Catuneanu O. Principles of Sequence Stratigraphy

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270 6. SEQUENCE MODELS

turbidites (muddier, as the ones related to lowstand

normal regressions and early transgressions) travel

farther into the basin, allowing for longer channels,

usually with a higher sinuosity and higher levees, to

develop. The sandy nature of channel fills, in contrast to

their finer-grained levee and overbank facies, is docu-

mented by the inversion of topography that follows

compaction, which confers a positive relief to the reser-

voir as seen on seismic reflection data (e.g., Fig. 5.47).

Turbidity-flow Levees and Overbank Sediment Waves

Levees may be associated with either slope or basin-

floor channels, and may form in relation to both high-

and low-density turbidity currents, commonly where

the flows are overloaded and the depositional trend is

aggradational (e.g., Figs. 5.40, 5.48, and 6.36). As such,

high-density turbidity flows are particularly prone to

forming leveed channels on the continental slope,

whereas low-density turbidity flows are more likely to

generate leveed channel systems on the basin floor

(Fig. 6.30). Levees capture the finer-grained fraction of

the original sediment mixture, as the finer and lighter

sediments may escape from the channel during the flow.

Levees are better developed in relation to the lower-

density turbidity currents on the basin floor, as these

are richer in fine-grained sediment, tend to be longer-

lived and travel greater distances, thus providing more

time and better conditions for the fractionation and

spillover of the fine-grained sediment fraction from

channel onto overbank. Therefore, in the progression of

high- to low-density turbidity currents in Fig. 6.37, the

channels of the distal splay systems, which most likely

form during stages of lowstand normal regression and

early transgression, would be associated with the best

developed levees. At the same time, as the muddy

sediment is progressively lost from the sediment/

water mixture with increasing travel distance, levees

tend to thin down-system (Fig. 6.38). Where the levee

height decreases below a critical threshold, the flow

becomes unconfined, leading to a rapid dissipation of

energy and the formation of frontal (terminal) splays.

Levee deposits also tend to be thicker and sandier at

outer channel bends, as a function of flow momentum

and the associated spillover patterns (Posamentier,

2004a; Fig. 6.39). It has been noted that levee height is

commonly in a range of meters, with the outer bend

levees having two or three times the relief of the inner

bend levees (Piper and Normark, 1983; Posamentier,

2003). High-resolution seismic imaging of levee

morphology also reveals that the inner, steeper side of

levees may be marked by slump scars associated with

sediment instability and mass wasting along levee

walls (Fig. 5.47). The formation of levees is important

from an exploration prospective, because it allows the

concentration of sand into the channel and splay depo-

sitional elements.

progradation (downlap)

retrogradation

(marine onlap)

Shelf edge

2. Late forced regression

3. Lowstand normal regression

4. Early transgression

5. Late transgression

KEY

cohesive debris flows (mudflows)

high-density turbidites and grainflows

low-density (diluted) turbidity currents

Not to scale

Sea level

1. Early forced

regression

FIGURE 6.37 Idealized architecture of a submarine fan complex that may form during the base-level cycle

in Fig. 6.32. The fan progrades during the first four time steps (early forced regression to early transgression)

in response to the change in the type of gravity flow, based on the assumption that the travel distance of the

flow depends on its rheological behavior: mudflows travel the shortest distance, due to their discrete shear

strength; turbidites travel farther, due to their fluidal behavior, to a distance that is inversely proportional to

the flow density. The fan retrogrades during transgression, as a result of the gradual change from fluidal to

plastic behavior (turbidites to mudflows, respectively) which accompanies the decrease with time in the

sand/mud ratio. According to this general scenario, submarine fans are more likely to onlap the continental

slope during transgressions (the ‘marine onlap’ of Galloway, 1989). Note that the vertical stacking of gravity-

flow products in this diagram is the artifact of the 2D style of representation. In a 3D basin, autocyclic shifts

in the locus of sedimentation of the different lobes may result in their accumulation along different dip-

oriented profiles. This is a common problem with all 2D models. However, the general principle that relates

the travel distance to the rheological behavior of the flow still remains valid. This idealized model of the rela-

tive locus of deposition of the various deep-water facies may be altered by variations in sediment supply that

could modify the types of gravity flows, as well as by uneven seafloor topography.

