Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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the slope: consequently there may be little sedimenta-

tion associated with falling stage and lowstand except

for some redeposited material at the base of the slope.

Carbonate ramps are generally areas of lower car-

bonate productivity (Bosence & Wilson 2003). The

productivity may be unable to keep up with sea-level

rise during transgression and the transgressive sys-

tems tract will consist of retogradational parase-

quence sets. A maximum flooding surface marked by

condensed beds and/or hardgrounds is likely to form.

With a reduced rate of sea-level rise in the highstand

the parasequence sets will be aggradational to progra-

dational. In contrast to rimmed shelves, sedimenta-

tion continues during deposition of the falling stage

and lowstand systems tracts: the parasequences show

a progradational downstepping geometry, while expo-

sure on the inner part of the ramp results in solution

and karst formation.

Parasequences in carbonate depositional systems

normally show a shallowing-up character. They typi-

cally consist of beds deposited in the lower subtidal

zone comprising wackestones that coarsen up into

packstones and then to grainstones deposited in the

higher energy wave-reworked zone of shallower

water. Parasequences that form part of a retrograda-

tional or aggradational package tend to be thicker

with higher proportions of finer grained facies,

whereas parasequences in progradational parase-

quence sets tend to become thinner upwards with

more shallow water grainstone facies present.

23.5 SEQUENCE STRATIGRAPHY

IN NON-MARINE BASINS

In basins that are not connected to the oceans the

relative sea level does not act as a control on sedimen-

tation. The deposition in a basin that has a permanent

central lake is affected by the water level in that lake

in a manner that is similar to a relative sea-level

control. Climate directly controls the volume of

water in lakes. A shift to a more arid climate causes

a reduction in water supply and an increase in eva-

poration and the result is a fall in the lake level.

Wetter climatic conditions mean that rivers supply

more water, evaporation is reduced and the lake

level consequently rises. These base-level fluctuations

can be of greater magnitude than global eustasy, and

accommodation in the fluvial and lacustrine deposi-

tional systems within the basin is also determined by

tectonic subsidence. In areas of accumulation of

wind-blown sand accommodation is controlled by

the level of the water table as it limits the extent of

wind deflation (Kocurek 1996).

23.6 ALTERNATIVE SCHEMES IN

SEQUENCE STRATIGRAPHY

It is perhaps a consequence of the relatively recent

development of the concepts involved in sequence

stratigraphy that there is not general agreement on

definitions and terminology amongst those applying it

to field and subsurface geology. A summary of the

history and development of models, conceptual

approaches and methods in sequence stratigraphy is

provided in Nystuen (1998). Schemes that place the

sequence boundary in a completely different part of

the cycle, such as the ‘genetic sequence’ approach of

Galloway (1989) in which the ‘sequence boundary’ is

placed at the maximum flooding surface, have fallen

out of favour. The approach presented in this chapter

is based partly on the original ‘Exxon model’ devel-

oped as a tool for analysis of subsurface stratigraphy

seen on seismic reflection profiles (Payton 1977; Vail

et al. 1977; Jervey 1988) and later extended to sedi-

mentary successions (Van Wagoner et al. 1988,

1990). Revisions to these models presented by Coe

(2003) and Cateneaunu (2006) have been incorpo-

rated, and some of the original ideas from the Exxon

scheme, such as the concept of ‘type 1 and type 2’

sequences, depending on whether there was erosion

at the sequence boundary (type 1) or not (type 2),

have not been explored. As noted in section 23.2.2

there are various approaches used in the analysis of

events during sea-level fall, particularly the problem

of where to place a sequence boundary in a setting

where there is continuous deposition on the margin

(e.g. Hunt & Tucker 1990; Posamentier & Morris

2000). The shoreline trajectory concept, which is

summarised in Helland-Hansen & Martinsen (1996),

provides an alternative way of treating stratal rela-

tionships (particularly on seismic reflection profiles)

that is based on considering the relative rates of crea-

tion/reduction of accommodation and sediment sup-

ply along with the physiography of the margin.

Different approaches are best suited to particular

situations, depending on the scale of investigation

and the geological setting, and no single scheme can

be considered to be ideal for all circumstances.

