Boggs S. Principles of Sedimentology and Stratigraphy
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346
Chapter
1
0 I Siliciclastic Marine Environments
gure 10.9
Current off the Asian coast, and the Agulhas Current off the southeast tip of Africa
are examples of some prominent ocean currents.
These semipermanent ocean currents intrude onto some shelves with suf
cient bottom velocity to transport sandy sediment. About 3 percent of modem
shelves are dominated by these ocean currents, which operate most effectively on
the outer shelf. Modern examples of such shelves include the northweste Gulf
of Mexico, which is affected by the Gulf Stream System; shelves swept by the
Panama and North Equatorial Current off the northeast coast of South America;
the Ta iwan Strait between Ta iwan and mainland China, which is intruded by a
branch of the Kuroshio Current flowing north from the Philippines; and the outer
shelf of southe Africa, which is crossed by the southward-flowing Agulhas Cur
rent of the weste Indian Ocean. These currents commonly contribute little if any
new sediment to the shelf, but they are capable of transporting significant vol
umes of fine sediment along the shelf. Some achieve bottom velocities eat
enough to transport sandy sediment and create sand waves and other bedforms,
for example the Kuroshio Current (Boggs, Wang, and Lewis, 1979) and the Agul
has Current (Fleming, 1980).
Sediment on such shelves raked by intruding ocean currents is largely relict,
but it is commonly reworked by intruding currents to form sand waves (dunes),
sand ribbons, and coarse sand and gravel lag deposits. The best-documented ex
ample of this type is the southeastern shelf of South Africa, which is intruded by
the Agulhas Current of the Indian Ocean (Fleming, 1980). Sand waves up to 17m
high with wave lengths up to 700 m occur sand-wave fields as much as 10
wide and 20 km long (Fig. 10.9). The Taiwan Strait between Ta iwan and China is
another broad shelf invaded by ocean currents that create extensive sand-wave
fields (Boggs, 1974).
Shelf ansport by Density Cuents
Density currents are created by density diffences within water masses. The
buoyant plumes and underflows described above and illustrated in Figure 10.5, as
well as nepheloid flows, are all density currents driven by density differences aris
ing from suspended sediment. In arid climates, excessive nearshore evaporation
may generate dense brines that flow seaward along the bottom as an underflow.
Such underflows could conceivably occur also on shelves whe cold surface wa
ters sink and flow seaward. Density currents generated as a result of variations in
A
Sediment transport by the Agulhas Current o
the southeastern tip of Africa. Sand in the current
controlled central shelf (B) migrates under the
influence of the Agulhas Current; sand-wave
fields are up to 20 km long and
1
0 km wide, and
individual sand waves are up to
1
7 m high. Black
streaks indicate sand ribbons. The stippled pat
tern indicates coarse lag deposits in the sand-de
pleted outer shelf (C). The nearshore sediment
wedge (A) is dominated by wave processes.
[From Fleming, B. ,
1
980, Sand transport and
bedforms on the continental shelf between Dur
ban and Port Elizabeth (southeast Africa conti
nental margin): Sed. Geology, v. 26, Fig.
1
5, p.
1
94, reproduced by permission of Elsevier Sci
ence Publishers, Amsterdam.]
+100
I
MSL
0
12
10.2 The Shelf Envinment
347
temperature or salinity are not significant agents of shelf transport, and no mod-
e shelf is dominated by such processes.
Eects of Sea-Level Change on Shelf anspo
Because geographic environments shift rapidly and change their form during sea
level fluctuations, sea-level changes constitute an important, and sensitive, depo
sitional variable on shelves. They can affect both erosional and depositional
presses and thus the kinds of sediments deposited on shelves. Among other
things, sea-level changes are an important factor in establishing the stratigraphic
architecture of shelf sediments, a topic that we explore in Chapter 13. See also
Wa lker and James (1992) and Reading and Levell (1996).
Biological Activities on Shelves
Mode contental shelves are among the environments most densely populated
by organisms, and the geologic record suggests that ancient epeiric seas were also
inhabited by large populations of organisms. The shelf floor is habitat for highly
diverse invertebrate organisms such as molluscs, echinoderms, corals, sponges,
worms, and arthropods. Both infauna and vagrant and sessile epifauna are repre
sented. Organisms are most abundant in lower-energy areas of the shelf, and the
eatest populations occur on the ier shelf just below the wave base.