SEQUENCES IN DEEP-WATER CLASTIC SYSTEMS 271

Beyond levees, for several kilometers into the over-

bank environment, linear features defined as ‘sedi-

ment waves’ may form in relation to overspill and

flow stripping from the main channel (Posamentier

et al., 2000; Posamentier and Kolla, 2003; Posamentier,

2003; Figs. 6.39 and 6.40). These sediment waves are

morphologically similar to the straight-crested dune

fields described for fluvial and shallow-water systems

in the context of flow regime charts, excepting that

they develop over much larger scales. The spacing

between the deep-water sediment waves may vary

greatly, from 50 to 250 m (Posamentier, 2003), probably

reflecting the variable energy (velocity) of the over-

bank flow. From the orientation of sediment waves in

relation to the main channel, it may be inferred that

they form preferentially in the extension of outer

bends, with their crests oriented perpendicular to the

direction of overbank flow (Fig. 6.39). Sediment waves

are not necessarily associated with crevasse splays,

and, being related to secondary flows that escape the

main channel while the system is active, are generally

composed of finer-grained sediment fractions relative

to the levee deposits. The petroleum reservoir quality

of deep-water depositional elements therefore decreases

away from the main channel, from channel fills to levees

and overbank sediment waves.

Turbidity-flow Splay Complexes

Splays may be classified into crevasse (lateral)

splays (lateral relative to a channel—less important;

Fig. 6.41) and frontal (terminal) splays (in front of a

FIGURE 6.39 Reflection dip magnitude map showing sediment

waves related to ‘flow stripping’ in the overbank area of channelized

basin-floor turbidity flows (Pleistocene leveed channel, offshore

eastern Borneo, Kalimantan, Indonesia; modified from Posamentier,

2004a; image courtesy of H.W. Posamentier).

FIGURE 6.40 Seafloor reflection dip magnitude map showing

sediment waves related to ‘flow stripping’ in the overbank area of

channelized basin-floor turbidity flows (offshore Nigeria; modified

from Posamentier and Kolla, 2003; image courtesy of H.W.

Posamentier).

100 ms

Decreasing levee height

FIGURE 6.38 Seismic traverse (lower image) along the levee crest

of the turbidity-flow channel shown on the seismic horizon map

(upper image) (basin-floor setting, Gulf of Mexico; modified from

Posamentier and Kolla, 2003; images courtesy of H.W. Posamentier).

The datum in the seismic traverse (green line) marks the top of levee

deposits. The levee height decreases in a basinward direction up to

the point where no more fine-grained sediment is available in the

turbidity flow to sustain the formation of levees. Where the levee

height decreases below a critical threshold, the flow becomes uncon-

fined and a frontal splay forms at the end of the leveed channel.

272 6. SEQUENCE MODELS

channel, representing the most distal depositional

element of the system – more important; Figs. 5.42,

6.42, and 6.43), and generally form where the energy of

a flow drops to the point that triggers the accumula-

tion of the bulk of its sediment load. As opposed to the

mudflow lobes, which freeze on deceleration and tend

to be more proximal relative to the shelf edge, turbid-

ity-current splays involve a gradual loss of sediment

from the flow until the flow energy is totally

exhausted, and therefore tend to be located farther

away relative to the shelf edge. Within a given diver-

gent continental margin setting, the location of frontal

splays depends on the type of turbidity current, as

discussed in the case of channels. The frontal splays of

high-density turbidity flows may form aprons at the

base of the continental slope, with shorter leveed chan-

nels on the continental slope, whereas the frontal

splays of the low-density turbidity currents form

basin-floor lobes, with well-developed leveed chan-

nels associated with them. The frontal splays of

high-density turbidity currents tend to be larger,

because sediment supply is higher and only little sedi-

ment of the original mixture is trapped within

channel-levee systems. The opposite is valid for the

frontal splays of low-density turbidity currents, whose

size decreases with the travel distance of the flow.

At the same time, distal splays may have a higher

textural maturity (cleaner sand, even though in lesser

amounts and potentially finer-grained), as the finest-

grained fractions of the original sediment/water

mixture separate from the flow and are trapped within

levees and overbank sediment waves in the process of

sediment transport.

