more likely, however, that slope erosion was initiated,
or greatly intensified, by the deposition of unstable masses of sediment on the upper
continental slope in the vicinity of river mouths. Rapid accumulation of river-borne
sediment on the upper slope must have taken place repeatedly during glacial stages
of the Quaternary when eustatically lowered sea levels permitted Alaskan and
Siberian rivers to debouch on the outer edge of the Bering Sea shelf. The courses
of the larger of these rivers, i.e., the Yukon and Kuskokwim of Alaska, and the
Anadyr of Siberia, (Fig.l), across the emerged shelf were most likely structurally
controlled. Thus the cutting of some or all of the larger submarine canyons may be
related to a structurally influenced outer-shelf location for the mouths of major
subarctic rivers.
Marine Geol., 6 (1968) 297-330
THE AMERICAN CONTINENTAL MARGIN IN THE BERING SEA
321
KOTENEV'S (1966) discovery of a portion of the submerged channel of the
"Paleo-Anadyr" River east of Cape Navarin suggests that either Navarin or Pervenets
Canyons (Fig.2) could have been wholly or in part excavated by moving sediments
derived from the Anadyr. The Pleistocene mouth of this river, as well as all others,
undoubtedly shifted somewhat with each succeeding glacial-eustatic oscillation of
sea level. Considering that the Pleistocene was approximately 3 million years in
length (HovKINS et al., 1965a; CURRY, 1966; MATHEWSand CURTIS, 1966; OPDYKE
et al., 1966) and comprised perhaps as many as ten glacial stages, there was ample
time and opportunity for shifts to have taken place. Thus sediments contributed
by the Anadyr River could have partly excavated both Navarin and Pervenets
Canyons at slightly different times within the Pleistocene.
The Pleistocene channel of the Yukon River cannot be topographically traced
across the Bering Sea shelf with any degree of certainty. This is attributable to channel
infilling by open-water shelf sedimentation and channel blockage by the formation
of deltaic wedges during periodic still-stands of the many Pleistocene marine transgression that overran this shelf (Hot'I<rNs, 1959; GERSHANOVICH,1963; SCHOLLand
SAINSBCRY, 1961; CREAGER and MCMANUS, 1965, 1967; HOPKINS et al., 1965b;
Hov~rys, 1967). The Yukon could easily have emptied in the vicinity of Zhemchug 1
or Pribilof Canyons; the Kuskokwim, which may have shared some of the drainage
of the Yukon during the Pleistocene, could have reached the head of Pribilof Canyon.
The only large river that could have easily reached the head of Bering Canyon
is the Kuskokwim (Fig.1 and 2). Smaller rivers presently entering Bristol Bay from
mainland Alaska and the Alaska Peninsula could have also contributed sediment to
the head of Bering Canyon. Volcanic and glacial debris must have entered the
canyon system via its numerous tributary valleys originating on the r, orthern slope
of the Aleutian Ridge (Fig.2).
If, as speculated above, the major episode of canyon cutting is related to rapid
outer-shelf sedimentation, it does not necessarily follow that this period is restricted
to the Pleistocene. HOPKINS (1959, 1967) has shown that throughout most of the
Tertiary parts of the northern region of the Bering Sea shelf formed a subaerial
landscape of low relief. Not until Late Pliocene or Early Pleistocene was the northern
region more permanently flooded and the modern Bering Strait formed. Thus
uplift of the shelf, or coastal outbuilding by littoral processes, may have periodically
permitted Alaskan and Siberian rivers to empty at, or near, the shelf edge during the
Late Tertiary.
1 The results of some preliminary calculations suggest that the total volume of sediment probably
carried by the Yukon during the Pleistocene is approximately 30 times that of the volume of Zhemchug
Canyon. This compares favorably with the ratio of the probable amount of sediment carried down
Monterey Canyon (off the California coast) to the volume of this canyon (MENARD, 1960). The
implication of this agreement is difficult to assess, but presumably it indicates that Zhemchug Canyon
could have been carved by sluicing Yukon sediments within the span of the Pleistocene (3 • 106 years).
