Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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stratigraphy has resulted in the introduction of a large

number of new terms into sedimentary geology and

although at first this new terminology seems to be

quite daunting, it is generally quite logical and easy to

relate to the concepts. To illustrate and introduce the

application of sequence stratigraphic terminology,

and to show how facies distributions can relate to

changes in relative sea level, two types of continental

margin are considered, each with certain conditions

regarding rates of sea-level change and sediment sup-

ply. These two case studies are the ones most often

illustrated in texts on sequence stratigraphy (e.g. Coe

2003; Cateneaunu 2006), but variations on these

two are not only possible, but common, depending on

the rates of sea-level rise and fall, changes in sediment

supply, and the shape of the bathymetry of the shelf.

1 A continental margin with a distinct shelf break:

sediment supply is assumed to be constant, and the

sea level falls to below the edge of the shelf so that

there is erosion on the shelf during sea-level fall

(Fig. 23.5). Beyond the edge of the shelf lies a slope

and a deeper basin area that receives sediment during

certain stages of the sea-level cycle.

2 A continental shelf that is a sloping ramp with no

distinct change in slope: the sediment supply is again

considered to be constant and is relatively high such

that deposition occurs throughout the cycle (Fig. 23.5).

A deeper basin area may exist, but is not strongly influ-

enced by sea-level fluctuations on the ramp.

23.2.1 Shelf-break margin depositional

sequence (Fig. 23.6)

Highstand

The highstand is the period of high sea level during

the cycle and the beds deposited during this period are

called the highstand systems tract (HST). (A ‘sys-

tems tract’ is the term used in sequence stratigraphy

for strata deposited during a stage of a depositional

sequence.) The beds show either an aggradational or

a progradational pattern as the shoreline shifts sea-

wards across the shelf. Sediment is supplied by rivers

from the hinterland and most of the accumulation

occurs on the shelf with little sediment reaching the

deeper basin.

Shelf-break margin

Ramp margin

high sea level

low sea level

shelf

slope

deeper basin

shelf extends into

deeper water

shelf break

no shelf break

Fig. 23.5 Two main types of continental

margin are recognised, each resulting in

different stratigraphic patterns when

there are sea-level fluctuations: (a) shelf-

break margins have a shallow shelf area

bordered by a steeper slope down to the

deeper basin floor; (b) ramp margins do

not have a distinct change in slope at a

shelf edge.

358 Sequence Stratigraphy and Sea-level Changes

Sequence boundary

During sea-level fall erosion of the shelf occurs as rivers

erode into the sediment deposited during the previous

cycle: where erosion is localised the rivers cut incised

valleys. This erosion creates an unconformity, which

in this context is also called a sequence boundary

(SB). It marks the end of the previous depositional

sequence and the start of a new one: depositional

sequences are defined as the packages of beds that

lie between successive sequence boundaries. If the

sea level falls to the level of the shelf edge then both

the detritus from the hinterland being carried by the

rivers and the material eroded from the shelf are

carried beyond the edge of the shelf. This sediment

forms a succession of turbidites on the basin floor that

are deposited during the period of falling sea level,

forming a basin floor submarine fan. There is no

unconformity within the basin floor succession to

mark the sequence boundary, so the start of the

next sequence is marked by a correlative confor-

mity, a surface that is laterally equivalent to the

unconformity that forms the sequence boundary on

the shelf.

Lowstand

The interval of low sea level is called a lowstand and

the deposits of this period are called the lowstand

systems tract (LST). The relative sea level is rising

slowly but the rate of sediment supply is relatively

high. The rivers cease to erode on the shelf but the

shelf continues to be by-passed: sedimentation con-

tinues to occur on the basin-floor fan as turbidites

(also referred to as lowstand fan deposits). Sediments

also start to build up above the fan at the base of the

slope to form a lowstand wedge (not shown in

Fig. 23.6). The pattern of beds in these deposits is

initially progradational, becoming aggradational in

the lowstand wedge as the rate of sea level increases.

Transgressive surface

The point at which the rate of creation of accom-

modation due to relative sea-level rise exceeds the rate

of sediment supply to fill the space is called the trans-

gressive surface (TS). It marks the start of retrogra-

dational patterns within the sedimentary succession

as accommodation outpaces sediment supply. If sedi-

ment supply is relatively low the transgressive surface

may be erosional: surfaces of erosion formed during

pelagic deposits

in deep water

deposition on shelf

Highstand

Falling stage

Lowstand

Transgression

Highstand

submarine fan deposits

in deep water

fluvial incision and

erosion on shelf

reduced deposition

in deep water

estuarine deposition

pelagic deposits

in deep water

deposition near shore

pelagic deposits

in deep water

deposition on shelf

condensed facies on shelf

sequence boundary

highstand systems tract

lowstand system tract:

transgressive system tract

highstand systems tract

Fig. 23.6 Variations in sea level that follow the pattern in

Fig. 23.4 affecting a shelf-break margin result in a series of

systems tracts formed at different stages: the lowstand is

characterised by deep-basin turbidites if the sea level falls to

the shelf edge.