368 Sequence Stratigraphy and Sea-level Changes

23.7 APPLICATIONS OF SEQUENCE

STRATIGRAPHY

One of the advantages of using the sequence strati-

graphic approach for the analysis of sedimentary suc-

cessions is that it can be applied to different types of

data and be used to help combine information from

different sources. It can be applied in the field using

graphic sedimentary logs of the strata, it can be used

in the subsurface in a similar way using borehole core

and wireline log data (22.4.3) and it can be applied to

seismic reflection profiles that provide images of sub-

surface stratigraphic relationships (22.2.5). Patterns

can be related to a general model (e.g. Fig. 23.14) and

predictions made about likely trends both laterally

and vertically.

23.7.1 Sequence stratigraphic analysis

of seismic sections

The sequence stratigraphic approach first became

well established as a means of analysing seismic

reflection profiles (Payton 1977; Vail et al. 1977;

Jervey 1988). Patterns in the stratal geometries can

be readily related to changes in relative sea level:

onlap relationships (22.2.5) on the shelf are charac-

teristic of transgression, a widespread unconformity

can be recognised and interpreted as a sequence

boundary, prograding clinoforms form during high-

stand, and so on. The scale of the resolution of seismic

reflection profiles (22.2.4) means that individual

parasequences can rarely be recognised, so interpre-

tation is at the scale of depositional sequences. Anal-

ysis is carried out by identifying key stratigraphic

surfaces, such as sequence boundaries identifiable as

unconformities on the shelf and maximum flooding

surfaces that can be recognised by the change in

stratal geometries from retrogradational to prograda-

tional. The vertical interval between maximum flood-

ing surfaces can be used as a means of estimating the

magnitude of relative sea-level change, provided that

post-depositional compaction of the sediment (18.2.1)

is taken into account. Shoreline trajectories deter-

mined from the geometry of the strata between the

stratigraphic surfaces can be used to infer information

about rates of relative sea-level change and sedimen-

tation.

23.7.2 Sequence stratigraphic analysis

of graphic sedimentary logs

The procedure for carrying out an analysis of a suc-

cession of sedimentary rocks in terms of changes

in relative sea level must start with facies analysis of

the entire sections. In outcrop and cores this is

achieved by examining the lithology, sedimentary

structures and biogenic features within each bed and

using them to determine the environment of deposi-

tion of each bed. The principles of relating deposi-

tional facies and facies associations to environment

of deposition that have been considered in earlier

chapters in this book are used. Trends in facies

Highstand systems tract

progradational parasequence set

Transgressive systems tract

retrogradational parasequence set

Lowstand systems tract

submarine fan turbidites

Maximum flooding surface

condensed facies

Sequence boundary

unconformity and

correlative conformity

Highstand systems tract

progradational parasequence set

Continental margin

Ocean basin

Fig. 23.14 A summary of the stratal geometries and the patterns within systems tracts in a depositional sequence.

Applications of Sequence Stratigraphy 369

and their associations can then be used to identify

parasequences: the beds in a parasequence can be

expected to show a shallowing-up pattern, that is,

going up through the succession the facies indi-

cate shallower water deposition. Flooding surfaces

can be recognised by relatively abrupt changes

from shallower to deeper water facies. Once parase-

quences have been identified they can be grouped into

parasequence sets and the trends within the sets

determined. A progradational trend is indicated if

the higher parasequences indicate generally shal-

lower water than the lower ones in the set; a deepen-

ing-up trend through the parasequence set is the

signature of a retrogradational trend and if all the

parasequences in the set show the same range of

facies the trend is aggradational. Trends within para-

sequence sets can then be used to establish the sys-

tems tracts and hence the whole succession can be

divided up into systems tracts and depositional

sequences (Fig. 23.15).

The recognition of key surfaces such as seque-

nce boundaries and maximum flooding surfaces

is an important step in the analysis of the sedimen-

tary succession (Figs 23.14 & 23.15). The nature

of the sequence boundary depends on the relative

landward/seaward location on the margin: on the

middle to outer regions the surface may mark an

abrupt change in facies from offshore or offshore

transition facies of the preceding highstand systems

tract to estuarine deposits of the transgressive

systems tract above the sequence boundary. The

presence of estuarine facies is generally a very

useful indicator in a sequence stratigraphic analysis

because estuaries form by flooding a river valley

(13.6) and are therefore characteristic of transgres-

sion. Further landward the sequence boundary may

be entirely within fluvial deposits and recognition

can be difficult: the best indicator is the identifica-

tion of an incised valley with fluvial channel deposits

concentrated within in it during the transgression.

There is no erosional surface within deep basin depos-

its and the sequence boundary is identified as a corre-

lative conformity: below this sedimentation is

dominantly fine-grained and pelagic but at the

boundary the onset of turbidites deposited on a basin

floor fan marks the onset of erosion and by-pass on

the shelf.