Marine Geol., 6 (1968) 297-330
:322
t). VV. SCIIOI.I., E. C. BLJFFINGTON AND 1;,. '~i. I.tOPKIN5.
Secondary marginal sedimentation
Concurrent with intense canyon cutting, a broad continental rise consistin~
of laterally coalescing submarine fans was formed at the base of the slope. Deposition
of this rise unit represents a secondary phase of marginal growth. The rise unit i~:;
constructed of turbidites laid down in the vicinity of canyon mouths and their
outlying leveed deep-sea channel systems (Fig. 12).
Except near submarine canyons, the surface of Umnak Plateau and the adjacent
continental slope of the southeastern bathymetric province was the site of essentially
continuous Pleistocene sedimentation and marginal outbuilding. Accumulation
of this sediments, the surface-mantling unit, which is as much as 350-m thick, not
only represents continuation of slope sedimentation into the Quaternary, but also
a secondary phase of marginal outbuilding in that the unit unconformably overlies a
region-wide surface cut across main-sequence strata. The origin of the unconformity,
which is well developed even on gentle slopes, is not fully understood; presumably
the surface is related to canyon cutting. The thickness of the unit over the southeastern
province can be related to the nearness of repeatedly glaciated landmasses and to
the high organic productivity of overlying water masses. The apparent absence of
the surface-mantling unit farther to the northwest can partially be ascribed to lack
of discovery, but the general steepness of the slope elsewhere along the margin may
have inhibited accumulation of detectable thickness of Pleistocene sediments subsequent to slope dissection, i.e., canyon cutting.
8,
,I :
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IN M E T E R S
CORE AND CAMERA STATION
.....................
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provinces.
Marine Geol., 6 (1968) 297-330
I'J
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T'
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5oo~~
.
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0
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325
THE AMERICAN CONTINENTAL MARGIN IN THE BERING SEA
ORIGIN OF THE ALEUTIAN BASIN
SUOR (1964) has suggested the Aleutian Basin is a former embayment of the
north Pacific that was isolated from this oceanic region by the formation of the Aleutian Ridge (see also MENARD, 1967). His contention is based on the facts that the
basin floor is underlain by an exceptionally thick sediment layer, and a thicker than
normal seismic "second layer" of sedimentary and/or volcanic rocks overlying a
normal, though somewhat depressed, oceanic crust (Fig.14). The sediment layer
is approximately 3-km thick and is presumed to represent sediment infilling. ]nfilling
of the Aleutian Basin is forming "new" continental crust, and has isostatically depressed the Mohorovicic discontinuity by about 4 km. SHOR (1964) assigns no age to
the formation of the sediment-impounding Aleutian Ridge, which is largely composed
of a complicated sequence of Cenozoic to Late Paleozoic volcanic and volcanoclastic
sedimentary units (COATS, 1962).
BURK (1965, pp.147-157), based on his study of the Alaska Peninsula and
outlying Shumagin-Kodiak shelf, suspects that much of the Aleutian Ridge is of
Tertiary construction, despite the exposure of Mesozoic ard Paleozoic terrestrial
NORMAL
OCEANIC
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AFTER SHOR
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DISTANCE IN KM
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500
600
Fig.14. Composite seismic refraction section across the Aleutian Basin and its northern continental margin. (Modified after SHOR, 1964).
Marine G e o l . , 6 (1968) 297-330
326
i). w . S C t l O L I . , E. (7. B U F F 1 N G T O N A N D tL 3.1. HOPKIN:',
deposits on the ridge. He further suggests that Late Cretaceous deformalion of Ihc
north Pacific continental margin formed the present Shumagin-Kodiak shelf, the
Koryak-Kamchatka shelf of eastern Siberia, and the outer Bering Sea shelf connecting~
these two regions.
