Walker J. Facies models, Canada, 1992

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DALRYMPLE

Hayes, M.O., 1979, Barrier island mor-

phology as a function of tidal and wave

regime,

Leatherman, S.P., ed.,

Barrier islands

from the Gulf of St.

Lawrence to the Gulf of Mexico: New

York, Academic Press, p. 1-27.

Hein, F.J., 1987, Tidalllittoral offshore shelf

deposits

Lower Cambrian Gog

Group, southern Rocky Mountains,

Canada: Sedimentary Geology, v. 52,

p. 155-1 82.

Houbolt, J.J.H.C., 1968, Recent sediments

in the southern bight of the North Sea:

Geologie en

Mijnbouw, v. 47, p. 245-

273.

Houthuys, R. and Gullentops, F., 1988, The

Vlierzele Sands (Eocene, Belgium):

tidal ridge system,

de Boer, P.L., van

Gelder, A. and Nio, S.D., eds., Tide-in-

fluenced sedimentary environments and

facies: Boston, D.

Reidel Publishing

Company, p. 139-1 52.

Howarth, M.J., 1982, Tidal currents of the

continental shelf,

Stride, A.H., ed.,

Offshore tidal sands: processes and de-

posits: New York, Chapman and Hall,

p. 10-26.

Johnson, H.D. and

Baldwin, C.T., 1986,

Shallow siliciclastic seas,

Reading,

H.G., ed., Sedimentary environments

and facies: Oxford, Blackwell Scientific

Publications, p. 229-282.

Johnson, M.A.,

Kenyon, N.H., Belderson,

R.H. and Stride, A.H., 1982, Sand trans-

port,

Stride, A.H., ed., Offshore tidal

sands: processes and deposits: New

York, Chapman and Hall, p. 58-94.

Kreisa, R.D., Moiola, R.J. and

Nottvedt, A.,

1986, Tidal sand wave facies, Rancho

Rojo Sandstone (Permian), Arizona,

Knight, R.J. and McLean, J.R., eds.,

Shelf sands and sandstones: Canadian

Society of Petroleum Geologists,

Memoir

1 1, p. 277-291.

Kvale, E.P. and Archer, A.W., 1990, Tidal

deposits associated with low-sulphur

coals, Brazil

Fm. (Lower Pennsyl-

vanian), Indiana: Journal of Sedimen-

tary Petrology, v. 60, p. 563-574.

Larsonneur, C., 1975, Tidal deposits, Mont

Saint-Michel Bay, France,

Ginsburg,

R.N., ed., Tidal deposits: a

casebook of

recent examples and fossil counter-

parts: New York, Springer-Verlag, p. 21

30.

Levell, B.K., 1980, A late Precambrian tidal

shelf deposit, the Lower Sandfjord

Formation, Finnmark, North Norway:

Sedimentology, v. 27, p. 539-557.

McCave, I.N., 1971, Sand waves in the

North sea off the coast of Holland:

Marine Geology, v. 10, p. 199-225.

Mutti, E.,

Rosell, J., Allen, G.P., Fonnesu,

F. and Sgavetti, M., 1985, The Eocene

Baronia tide dominated delta-shelf

system in the Ager Basin,

Mila, M.D.

and

Rosell, J., eds., Excursion guide-

book: 6th European Regional Meeting,

International Association of

Sedimen-

tologists, Lleida, Spain, p. 579-600.

Open University Course Team, 1989,

Waves, tides and shallow-water pro-

cesses: Oxford, Pergamon Press, 187 p.

Reineck, H.E. and Singh, I.B., 1980,

Depositional sedimentary environments:

New York, Springer-Verlag, 549 p.

Reineck, H.E. and Wunderlich, F., 1968,

Classification and origin of flaser and

lenticular bedding: Sedimentology, v.

1 1,

p. 99-1 04.

Sellwood, B. W., 1975, Lower Jurassic

tidal-flat deposits, Bornholm, Denmark,

Ginsburg, R.N., ed., Tidal deposits: a

casebook of recent examples and fossil

counterparts: New York, Springer-

Verlag, p. 93-1 01.

Stewart, H.B. and Jordan, G.F., 1964,

Underwater sand ridges on Georges

Shoal,

Miller, R.L., ed., Papers in

marine geology:

Shepard commemora-

tive volume: New York,

McMillan,

p. 102-1 16.

Stride, A.H., Belderson, R.H.,

Kenyon, N.H.

and Johnson, M.A., 1982, Offshore tidal

deposits: sand sheet and sand bank

facies,

Stride, A.H., ed., Offshore tidal

sands: processes and deposits; New

York, Chapman and Hall, p. 95-125.

Swift, D.J.P., 1975, Tidal sand ridges and

shoal retreat massifs: Marine Geology,

v. 18, p. 105-134.

Tankard, A.J. and

Hobday, D.K., 1977,

Tide-dominated back-barrier sedimenta-

tion, early Ordovician Cape Basin, Cape

Peninsula, South Africa: Sedimentary

Geology, v. 18, p. 135-1 59.

Terwindt, J.H.J., 1988, Palaeo-tidal recon-

structions of inshore tidal depositional

environments,

De Boer, P.L., van

Gelder, A. and Nio, S.D., eds., Tide-in-

fluenced sedimentary environments and

facies: Boston, D.

Reidel Publishing

Company, p. 233-263.

