Catuneanu O. Principles of Sequence Stratigraphy

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with shoreline shifts (i.e., shifts of sediment entry

points into the marine basin). For example, deep water

is generally correlated with low depositional energy

and fine-grained sediments. However, depending on

seafloor gradients and sediment supply, the manifesta-

tion of gravity flows may distort this relationship,

leading to increases in depositional energy and grain

size in a manner that is independent of water depth.

Gravity-driven sediment dispersal to deeper-water

environments may either materialize as discrete short-

term events, or, where sediment supply is continuous

and the prograding clinoforms are sufficiently steep,

as long-term diffusion. The latter mechanism of sedi-

ment transport is commonly invoked to explain the

formation of healing-phase wedges (see Chapters 5

and 6 for details), and may simply mean an increase in

the frequency of small-magnitude gravity flows to the

point that resembles continuous sediment transfer

from staging areas (e.g., upper delta front) to deposi-

tional areas (e.g., lower delta front to prodelta).

Figure 7.14 provides a simple model of deposition on

a subsiding seafloor that is subject to differential subsi-

dence, in which deltaic progradation generates a coarsen-

ing-upward succession in both shallowing and coeval

deepening waters, in areas proximal and distal relative

to the shoreline, respectively. In this example, the reverse

grading is directly associated with the progradation of

sediment entry points (regressive shoreline shift) and

with the increase in depositional energy through time in

relation to the steepening of the seafloor gradient, irre-

spective of water-depth changes. Under these circum-

stances, the contribution of gravity flows to the transport

of sediment to the deeper areas, regardless of bathy-

metric trends, is increasingly important through time

(Figs. 6.15, 7.12 and 7.13). The deepening of the water

in the lower delta front––prodelta area does not

prevent the progradation of increasingly coarser sedi-

ment, because the depositional energy is actually increas-

ing in response to the change in sediment supply and

slope gradients. Water depth is therefore not the primary

control on grain size and depositional energy, but merely

a consequence of the interplay of sedimentation and

base-level changes.

Forward modeling simulations, as exemplified in

Figs. 7.2–7.11, are useful to observe the types of strati-

graphic architecture that may result under various

combinations of sea level, subsidence and sediment

supply conditions. At the opposite end of the spec-

trum, inverse modeling is used to reconstruct synde-

positional conditions starting from the preserved rock

record, including the curve of base-level changes at the

shoreline and the history of bathymetric changes

within the basin. Figure 7.15 shows an example of a

preserved stratigraphic succession, which provides the

starting point for inverse modeling. Age constraints,

facies analyses, and the nature of contacts provide the

basis for the interpretation of systems tracts, which are

indicated on the cross-section in Fig. 7.15. Figure 7.16

presents the reconstructed curve of relative sea-level

changes at the shoreline, obtained by measuring the

amounts of available accommodation at the shoreline

for each time step, based on the observed magnitudes

of coastal aggradation or erosion. This is the reference

curve relative to which the timing of systems tracts

and bounding surfaces is defined, as explained in the

previous section of this chapter. Note that the inverse

modeling approach taken to reconstruct this curve is

very different from the forward modeling approach in

Fig. 7.10. Here, no ‘static’ curves are available for inter-

polation (see locations A, B, and C in Figs. 7.2–7.4), and

in fact such static curves are impossible to reconstruct

based on the data provided in Fig. 7.15. Hence, the

curve of relative sea-level changes at the shoreline is the

only reference curve that can be constructed starting

from the preserved rock record. Also note the relation-

ship between shoreline shifts and bathymetric changes

in Fig. 7.17. For example, the highstand systems tracts,

which prograde and display coarsening-upward

profiles, tend to include a deepening-upward succession

at their base, overlain by a shallowing-upward package

(both part of the overall coarsening-upward trend).

