Boggs S. Principles of Sedimentology and Stratigraphy
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336
Chapter 10 I Siliciclastic Marine Environments
Fire 10.3
Subdivisions of the conti
nental shelf. [Modified from
Galloway, W. E., and D. K.
Hobday, 1983, Terrigenous
clastic depositional systems,
Fig. 7.1, p. 144, reprinted
by permission of Springer
Verlag, Heidelberg.]
North Sea, Yellow Sea, and Timor-Arafura Sea). We may assume that similar sedi
mentological processes operated on continental shelves and epiconnental sea
ways, but, in fact, differences exist between these two environments. For example,
epicontinental seas received sediments from nearly all sides, whereas continental
shelves receive sediments from only one side. Furthermore, the wave and current
regimes in epicontinental seas may have been different from those on shelves.
addition, modern continental shelves may not provide a good analog for ancient
marginal seas because rapid rise of sea level following the final episode of Pleis
tocene glaciation has stranded coarse sediment in deeper parts of the shelves, cre
ating conditions of sediment-water disequilibrium. Thus, sediment grain size on
some parts of modern shelves is not consistent with present water depth, energy
conditions, and sedentaon processes on the shelves.
Both siliciclastic and carbonate sediments can accumulate in the marine
shelf environment, although most modern continental shelves are covered by
siliciclastic sediments. Carbonate sediments (Chapter 11) are restricted to a few
shelves, mainly (but not exclusively) tropical areas.
Physiography and Depositional Setting
The siliciclastic shelf environment is bounded by various coastal environments on
the landward side and by the continental slope on the seaward side. It can di
vided into the shallow inner shelf, which is dominated by tidal, wind-driven,
and storm-wave processes; the middle shelf; and the deeper-water outer shelf
(Fig. 10.3). Wright (1995) suggests that the inner shelf extends offshore to depths
of about 30 m; however, the boundaries between the inner, middle, and outer
shelves are not well defined and their positions uctuate with changing sea level.
In fact, during greatly lowered sea level, the normal inner and middle shelves are
subaerially exposed, and the outer shelf may be exposed or covered only by very
shallow water.
The width of shelves varies according to their plate-tectonic settings (Sh
ard, 1973; Eisma, 1988). Shelves along the fore-arc region of convergent continen
tal margins tend to be very narrow. By contrast, broad shelves and platforms occur
in the back-arc basins of convergent margins, on divergent or trailing-edge conti
nental margins, and on cratonic downwarps that open to the sea. Because conti
nental margins may evolve from diverse structural origins, the barrier forming the
shelf break, which marks the transition from shallow shelf to steep slope, may also
have diverse origins as shown in Figure 10.4. It may have formed initially as a
basement ridge, fold belt, fault block, reef, volcanic ridge, or a diapir (a salt or
mud intrusion). With time, sediments fill the depression behind the shelf-margin
.. · .
.
10.2 e Shelf Environment
337
Basement
(A)
Shelf
ridge
Sea level
Vo lcanic ridge
(
C)
Fault block
barrier and may drape over the shelf break onto the slope. During this process,
shelf sediments build or aggrade upward to wave base, the depth below which
wave-generated processes have little effect on sediment movement. Once wave
base is reached by an aggrading prism of sediment, the shelf achieves a state of
near equilibrium (Swift and Thorne, 1991) by which little further deposition
curs unless additional subsidence takes place or sea level rises (see also Wright,
1995, Chapter 2). That is, under equibrium conditions, the sediment simply
moves across the shelf (bypassing) on its way toward the slope. The mechanisms
responsible for movement of sediment across the shelf are considered in the next
section. Although shelves are fundamentally low-relief platforms, the shelf sur
face can vary considerably. It may be relatively smooth or covered by a variety of
small- to large-scale bedforms. It may also contain banks, islands, or shoals near
its offshore edge (Bouma et a!., 1982).
