Walker J. Facies models, Canada, 1992

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DALRYMPLE

current speeds decrease, and the

distal parts of the sand sheet may con-

tain storm-generated structures.

Within active sand ridges, it is be-

lieved that the sediments coarsen

upward because wave action is most

intense on the ridge crests. During a

transgression, however, the progres-

sive increase in water depth causes

current speeds at any location to de-

crease through time. This causes the

bedform zones (Fig. 26) to shift in an

upcurrent direction. In the process, the

sand ridges become moribund. Sedi-

ment is eroded from the ridge crests

by storm waves, and deposited as a

drape on the ridge flanks (Fig. 24).

Shell accumulations may also form on

the ridge crest at this time. It is pos-

sible, based on modern examples, that

some ridge topography will remain, but

the lack of published ancient examples

with a preserved topography suggests

that complete flattening occurs in

many cases. As the transgression pro-

ceeds, there is an upward change

from cross bedding to finer-grained,

rippled and burrowed sediment as

current speeds decrease. The subdued

or flattened ridge is eventually mantled

by mud (Fig. 24).

In some circumstances transgres-

sion leads to the abandonment of est-

uary-mouth ridges on the shelf. For

example, Yang and Sun (1988) and

Yang (1989) have proposed that the

sand ridges seaward of the

Chang-

jiang River in the East China Sea origi-

nated in an estuarine setting, and

were stranded on the shelf as the

coast moved westward during the

Holocene transgression (Fig. 27).

Seismic data show that the internal

structure of these ridges is similar to

that in North Sea shelf ridges (Fig.

21),

but the East China Sea ridges are in-

active now, and are draped by a layer

of shelf mud. Their relief is subdued but

not obliterated. The extent to which

these estuarine ridges have been

re-

moulded on the shelf is unknown.

Transgressive stratigraphy

The transgressive systems tract rests

on a bounding disconformity (Fig.

24)

which was formed subaerially during

the preceding lowstand. The base of

any incised valleys may contain

fluvial

sediments that represent part of the

lowstand systems tract, unless they

have been scoured out by tidal

cur-

INTERTIDAL

SALT MARSH

LEGEND

Inclined Cross-beddlng (Sandwaves)

A'A

A'/\

Current Ripples

Trough Cross-bedding

(3-D

Megadpples)

Wave Ripples

Tabular

Cross-beddlng

(2-D

Megarippks)

111

Bloturbatlon

Parallel Lamlnatlon (U.F.R. Plane Bed) Mud Drapes

.hch

Tidal Bedding

-the

Plant ~oots

...

Pebbles

.*a

Flgure 23 Vertical facies succession which would be developed if a tide-dominated

estuary such as the Cobequid Bay-Salmon River estuary filled by progradation.

mud;

fine sand; MS

medium sand; CS

coarse sand;

gravel; M.H.W.

mean high

water; M.L.W.

mean low water; U.F.R.

upper flow regime. The sediments underlying

this succession may consist of preserved remnants of transgressive estuarine or

fluvial de-

posits, or of older material, depending on the amount of tidal-current erosion during the

transgression. From Dalrymple

eta/.

990).

11.

TIDAL SYSTEMS

Ravinement Surface

Lagoonal

Tidal

Flats

Bounding Discontinuity

Figure

Hypothetical, coast parallel section showing the stratigraphy of a transgressive systems tract, assuming complete preservation of

the estuarine deposits for the purposes of illustration. Less than complete preservation is more likely. The estuarine portion of this figure is

based on the Cobequid Bay

Salmon River and Severn River estuaries, while the marine part is based on the southern North Sea. Autocyclic

erosion surfaces are generated by tidal channel migration, whereas

allocyclic erosion surfaces are related to relative changes in sea level.