Organisms on siliciclastic-dominated shelves are particularly important as
agents of bioturbation. Both type and abundance of bioturbation structures vary
with sediment type and water depth. As discussed in Chapter 4, many burrows in
the nearshore high-energy zone are escape structures that tend to be predominantly
vertical. The burrow style changes to oblique or horizontal feeding structures with
deepening of water across the shelf. general, muddy sediments of the shelf are
more highly bioturbated than are sandy sediments, and physical sedimentary struc
tures in these sediments may be almost completely obliterated by bioturbation. By
contrast, only a few species of organisms can survive in the very hi energy
nearshore shelf and beach zone. Therefore, sandy sediments of the beach-shelf
transition zone are dominated by physical structures such as cross-bedding rather
than bioturbation structures. Nonetheless, some bioturbation structures may be
psent if they escape destruction by reworking. Sandy layers deposited in deeper
water on the shelf may be bioturbated to some degree in their upper part. In addi
tion to their importance as bioturbation agents, some organisms produce fecal pel
lets from muddy sediment; these pellets may become hardened and coherent
enough to behave as sand grains. Organisms with shells or other fossilizable hard
parts also leave remains that may be preserved to become part of the sediment
record.
Ancient Siliciclastic Shelf Sediments
Although recognition of ancient she sediments is aided by study of modern con
tinental shelves, modern shelves are not necessarily good analogs of ancient
sh
elves. For example, the prevalence of relict sediments on modern shelves may
be atypical. Conversely, some structures believed to be diagnostic of ancient shelf
sediments, notably storm-generated hummocky cross-stratification, have appar
ently not been recoized in modern shelf environments. In general, ancient shelf
sediments appear to be distinguished by the following: (1) tabular shape, (2) ex
tensive lateral dimensions (thousands of square kilometers) and great thickness
(hundreds of meters), (3) moderate compositional maturity of sands with quartz
do
minating feldspars and rock fragments, (4) generally well-developed, even, lat
erally extensive beddg, (5) storm beds in some shelf deposits, (6) wide diversity
348
Chapter 10 I Siliciclastic Marine Environments
Figure 10.10
Idealized diagrams illustrat
ing typical fining-upward
transgressive shelf succes
sions on (A) a tide-dominat
ed shelf and (B) a
storm-dominated shelf, and
a coarsening-upward re
gressive shelf succession (C)
on a storm-dominated shelf.
[After Galloway, W. E., and
D. K. Hobday, 1983, Ter
rigenous clastic depositional
systems. Fig. 7.14, p. 159,
Fig. 7.15, p. 160, Fig. 7.17,
p. 162, reprinted by permis
sion of Springer-Verlag, Hei
delberg.]
and abundance of normal marine fossil organisms, and (7) diagnostic associations
of trace fossils.
More specific characteristics are related to deposition under tide-dominated
or storm-dominated conditions. The deposits of ancient tide-dominated shelves
are characterized particularly by cross-bedded sandstone. Paleocurrents are main
ly unimodal, apparently because of ancient regional net-sediment-transport paths,
although bipolar cross-stratification is present locally (Dalrymple, 1992). Reactiva
tion surfaces are abundant. Ancient storm-dominated shelf deposits likely contain
a greater proportion of mud than do tide-dominated deposits, and hummocky
cross-stratification and storm layers are common (but not in modern deposits!).
Several kinds of vertical successions may thus be generated in shelf sedi
ments, depending upon whether deposition takes place during transgression or
regression and depending upon the dominant type of shelf processes operating
during deposition. It is difficult to generalize about these successions except to say
that transgression tends to produce fining-upward successions that may begin
with coarse lag deposits, and regression produces coarsening-upward succes
sions. Some idealized vertical shelf successions produced under different postu
lated sedimentation conditions are illustrated in Figure 10.10. These successions
should be considered only as working models. Actual transgressive and regres
sive successions may differ markedly in detail from these idealized profiles.
Ancient shelf deposits are known from stratigraphic units of all ages and all
continents. They are probably the most extensively preserved rocks in the geolog
ic record. Readers interested in pursuing case histories of specific shelf deposi
can consult the extensive list of pertinent references provided by Leeder (1999, p.
463); these references include some of the best accounts of ancient shelf deposi
on the basis of study of Cretaceous sediments deposited in the Western Interior
Seaway of North America. Additional accounts of these deposits, derived from
study of magnificent exposures stretching from Colorado to Alberta, can be found
in Bergman and Snedden (1999).