Shelf Sediment a nspo and Deposition
Waves are constantly moving from deeper water in the open ocean across the shelf
to the shore zone; there, they eventually break and become translated into wave
swash and longshore currents. Given that the direction of wave movement across
the shelf is dominantly shoreward, how is it that sediment can apparently move
across the shelf in a seaward direction? Johnson (1919) suggested the idea of a
"graded shelf," which he believed to display progressive decrease in grain size
om coarse at the shoreline to very fine at the shelf edge, in response to presumed
decrease in water energy seaward. The graded-shelf concept for modern shelves
eventually fell into disfavor when it became obvious with additional study in the
1930s-1960s that the grain-size distribution on many modem shelves is patchy or
irregular. The concept of relict shelf sediments was introduced to explain such ir
regular disibution pattes as the presence of coarse sands and gravels in deep
water. Relict sediments are deposits that are apparently not in equilibrium with pre
nt hydrodynamic condions. They were deposited on the shelf by fluvial glacial
processes during low stands of sea level. Relict sediments were initially believed to
main on the shelf floor without significant reworking as they were inundated by
sing sea level (Shepard, 1932; Emery, 1968). It was subsequently recognized, how
ever, that some so-called relict sediments had likely been reworked to some extent
Fi 10.4
Various kinds of structural
barriers that form the sea
ward margins of continental
shelves. [After Hedberg, H.
D., 1970, Continental mar
gins from viewpoint of the
petroleum geologist: Ameri
can Association Petroleum
Geologists Bull., v. 54, Fig.
18, p. 22, reproduced by
permission.]
338
Chapter 10 I Siliciclastic Marine Environments
Figure 10.5
Schematic representation of
the major physical processes
that operate on the shelf to
transport sediment. Based on
Nittrouer
and Wright, 1994;
Swift et al., 1986; Swift and
Thorne, 1991; and Vincent,
1986.
dung sea-level rise. This reworng caused the sediments to be brought into par
tial or complete equilibrium with present shelf processes and conditions. Such re
worked sediment, having partly relict and partly modem characteristics, was
called palimpsest (Swi, Stanley, and Curray, 1971).
Studies of shelf sedimentation since the 1960s suggest that purely relict sedi
ments may be less common than originally thought and that Johnson's concept of
a graded or equilibrium shelf may still have merit (e.g., Joson and Baldwin,
1996). Also, many ancient shelves may not have been affected by drastically low
ered sea levels. Therefore, it becomes necessary to consider what mechanisms
exist on the shelf that allow seaward transport of sediment across shelves during
high stands of sea level, as at the present time. We know that modem shelves are
affected by waves and storms, tidal currents, major surface ocean currents that in
trude onto the edges of some shelves, and possibly density currents (e.g., Swift,
Stanley; and Curray, 1971). We commonly divide shelves into two main types:
wave- and storm-dominated, also referred to as weather-dominated, and tide
dominated, although most shelves a actually inuenced by a mixture of processes.
Approximately 80 percent of modem shelves are wave- and storm-dominated and
17 percent are tide-dominated (Swift, Hans, and Vincent, 1986). About 3 percent of
shelves are dominated by intruding ocean currents; density currents play only a
minor role in shelf transport.
Wave- and Storm-Dominated Shelves
A complex spectrum of transport presses operate on wave- and storm-dominated
shelves, including fair-weather waves and swells, storm waves, wind-driven sur
face currents, river-generated plumes, and density currents. These processes a
summarized graphically in Figure 10.5. During fair weather, waves generated locally
by wd move across the shelf from deeper water onto the shallow-water inner
shelf. As briefly described under beach processes in Chapter 9, these waves have
oscillatory moon that causes water to move in nearly circular orbits. As a wave
passes, water moves forward in the crest of the wave, then downward, and finally
backward under the trough and upward. Thus, individual water particles do not
move forward with the passing wave but simply trace an orbital path (Fig. 10.6a).
Fa ir- We ather Wa ves
Orbital motion of water generated by passage of these orbital waves dies out
downward at a depth equal to about one-half of the wave length (distance between
(a)
!
0
·
0
�
i
!
€
0
0
0
)
Grain movement
Time
Grain movement
Figu 10.6
10.2 The Shelf Environment
339
Behavior of oscillatory waves in shoaling water. (a) Flatten
ing of orbits as waves enter water shallower than about
one-half wave length. (b) Time-velocity record of bottom
flow during passage of a shoaling wave. The landward
stroke as the crest passes has higher veloci and moves
more sediment than does the return stroke associated with
the passage of the trough. [After Swift, D. J. P., and J. A.
Thorne, 1991, Sedimentation on continental margins, 1: a
general model for shelf sedimentation, in Swift, D. J. P., G.