This discussion demonstrates the complexity of

turbidite reservoirs, whose distribution (location

within the basin) and quality (grain size and textural

maturity) depend on a number of competing mecha-

nisms that control sediment supply and the type of

gravity flow. The ratio between the two main deposi-

tional elements of the turbidity flow deposits, i.e.,

leveed channels and frontal splays, is primarily a func-

tion of the density of the current, which in turn is a

reflection of the initial sediment composition (sedi-

ment/water and sand/mud ratios). The reservoirs of

the high-density currents are dominated by proximal

frontal splays, which include the bulk of the original

3 km

FIGURE 6.41 Seismic amplitude

extraction maps showing examples of

turbidity-flow crevasse splays in a

deep-water basin-floor setting (Gulf of

Mexico; images courtesy of H.W.

Posamentier).

SEQUENCES IN DEEP-WATER CLASTIC SYSTEMS 273

sand-rich sediment. In contrast, the reservoirs of the

low-density currents are volumetrically dominated by

leveed channel fills, which use most of the sand of the

original mixture (Figs. 6.30 and 6.44). The remaining

part of the sand forms relatively smaller distal frontal

splays, located where levees thin below a critical

height, whereas most of the mud contributes to the

formation of levee deposits and overbank (basin-floor)

sediment waves (Posamentier and Walker, 2002;

Posamentier and Kolla, 2003). Frontal splays generally

include sheet-bedded sandstones, in contrast to the

mudflow deposits that may take the form of sheets or

lobes with chaotic internal structure, and both deposit

types may achieve thicknesses in excess of 100 m

(Posamentier and Walker, 2002; Posamentier and

Kolla, 2003).

Mudflow (Cohesive Debris Flow) Macroforms

Mudflow macroforms, primarily in the shape of

sheets and lobes, are most likely to accumulate in the

deep-water environment during stages of early forced

regression and late transgression, when sediment entry

points into the marine basin are far from the shelf

edge, and the wave or hydraulic energy regimes

generate instability in the shelf edge region (Figs. 5.26,

5.57, and 6.30). During such stages of the base-level

cycle, the dominance of fine-grained sediments in the

distal shelf environment, which serves as the staging

area for gravity flows, promotes the manifestation of

mudflows over turbidity currents.

Mudflow deposits may form a significant portion of

the deep-water submarine fan complexes (e.g., Fig. 6.44),

and therefore they are often encountered in the process

of offshore petroleum exploration. Due to their domi-

nant fine-grained composition, mudflow deposits do

not form petroleum reservoirs, and so their recogni-

tion as such, and separation from turbidity-flow depo-

sitional elements, is a critical risk-reducing factor in

the pre-drilling stage of exploration. Criteria for the

identification of mudflow deposits have been devel-

oped based on their geomorphological, stratigraphic,

and structural features that can be observed on 3D

seismic data (e.g., Posamentier and Kolla, 2003), which

in turn are explained by the plastic rheological behav-

ior of the sediment/water mixture during the manifes-

tation of the flow. These criteria include significant

erosional relief at the base of mudflow sheets or lobes

(Fig. 5.33); the presence of grooves at the base of

2 km

FIGURE 6.42 Seismic amplitude

extraction maps showing examples of

turbidity-flow frontal splays in a deep-

water basin-floor setting (Gulf of

Mexico; images courtesy of H.W.

Posamentier).

274 6. SEQUENCE MODELS

mudflow deposits (Fig. 5.34), or preserved upslope as

a result of flow motion (Fig. 6.36); internal convolute

architecture of mudflow deposits (Fig. 5.35); and the

presence of internal thrust faults and associated

compressional ridges at the top of mudflow sheets or

lobes (Fig. 5.36).

The dense nature of mudflows, in which particles

are maintained in suspension during the manifestation

of the flow by the cohesiveness of the sediment/water

mixture (matrix strength) rather than the turbulence of

the water or any other clast-supporting mechanism,

confers on the flow a non-channelized character, even

though transverse cross-sectional views through

mudflow lobes may resemble the morphology of a

channel (Fig. 6.45). In such cases, the channel shape is

an artifact of erosional relief (Fig. 5.33), and it is not

representative for the style of sediment transport. A

key characteristic of mudflows, which explains some

of their diagnostic features, is their tendency to freeze

on deceleration. This behavior, caused by the discrete

shear strength of the sediment/water mixture, is

responsible for the contorted seismic facies observed

FIGURE 6.43 Seismic expression

of a basin-floor frontal splay (distrib-

utary channel complex) on an ampli-

tude extraction map (Gulf of Mexico;

modified from Posamentier, 2003;

images courtesy of H.W. Posamentier).

