Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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organic-rich shales. Seasonal temperature variations

in permanent saline lakes result in a fine layering.

This is due to direct precipitation of calcium carbonate

as aragonite needles in the hot summer to form white

laminae, which alternate with darker laminae formed

by clastic input when sediment influx is greater in the

winter.

10.4 EPHEMERAL LAKES

Large bodies of water that periodically dry out are

probably best described as ephemeral lakes,

although the term playa lake is also commonly

used (Briere 2000). Terms such as ‘saline pan’ are

also sometimes used to describe these temporary lake

environments. It is perhaps simpler to just use the

term ‘ephemeral lakes’ as this unambiguously implies

that the water body is temporary. They occur in semi-

arid and arid environments where the rainfall is low

and the rate of evaporation is high.

Many desert areas are subject to highly irregular

rainfall with long periods of dry conditions inter-

rupted by intense rainfall that may occur only every

few years or tens of years. After rainfall in the catch-

ment area, the rivers become active (9.2.3) and flash

floods supply water and sediment to the basin centre

where it ponds to form a lake. Once the lake has

formed, particles suspended in the water will start to

deposit and form a layer of fine-grained muddy sedi-

ment (Lowenstein & Hardie 1985). Evaporation of the

water body gradually reduces its volume and the area

of the lake starts to shrink, leaving areas of margin

exposed where desiccation cracks (4.6) may form in

the mud as it dries out. With further evaporation the

ion concentration in the water starts to increase to

the point where precipitation of minerals occurs. The

Increasing amount of evaporation

Normal waters

Ca~Mg

HCO

~ Ca + Mg

Sulphate and

chloride waters

Ca >> Mg

HCO

<< Ca + Mg

Sodium-enriched

carbonate waters

Ca >> Mg

HCO

>> Ca + Mg

Calcium

carbonate

CaCO

Halite

NaCI

Mg salts

& others

Gypsum

CaSO

.2H

Calcium

carbonate

CaCO

Halite

NaCI

& other

salts

Sodium sulphates

e.g. mirabilite

.10H

Calcium

carbonate

CaCO

Halite

NaCI

& other

salts

Soda salts

e.g. trona

NaHCO

.NaCO

.2H

Gypsum

CaSO

.2H

Fig. 10.8 Three general types of saline lake can be distinguished on the basis of their chemistry.

Fig. 10.9 A salt crust of minerals formed by evaporation in

an ephemeral lake.

158 Lakes

least soluble minerals will precipitate first, followed by

other evaporite minerals until the lake has dried

up completely (Fig. 10.9). The resulting deposit con-

sists of a layer of mud overlain by a layer of evaporite

minerals (Fig. 10.10) The minerals formed by eva-

poration will be determined by the chemistry of the

waters and show the same ranges of composition as

is found in saline lakes. Subsequent flooding of the

lake floor following another flood event does not

necessarily result in solution of the evaporite minerals

on the surface as they may become quickly blanketed

by mud.

Repeated flooding and evaporation results in a ser-

ies of depositional couplets of a layer of mud over-

lain by a layer of evaporite minerals: these couplets

are typically a few millimetres to centimetres thick

and are a characteristic signature of ephemeral lake

deposition (Lowenstein & Hardie 1985). Ephemeral

lake deposits occur in arid environments and

are therefore likely to be associated with other facies

formed in these settings: these will include aeolian

sandflat and dune deposits, alluvial fan facies and

material deposited by flash floods from ephemeral riv-

ers. These facies are likely to be found interfingering

with ephemeral lake deposits to form a facies associa-

tion characteristic of arid depositional environments.