Oertel, R. W. Tillman, and J. A. Thorne (eds.), Shelf sand
and sandstone bodies: Geomet, facies, and sequence
stratigraphy: International Association of Sedimentologists
Spec. Publ. 14, Blackwell, Oxford, Fig. 5, p. 12, repro
duced by permission.]
wave crests). This depth is referred to as the wave base (Fig. 9.21). Fair-weather wave
ba is commonly on e order of 10-15 m. Because orbital motion in deep water is
unimpeded by the bottom, orbits a nearly circular. a wave moves into shallow
water, where depth is less than one-half the wave length, the bottom begins to inter
fere with orbital moon and thus begins to affect the shape of the orbits. By the time
that waves reach very shallow water, where depth is Jess than about 1/20 the wave
length, the moon of the particles is strongly affected by interaction with the bottom,
and the orbits become much more elliptical (Fig. 10.6a). They become progressively
atter downward below the surface til near bottom they are essentially linear, gen
erang a to-and-o oscillating motion as waves pass. This motion produces bidirec
tional flow of water along the seaoor as each wave passes over the surface. The
velocity of this bottom flow referd to as the orbital veloci because it vaes di
rectly as a function of the magnitude of the orbital diameter and indirectly as a func
on of the wave period (the me required for passage of one wave lgth).
Orbital velocity is commonly greater in one direction than the other. This dif
ference in velocity becomes important when the stronger velocity flow exceeds the
threshold of movement for grains, resulting in net transport of grains in one direc
on (Fig. 10.6b). Because the fair-weather wave base is shallow, movement of sed
iment by fair-weather waves takes place mainly on the innermost shelf and is
dominantly in an onshore direction. The eventual breaking of waves in very shal
low water generates currents (wave swash), which also move in an onshore direc
tion, as well as longshore currents directed laterally along shore (Chapter 9). This
overall shoreward movement of water in the nearshore zone above the fair-weather
wave base creates a "littoral energy fence" (Swift and Thorne, 1991) that tends to
ap sediment in the nearshore zone. For sediment to move from land onto the
shelf, it must escape through the littoral energy fence by a mechanism such as
river-mouth bypassing that occurs as a flood-stage jet of sediment-laden water
from a river mouth (Fig. 10.5) or rip currents (Fig. 9.22).
Swells, Storm Wa ves, and Wi nd-Forced Currents
Swells are low-relief, long-period, long-wave-length waves generated by storms
at may originate far out to sea. When swells move onto the shelf they tend to
340
Chapter 10 1 Siliciclastic Marine Environmen
"sr" the bottom to greater depths than do fair-weather waves. Storms (seasonal
storms, typhoons, hurricanes) that move across a shelf have an even greater effect
on shelf transport and deposition. First, highly energetic storm waves in the shore
zone, perhaps accompaed by elevated tides, vigorously erode the beachface and
upper shoreface. During such strong storms, beach sediment is flushed seaward,
probably mainly by a combination of storm-enhanced rip currents and do
welling currents that evolve from wind-driven or wind-forced currents.
Wind-forced currents are unidirectional currents generated by wind shear
stress as wind blows across the water surface, gradually putting into motion deep
er and deeper layers of water (Ekman transport). Deeper layers of water are de
flected by the Coriolis force, so that their direction of movement diverges from
that of surface layers. The Coriolis force is generated by Earth's rotation, causing
moving objects to be deflected to the right in the Northe Hemisphere and to the
left in the Southern Hemisphere. If the velocity and duration of wind are great
enough, water movement may extend to the seabed with enough velocity to trans
port sediments. Strong winds commonly create wind-forced curren that flow
parallel to shore and therefore do not provide much offshore sediment transport.
If, however, currents moving along the shoreline are deected landward owing to
the Coriolis force, an onshore pile-up of water takes place (e.g., the Oregon Coast
during winter). Piling up of water onshore creates an elevation of the water sur
face-a coastal setup (Fig. 10.5)-of perhaps a meter or two. This setup can ap
parently be enhanced by very low atmospheric pressure. The different water
levels at the coast and offshore result in a hydrostatic pressure difference on e
ocean floor at drives a bottom flow seaward (downwelling). As the bottom
water flows seaward, it is deflected laterally to form a geostrophic current. This
current initially moves obliquely offshore but subsequently veers around, owing
to the Corio lis force, to assume a direction roughly parallel to the bathymetric con
tours or isobaths, that is, roughly parallel to the shoreline.