Within carbonate successions the principles of

analysing all the strata in terms of water depth and

then identifying parasequences, patterns within

parasequence sets and systems tracts also apply. On

ramps and shelves the sequence boundary can be

picked out by evidence of subaerial exposure in the

form of karstic surfaces. In the deeper water environ-

ment the correlative conformity may not be easy to

identify because the reduced sediment supply during

lowstand means that there is not necessarily a signifi-

cant increase in sedimentation at the sequence

boundary. The maximum flooding surface may be

marked by condensed beds and/or hardgrounds if

sea-level rise is relatively rapid, but in other cases it

may not be possible to define a distinct surface between

the transgressive and highstand systems tracts.

23.7.3 Sequence stratigraphic analysis

of geophysical logs

The most useful petrophysical logging tool in

sequence stratigraphic analysis is the gamma-ray log

(22.4.1). This tool is used to assess the relative pro-

portions of sand and mud within the succession and,

in general terms, sandier successions indicate shal-

lower marine deposition than muddy sediments. A

trend of decreasing value upwards on a gamma-ray

log can therefore be taken to indicate increase in sand

content and hence interpreted as a shallowing-up

succession (Fig. 23.16). An abrupt increase in the

reading means a higher mud content and this can

be used as evidence for a flooding surface (i.e. a para-

sequence boundary). Trends in sand and mud content

on gamma logs within parasequence sets can be used

to identify patterns of progradation, retrogradation

and aggradation (Fig. 23.16) through the succession.

Recognition of channel-fill successions, characterised

by sharp increases in sand content (low gamma log

value) at the base followed by a trend over several

metres to become more mud-rich (increasing gamma

log values), can be important to help identify fluvial

and estuarine facies which, in turn, can be indicators

of sequence boundaries. Maximum flooding surfaces

may also be picked up on spectral gamma logs

(22.4.1) by a characteristic increase in the potassium

content indicating a concentration of glauconite.

Other geophysical logs, such as imaging tools, provide

more information that helps to interpret the environ-

ment of deposition and hence aids in stratigraphic

analysis.

370 Sequence Stratigraphy and Sea-level Changes

Highstand

systems tract

Offshore

Sequence

boundary

Erosion surface

Fluvial

Lowstand

systems tract

Estuarine

Shoreface

Shoreface and

offshore

transition

Shoreface and

offshore

transition

Offshore

transition

Offshore

transition

Offshore

transition and

offshore

Offshore

Maximum

flooding surface

Condensed

facies

MUD

(a) (b)

yalc

tlis

SAND

GRAVEL

narg

bbep

bboc

luob

Idealised composite sequence

elacS

ygolohtiL

seicaflanoitisopeD

secneuqesaraP

Maximum

flooding surface

Condensed

facies

Offshore

transition and

offshore

Offshore

transition and

offshore

Offshore

transition

Shoreface and

offshore

transition

Highstand

systems tract

Shoreface and

offshore

transition

Shoreface

Sequence

boundary

Erosion surface

Transgressive

systems tract

Estuarine

MUD

yalc

tlis

SAND

GRAVEL

narg

bbep

bboc

uob

Idealised composite sequence

elacS

ygolohtiL

seicaflanoitisop

secneuqesaraP

Fig. 23.15 Schematic sedimentary log through an idealised depositional sequence: in practice, the succession seen in outcrop

or in the subsurface will often include only parts of this whole pattern, with considerable variations in thicknesses of the systems

tracts. (a) Lower part of the succession. (b) Continuation into the upper part of the succession.

Applications of Sequence Stratigraphy 371

23.7.4 Correlation of sections using

sequence stratigraphic principles

The benefits of using a sequence stratigraphic

approach become apparent when it comes to correlat-

ing outcrop sections or boreholes kilometres to tens

of kilometres apart. In Chapter 19 the limitations

of lithostratigraphic analysis were discussed: it is

clear that this approach does not provide a time

framework and hence is of limited value. Biostrati-

graphic techniques (Chapter 20) provide essential

information for correlating strata on the basis of

their age, but the resolution available may mean

that hundreds of metres of strata fall within the

same biozone and also correlation between terrestrial,

shallow marine and deep marine environments

may not always be easy because different fossils are

found in beds deposited in these different environ-

ments. Radiometric dating is of even more limited

value in correlation because of the problems in finding

material that can be used to provide a depositional age

(21.2). By using changes in relative sea level as the

template for analysis the sequence stratigraphic

approach overcomes some of the problems inherent

in other techniques.