Depositional Sequences and Systems Tracts 359

transgression are called ravinement surfaces and

they form because high wave energy in the shallow

water that floods over the land surface can result in

erosion.

Transgressive systems tract

Deposits on the shelf formed during a period of relative

sea level rising faster than the rate of sediment sup-

ply are referred to as the transgressive systems

tract (TST). They show a retrogradational pattern

within the beds as the shoreline moves landwards.

Sediment is no longer supplied to the basin floor

because there is now sufficient accommodation on

the shelf. The relative sea-level rise results in the

formation of estuaries as the incised valleys are

flooded with seawater: estuarine sedimentation

(13.6) is characteristic of the transgressive systems

tract. The rise in base level further upstream creates

accommodation for the accumulation of fluvial depos-

its within the valleys.

Maximum flooding surface

As the rate of sea-level rise slows down the deposi-

tional system reaches the point where the accommo-

dation is balanced by sediment supply: when this

happens transgression ceases and the shoreline initi-

ally remains static and then starts to move seawards.

This point of furthest landward extent of the shoreline

is called the maximum flooding surface (MFS):it

should be noted that it does not represent the highest

sea level in the cycle, which occurs later in the high-

stand systems tract. As the point of the maximum

flooding surface is approached the outer part of the

shelf is starved of sediment because there is abundant

accommodation near the shoreline: very low sedi-

mentation rates on the shelf can be recognised by a

number of features including concentrations of authi-

genic glauconite and phosphorites (11.5.1, 3.4), con-

densed beds rich in fossils (11.3.2) and evidence of

sea-floor cementation from hardgrounds and firm-

grounds (11.7.4).

Highstand

A return to aggradational and progradational pat-

terns of shelf sedimentation marks the onset of the

highstand systems tract above the maximum flooding

surface. Continued relative sea-level rise creates

accommodation within the continental realm: fluvial

deposition is no longer confined within incised

valleys (cf. transgressive systems tract) resulting in

deposition in rivers and on overbank areas over wide

areas of the coastal plain.

23.2.2 Ramp margin depositional sequence

(Fig. 23.7)

Highstand

Highstand deposition on a ramp margin is essentially

the same as for the shelf-break example, with an

aggradational to progradational pattern of deposition

on the inner part of the margin.

Sequence boundary

The sequence boundary in the ramp succession is

placed on the surface on which there is the first

evidence of erosion caused by sea-level fall (Coe

2003). Erosion starts at the landward end, but further

seaward there will be continued sedimentation, with

the geometry of the strata changing from building

up to stepping down (i.e. from geometry IV to geome-

try VI on Fig. 23.3). This change in stratal geometry

may not always be easy to recognise in practice. It is

worth noting that in the original schemes for shelf-

break margins summarised in publications such as

Van Wagoner et al. (1990) the highstand was con-

sidered to continue into the initial stages of sea-level

fall and the sequence boundary placed at the point

when erosion was widespread across the shelf: if this

approach is applied to a ramp margin it would be

placed at some point within the succession deposited

during sea-level fall. Hunt & Tucker (1992) suggested

an alternative scheme in which the sequence bound-

ary is placed above all the strata deposited during the

period of falling sea level, but this creates the situation

where the sequence boundary lies within the package

of strata deposited in the basin in the shelf-break

setting.

Falling stage systems tract

Sediments deposited during the period from the onset

of the relative fall in sea level until the point where it

stops falling are considered to form the falling stage

systems tract (FSST) (Plint & Nummedal 2000). The

360 Sequence Stratigraphy and Sea-level Changes

sediment is supplied by rivers from the hinterland and

also as a result of erosion of the landward part of the

ramp. Erosion by rivers may create incised valleys

between the landward part of the ramp and the shore-

line. The shoreline moves seawards and steps down as

the relative sea level falls and hence the geometry of

the strata is progradational and down-stepping. Note

that under the alternative scheme suggested by Hunt

deposition on inner ramp

Highstand

Falling stage

Lowstand

Transgression

fluvial incision and

erosion

reduced deposition

on outer ramp

estuarine deposition

deposition near shore

condensed facies

Highstand

mud deposition

on outer ramp

falling stage systems tract

highstand systems tract

lowstand system tract

deposition shifts down ramp

transgressive system tract

deposition on inner ramp

mud deposition

on outer ramp

highstand systems tract

Fig. 23.7 Variations in sea level that

follow the pattern in Fig. 23.4 affecting a

ramp margin result in a series of systems

tracts formed at different stages: during

lowstand the deposition is shifted down

the ramp.