Hence, the maximum water depth, as inferred from

300 7. TIME ATTRIBUTES OF STRATIGRAPHIC SURFACES

FIGURE 7.12 Sandy turbidites (mainly divisions A and B of the

Bouma sequence) accumulated in a relatively deep-water lower

delta front––prodelta setting, via the manifestation of hyperpycnal

flows (river-dominated Ferron delta, Utah). Conventional views

account for a linear water-depth/depositional-energy/grain-size

relationship. Gravity-driven flows (either short-term ‘events’ or

long-term diffusion-style of sediment dispersal) alter the assumed

linear relationship between water depth and depositional energy,

explaining the progradation of coarser sediments into deeper

(or deepening) water.

FIGURE 7.13 Sandy turbidites accumulated in the lower delta front––prodelta setting of a river-dominated

deltaic succession (Panther Tongue, Gentle Wash Canyon, Utah). Note the importance of hyperpycnal (gravity)

flows in the transport of sediment to deeper areas; hence, no direct relationship may be established between

water depth and grain size, or between water-depth changes and grading trends. Instead, grain size correlates

with depositional energy, which if often independent of bathymetry. A, B––sole marks at the base of turbidite

rhythms; C, D––sandy turbidites, dominated by the divisions A and B of the Bouma sequence; E, F––cyclicity

of turbidite rhythms in the distal portion of the deltaic system.

benthic foraminiferal paleobathymetry for example,

often occurs within the highstand (normal regressive)

progradational wedge (Naish and Kamp, 1997;

T. Naish, pers. comm., 1998). Similar conclusions have

been reached by Vecsei and Duringer (2003), who have

demonstrated that the ‘maximum depth interval’ within

the Middle Triassic marine succession of the Germanic

Basin is younger than the maximum flooding surface,

hence occurring within the highstand systems tract

(Fig. 7.18). In this case study, the difference in age

between the maximum flooding surface and the maxi-

mum water-depth interval is attributed to variations in

sedimentation rates between the basin margin and the

basin center, which is in agreement with previous results

of numerical modeling (e.g., Catuneanu et al., 1998b).

Figure 7.19 presents the time-distance (Wheeler)

diagram for the cross-section in Fig. 7.15. This type of

chronostratigraphic chart is useful to illustrate the

shifts in the locus of sediment accumulation, as well as

the patterns of facies migration through time. With the

inverse modeling approach (Figs. 7.15–7.19) the best

we can do is to reconstruct the ‘dynamic’ curve of

base-level changes at the shoreline, without being able

to quantify the relative contributions of eustasy and

subsidence to this curve. The forward modeling simula-

tion (Figs. 7.2–7.11), as well as the example in Fig. 7.14,

account for syndepositional differential subsidence

that implies an increasingly important role for gravity

flows through time, in relation to the gradual steepen-

ing of the seafloor gradient. Nevertheless, the same

coarsening-upward trends in both deepening and

shallowing water may also be generated without

changes in seafloor gradients, with accommodation

exclusively generated by sea-level rise (Fig. 7.20). This

means that the accumulation of marine coarsening-

upward successions under changing bathymetric

conditions, including deepening water, does not

necessarily require an increase in the participation of

gravity flows, but can also be explained solely by

changes in sediment supply as a result of shoreline

302 7. TIME ATTRIBUTES OF STRATIGRAPHIC SURFACES

Sea level. Assume no eustatic

changes from time (1) to time (2)

shoreline shift

(1)

(2)

(1)

(2)

DEEPENINGSHALLOWING

Sea floor at time (1)

Sea floor at time (2)

Position of the initial seafloor surface (1) at time (2), due to subsidence. Arrows indicate the

amounts of differential subsidence that took place from time (1) to time (2).

coarser sediment (proximal)

mid-range grain size

finer sediment (distal)

FIGURE 7.14 Progradation of a coarsening-upward succession onto a seafloor that is subject to varying

bathymetric conditions. The threshold ‘T’ separates areas of water shallowing and deepening. This threshold

is placed where sedimentation and subsidence are in a perfect balance. Landwards from T, sedimentation

outpaces subsidence. Seawards from T, accommodation is created more rapidly than it is consumed by

sedimentation. The succession that accumulates in the deepening water is still coarsening-upward, as

the sediment entry points shift towards the sea (regression). This is the case of the Mahakam delta in

Indonesia (sedimentation > subsidence nearshore, and subsidence > sedimentation offshore).