Evaporite minerals also form within the sediments

surrounding ephemeral lakes. In these areas the

cycle repeats:

generates

mudstoneevaporite

couplets

Influx of water and suspended sediment

Settling out of suspension as a layer of mud

Lake shrinks, desiccation at margins

Concentration of ions leads to evaporite precipitation

Complete evaporation of lake waters

mudcracks

Fig. 10.10 When an ephemeral lake receives an influx of water and sediment, mud is deposited from suspension to form a thin

bed that is overlain by evaporite minerals as the water evaporates. Repetitions of this process create a series of couplets of

mudstone and evaporite.

Ephemeral Lakes 159

sediment is saturated with saline groundwater, which

evaporates at the ground surface, concentrating the

dissolved minerals and leading to the crystallisation

of evaporite minerals. These regions are sometimes

referred to as inland sabkhas (cf. coastal sabkhas:

15.2.3) and the most common mineral to be formed is

gypsum, which grows within the sediment in an

interconnected mass of bladed crystals known as

desert rose.

10.5 CONTROLS ON LACUSTRINE

DEPOSITION

The characteristics of the deposits of lacustrine envi-

ronments are controlled by factors that control the

depth and size of the basin (which are largely deter-

mined by the tectonic setting), the sediment supply to

the lake (which is a function of a combination of

tectonics and climatic controls on relief and weath-

ering) and the balance between water supply and loss

through evaporation (which is principally related to

the climate). If the climate is humid a lake will be

hydrologically open, with water flowing both in and

out of it. Such lakes can be considered to be overfilled

(Bohacs et al. 2000, 2003), and their deposits are

characterised by accumulation both at the margins,

where sediment is supplied to deltas and beaches, and

in the deep water from suspension and turbidity cur-

rents. The lake level remains constant, so there is no

evidence of fluctuations in water depth under these

conditions.

A balanced fill lake is one where the fluvial input

is approximately balanced by the loss through eva-

poration. These lakes are sensitive to variations in the

climate because a reduction in water input and/or an

increase in evaporation (drier and/or warmer condi-

tions) will result in a fall in the water level below the

sill and the system becomes hydrologically closed.

The area of the lake will contract, shifting the lake

shoreline towards the basin centre and leaving a pe-

ripheral area exposed to subaerial conditions where

desiccation cracks may form, plants colonise the sur-

face and pedogenic processes modify the sediment. A

fall in the water level will also bring parts of the lake

floor that were previously below the wave base into

shallower water where wave ripples rework the sedi-

ment. These changes will be reversed if the climate

reverts to wetter and/or cooler conditions, as the lake

level rises and the lake margin is reflooded. The deposits

of lakes subject to climatic and lake level fluctuations

will exhibit frequent vertical changes in facies.

Saline and ephemeral lakes are underfilled

(Bohacs et al. 2000, 2003). Some saline lakes may

become relatively fresh if there is a change to wetter,

cooler conditions, allowing the formation of a strati-

fied water body and consequently the accumulation of

organic-rich sediment on the lake floor. A return to a

drier climate increases evaporation and concentration

of ions leading to evaporite deposition. Cycles of cli-

mate change can be recognised in some lake deposits

as alternations between dark, carbonaceous mudrock

(sometimes oil shales: 3.6.3) and beds of gypsum and

other evaporite minerals. The areas of shallow saline

and ephemeral lakes can show considerable varia-

tions through time as a result of changes in climate.

The rate of sediment supply is significant in all

lacustrine environments. If the rate of deposition of

clastic, carbonate and evaporite deposits is greater

than the rate of basin subsidence (Chapter 24) the

lake basin will gradually fill. In overfilled lake settings

this will result in a change from lacustrine to fluvial

deposition as the river waters no longer pond in

the lake but instead flow straight through the former

lake area with channel and overbank deposits accu-

mulating. Balanced fill and underfilled basins will also

gradually fill with sediment, sometimes to the level of

the sill such that they also become areas of fluvial

deposition.