Thomas, R.G., Smith, D.G., Wood, J.M.,

Visser,

J.,

Calverley-Range, E.A. and

Koster, E.H., 1987, Inclined heterolithic

stratification

terminology, description,

interpretation and significance: Sedi-

mentary Geology, v. 53, p. 123-179.

Thompson, R.W., 1968, Tidal flat sedimen-

tation on the Colorado River delta,

northwestern Gulf of California: Geo-

logical Society of America, Memoir 107,

413 p.

Twichell, D.C., 1983,

Bedform distribution

and inferred sand transport on Georges

Bank, United States Atlantic continental

shelf: Sedimentology, v. 30, p. 695-710.

Van Wagoner, J.C., Mitchum, R.M.,

Campion, K.M. and Rahmanian, V.D.,

1990, Siliciclastic sequence stratigraphy

in well logs, cores, and outcrops: con-

cepts of high-resolution correlation of

time and facies: American Association

of Petroleum Geologists, Methods in

Exploration Series, No. 7, 55 p.

Visser, M.J., 1980, Neap-spring cycles re-

flected in Holocene

subtidal large-scale

bedform deposits: a preliminary note:

Geology, v. 8, p. 543-546.

Wang,

Y.,

1983, The mudflat system of

China,

Gordon, D.C., Jr. and

Hourston,

AS., eds., Proceedings of the

symposium on the dynamics of turbid

coastal environments: Canadian Journal

of Fisheries and Aquatic Science, v. 40,

supplement 1, p. 160-171.

Williams, G.E., 1991, Upper Proterozoic

tidal rhythmites, South Australia: sedi-

mentary features, deposition, and impli-

cations for the Earth's paleorotation,

Smith, D.G., Reinson, G.E., Zaitlin, B.A.

and Rahmani, R.A., eds., Clastic tidal

sedimentology: Canadian Society of

Petroleum Geologists, Memoir 16,

p. 161-178.

Wright, L.D., Coleman, J.M. and

Thorn,

B.G., 1975, Sediment transport and de-

position in a macrotidal river channel,

Ord River, Western Australia,

Cronin,

L.E., ed., Estuarine research, v.

II: New

York, Academic Press, p.

309-322.

Yang, C.-S., 1989, Active, moribund and

buried tidal sand ridges in the East

China Sea and the southern Yellow

Sea: Marine Geology, v. 88, p. 97-1 16.

Yang, C.-S. and Sun, J., 1988, Tidal sand

ridges on the East China Sea shelf,

De Boer, P.L., van Gelder, A. and Nio,

S.D., eds., Tide-influenced sedimentary

environments and facies: Boston, D.

Reidel Publishing Company, p. 23-38.

INTRODUCTION

Shallow marine systems include a con-

tinuum of depositional environments,

from beach and shoreface, through

inner to outer shelf settings. The envi-

ronments are laterally gradational, and

are linked by a variety of geological

processes. Wave- and

storm-domi-

nated shallow marine sediments can

occur in highstand, lows nd and trans-

gressive systems tract

. Highstand de-

posits are commonly typified by sand-

ier-upward facies successions that ac-

cumulate during coastal progradation

and shelf aggradation.

Lowstand shelf

deposits also shoal upward; they com-

monly form isolated sandbodies,

bounded by erosion surfaces and en-

closed in marine mudstone. Trans-

gressive systems tract deposits

commonly consist of coarse lags that

pass upward into transgressive

mud-

stones. The sedimentary record may

be even more complex if tidal and

oceanic currents were operating; the

distinctive suite of tidally influenced

facies is discussed in Chapter 11.

Wave- and Storm-Dominated

Shallow Marine Systems

Roger G. Walker, Department of Geology,

McMaster University, Hamilton, Ontario C8S 4M1

Guy Plint, Department of Geology, University of Western Ontario,

London, Ontario

N6A

587

The geological record of shoreline

and shallow marine systems is further

complicated by their sensitivity to even

minor fluctuations of sea level. This

sensitivity was not fully appreciated

when the second edition of

Facies

Models

was written (Walker, 1984), but

its effect on the distribution and geom-

etry of sandbodies is now much better

understood. For example, erosion can

result from both falling and rising sea

levels, and can be as important as de-

positional processes in controlling

sandbody geometry.

The abundance of erosion surfaces

(bounding discontinuities) makes an

allostratigraphic (Chapter

1) approach

to shoreline and shallow marine sys-

tems very important. Readers inter-

ested in the history of studies of shal-

low marine systems, and in some of

the older case histories, are referred to

the second edition (Walker, 1984).

Modelling depositional systems in-

volves the comparison of modern and

ancient examples. Sandbodies around

modern shorelines and on modern

shelves have been severely influenced

by the Holocene transgression. This

began about 18,000 years ago as a

result of the onset of deglaciation, and

involved a relative sea level rise of

more than 100 m. Consequently,

modern transgressive systems tract

deposits can be examined, but few

prograding highstand systems are

available for study.

DEFINITIONS AND TERMS

USED IN THIS CHAPTER

The term

bar

includes all ancient elon-

gate, narrow accumulations of coarse

sediment, and the term

sandbody

in-

cludes accumulations of sandstone

and/or conglomerate.

Shallow marine

will be used as a general term for envi-

ronments affected by waves, regard-

less of their tectonic setting. It includes

modern shelves as well as epeiric seas

and the shallow parts of foreland

basins.