300

(m)

(Km)

last sea level

fluvial

supratidal

intertidal

subtidal

gravity flows

Location 2

Location 1

subaerial unconformity

basal surface of forced regression

FSST

LST

HST 1

TST

HST 2

FIGURE 7.15 Stratigraphic architecture

of five systems tracts (cross-section cour-

tesy of H. W. Posamentier). The inter-

preted succession of systems tracts is

as follows: highstand systems tract #1

(HST 1)––time steps 1–4; falling-stage

systems tract (FSST) ––time steps 5–7;

lowstand systems tract (LST)––time steps

8–9; transgressive systems tract (TST)––time

steps 10–12; and highstand systems tract #2

(HST 2)––time steps 13–15.

shifts (Fig. 7.20). Gravity flows, in addition to tidal

currents, storm surges and hypopycnal plumes, are, of

course, still important as a mechanism of sediment

transport beyond the fairweather wave base, but their

contribution to the distribution of sediment within the

basin does not have to change through time in order to

explain depositional patterns such as the ones illus-

trated in Fig. 7.20. In the absence of differential subsi-

dence, the progradation of a coarsening-upward

succession in deepening water requires that the rates of

coastal aggradation (base-level rise at the shoreline, as

inferred from the thickness of the delta plain topset) be

higher then the rates of aggradation in the basinal to

prodelta areas (as reflected by the thickness of the deltaic

bottomset; Fig. 7.20). This condition is not necessary in

basins affected by differential subsidence, where progra-

dation in deepening water may occur even without

coastal aggradation (Fig. 7.14).

Discussion

In conclusion, the relationship between shoreline

shifts, sediment grading, and water-depth changes is

much more complex than commonly inferred. Shoreline

shifts exert a critical control on grading, as sediment

entry points prograde and retrograde relative to discrete

locations within the marine basin where changes in

grain size are observed. On the other hand, the corre-

lation between grain size and water depth, generally

assumed to be linear, is altered by fluctuations in sedi-

ment supply and depositional energy. Sediment supply

is to a large extent controlled by shoreline shifts, while

depositional energy is affected by the various trans-

port mechanisms that shed riverborne sediment

beyond the fairweather wave base, into the deeper-

water environment. Such transport mechanisms are

discussed in more detail in Chapter 6 (section on

Sediment supply and transport mechanisms), and include

SHORELINE SHIFTS, GRADING, AND BATHYMETRY 303

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−20

−40

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−80

0123456789101112131415

Relative Sea Level at the Shoreline (m)

Time (× 10

years)

Fall (−)

Rise (+)

Normal

regression

(HST)

Normal

regression

(LST)

Normal

regression

(HST)

Forced

regression

(FSST)

Transgression

(TST)

FIGURE 7.16 Reconstructed curve of relative sea-level changes at the shoreline, based on the amount of

coastal aggradation or erosion at each time step. Changes in accommodation at the shoreline are measured

based on the vertical distance between consecutive shoreline positions. In the case of the early highstand

systems tract (HST 1 in Fig. 7.15), whose top is eroded by the subaerial unconformity, the position of the

normal regressive shorelines (not preserved) is reconstructed based on the overall architecture and the

inferred progradation rates (the latter are inversely proportional to the rates of coastal aggradation).

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500

480

460

440

420

400

380

360

340

320

300

280

260

240

220

200

180

160

140

120

100

Subaerial Exposure

Normal Regression

(HST 1)

Forced

Regression

(FSST)

Normal

Regression

(LST)

Transgression

(TST)

Normal

Regression

(HST 2)

Water Depth (m)

Time (× 10

years)

Location 2

Location 1

FIGURE 7.17 Water-depth changes at the reference locations 1 and 2 in Fig. 7.15. Note that in a vertical

profile, a normal regressive systems tract (prograding, coarsening-upward deposits) may be associated with

both deepening- and shallowing-upward trends. See text for more details. Abbreviations: LST––lowstand systems

tract; TST––transgressive systems tract; HST––highstand systems tract; FSST––falling-stage systems tract.

