Catuneanu O. Principles of Sequence Stratigraphy

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180 5. SYSTEMS TRACTS

plan-view morphology of highstand rivers, whose

channel meander pattern may be inherited by younger

systems that are trapped within the confines of incised

valleys. As highstand rivers tend to be the lowest

energy fluvial systems of a stratigraphic sequence,

they are commonly sluggish, of meandering type,

being characterized by channels of moderate to high

sinuosity. Erosion along such channels leads to the

formation of incised meander belts that may preserve the

morphology of highstand rivers beyond the end of base-level

rise, and after changes in slope gradients and fluvial

energy occur during subsequent stages of base-level

fall, lowstand, or even transgression (Fig. 5.19). This

means that the earliest fluvial strata that accumulate

on top of a subaerial unconformity are not necessarily

braided stream deposits, as commonly inferred by

standard sequence stratigraphic models (e.g., Figs. 4.32

and 4.38). Instead, in the case of incised meander belts,

the valley fill, which generally consists of lowstand

and/or transgressive strata, is more likely to start with

meandering stream deposits (Fig. 5.19). The abrupt

shift from meandering to braided systems across a

subaerial unconformity (e.g., Fig. 4.16C) is easier to

achieve in the case of unincised fluvial systems, as

rivers that are not constrained by an erosional land-

scape can adjust their morphology more rapidly to

new energy regimes. Such changes in morphology are

more difficult to achieve in the case of incised valleys

(Fig. 5.19), where increases in energy levels with time

across the sequence boundary are not readily accom-

panied by corresponding changes in fluvial style. A

good example is provided by the Orange River along

the South Africa–Namibia border, which retained the

meander pattern inherited from its early stage of

evolution as a low energy system despite the subse-

quent differential uplift and steepening of the slope

gradient. During its approximately 100 Ma history, the

Orange River started as an unincised meandering

system flowing on a relatively flat landscape, which

gradually evolved as an incised meander belt in

response to continental-scale tectonic uplift (J.D. Ward,

pers. comm., 1997). Even today, the river preserves

3 km

90 ms

112 ms

FIGURE 5.17 Time slice from the amplitude domain of a 3D seismic volume, illustrating a Late Pleistocene

incised valley located approximately 72 m (90 ms) beneath the modern seafloor, offshore northeast Java

(Indonesia) (modified from Posamentier, 2001; images courtesy of H.W. Posamentier). Fluvial incision was

caused by a base-level fall in excess of 110 m, which led to the subaerial exposure of the entire continental

shelf and upper slope. The main trunk of the valley, shown in image A, is approximately 90 km long. The

valley system is characterized by short and incised tributary valleys that display a dendritic pattern in plan

view. The paleoflow is inferred to be southward, as the river system appears to widen toward the south

(Posamentier, 2001). The inset map of image A shows a time slice at a deeper level (approximately 90 m/

112 ms subsea) that captures the morphology of older (probably highstand), unincised alluvial systems. Note

that the unincised (highstand) and incised (falling-stage) systems have similar meander belt morphologies,

excepting that the latter are associated with incised tributaries. The similarity in the morphology of unincised

and incised meander belts is due to the fact that the falling-stage valleys are in fact incised, former highstand

rivers. In other words, the meander pattern of incised valleys is inherited from the time the rivers were unin-

cised. Image B details the lower portion of image A.

FALLING-STAGE SYSTEMS TRACT 181

the original meander pattern to a large extent,

although a braided style that is more in line with its

current energy level is emerging in some areas as a

result of meander cut-off, valley widening, and

erosion of valley walls (Fig. 5.20).

Unincised fluvial systems of the falling stage are

now documented as having a much more common

occurrence in the rock record than originally inferred

by early, standard sequence stratigraphic models. This

is especially the case in shallow-marine basins with

Time Slice 164 msec

1 km

Lowstand

Shoreline

Time Slice 160 msec

1 km

Incised Valley

Fault Scarp

Incised Valley

Delta

End of

Incised Valley

Unincised

Alluvial

System

Unincised

Alluvial

System

Alluvial Plain

Interfluve Area

Detached shorelines

Incised

valley

Bypass channel

Structural (fault) scarp

Coastal plain

Delta

FIGURE 5.18 Incised vs. unincised falling-stage alluvial systems on a continental shelf affected by normal

faulting (images courtesy of H.W. Posamentier). The block diagram provides an interpretation for the geology