MORPHOLOGICAL ELEMENTS

The main morphological elements of a

VERTICAL SCALE GREATLY EXAGGERATED

WAVES BEGIN TO BUILD UP

SPILLING

MEAKERS

SmlALING WAVES

HIGH

LOW

.QIIJW&A~R-W~VfiAf.E-

,=,

MUWY

SUBSTRATE

SANDY SUBSTRATE

Skollthor

Crvzlono

1chnofocl.r

Zoophycos

- -

Figure

Shoreline to shallow marine profile locating foreshore,'shoreface and offshore areas, as well as fairweather wave base and ap-

proximate ichnofacies occurrences. Note that fairweather waves of wavelength

cannot agitate the bed at depths greater than approximately

U2.

Fairweather wave base lies at depths of about

5-15

220 WALKER, PLlNT

modern shoreline-to-shallow-marine

depositional system are shown in

Figure 1. The foreshore, which is es-

sentially synonymous with beach, con-

sists of the portion above low tide line,

and is dominated by the swash and

backwash of breaking waves. The

shoreface lies consistently below low

tide level and is characterized by

day-

to-day sand transport above fair-

weather wave base. The depth of fair-

weather wave base varies from about

5-15 m, depending on the general wave

climate of the basin. The shoreface nor-

mally has a concave-upward profile,

which is in equilibrium with the waves

that shape it. The shoreface gradient of

about

1:200 (about 0.3") decreases

seaward, and the sand-dominated

shoreface passes into the

mud-domi-

nated offshore zone where the gradient

is closer to

1:2000 (about 0.03"). In a

prograding sedimentary succession, it

is impossible to identify the topographic

change that marks the transition from

offshore to shoreface. In practice, the

base of the shoreface can be defined at

the point where interbedded sand-

stones and mudstones pass upward

into relatively clean sandstones.

A variety of small bedforms, bars and

runnels exist in the shoreface. Their mi-

Figure

Bathymetry (in metres) of the

Assateague ridge field, offshore Maryland,

U.S.A. Note typical shoreface (S),

nearshore

(N)

and offshore

(0)

ridges, with

their typical bathymetric cross sections

(which are presented at a greater scale

than the map itself). Ridges appear to be

more symmetrical in the shoreface, and

more asymmetrical offshore. Simplified

from Swift and Field (1

981).

gration produces both scoured surfaces

and medium-scale angle-of-repose

cross stratification, and these features

can be preserved in the stratigraphic

record.

In many places, large shoreface

ridges (Swift and Field, 1981) originate

in the middle to lower shoreface, and

trend obliquely seaward (Fig. 2). Off

the coast of Maryland

(US.

Atlantic

shelf), for example, the shoreface ridges

pass seaward into shelf ridges with

heights of 6-9 m, lengths of 8-14 km

and widths of 1-2 km. Both shelf and

shoreface ridges rest on an erosion

surface cut during the Holocene trans-

gression. The more seaward ridges are

now isolated on the shelf where they

form fields of ridges and swales in

nearshore and offshore areas. The

morphology and internal structure of

these ridges are slowly being modified

by storm currents (Fig. 2).

The ridges and bars observed in

shallow marine settings today origi-

nally formed at or close to the shore-

line. They were subsequently drowned

during the Holocene transgression and

now appear "abandoned" on the shelf.

There is no well-documented example

of a modern sandy "offshore bar" that

has developed on a contemporaneous

muddy substrate in a totally offshore

setting.

PHYSICAL PROCESSES IN THE

BEACHANDUPPERSHOREFACE

Sediment transport in the beach and

upper shoreface is driven by waves.

Good reviews of the processes are

given by Komar (1976, 1983) and

Swift et

a/. (1 986). As waves enter shal-

low water, they begin to move se-

diment on the

seafloor at a maximum

depth roughly equal to half their wave-

length (Fig. 1). The wave-induced or-

bital water motion close to the bed is

asymmetrical, resulting in a net onshore

movement of sand.

In the foreshore and upper shoreface,

the onshore, longshore and offshore

(rip) currents are shown in Figure 3.

Waves transport water onshore; when

they break on the beach they drive a

thin sheet of water rapidly up the beach-

face. This swash is capable of moving

sand and gravel, and typically produces

plane lamination parallel to the beach-

face. There are two dominant current

systems

longshore currents pro-

duced by oblique wave approach to the

shoreline and a cell circulation of rip cur-

rents and longshore currents (Komar,

1976; Fig. 3).

Longshore currents are driven by

waves breaking at an angle to the

shoreline (Fig. 3). Measurements show

that the current velocity drops quickly

to zero outside the breaker zone, indi-

cating that waves rather than tidal cur-

rents or oceanic circulation control the

flow. Predicting sand movement by

these longshore currents is very im-

portant in studies of coastal stability

and erosion (Komar, 1983; Everts,

1 987; Pilkey and Davis, 1987). The dip

BEACH

:;:::

.:..:.::::..

:...

:...:.

.......................

:.:.

.......

.....

........................

..:..:::*

:.::::..::::::

................

.............

LONGSHORE CURRENTS

/---

fl-

L-

,,-kJ

,,-_,,-

?&E

//J/

4.C

BREAKER

"A&.......

acrn--r4-.na-uwr-*/

ttt

.CJ.

tfttt

MASS

TRANSPORT

OBLIQUE WAVE

APPROACH

Figure

In the shoreface, there is mass onshore transport of water, driven by waves.

Landward of the breaker zone, this water moves as longshore currents. If wave crests are

roughly parallel to shore, the longshore currents flow in opposite directions to form cells.