FSST

MFS

MRS

MDI

HST

LST

TST

HST

LST

BSFR

MFS

MDI

Coastal

sands

Continental and coastal plain

sediments

Open-marine

sediments

FIGURE 7.18 Sequence stratigraphic model for a low-gradient ramp in an intracratonic basin (modified

from Vecsei and Duringer, 2003). Note that the maximum depth interval is younger than the maximum flooding

surface, and occurs within the highstand systems tract. The difference in age between the maximum flooding

surface and the maximum depth interval is attributed to differences in sedimentation rates between the basin

margin and the basin center. Abbreviations: HST––highstand systems tract; FSST––falling-stage systems tract;

LST––lowstand systems tract; TST––transgressive systems tract; SB––depositional sequence boundary;

BSFR––basal surface of forced regression; MRS––maximum regressive surface; MFS––maximum flooding surface;

MDI––maximum depth interval; R––regression and coarsening upward; T––transgression and fining upward.

tidal currents, storm currents, gravity flows, and

hypopycnal plumes.

The progradation of a coarsening-upward succes-

sion into a deepening basin is possible during both

forced and normal regressions, irrespective of the trends

of destruction or creation of accommodation at the

shoreline. Deepening of the water in distal deltaic

environments (lower delta front to prodelta) may be

caused by (1) topsets being thicker (aggrading faster)

than bottomsets, in normal regressive settings (Figs. 7.20

and 7.21), and/or (2) differential subsidence, in forced

or normal regressive settings (Figs. 7.14 and 7.21). In

either case, deltaic progradation in deepening water (that

is, beyond the point of balance between accommodation

SHORELINE SHIFTS, GRADING, AND BATHYMETRY 305

Wheeler Diagram (Chronostratigraphic Chart)

‘Coastal’ onlap

Seaward

Landward

Condensed Section

Bypass & Erosion

Sedimentation (including

condensed sections)

Fluvial onlap

Onlap

Downlap

300

(m)

(Km)

last sea level

fluvial

supratidal

intertidal

subtidal

gravity flows

MFS

MRS

Location 2

Location 1

subaerial unconformity

basal surface of forced regression

FSST

LST

HST 1

TST

HST 2

FIGURE 7.19 Wheeler diagram illustrating the time/distance relationships of the facies shown in Fig. 7.15

(courtesy of H. W. Posamentier). Note that the ‘coastal’ onlap curve, which was used in the early days of

seismic and sequence stratigraphy for the construction of the global cycle chart, is in fact composed mainly of a

combination of fluvial and marine onlap. For this reason, the assumption that ‘coastal’ onlap indicates stages of

sea-level rise is misleading, and resulted in the representation of sea-level fall as instantaneous events.

SHALLOWING

(T2)

SHALLOWING

(T3)

DEEPENING

(T2)

DEEPENING

(T3)

(1)

(2)

(3)

(4)

Sea level 3-4

Sea level 4

Sea level 1

Sea level 1-2

Sea level 2

Sea level 3

Time Time

Time

Base-level rise

Water depth increase

Grain size increase

MFS

MRS

1. Highstand normal regression (HST): the rates of base-level rise at the shoreline decrease with time

2. Lowstand normal regression (LST): the rates of base-level rise at the shoreline increase with time

- Time 4 (highstand): shallowing along the entire profile, in the absence of differential subsidence

- most of the maximum flooding surface (MFS) forms in deepening water

- the surface corresponding to the peak of deepest water is highly diachronous and forms within the HST

- Time 1 (lowstand): shallowing along the entire profile, in the absence of differential subsidence

