Boggs S. Principles of Sedimentology and Stratigraphy

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326

Chapter 9 I Marginal-Marine Environments

Figure 9.36

This lagoonal succession begins with a basal section of sandstones, siltstones, and

coals that represent marsh-tidal-flat deposits (Young and Reinson, 1975). These

deposits lie on the eroded surface of tidal-inlet deposits of he underlying Bl

Reserve Formation. The basal coal-bearing beds are succeeded upward by gray

shales containing oysters, wch are brackish-water faunas, and disseminated car

bonaceous materials and imprints of plant remains. These depits are interpret

as subtidal lagoonal sediments. They are overlain by fine-grained, planar and

cross-laminated sandstones, which are probably washover deposits from the bat·

rier beach. More subtidal shales lie above these washover sands, and the suc

cession is capped by sandstones and siltstones with coal lens of probable

marsh-tidal-flat origin. Overall, the Blood Reserve and St. Mary River formations

appear to represent a progradational barrier-island complex, in which the lagoon

al deposits of the St. Mary River Formation form the topmost part of the regressive

succession.

9.6 TIDAL-FLAT SYSTEMS

Introduction

Tid al flats form

primarily

on mesotidal and macrotidal coasts (Fig. 9.36) where

strong wave activity is absent. They develop either along open coasts of low relief

and relatively low wave energy or behind barriers on high-energy coasts where

protection is afforded from waves by barrier islands, spits, reefs, and other stc

tures. Thus, they occur within estuaries, bays, the backshores of barrier-island

complexes, and deltas, as well as along open coasts. They are particulady com

mon in the modern ocean along the coasts of Europe, Africa, South Asia; North

and East Asia, Australia, New Zealand, large Pacific islands, and weste and

eastern America (Eisma et a!., 1998).

Ti dal ats are marshy and muddy to sandy areas partially uncovered by the

rise and fall of tides. They constitute almost featureless plains dissected by a net

work of tidal channels and creeks that are largely exposed during low tide (e.g.,

Fig. 9.37). As tide level rises, flood-tide waters move into the channels until at high

tide the charu1els are overtopped and water spreads over and inundates the adja

cent shallow flats. Ebb tide again exposes the channels and intervening ats. 

temperate regions, salt marshes commonly cover the upper parts of tidal ats, and

muds and silts accumulate near high-water level. At the same time, mixed mud

Global classification of coastlines by tidal

range (microtidal 0-2 m; mesotidal 2 m;

macrotidal >4 m). [From Klein, G. deV.,

1985, Intertidal flats and intertidal sand

bodies, in Davis, R. A. Jr. (ed.), Coastal sedi

mentary environments, 2nd ed.: Springer

Verlag, New York, Fig. 3.1 , p. 189, Redrawn

from Davies, j. L., 1964, Zeitschrift fUr Geo

morphologie, v. 8, Fig. 4, p. 136.]

- Macrotidal

� Mesotidal

D Microtidal

Figure 9.37

9.6 Tidal-Flat Systems

327

Tidal flat in the Ashe Island area, about 70 km (45 mi) south of Charleston, South Carolina, exposed at

low tide. Note tidal channels and areas covered by shallow water (dark patches) on the flats. National

Oceanographic and Atmospheric Administration (NOAA) photograph. Downloaded from the Internet

4/23/04.

and sand are deposited in the middle tidal-flat region, and sands accumulate in

channels and on the lower parts of the tidal flat.  arid to semiarid regions, tidal

ats may become desiccated and marked by mudcracks and by gypsum and halite

crystals that form in muds. The surface of tidal flats in subarcc regions may be

marked by surficial scars, caused by ice floes and ice-pushed boulders, and ice-rafted

pebbles and cobbles. Modern tidal flats are primarily sites of siliciclastic deposi

tion; however, carbonate sediments and, in a few areas, evaporites accumulate on

some modern dal flats such as those in the Bahamas, the Persian Gulf, Florida

Bay, and the western coast of Australia (e.g., Hardie and Shinn, 1986).