10.6 LIFE IN LAKES AND FOSSILS

IN LACUSTRINE DEPOSITS

Palaeontological evidence is often a critical factor in

the recognition of ancient lacustrine facies. Fresh-

water lakes may be rich in life with a large number

of organisms, but they are of a limited number of

species and genera when compared with an assem-

blage from a shallow marine environment. Fauna

commonly found in lake deposits include gastropods,

bivalves, ostracods and arthropods, sometimes occur-

ring in monospecific assemblages, that is, all organ-

isms belong to the same species. Some organisms,

such as the brine shrimp arthropods, are tolerant of

saline conditions and may flourish in perennial saline

lake environments.

Algae and cyanobacteria are an important compo-

nent of the ecology of lakes and also have sedimento-

logical significance. A common organism found in

160 Lakes

lake deposits are charophytes, algae belonging to the

Chlorophyta (3.1.3), which are seen in many ancient

lacustrine sediments in the form of calcareous

encrusted stems and spherical reproductive bodies.

Charophytes are considered to be intolerant of high

salinities and the recognition of these millimetre-

scale, often dark, spherical bodies in fine-grained

sediment is a good indicator of fresh or possibly brack-

ish water conditions.

Cold, sediment-starved lakes in mountainous or polar

environments may be sites of deposition of siliceous

oozes (3.3). The origin of the silica is diatom phyto-

plankton, which can be very abundant in glacial

lakes. These deposits are typically bright white cherty

beds that are called diatomites , and they are basi-

cally made up entirely of the silica from diatoms.

10.7 RECOGNITION OF LACUSTRINE

FACIES

If the succession is entirely terrigenous clastic mate-

rial, it is not always easy to distinguish between the

deposits of a lake and those of a low energy marine

environment such as a lagoon (13.3.2 ), the outer part

of a shelf (Chapter 14) or even the deep sea (Chapter

16). Shallow lake facies will have similar character-

istics to lagoonal deposits, with wave ripple sands

interbedded with muds deposited from suspension,

while the deeper environments of a lake resemble

those of seas with similar or greater depths, as they

include deposits from suspension and turbidites. The

main criteria for distinguishing between lacustrine

and marine facies are often the differences between

the organisms and habitats that exist in these envi-

ronments.

There are a number of groups of organisms that are

found only in fully marine environments: these

include corals, echinoids, brachiopods, cephalopods,

graptolites and foraminifers, amongst others (3.1.3).

The occurrence of fossils of members of these groups

therefore provides evidence of marine deposition.

There are many genera of other phyla that can be

used as indicators if found as fossils, in particular

there are groups of bivalve and gastropod molluscs

that are considered to be freshwater forms, and fish

that are thought to be exclusively lake-dwellers.

Some of the more reliable indicators of freshwater

conditions are algal and bacterial (3.1.3) fossil

forms. Reliance on fossils to provide indicators of

lacustrine environments becomes more difficult in

rocks that are from further back in the stratigraphic

record, and in Precambrian strata it may be almost

impossible to be sure.

A feature of lakes that is much less commonly

found in marine settings is the stratification of the

water body (Fig. 10.3). The lack of mixing of the

oxygenated surface water with the lower part of

the water column results in anaerobic conditions at

the bottom of a deep, stratified lake. Animals are

unable to tolerate the anaerobic conditions so the

lake floor is devoid of life, and therefore there is no

bioturbation (11.7). Deep lake deposits may therefore

preserve primary sedimentary lamination that in

marine environments is typically destroyed by bur-

rowing organisms. The anoxia also prevents the aero-

bic breakdown of organic material that settles on the

lake floor, allowing the accumulation of organic-rich

sediments. The deposits of saline and ephemeral lakes

usually can be distinguished from marine facies by the

chemistry of the evaporite minerals.