Figure

Map showing the net tidal-

transport paths and distribution of grain

sizes on the continental shelf around the

British

Isles (coarse stipple

gravel; fine

stipple

sand; blank areas

mud). Each

sediment transport path, which extends

from a bed load parting (heavy lines) to a

bed load convergence is tens to hundreds

of kilometres long. Deposition occurs in

areas where sediment moves into an area

with lower speeds, and not necessarily at

sites of bed load convergence. Note the

transport into the Severn and Thames es-

tuaries. After Johnson

a/.

(1982) and

Stride

a/.

982).

21 0 DALRYMPLE

rents. They will be overlain by

tide-

dominated estuarine deposits which

comprise the lower portion of the

transgressive systems tract. Exxon's

assignment of estuarine deposits pri-

marily to the

lowstand systems tract

(Van Wagoner

et al.,

1990) seems il-

logical because the estuary only exists

because of transgression. The contact

between the

fluvial and estuarine sedi-

ments is a flooding surface (Chapter

I), although it need not be abrupt ev-

erywhere.

The top of the estuarine sediments

may or may not be marked by an

erosion surface. In the East China Sea

example discussed above (Fig.

27),

the estuarine ridges are stranded on

the shelf without major erosion, and

are draped by shelf muds. There

should be no transgressive surface of

erosion in such cases. By contrast,

sand ridges are not present seaward

of the mouth of the Severn River,

England. Instead, this area is the site

of a bed load parting (Fig.

25), and the

seafloor is eroded by strong tidal cur-

rents. The marine erosion surface that

is formed as the bed load parting mi-

grates landward during the transgres-

sion is the tidal equivalent of the

ravinement surface

formed by shore-

face erosion in wave-dominated set-

tings (see Chapters 10, 12). In such

situations, the shallow marine sand

sheet that forms from reworked older

sediment will rest disconformably on

any preserved remnants of estuarine

deposits (Fig. 24).

The coastline between major estu-

aries is typically characterized by a

mixed energy (wave plus tide) regime

which creates short barrier islands

with numerous tidal inlets. The Dutch

and German coasts provide an exam-

ple of this (Fig.

6).

Landward move-

ment of the barrier island shoreface

will generate a true ravinement sur-

face, in the manner described in

Chapter 10. As a result, the shelf

ridges seaward of the Dutch coast

erosionally overlie and locally contain

remnants of lagoonal, tidal flat deposits

(Cameron

al.,

1989). The disconti-

nuity which separates the lagoonal and

shelf ridge sediments was originally

formed by shoreface erosion, but sub-

sequent scour in the ridge troughs has

modified its form (Fig. 24).

Continued transgression leads to

abandonment of the shelf ridges and

---

their ultimate burial beneath shelf

muds. At some level within the trans-

gressive muds, and probably close to

their base, there is a

maximum flood-

ing surface

(Fig. 24) which is the

upper bounding discontinuity of the

transgressive systems tract.

If estuarine ridge sediments survive

the transgression, distinction between

estuarine and shelf sands is crucial for

a correct interpretation of the strati-

graphic succession. In addition to po-

tentially having differing grain-size

trends (estuarine ridges fine upwards,

whereas active shelf ridges coarsen

upwards beneath an upward-fining

transgressive drape), estuarine sands

are likely to contain more mud in the

form of mud drapes

and/or mud

pebbles than their shelf counterparts

because of the higher suspended sed-

iment concentrations in estuaries.

Neap-spring bundle sequences will be

more clearly developed in estuaries

because of less disruption of the cy-

clicity by storms. The shelly fossils and

ichnofauna (Chapter 4) of estuarine

deposits should show evidence of

Figure

Idealized distribution of bedforms along a sediment transport path, with the

typical maximum (spring-tide), near-surface current speeds

(cmts) associated with each

bedform type. Sand ridges will be developed in the sand-wave zone if there is sufficient

sand. From

Belderson

a/.

982).

brackish water conditions. Estuarine

sands are also intimately associated

with tidal flat sediments, and should be

contained within an incised valley.