Depositional Sequences and Systems Tracts 361

& Tucker (1992), these deposits would be considered

to be below the sequence boundary and they refer to

them as the forced regressive wedge systems tract

(FRWST).

Lowstand

The lowstand systems tract is deposited during the

early stages of sea-level rise. The inner part of the

ramp is no longer erosional and the water level starts

to rise in the incised valleys. The geometry of the

strata in the outer part of the ramp is progradational,

becoming aggradational as the rate of sea-level rise

increases and the shoreline stops moving seawards

and becomes stationary.

Transgressive surface, transgressive systems

tract, maximum flooding surface, highstand

The processes and patterns of sedimentation during

these stages of rising sea level are essentially the same

as for the shelf-break margin depositional sequence.

23.3 PARASEQUENCES: COMPONENTS

OF SYSTEMS TRACTS

23.3.1 Parasequences

Each of the systems tracts in Figs 23.6 and 23.7

is shown as consisting of several separate packa-

ges of strata. In detail each package is itself a cycle

of beds showing a progradational pattern and

these depositional cycles are known as parase-

quences in sequence stratigraphy terminology. They

form because the actual variation in sea level is

not the smooth sinusoidal curve shown in

Fig. 23.4: sea level does not rise or fall at steadily

varying rates as the smooth curve indicates, but

occurs in a series of shorter stages. A curve shape

generated by superimposing a short-term sinusoidal

pattern onto the longer term trend can account for

the variations in accommodation indicated by the

presence of parasequences (Fig. 23.8) During the

Depositional sequence

(Between sequence boundaries)

Highstand

systems tract

Lowstand

systems

tract

Parasequences

deposited during

short-term

sea level rises

tag

ste

Background subsidence

Short-term sea level

fluctuation superimposed

upon long-term rise and fall

Fall RiseRelative sea level

Time

decrease accommodation increase

Fig. 23.8 A higher frequency sea-level fluctuation curve superimposed on the curve in Fig. 23.4 produces a pattern of

short-term rises and falls in sea level within the general trends of transgression and regression. These short-term fluctuations

result in creation of small amounts of accommodation being created, even during the falling stage.

362 Sequence Stratigraphy and Sea-level Changes

rising limb of the depositional sequence curve the

superimposed curve results in short periods of accel-

erated relative sea-level rise separated by periods

when sea level is rising more slowly or not at all.

Superimposition of curves on the falling limb creates

short periods when the relative sea-level rise is

enough to create some accommodation.

Parasequence boundaries

Parasequences are hence packages of beds deposited

as a consequence of the creation of a small amount of

accommodation that is then filled up by sediment.

Within a succession, the boundary of a parasequence

is marked by a facies shift from shallower to deeper

water deposition (e.g. from foreshore to offshore tran-

sition facies), which signals a sudden increase in rela-

tive sea level. This surface marking the parasequence

boundary is called a flooding surface (not to be

confused with a ‘maximum flooding surface’).

Parasequence thickness

The thickness of a parasequence measured between

flooding surfaces is determined by the relative sea-

level rise: it is usually in the range of metres to tens

of metres (Fig. 23.9). The accommodation created is

filled by sediment until the next flooding surface

occurs, and there are three different possibilities for

the succession of beds within the parasequence. If the

sea level is static between flooding events the thick-

ness of the beds representing each of the facies belts

will be equivalent to the depth ranges of those facies

belts: for example, if the foreshore covers the range

from 0 to 2 m depth, the foreshore deposits will be

2 m, and if the shoreface is from 2 to 15 m deep, the

shoreface facies will be 13 m thick, and so on. How-

ever, if the sea level is continuing to slowly rise during

the period of deposition of the parasequence, as could

happen in a transgressive systems tract, each of these

facies units will be thicker: this can be considered to

be an expanded succession within a parasequence.

On the other hand, falling sea level during parase-

quence formation would result in thinner units, per-

haps only 1 m of foreshore facies and 6 m of shoreface

deposits: such a foreshortened succession within a

parasequence would be characteristic of falling stage

systems tract. The facies present in a parasequence

and the thickness of beds will also be determined by

the position on the shelf where deposition is occur-

ring: in a more distal setting the parasequence will be

made up of finer grained facies (Fig. 23.10).