observed on the two 3D seismic amplitude time slices from the Late Pleistocene continental shelf of offshore

east Java, Indonesia (see also Fig. 2.66 for additional information). Times 1 and 2 in the block diagram show

the positions of the sea level at the onset and end of forced regression. The boundary between coeval incised

and unincised alluvial systems on the continental shelf is controlled by the topographic break associated with

a fault scarp. The process of valley incision affects the more elevated footwall of the normal fault. Since the

shoreline at the end of forced regression remained inboard of the shelf edge, the downstream portion of the

fluvial system seaward from the fault scarp is unincised. Note that the fault scarp has the same effect on fluvial

processes as the depositional/stratigraphic scarp of a highstand prism (compare this case study with Fig. 5.16).

182 5. SYSTEMS TRACTS

Incised tributar

Incised

tributaries

Incised

tributaries

Incised tributaries

Point

bar

Point

bars

Point barIncised meanders

Incised meander belt

Modern analog: San Juan River, Utah

Malay Basin, Pleistocene

FIGURE 5.19 Incised meander belts formed during stages of base-level fall. A—subsurface example

depicting a time slice through a 3D seismic volume, placed approximately 77 m subsea in the Malay Basin,

offshore Malaysia (detail from Fig. 5.12; image courtesy of A.D. Miall). The Pleistocene meandering stream

deposits captured in this image are interpreted as lowstand (normal regressive) strata, and form the lower

portion of an incised-valley fill that rests on a subaerial unconformity (Miall, 2002; Fig. 5.12). Note the incised

tributaries that feed into the main meander belt. These tributaries are slightly older (formed during base-level

fall) than the point bars (younger, lowstand deposits accumulated during early base-level rise). B—interpre-

tation of the features observed in image A. C—modern example of an incised meander belt (modified from

Press et al., 2004). Note that these incised meander belts preserve the morphology of pre-existing low energy

and high sinuosity highstand rivers which became trapped within their own valleys during subsequent

falling-stage incision. As a result, renewed fluvial aggradation at the onset of base-level rise takes place in

meandering rather than braided rivers, even though lowstand fluvial systems are commonly expected to be

of significantly higher energy relative to the highstand ones.

Atlantic

Ocean

FIGURE 5.20 Satellite photograph of the Orange River (arrows) along the Namibia – South Africa border

(image courtesy of J.D. Ward). This incised fluvial system retains the meander pattern inherited from its early

stages of evolution, when the river was of low-energy and flowing across a relatively flat landscape, in spite of the

subsequent steepening of the slope gradient and the associated increase in fluvial energy. See text for more details.

FALLING-STAGE SYSTEMS TRACT 183

gently sloping ramp margins, such as intracratonic

basins and filled foreland systems, or in continental

shelf settings where the forced regressive shoreline

does not fall below the elevation of the shelf edge

(Posamentier, 2001; Fig. 5.21). The under-representation

of falling-stage bypass fluvial systems in the sequence

stratigraphic literature may be due to a number of

factors, including inadequate well spacing, lack of high-

resolution 3D seismic data, overlooked horizon slice

imagery, and interpretation approaches unprepared

to challenge conventional thinking. Thus, it appears

that many published examples of incised valleys may

not, in fact, be incised systems (Posamentier, 2001).

A look at the continental shelf of the Java Sea during

the last 0.5 Ma before present reveals that extensive

fluvial valley incision, across the shelf, could only have

occurred during three, rather short time intervals

when the entire shelf and upper slope were exposed

subaerially (Posamentier, 2001; Fig. 5.22). Consequently,

incised valleys such as the one captured in Fig. 5.17 are

rather the exception than the rule, and for the majority

of Late Pleistocene time, fluvial systems across the

Java shelf were mainly characterized by unincised

channels (Fig. 5.22).

The separation between incised and unincised

falling-stage fluvial systems is fundamental for the

design of successful petroleum exploration and

production strategies, as the fill of incised valleys and

unincised channels are inherently different play types.

The differentiation between these play types is impor-

tant not only for the evaluation of the petroleum

potential of fluvial deposits, but it has implications for

the evaluation of all deposits of the falling-stage

systems tract. For example, the fact that not all episodes

of base-level fall result in valley incision and full

subaerial exposure of the shelf is highly significant for

the development of deep-water reservoirs, and hence

for the design of deep-water exploration strategies.