Characteristics of lake deposits

. lithologies – sandstone, mudstone, fine-grained

limestones and evaporites

. mineralogy – variable

. texture – sands moderately well sorted

. bed geometry – often very thin-bedded

. sedimentary structures – wave ripples and very fine

parallel lamination

. palaeocurrents – few with palaeoenvironmental sig-

nificance

. fossils – algal and microbial plus uncommon shells

. colour – variable, but may be dark grey in deep lake

deposits

. facies associations – commonly occur with

fluvial deposits, evaporites and associated with aeo-

lian facies

Further Reading 161

Studies in Geology 46, American Association of Petroleum

Geologists, Tulsa, OK; 3–34.

Bohacs, K.M., Carroll, A.R. & Neal, J.E. (2003) Lessons from

large lake systems – thresholds, nonlinearity, and strange

attractors. In: Extreme Depositional Environments: Mega End

Members in Geologic Time (Eds Chan, M.A. & Archer, A.

W.). Geological Society of America Special Paper 370,

Boulder, CO; 75–90.

Carroll, A.R. & Bohacs, K.M. (1999) Stratigraphic classifica-

tion of ancient lakes: balancing tectonic and climatic con-

trols: Geology, 27, 99–102.

Matter, A. & Tucker, M.E. (Eds) (1978) Modern and Ancient

Lake Sediments, Special Publication 2, International Asso-

ciation of Sedimentologists. Blackwell Scientific Publica-

tions, Oxford, 290 pp.

Talbot, M.R. & Allen, P.A. (1996) Lakes. In: Sedimentary

Environments: Processes, Facies and Stratigraphy (Ed. Read-

ing, H.G.). Blackwell Scientific Publications, Oxford;

83–124.

Tucker, M.E. & Wright, V.P. (1990) Lacustrine carbonates.

In: Carbonate Sedimentology. Blackwell Scientific Publica-

tions, Oxford; 164–190.

162 Lakes

The Marine Realm:

Morphology and Processes

The oceans and seas of the world cover almost three-quarters of the surface of the planet

and are very important areas of sediment accumulation. The oceans are underlain by

oceanic crust, but at their margins are areas of continental crust that may be flooded by

seawater: these are the continental shelves. The extent of marine flooding of these

continental margins has varied through time due to plate movements and the rise and

fall in global sea level related to climate changes. The sedimentary successions in these

shallow shelf areas provide us with a record of global and local tectonic and climatic

variations. There is considerable variety in the sedimentation that occurs in the marine

realm, but there are a number of physical, chemical and biological processes that are

common to many of the marine environments. Physical processes include the formation

of currents driven by winds, water density, temperate and salinity variations and tidal

forces: these have a strong effect on the transport and deposition of sediment in the

seas. Chemical reactions in seawater lead to the formation of new minerals and the

modification of detrital sediment. The seas also team with life: long before there was life

on land organisms evolved in the marine realm and continue to occupy many habitats

within the waters and on the sea floor. The remains of these organisms and the evidence

for their existence provide important clues in the understanding of palaeoenvironments.

11.1 DIVISIONS OF THE MARINE

REALM

The bathymetry, the shape and depth of the sea

floor (Fig. 11.1), is fundamentally determined by the

plate tectonic processes that create ocean basins by

sea-floor spreading. The spreading ridges are areas

of young, hot basaltic crust that is relatively buoyant

and typically around 2500 m below sea level. Away

from the ridges the water depth increases as the older

crust cools and subsides, and most of the ocean floor

is between about 4000 and 5000 m below sea level.

The deepest parts of the oceans are the ocean

trenches created by subduction zones, where water

depths can be more than 10,000 m. At the ocean

margins the transition from ocean crust to continental

crust underlies the continental rise and the

continental slope, which are the lower and upper

parts of the bathymetric profile from the deep ocean

to the shelf. The angle of the continental slope is

relatively steep, usually between about 2

and 7

while the continental rise is a lower angle slope

down to the edge of the abyssal plain.

The continental shelf itself is underlain by conti-

nental crust, and the junction between the shelf and

the slope usually occurs at about 200 m below sea

level at present-day margins (the shelf edge break).