One example of shelf deposits from the Western Interior Seaway is shown in
Figure 10.11. Baneee (1991) describes Cretaceous sandstones of southern Alberta,
Canada, which he interprets as tidal sand sheet facies. One of these is the (lower
most) Sunkay Member of the Lower Cretaceous Alberta Group. This sandstone
unit, approximately 10 m thick, rests with a sharp erosional base on nonmarine
A
Transgressive Tide
Dominated Shelf
v
v
Bioturbated
silts and muds
Large·scale cross
stratified sands
B
Tr ansgressive Storm
Dominated Shelf
c
Regressive Storm�
Dominated Shell
Burrowed, glauconitic,
si�y muds
Hummky cross
stratified sands
Hummocky cross
stratified sands
Parallel lamination
Storm-graded
beds within
shelf muds
Burrowed shelf
mud
m
11
-
10
9
8-
!
7
-
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c
6-
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5-
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4-
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Fluvial (?)
1111 I
EE��
Dinoflagellates
(Albian)
Storm layer
Storm layer
Storm layer
Foraminifer
(S accamina?)
Tr ansgressive
lag
10.3 The Oceanic (Deep-Water) Environment
349
�
quartchert pebbles
�
sandstone
shale
interbedded lithology
high-anglj
cross bed
low-angle
mud-draped foresets
!laser
parallel
} """'"'
wavy
gu 10.11
Vertical succession of sandy
tidal shelf deposits in the
Sun kay Sandstone Member of
the Lower Cretaceous Alberta
Group, southern Alberta,
Canada.
Symbols in the grain
size scale are gvl = gravel,
cs = coarse sand,
ms = medium sand,
fs = fine sand, and m = mud
(silt-clay). [After Banerjee, 1.,
1991 , Tidal sand sheets of the
late Albian Joli Fou-Kiowa-Skull
Creek marine transgression,
Western
Interior Seaway of
North America in Smith, R. G.,
et al. (eds.), Clastic tidal sedi
mentology: Canadian Soc. Pe
troleum Geologists Mem. 16,
Fig. 9, p. 334.]
shales of the Beaver Mines Formation d is overlain by black marine shales. The
Sunkay Member consists mainly of sdstone, with a thin lag gravel deposit at
the base and a few thin interbedded conglomerate layers, interpreted as storm
layers. The succession as a whole fines upward. The basal sandstone units are
cross-bedded; the uppermost ones are characterized by flaser and wavy bedding.
Foraminifers and dinoflagellates are present in some layers. Bioturbation is not
common.
Banerjee interprets these sandstones as tidal sand sheets. 11dal interpretation
based in part on the presence of the following: cross-stratification (>20° dip and
sets 300 em thick), with thin shale drapes adhering to foreset surfaces, in the
lower part of the section; and flaser, wavy, and lenticular bedding in the upper
part. Tr ansgression deepened the water at this site, causing black marine shales to
be deposited on top of the tidal sandstones.
10.3 THE OCEANIC (DEEP-WATER) ENVIRONMENT
Inoduion
e discussion of depositional environments to this point, I have focused on the
continental, marginal-marine, and shallow-marine enviroents because much of
the preserved sedimentary record was deposited in these environments. In terms
350
Chapter 10 I Siliciclastic Marine Environments
of size of the environmental setting, however, these nonmarine and shallow-water
marine environments actually cover a much smaller area of Earth's surface than
do deep-water environments. By far, the largest portion of Earth's surface lies
seaward of the continental shelf in water deeper than about 200 m. Approximately
65 percent of Earth's surface is occupied by the continental slope, the continental
rise, deep-sea trenches, and the deep ocean oor. Even so, most textbooks that dis
cuss sedimentary environments typically give only modest coverage to oceic
environments. This bias probably exists because, as a whole, deep-water sedi
ments are much more poorly presented in the exposed rock record than are shal
low-water sediments. Deep-water deposits are less abundant than shallow-water
deposits in the exposed rock record because sedimentation rates overall are slow
er deeper water; thus, the sediment record is thinner. [ exception is the sub
marine fan environment near the base of the slope where sedimentation tes
from turbidity currents can exceed 10 m/1000 yr and turbidite sediments can
achieve thicknesses of thousands of meters (e.g., Bouma, Normark, and Barnes,
1985)]. Also, part of the sediment record of the deep seafloor may have been de
stroyed by subduction in trenches, and those deep-water sediments that have es
caped subduction have required extensive faulting and uplift to bring them above
sea level where they can be viewed. Deepwater sediments other than turbidites
have not been studied as thoroughly as shallow-water sediments-perhaps in
part because deep-water sediments have less economic potential for petroleum.