Thus, stages of incomplete exposure of the shelf, as

inferred from the presence of unincised fluvial systems,

are prone to the accumulation of mud-rich sediments

in the deep-water environment, as opposed to stages

Incised

valley

Bypass channel

Depositional scarp

Highstand prism

Coastal plain

Delta

2 km

FIGURE 5.21 Unincised (bypass) fluvial system on a continental shelf that is not fully exposed by the fall

in base level (offshore Java, Indonesia; modified from Posamentier, 2001; seismic image courtesy of H.W.

Posamentier). The seismic amplitude horizon slice shows a high-sinuosity unincised channel flowing to the

right (southeast) across the Miocene shelf of the Java Sea.

(

400 300 200 100 0

140

110

Meters below

modern sea level

Low

High

Mean

FIGURE 5.22 Late Pleistocene to Holocene sea-level curve based on oxygen isotope data (modified from

Bard et al., 1990). The 110 m threshold marks the depth of the Java shelf edge below the modern sea level, and

it has been identified as the level below which the Java continental shelf is fully exposed. A fall of less than

110 m below the modern sea level results in the formation of unincised fluvial systems, whereas a sea-level

fall in excess of 110 m below the present level results in the formation of incised valleys across the shelf

(Posamentier, 2001). Note that the last 0.5 Ma in the evolution of the Java shelf are by far dominated by the

presence of unincised fluvial systems.

184 5. SYSTEMS TRACTS

of full exposure of the shelf that are likely to result in

the formation of deep-water reservoirs – see further

discussion below on the effects of low- vs. high-magni-

tude falls in base level on deep-water sedimentation.

The definition of criteria for the correct identification

of incised vs. unincised fluvial systems is therefore

highly important. A summary of features that can be

used to distinguish between these two play types is

presented in Fig. 5.23. A detailed imagery of subsurface

fluvial systems, as afforded by 3D seismic, well-log and

core data, is necessary to document their stratigraphic

makeup and architecture, aspect ratio, the characteris-

tics of tributary systems, and the position of the fluvial

deposits under analysis within the overall stratigraphic

context. From the latter perspective, incised-valley fills

form stratigraphic ‘anomalies,’ being genetically unre-

lated to the adjacent facies, whereas unincised channel

fills integrate within the paleogeography of the juxta-

posed and underlying depositional environments

(Figs. 5.24 and 5.25). Note that all incised valleys regard-

less of origin (e.g., climate- vs. base level-controlled)

share similar features (e.g., compare Figs. 3.7 and 5.17),

FIGURE 5.24 Synthetic gamma ray logs illustrating the stratigraphic context of (I) incised-valley fills and (II)

unincised channel fills (modified from Posamentier and Allen, 1999). Not to scale. Incised-valley fills occupy an

anomalous position in the stratigraphic context, being genetically unrelated to the juxtaposed facies—in this

example, the valley fill is fully engulfed within outer shelf shales. Unincised valley fills are genetically related to

the juxtaposed and underlying facies—in this example, the channel may be part of an alluvial or delta plain envi-

ronment that progrades over delta front facies. In both block diagrams, 1 represents the sea level at the onset of

forced regression, and 2 the sea level at the end of forced regression. K—upstream-migrating fluvial knickpoint.

Incised-valley fills

Unincised fluvial or

distributary channel fills

Width:thickness

aspect ratio

Stratigraphic

architecture

Well log response

Gas/oil production

Well log markers

Complex, involving depositional

systems ranging from fluvial to

estuarine and open marine

Tributaries

Simple, commonly including

only fluvial deposits

Low, commonly less than 200:1 High, potentially close to 1000:1

Incised Unincised

Anomalous, showing a lack of

correlation with juxtaposed units

Good correlation with juxtaposed

units

Commonly truncated by

valley incision

Preserved in a relatively

conformable succession

Potentially very high Average

Systems

Criteria

FIGURE 5.23 Summary of criteria that

may be used to differentiate between incised-

valley fills and unincised or distributary

channel fills. Inadequate data (e.g., a lack of

high-resolution 3D seismic data, well logs,

or core material) may lead to confusions

between these two play types, with negative

consequences for the strategy of exploration

of the entire falling-stage systems tract – see

text for details. The presence of incised trib-

utaries in association with incised valleys is

one of the easier and unequivocal features

to observe in the preliminary stages of

stratigraphic analysis (e.g., Figs. 3.7, 5.17,

and 5.19). Note that incised-valley fills tend

to form stratigraphic ‘anomalies,’ which

disrupt the continuity of stratigraphic mark-

ers and the predictable association of facies

that is expected according to Walther’s law

(compare the two scenarios in Fig. 5.24).