Parasequence sets

Although an individual parasequence shows an inter-

nal progradational bed pattern, they may be arran-

ged into parasequence sets that may overall show

progradational, aggradational or retrogradational

offshore transition

m - 10s m

shoreface

foreshore

MUD

clay

silt

SAND

GRAVEL

gran

pebb

cobb

boul

inner shelf parasequence

Scale

Lithology

Structures etc

Notes

Fig. 23.9 A schematic graphic sedimentary log of a para-

sequence in an inner shelf shallow-marine setting.

Parasequences: Components of Systems Tracts 363

patterns (Fig. 23.10). In a progradational parasequence

set each successive parasequence going up through the

set shows shallower water facies, that is, an overall

shallowing-up trend. Retrogradational parasequence

sets are made up of parasequences that show a trend

of becoming deeper up through the succession, that is,

the top bed of the higher parasequence is a deeper water

facies than the beds that form the top of the parase-

quence below. In aggradational parasequence sets all

the individual parasequences show the same cycle of

facies with no overall trend of shallowing or deepening.

Parasequence sets and systems tracts

The trends within parasequence sets are indicative of

the systems tract that they make up (Fig. 23.11): a

highstand systems tract will have parasequence sets

with aggradational to progradational trends, a trans-

gressive systems tract is characterised by parase-

quence sets with a retrogradational trend, and a

lowstand by progradational to aggradational trends

within parasequence sets. A systems tract may be

made up of one or more parasequence sets, but all

sets in the same tract will show the same trends. In

falling stage systems tracts the parasequences are not

stacked vertically but are instead arranged laterally

(Fig. 23.12). The relationship between parasequences

within a fallings stage systems tract depends on the

rate of sea-level fall in relation to the rate of sediment

supply: with a high sedimentation rate or slow sea-

level fall the individual parasequences are stacked

against each other to form an attached falling

stage systems tract, whereas faster sea-level fall

and/or lower sediment supply results in a detached

falling stage systems tract (Fig. 23.12).

23.3.2 Sequences and parasequences:

scales and variations

Building up from parasequences, which are metres to

tens of metres thick, to parasequence sets and syst-

ems tracts, which are several tens of metres thick,

depositional sequences can be expected to be in the

order of a hundred metres or more in thickness. In

practice, depositional sequences vary from a few

metres to hundreds of metres thick depending on the

amount of accommodation that is created. The great-

est amount of accommodation occurs in places where

there is a large amount of tectonic subsidence that

occurs in certain types of sedimentary basin (Chapter

24). Accommodation tends to be much less in regions

that are tectonically stable and where the sea-level

variations are due to global eustatic fluctuations. The

mechanisms and magnitudes of relative sea-level

change are discussed in section 23.8.

The two conceptual models for the arrangement of

strata in depositional sequences present in section

23.2 are idealised and merely represent possible

Fig. 23.10 A schematic graphic sedimentary log of a

parasequence in an offshore shelf shallow-marine setting.

364 Sequence Stratigraphy and Sea-level Changes

scenarios. In addition to variations in the scales, there

is huge scope for variations in the character of the

deposits and the way that the strata are arranged

depending on the variables that control sequence

development. These variables are (a) the magnitude

of relative sea-level change, (b) the rate of relative sea-

level change, (c) the supply of sediment and (d) the

physiography of the margin. Different combinations of

these variables can generate any number of stratal

patterns, but there are several points to consider.

1 Rapid changes in relative sea level can lead to the

complete omission of systems tracts. For example, a

rapid sea-level rise can result in a succession of a

lowstand systems tract directly overlain by highstand

deposits, with no deposition during transgression.

Also, highstand deposits may be absent if relative

sea level falls abruptly after transgression and simi-

larly the lowstand systems tract may not be repre-

sented if there is a quick turnaround from fall to rise of

relative sea level.

2 The physiography of the margin and the magni-

tude of the relative sea-level changes determine the

proportion of the deposition that occurs in shallow

marine environments. Margins with a very wide shelf

with a shelf break below the lowest point of sea level

experience predominantly ramp-type geometries and

most of the sedimentation is accommodated on the

shelf, with little reaching the basin. Conversely, nar-

row shelves do not accommodate very much sediment

and it may be that only highstand deposits are pres-

ent, with the bulk of the deposition occurring on the

basin floor.

3 Sedimentation rates are important because if they

are low then accommodation will not be filled up and

progradation will be subdued, but at high sedimenta-

tion rates progradational geometries will tend to dom-

inate the succession.