Several investigators consider that the Aleutian Basin, or a part of it, was
formed by "oceanization" or f\mndering of a segment of the Alaska-Siberian continental crust. This point of view is implicit in the largely theoretical writings of
BELOUSSOVand RUDITCH (1961), BELOUSSOV(1962), ATLASOVet al. (1966), and other
Russian authors. Firmer geologic evidence pointing to partial or complete oceanization of the basin has recently been compiled by GRANTZ (1960, who emphasizes
that the present Aleutian Ridge is a Cenozoic structure superimposed on an older
arc-shaped fragment of continental crust. Evidence supporting this conclusion includes the following:
(1) Inclusions of sialic igneous and metamorphic rock and quartz-bearing
sandstone in some of the Tertiary effusives of the western Aleutian Islands, and
erratics of similar rock types in associated glacial and marine terrace deposits of Late
Cenozoic age.
(2) The dominantly andesitic composition of Middle and Late Tertiary volcanic
rocks forming the ridge.
(3) The occurrence of plant-bearing Upper Paleozoic rocks on Adak Island.
The results of the present study do not unequivocally resolve the question of
the origin of the Aleutian Basin and its northern marginal slope. However, it is clear
that the North American-Siberian continental margin in the Bering Sea underwent
substantial subsidence during Cenozoic time. The amount of subsidence beneath
the average position of the shelf edge must have been at least 1.5 km, to judge from
the maximum thickness of the flat-lying beds of the main layered sequence underlying
the outer shelf. If the surface of the acoustic basement is an erosional unconformity
that extends down the continental slope, then the minimum subsidence of the slope
during the Tertiary would be 3-4 kin. Subsidence of a similar magnitude is suggested
by the structure of Umnak Plateau. Crustal foundering of the Aleutian Basin is an
adequate mechanism to account for the subsidence of its northern margin. Alternately, if the basin is a former segment of the north Pacific floor, then Cenozoic
downwarping of its northern margin can be linked with isostatic subsidence of the
basin in response to sediment infilling behind the Aleutian Ridge.
Until more is known about the lithologic character of the Mesozoic acoustic
basement, the Late Mesozoic-Early Tertiary history of this area and of the adjacent
floor of the Aleutian Basin will remain unknown. However, exposures of what are
believed to be bathypelagic (lower continental slope or rise region) volcanic and
sedimentary rocks of Neocomian to Middle Senonian age in northern Kamchatka
and the Koryak Mauntains, imply that a deep intra-crustal trough or a more extensive
deep-water Bering Sea existed at least by early Late Cretaceous time (RoTMA~, 1963;
BURK, 1965, pp.154-155; ROTMANand MARKOVSKY,1965; L. I. Krasny, V. K. Rotman,
and B. A. Markovsky, personal communication, 1966). If, as BURK (1965) suggests,
Marine GeoL, 6 (1968) 297-330
THE AMER1CANCONTINENTALMARGININ THE BERINGSEA
327
the structural framework of the outer Bering shelf was laid out in conjunction with
the Late Cretaceous marginal deformation that formed the present KamchatkaKoryak and Shumigan-Kodiak shelves, then it is clear that the Bering Sea shelf
was later tectonically "decoupled" from these adjacent areas as it shared in little
of the subsequent Cenozoic uplift and volcanism that formed their flanking coastal
ranges.
During the Cenozoic most of the Bering Sea shelf can, in the main, be viewed
as a slowly subsiding continental platform. It remains to be determined whether the
outer segment of this block fell away in Late Mesozoic time to form the Aleutian
Basin and the relief of the present continental margin, or whether an existing Late
Mesozoic margin was simply downwarped during the Cenozoic in response to isostatic loading of an oceanic crust by a thick accumulation of impounded sediment.
ACKNOWLEDGEMENTS
The bulk of this study was supported by the U.S. Naval Weapons Center
(formerly the U.S. Naval Ordnance Test Station), China Lake, Calif., and the U.S.