- most of the maximum regressive surface (MRS) forms in deepening water

- the surface corresponding to the peak of shallowest water is highly diachronous and forms within the LST

delta plain

delta front (sandy)

prodelta (silty)

basinal facies (pelitic)

surface of deepest

water

delta plain

delta front (sandy)

prodelta (silty)

basinal facies (pelitic)

surface of shallowest

water

(1)

(4)

FIGURE 7.20 Base level, water depth, and grading trends in highstand and lowstand normal regressive

settings. No differential subsidence is taken into account, and the creation of accommodation is entirely attributed

to sea-level rise (in contrast to Fig. 7.14). Similar illustrations of deltaic progradation, with the lower delta front

prograding into deepening water, have been produced by Berg (1982) and Bhattacharya and Walker (1992; their

Fig. 25), accounting for thinner deltaic bottomsets relative to the topsets (see also Figs. 7.9 and 7.15). The

changing rates of base-level rise at the shoreline are matched by the rates of topset (delta plain) aggradation,

and are compensated by inversely proportional rates of progradation. ‘T’ is the threshold of no water-depth

change at each time step, where accommodation and sedimentation are in a perfect balance. This threshold

shifts along dip through time as a function of the decreasing or increasing rates of base-level rise. The addition

of differential subsidence would place the threshold T closer to the shoreline at each time step, as the area subject

to water deepening would expand towards the land. Note that surfaces of deepest and shallowest water join the

maximum flooding and maximum regressive surfaces in the vicinity of the shoreline, but diverge from them in

a basinward direction. Also note the opposite bathymetric trends of the highstand and lowstand systems tracts:

initial deepening followed by shallowing, and shallowing followed by deepening, respectively. This means that

both maximum flooding and maximum regressive surfaces form in deepening water.

and sedimentation: threshold ‘T’ in Fig. 7.20) results in

the formation of longer clinoforms through time (also

inferred by Berg, 1982, and Bhattacharya and Walker,

1992, accounting for thinner deltaic bottomsets relative to

topsets). The higher the rates of aggradation at the shoreline,

or the more pronounced the pattern of differential subsidence,

the more evident the offset between grading and bathymetric

trends (threshold ‘T’ closer to the shoreline in Fig. 7.20).

These principles may also apply to transgressive settings,

where proper deltas develop (Figs. 5.51–5.54). In fact, the

high rates of base-level rise that are commonly associated

with transgressive coastlines make transgressive deltas

most likely candidates for progradation into deepening

water, in areas relatively close to the shoreline.

The discussion in this section was focused on

tectonic settings where subsidence rates are either

constant along dip (Fig. 7.20) or increase in a basinward

direction (Figs. 7.2 and 7.14). In such basins, the accu-

mulation of fining-upward successions in deepening

water during transgressions is commonly the norm

(with the exception of transgressive deltas, as discussed

above), and departures from conventional thinking

(i.e., which accounts for a linear grain-size/water-depth

correlation) apply during regressions, when coarsening-

upward successions may prograde into deepening water.

Such situations may be common within the context of

divergent continental margins, other extensional

basins, or any basin that fulfills the conditions outlined

above. Clastic sedimentation in foreland basins,

however, may follow a different pattern, due to the

fact that subsidence rates decrease in a basinward direc-

tion (Fig. 2.63). Under these circumstances, the progra-

dation of coarsening-upward successions in shallowing

water during regressions may become the norm, while

the ‘anomaly’ may rather consist of the accumulation

of fining-upward deposits in shallowing water during

transgressions, in relation to the retrogradation of

sediment entry points and the pattern of subsidence

across the basin. For example, if subsidence rates vary

between 10 and 0 m/Ma along the dip of the foredeep

(proximal to distal), no sea-level changes are recorded,

and sedimentation rates decrease from 3 to 1 m/Ma in

a basinward direction in response to the retrogradation

of the shoreline, then the fining-upward transgressive

deposits accumulate in deepening water proximally,

but in shallowing water distally. Carbonate platforms

on continental shelves may also record bathymetric

‘anomalies,’ such as the growth of distal-shelf barrier

reefs during slow transgressions, which may keep up

with the rise in base level (i.e., no change in water

depth), although the shoreline is transgressive and the

rest of the flooded shelf experiences a deepening of the

water (Fig. 6.49).