Also note the position of the turbid-

ity-flow channel during previous

time steps, and the updip migration

of avulsion nodes with time. This

pattern of migration of avulsion nodes

is attributed to allogenic controls

(e.g., changes in discharge and sedi-

ment yield of gravity flows triggered

by base-level shifts), as opposed to

the pattern of downdip migration of

avulsion nodes which is interpreted

as the result of autogenic mechanisms

(Posamentier, 2003).

SEQUENCES IN DEEP-WATER CLASTIC SYSTEMS 275

Sediment waves

Leveed channel

50 ms

1 km

Basinward

FIGURE 6.44 Uninterpreted and

interpreted seismic lines showing a

typical (and complete) basin-floor

succession of gravity-flow deposits

formed in response to a full cycle of

base-level changes (modified from

Posamentier and Kolla, 2003; seismic

line courtesy of H.W. Posamentier).

A—mudflow deposits (chaotic inter-

nal facies) interpreted to correspond

to the early falling-stage systems

tract; B—turbidity-flow frontal splay

(well-defined parallel reflections),

interpreted to represent the late

falling-stage systems tract; C—leveed

channel and correlative overbank

facies (strong reflections associated

with the sandy channel fill and weak

reflections/transparent facies associ-

ated with the finer-grained overbank

deposits), interpreted to include the

lowstand systems tract and the early

portion of the transgressive systems

tract; D—mudflow deposits (chaotic

internal facies), interpreted to repre-

sent the late portion of the transgres-

sive systems tract. Note the gradual

progradation of gravity-flow deposits

into the basin from A to C, and the

retrogradation from C to D (compare

with Fig. 6.37).

100 ms

5 km

FIGURE 6.45 Transverse and pla-

nar sections (upper and lower seis-

mic images, respectively) through a

Pleistocene mudflow lobe accumu-

lated at the toe of the continental slope

(Gulf of Mexico; images courtesy of

H.W. Posamentier). Note that in trans-

verse view the lobe resembles the

shape of a channel due to the

substantial erosional relief created by

the motion of mudflow deposits.

However, the flow is not ‘channel-

ized’ in a conventional sense.

within mudflow deposits (Fig. 5.35), and also for

the development of thrust faults and compressional

ridges (Fig. 5.36). These stratigraphic and structural

features relate to the fact that the frontal part of the

flow tends to freeze first, as approaching lower gradi-

ents towards the base of the continental slope, creating

a barrier which the remaining part of the flow

crashes into.

Cyclicity of Deep-water Systems in Relation to

Shoreline Shifts

As sediment supply to the deep-water environment

depends strongly on the proximity of the shoreline to

the shelf edge (staging area), and also on the trajectory

of the shoreline (transgressive, forced regressive or

normal regressive), a predictive stratigraphic cyclicity

of deep-water gravity flows and depositional elements

may be established in relation to shoreline shifts

(Figs. 5.37, 5.63, 6.30, 6.32, 6.37, and 6.44). This section

summarizes not only the predicted changes in the type

of gravity flows that are expected during consecutive

stages of a base-level cycle, but also the contrasts in

depositional elements and depositional trends

between the slope and the basin-floor settings of the

deep-water environment during each stage of shore-

line shift (Fig. 6.30).

Highstand Normal Regressions

No significant gravity flows into the deep-water

environment are expected during highstand normal

regressions, as sediment entry points are far from the

shelf edge and the bathymetric conditions in the outer

shelf to upper slope settings are relatively stable (Fig. 5.7).

Pelagic sedimentation is the dominant process on the

distal continental shelf, and in the deep-water environ-

ment, which results in the development of condensed

sections throughout the outer shelf and the deep-

water (continental slope and basin-floor) portions of

the basin (Figs. 5.7, 5.63, 6.30, and 6.32).

Early Forced Regressions

Even though the shoreline is still far from the shelf

edge, the rapidly changing bathymetric conditions in

the distal shallow-water environment coupled with

the lowering of the storm wave base during base-level

fall trigger instability and sediment reworking around

the shelf edge, and implicitly the manifestation of

gravity flows. These flows involve mainly the fine-

grained sediment accumulated on the outer shelf

during the highstand normal regression. As a result,

the dominant gravity flows in the early stages of forced

regression are represented by mudflows (Figs. 5.26,

5.63, and 6.32). These cohesive debris flows have a

discrete shear strength (plastic behavior) which causes

the flow to ‘freeze’ on deceleration (Stow et al., 1996).