Continental shelves are very gently sloping with gra-

dients ranging from steep shelves of 1 in 40 to more

typical gradients of 1 in 1000. They may extend for

tens to hundreds of kilometres from the coastline to

the shelf edge break. Large areas of continental crust

that are covered by seawater, which are mainly bor-

dered by land masses and connected by straits to the

oceans, are called epicontinental seas (sometimes

called epeiric seas). The areas of epicontinental seas

are greatest when relative sea levels are at the highest

worldwide. A nomenclature for the division of the

marine realm based on these depth zones is shown

in Fig. 11.2. The shelf area, down to 200 m water

depth, is called the neritic zone, the bathyal zone

corresponds to the continental slope and extends from

200 m to 2000 m water depth, while the abyssal

zone is the ocean floor below 2000 m. A depth limit

land

continental

shelf

continental

slope

continental

rise

abyssal plain

100 km

2km

Continental crust

Oceanic crust

Fig. 11.1 A cross-section from the

continental shelf through the continental

slope and rise down to the abyssal plain.

sea level

bathyal zone

200 m

abyssal zone

~4000 m

sea level

mean high water

mean low water

fair weather wave base

storm wave base

offshore-transition

shoreface

foreshore

offshore

neritic zone

~5000 m

hadal zone

shelf edge break

Fig. 11.2 Depth-related divisions of the marine realm: (a) broad divisions are defined by water depth; (b) the shelf is described

in terms of the depth to which different processes interact with the sea floor, and the actual depths vary according to the

characteristics of the shelf.

164 The Marine Realm: Morphology and Processes

to this zone can also be applied at about 5000 m,

below which the deepest parts of the oceans are called

the hadal zone.

The shelf (neritic environment) can be usefully

further divided into depth-controlled zones (Fig. 11.2),

although in this case the divisions are not defined

by absolute depths, but the depths to which certain

processes operate. Their range therefore varies

according to the conditions in a particular basin

because the depths to which tidal processes, waves

and storms affect the shelf vary considerably. The

foreshore is the region between mean high water

and mean low water marks of the tides. Depending

on the tidal range (11.2.2) this may be a vertical

distance of anything from a few tens of centimetres

to many metres. The seaward extent of the foreshore

is governed also by the slope and it may be anything

from a few metres, if the shelf is steeply sloping and/or

the tidal range is small, to over a kilometre in places

where there is a high tidal range and a gently sloping

shelf. The foreshore is part of the beach environment

or littoral zone (13.2).

The shoreface is defined as the region of the shelf

between the low-tide mark and the depth to which

waves normally affect the sea bottom (4.4.1), and this

is the fair weather wave base. The lower depth that

the shoreface reaches depends on the energy of the

waves in the area but is typically somewhere between

5 and 20 m. The width of the shoreface will be gov-

erned by the shelf slope as well as the depth of the fair

weather wave base and may be hundreds of metres to

kilometres across. In deeper water it is only the larger,

higher energy waves generated by storms that affect

the sea bed. The depth to which this occurs is the

storm wave base and this is very variable on different

shelves. In some places it may be as little as 20 m

water depth but can be 50 to 200 m water depth

if the shelf borders an ocean with a large fetch for

storm waves. This deeper shelf area between the fair

weather and storm wave bases is called the offshore-

transition zone. The offshore zone is the region

below storm wave base and extends out to the shelf-

edge break at around 200 m depth.

The activities of a number of physical, chemical and

biological processes are determined by water depth,

and in turn these influence the sediment accumula-

tion on the different parts of the sea floor. The following

sections consider some of these processes and how they

affect depositional environments.