This discussion indicates that more caution needs to

be exercised when using bathymetric terms to describe

observed trends of sediment grading, or observed or

inferred shoreline shifts, and that interpretations of

bathymetric changes should be confirmed independ-

ently, such as by using biostratigraphic or ichnological

data. In this context, the equivalence between transgres-

sive and ‘deepening-upward’ trends, or between regres-

sive and ‘shallowing-upward’ trends, as promoted by

Embry (2002, 2005), may be misleading if used as a

generalization. Such equivalence is only safely valid

for shallow areas in the vicinity of the shoreline (e.g.,

between the shoreline and the threshold ‘T’ in Fig. 7.20).

Beyond the threshold ‘T’ in Fig. 7.20, the deltaic clino-

forms (coarsening-upward, regressive succession)

prograde into deepening water. In fact, any time a well-

developed topset forms, the likelihood is that the toe

of the delta front clinoforms progrades into deepening

water.

Ultimately, the transgressive or regressive shoreline

shifts, followed closely by overall grading trends within

the marine basin (fining- and coarsening-upward,

respectively), reflect the balance between accommoda-

tion and sedimentation at the shoreline. This balance

controls the patterns of sediment supply into the basin,

and implicitly grading trends, as sediment entry points

retrograde or prograde with time. Elsewhere within the

basin, water depth depends on the interplay of local

accommodation and sedimentation, which may differ

significantly from the conditions established in the

vicinity of the shoreline. Therefore, water-depth

changes may follow patterns that are independent of

grading trends. However, such patterns are predictable

to some extent, depending on the subsidence regimes

of each sedimentary basin, and therefore they may be

modeled using numerical simulations. The differentia-

tion between grading and water-depth changes is

important because the definition of some sequence

stratigraphic surfaces revolves around these concepts

SHORELINE SHIFTS, GRADING, AND BATHYMETRY 307

(1)

(2)

Sea level

Sea floor

FIGURE 7.21 Schematic representation of the dominant controls

on water deepening during the progradation of coarsening-upward

successions. These two controls may act independently or in

conjunction: (1)––coastal aggradation, at rates higher than the rates

of aggradation of the distal prograding system (topset > bottomset);

(2)––differential subsidence, with rates increasing basinward

(subsidence > sedimentation + eustatic fall).

(see numerical modeling in the following section of

this chapter).

The next question is what criteria are best to employ

for tracing sequence stratigraphic surfaces in the rock

record as we move away from coeval paleoshorelines,

and, linked to this issue, what is the temporal signifi-

cance of surfaces defined on the basis of different criteria?

METHODS OF DEFINITION

OF STRATIGRAPHIC SURFACES

Introduction

All seven sequence stratigraphic surfaces are linked

to the reference curve of base-level changes at the shore-

line (Fig. 4.7). Three of these surfaces (the subaerial

unconformity, the wave/tidal transgressive ravinement

surface, and the regressive surface of marine erosion)

form during particular stages of shoreline shifts,

whereas the other four (the correlative conformities of

the onset and end of forced regression, the maximum

regressive surface and the maximum flooding surface)

are related to changes in the direction and/or type of

shoreline shift.

A general consensus is now reached with regards to

the temporal significance of sequence stratigraphic

surfaces that form during particular stages of shoreline

shift. The subaerial unconformity corresponds to a hiatus

that forms and extends basinward for as long as the

shoreline is subject to forced regression. The regressive

surface of marine erosion is a highly diachronous scour

that, again, forms during the forced regression of the shore-

line. Similarly, the transgressive ravinement surfaces are

highly diachronous scours that form in parallel with the

transgression of the shoreline. The concept of diachrone-

ity is illustrated schematically in Fig. 7.22.