Shelf

edge

Incised valley

Highstand prism

Delta

fluvial and/or estuarine deposits

marine shale

subaerial unconformity

wave ravinement

Incised

valley

Bypass channel

Depositional scarp

Coastal plain

Delta

Prograding,

shallow marine

deposits

(I) Incised-valley fill (II) Unincised channel fill

FALLING-STAGE SYSTEMS TRACT 185

so the mere identification of incised valleys is not suffi-

cient for an immediate interpretation of the allogenic

controls responsible for the process of valley incision.

More criteria that may help to interpret the nature

of the allogenic controls on fluvial processes are

discussed further in Chapter 6.

Diagnostic for the falling-stage systems tract are the

shallow-marine deposits with rapidly prograding and

offlapping stacking patterns (Fig. 5.10), which are age-

equivalent with the bulk of the deep-water submarine

fans (e.g., Hunt and Tucker, 1992; Plint and Nummedal,

2000; Figs. 5.26 and 5.27). This systems tract was inde-

pendently described by Hunt and Tucker (1992) who

specifically referred to the slope and basin-floor

settings, and by Plint and Nummedal (2000) who stud-

ied the processes and products of forced regressions

(m)

CLAY

Sandstone

Vf F M C

CLAY

Sandstone

Vf F M C

CLAY

SIL

Sandstone

Vf F M C

CLAY

Sandstone

Vf F M C

DATUM

Top of

preserved section

FA 4

FA 3

FA 2

FA 1

FA 5

FA 2

FA 6

FA 5

FA 1

FA 3

FA 7

FA 5

FA 8

Base of

exposed section

TST

TST 3

LST 4

TST 3

TST 1

HST 1

LST 1

LST 3

Upper sequence boundary

of the Bahariya Formation

Upper sequence

boundary of the

Bahariya Fm.

WRS/MRS

MRS

MFS

WRS/SU

CLAY

Sandstone

Vf F M C

FIGURE 5.25 Sequence stratigraphic framework, and incised-valley systems, of the Lower Cenomanian

Bahariya Formation in the Bahariya Oasis, Western Desert of Egypt (from Catuneanu et al., in press). For scale,

the cross section covers a horizontal distance of about 100 km. The erosional relief generated by the process

of valley incision explains the abrupt facies shifts that may occur laterally over relatively short distances, as

well as the absence of some systems tracts in areas most affected by fluvial erosion. The magnitude of base-

level fall associated with the formation of each subaerial unconformity (sequence boundary) may be esti-

mated from the amount of valley incision as inferred from the thickness variation of lowstand deposits across

the Bahariya Oasis. Accumulation of lowstand fluvial deposits contributes to the peneplanation of the

incised-valley topographic relief. Abbreviations: LST—lowstand systems tract; TST—transgressive systems

tract; HST—highstand systems tract; SU—subaerial unconformity; MRS—maximum regressive surface;

MFS—maximum flooding surface; WRS/MRS—wave-ravinement surface that replaces the maximum regres-

sive surface; WRS/SU—wave-ravinement surface that replaces the subaerial unconformity. Facies associa-

tions (FA): FA 1—aggrading and prograding delta plain; FA 2—backstepping parasequences; FA 3 and

4—low-energy fluvial systems; FA 5 and 8—high-energy fluvial systems; FA 6—outer-shelf glauconitic shales;

FA 7—beach deposits (see Catuneanu et al., in press, for full details).

Sea level

lower shoreface

erosion (RSME)

shoreline - upper shoreface sands

deep-water muds

Early forced regression: depositional processes and products

fall riserise

detached

offlapping lobes

storm wave

reworking

fluvial

incision / bypass

sediment starvation

on floodplains

mudflows

outer shelf and shelf edge

instability

Not to scale

highstand

prism

slump

backfilled

canyon

- fluvial incision and/or bypass

- delta plain bypass or erosion (no topset)

- delta front progradation and offlap

- erosion in the lower shoreface

- outer shelf and shelf edge instability

- dominant gravity flows: debris flows / mudflows

Key features:

FIGURE 5.26 Depositional processes and products of the early falling-stage systems tract (modified from Catuneanu, 2003). Most of the sand

that accumulates during this stage is captured within detached and offlapping shoreline to upper shoreface systems. A significant amount of

finer-grained sediment starts to accumulate in the deep-water environment as mudflow deposits. Two sequence stratigraphic surfaces form

during base-level fall: the subaerial unconformity, which gradually expands basinward as the shoreline regresses; and the regressive surface of

marine erosion (RSME) cut by waves in the lower shoreface. The basal surface of forced regression is taken at the base of all forced regressive

strata, including the early fall mudflow deposits. In places, this surface may be reworked by the RSME (Fig. 4.23).