This property translates into transparent to chaotic

facies on seismic lines, significant erosion of the under-

lying substrate, basal grooves, internal thrust faults

and associated compressional ridges at the top of such

deposits (Figs. 5.33–5.36). Due to their plastic rheolog-

ical behavior, mudflows travel shorter distances rela-

tive to the turbidity currents, and their products may

be part of, but not necessarily restricted to, slope

fans/aprons (Fig. 6.37). Seafloor processes are domi-

nated by erosion on the continental slope (e.g., see

grooves in Fig. 6.36) and sediment accumulation

towards the toe of the slope and in the proximal basin-

floor setting (Figs. 5.26 and 6.30).

Late Forced Regressions

These are the most important stages for the construc-

tion of submarine fans, when the largest amount of

terrigenous sediment is delivered to the deep-water

environment. The proximity of sediment entry points to

the shelf edge, together with the negative accommoda-

tion on the largely subaerially exposed continental

shelf (Fig. 5.27), result in large amounts of sand being

transferred into the deep basin via high-density turbid-

ity currents (Figs. 5.63, 6.32, and 6.36). Sedimentation

rates in the deep-water environment during late

forced regressions are the highest among all stages of

the base-level cycle, and the bulk of sandy turbidites

accumulate in proximal frontal splays located close

to the limit between the slope and the abyssal plain

(Fig. 5.42). In contrast to the underlying mudflow

deposits, these sandy lobes are characterized by

higher-amplitude reflections on seismic lines, with

well-defined layer-cake architecture (Figs. 5.32 and 6.44),

indicating (1) continuous deceleration of the flow with

the shallowing of the seafloor gradient, (2) gradual

deposition of the sediment load until the energy of the

flow is totally exhausted, and (3) the relatively coarse

nature of the sediment.

As the turbidity currents have a fluidal behavior,

with no inherent shear strength, they do not freeze on

deceleration. This flow property allows them to

travel farther into the basin, and on lower seafloor

gradients, relative to the underlying mudflow deposits

(Figs. 6.37 and 6.44), although they may also be

found superimposed in some locations (Fig. 5.32).

As with the products of mudflows, high-density

turbidites may also be part of, even though not restricted

to, slope fans/aprons. High-density turbidites are

generally expected to aggrade in both continental

slope and basin-floor settings due to the overloaded

nature of the flow, even on steep gradients where

the flow energy is highest. Continental slopes are

dominated by the aggradation of leveed channels

(Figs. 5.39–5.42, and 6.36), whereas basin floors

276 6. SEQUENCE MODELS

SEQUENCES IN DEEP-WATER CLASTIC SYSTEMS 277

receive sediments predominantly within frontal splays

(Figs. 5.27, 5.42, 6.30, and 6.44).

Lowstand Normal Regressions

The creation of accommodation on the continental

shelf during early stages of base-level rise results in a net

decrease in the amount of sand that is delivered to the

deep basin, as a significant amount of riverborn sedi-

ment (and particularly its coarser fractions) is now

trapped in amalgamated fluvial channel fills and

aggrading coastline systems. Trapping of terrigenous

sediment on the continental shelf, starting with the onset

of base-level rise, triggers the change from high- to low-

density turbidites across the correlative conformity sensu

Hunt and Tucker (1992) (Fig. 5.63). While the density of

turbidity flows is measured by the sediment/water

ratio, the change from high- to low-density flows at the

onset of base-level rise means not only a decrease in the

volume of sediment that is made available to the deep-

water environment, but also a decrease in the sand/mud

ratio of the sediment involved in the flow. Thus, the

terrigenous sediment influx that feeds the late forced

regressive high-density turbidity currents includes all

sediment fractions of the riverborne sediment, as accom-

modation across the continental shelf is negative. In

contrast, as accommodation becomes positive on the

continental shelf, the coarser riverborne sediment is

trapped first within aggrading fluvial to coastal systems,

leaving the finer-grained fractions still available to feed the

lowstand normal regressive low-density turbidity flows.

This explains the abrupt decrease in the sand/mud ratio

across the correlative conformity indicated in Fig. 5.63.

The surplus of riverborne sediment that exceeds the

amount of available accommodation on the continen-

tal shelf is therefore fed to submarine fans by low-

density turbidity currents (Fig. 5.44). As these diluted

flows carry an overall finer-grained sediment load

relative to the previous late forced regressive currents,

the formation of levees on the basin floor is sustained

over larger distances, enabling the flows to travel

farther into the basin across the abyssal plain (Fig. 6.37).