These geostrophic flows can achieve velocities at water depths of 10-20 m of
as much as 60 cm/s (Walker and Plint, 1992). Flows of this magnitude may not be
capable of transporting much sandy sediment unless they are accompanied by
strong wave-driven oscillatory motion at the bed. Oscillatory wave motion can
provide the shear stress needed to lift grains off the bottom, and those grains are
then transported by the geostrophic currents (Snedden, Nummedal, and Amos,
1988). Unidirectional and oscillatory currents operating together are called com
bined flows. Tropical storms and hurricanes accompanied by strong winds that
blow directly onshore can generate higher coastal setups than those generated by
seasonal storms, and thus create much songer seaward-flowing currents. Cur
rent-flow velocities of as much as 2 m/s have been reported on shelves during
some tropical storms (Morton, 1988), and currents reaching velocities of 2 m/s
have also been recorded flowing down submarine canyons (Hubbard, 1992). Sed
iment moves obliquely offshore, and some sand moves from the beach shoreface
into offshore settings. Note again, however, that such transport by geostrophic
currents tends to be mostly parallel to isobas and not directly seaward. There
fore, sediment may not be transported by such currents to any great distance out
ward onto the shelf.
Storm waves also affect sediment movement in deeper water on the middle
and outer shelf. Because of their longer wave length and period, storm wav
moving across a shelf may be able to rake the seafloor to depths of as much as 200
m (Komar et al., 1972). The orbital velocities generated by these storm waves may
be nearly equal so that no net seaward transport of sediment occurs; however, the
waves resuspend bottom mud and tend to spread or dissipate it around the
seafloor. the other hand, net sediment transport in either a seaward
or land
ward direction can occur.
Sediment Plumes
10.2 The Shelf Environment
341
Sediment discharged at river mouths into the ocean may be carried onto the shelf
eier as buoyant plumes or underflows (bottom plumes), as suggested in Figure
10.5. Buoyant (hypopycnal) plumes commonly develop where low-salinity water
issuing from rivers and estuaries flows out on top of higher-salinity seawater.
These plumes generally do not reach farther than the inner or middle shelf before
being carried parallel to the coast owing to the Coriolis force (e.g., Nittrouer and
Wright, 1994). There the suspended fine sediment (coarser sediment drops out
near the river mouth) gradually settles to the shelf floor.
Underows (hyperpycnal flows) are generated where the mouths of rivers
carry unusually high suspended-sediment loads, causing e inflowing river
water to be more dense than the ocean water. These ows are, in fact, the steady
flow turbidity currents discussed in Chapter 2. Many of these underflows may be
checked
rather quickly, resulting in most of the sediment being deposited near the
river mouth. Some may be carried greater distances onto the inner shelf, however,
and apparently need not be confined by submarine canyons (Seymour, 1990).
Nepheloid Flow
A nepheloid layer is a turbid body of suspended sediment that may reach heights
of several hundred meters above the seafloor. These layers were first surveyed
and named by Ewing and orndike (1965), who discovered them by using an op
tical nephelometer to measure light scattering at various levels in the water col
umn. A nepheloid layer is more dense than the surrounding ambient water but
not dense enough to sink rapidly. Thus, sediment may remain suspended in such
layers for a long period of time. Most of e material of nepheloid layers consist
s
of very fine clay particles. Some of this material may have reached the nepheloid
layer directly by settling through the water column from buoyant plumes. Most of
it is probably fine sediment resuspended from the ocean floor owing to erosion of
e seabed by storm waves (Fig. 10.5), possibly swell conditions, or it is fine ma
rial injected into the water column by turbidity currents or other mechanisms.
ing to its low settling velocity, fine sediment may remain in suspension in the
nepheloid layer for periods ranging from days to weeks in the lowest 15 m of the
water column and from weeks to months in the lowest 100 m (Kennett, 1982).
The turbid nepheloid layer is slowly advected (flows laterally) seaward as a kind
of density ow. Sedimentation and resuspension would likely occur many times
before fine sediment could be moved completely across a shelf by this process.
Sediment Characteristics of Stonn-Dominated Shelves
mentioned, storm-dominated shelves predominate most of the world's coas.
ey are characterized by low tidal cuent velocities (commonly <25 cm/s), and
fair-weather wave base is normally shallow (�10 m). Because of these characteris
tics, little coarse sediment moves on these shelves except during intense storms.