11.2 TIDES

11.2.1 Tidal cycles

According to Newton’s Law of Gravitation, all objects

exert gravitational forces on each other, the strength

of which is related to their masses and their distances

apart. The Moon exerts a gravitational force on the

Earth and although ocean water is strongly attracted

gravitationally to the Earth, it also experiences a small

gravitational attraction from the Moon. The water

that is closest to the Moon experiences the largest

gravitational attraction and this creates a bulge of

water, a tidal bulge, on that side of the Earth

(Fig. 11.3). The bulge on the opposite side, facing

away from the Moon, can be thought of as being the

result of the Earth being pulled away from that water

mass by the gravitational force of the Moon.

Fig. 11.3 The gravitational force of the

Sun and Moon act on the Earth and on

anything on the surface, including the

water masses in oceans.

Moon

Sun

Gravitational

force

gravitational

force

Earth rotates under

each tidal bulge once

each day  results

in two tides each day

Moon completes orbit

of Earth in 1 month.

When aligned with

Sun (twice a month),

spring tides occur.

When perpendicular,

neap tides occur

tidal bulge

in oceans

centre of mass

of EarthMoon

system

Earths orbit of

Sun is elliptical.

When the Earth

is closest (spring

and autumn

equinoxes), tides

are highest

Earth

Tides 165

If the land areas are ignored the effect of these

bulges is to create an ellipsoid of water with its long

axis oriented towards the position of the Moon. As the

Earth rotates about its axis the bulges move around

the planet. At any point on the surface the level of the

water will rise and fall twice a day as the two bulges

are passed in each rotation. This creates the daily

or diurnal tides . During the daily rotation, a point

on the Earth will pass under one high bulge and

a slightly lower bulge 12 hours or so later: this is

referred to as the diurnal tidal inequality, the two

high tides in a day are not of equal height. The two

tides in the diurnal tidal cycle are just over twelve and

a half hours because the Moon is orbiting the Earth as

the planet is rotating, changing its relative position

each day.

The Moon rotates around the Earth in the same

plane as the Earth’s orbit around the Sun. The Sun

also creates a tide, but its strength is about half that

of the Moon despite its greater mass, because the

Sun is further away. When the Sun and Moon are

in line with the Earth (an alignment known as

syzygy) the gravitational effects of these two bodies

are added together to increase the height of the tidal

bulge. When the Moon is at 90

to the line joining the

Sun and the Earth (the quadratic alignment), the

gravitational effects of the two on the water tend

to cancel each other. During the four weeks of the

Moon’s orbit, it is twice in line and twice perpendicu-

lar. This creates neap–spring tidal cycles with the

highest tides in each month, the spring tides, occur-

ring when the three bodies are in line. (The term

‘spring’ in this context is not referring to the season

of the year.) A week either side of the spring tides

are the neap tides, which occur when the Moon and

Sun tend to cancel each other and the tidal effect is

smallest.

Superimposed on the diurnal and neap–spring cycles

is an annual tidal cycle caused by the elliptical

nature of the Earth’s orbit around the Sun. At the

spring and autumn (Fall) equinoxes, the Earth is clo-

sest to the Sun and the gravitational effect is stron-

gest. The highest tides of the year occur when there

are spring tides in late March and late September. In

mid-summer and mid-winter the Sun is at its furthest

away and the tides are smaller. This pattern of three

superimposed tidal cycles (diurnal, neap–spring and

annual) is a fundamental feature of tidal processes

that controls variations through time of the strength

of tidal currents.

11.2.2 Tidal ranges

The tidal bulge created in the open ocean is only a few

tens of centimetres, but of course the difference

between high and low tide is many metres in some

places, so there must be a mechanism to amplify the

vertical change in sea level. The tidal bulge can be

considered as a wave of water that passes over the

surface of the Earth. In any waveform resonance

effects are created by the shape of the boundaries of

the ‘vessel’ the wave is moving through. In oceans

and seas the shape of the continental shelf as it shal-

lows towards land, indentations of the coastline and

narrow straits between seas can all create resonance

effects in the tidal wave. These can increase the ampli-

tude of the tide and locally the tidal range is increased

to several metres by tidal resonance effects. The high-

est tidal ranges in the world today are in bays on

continental shelves, such as the Bay of Fundy, on

the Atlantic seaboard of Canada, which has a tidal

range of over 15 m (Dalrymple 1984).