This explains why low-density turbidity flows are

commonly associated with smaller and more distal

frontal splays, a feature that reflects their characteristic

low sand/mud ratio. Due to the net decrease in

the amount of riverborn sediment that is available to

the deep-water environment, sedimentation rates in the

submarine fans during lowstand normal regressions

also decrease accordingly. The low-density turbidity

flows of lowstand normal regressions tend to be

underloaded on the continental slope, generating

entrenched channels (Fig. 5.45), and overloaded on the

basin floor where leveed channels form the dominant

depositional element (Figs. 5.44–5.48, 6.30, and 6.44).

The underloaded character of low-density flows on

continental slopes relates to their low sediment/water

ratio, as the amount of sediment load is insufficient to

compensate for the high energy of the flow on rela-

tively steep seafloor gradients. Such flows may only

become overloaded (and hence aggradational) on

nearly flat surfaces, where the energy flux drops below

the threshold required to maintain the entire amount

of sediment load in suspension.

Early Transgressions

In the early stage of transgression, the shoreline is

still in the shelf edge region (Fig. 5.56), and therefore

there is a chance that some riverborn sediment, together

with the sediment reworked in the upper shoreface by

wave-ravinement processes, may be transported into

the deep basin by low-density turbidity currents. Such

currents are, however, expected to be even more

diluted relative to the previous regressive flows

because the bulk of the riverborn and coastal erosion-

derived sediment is now trapped in backstepping

beaches, estuary-mouth complexes, and healing-phase

wedges (Figs. 5.56, 5.63, and 6.32). In contrast to the

case of lowstand normal regressions, the sand-trap-

ping efficiency of the transgressive fluvial and coastal

systems is much higher, being enhanced by the higher

rates of base-level rise which allow accommodation to

outpace sedimentation. The diluted turbidity currents

of the early transgressive stage may travel farther into

the basin relative to the previous lowstand normal

regressive flows, due to the lower density of the sedi-

ment/water mixture (Fig. 6.37). At the same time, sedi-

mentation rates in the deep-water environment are also

expected to decrease, following the trend that started

with the onset of base-level rise. This trend is driven by

the increase with time in the rates of base-level rise

(Fig. 3.19), which results in (1) increasingly efficient

trapping of riverborn sediment within fluvial and

coastal systems; (2) a corresponding decrease in the

amount of sand, and sediment in general, that is avail-

able to the deep-water environment; (3) dilution of the

turbidity currents, accompanied by a decline in the

sand/mud ratio and a corresponding decrease in reser-

voir quality of turbidites; (4) decrease in sedimentation

rates in the deep-water environment; and (5) increase

in the travel distance of gravity flows (i.e., the ‘progra-

dational trend’ noted in Figs. 5.63 and 6.37). In parallel

with the continued trend of progradation of turbidites

into the basin, deep-water healing-phase wedges start

forming following the onset of transgression, from the

toe of the slope and gradually onlapping the continen-

tal slope (Figs. 3.22 and 5.56). The accumulation of

deep-water healing-phase deposits continues through-

out the transgressive stage, and the characteristics of

this type of ‘transgressive slope aprons’ are discussed

further in the following section.

278 6. SEQUENCE MODELS

Similar to the lowstand normal regressive flows, the

low-density turbidity currents of early transgressions

may only become overloaded on nearly horizontal

basin floors, where the energy of the flow drops grad-

ually to zero. As a result, the early transgressive flows

tend to be entrenched on the continental slope

(Fig. 5.45), with age-equivalent leveed channels on the

basin floor (Figs. 5.46–5.48, 5.56, 6.30, and 6.44).

Late Transgressions

During late transgressions the shoreline is far from

the shelf edge (Fig. 5.57), which, coupled with the effi-

cient sediment trapping within nonmarine and coastal

systems, reduces dramatically the chance of any river-

born sediment to reach the deep-water portion of the

basin. The fine-grained sediments that accumulate on

the shelf and upper slope are, however, subject to

reworking due to the general hydraulic instability

created by rapidly increasing water depths at the shelf

edge, which explains the manifestation of mudflows in

the deep-water environment (Figs. 5.57, 5.63, and 6.32).