Navy Underseas Warfare Center (formerly the U.S. Navy Electronics Laboratory)
San Diego, Calif. Special thanks are due to Dr. Pierre St. Amand, Head, Earth and
Planetary Sciences Division, U.S.N.W.C., for assistance in arranging support.
We are pleased to acknowledge Captain James Sedam, Master, U.S.N.S.
"Charles H. Davis" (AGOR-5), for his assistance and display of seamanship during
the conduct of the field operations. We were also ably assisted by Messrs. Carl Shipek,
U.S.N.U.W.C. who provided deep-sea photographic apparatus, Robert E. Boyce,
U.S.N.U.W.C., San Diego, who performed laboratory analyses on core samples of
bottom sediments, and Messrs. Mark Holmes and Douglas Morrisson, Department of
Oceanography, University of Washington, who recovered samples of exposed bedrock
from Pribilof Canyon during a cruise of the R/V "Thomas G. Thompson". Dr.
Roland yon Huene, U.S. Geological Survey, gave considerable time to joint discussions regarding the interpretation of our reflection records. Similar and profitable
discussions were held with Dr. George G. Shor Jr., Scripps Institution of Oceanography, and Dr. Gene A. Rusnak, U.S. Geological Survey. The manuscript has
benefited significantly from critical readings by Drs. Arthur Grantz, U.S. Geological
Survey, Edwin L. Hamilton, U.S.N.U.W.C., John V. Byrne, Oregon State University,
and Joseph R. Curray, Scripps Institution of Oceanography.
REFERENCES
ATLASOV,I. P., EGIAZAROV,B. Kh., DIBNER,V. D., ROMANOVICH,B. S., VAKER,V. A., DEMENITSKAYA,
R. M., LEVIN,D. V., KARASIK,A. M., HAKKEL,YA. and LITVIN,V. M., 1966. Tectonic Map
of the Artic and Subarctic, Scale 1 : 5,000,000. Commun. Sci., Comm. Carte G6ol. Monde,
Paris.
Marine Geol., 6 (1968) 297-330
328
~). xv. st I I O I , L , I~. (L I ] L i F F I N G ' I O N A N D I). x,I. I I O ] ' k t \
~.
BARTH, T. F. W., 1956. Geology and petrology of the Pribilof islands, Alaska. t_.'.S., G~;ol. Sm~.
Bull., 1028 (F) : 101 -160.
BELOUSSOV,V. V., 1962. Basic Problems of Geotectonics. McGraw-Hill, New York, N.Y., 816 pp.
BELOUSSOV,V. V. and RUDITCH, E. M,, 1961. Island arcs in the development of the earth's structure
(especially in the region of Japan and the Sea of Okhotsk). J. Geol., 69 : 647-658.
BLAKE, C. and HOPKINS, D. M., (Translator and Editor), 1962. Stratigraphic Tabh¢~s and Geologic
Mapsofthe Northeast U.S.S.R. Am. Geol. [nst., Washington, D.C., 18 pp.
BOYCE, R. E., 1967. Electrical Resistivity of ~¢odern Marbte Sediments from the Bering Sea. Thesis,
San Diego State College, San Diego, Calif. 172 pp. (unpublished).
BURK, C. A., 1965. Geology of the Alaska Peninsula: Island arc and continental margin, 1, 2. Geo/.
Soc. Am., Mere., 99 : 250 pp.
BYERS JR., F. M., 1959. Geology of Umnak and Bogoslof Islands, Aleutian Islands. Alaska. U,$'.,
Geol. Surv., Bull., 1028 (L) : 267-369.
BYRNE, J. V. and EMERY, K. O., 1960. Sediments of the Gulf of California. Bull. Geol. Soc. Am.,
71 : 983-1010.
CALVERT,S. E., 1966. Accumulation of diatomaceous silica in the sediments of the Gulf of California.
Bull. Geol. Soc. Am. ,77 : 569 596.
CHIKOV, V. M., 1965. Cross faults as shown by the Koryak fold region. Dokl. Akad. Nauk S.S.S.R.,
EarthSci. Sect., 161 : 115-117.