Much of what is known about ancient tidal-flat sediments comes

from re

search on modern tidal flats. Modern tidal flats have been studied intensively in

many parts of the world sce the 1950s, particularly in Germany, the North Sea

coastline of The Netherlands, England, the Bay of Fundy in Nova Scotia, the Yel

low a of Korea, and the Gulf of California. Eisma et al. (1998) describes and dis

cusses many of the major tidal flats of the world. Oil and gas deposits have been

discovered in both siliciclastic and carbonate dal facies, and uranium is present

 some sandy tidal facies. Therefore, dal deposits have economic significance as

well as general scientific interest.

Depositional Setting

Although dal currents may operate in the ocean to depths of 2000-2500 m, the

dal-flat environment is coned to the shallow margin of the ocean. The vertical

328

Chapter 9 I Marginal

Marine Environments

Figure 9.38

distance between the high- and low-tide line in most mode tidal environments

commonly ranges from 1 and 4 m (mesotidal coasts), depending upon the locality,

although tidal ranges of 10-15 m or more (macrotidal coasts) occur in some locali

ties, such as the Bay of Fundy. The total width of tidal flats may range from a few

kilometers to as much as 25 km. To pographic relief within the tidal-at environ

ment is generally rather small except for tidal channels, and slopes of the tidal flat

are gentle although commonly irregular.

The Hdal-flat environment is divided into three zones: subtidal, intertidal,

and supratidal (Fig. 9.38). The subtidal zone encompasses the part ,of the tidal flat

that normally lies below man low tide [eve!. It is inundated with water most of

the time and is normally subjected to the highest tida1-current velities. Tidal in

fluence in this part of the environment is particularly important within tidal chan

nels, where bedload transrt and deposition are predomu1ant, although this

zone is also influenoed to some extent by wave process. The intertidal zone lies

between mean high and ow tide levels. It is subaerially exposed either once 

twice each day,

dep�nding

upon local wind and tide conditions, but it commonly

does not support significant vegetation. Both bed!oad and suspension sedimen

tation take place in this zone. The Spratidal zone lies above normal high-Hde

level but is incised by tidal channels and flooded by extJ·eme tides. This part of

the tidal flat is exposed to subaerial onditions most of the time but may be flood

ed by spring tides twice each month or by storm tides at irregular intervals. d

imentation is dominantly from suspension, On some tidal flats, the supratidal

zone is a satt�marsh environment incised by tidal channels. In Mid or semiarid

climates, it s commonly an environment of evaporite deposition and s often re

ferred to as a sabkha.

Sedimentary Processes and Sediment

Characteristics of Tidal-Flats

Physical sedimentation on siliciclastic tidal flats takes place in response to both

tidal processes and waves, producing sediments with characteristic grain-size and

structural properties in different parts of the tidal flat (Fig. 9.39). Sedimentation in

the chaels of tidal flats is dominated by tidal currents, but wind-driven waves

and the currents generated by these waves also play an important role in deposi

tion on the flats between charmels (e.g., Ridderinkhof, 1998). Tidal currents move

up the gentle slope of the tidal flat during flood tide and back down during ebb

Schematic diagram showing

the relationship of subtidal, in

tertidal, and supratidal zones

in the tidal-flat environment.

Note that mud is the domi

nant deposit in the upper part

of the intertidal zone, mixed

mud and sand predominate in

the lower intertidal zone, and

sand is deposited in the subti

dal zone and in tidal channels.

Muddy marsh deposits char

acterize the supratidal zone.