Shelf sands will occur as an

areally

extensive sheet that is probably less

than 15 m thick on average.

Highstand systems tracts

As the rate of sea level rise slows at

the end of a transgression, estuaries

become filled. Consequently, there are

no highstand estuaries, and

fluvial

sediment is delivered directly to the

shoreline and shelf. Therefore, tide-

dominated, highstand systems tracts

are characterized by progradation,

either of tide-dominated deltas

andlor

open-coast tidal flats. Both of these

should extend seaward into a regres-

sive shelf.

Tide-dominated deltas

Deltas which experience strong tidal

action represent one of three end

members within the continuum of delta

types (Chapters 1, 9; Coleman and

Wright, 1975; Galloway, 1975). Sev-

eral examples have been described,

including the Klang-Langat (Malaysia;

Coleman et

a/., 1970), the Colorado

River delta (Gulf of California; Meckel,

11. TIDAL SYSTEMS 21 1

1975), and the Ord River (northern

Australia; Wright et

a/., 1975), al-

though this latter system may be an

estuary. The Mahakam delta (Indo-

nesia; Allen

eta/., 1979) also displays

strong tidal influence. These deltas

tend to occur at the heads of

embay-

ments where the tidal range is ampli-

fied, or adjacent to narrow straits

where tidal current speeds are high.

The subaqueous delta plain of tide-

dominated deltas is characterized by

tidal sand ridges which are oriented at

a high angle to the general trend of the

coastline (Fig.

28), unless the delta

borders a strait. Near the shoreline

these ridges may have a relief of up to

15-20 m, but the relief gradually de-

creases away from the distributar-

ies and they pass outward into the

smooth, muddy delta front and

prodelta

in depths of 10-30 m. These ridges are

best developed where the ratio of tidal

fluvial discharge is high. Thus, the

abandoned portion of the delta plain

which receives little

fluvial discharge is

the part most likely to contain tidal

sand bars (Allen et

a/., 1979), espe-

cially if the tidal range is mesotidal. In

macrotidal areas, even the active dis-

tributaries contain sand ridges.

The subaerial portion of the delta

Figure

Evolution of the tidal sand-ridge field in the East China Sea during the Late

Wisconsinan-Holocene transgression. These ridges overlie incised channels which are inter-

preted to be filled with

fluvial and estuarine deposits. From Yang and Sun

988).

plain is characterized by broad tidal

flats that grade seaward into the sand

bars, and landward into either vege-

tated marshes if the climate is humid

(the Klang-Langat delta), or evapora-

tive flats if the climate is arid (Colorado

and Ord Rivers). The tidal flats and

marshes are dissected by tidal gulleys

that have highly meandering channel

patterns. Such gulleys are especially

abundant in inactive portions of the

delta plain (Fig. 28).

Progradation of the active distribu-

taries generates a gradational,

upward-

coarsening succession which passes

from

prodelta muds into interbedded

sands, silts and muds of the delta front.

These are in turn gradationally overlain

by the fine to medium sands of the tidal

ridges (Fig.

298; Coleman and Wright,

1975; Meckel, 1975; Allen

etal., 1979).

Tidal modulation of the sediment dis-

charge (sediment output is impeded

during the flooding tide and enhanced

on the ebb tide) may produce tidal

rhythmites in the portions of the delta

front where wave action does not

disrupt the regularity of the stratification

(Williams, 1991). The medium sands of

the ridges contain abundant cross bed-

ding, with ebb-oriented paleocurrents

predominating near the active distribu-

taries. Continued progradation causes

the distributary

channel(s) themselves

to extend into the sand ridge field, ero-

sionally truncating the ridge deposits

locally (Fig.

29A). The distributary

channel sediments fine upward and

contain more slack-water mud drapes

than do either the nontidal

fluvial chan-

nels up river (Allen

etal., 1979), or the

sands of the ridges further seaward

where wave action inhibits mud depo-

sition.