CoaTs, R. R., 1962. Magma type and crustal structure in the Aleutian Arc. In: G. A. MCDONALDand
H. KUNO (Editors), The Crust of the Pacific Basin--Geophys. Monograph, 9 : 92-115.
Cox, A., HOPKINS, D. M. and DARYMPLE,G. B., 1966. Geomagnetic polarity epochs: Pribilof Islands,
Alaska. Bull. Geol. Soc. Ant., 77 : 883-910.
CREAGER,J. S. and McMANuS, D. A., 1965. Pleistocene drainage patterns on the floor of the Chukchi
Sea. Marine Geol., 3 : 279-290.
CgEaGEg, J. S. and McMANuS, D. A., 1967. Geology of the floor of Bering and Chukchi Seas. In:
D. H. HOPKINS (Editor), The Bering Land Bridge. Stanford Univ. Press, Stanford, Calif..
495 pp.
CVRRaY, J. R., MOORE, D. G., BELDEgSON,R. H. and STRtOE, A. H., 1966. Continental margin of
western Europe: slope progradation and erosion. Science, 154 : 770-771.
CURRAY, J. R., 1965. Structure of the continental margin off central California. Trans. N.Y. Aead.
Sci., Ser. 11, 27 : 794-801.
CURRY, R. R., 1966. Glaciation about 3,000,000 years ago in the Sierra Nevada. Science, 154 : 770-771.
DtJTRO, J. and PAYNE, T., 1956. Geologieal Map of Alaska, Scale 1:2,500,000. U.S. Geol. Surv.,
Washington D.C.
EMILIANI,C., 1955. Pleistocene temperatures. J. Geol., 63 : 538-578.
EMILIANI, C., 1964. Paleotemperature analysis of the Caribbean cores A254-BR-C and CP-28. Bull.
Geol. SoP. Am., 75 : 129--144.
EW~NG, M., LUDWIC;, W. J. and EWING, J., 1965. Oceanic structural history of the Bering Sea. J.
Geophys. Res., 70 : 4593-4600.
EWING, J., EWING, M. and LEYDEN, R., 1966. Seismic profiler survey of the Blake Plateau. Science,
154 : 770-771.
GAZES, O. and GIBSON, W., 1956. Interpretation of the configuration of the Aleutian Ridge. Bull.
Geol. Soc. Am,, 67 : 127-146.
GATES, G. O. and GgYc, G., 1963. Structures and tectonic history of Alaska. In: O. E. CmLDS and
B. WARREN(Editors), Backbone of the Americas. Am. Assoc. Petrol. Geologists, Tulsa, Okla.,
pp.264-277.
GERSr~ANOVICH,D. E., 1962a. New data on the Recent deposits of the Bering Sea. Tr. Vses. Nauehn.
Issled. Inst. Razved. Okeanol., 46 : 128-164 (in Russian).
GERSHaNOVlCH, D. E., 1962b. Relief and Recent deposits of the Bering Sea shelf. Tr. Vses. Nauehn.
Issled. Inst. Razved. Okeanol., 46 : 164-184 (in Russian).
GERSHANOVICH,D. E., 1963. The relief of the main fishing industry regions (the shelf and the continental slope) and some characteristics of the geomorphology of the Bering Sea. Tr. Vses.
Nauehn. Issled. Inst. Razved. OkeanoL, 48 : 13-76 (in Russian).
GERSHANOWCH, D. E., 1965. Thickness of contemporary bottom sediments and rate of sediment
formation in the Bering Sea. Tr. Vses. Nauehn. Issled. Inst. Razved. Okeanol., 48 : 261-269
(in Russian).
Marine Geol., 6 (1968) 297-330
THE AMERICAN CONTINENTAL MARGIN IN "[HE BERING SEA
329
GERSHANOV1CH,D. E., 1967. Late Quaternary deposits of the Bering Sea and the Gulf of Alaska. In:
D. M. HOPKINS (Editor), The Bering Land Bridge. Stanford Univ. Press, Stanford, Calif.,
pp.32-45.