23.4 CARBONATE SEQUENCE

STRATIGRAPHY

A fundamental difference between the behaviour of

carbonate depositional systems and terrigenous clas-

tic settings is the control on the supply of sediment. In

clastic depositional systems the sediment supply is

mainly detritus coming from a hinterland area sup-

plemented at times by the erosion and reworking of

material from the margin. In small clastic depositional

systems, such as alluvial-fan supplied fan-deltas at

active basin margins, there may be a close link

between relief, sediment supply and relative sea

Fig. 23.11 The stacking patterns of

parasequences to form parasequence sets

are characteristic of different systems

tracts.

Progradational parasequence set

sediment input > accommodation

Retrogradational parasequence set

sediment input < accommodation

Aggradational parasequence set

coastal plain

foreshore

shoreface

offshore transition

offshore

sediment input = accommodation

parasequence

Carbonate Sequence Stratigraphy 365

level. However, although there are some links

between climate, sea level and rates of erosion, the

sediment supply through larger scale, continent-wide

fluvial systems, however, is largely independent of

relative sea-level fluctuations. In contrast, supply of

material in carbonate settings is governed mainly by

the factors that control biogenic productivity

(15.1.1), such as water temperature, salinity, nutri-

ent supply, suspended sediment content, water depth

and the area of the shelf that is available for produc-

tion (Tucker & Wright 1990). Fluctuations in relative

sea level determine both the water depth and the shelf

area and so there is a direct link between relative sea

level and sediment supply in carbonate depositional

systems. There is a lot of production of sediment if

there is a large area of shallow water, but if this area

is reduced, the amount of sediment supply also drops.

The pattern of strata in a carbonate rimmed shelf

during a cycle of relative sea-level rise and fall is

shown in Fig. 23.13. The character of the systems

tracts will depend on the relative rates of sea-level rise

and carbonate production. If the sea-level rise is rapid

the shelf floods and there is a thin retrogradational

package of beds making up the transgressive systems

tract: the outer shelf area may be starved of sediment

allowing the development of condensed beds and

hardgrounds that will mark the maximum flooding

surface. The following highstand systems tract will be

aggradational to progradational. Under conditions of

slow relative sea-level rise the carbonate production

keeps pace and the pattern of beds is aggradational to

progradational: the transgressive systems tract will

pass without a maximum flooding surface into a pro-

grading highstand systems tract. A fall in sea level

that results in the exposure of the shelf or ramp

carbonate sediment can normally be recognised by

the presence of solution, karstification, soil develop-

ment and/or subaerial evaporite formation. Smaller

falls in sea level can result in a restriction of circula-

tion to the shelf lagoon, leading to hypersaline condi-

tions in arid regions and reduced salinity in humid

climates. Either way, the change in water chemistry

will be reflected in the organisms in the lagoon as

biodiversity is reduced under these conditions. Fall of

the sea level below the shelf edge dramatically reduces

the area of the carbonate factory to a small patch on

Falling stage

high sea level

attached falling stage systems tract

Falling stage

detached falling stage systems tract

low sea level

deposition shifts down ramp to

form continuous unit

deposition shifts down ramp with shoreface

deposits of low sea level detached from

those of high sea level

high sea level

low sea level

Fig. 23.12 A falling-stage systems tract

(FSST) on a ramp margin can show dif-

ferent patterns of deposition: if the sea

level falls relatively slowly with respect to

sediment supply deposition forms a con-

tinuous succession across the ramp as an

attached FSST; relatively fast sea-level

fall results in a detached FSST.

366 Sequence Stratigraphy and Sea-level Changes

Fig. 23.13 The responses of a carbo-

nate rimmed shelf to changes in sea

level. An important difference with

clastic systems tracts is that the carbo-

nate productivity varies because most

carbonate material is biogenic and

forms in shallow water. During high-

stand and transgressive systems tracts

wide areas of shallow water allow more

sediment to be formed, whereas at fall-

ing stage and lowstand production of

carbonate sediment is much lower.

pelagic deposits

in deep water

shallow carbonate deposition

Highstand

Falling stage

Lowstand

Transgression and

highstand

sequence boundary

highstand systems tract

lowstand system tract

transgressive systems tract

and highstand systems tract

reef and shoal deposits on shelf edge

slope deposits

sabkha/mudflats

pelagic deposits

in deep water

exposed shelf -

karst formation

narrow platform on shelf edge

slope deposits

pelagic deposits

in deep water

reef, shoal deposits on narrow platform

slope deposits

exposed shelf -

karst formation

pelagic deposits

in deep water

shallow carbonate deposition

reef, shoal deposits on shelf edge

slope deposits

sabkha/mudflats

Carbonate Sequence Stratigraphy 367