In the abandoned parts of the delta

plain which are undergoing local trans-

gression due to compactional subsi-

dence, tidal current scour in the chan-

nels between the ridges erosionally

truncates the underlying delta front

sediments (Fig.

29C). As a result, the

sand ridges in these areas are com-

monly erosionally based (Allen et a/.,

1979), and fine upward like estuarine

ridges. Flood-oriented cross bedding

and shell accumulations will be more

common here than in areas adjacent

to the active distributaries.

In all parts of the delta plain, the

ridge sands are gradationally overlain

by an upward-fining succession of tidal

21 2

DALRYMPLE

flat and marsh sediments (Fig.

29).

Erosionally based tidal creek deposits

that contain lateral-accretion bedding

are common. In humid tropical areas,

saltwater and freshwater peat swamps

are widespread on the delta plain, and

organic detritus is a major constituent

of the "mud" drapes. In arid areas, the

tidal flats are barren, mud cracks are

abundant, and evaporites may be de-

veloped.

Open-coast tidal flats

In coastal areas adjacent to a major

delta, alongshore transport of sedi-

ment by tidal and shelf currents leads

to the development of open-coast tidal

flats like those downdrift from the

Colorado River delta (Thompson,

1968).

These tidal flats are generally

muddy near the delta, but become

progressively more sandy as the dis-

tance from the delta increases (Fig.

28)

and eventually grade alongshore

into beaches because of the increas-

ing influence of waves. Variations in

the supply of sediment due to delta

switching cause periods of tidal flat

progradation to alternate with epi-

sodes of wave erosion when beach

ridges composed of sand

and/or shells

are formed. Tidal flat

chenier plains

formed in this manner are present in

areas adjacent to the Colorado River

delta (Thompson,

1968),

and at var-

ious places along the Chinese coast

(Wang,

1 983).

Regressive shelves

Sediment supply to the shelf during a

highstand should lead to gradual shal-

lowing and the development of regres-

sive conditions. Unfortunately, no

modern examples of regressive, pro-

grading tidal systems are sufficiently

well documented to serve as a model.

There are also surprisingly few ancient

examples (Mutti

a/.,

1985),

in con-

trast to the abundance of regressive

storrn/wave-dominated shelves (Chap-

ter

12).

Because of this, the following

description cannot be considered as a

facies model in the sense discussed in

Chapter

Because the processes which

produce unidirectional, net sediment

transport paths are not dependent on

the history of relative sea level, regres-

sive shelves are expected to show re-

gional changes in facies which are

similar to those on transgressive

shelves (Fig.

26),

although there will

be notable differences. As sediment is

most likely input at the upstream end

of a transport path, the erosional zone

with furrows and sand ribbons will not

be present. Instead, there may be

direct passage from deltaic sand

ridges into shelf ridges with little clear

separation. Sand waves should be

abundant. From here, the sediment

will fine down the transport path,

passing outward into shelf muds. The

overall grain size is likely to be finer

than in transgressive settings, and

mud will be relatively abundant.

Progradation of an individual shelf

ridge produces an upward-coarsening

succession (Fig.

30)

that may reach

m in thickness. It begins with bio-

turbated muds and sands that contain

Cruziana

ichnofacies (Chapter

4).

The sand beds become thicker and

the proportion of muddy interbeds de-

creases upward. The succession is

capped by cross-bedded medium to

fine sand of the ridge crest, which is in

turn abruptly overlain by shelf mud of

the next succession. Evidence of tidal

_----------__--

PRODELTA

Figure

Schematic map of a tide-dominated delta, based on the Ganges-Brahmaputra,

Ord, Colorado and Klang-Langat deltas. Note the morphological distinction between the

active and inactive (abandoned) portions of the delta plain, and the outward transition from

salt marsh, through mud flats and sand ridges, to the muddy prodelta. In areas adjacent to

the embayment, open-coast tidal flats and chenier plains are developed.

11.