These mudflows, owing to their plastic behavior

(discrete shear strength), travel shorter distances rela-

tive to the previous turbidity currents, and so they

may backstep and with time onlap the continental

slope, forming a ‘transgressive slope apron’ associated

with ‘marine onlap’ (Galloway, 1989; Figs. 4.2 and 6.37).

This landward shift with time of the point of sediment

onlap onto the continental slope is referred to as a

‘retrogradational trend’ in Figs. 5.63 and 6.37.

The transgressive mudflow deposits may display

the same characteristics on seismic lines and horizon

slices as the cohesive debris flows of the early forced

regression (Figs. 5.33–5.36, and 6.44). As in the case of

early forced regressive mudflows, gravity-flow-

related seafloor processes during late transgressions

are dominated by erosion on the continental slope

(e.g., see grooves in Fig. 6.36) and sediment accumula-

tion towards the toe of the slope and in the proximal

basin-floor setting (Figs. 5.57 and 6.30).

In addition to the gravity flows described above,

which build submarine fans associated primarily with

point-sources of sediment supply, the deep-water

environment may also accumulate healing-phase wedges

during shoreline transgression (Figs. 3.22, 5.56, and 5.57).

These wedges may also include significant volumes of

sediment (Fig. 3.22), but in contrast to the submarine

fans, sediment sources may be considered linear and

the transport of sediment is primarily by diffusion

rather than channelized (see the ‘overbank’ location of

the cross-section in Fig. 3.22 relative to the closest chan-

nelized flow; Figs. 5.56 and 5.57; more details about

healing-phase wedges are provided in Chapter 5). The

deep-water healing-phase wedges develop during the

entire stage of shoreline transgression, by gradually

onlapping the continental slope (Figs. 5.56 and 5.57).

This landward shift (backstepping) with time of the

point of sediment onlap onto the continental slope also

qualifies as ‘marine onlap,’ as in the case of late trans-

gressive mudflow deposits (Fig. 4.2). It can be noted

that two types of ‘transgressive slope aprons’ may be

identified in deep-water settings, each associated with

retrogradation and marine onlap: one type consists of

late transgressive mudflow deposits, which are part of

submarine fan complexes (Fig. 6.37), and a second type

consists of healing-phase deposits that have a poten-

tially better spatial development along strike (Figs. 3.22,

5.56, and 5.57). The type of transgressive slope apron

that may be intercepted along dip-oriented 2D seismic

lines depends on the location of the cross-sectional

profiles relative to the submarine fan complexes and

their feeding canyons or channels. As such, 2D transects

that are placed in the ‘interfluve’ areas of submarine fan

complexes are most likely to capture the architecture of

transgressive healing-phase wedges (Fig. 3.22).

Summary

The change in the type of gravity flows that operate

during a full cycle of base-level shifts (Figs. 5.37, 5.63,

6.30, 6.32, and 6.44) triggers changes in the locus of

deposition of submarine lobes, as illustrated in Fig. 6.37.

Assuming a smooth bathymetric profile of the seafloor

and no significant changes in sediment supply during

the base-level cycle, a progradation through time of the

submarine fan lobes is generally expected from the

mudflow deposits of the early forced regression to

the low-density turbidites of early transgression.

Following this progradational trend, a retrogradation

of the submarine fan lobes is expected during trans-

gression, accompanying the transition from low-

density turbidites to mudflows (Fig. 6.37). Note that

the 1D and 2D models (Figs. 5.63 and 6.37) do not

necessarily reflect the reality of a 3D basin, because

they ignore the autocyclic lateral shifts in the location

of the various architectural elements of the deep-water

depositional systems. This means that the composite

profile in Fig. 5.63 may not necessarily reflect the verti-

cal facies shifts in a single location, but it may be

composed of several sections of different ages that are

located in different areas of the submarine fan

complex. Keeping this in mind, one has to be careful

with the interpretation of the deep-water basal surface

of forced regression (Fig. 4.27) which, unless it corre-

sponds indeed to the base of the earliest gravity-flow

products of forced regression, may be confused with

the facies contacts at the base of younger lobes of the

submarine fan complex.