Water is transported seaward as rip currents where two longshore currents converge (left

half of figure). Seaward of the breaker zone, the rip currents spread in the rip heads and die

out.

wave crests approach the beach obliquely, the longshore currents tend to move in one

direction (right half of figure). Rip cell portion of the figure is based on Komar (1976, fig. 7-1).

12. WAVE AND STORM SYSTEMS 22 1

of the beachface is influenced by grain

size (steeper on coarser grained

beachfaces) and by wave conditions.

During fair weather sand tends to

move onshore, steepening the beach

profile, whereas storms remove sand

from the beach and flatten the profile.

In the cell circulation, water returns

seaward in rip currents that can trans-

port sand and gravel, and cut chan-

nels in the upper shoreface. However,

the currents die out seaward of the

breaker zone, and do not transport se-

diment more than a few hundred

metres offshore.

STORM-INDUCED SHELF

CURRENTS

Although storm conditions exist for only

a small part of the year at any given

point on the shelf, their effects in terms

of erosion and sediment transport are

disproportionately large. During a

storm, frictional coupling between the

wind and the water surface causes

water to be driven forward. The moving

surface waters entrain deeper and

deeper layers, all of which are deflected

to the right (in the northern hemisphere)

Coriolis force.

Sediment-moving cur-

rents on the

seafloor therefore flow at

angles of up to

90' to the wind direc-

tion. A very clear discussion of these

wind-driven currents is given by the

Open University Course Team (1 989).

Water blown onshore by the wind

also results in an ocean surface that is

higher at the coast than offshore (a

coastal set

up).

The superelevation can

also be augmented by very low atmo-

spheric pressures. The different water

levels at the coast and offshore result

in hydrostatic pressure differences on

the ocean floor. These pressure

differ-

HIGH

LOW

I I

PRESSURE GRADIENT

Flgure

Coastal set-up ("storm surge") creates a seaward pressure gradient

(A).

Bottom

water flows seaward as a result, but is deflected to the right (northern hemisphere) by

Coriolis force to form a geostrophic flow

(6) parallel to isobaths. The extent

the deflection

is controversial (see

"The sand transport problem" in text). In theory it depends upon the lati-

tude, mass of water in motion and its velocity, as well as the duration of the flow. No scale is

implied in the diagram, which is constructed after Swift and Niedoroda (1985) and Strahler

963).

ences drive a bottom flow down the

pressure gradient (relaxation flow, Fig.

4).

This flow is also deflected by

Coriolis force, resulting in a

geostro-

phic flow lhat moves obliquely off-

shore, veering through time to flow

shore-parallel. The flow persists as

long as there is a pressure gradient,

typically for a few days in the case of a

winter storm on the present Atlantic

shelf (Swift

et al.,

1986). We empha-

size that the flow is a continuous re-

sponse to the pressure gradient; it is

not

a sudden surge of water related to

the end of the storm winds.

Storms and hurricanes

on modern shelves

Flow velocities associated with annual

northeasterly storms on the Atlantic

shelf have been measured by Swift

and colleagues (Swift

et al.,

1981;

Swift and Field, 1981). In depths of

10-

20 m, near-bottom velocities are up to

about 60

cmlsec. These flows, by them-

selves, are incapable of moving much

sand. However, the currents are usually

accompanied by a powerful

wave-

driven oscillatory motion at the bed

(Swift

et a/.,

1986). Unidirectional and

oscillatory currents, operating together,

are termed

combined flows.

The waves

provide the high shear stresses

needed to lift the sediment off the bed,

and the grains are gradually dispersed

in the direction of the geostrophic

bottom currents.

Tropical storms and hurricanes gen-

erate much higher flow velocities close

to the bed. Classic examples have

been reviewed by Morton

(1988), and

recent data from the Texas shelf have

been discussed by Snedden

et al.

(1 988). Hurricane Camille, in 1969,

was one of the most intense Gulf of

Mexico hurricanes in modern times.

Current meters in the shoreface to the

east of the path of Camille showed a

longshore flow strengthening and

veering offshore, with peak velocities

of about 160

cmlsec. Farther out on

the shelf, just east of the

birdfoot delta

of the Mississippi, the storm waves

appear to have liquefied the bed,

causing sediment flowage and slum-

ping. Parts of the bed appear to have

been lowered by about

rn,

and other

parts raised by 10 m.

Tropical Storm Delia (1973) moved

northwestward across the Texas shelf.

Near bottom current measurements

222 WALKER, PLlNT

were made from an offshore drilling

platform situated 50 km off Galveston

Island in 21 m of

water (Fig. 5; Forri-

stall

et a/.,

1977). Shore-parallel flows

reached nearly 2

m/sec, and offshore

flows were between 50 and 75

cmlsec.

The coastal set up of 2 m at Galveston

Island did not occur until 30 hours after

the storm reached the shoreline (land-

fall, Fig. 5), whereas the maximum

shore-parallel velocity occurred about

38 hours before peak coastal set up

(Fig.

5).

Thus the shore-parallel (geo-

strophic) flow was driven by wind

stresses, not storm surge ebb. Hurri-

cane Alicia (1983) followed roughly the

same path as Delia, causing a coastal

set up of about 3 m. Longshore and

slightly offshore currents were mea-

sured up to 1

rntsec, and flows of about

cmlsec were sustained for about

two days. Hurricane Alicia appears to

have formed a graded bed, probably by

in situ reworking and size fractionation

of pre-existing shelf sediments (Morton,

1988). However, the graded bed had

largely been reworked by organisms

only three months later.