TIDAL

SYSTEMS

A-

FLUVIAL

B-

PROGRADING

C-

ABANDONED

DISTRIBUTARY

SAND RIDGE DELTA PLAIN

SALT MARSW SALT MARSW

SUPRATIDAL FLATS SUPRATIDAL FLATS

TIDAL

CHANNELS

TIDAL FLAT

TIDAL

RIDGE

DISTAL

RIDGE/

DELTA FRONT

PRODELTA AND

SHELF MUD

Sandatone Mudstone Coal

Mudstono or

Plant Fragment,

Planar-tabular Trough

Ripple

-~ros8 Bedding Cross Bedding Lamination

Shell Debris

Rootlets

Desiccation

MFS

Maximum Flooding

Cracks Surface

Upward-fining

Successions

Rgure

Three hypothetical, vertical facies successions within tide-dominated deltas,

based on Coleman and Wright

(1975), Meckel (1975) and Allen

et a/.

(1979). In this

example, the climate is humid, leading to the development of coal (black) in the subaerial

portions of the delta plain. MFS

maximum flooding surface.

BAR

CREST

BAR

SLOPE

Figure

Schematic vertical succession

through a prograding sand ridge in a re-

gressive shelf setting, based on the

Eocene of the Ager Basin, northern Spain.

The vertical succession is typically about

5 m thick. After Mutti

etal.

(1985).

Figure

(right) Stratigraphic section

through Ordovician tidal fiat deposits,

Capetown, South Africa, showing repeated

upward-fining tidal flat successions (A.J.

Tankard, pers. comm., 1991). The sandier

middle portion of the section contains a

greater proportion of

subtidal sands, and

accumulated in deeper water, on average,

than the base and top of the formation.

After Tankard and

Hobday (1 977).

metres

DALRYMPLE

activity is provided by reactivation sur-

faces, mud drapes and local bipolar

paleocurrents. Bioturbation of the

sands may range from negligible to

intense, but is probably greater on

average than in estuarine sediments.

Unlike the relatively thin nature of

transgressive shelf units, regressive

shelf deposits are typically several

tens to hundreds of metres thick.

Regressive stratigraphy

The stratigraphy of tide-dominated,

highstand systems tracts remains

largely speculative because of the

scarcity of documented examples. The

shelf, deltaic and coastal tidal flat de-

posits should

downlap onto the maxi-

mum flooding surface at the top of the

underlying transgressive systems

tract, in the same manner as is seen in

Figure

32 Reconstructed paleogeography of the Rancho Rojo Sandstone, Arizona. The

storm-dominated

shelves

(Chapter

upward decrease in set thickness and grain size from facies

to facies

is interpreted to

12)'

The

overall

succession

will

represent progressive deepening, erosional transgression of an estuary, and establishment

'pward coarsening

and

'pward

of shallow marine conditions. After Kreisa

eta/.

(1986).

shallowing, from offshore andlor pro-

delta mids, into the burrowed to

cross-bedded sands of the shelf or

deltaic sand ridge zone (Fig.

29B).

Numerous small, upward-coarsening

sand ridge successions (Fig. 30) may

be present. In deltaic settings, these

ridge deposits will be erosionally over-

lain by distributary channels (Fig.

29A).

Tidal current scour in the abandoned

portion of the delta plain may also trun-

cate the sand ridges (Fig.

29C). Along

nondeltaic shorelines, open-coast tidal

flats will overlie the shelf sediments.

The contact may be marked by an ero-

sional discontinuity produced by wave

erosion during the formation of chenier

ridges. Above these various disconti-

nuities, the sediments will generally

fine upward, due to deposition on tidal

flats or in distributary channels. The

upper bounding discontinuity of the

systems tract will be a terrestrial

erosion surface produced during the

subsequent sea level fall.