The other four surfaces are linked to the four events

of the base-level cycle (Fig. 4.7), which correspond to

the change from highstand normal regression to

subsequent forced regression (correlative conformity

of Posamentier and Allen, 1999); from forced regres-

sion to subsequent lowstand normal regression

(correlative conformity of Hunt and Tucker, 1992);

from regression to transgression (maximum regressive

surface); and from transgression to regression (maxi-

mum flooding surface). As ‘event-significant’ surfaces,

all four are assigned a ‘low’ diachroneity in Fig. 4.9.

Both types of correlative conformities are defined on

the basis of regional stratal stacking patterns, and are

taken at the base and at the top of marine forced

regressive deposits, respectively (Fig. 6.2). The timing

of these correlative conformities depends primarily on

the interplay of subsidence and eustasy at the shoreline.

As the rates of subsidence may vary along strike, correl-

ative conformities may be somewhat diachronous.

The timing of the maximum regressive and maximum

flooding surfaces depends not only on the interplay of

subsidence and eustasy at the shoreline, but also on vari-

ations in sedimentation rates along the coastline. It may

be inferred, therefore, that these surfaces can be even

more diachronous than the correlative conformities,

and so somewhat less useful for the construction of

a chronostratigraphic framework, particularly along

strike-oriented profiles. However, as pointed out by

Posamentier and Allen (1999), ‘even though the age of

the bounding surfaces separating the systems tracts

might be diachronous on the scale of the basin, the

difference in age is commonly below biostratigraphic

resolution. Even if their diachroneity can be established

on a regional scale, the surfaces and systems tracts can

be physically continuous over wide areas and record

important phases of the evolution of the basin.’ This is

particularly true in basins without rapid or local changes

in subsidence rates along strike, such as divergent conti-

nental margins or intracratonic basins, where systems

tracts and their bounding (event-significant) surfaces

may be continuous over large areas. In more tectonically-

active basins, however, characterized by significant

changes in subsidence patterns along strike, systems

tract boundaries may be much more diachronous, and

with a more limited lateral extent.

Correlative Conformities

Onset-of-fall Correlative Conformity

The correlative conformity of Posamentier and

Allen (1999) (equivalent to the ‘basal surface of forced

regression’ of Hunt and Tucker, 1992) approximates

the seafloor at the onset of forced regression of the

shoreline, and therefore it is generally regarded as a

308 7. TIME ATTRIBUTES OF STRATIGRAPHIC SURFACES

Time

Distance

Diachronous/time-transgressive

Isochronous/time-line

α = diachroneity rate

FIGURE 7.22 Isochronism and diachronism of stratigraphic

surfaces. A diachronous surface is time–transgressive, i.e., ‘of varying

age in different areas or that cuts across time planes or biozones’

(Bates and Jackson, 1987). An isochronous surface is a time line,

i.e., it forms everywhere at the same time.

time line in the rock record, particularly along dip.

In reality, a low diachroneity rate is recorded in rela-

tion to the rates of sediment transport in an offshore

direction (Catuneanu et al., 1998b), as it takes time for

the first gravity flows associated with forced regres-

sion to reach the deeper parts of the basin. The rates of

offshore transport of terrigenous sediment along the

depositional dip within a marine basin vary from

−1

–10

m/s in the case of low-gradient shelf settings

to 10

–10

m/s in the case of turbidity flows associated

with steeper gradients in continental slope settings

(Reading, 1996). This low diachroneity rate is generally

undetectable relative to the resolution of the current

biostratigraphic or radiometric dating techniques.

Along strike, however, the degree of diachroneity may

be more significant, and may vary greatly depending

on subsidence patterns.

The definition provided by Posamentier and Allen

(1999) for their ‘correlative conformity’ is ‘the sedimen-

tary surface at the onset of relative sea-level (or base-

level) fall.’ This definition omits to specify that reference

is made to changes in relative sea level at the shoreline.