GNmEDENKO, G. S. and ROZENBLYUM,I. S., 1966. The Precambrian problem in the northeastern
Koryak Mountains. Dokl. Akad. Nauk S.S.S.R., Earth Sci. Sect., 167 : 40-42.
GRANTZ, A., 1966. Strike-Slip Faults in Alaska. Thesis, Dept. Geol. Stanford Univ., Stanford, Calif.,
82 pp. (unpublished).
HAMILTON,E. L., 1959. Thickness and consolidation of deep-sea sediments. Bull. Geol. Soc. Am.,
70 : 1399-1424.
HAMILTON,E. L., 1964. Consolidation characteristics and related properties of sediments from Experimental Mohole (Guadalupe Site). J. Geophys. Res., 69 : 4257-4269.
HAMILTON,E. L., 1967. Marine geology of abyssal plains in the Gulf of Alaska. J. Geophys. Res.,
72 : 4189-4214.
HOPKINS,D. M., 1959. Cenozoic history of the Bering Land bridge. Seienee, 129 : 1519-1528.
HOPKINS, D. M., 1967. The Cenozoic history of Beringia--a synthesis. In: D. M. HOPKINS (Editor),
The Bering Land Bridge, Stanford Univ. Press, Stanford, Calif., pp.451-484.
HOPKINS, D. M., EINARSSON,T. and DOELL, R. R., 1965a. The stratigraphy of Tjornes, northeastern
Iceland: its significance for the history of the Bering Land Bridge. Abstr. Intern. Assoc. Quaternary Res., VII Congress, Boulder and Denver, Colo., p.223.
HOPICINS, D. M., MACNEIL, F. S., MERKLIN, R. L. and PETROV, O. M., 1965b. Quaternary correlations across Bering Strait. Science, 147 : 1107-1114.
HouTz, R. E. and EWING, J., 1963. Detailed sedimentary velocities from seismic reflection profiles
in the western North Atlantic J. Geophys. Res., 68 : 5233-5258.
KOSTVLEV, YE. N. and BURLIN,YU. K., 1966. Geologic evolution of the Anadyr Depression. Dokl.
Akad. Nauk S.S.S.R., Earth Sci. Sect., 166 : 1177-1179.
KOTENEV, B. N., 1965. Submarine valleys in the zone of the continental slope in the Bering Sea. Tr.
Vses. Nauchn. Issled. Inst. Razved. Okeanol., 58 : 35~14 (in Russian).
KOTENEV, B. N., 1966. The submerged valleys of the Paleo-Anadyr. Priroda, 8 : 118-119 (in Russian).
LISITZlN, A. P., 1964. Bottom sediments of the Bering Sea. In:P. L. BEZREKOV(Editor), Geographical
Description of the Bering Sea. Israel Program Sci. Transl., Jerusalem; U.S. Dept. C o m m e r c e , Natl. Sci. Found., Washington D.C., pp.65-188.
LISITZIN, A. P., 1966. Processes of Recent Sedimentation in the Bering Sea. Nauk, Moscow, 574 pp.
(in Russian).
MATnEWS, W. H. and CURTIS, G. H., 1966. Date of the Pliocene-Pleistocene boundary in New
Zealand. Nature, 212 : 979-980.
MENARD, H. W., 1955. Deep-sea channels, topography and sedimentation. Bull. Ant. Assoc. Petrol
Geologists, 39 : 236-255.
MENARD, H. W., 1960. Possible Pre-Pleistocene deep-sea fans off central California. Bull. Geol. Soc.
Am., 71 : 1271-1278.
MENARD, H. W., 1964. Marine Geology of the Pacific. McGraw-Hill, New York, N.Y., 271 pp.
MENARD, H. W., 1967. Transitional types of crust under small ocean basins. J. Geophys. Res.,
72 : 3061-3073.