Changes in relative sea level usually occur over

relatively large areas, and can even be global (see

23.8 for further discussion): they affect sedimentation

in environments ranging from rivers on coastal

plains, to coastlines, shelves and the areas of the

deep seas adjacent to continental margins. If there is

evidence of a sea-level rise or fall in one of these

environments, then there should also be a signature

in the others as well. Correlation can be carried out on

the basis of comparing patterns in different sections: a

progradational pattern seen in coastal facies will be

matched by a progradational pattern in the offshore

and offshore transition zone and the two may there-

fore be correlated, even though the lithologies are

quite different and they may not contain the same

fossils. Sequence boundaries, seen as erosional uncon-

formities on the shelf or as correlative conformities

in deeper water, may be traced over large areas.

Maximum flooding surfaces may be traced by identi-

fying evidence for sediment starvation on the outer

parts of the shelf and a change from retrogradational

to progradational patterns in parasequence sets

nearer shore. A theoretical example of the process of

correlation using sequence stratigraphy is shown in

Fig. 23.17.

Correlation of sections using the recognition of pat-

terns and key surfaces can be undertaken only if there

is some additional information to help the process.

Biostratigraphic information is needed in many cases

to provide an overall temporal framework: it is neces-

sary to know if the sections being correlated are

approximately the same age, but higher resolution

can be provided by looking at the sequences because

it may be possible to identify several depositional

sequences within a single biozone. In the subsurface

a general correlation between boreholes can be

achieved by locating them on seismic reflection pro-

files and then tracing reflector surfaces between them.

One of the most useful applications of sequence

stratigraphy is that it can be used as a predictive

tool. Consider the four sections in Fig. 23.17: even if

only the three shelf successions are present, it is still

parasequence

boundaries

depth (m)

600

700

800

600

700

800

coarsening-up (progradational)

fining-up (retrogradational)

GR GR

parasequences

depth (m)

Fig. 23.16 Interpretation of gamma-ray logs in terms of

parasequences and systems tracts.

372 Sequence Stratigraphy and Sea-level Changes

possible to make a prediction that a succession similar

to that seen in the deep basin section will be present

because the presence of a sequence boundary uncon-

formity indicates non-deposition on the shelf and so

accumulation of turbidites in the deep water can be

expected. Similarly, using the same logic, if any of the

other sections are missing a prediction can be made of

what might be expected to occur there. This predictive

facility is especially valuable in subsurface strati-

graphic analysis. A seismic reflection survey provides

information about the general patterns of the strata,

but even a single borehole can provide enough infor-

mation to make an informed guess about the distribu-

tion of facies across the area using sequence

stratigraphic principles in both the sedimentary sec-

tion and the seismic profile. With more data from

boreholes the depositional model for the area becomes

more sophisticated and, hopefully, more accurate.

23.8 CAUSES OF SEA-LEVEL

FLUCTUATIONS

In the introduction section to this chapter two main

causes of changes of relative sea level were identified:

tectonics and global eustasy. Uplift and subsidence are

never on a global scale and are always localised to

some extent, affecting just part of a continent at its

broadest scale. There are, however, aspects of plate

tectonic processes that affect the sea level on a global

scale and these also have to be considered. Eustasy is a

global phenomenon involving changes in the volume

of water in the world’s oceans, so every shoreline will

experience the same amount of sea-level rise or fall at

the same time. The different mechanisms resulting in

sea-level fluctuations are considered in the following

sections.

23.8.1 Local changes in sea level

Tectonic forces and related thermal effects acting on

the margins of continents result in the land mass

being raised or lowered relative to sea level

(Fig. 23.18). In rifted basins, blocks move down in

the rift, but along the flanks blocks may be uplifted:

this can give rise to either relative rises or fall in sea

level depending on which faulted block the coast is

situated. Along passive margins at the edges of ocean

basins the continental crust cools and contracts

through time resulting in a relative rise in sea level

along these margins. Relative rises and falls in sea

level may occur also at active continental margins

where ocean crust is being consumed at a subduction

zone. In all these cases, the sea-level changes are

localised to the region affected by the thermal or

tectonic event. In the case of rift basins this may be

a region only a few kilometres across, but on passive

margins the subsidence may affect the whole margin

of the continent. The influence of these localised

events does not extend to other coastlines around

the world.