ANCIENT EXAMPLES OF

TIDE- DOMINATED DEPOSITS

Coastal

successlons

There are numerous descriptions of

ancient tidal flat deposits

(e.g.,

Tankard

and

"Obday'

Figure

A) Distribution of Eocene tidal sand ridges (heavy lines) in

the

Vlierzele Sands

Kvale

and

Archer'

990)'

Most

(Eocene, Belgium).

and C) Vertical profile and section through a sand ridge. Note the in-

these

display many

all

the

fea-

clination of the stratigraphic units (C), and the overall upward-coarsening succession

(6).

tures

tidal

flat progradation

The enigmatic massive sands of units D and F are interpreted as rapidly formed storm de-

(Fig. 121, even

the overall stratbra-

posits, even though they do not have the typical features of storm beds (see Chapter 12).

phic setting is transgressive. The paleo-

From Houthuys and Gullentops (1988).

_-_._

11. TIDAL SYSTEMS

--215

geographic and sequence-stratigraphic

context of many of the documented

tidal flat successions is not well

known, but most are interpreted as

la-

goonal and estuarine deposits, and

thus occur within transgressive

systems tracts.

Sellwood (1975) de-

scribes one such tidal flat deposit in

the Jurassic of Bornholm, Denmark.

Here, a single 13 m-thick, upward-

fining, tidal flat succession overlies

fluvial deposits and is in turn erosion-

ally overlain by shallow-marine sand-

stones. Open-coast and

delta-associ-

ated tidal flats and their accompanying

chenier ridges in a highstand systems

tract have been identified much less

commonly. However,

Kvale and

Archer (1990) have described an 8

thick unit of tidal flat sediments that is

interbedded between coals in a deltaic

succession. Tidal rhythmites are well

developed. This interval of tidal sedi-

ments, which shows an unusual sym-

metric, upward-coarsening upward-

fining succession, represents a trans-

gressive-regressive episode in the

overall progradation of the delta plain.

Thicker sections containing a large

number of stacked, tidal flat succes-

sions have been described from many

localities, including the Ordovician of

South Africa (Tankard and

Hobday,

1977). In this example, the 55 m-thick

Graafwater Formation consists of re-

peated, erosionally based, upward-

fining (progradational) tidal flat

successions that are typically 1-5 m

thick (Fig. 31). This remarkable vertical

stacking of tidal flat sediments implies

that a close balance existed between

accumulation and subsidence. Similar

successions in carbonate rocks have

received considerable attention in

recent years (Chapter 16) because it is

believed that they contain a detailed

record of sea level fluctuations.

Shelf successions

The deposits of ancient tide-domi-

nated shelves are characterized by

cross-bedded sandstone. In most ex-

amples, the

paleocurrents are essen-

tially

unimodal, although bipolar cross

stratification is present locally. This pa-

leocurrent pattern is usually attributed

to the presence of regional, net sedi-

ment transport paths,

and/or to the su-

perposition of a unidirectional oceanic

current. Reactivation surfaces are

abundant, but regularly spaced mud

drapes and neap-spring cyclicity are,

not particularly common.

Tidal shelf deposits are known from

both transgressive and regressive

(highstand) systems tracts, even

though several authors (Banerjee,

1991) have suggested that tidal

deposi-

Tidal Bars

tion is favoured by transgressions.

The Permian Rancho Rojo Sand-

stone of Arizona (Kreisa

al.,

1986) il-

lustrates the main features of a

transgressive, tidal systems tract (Fig.

32). Like most transgressive deposits,

this unit is thin (maximum 18 m), and

disconformably overlies and

onlaps ter-

restrial sediments. The lowest part

(facies 1) is interpreted to lie within an

incised valley, and contains very large,

simple and compound cross bedding

(3-12 m-thick sets) with a bipolar

(east-

west) paleocurrent pattern. Re-

activation surfaces, mud drapes and

mud

intraclasts are abundant. This is

erosionally overlain by a unit of

smaller-

scale cross bedding (sets el m thick)

that has a bipolar (north-south)

pale-

ocurrent (facies 2). Re-activation sur-

faces occur, but mud drapes are not

reported. This is followed by rippled

and

bioturbated marine sands (facies

which become finer grained upwards.