High-tide 

water level

Tidal channel

_

•

Flgure 9.39

Supratidal

Hi   -

Low   -

Supratidal

9.6 Tidal-Flat Systems

329

Schematic diagram of a typical siliciclastic tidal flat. The tidal flat fines toward the high-tide level,

passing gradationally from sandflats, though mixed flats, to mudflats and salt marshes. An example

of the upward-fining succession produced by tidal-flat progradation is shown in the upper-left cor

ner. [From Dalrymple, R. W., 1992, Tidal depositional systems, in Walker, R. G., and N. P. james

(eds.), Facies models: Geol. Assoc. Canada, Fig. 12, p. 201 , reproduced by permission.]

tide. The tidal velocities achieved during reversing tides are commonly asymmet

rical, and the velities of fld tides may differ significantly from those of ebb

tides. Within the charu1els, tida• currents can reach velocities of 1.5 m/s or more,

and vefocities on the ats range from 30 to 50 cm/s (Reineck and Singh, 1980).

The

se velities are adequate to cause transport of sandy sediment and produce

ripple and dune  forms, cross�bedding, and plane bedding. Thus, sand deposi

tion dominates  the shallow subtidal zone as well as in the lower intertidal zone

and the channels.

Channel sands are characterized by ripples and internal cross-bedding that

may display bimodal directions of foreset dip, generated by reversing tides. The

sands thus display herringbone cross-stratification; that is, cross-laminated sedi

ment deposited dming  tide dip in the opposite direction to those formed al

most immediately afterward during ebb tide (see Fig. 9.40). Reversing tides

during an asymmetric tidal cyde can also cause erosion of ripple crests during the

next tidal cycle, producing reacti'ation surfaces (Fig. 9.41).

As water fls tidal channels during rising tide, it spills out from the channels

and spreads at relatively low velocities across the tervening flats between chan

nels. Both fi ne sand and mud can be deposited on these low-energy, flat areas.

330

Chapter 9 I Marginal-Marine Environments

Figure 9.40

Progradational succession of tidal-flat deposits based on Middle

Member, Wood Canyon Formation (late Precambrian), Nevada.

Suspension sediment transport was dominant in the upper tidal

flat, alternating bedload and suspension transport occurred in lhe

middle tidal flat, and bedload transport was dominant in the lower

tidal flat. [After Klein, G. deV., 1977, Clastic tidal facies, Fig. 76, p.

85, reproduced by permission of IHRDC Publications, Boston.]

Upper tidal

flat

Middle tidal

flat

Lower tidal

flat

Shale

Alternating shale

and sandstone



�

ser bedding

Herringbone

cross-stratified

sandstone

A. Constructional event

•

Dominant tidal phase

• •

water surfa

figure 9.41

Schematic representation of reactivation sur

face developed owing to alternation of a

dominant tidal phase (constructional event)

with a subordinate phase (destructional

event). [After Klein, G. deV., 1970, Deposi

tional and dispersal dynamics of intertidal

sand bars: Jour. Sed. Petrology, v. 40, Fig.

28, p. 1118, reproduced by permission of

SEPM (Society for Sedimentary Geology),

Tulsa, Okla.]

B. Destructional event

•

Subordinate tidal phase

C. Constructional event

Dominant tidal phase

D. Destructional event

• •

Subordinate tidal phase

9.6 dal-Flat Systems

331

Some tidal flats are dominated by mud whereas others are dominated by sand.

Sandy and muddy sediment deposited on the flats between channels is character-

ized by small-scale ripple cross-lamination, flaser bedding, wavy bedding, lentic-

ular bedding, and, more rarely finely laminated bedding (Fig. 9.39).

The supratidal zone is only slightly affected by tidal currents and only mar

ginally affected by waves. The supratidal zone is the zone of lowest energy on the

tidal at. The deposits of this zone are mainly mud but may include abundant

plant debris in supratidal marshes, which may eventually form peat. Desiccated,

cracked muds are characteristic features of the supratidal zone.