23.8.2 Glacio-eustasy

Melting of continental ice caps at the poles can release

large quantities of water to the oceans that can poten-

tially raise the sea levels around the world by several

tens of metres. At the South Pole the Antarctic con-

tinent hosts most of the world’s fresh water as ice

sheets and ice caps, which are thousands of metres

thick in places. In the northern hemisphere there is

much less continental ice on Greenland, with the

remainder of the polar ice being floating sea ice that

does not change the level of the sea when it melts

because the mass above water level is compensated by

the reduction in density when the mass of ice changes

to water.

There is abundant evidence that the ice sheets at

the poles expanded and contracted in volume during

the Quaternary, resulting in worldwide changes in

sea level of tens of metres. Glacial deposits from the

Pleistocene are evidence of times when there were

areas of northern Eurasia and the North American

continent covered by ice sheets. Elsewhere morpholo-

gical features such as raised beaches testify to periods

of higher sea level in interglacial periods. The fluctua-

tions between glacial and interglacial conditions are

attributed to periodic cooling and warming of the

global climate during the Pleistocene. The connection

between climate change, glacial accretion/wastage

and global sea-level changes is well established as a

glacio-eustatic mechanism in the Quaternary

(Chappell & Shackleton 1986; Matthews 1986).

Glacio-eustasy is therefore a mechanism that can

explain global sea-level changes for periods of Earth

history when there were ice caps at the poles to store

water during cooler climate periods. The volumes of

water involved are sufficient to cause a rise and fall in

Causes of Sea-level Fluctuations 373

Offshore

Fluvial

Estuarine

Shoreface

Offshore

transition

Offshore

transition

Offshore

transition

Shoreface

Estuarine

clay

tlisilt

gran

pebb

cobb

boul

Offshore

Estuarine

Shoreface

Offshore

Shoreface

tlis

Maximum flooding surface

Maximum flooding

surface

Sequence boundary

Sequence

boundary

Correlation across a shelf from proximal (left) to

distal, the log on the right representing deposition

off the shelf in the deeper basin.

deepening-up trend

regression

deepening-up trend

regression

shallowing-up trend

regression

shallowing-up trend

regression

clay

gran

pebb

cobb

boul

Fig. 23.17 A hypothetical example of correlation between logs in different parts of the coastal and marine environments using

sequence stratigraphic principles. Note that correlation is on the basis that in different places on the shelf and in the basin

there will be different facies deposited at the same time.

Pelagic

Submarine

fan

Submarine

fan

Submarine

fan

Pelagic

tlis

Offshore

Shoreface

Offshore

transition

Offshore

transition

tlis

Submarine fan deposited

during period of erosion

on shelf (sequence

boundary)

Pelagic deposition in

deep sea during

deposition on

the shelf

Sequence

boundary

deepening-up trend

regression

shallowing-up trend

regression

clay

gran

pebb

cobb

boul

clay

gran

pebb

cobb

boul

Fig. 23.17 (cont’d)

Causes of Sea-level Fluctuations 375

sea level of over a hundred metres. The time period

over which glacio-eustasy operates is also significant

because climate changes apparently occur very

quickly (Plint et al. 1992). The resulting sea-level

change may take place over a few thousand years

and the interval between glacial and interglacial per-

iods in the Pleistocene is in the order of tens to hun-

dreds of thousands of years. Climatically driven

glacio-eustasy can therefore provide quick and fre-

quent global sea-level fluctuations.

23.8.3 Thermo-tectonic causes

of sea-level change

The configuration of the tectonic plates around the

globe is constantly changing with long-term patterns

of amalgamation to form supercontinents and subse-

quent dispersal as the continental mass breaks up into

smaller units. During supercontinent break-up new

oceanic spreading centres develop: this young oceanic

crust is hot and relatively buoyant and mid-ocean

CAUSES OF SEA-LEVEL CHANGE

Subsidence in basin causes

relative sea-level rise

Local tectonics

e.g.

Decrease in ice

volume raises

sea level

Increase in ice volume

lowers sea level

Continental ice caps

Global scale thermo-tectonic

Formation and break-up of supercontinents Changes in rates of formation of ocean crust

Changes in seawater

temperature

cause thermal

expansion/contraction

Seawater temperature

10s of m sea-level

change over 100 ka

10100 m sea-level

change over

10100 Ma

Centimetres to a few metres

change over hundreds to

thousands of years

Exchange with water on continents

Lakes

Groundwater

Very variable rates

and magnitudes of

relative sea-level

changes

Fast mid-ocean ridge spreading

More hot, buoyant oceanic crust occupies

more space in the ocean basin

Seawater displaced onto continental shelves

Slow mid-ocean ridge spreading

Oceanic crust cools and contracts

Centimetres to metres change over

hundreds to thousands of years

Fig. 23.18 There are a number of possible causes of sea-level change related to tectonic and climatic factors; the approximate

magnitudes of change and the rates at which it will occur are indicated in each case.