Owing to the advent of seafloor spreading and global plate tectonics concepts,
however, the deep seaoor has taken on enormous significance for geologists.
Consequently, intensive research has been focused on the continental margins and
deep seafloor since the early 1960s. Also, the continuing need to add to our fossil
fuel reserves is pushing petroleum exploration into deeper and deeper water, and
the possibility of mining manganese nodules and metallerous muds from the
seafloor is also causing increased economic interest in the deep ocean.
Deep-sea research has been particularly stimulated by the Deep Sea Drilling
Program (DSDP), which began in 1968 and shifted to the Ocean Drilling Pgram
(ODP) in 1984. Since initiation of these programs, several hundred holes have bn
drilled by DSDP and ODP teams throughout the ocean basins of the world to an
average depth below seaoor of about 300 m ( �1000 ft) and to maximum dths
exceeding 1000 m. In addition to deep coring by DSDP and ODP, many thousands
of shallow piston cores have been collected from the seafloor throughout the ocean
by marine ologists from major oceanographic institutions of the world. Also,
hundreds of thousands of kilometers of seismic profiling les (Chapter 13) have
been run in crisscross pattes across the ooean floor in an attempt to unravel the
sub-bottom structure of the ocean. Although much of this research has been aimed
at
understanding the larger scale feahtres of the ocean basins that illuminate the
origin and evolutionary history of the ocean basins along plate tectonics concepts,
many data on sedimentary facies and sedimentary environments have also been
collected. Much additional new information on ocean circulation and sediment
transport systems has also been generated by oceanographers who study ocean
bottom currents and bottom-water masses. Thus, a significant increase in under
standing of the ocean basins and the deep ocean oor has come about since the
1950s. We shall concentrate discussion here on the fundamental pcesses of sedi
ment transport and deposition on continental slopes and the deep ocean oor and
the principal types of facies developed in these environmen.
Depositional Setting
Continental Slope
e continental slope extends from the shelf break, which occurs at an average
depth of about 130 m in the modern ocean, to the deep seaoor (Fig. 10.12). The
10.3 e Oceanic (Deep-Water) Environment
351
Continental She
Average width 75 km
Average slope 1.7 m/km (0.1 °)
Cont.
:
Slope
1
width 1
110-100 km 1
1 av. slope
1
1 70 m/km
1
I Wl
I
Continental
Ri
Width 0 - 600 km
Slope 1 - 10 m/km (0.05 - 0.6")
i Abyssal
I
I
I Slope
I 1 m/km
I (0.05°)
lower boundary is typically located at water depths ranging from about 1500 to
4000 m, but locally in deep trenches it may extend to depths exceeding 10,000 m.
Continental slopes are comparatively narrow {10-100 km wide), and they dip sea
ward much more steeply than does the shelf. The average inclination of modem
continental slopes is about 4°, although slopes may range from less than 2° off
major deltas to more than 45° off some coral islands.
The origin and internal structure of continental slopes are not of primary
conce here; however, a brief description of differences in the characteristics of
continental slopes on passive {Atlantic-type) and active (Pacific-type) continental
margins is pertinent to succeedg discussion. As shown in Figure 10.4, various
kinds of barriers form the boundary between the continental shelf and the conti
nental slope. The principal kinds of passive continental margins can include mar
gins where siliciclastic sediments drape over basement ridges, folds, or faults (Fig.
10.4 A, B, C); margins with a carbonate bank or platform (Fig. 10.40}, and margins
dominated by salt tectonics (flowage of salt to produce salt domes and diapirs;
Fig. 10.4F). More than one of these passive-margin types may be present within a
given geographic area. Active continental margins may be characterized by the
psence of a fore-arc region only or by both fore-arc and back-arc regions, as, for
example, the Japan margin (Fig. 10.13). Sedimentation can take place in bo back
a and fore-arc basins, on back-arc and fore-arc slopes, and in the fore-arc trench.