These examples show that storms

and hurricanes move sediment in

shallow marine settings. The direction

of sediment movement appears to be

obliquely offshore with some sand

moving from the shoreface into off-

shore settings.

SEDIMENTARY STRUCTURES

IN SHOREFACE AND SHALLOW

MARINE SETTINGS

In these environments, structures form

in response to unidirectional and oscil-

latory (wave) currents; these flows

commonly co-exist, to form combined

flow sedimentary structures.

Ripples and dunes

In sand-sized sediment, oscillation

ripples are ubiquitous, and are charac-

terized by straight crests and symmet-

rical profiles. Symmetrical gravel ripples,

which can form at depths of up to 160

m, have also been reported in modem

(Forbes and Boyd, 1987; Hart and

Plint,

1989) and ancient (Leckie, 1988) sedi-

ments. Combined flow ripples (Fig. 6)

tend to have symmetrical profiles, but

an internal cross lamination that dips

consistently in one direction. In the geo-

logical record, where the ripple profile

to distinguish.

Two- and three-dimensional dunes

are common in modern shorefaces

where they are formed by fairweather

longshore and onshore currents (Clifton

a/.,

1971; Greenwood and Mittler,

1985). Their migration forms

medium-

scale cross bedding. In wave- and par-

ticularly storm-dominated shorefaces,

however, the cross bedding tends to

be obliterated by storm waves and re-

placed by parallel lamination and/or

swaly cross stratification (Greenwood

and Sherman, 1986). Dunes are also

observed on modern shelves, with

wavelengths of 1-40 m, and dune

heights of several metres. The smaller

3-D dunes actively migrate during

storm-induced geostrophic flows (Swift

et a/.,

1981), but may be reworked to

ripples during waning flows. The larger

2-D dunes, with spacings of 20-400 m,

migrate more slowly and are not re-

formed from storm to storm.

Hummocky cross

stratification (HCS)

This structure (Fig. 7), which forms

mainly in fine- and very fine-grained

sandstones, was not commonly identi-

fied in the geological record until the

name

hummocky cross stratification

(MCS) was created by John Harms

(Harms

a/., 1975). He compared the

hummocky

bedform to large-scale

wave ripples, and interpreted HCS as a

storm deposit. HCS has now been rec-

ognized in many basins of different

ages (Duke,

1985), and the storm inter-

pretation is widely accepted. Debate

continues on the mechanics of its for-

mation; some authors favour combined

x;d'all

0000

0600

I1200 18:W

24a

SEPTEMBER

1973

Figure

Windspeed, longshore (shore-

parallel) and offshore currents measured

during Tropical

Storm Delia, September

3-5,

1973. Data recorded at a drilling platform

km offshore from Galveston Island, Texas, in

flows whereas the experimental results

of Southard

et a/.

(1990) favour strong,

purely oscillatory flows. Because HCS

is probably formed during major storms,

there is no direct observational evi-

dence from modern shallow seas.

Cores of possible HCS have been ob-

tained from the shoreface (Greenwood

and Sherman,

1986), and it has been

tentatively identified in offshore areas.

In the geological record, HCS sand-

stones are commonly interbedded with

bioturbated mudstones. The sandstone

beds are commonly 10-50 cm thick, with

a few beds thicker than 1 m. Most beds

have sharp bases, and a few also have

directional sole marks (which often

suggest flow perpendicular to regional

shoreline trends). In most examples,

there is no associated medium-scale

angle-of-repose cross bedding; this is

probably a function of grain-size control,

with the cross bedding forming mainly in

medium- and coarse-grained sands.

HCS is generally preserved in areas of

weak tidal activity that lie below

fair-

weather wave base. The main problem

presented by HCS sandstones

inter-

bedded with bioturbated mudstones is

not so much the formation of HCS as the

mechanism of sand transport to the final

depositional site; this is discussed later.

Swaly cross stratlflcation

In the geological record, some HCS

sandstone beds amalgamate together,

Figure

Sketch showing the various

may not be preserved, combined

flow

a depth of 21 m. Redrawn and simplified

types of ripple cross stratification formed

and current ripples can be very difficult

from

Forristall

etal.

(1977).

waves. Modified from de Raaf

etal.

(1977).

12. WAVE AND STORM SYSTEMS 223

and the interbedded mudstones are

gesting a generally shallower, more

and no distinct parting planes or

amal-

thin or absent. There appears to have

agitated environment. If the HCS

gamation surfaces, a thick

sandbody

been little opportunity for the deposi-

sandstones amalgamate so thoroughly

several metres thick can form (Fig. 8).

tion and preservation of mud, sug-

that there is no interbedded mudstone,

The term

swaly cross stratification

(SCS) was created by Leckie and

Walker (1982) for sandbodies by defi-

nition thicker than

m. The internal

stratification is dominantly flattish to

very gently undulating, and the swales

cut into this lamination. The swales

are circular to elliptical in plan view, a

metre or more in diameter and a few

tens of centimetres deep. The infilling

laminae conform to the shape of the

swale, gradually flattening out upward.

The swales rarely pass laterally into

hummocks, although there are enough

convex-upward laminae in a swaly

sandbody to indicate a descriptive

(and probably genetic) similarity to

classical HCS.

In many examples from the geolog-

ical record, SCS sandbodies occur

stratigraphically above HCS sand-

stones and interbedded mudstones.

Commonly, the sandbodies are 10-15

m thick and contain SCS throughout.

Alternatively, the lower part contains

Figure

Hummocky cross stratification, Cardium Formation, Ram River, Alberta. Arrows

~CS, with cross-bedded and horizon-

show curvature of lamination within the

HCS.