Examples of modern storm-dominated shelves include the Atlantic shelf off the
easte coast of the United States, the Pacific shelf o Oregon and Washington,
and the Bering a.
Sedimentation pattes on storm-dominated shelves may be quite complex,
depending in part upon the extent to which the shelves are mantled by relict sedi
ments or modern sediments. Shelves with abundant relict sediment, such as the
Atlantic, are characterized particularly by sand bodies. The most controversial
sand bodies, with respect to their origin, are shelf sand dges. Modern shelf sand
ridges are elongate, coastal- to shelf-sand bodies that are larger than subaqueous
dunes (Chapter 4), with lengths on the order of 10 km and heights that are more
342
Chapter 10 I Siliciclastic Marine Environments
Figure 10.7
Schematic comparison of
idealized coarse-grained
storm beds and fine-grained
hummocky cross-stratified
beds on storm-dominated
shelves. The lengths of the
current vectors are propor
tional to the strength of the
current in a given direction
rather than duration. [From
Cheel, R. ]., and D. A. Leck
ie, 1992, Coarse-grained
storm beds of the Upper
Cretaceous Chungo Mem
ber (Wapiabi Formation),
southern Alberta, Canada:
)our. Sed. Petrology, v. 62,
Fig. 14
,
p. 943, reproduced
by permission of Society of
Economic Paleontologists
and Mineralogists, Tulsa,
Okla.]
than 20 percent of the water depth (Snedden and Dalrymple, 1999). These ridges
are also present on tide-dominated shelves and share many similarities. Snedden
and Dalrymple suggest that shelf sand ridges pass rough three stages of devel
opment: an initial irregularity (the ridge nucleus) forms by coastal or shelf
processes; storm- or tide-driven nearshore/shelf currents interact with this irregu
larity, causing upward growth and down-current migration of the incipient ridge;
and the ridge evolves as a result of continued current action. Shelf sand ridges are
most likely to form during transgressions. Thus, most shelf sand ridges probably
overlie transgressive ravinement (erosion) surfaces.
Shelves with a greater component of modern vs. relict sediments, such as the
Pacific shelf off Oregon and Washington, are typically characterized by less relief
and a greater proportion of finer grained sediments (muds) than are Atlantic-te
shelves. Although sediment on these shelves may display a general trend of sea
ward fining, relict sands or gravels show through extensive "windows" in a dis
continuous blanket of muddy modern sediment. Also, mixing of relict sands and
modern muds may take place in some areas. Muds are typically thoroughly bio
turbated.
Coarse-grained "storm layers" and hummocky cross-stratification are sedi
mentary structures that appear to be especially characteristic of storm-dominated
shelf sediments. "Storm layers" are commonly thin layers consisting of concena
tions of coarser grains interlayered or embedded in finer grained muds (Fig. 10.7).
The coarser material typically consists of coarse silt, fine sand, shell fragments or,
less commonly, gravel. The layers characteristically show vertical size grading.
The exact origin of these layers is controversial; however, Cheel and Leckie (1992)
suggest that they form by a two-stage process: transport from a beach by offshore
directed, storm-generated combined (geostrophic) ows, followed by reworking
and selective sorting of bed material by asymmetrical oscillatory currents generated
by shoaling swell waves propagating onshore (Fig. 10.7). Storm layers, also called
tempestites, are described in considerable detail by Aier (1985). They are best
developed on the inner shelf, but they have been found as far as 40 km from the
coast (Reineck and Singh, 1980).
Shallow-marine Storm Beds
Coarse-grained
Storm Beds
Interpretation
Fine-grained
Storm
Background and
Waning Storm sedimentation
Background
sedimentation
s t. IR I
Fl
ne
.
ymme nca lPP
·:� Conglomerate
Coae *· .
Sole Marks
D Sandstone
Intraclast
• Shale
·��
Hummocky
cross-stratification
Horizontal Lamination
Unidirectional current
Oscillatory __
�urrent
Current vectors
symmetrically
oscillating
asymmetrically
oscillat
l
n
10.2 The Shelf Environment
343
Hummocky cross-stratification has been identified in few, if any, modem
shelf sediments, but it has been described in numerous ancient shelf sediments
ranging in age from Precambrian to Pleistocene. It is thought to be particularly
characteristic of shelf sediments, although it has been described also in shoreface
(beach) and some lake sediments. As discussed in Chapter 4, it consists of curving,
gently dipping laminae, both convex-up (hummocks) and concave-up (swales),
that
intersect at a low angle (Fig. 10.7). It is commonly interbedded with biotur
bated mudstones. Most workers appear to agree that hummocky cross-stratification
fos as a result of storm waves acting in some manner below fair-weather wave
base; however, the exact mechanism of formation remains controversial (see re
view in Duke, Amott, and Cheel, 1991).