In addition to the influence of land masses, the

movement of water between high- and low-tide con-

ditions is also affected by the Coriolis force (6.3):

water masses moving in the northern hemisphere

are deflected to the right of their path and in the

southern hemisphere to the left. These effects break

up the tidal wave into a series of amphidromic cells

and at the centre of each cell there is an amphidro-

mic point around which the tidal wave rotates

(Fig. 11.4). At the amphidromic point there is no

change in the water level during the tidal cycle. All

oceans are divided into a number of major amphidro-

mic cells and there are additional, smaller cells in shelf

areas such as the North Sea and small seas such as

the Gulf of Mexico. Tidal ranges are therefore very

variable and within a body of water the pattern of

tides can be very complex: in the North Sea, for

example, the tidal range varies from less than a

metre to over 6 m (Fig. 11.4). For sedimentological

purposes it is useful to divide tidal ranges into the

following categories: up to 2 m mean tidal range the

regime is microtidal, between 2 and 4 m range it is

mesotidal and over 4 m is macrotidal.

11.2.3 Characteristics of tidal currents

The horizontal movement of water induced by tides is

a tidal current: tidal currents are weak in microtidal

166 The Marine Realm: Morphology and Processes

regimes, more pronounced if the range is mesotidal

and are capable of carrying large quantities of sediment

in macrotidal regimes. Nearshore tidal currents show

a number of features that produce recognisable char-

acteristics in sediment deposited by them (Fig. 11.5).

First, tidal currents regularly change direction from

the flood tide current, which moves water onshore

between the low and high tide, and the ebb tide

current, which flows in the opposite direction as the

water level returns to low tide. These are bipolar

currents acting in two opposite directions. Second,

the tidal flow varies in velocity in a cyclical manner.

At times of high and low tide, the water is still, but as

the tide turns, the water starts to move and increases

in velocity up to a peak at the mid-tide point in each

direction. Third, the strength of the flow is directly

related to the difference between the levels of the high

and low tides. As the tidal range varies according to

the series of cycles (see above) the velocity of the

current varies in the same pattern. The strongest

tidal currents occur when there are the highest spring

tides at the spring and autumn equinoxes.

The rotational pattern of the tidal wave within

amphidromic cells results in a flow of water that

follows a circular or elliptical pattern. These rotary

tides can be important currents on shelves and in

epicontinental seas. During the course of the tidal

cycle the current varies in strength, but does not

change direction and there may not be a period of

slack water (Dalrymple 1992). These offshore tidal

currents are important processes in the transport

and deposition of sediment on some shelf areas (14.3).

11.2.4 Sedimentary structures generated

by tidal currents

Bipolar cross-stratification

An analysis of current directions recorded by cross-

bedding in sands deposited by tidal currents may

mesotidal

(24 m

range)

microtidal

(< 2 m range)

macrotidal

(> 4 m range)

> 6 m range

rotation of the

tidal wave about

amphidromic points

Fig. 11.4 The North Sea of northwest Europe has a variable

tidal range along different parts of the bordering coasts.

Amphidromic points mark the centres of cells of rotary tides

that affect the shallow sea.

Fig. 11.5 During the diurnal tidal

cycle the direction of flow reverses

from ebb (offshore) to flood (onshore).

The current velocity also varies from

peaks at the mid points of ebb and

flood flow, reducing to zero at high

and low tide slack water before

accelerating again.

Twice daily tidal cycle

Water level

Low tide

High tide High tide

offshore

flow

onshore

flow

maximum

flow

velocity

slack

water

high tide level

low tide level

flow offshore as tide falls

ebb tide current

flow onshore as tide rises

flood tide current

Time

Tides 167