Sea level

- fluvial bypass and/or incision

- delta plain bypass or erosion (no topset)

- delta front progradation and offlap

- dominant gravity flows: high-density turbidites

shoreline and shoreface sands

longshore currents

deep-water sands

Late forced regression: depositional processes and products

fall riserise

detached

offlapping lobes

Not to scale

highstand

prism

Incised valleys

and tributaries

remnants of

early forced regressive lobes

offlapping

shelf edge delta

leveed

channels

frontal

splays

paleosols / wind degradation

Key features:

Sandy

turbidites

(large volume)

FIGURE 5.27 Depositional processes and products of the late falling-stage systems tract (modified from Catuneanu, 2003). The sediment mass

balance changes in the favor of the deep-sea submarine fans, which capture most of the sand. The subaerial unconformity keeps forming and expand-

ing basinward until the end of base-level fall. Once the shoreline falls below the shelf edge, the regressive surface of marine erosion stops forming as

the seafloor gradient of the continental slope is steeper than what is required by the shoreface profile to be in equilibrium with the wave energy. Note

that fluvial systems are likely to incise into the highstand prism but may only bypass the rest of the subaerially exposed shelf, unless the base level

falls below the elevation of the shelf edge. The turbidity currents of the deep basin are dominantly of high-density type, due to the massive amount

of sediment supply, and hence they tend to be overloaded and aggradational (sediment load > energy of the flow) along their entire course.

FALLING-STAGE SYSTEMS TRACT 187

in a shelf-type setting. Posamentier and Morris (2000)

also provide a comprehensive synthesis of the diag-

nostic features of shallow-marine forced regressive

deposits, and Posamentier and Kolla (2003) discuss the

depositional elements that are most likely to accumu-

late in the deep-water environment during base-level

fall. As pointed out by Plint and Nummedal (2000), the

pattern of stratal offlap that is characteristic of forced

regressions (Fig. 3.22) may be obliterated by subse-

quent subaerial or transgressive ravinement erosion.

In such cases, the most practical feature that helps to

identify a falling-stage systems tract is the presence of

sharp-based shoreface sandbodies in wave-dominated

nearshore areas (Plint and Nummedal, 2000; Figs. 4.21

and 4.28). Additional criteria for the recognition of

shallow-marine forced regressive deposits include: the

presence of zones of separation (detachment) between

successive shoreface deposits (Figs. 5.26–5.30); the

occurrence of long-distance regression (Fig. 5.27); the

absence of alluvial plain, coastal plain, or delta plain

deposits at the top of shoreface deposits (e.g., contrast

Figs. 3.27 and 3.35); the presence of a seaward-dipping

subaerial unconformity on top (Fig. 4.15); the presence

of progressively lower-relief clinoforms in a basinward

direction (Fig. 5.31); a potential increase in average

grain size in a seaward direction, as the gradient

Sea ice

FIGURE 5.29 Detached paleoshorelines (arrows) left behind by

the forced regression associated with the fall in base level caused by

Holocene glacio-isostatic rebound. The horizontal distance shown in

the photograph is about 6 km. Dundas Peninsula, Melville Island,

Canadian Arctics.

Sea ice

FIGURE 5.30 Detached and downstepping beaches (arrows)

associated with base-level fall and the forced regression of the shore-

line caused by Holocene glacio-isostatic rebound. The horizontal

distance shown in the photograph is about 6 km. Dundas Peninsula,

Melville Island, Canadian Arctics.

detached and downstepping

shoreface deposits (on shelf)

frontal

splay

(basin

floor)

FIGURE 5.28 Amplitude extraction map

along a seismic horizon, showing detached

and downstepping forced regressive

shoreface deposits on the continental shelf

(from Catuneanu et al., 2003a; image cour-

tesy of PEMEX). The color code uses blue

for sand and orange for shale.

188 5. SYSTEMS TRACTS

of fluvial systems may steepen during base-level fall

(Fig. 5.13); and the presence of ‘foreshortened’ strati-

graphic successions (Posamentier and Morris, 2000).