The quality and distribution of the deep-water

reservoirs depend primarily on sediment supply

(which in turn is controlled by other first-order mech-

anisms, as discussed above), basin physiography, and

types of gravity flows. Given a smooth bathymetric

profile of the basin, slope fans/aprons may include more

texturally immature sediments, due to the shorter trans-

port distance, and may form as a result of mudflows or

high-density turbidity currents. The products of the

latter flows, in spite of the limited degree of sorting,

may form potentially the best and the largest reser-

voirs of the deep-water systems, as they are related to

the high sediment supply, with the highest sand/mud

ratio, which is commonly associated with the late

stages of forced regression. The dominant depositional

element of this type of reservoirs is represented by

frontal splays. Basin-floor fans are mainly related to

lower-density turbidity currents, which are able to

travel greater distances, and which produce reservoirs

mainly dominated by leveed channels. These types of

fans also have frontal splays, which may be more

texturally mature (as mud is separated and trapped

within levees in the process of sediment transport) but

volumetrically less important relative to the leveed

channels.

Deep-water clastic systems have received less

attention in the past relative to their fluvial to shallow-

water correlatives, partly because of the technical

difficulties in exploring and drilling deeper offshore

areas. Technological advances in seismic exploration

and drilling techniques allowed for a change in focus

in recent years, bringing turbidite reservoirs to the

forefront of petroleum exploration. Offshore explo-

ration is of course more challenging and expensive,

so every effort should be made prior to drilling to

generate detailed and accurate stratigraphic models.

Simple models like the ones illustrated in Figs. 5.63,

6.32, and 6.37 only capture general theoretical princi-

ples, and need to be re-evaluated on a case-by-case

basis, taking into account the realities of each particu-

lar basin.

The relative inaccessibility of the present day

deep-water environments deprives the geologist of the

first-hand observation of modern processes, which

explains why deep-water systems are generally less

understood relative to their fluvial, coastal, and shal-

low-water correlatives. The lack of easily accessible

modern analogues in deep-water environments is,

however, compensated by the technological advances

in the fields of seismic data acquisition and processing,

which allow for the high-resolution imaging of the 3D

architecture and evolution through time of deep-

water systems (e.g., Figs. 5.33–5.36 and 5.39–5.48).

Recent work on the characterization of deep-water

petroleum reservoirs and other depositional elements

has been published by Posamentier and Kolla (2003)

and Weimer and Slatt (2004). In the absence of easy

access to modern analogues, to observe gravity

flows in action in present day deep-water environ-

ments, outcrop analogues are particularly useful to

study the small-scale sedimentology and physical

(reservoir) characteristics of turbidites and other

gravity-flow-related facies (Figs. 4.27, 6.46, and 6.47),

as well as their larger-scale architecture (e.g., Wickens,

1994; Scott, 1997; Scott and Bouma, 1998; Bouma and

Stone, 2000).

SEQUENCES IN CARBONATE SYSTEMS

Introduction

The application of sequence stratigraphy to carbon-

ate depositional systems was a topic of debate in the

late 1980s, particularly with respect to how a sequence

framework developed essentially for clastic systems

can be adapted to reflect the realities of carbonate envi-

ronments (Vail, 1987; Sarg, 1988; Schlager, 1989).

Following up on these early contributions, significant

progress was made in the early 1990s when the funda-

mental principles of carbonate sequence stratigraphy,

as well as the differences between the clastic and

carbonate stratigraphic models, were elucidated

(Coniglio and Dix, 1992; James and Kendall, 1992;

Jones and Desrochers, 1992; Pratt et al., 1992; Schlager,

1992; Erlich et al., 1993; Hunt and Tucker, 1993; Long,

1993; Loucks and Sarg, 1993; Tucker et al., 1993). The

current status of carbonate sequence stratigraphy has

been summarized by Schlager (2005).

‘Principles’ of sequence stratigraphy, and the defini-

tion of the fundamental sequence stratigraphic

concepts, are independent of the type of depositional

environments established within a sedimentary basin,

and are discussed in this book based primarily on the

processes and products of clastic environments.

Nevertheless, the types of shoreline shifts, the systems

tract nomenclature in relation to base-level changes,

the types of stratigraphic surfaces or stratigraphic

sequences, may all be applied to carbonate depositional

systems as well. Notable differences, however, between

the stratigraphic models of clastic and carbonate

systems relate mainly to the geometry of systems

tracts and the sediment budget across the basin during

the various stages of the base-level cycle. Such differ-

ences stem from the all-important sedimentation vari-

able, whose interplay with accommodation controls

the type of shoreline shifts, the depositional trends

within the basin, and implicitly the formation and

architecture of systems tracts.

In contrast with basins dominated by siliciclastic

environments, whose bulk of sediment is terrigenous

SEQUENCES IN CARBONATE SYSTEMS 279