Also note low angle convergence of curved

tally bedded sandstones at the top

laminae at C.

(upper shoreface and beach). These

may be capped by coaly horizons with

in situ root traces. This stratigraphic

context suggests that many or most

SCS sandbodies represent prograding

storm-dominated shorefaces, in which

storm processes have overprinted all

record of fairweather sedimentation,

except in the upper shoreface and

beach.

SHELF

MUDSTONES

The attention paid to sandstones out-

weighs that devoted to mudstones, al-

though the latter comprise the bulk of

shelf sediments. Mud and silt washed

into the sea by rivers can be trans-

ported for many tens of kilometres in

suspension. Final deposition onto the

bed is commonly accelerated by

filter-

feeding organisms that ingest mud

from the water column and deposit it

as fecal pellets. Where the concentra-

tion of suspended mud is very high,

ocean waves are damped and

the

shoreface itself may consist

mud

(Rine and Ginsberg,

1985).

Physical

and biological structures in mudstones

Figure

Swaly cross stratification in Permian rocks at Thirroul, near Woollongong,

N.S.

W.,

a re commonly difficult to study in

Australia. Much of the stratification is relatively flat

(F),

with isolated swales (S). Note low angle,

outcrop, but can be well displayed in

curved intersecting laminae

upper one-third of the photograph. Scale is

cm long.

unweathered polished slabs or in

224 WALKER, PLlNT

cores. In most shelf mudstones, all

physically formed structures have

been partly or completely destroyed by

the burrowing and grazing activities of

organisms, as discussed in Chapter

In situations where the water close to

the bed was depleted in oxygen, and

benthonic organisms consequently

scarce, primary sedimentary structures

may be preserved. The most common

feature consists of a subtle colour

banding that reflects milimetre- to

cen-

timetre-scale graded siltstone-mud-

stone beds (Fig. 9; Davis

et al.,

1989).

The sharp-based graded beds were

probably deposited from waning,

storm-

generated flows; the muddy portion of

each bed is probably partly storm em-

placed, and partly reflects pelagic de-

position between storms.

THE SAND TRANSPORT PROBLEM

For medium to large shallow marine

sandbodies, the most important problem

can be simply stated:

did the sand move

from the shoreline to the sandbody, or

did the

sandbody form at the shoreline

and become subsequently abandoned

on the shelf by transgression?

Where mudstones interbedded with

the sandstones suggest an offshore

depositional environment, the question

arises as to the direction and process

of sand transport from the shoreface

to its depositional site. Directions of

transport can be perpendicular, oblique

or parallel to the shoreline. Transport

mechanisms might include combined

flows driven by waves

and/or pressure

differences, or density currents similar

to turbidity currents.

Parallel-laminated to current-rippled

sandstone beds

Some ancient sandstones, deposited in

offshore environments, are character-

ized by sharp bases with sole marks,

parallel lamination, and ripple cross

lamination (Fig. 10). In the Bouma ter-

minology for turbidites, these beds

would be described as BC beds (see

Chapter 13). Although the sedimentary

structures indicate waning flow, it is not

always clear whether the flows were

unidirectional or oscillatory.

Duke (1990) has suggested that the

parallel lamination may be formed by

oscillatory currents,' and that the ripples

are combined flow ripples. The sharp

bases would reflect scouring of the

muddy shelf floor during the rising

phase of the storm, followed by the em-

placement of sand as the storm waned.

The sedimentary structures would re-

flect diminishing wave orbital velocities

as the storm waves died down. This in-

terpretation can be applied to situations

where sand moves incrementally, storm

by storm, to its final depositional site.

In other situations, apparently unidi-

rectional currents also form

beds

with sharp bases. The best-known

example is in the Jurassic of Alberta,

where the BC beds and interbedded

mudstones form a section about

thick. This section is overlain by HCS

beds (20 m) and finally by SCS

shore-

face sandstones and coals (Hamblin

and Walker, 1979). It was suggested

that the BC beds were

emplaced by tur-

bidity currents that deposited sediment

below storm wave base. The

sharp-

based HCS beds have identical sole

Figure

Core photo shows sharp-based

siltstone beds, many with subtle colour gra-

ding indicating more mud in the upper part of

the

bed.

Note faint flat to very gently inclined

lamination within

some beds. There is an ab-

sence of ripple cross lamination. Viking For-

mation at Judy Creek, Alberta: well

10-36V-

64-1

3W5,4745

feet. Scale in centimetres.

mark directions to the BC beds below,

and hence were also believed to be tur-

bidity current deposits. However,

because the sand was deposited above

storm wave base, the normal turbidite

sedimentary structures were reworked

into HCS. The problems raised by this

interpretation are 1) that there are no

actualistic examples of turbidity currents

on modem shelves, and 2) that gener-

ating mechanisms for turbidity currents

at or close to the shoreface are difficult

to envisage.

Arguments concerning the direction

of sand emplacement are fuelled by

the geological observation that many

ancient storm-emplaced sandstone

beds have sole marks indicating domi-

nantly shore-perpendicular flow direc-

tions (Leckie and Krystinik, 1989; but

see Hart

et al.,

1990 and Plint and

Norris, 1991 for contrasting

observa-

Flgure

Sharp-based bed of fine sand-

stone showing parallel lamination throughout

most of the bed, and a single layer of ripple

cross lamination at the top. Cardium

Formation at Ricinus field, Alberta; well

3-5-

35-8W5,8845

feet.