The entire succession reflects a de-

crease in tidal energy, due to In-

creasing water depth as the trans-

gression proceeded. The

90" change in

paleocurrent direction, the decrease in

the abundance of mud drapes, and the

erosional contact between facies 1 and

are interpreted as resulting from

transgression of an estuary, followed by

shelf sedimentation.

Bioclastic Cross

Bedded Facies

Tsunami Layer

Figure

Diagrammatic cross section of the Baronia tide-dominated deltaic and regressive shelf depositional system, Ager Syncline,

northem Spain. After Mutti

985).

DALRYMPLE

Although abundant in modern

shallow seas, tidal sand ridges are

rarely described from the stratigraphic

record. One of the most convincing

examples occurs in Eocene sands

in Belgium (Fig. 33; Houthuys and

Gullentops,

1988), which are 10-20 m

thick in the axes of the ridges. Cross

bedding is the dominant structure, and

there is abundant evidence of regu-

larly varying current speeds and direc-

tions in the form of thin mud layers,

reactivation surfaces and bipolar

paleo-

currents. The cross bedding is ori-

ented parallel or slightly oblique to the

ridge axis, as is expected from modern

ridges, and the boundaries between

cosets dip at low angles

(2-3"; Fig.

33C) perpendicular to the ridge length.

These surfaces are presumably equiv-

alent to the inclined seismic reflectors

seen in modern ridges (Fig. 21). Both

vertical and horizontal burrowing are

locally abundant. The ridge sediments

erosionally overlie an

upward-coars-

ening, regressive succession of

marine clays and bioturbated fine

sands (Fig.

338). The limited thick-

ness of the succession suggests that

deposition occurred during a sea level

fall, and represents a

forced regres-

sion

(Chapter 12).

A particularly good example of a

tide-dominated regressive shelf and

delta is provided by the Eocene

Baronia deltaic system of northern

Spain (Fig. 34; Mutti

a/.,

1985),

which consists of up to 275 m of in-

terbedded sandstones and mud-

stones. The basal part is transgres-

sive, and

onlaps onto bedrock highs

(Fig. 34). This lower unit accumulated

in an eastward-closing embayment,

and consists primarily of upward-

coarsening, shelf sand ridge deposits

(Fig.

30), each 5-6 m thick, that are

encased in bioturbated shelf muds.

Regressive periods are characterized

by seaward-prograding ridges com-

posed of

siliciclastic sands, whereas

transgressive episodes are marked by

landward-migrating ridges composed

of cross-bedded

bioclastic material.

The lower unit is capped by a deeper

water, bioturbated sandy

mudstone

that represents the culmination of the

transgression.

Following a hiatus that may be

related to tectonic activity,

prograda-

tion began from a deltaic sediment

source along the southern margin of a

through-going seaway (Fig. 34, middle

unit). Deltaic sand ridges (the "estuary

mouth shoals" of Mutti

a/.,

1985)

and regressive shelf ridges prograded

to the east, in the direction of the dom-

inant tidal currents, producing an up-

ward-coarsening succession that

down-

laps onto the underlying discontinuity

(Fig.

34).

This regressive succession is

then erosionally overlain by upward-

fining, delta distributary channels

("estuary channelsn of Mutti

a/.,

1985). Individual channels are 5-20 m

thick, but occur in amalgamated com-

plexes up to 40 m thick. Almost all of

the cross bedding dips seaward, and

mud drapes are abundant. These

channels are overlain by upward-fining

tidal flat deposits. Delta progradation

is terminated by another transgres-

sion, which is followed by a final phase

of deltaic and tidal flat progradation

from the west (Fig. 34, upper unit).