This omission relates to the inherited generic nature

of the reference curve of relative sea-level (base-level)

changes, as discussed earlier in this chapter, and allows

for alternative interpretations of the temporal attributes

of this correlative conformity, other than the ones

intended by its authors. As subsidence is differential

along both dip and strike, taking into account the onset

of relative sea-level fall in every discrete location

within the basin (rather than only at the shoreline)

results in the definition of a highly diachronous surface

that diverges from the earliest clinoform of forced

regression referred to by Posamentier and Allen (1999)

(see full modeling results in Catuneanu et al., 1998b—

their Figs. 10–14). This diachronous surface may not

extend across the entire basin, but only in areas where

subsidence rates are within the range of variation of

the rates of sea-level change (Catuneanu et al., 1998b).

Consequently, depocenters that record high rates of

subsidence, outpacing the rates of sea-level change at

all times, experience continuous relative sea-level rise,

thus not allowing for the formation of surfaces defined

at the onset of local stages of relative fall. Such theoret-

ical surfaces do not have a signature in the rock record

that can be defined on the basis of stratal stacking

patterns (i.e., they do not form sequence or systems

tract boundaries), and hence they have little value for

sequence stratigraphy. These are not the correlative

conformities referred to by Posamentier and Allen

(1999), which are independent of the offshore variations

in the rates of relative sea-level change, thus extending

across the entire basin at the base of all forced regressive

marine deposits. This discussion shows how important

it is to specify where the reference curve of relative sea-

level changes is taken, i.e., at the shoreline, in order to

avoid any possible confusion. Once again, the shoreline

trajectory (transgressive, normal regressive or forced

regressive) represents the fundamental switch that

controls sediment supply to the marine basin, and the

timing of all systems tracts and bounding surfaces, in

a manner that is independent of the offshore variability

in subsidence rates.

End-of-fall Correlative Conformity

The correlative conformity of Hunt and Tucker (1992)

is also defined on the basis of stratal stacking patterns,

separating offlapping forced regressive lobes from the

overlying aggradational lowstand normal regressive

deposits (Haq, 1991: ‘a change from rapidly prograding

parasequences to aggradational parasequences’). This

definition implies a diachronous correlative conformity,

younger basinward, with a diachroneity rate that

matches the rate of offshore sediment transport. As this

diachroneity rate is low, as explained above for the basal

surface of forced regression (correlative conformity of

Posamentier and Allen, 1999), this surface is also often

approximated with a time line in the rock record

(Embry, 1995: ‘The subaerial unconformity is devel-

oped and migrates seaward during base level fall and

reaches its maximum extent at the end of the fall. …,

the depositional surface in the marine realm at this

time of change from base level fall to base level rise is

the correlative conformity,’ portrayed as a time line in

his fig. 1). The quasi-time line significance of such

surfaces is of course only valid along depositional dip

sections, as varying subsidence rates along strike may

offset the transition between base-level fall and base-

level rise along the shoreline.

As in the case of the correlative conformity of

Posamentier and Allen (1999), the existing definitions

for the correlative conformity of Hunt and Tucker (1992)

fail to specify that this surface marks the end of base-

level fall at the shoreline, along each dip-oriented

profile. As subsidence rates vary throughout the basin,

connecting the dots that signify the end of base-level

fall in each discrete location would generate a highly

diachronous surface that diverges from the sequence

boundary defined by Hunt and Tucker (1992) on the

basis of stratal stacking patterns (see full modeling

results in Catuneanu et al., 1998b—their Figs. 10–14).

If we consider the situation envisaged by Vail et al.

(1984) for ‘type 2’ sequences, with the base level falling

at the shoreline but rising at the shelf edge, the correl-

ative conformity that accounts for a base-level fall-to-rise

transition in each discrete location would only develop

within a limited area on the continental shelf, beyond

which subsidence rates become too high to allow

METHODS OF DEFINITION OF STRATIGRAPHIC SURFACES 309