Although most tidal flats are sites of siliciclastic deposition, some tidal flats

are dominated by deposition of carbonate sediments. Thus, deposition on these

tidal flats is characterized by chemical and biological processes as well as physical

processes. Lime muds and sand-size skeletal fragments generated within the sub

tidal zone may be transported into the intertidal and supratidal zones by waves

and currents. In arid and semiarid climates, gypsum, anhydrite, and dolomite

may be chemically precipitated in the supratidal and upper intertidal zones owing

to strong evaporation. Organisms such as pelecypods, crustaceans, polychaete

worms, foraminifers, diatoms, and blue-green algae (cyanobacteria) inhabit tidal

flats and produce fecal pellets, cause extensive bioturbation of sediment, and gen

erate burrows belonging to the Skolithos ichnofacies (Chapter 4). In the supratidal

and intertidal zones blue-green algae are particularly important agents that trap

and bind fine sediment to produce stromatolites.

Transgression and regression cause deposits of laterally adjacent tidal-flat

environments to become superimposed, generating characteristic successions of

vercal facies. Progradation produces a generalized fining-upward succession

at begins with subtidal and lower intertidal cross-bedded sands, followed up

ward by mixed sand and mud in the middle intertidal zone and mud and peat in

the upper intertidal and supratidal zones. A typical vertical regressive (prograda

tional) succession developed on a siliciclastic tidal flat is illustrated schematically in

Figure 9.40; see also Figure 9.39. Tr ansgression presumably generates a coarsening

upward succession that displays the same general facies but in reverse order;

however, transgression may rework and destroy intertidal deposits. Similar pat

terns of subtidal, intertidal, and supratidal carbonate facies could be expected to

develop on coasts characterized by carbonate tidal flats (e.g., Hardie and Shinn,

1986).  a local scale, lateral channel migration may also produce small-scale

coarsening-upward vertical successions.

Ancient Tidai�Aat Sediments

dal-flat deposits have several distctive characteristics that help to differenate

th om sediments of most other environments; however, eir overa character

istics are similar to those of estuarine deposits. The most important criteria for

cognition of ancient dal flat deposits are commonly regarded to include the fol

lowing: (1) bimodal direcon of current-formed cross-bedding resulting om revers

ing tidal currents, (2) the occurrence of sediments that reflect repeated, small-scale

alteations in sediment ansport conditions (tidal rhythmites or tidalites; e.g.,

Kle, 18) and the joint occurrence of large-scale (channel) and small-scale (sandat

and mudflat) structural units in superposition or juxtaposition, (3) abundant reacti

vation surfaces and flaser bedding, and (4) a high frequency of erosional contacts

and abrupt facies changes. Other supporting criteria include the pical vertical suc

ceion of facies discussed above, the high degree of bioturbation of many tidal-flat

sediments, the presence of mudcracked stromatolites, and oer evidence of subaer

ial exposure such as raindrop imprints, hail marks, and animal or bird tracks.

Deposits influenced by tidal action occur in several of the environmental set

tings already discussed (e.g., delta, barrier-island systems, estuaries, lagoons) and

332

Chapter 9 I Marginal-Marine Environments

Figure 9.42

Representative lithostratigraphic column of the

upper member of the Baraichari Shale Formation

(late Miocene-Pliocene), Bengal Basin, Bangladesh,

interpreted as a cyclic succession of progradational

tidal-flat deposits. [After Alam, M. M., 1995, Tide

dominated sedimentation in the upper Te rtiary suc

cession of the Sitapahar anticline, Bangladesh, in

Flemming, B. W., and A. Bartholoma (eds.), Tidal

signatures in modern and ancient sediments, Inter

national Association of Sedimentologists Spec. Pub.

24, Blackwell Science, Oxford, Fig. 4, p. 332, repro

duced by permission.]

Explanation

•

�

Shale

Shale with sandstone/

siltstone streaks

Lenticular bedding

�� Wavy-bedded sandstone

�

Flaser-bedded sandstone

� Ripple-laminated sandstone

Fining-upward

cycle formed by

progradation of

upper inteidal muds

over lower inteidal

sands

also form on shallow, tide-dominated shelves (to be discussed). Numerous exam

ples of such tidal deposits have en reported from stratigraphic units of virtually

all ages from Precambrian to Holene. Fewer examples of ancient tidal deposits

formed specifically in tidal-fat environments have been reported but such de

posits are certainly known, for instance, those described in Alexander, Davis, and

Henry (1998).