MOORE, D. G., 1968. Reflections profiling studies of the California continental borderland; structure
and Quaternary turbidite basins. Geol. Soe. Am., Spec. Papers, in press.
MOORE, D. G. and CURRAV,J. R., 1963. Structural framework of the continental terrace, northwest
Gulf of Mexico. J. Geophys. Res., 68 : 1725-1747.
NAGOL'SK1Y,N. N., 1965. More facts on the Paleogene of the central Koryak Mountains. Dokl. Akad.
Nauk S.S.S.R., Earth Sci. Sect., 163 : 84-86.
NALIVKIN, D. (Editor), 1960. The Geology of the U.S.S.R. Pergamon, New York, N.Y., 170 pp.
NICHOLS, H. and PERRY, D., 1966. Bathymetry of the Aleutian Are, Alaska: Seale 1 : 400,O00--Dept.
Commerce, EnvironmentaI Sci. Serv. Admin., Coast Geodetic Surv., Monograph, 3 : 6 maps.
OPDYKE, N. D., GLASS,B., HAYS, J. D. and FOSTER,J., 1966. Paleomagnetic study of Antarctic deepsea cores. Science, 154 : 349-356.
PUSHCHAROVSKV,YU. M. (Editor), 1963. Tectonic Map of the aretic scale 1 : 10,000,000. Geol. Inst.
Acad. Sci. U.S.S.R., Moscow (in Russian).
ROTMAN, V. K., 1963, On certain peculiarities of geosynclinal volcanogenic sedimentary formations
in the light of recent data. Dokl. Akad. Nauk S.S.S.R., Earth Sci. Sect., 140 : 936-938.
Marine Geol., 6 (1968) 297-330
330
I). "v\. S('tI()L|, [. C. BUFFINGTON AND I). M. li()PKIN?'
ROTMAN, V. K. and MARKOVSKY,B. A., 1965. Geosynclinal alkalic basalts of the northwestern parl
of the Pacific Ocean belt. Dokl. Akad. Nauk S.S.S.R., Earth Sci. Sect., 165 : 19 -21.
SAmOVA, KH, M. and L~sIrzlN, A. P., 1961. Sedimentary stratigraphy and paleogeography of ~hc
Bering Sea during the Quaternary period. Dokl. Akad. Nattk S.S.S.R., Earth Sci. Sec/..
139 : 59-63.
SCHOLL, D. W. and SAINSmJR~,C. L., 1961. An Arctic sea valley carved in a modern epicontinental
sea. Bull. Geol. Soc. Am., 72 : 1433--1436.
SCHOLL, D. W., BUFFINGTON,E. C. and HoPKINs, D. M., 1966. Exposure of basement rock on the
continental slope of the Bering Sea. Science, 153 : 992-994.
SHEPARD, F. P., 1963. Submarhle Geology, 2nd ed. Harper and Row, New York, N.Y., 554 pp.
SHEPARD, F. P. and DILL, R. F., 1966. Submarine canyons and other sea valleys. Rand McNally,
Chicago, 381 pp.
SHort JR., G. G., 1964. Structure of the Bering Sea and the Aleutian Ridge. Marine Geol., 1 : 213-219.
UcrtvPI, E. and EMERY, K. O., 1967. Structure of continental margin off Atlantic coast of United
States. Bull. Am. Assoc. Petrol. Geologists, 51 : 223-234.
UO1NTSEV, G. B., BIOCriENKO,1. G. and KANAEV, V. R., 1964. Bottom relief of the Bering Sea. In:
P. L. BEZRUKOV(Editor), GeolographiCal Description of the Bering Sea. Israel Program Sci.
Transl., Jerusalem; U.S. Dept. Commerce; Natl. Sci. Found., Washington, D.C., pp.14-64.
YANSmN, A. L. (Editor), 1966. Tectonic Map of Eurasia: Scale 1 : 5,000,000. Geol. Inst. Acad. Sci.
U.S.S.R., Moscow (in Russian).
Marine Geol., 6 (1968) 297-330