Many Late Precambrian and Cam-

brian tidal shelf deposits are not easily

divisible into transgressive and regres-

sive intervals. Such units are hundreds

to thousands of metres thick, and con-

sist largely of cross-bedded sandstone

with little or no mud. For example,

the

Loyver Sandfjord Formation (Late

Precambrian) of northern Norway

(Levell, 1980) is 1.5 km thick, and con-

tains a variety of sizes and styles of

simple and compound cross bedding.

The

paleocurrents are predominantly

unimodal, although reversals occur.

Numerous reactivation surfaces are

present. Storm-generated event beds

are rare. A clearly defined sequential

ordering of grain sizes or facies is

lacking in this and similar units, but

from their thickness it is evident that

deposition must have occurred over

numerous sea level cycles.

Portions of the Gog quartzites (Cam-

brian) of the southern Canadian

Rockies have similar characteristics,

but with somewhat greater amounts of

shale.

Hein (1987) interprets the rela-

tively disordered succession as re-

sulting from the lateral migration of

offshore sand ridges, without a strong

overprint related to fluctuations of sea

level.

PRESENT STATUS AND FUTURE

DIRECTIONS

The recent recognition of neap-spring

tidal-bundle sequences and tidal

rhyth-

mites has provided major new criteria

for the identification of tidal sediments.

Care must be exercized in the applica-

tion of these criteria because not every

thinly interbedded unit of sand and

mud is a tidal

rhythmite, nor do mud-

draped foresets necessarily indicate

the presence of tidal bundles. How-

ever, if tidal periodicities can be deter-

mined with confidence (De Boer

a/.,

1989), then unequivocal evidence of

tidal deposition is present. In the

absence of these diagnostic struc-

tures, the presence of abundant reacti-

vation surfaces and mud drapes, bi-

polar

paleocurrents, the various types

of so-called "tidal" bedding (Fig.

lo),

and emergence indicators may be

used to infer a tidal origin. As with any

environmental reconstruction, tidal in-

terpretations should be based on the

entire association of structures within

the deposit.

Despite the large body of literature

on tidal sedimentation, not all environ-

ments are known in equal detail. Tidal

flats, which are the most accessible of

tidal environments, are best known,

and tide-dominated estuaries are be-

coming better understood through

work in such areas as the Bay of

Fundy and the Severn River. The mor-

phological and textural attributes of

transgressive shelves are well docu-

mented, but our understanding of the

internal structure of

subtidal sand

waves and sand ridges remains large-

ly theoretical. Our greatest knowledge

gap lies with regressive tidal settings,

for which there are few modern or

ancient examples. Is this due to a real

scarcity of such deposits, or to the

absence of appropriate models?

Tidal facies models which include the

influence of relative sea level changes

are only beginning to emerge. Re-

search on this aspect of tidal sedimen-

tation has lagged behind that in

wave-dominated systems. Models for

transgressive systems tracts are the

most firmly established (Fig. 24)

because of the abundance of modern

transgressive environments, but remain

partially theoretical. The models for

highstand systems tracts are even less

well constrained, being based mainly

on a few ancient examples (Fig. 34).

None of these models, with the excep-

tion of that for an individual tidal flat

succession (Fig

12), has the generality

that the models for

wavelstorm-domi-

nated coasts and shelves have.

ACKNOWLEDGEMENTS

thank Roger Walker for his helpful

editorial comments on earlier versions

of the manuscript. Ela Rusak Mazur

and John Glew drafted the figures.

research on tidal sedimentation has

been funded by the Natural Sciences

and Engineering Research Council of

Canada.

Weimer, R.J., Howard, J.D. and Lindsay,

D.R., 1982, Tidal flats and associated

tidal channels,

Scholle, P.A. and

Spearing, D., eds., Sandstone deposi-

tional environments: American Asso-

ciation of Petroleum Geologists, Memoir

31,

p. 191 -245.

review of tidal flat sedimentation, with

modem and ancient examples.

Other references

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Allen, J.R.L., 1980, Sand waves; a model

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