Alam (1995) describes a stratigraphic succession in the upper member of the

Baraichari Shale Formation (Late Miocene-Pliocene) from the Bengal Basin,

Bangladesh, that is characterized by many of the classic features of tidal-flat de

posits (Figure 9.42). This cyclic succession contains repeated fining-upward units

of fine-grained sandstone overlain by gray shale that commonly contains thin lay

ers and streaks of sandstone or siltstone. Wavy bedding, flaser bedding, and rip

ple lamination characterize many of the sandstone beds. The sandstone units a

interpreted as lower intertidal deposits and the shales as upper intertidal deposits.

Stacking of sand and mud uts took place during repeated cycles of progradation

(regression), as upper intertidal deposits advanced over lower intertidal deposits.

Further Reading

333

Estuarine and Lagoonal Systems

Ashlev, G. M. (ed.), 1988, e hydrodynamics and sedimentol

ogy of a back-barrier lagoon-salt marsh system, Great Sound,

New Jersey: Marine Geology, v. 82, p. 1-132.

Kjerfve, B. (ed.), 1994, Coastal lagoon processes: Elsevier, Ams

terdam, 577 p.

Mehta, A. J. (ed.), 1993, Nearshore and estuarine cohesive sedi

ment transport: American Geophysical Union, Wa shington

D.C., 581 p.

ordstrom, K. , and C. T. Roman (eds.), 1996, Estuarine shores:

John Wiley & Sons, Chichester, 486 p.

Perillo, G. M. E. (ed.), 1995, Geomorphology and sedimentology

of estuaries, Developments in Sedimentology 53: Elsevier Sci

ence B.V., Amsterdam, 471 p.

Prandle, D. (ed.), 1992, Dynamics and exchanges in estuaries and

the coastal zone: American Geophysical Union, Washington

D.C., 647p.

dai-Aat Systems

Alexander, C. R., R. A. Davis, and  J. Henry (eds.), 1998,

Tidalites: Processes and products: SEPM Special Publication

No. 61, 171 p.

Black, K. S., D. M. Paterson, and A. Cramp (eds.), 1998, Sedimen

tary processes in the intertidal zone: Geological Society Spec.

Pub!. 139, e Geologal Society, London, 409 p.

Eisma, D., P. L. de Boer, G. C. Cadee, K. Dijkema, H. Rid

derinkhof, and C. Philippart, 1998, Intertidal deposits: River

mouths, tidal flats, and coastal lagoons: CRC Press, Boca

Raton, 524 p.

Flemming, B. W., and A. Bartholo

�

a (eds.), 1995,

Tidal signa

tures in modern and ancient sediments, Internatwnal Associ

ation of Sedimentologists Spec. Pub. 24, Blackwell Science,

Oxford, 358 p.

Hardie, L. A., and E. A. Shinn, 1986, Carbonate depositional en

vironments modem and ancient-Part 3: Tidal flats: Col

orado School of Mines Quarterly, v. 81, No. 1, 74 p.

Open University Course Team, 1989, Waves, tides and shallow

water processes: Pergamon Press, Oxford, 187 p.

Smith, D. G., G. E. Reison, B. A. Zaitl, and R. A. Rahmani (eds.),

1991, Clastic tidal sedimentology: Canadian Soc. Petroleum

Geologists Mem. 16, 307 p.