Continental slopes may have a smooth, slightly convex surface morphology,
such as that found on passive siliciclastic margins (e.g., Fig. 10.4A) or they may be
irregular on a small to very large scale. Active-margin slopes tend to be particular
ly irregular. For example, the Pacific slope off Japan, which descends to a depth of
about 7000 m into the Japan Trench, is characterized by structural terraces and
basins together with anticlinal welts and fault-bounded ridges arranged in an ech
elon pattern roughly parallel to the Japan coast (Boggs, 1984). These ridges and
folds form prominent structural "dams" behind which sediments are ponded. In
general, structural barriers on highly irregular slopes can hibit movement of
bottom sediment across the slope and create catchment basins for sediment.
Figure 10.12
Principal elements of the
continental margin. [After
Drake, C. L., and C. A. Burk,
1974, Geological signifi
cance of continental mar
gins, in Burk, C. A., and C.
L. Drake (eds.), The geology
of continental margins, Fig.
9, p. 8, reprinted by permis
sion of Springer-Verlag, Hei
delberg.]
Figure 10.13
JAPAN BASIN
\
YAMATO
BASIN
\
•
HON
SHU
(
J
apan
)
Subduction Complex __
Trench
Schematic representation of an ac
tive continental margin (Japan),
showing both the fore-arc and
back-arc characteristics of the mar
gin. [From Boggs, S., Jr., 1984,
Quaternary sedimentation in the
Japan arc-trench system: Geol. Soc.
America Bull., v. 95, Fig. 2, p. 670.]
352
Chapter lO I Siliciclastic Marine Environments
Modem continental slopes are gashed to various degrees by submarine
canyons oriented approximately normal to the shelf break (see Fig. 10.16, 10.19),
which provide accessways for turbidity currents moving across the slope. Most sub
marine canyons have their heads near the slope break and do not cross the shelf;
however, a few major canyons on modem shelves extend onto the shelf and head up
ve close to shore. Some large canyons also extend seaward beyond the base of the
slope to form deep-sea annels that may meander over the nearly flat ocean floor
for hundreds of lometers. The Toyama Deep a Channel in the Japan a, for ex
ample, winds its way across the seaoor from the mouth of the Toyama Trough for
approximately 500 before emptying onto the Japan a abyssal plain.
The origin of submarine canyons has been debated since the early part of the
twentieth century (Pickering, Hiscott, and Hein, 1989, p. 134-136). Although
downcutting by rivers that extended across the shelf during periods of lowered
sea level may have initiated the formation of some canyons on the shelf, turbidity
currents are the main agents of canyon cutting on the slope and deeper seaoor.
Canyon development may be initiated by local slope failure (slumping), followed
by headward growth of erosional scars. Turbidity currents are erosive in their ini
tial stages and thus can deepen and lengthen the incipient canyons over time,
aided by further slumping on the upper part of the slope. The locations and
shapes of some submarine canyons may have been influenced by the presence of
faults and folds (Green, Clarke, and Kennedy, 1991 ).
Continental Rise and Deep Ocean Basin
The continental rise and deep ocean basin encompass that part of the ocean lying
below the base of the continental slope. To gether, they make up about 80 percent
of the total ocean seafloor. The deeper part of the ocean seaward of the continental
slope is divided into two principal physiographic components: the deep ocean
floor, which is characterized by the presence of abyssal plains, abyssal hills (vol
canic hills <1 km high), and seamounts (volcanic peaks > 1 high); and oceanic
ridges. Off passive contental margins, a continental rise (Fig. 10.1) is present at
the base of the slope. The continental rise is a gently sloping surface that leads
gradually onto the deep ocean oor and is built in part from submarine fans ex
tending seaward from the foot of the slope. It commonly has little relief other than
that resulting from incised submarine canyons and protruding seamounts. Conti
nental rises are generally absent on convergent or active margins whe subduc
tion is taking place, such as along much of the Pacific margin. margins of this
type, a long, arcuate deep-sea trench commonly lies at
the foot of the continental
slope, and the rise is absent. Trenches in less active subduction zones, such as
along the Oregon-Washington coast, may be filled with sediment. Abyssal plains
are extensive, nearly flat areas punctuated here and there by seamounts. Some
abyssal
plains are also cut by deep-sea channels, as mentioned. Mid-ocean ridges
extend across some 60,000 km of the modern ocean and overall make up about
30-35 percent of the area of the ocean. Mid-ocean ridges are particularly promi
nent in the Atlantic, where they rise about 2.5 above the abyssal plains on ei
ther side. Rocks on these ridges are predominantly volcanic; the ridges are cut by
numerous transverse fractu zones along which significant lateral displacements
may be apparent. Ridges play a crucial role in the seafloor spreading process, but
they are not particularly active areas of sedimentaon. They do have a very im
portant effect on circulation of deep bottom currents in the ocean and thus have an
indirect effect on sedimentation in the deep ocean.