Tide-Dominated Shelves
Ti dal Processes
As discussed in Chapter 9, tidal processes can affect sedimentation on tidal flats,
in estuaries, and on deltas. They also strongly influence sedimentation on some
continental shelves. Vertical rise and fall of tides is accompanied by horizontal
movements of water (Howarth, 1982; The Open University Te am, 1989) that we
refer to as tidal currents. The currents generated on the shelf by tides are bidirec
onal but asymmetrical with respect to velocity. at is, flood-tide and ebb-tide
velocities are commonly different. Asymmetrical currents may result in net sedi
ment transport in the direction of the stronger current. If both current phases are
able to transport sand, herringbone cross-stratification and reactivation surfaces
may form (see tidal flat systems, Chapter 9). Also, tidal rise and fall in some envi
roents can produce couplets of sand-mud laminae, with mud deposited on top
of sand during a standstilL Stacking of these couplets produces tidal successions
referred to as tidal rythmites or tidalites (e.g., Dalrymple, Makino, and Zaitlin,
1991; Smith et al., 1991; Alexander, Davis, and Henry, 1998). Such beds may show
Box 10.1 Tides
Tides are generated by the gravitational attraction of the moon and sun for
Earth in conjunction with the rotation of Ear. Tidal influence is manifested at
any given coastal locality by daily rise and fall of the sea over an average range
of about 1 m on open coasts, but tidal range may exceed 15 m in some en
closed basins (e.g., the Bay of Fundy, Nova Scotia). Some localities (e.g., the At
lantic coast of the United States} experience semidiumal tides, characterized
by two highs (of approximately equal height) and two lows (of approximately
equal height) each day. Others, owing to configuration of the generation basin
and complexities of the shoreline (e.g., parts of the Gulf of Mexico coast), have
diual tides, distguished by one high and one low each day. Still others
(e.g., the United States, Pacific coast) have mixed tides-two highs (of unequal
height) and two lows (of unequal height) each day. Alignment of the sun and
moon (new or full moon) gives rise to spring tides that are about 20 percent
higher than normal. When the sun and moon are at right angles in relation to
e Earth, neap tides that are about 20 percent lower than normal result. Tidal
currents on continental shelves are propagated as a large wave or tidal bulge
generated in deep ocean basins (Fox, 1983). In major ocean basins, this tidal
bulge rotates around a central point of no tidal movement called an
amphidromic point. The tidal wave follows an elliptical path that is almost cir
cular in the open ocean. more restricted areas, the ellipse is strongly elongat
, forming a narrow, rectilinear pattern.
3 Chapter 10 I Siliciclastic Marine Environments
cyclic changes in thickness that may represent neap-tide and spring-tide varia
tions in tidal current velocity.
Tidal-current velocity decreases with water depth; thus, tidal-cuent trans
port is most important in shallow water. Tidal-current velocities ranging up to
about 2 m/ s have been measured in some enclosed basins-the Bay of Fundy, for
example. Tidal currents on some shelves, such as those around the British Isles,
have velocities that may exceed 1.5 m/s; however, tidal velocities on most shelves
are less than about 1 m/s. Even so, close to the seabed many tidal currents are
strong enough to rework and transport significant quantities of sand and possibly
gravel. On the other hand, tidal-current velocities on some shelves are so low that
they are below the threshold velocities required for sediment entrainment and
transport. Much of the movement of sediment by tidal currents occurs when tidal
currents are aided by wave action. The orbital motion of waves may be sufficit
to lift grains off the seaoor, which are then transported some distance by currents
too weak to move the grains unaided (Komar, 1976; The Open University Te am,
1989). An outstanding example of a modem shelf dominated by strong tidal cur
rents is under the North Sea, which lies between the United Kingdom and the
coasts of Denmark and Norway.