Siliciclastic Marine

Environments

334

10.1 INTRODUCTION

he marine environment is that part of the ocean lying seaward of the zone

dominated by shoreline processes. Water depth in the marine realm ranges

from a few meters to more than 10,000 m. The salinity of seawater in e

open ocean averages about 35 %o, although higher or lower salinities can occur lo

cally in restricted bodies of the ocean. Marine life forms are characterized by gen

erally high diversity and large populations, and most are low-tolerance organisms

adapted to conditions of normal salinity. The energy of the bottom water lying

immediately above the ocean floor is generally low, except on the shallow conti

nental shelf, which is affected by a variety of tidal processes and wind- and storm

wave

activity, and on some parts of the deeper ocean floor that are swept by

bottom currents.

The

major subdivisions of the oceanic realm are the continental margin and

the ocean basin. These regions in tum can be further subdivided as shown 

Figure 10.1. The continental shelf extends seaward from the shoreline at a gentle

slope of about 1 o to a point where a perceptible increase in rate of slope, the shelf

break, takes place. The shelf break occurs in the modern ocean at an average dis

tance from shore of about 75 km, although the distance ranges from a few tens of

meters to more than 1000 km. Av erage water depth at the shelf break  about 130

The continental slope descends from the shelf break to the deep seafloor with a

typical

slope of about 4 °. On passive, or divergent, continental margins, the foot of

the continental slope merges with the continental rise, which is a gently sloping

surface created by coalescing submarine fans at the base of the slope. The conti

nental rise passes gradually into the floor of the ocean basin. Parts of the deep

ocean oor consist of nearly flat areas called abyssal plains, which are covered by

sediment. Other parts of the ocean oor are characterized by volcan hills that

rise above the seafloor to elevations ranging from a few hundred meters to more

than 1000 m. The central part of the major ocean basins is occupied by a gigantic

mid-ocean ridge that may protrude more than 2.5 km above the seaoor.  acve,

convergent, margins, the continental slope may descd into a deep-sea trench,

and the continental rise  abst.  the basis of water depth, we divide the ocean

into two major zones: the neric zone and the oceanic zone. e shallow neritic

zone extends from the shoreline to the shelf break. The oceanic zone extends from

shelf break to shelf break and encompasses the deeper part of the ocean.

10.2 e Shelf Environment

335

Passive

Active

- continental  Ocean basin ______ ___ ntinental

marg1n

!n

In this chapter, we consider marine environments characterized by transport

and

deposition of siliciclastic sediment. Carbonate marine environments are dis

cussed in Chapter 11.

10.2 THE SHEL F ENVIRONMENT

The neritic zone encompasses the shallow-water areas of the ocean lying shore

ward of the shelf break. Although the shelf break on modern shelves lies at an av

erage depth of about 130 m, as indicated, it may be located on some shelves at

depths as shallow as 18m or as deep as 915 m (Bouma et a!., 1982). In the modern

ocean, the shallow-marine environment occupies mainly the continental shelf

area around the margin of the continents, forming what is referred to as a

pericontinental, or marginal, sea (Heckel, 1972). At various times in the geologic

past, broad, shallow epicontinental, or epeiric, seas occupied extensive areas

within the continents (Fig. 10.2), somewhat like the present-day Hudson Bay area

of the North American Arctic region. The following discussion of the neritic envi

ronment is focused primarily on the connental shelf environment because we

can draw on the modern continental shelf environment as a model. Readers

should keep in mind, however, that many of the shallow-marine deposits pre

served in the geologic record may have been deposited in broad epicontinental

seas, for wch we may have no truly representative modern analogs, although

some mode continental shelves are very wide (e.g., continental shelves of the

Figure 10.1

Schematic cross-sectional

profile of the marine envi

ronment. Not to scale.

Figure 10.2

Schematic diagram illustrat

ing the difference between

pericontinental (continental

shelf) and epicontinental

shallow-marine environ

ments. [After Heckel, P. H.,

1972, Recongintion of an

cient shallow marine envi

ronments, in Rigby, J. K.,

and W. K. Hamblin (eds.),

Recognition of ancient sedi

mentary environments:

SEPM (Society for Sedimen

tary Geology) Spec. Pub.

16, Fig. 1, p. 227, reprinted

by permission.]