nanspo and Depositional Processes to and within Deep Wa ter
Most
sedimt deposited deeper water, other than wind-blown sediment, orig
inates on the shelf and must make its way across the shelf (Fig. 10.5, 10.14) to get
t
0
E
E
c
0
�
E
:
�
PROCESSES
Wind
Sediment plume
Floating ice
Rock fall
Creep
Slide
Slump
Debris flow
Grain flow
Fluidized flow
L
iquefied flow
Tu rbidity current
(high/low density)
Internal tides and
waves
Canyon currents
Bottom (contour)
currents
Deep surface
currents
Surface currents
and pelagic settling
Flocculation
Pelletization
Chemogenic
processes
(authigenesis and
dissolution)
CHARACTERISTICS
10.3 e Oceanic (Deep-Water) Environment
353
DEPOSITS
Pelagic mud
Hemipelagic mud
Glaciomarine
(dropstones)
Olistolith
Avalanche deposit
Creep deposit
Slide
Slump
Debrite
Grain flow deposit
Fluidized flow deposit
Liquefied flow deposit
Turbidite (coarse,
medium, and
fine-grained)
Normal current deposit
Contourite
Pelagic ooze
Hemipelagic mud
FeMn nodules, lami
nation, pavements,
and umbers
Figure 10.14
The various kinds of processes that operate in
the deep sea to transport and deposit sedi
ments. (After Stow, D. A. V., 1994, Deep sea
processes of sediment transport and deposi
tion, in Pye, K. (ed.), Sediment transport and
depositional processes, Blackwell Scientific Pub
lications, Oxford, Fig. 8.2, p. 261, reproduced
by permission.]
to deeper water environments. Across-shelf sediment movement (discussed under
shelf transport in preceding sections) includes transport of coarser sediment by
turbidity currents and seaward advection of fine sediment by sediment plumes
and nepheloid flows. A variety of processes is capable of transporting and de
positing sediment within the deep ocean, such as wind transport from continents,
airfall and submarine settling of pyroclastic particles generated by explosive vol
canism within and outside ocean basins, sediment plumes, floating ice, mass-ow
processes, various kinds of bottom currents, surface currents, and pelagic settling
(e.g., Stow, 1994). These processes are summarized in Figure 10.14. The locations
within the deep ocean where the various processes operate are summarized more
graphically in Figure 10.15.
Sediment Plumes, Wind Transport, Ice Raing, Nepheloid Tr ansport
Where continental shelves are narrow, freshwater surface plumes carrying fine
sedimt across the shelf (Fig. 10.5) can move considerable distances into deeper
water, possibly as far as 100 km offshore (Reineck and Singh, 1980), before mixing
354 Chapter 10 I Siliciclastic Marine Environments
Figure 10.15
Schematic representation of principal processes responsible for transport and deposition
of sediments to the deep ocea n. Note that most of the processes deposit fine sediment;
however, glacial (floating ice), turbidity current, and resedimentation processes can move
both coarse and fine sediment. Chemogenic refers to minor processes that are largely
chemical in nature. [After Stow, D. A V., H. G. Reading, and ). D. Collinson, 1996, Deep
seas, in Reading, H. G. (ed.), Sedimentary environments: Processes, facies and stratigra
phy, Blackwell Science Ltd., Oxford, p. 39553, reproduced by permission.]
and flocculation cause clay particles to settle. Winds blowing over continents, par
ticularly desert areas, can also transport fine suspended dust particles seaward,
where they settle out over the ocean hundreds of kilometers from shore. fact,
wind transport may be the primary mechanism by which day-size siliciclastic
sediment is transported to the distal part of the deep ocean.
During glacial episodes of the Pleistocene when sea levels were low and
many land areas were covered by ice, rafting of sediment of all sizes into deeper
water by icebergs was a parcularly important transport process. Ice transport is
still going on today on a more limited scale at high latitudes in the Arctic and
Antarctic regions (e.g., Anderson and Ashle 1991; Dowdeswell and Scourse,
1990; Kempema, Reimnitz, and Barnes, 1988). Melting of the floating ice dumps
sediments of mixed sizes, commonly referred to as glacial-marine sediment
(Chapter 9), onto the shelf and the deep ocean floor. The overall quantitative sig
nificance of iceberg transport into deep water through geologic time has probably
not been significant, but locally and at certain times it may have been important.