Sediments of Tide-Dominated Shelves
As discussed, tide-dominated shelves are distinguished by the presence of tidal
currents with velocities ranging from about 50 to more than 150 cm/s. Modern ex
amples include the North Sea; the Korea Bay of the Yellow Sea; the Gulf of Cam
bay, India; the shelf around the British Isles; Georges Bank in the outer part of the
Gulf of Maine; and the northern Australia shelf. Tide-dominated shelves are char
acterized particularly by sand bodies of various types and dimensions. Large sand
waves (dunes) a few meters to more than 20 m high with wave lengths of tens to
hundreds of meters typically occur in elds that may cover areas of 15,000 km
2
or
more. Sand waves may have symmetrical cross-sectional shapes if produced by
tidal currents with equal ebb and flood peak speeds; however, asymmetrical
shapes caused by unequal ebb and flood velocities are more common (Belderson,
Joson, and Kenvon, 1982). Tidal sand ridges similar to the shelf sand ridges p
sent on wave- and storm-dominated shelves are also common. For example, such
ridges,
covering areas up to 5000 km
2
, have been reported from the North a
shelf (e.g., Swift, 1975). In addition to sand waves and shelf sand ridges, tide
dominated shelves also include sand sheets, sand patches, and gravel sheets, all
charactezed by small-scale bedforms, and patches of bioturbated muds in areas
sheltered from tidal currents and waves (Stride et a!., 1982).
Because most of the shelf is constantly covered by water, the characteristics
of sand waves, sand ridges, and other bedforms on modern shelves must be stud
ied largely by indirect methods. Small-scale bedforms can be observed and pho
tographed by divers or by remote-controlled cameras. Larger bedforms are
invesgated by sonar bottom-profiling and side-scan sonar techniques (e.g.,
Belderson, Johnson, and Kenyon, 1982). Small-scale, inteal sedimentary struc
tures can be studied in cores of bottom sediment, and sub-bottom seismic prol
ing methods, described Chapter 13, may be used to study some large-scale
features such as bedding. None of these methods allows detailed examination of
modern shelf structures. The idealized distribution of bedforms along e
sedi
ment transport path on tide-dominated shelves is illustrated in Figure 10.8. At
high tidal velocities of about 150 cm/s, the seafloor may be eroded, leaving fur
rows and gravel waves. With progressively diminishing velocity farther down the
transport path, eroded sediments are deposited to form flow-parallel sand rib
bons, large dunes, small dunes, a rippled sand sheet, and finally sand patches.
Sand ridges may form in the dune belt if enough sand is present.
1
0.2 e Shelf Environment
345
Figure 10.8
Idealized sequence of bedforms devel
oped
along a sediment transport path
on a tide-dominated shelf. Maximum
spring-tide current velocities associated
with each bedform type are shown
along the edges of the diagram. Sand
ridges may form in the dune belt if suffi
cient sand is present. [After Belderson, R.
H., M. A. johnson, and N. H. Kenyon,
1
982, Bedforms, in Stride, A. H. (ed.),
Offshore tidal sands: Chapman and Hall,
London, Fig. 3.1, p. 28. Reproduced by
permission.]
Most tidal shelf sands are charactered by cross-bedding. Small-scale cross
bedding and ripple cross-lamination, produced by migration of ripples and small
dunes, and large-scale cross-bedding generated by migration of dunes and sand
ridges are both common. Foreset dip directions of cross-lamination may be bi
directional or unidireconal depending upon tidal inuence. Plane beds also devel
op under some upper-flow-regime flow conditions. Physical structures thus tend
to
dominate tidal shelf sands,
which typically display fewer bioturbation struc
tures than do muddy shelf sediments (deposited under other conditions), which
are commonly highly bioturbated with few physical structures except possibly
planar lamination.
Shelves Affe cted by Intruding Ocean Currents
Major surface ocean currents flow through the world oceans like gigantic rivers,
driven by the prevailing wind system around Earth (e.g., Open University Course
Te am, 1989). These prevailing winds, in conjunction with the configuration of the
continents and the Coriolis force arising from Earth's rotation, drive the ocean cur
nts into gigantic circulating cells, or gyres, that rotate clockwise in the Northe
Hemisphere (North Pacific and North Atlantic) and counterclockwise in the
Southern HemLsphere (South Pacific, South Atlantic, Indian Ocean). The Gulf
Stream System off the southeast and east coast of the United States, the Kuroshio