12. WAVE AND STORM SYSTEMS 225

tions). The apparent conflict between

flow on modem shelves (largely

shore-

parallel) and paleocurrent evidence in

ancient rocks (largely

shore-perpen-

dicular) has been discussed by Duke

(1990) and Duke et

a/. (1991). They

point out that the paleocurrent evi-

dence is largely derived from

small-

scale sole marks that were made by

the instantaneous

action of waves;

these are typically

shore-perpendic-

ular. If angle-of-repose cross stratifica-

tion is present, the paleoflow direc-

tions are commonly shore-parallel.

However, the dunes that formed the

cross stratification responded to a

longer term time-averaged flow and

better reflect the net, geostrophically

driven along-shelf flow.

ANCIENT SHALLOW

MARINE

SANDBODIES

The processes described earlier in this

chapter appear to have combined to

form three main types of shoreface

and shallow marine sandbodies in the

geological record. The first consists of

sheet-like sandbodies up to about 20

m thick, representing prograding

shorefaces. The second consists of

sheet-like or patchy bodies of in-

terbedded HCS sandstones and bio-

{

turbated mudstones, representing an

aggrading offshore marine environ-

ment. The third type consists of long

narrow bodies that may be much

coarser (medium sandstone to con-

glomerate) than the surrounding sedi-

ments. These long, narrow bodies

tend to be more or less parallel to re-

gional shoreline trends, and they also

tend to be under- and overlain by re-

gionally extensive erosion surfaces.

Features directly analogous to the off-

shore ridges and swales of modern

shelves have not been identified in the

geological record.

Prograding shorefaces and

ag-

grading offshore HCS sandstones are

characteristic of highstand systems

tracts; the long, narrow sandstone and

conglomerate bodies more commonly

form

lowstand and parts of transgres-

sive systems tracts.

PROGRADING STORM-DOMINATED

SHOREFACE SYSTEMS

Modern prograding shorefaces

Relatively few modern shorefaces are

actively prograding. The best example is

the broadly arcuate, 200 km-long

mi-

crotidal coast of Nayarit on the Pacific

coast of Mexico (Fig. 11). It has

pro-

graded up to about

during the last

3,600 years, at an average rate of about

mlyear (Curray et a/., 1969). Sand is

supplied to the strandplain by the Rio

Grande de Santiago and a number of

smaller rivers, as well as by marine

erosion of older transgressive deposits.

The well-sorted fine and medium sand is

distributed along the coast by a powerful

wave-driven longshore drift system,

forming a

sandbody about 7 m thick. The

shore face sands grade seaward into

silts and clays at a depth of about 10 m.

The importance of sand supply to the

shoreface by reworking of the inner shelf

has been discussed by Dominguez

eta!.

(1987) from a study of the southeastem

coast of Brazil. Here, strandplains are as-

sociated with several river mouths (Fig.

12). On the downdrift side of the river

mouths, the strandplain contains some

sand supplied directly from the rivers. On

the

updrift side, the mineralogy of the

sand is different, suggesting derivation

from the shelf by reworking in response

to an approximately 5 m relative fall in

sea level during the last 5,100 years.

This hypothesis is strengthened by the

fact that the Caravelas strandplain

(Dominguez et

a/,,

1987, fig.

is not as-

sociated with a river at all. It has been

built in a zone of convergent longshore

drift with sand supplied by erosion of the

shelf. This work in Brazil suggests that

regressive sheet sandbodies are most

likely to develop following a relative sea

level fall, when dispersed sand on the

inner shelf is eroded and concentrated in

the shoreface.

Ancient shoreface sandbodies

Many examples of sheet-like sand-

Figure

Coast

Nayarit, Mexico, showing

(B)

arcuate wave-dominated strandplain

with

280

sub-parallel

beach

ridges

which

have

creted during about

4500

years of coastal progradation. Changes in wind patterns, and

the

positions

river

mouths

have

resulted

localized

erosion and reorientation of the shoreline.

cross section showing a Holocene transgressive ravinement surface overlain by a transgressive

lit-

toral

sand, in turn overlain by the prograding beach and beach-lidge complex (regressive shoreface sandbody). Modified

from

Curray

a/.

(1969).

226

WALKER, PLlNT

bodies characterized by SCS, and in-

terpreted to have been deposited in

prograding wave-dominated

shore-

faces, have now been recognized (see

Heward, 1981

;

McCubbin, 1982; John-

son and

Baldwin, 1986 for recent re-

views). This literature is strongly

biased toward the Cretaceous of the

Western Interior Seaway of North

America, where the sandbodies form

settings include the Tertiary Cohansey

Sand of New Jersey (Carter, 1978)

and the Oligocene Frio Formation of

Texas (Tyler and Ambrose, 1986).

Each

sandbody typically extends

many hundreds of kilometres along

depositional strike, and may

prograde

over 300 krn seaward. Where sand-

body geometry can be well defined

using isopach maps, localized bulges

in the shoreline trend probably indicate

the former position of major river

mouths.

The SCS shoreface sandbodies

characteristically occur in the upper

parts of sandier-upward facies succes-

sions (Figs. 13,

14).