Fe sediment resuspended by storms on the outer shelf can move off the
shelf and down the slope in near-bottom nepheloid suspension. Less-dense sus
pensions may move seaward along density interfaces as midwater suspensions
that gradually settle to the bottom (Fig. 10.15). Fine sediment may also be injected
into the water column by turbidity flows moving downslope. Nepheloid flows are
reported to extend seaward for hundreds of kilometers and to water depths of
6000 m or more. Deep bottom currents ( be discussed) may aid in resuspending
sediment into nepheloid layers.
Cuents in Canyons
Tidal curren measured in submarine canyons at depths exceeding 1000 m may be
capable of transporting silt and fine sand (Shepard, 1979; Pickering, Hiscott, and
Hein, 1989, p. 143). Two types of currents have been detected in submarine valleys:
10.3 The Oceanic (Deep-Water) Environment
355
ordinary tidal currents that rarely exceed 50 cm/s and that flow alternately up
and down the valley in response to tidal reversal, and occasional surges of strong
downcurrent flow with velocities up to 100 cm/s. Shepard (1979) interprets the
surges as low-velocity turbidity currents. There are few data as yet to support the
quantitative importance of net down-canyon transport owing to tidal currents;
however, surge currents of the magnitude measured by Shepard are certainly ca-
pable of transporting ne sediment seaward. Together, these currents probably
help to winnow canyon deposits and keep them free from fine sediment.
Contour Currents
Density differences in surface ocean water caused by temperature or salinity vari
ations create vertical circulation of water masses in the ocean commonly referred
to as thermohaline circulation. Circulation is initiated primarily at high latitudes
as cold surface waters sink toward the bottom, forming deep-water masses that
flow along the ocean floor as bottom currents. The path of these bottom currents is
influenced by the position of oceanic ridges and rises and other topographic fea
tures such as narrow passages through fracture zones. Owing to density stratica
tion of ocean water, bottom currents adjacent to continental margins tend to flow
parallel to depth contours or isobaths and thus are often called contour currents.
The movement of these currents is also affected by the Coriolis force, which like
wise tends to deflect them (left in the Southern Hemisphere and right in the
Northe Hemisphere) into paths parallel to depth contours; thus, they are some
times also called geostrophic contour currents.
In the modem ocean, Antarctic bottom water runs down the continental
slope, circulates eastward around the Antarcc continent possibly several mes,
and then flows northward into the Atlantic, Indian, and Pacific ocea (Stow, 1994).
In e North Atlantic, deep bottom water flows south out of the Norwegian
Greenland seas, Labrador Sea, and other parts of the North Atlantic. Interaction of
ese deep-water masses creates a highly complex ocean circulation system.
Because contour currents are best developed in areas of steep topography
where the bottom topography extends throu the greatest thickness of stratified
water column (Kennett, 1982), they are particularly important on the continental
slope and rise. Photographs of the deep seafloor have revealed current ripples in
some areas and suspended sediment clouds and seafloor erosional features in oth
ers, both of which suggest that some contour currents can achieve velocies on or
near the seafloor great enough to erode the seabed and transport sediment. Evi
dence is now available (e.g., Hollister and Nowell, 1991) which suggests that the
speed of these bottom currents may be accelerated in some parts of the ocean to
velocities on the order of em/ s, perhaps because of the superimposed influence
of large-scale, wind-driven circulation at the ocean's surface. That is, eddy kinec
ergy may be transmitted from the surface of the ocean to the deep seafloor. In
tsification may also occur where the Coriolis Force causes deep flows to bank
up against the continental slope on the westem margins of ocean basins, where it
is unable to move upslope against gravity and thus becomes restricted and inten
sified (Stow, 1994). Where bottom currents are intensified, resulting motions near
e seafloor are so energetic that they have been referred to as "abyssal storms" or
"bthic storms" (Hollister and Nowell, 1991), particularly because huge amounts
of fine sediments are stirred up and transported by these energetic pulses. Con
tour currents are believed to have had a particularly important role in shaping and
modifying continental rises, such as those off the eastern coast of North America.
Pelac Rain
Calcareous- and siliceous-shelled planktonic organisms settle through the ocean
water column to the seafloor upon death, a process called pelagic rain or pelagic