376 Sequence Stratigraphy and Sea-level Changes

ridges are at about 2000 to 2500 m water depth.

It therefore takes up more space in the ocean basins

than older, colder ocean crust that sinks to a lower

level allowing 4000 to 5000 m of water above it.

If the total length of spreading ridges in the world’s

oceans is great, more of the space in the ocean

basins will be taken up by the ocean crust and this

will cause the ocean water to spill further onto the

continental margins (Fig. 23.18). These will there-

fore be periods of higher sea level. Conversely during

periods of supercontinent formation the total length

of spreading ridges will be reduced and result in

more capacity in the ocean basins for the water and

the sea level falls. Changes in the rate of sea-floor

spreading also result in eustatic sea-level changes for

much the same reasons. When spreading rates are

high there is more hot crust in the ocean basins and

sea level rises: slower spreading rates result in a fall in

sea level.

It is estimated that sea-level changes of a hun-

dred metres or more could be produced by these tec-

tono-eustatic mechanisms (Plint et al. 1992).

However, these changes in global tectonics take

place slowly. The cycle of supercontinent amalgama-

tion and break-up takes hundreds of millions of years

and the changes in spreading rates probably occur

over tens of millions of years. The rates of rise or fall

in sea level generated by these mechanisms would

therefore be slow.

23.8.4 Other causes of global

sea-level change

Glacio-eustasy is climatically controlled and in addi-

tion to changing the proportion of ice on polar ice

caps there is a second effect of changes in global

temperature on the world’s oceans. When water

warms up, the volume increases by thermal expan-

sion. An increase in global atmospheric temperature

would result in a warming of the oceans although a

deep water circulation is required to affect all the

ocean waters. However, a rise in temperature of sev-

eral degrees Celsius would only result in a few metres

eustatic sea-level rise, so these thermal volume

changes in the oceans would have a very limited

effect. A similarly small change of sea level is predicted

from changing the proportion of the world’s water

(hydrosphere) which is resident on the continents in

rivers, lakes and groundwater (Plint et al. 1992).

23.8.5 Cyclicity in changes in sea level

An analysis of data from seismic reflection profiles,

boreholes and outcrop sections around the world car-

ried out by geoscientists in Exxon led to the publica-

tion of a ‘global sea-level curve’ (Vail et al. 1977; Haq

et al. 1987, 1988). There are disputes about aspects

of these curves regarding the timing and evidence for

global synchroneity (e.g. Miall 1992, 1997), but they

appear to show that there is a hierarchy of cycles of

sea-level fluctuations through the Phanerozoic. Other

authors have also remarked upon the evidence for

cycles of sea-level rise and fall (Hallam 1963; Pitman

1978; Worsley et al. 1984) and proposed mechan-

isms for generating these cycles. If the shorter term

variations are subtracted from the longer term trends

a number of orders of cyclicity can be recognised

(Fig. 23.19).

First-order cycles

The smoothed global sea level for the whole of the

Phanerozoic (Fig. 21.19) shows the trend of a rise

during the Cambrian to a peak in the early Ordovi-

cian, followed by a steady decline through to the end

of the Palaeozoic (Boggs 2006). A slow rise in the

Jurassic followed by a steep rise during the Cretaceous

culminated in a peak in global sea level in the Late

Cretaceous, which has been followed by a steady fall

to present-day levels. There is a strong correlation

between this curve and the patterns of continental

dispersal and amalgamation through the Phanerozoic

(Vail et al. 1977; Worsley et al. 1984). Superconti-

nent break-up occurred in the early Palaeozoic and

was followed by a long period of dispersal of conti-

nents prior to amalgamation of the supercontinent

of Pangea in the Permian. Sea level rose in the

Cambrian as the new spreading centres formed

between the continental masses and then fell

again as the continents regrouped during the Late

Palaeozoic. Break-up of Pangea and in particular the

dispersal of the fragments that made up Gondwana

led to the formation of new active ocean ridges

between the continents of South America, Africa,

Antarctica, Australia and India. Sea level rose

sharply during this period until continents started to

amalgamate again in the Late Cretaceous with the

collision between India and Eurasia and further west

the closure of the Tethys Ocean to form the Alpine

mountain belt.

Causes of Sea-level Fluctuations 377