These succes-

sions (such as the one at Lundbreck,

INNER SHELF

FLUVIAL SAND

DEFLECTED

D~wN-

SAND IS ERODED FROM

INNER SHELF AND

REWORKED ONSHORE

STRAND-PLAIN

BUILT IN A

ZONE OF CONVERGENT

LONGSHORE DRIFT -NO

Flgure

Development of beach-ridge

strandplains on a wave-dominated coast

undergiong relative sea level fall. Sand sup-

plied by rivers is deflected alongshore and

contributes to the downdrift side of

the

strandplain. The updrift side is built largely

of sand reworked in the onshore direction

through erosion of the inner shelf, as a

result of increased wave scouring during

relative sea level fall. Note that a

strand-

plain can accumulate in an area lacking a

direct

fluvial input. Idealized after diagrams

of Dominguez et a/.

(1987).

Figure

Prograding shoreface succession in the Cretaceous Chungo Member, Wapiabi For-

mation, at Lundbreck, Alberta. Stratigraphic top to left. M

offshore bioturbated mudstones; H

interbedded HCS sandstones and bioturbated mudstones; S

SCS shoreface sandstones.

Thickness of H plus

is about

100

Note progressive increase in proportion of sand upward.

Breaker zone

ridge

SUCC~SS~O~

runnel

rip channels

Shoreface

Swaley

cross -stratification

elative sea level fall

lo-

erosive

based

sandy siltstone

Outer shelf

bioturbated

f-4

Wave-dominated shoreface successions. Left, a gradationally based succession

passing frdm outer through inner shelf deposits into sand-dominated shoreface and beach

sediments. Right, shows a succession characterized by a

sharp-based shoreface deposited

during a period of relative sea level fall (a forced regression). At such times, reduced accom-

modation space eliminates the shelf-to-shoreface transitional zone, and the shoreface sands

rest

abruptly on a wave-scoured erosion surface.

_/-

__---

C--

12. WAVE AND STORM SYSTEMS

-----

Alberta; Fig. 13; Rosenthal and

Walker, 1987) typically begin with

bio-

turbated mudstones and interbedded

fine to very fine sandstone beds a few

centimetres or tens of centimetres

thick; these may contain HCS, planar

lamination or ripple cross lamination.

Upward, the sandstone beds thicken,

the proportion of HCS increases and

beds become more amalgamated. The

mudstone interbeds finally disappear

and SCS predominates. Toward the

top of the SCS sandbody, there may

be a few metres of somewhat coarser

(upper fine to medium grained) trough

cross-bedded sandstone, representa-

tive of longshore bars and troughs. The

top 1-2 m of the succession typically

consists of fine sandstone with planar

lamination, representing the beach. It

may be capped by somewhat silty,

structureless and rooted sandstone

representing a backshore environment.

Although fine sandstone commonly

dominates the SCS shoreface succes-

sions, coarser sediment may also be

present. For example, coarse,

cross-

and planar-bedded sandstone and/or

conglomerate may be important near

river mouths (Clifton, 1981

;

DeCelles,

1987; Leckie and Walker, 1982; Plint

and Walker, 1987; Hart and Plint, 1989;

Plint and Norris, 1991).

Sharp-based shoreface sandbodies

In some progradational facies succes-

sions, the base of the SCS

sandbody

is sharp rather than gradational (Fig.

14). The contact with underlying thinly

interbedded shelf mudstones and

silt-

stones can also be marked by large

load casts

and/or elongate gutter casts

(Rosenthal and Walker, 1987; Davis

and Byers, 1989; Plint, 1991). The gut-

ter casts have the geometry of gutters

(eavestroughs), and are narrow sand-

filled scours oriented perpendicular to

regional shoreline trends. They are be-

lieved to be formed by wave scour on

the bed (Duke, 1990).

Lateral relationships between grada-

tional and sharp-based sandbodies

can be demonstrated in subsurface

studies. For example, in the Cardium

Formation of Alberta (discussed in

detail later in this chapter), Plint (1988)

showed that gradational-based

shore-

face successions could be traced lat-

erally into thinner, sharp-based sand-

bodies (Fig. 15). The slight drop in

stratigraphic position of the base of the

shoreface, along with its change to a

sharp base, was interpreted to

the

result of progradation due to a relative

sea level fall. This would cause wave

scouring of the inner shelf, resulting

in shoreface progradation over an

erosion surface. The transitional lower

shoreface zone would be eliminated.

Because the water was shallower, less

sediment would be needed for

aggra-

dation, and therefore the progradation

rate would increase. More information

is provided by the Marshybank Form-

ation of Alberta (Plint, 1990, 1991;

Plint and Norris,

1991), which contains

a succession of very thin

(3-5 m),

erosive-based SCS shoreface sand-

stones. These sandbodies prograded

for many tens of kilometres over a

muddy shelf in response to relative sea

level falls. Comparable sandbodies

have recently been described from the

Paleozoic of Greenland (Bryant and

Figure

Representative

log

and core cross section through shoreface sandstones of the Kakwa allomember of the Cardium Formation.

Section is located in Figure 16, and progradation was left to right (eastward). The base of the shoreface (stippled) is gradational in 7-10-62-7,

but is stratigraphically a little lower, and sharp-based, in 7-10-62-6 and

6-8-62-5. Thus the stratigraphic drop in shoreface position

coinci-

dent with the change from gradational to sharp base. Note also the erosion of well log marker

"an between 7-10-62-6 and 6-8-62-5. The sharp-

based shoreface is interpreted to result from a small lowering of sea level during progradation (a forced regression). The shoreface rises

stratigraphically between 6-8-62-5 and

10-3-62-4, and becomes gradational again; this is interpreted to be due to a slight relative sea level

rise during progradation. Modified

from

Plint

(1988).