The latter criterion describes situations where the

decompacted thickness of the regressive succession

is significantly less than the paleowater depth at the

time of deposition. For example, the Panther Tongue

Sandstone (Fig. 3.30) accumulated in 75–100 m deep

water, but its decompacted thickness is only 25 m. The

difference is primarily accounted for by subaerial

erosion during the forced regression of the shoreline

(Posamentier and Morris, 2000).

In a most complete scenario, a falling-stage systems

tract may include offlapping shoreface lobes on the

continental shelf, inner to outer shelf macroforms,

shelf-edge deltas that downlap the continental slope,

and slope and basin-floor submarine fans (Figs. 4.23,

4.24, and 5.4–5.6). These deposits do not necessarily

coexist. The type of falling-stage facies that accumu-

late at any given time depends largely on the position

of the base level relative to the shelf-edge elevation,

and implicitly on the location of the shoreline relative

to the shelf edge (Figs. 5.26 and 5.27). Furthermore, the

type of falling-stage deposits that get preserved in the

rock record depends on the magnitude of base-level

fall and on the location of the shoreline relative to the

shelf edge at the end of fall. Among all portions of the

falling-stage systems tract, the shallow-marine facies

that accumulate on the continental shelf during forced

regression are most susceptible to subsequent subaer-

ial erosion, especially in situations where the base

level falls below the elevation of the shelf edge.

For low-magnitude falls in base level, when the

base level remains above the elevation of the shelf

edge during forced regression (Figs. 4.23 and 5.6), the

falling-stage deposits typically include offlapping

deltaic and shoreface lobes, shelf macroforms, and

deep sea (slope and basin-floor) submarine fans. In

such cases, where a portion of the continental shelf is

still submerged, no shelf-edge deltas may form and

the deep-water fans are dominated by fine-grained

sediments (Fig. 5.26). The forced regressive deltas that

prograde the continental shelf during the falling

stage tend to thin in a basinward direction (Fig. 5.31)

and, assuming that equal volumes of sediment are

present in each successive deltaic lobe, they also

tend to become wider along strike (Fig. 5.13).

Reservoir connectivity of forced regressive shelf-delta

sands is therefore expected to improve toward the top

of the falling-stage systems tract. At the same time, the

gradual steepening of the fluvial slope gradient

during forced regression results in a coarsening

down dip of the sediment that is present in the offlap-

ping delta lobes, which further enhances the reservoir

quality of the late falling-stage deltaic sands

(Posamentier and Morris, 2000; Fig. 5.13). The

shoreface deposits that accumulate in a shelf setting

during forced regression are sharp-based, excepting

for the earliest falling-stage shoreface strata which are

gradationally based (Figs. 4.23 and 5.6). The preser-

vation potential of these shallow-marine falling-

stage strata is inversely proportional to the magnitude

of base-level fall. As the shoreline approaches the

shelf edge during forced regression, shelf-edge deltas

will form and supply coarser sediment to the deep-

water environment (Fig. 5.27). At the same time, the

linear paleoshoreline sandbodies abandoned on the

subaerially exposed shelf are now subject to fluvial

and wind degradation. For as long as the base level

does not fall below the elevation of the shelf edge,

fluvial systems may only incise the highstand prism,

bypassing the rest of the shelf (Posamentier, 2001;

Fig. 5.27).

For higher-magnitude falls in base level, when the

base level falls below the elevation of the shelf edge

(Fig. 5.4), a shelf-edge delta with offlapping geome-

tries will prograde and downlap onto the continental

slope, coeval with the manifestation of significant

gravity-flow events in the deep-water environment.

These gravity flows consist mainly of high-density

turbidity currents, which are potentially rich in sandy

riverborne sediment that is supplied by distributary

channels directly to the deep-water environment. As

the regressive surface of marine erosion is unlikely

to form beyond the shelf edge, on the relatively steep

continental slope (see discussion in Chapter 4), these

falling-stage deposits are bounded at the top by the

subaerial unconformity and its correlative conformity,

and at the base by the basal surface of forced regres-

sion (Fig. 5.4). In this scenario, fluvial systems are

likely to incise not only the highstand prism, but

also the rest of the shelf that was submerged at the

onset of forced regression (e.g., case C in Fig. 5.16).

The preservation potential of all shallow-marine

falling-stage deposits that accumulated on the shelf

during the earlier stages of forced regression is mini-

mal in this case.

Seaward decrease in clinoform height

FIGURE 5.31 Decreasing shallow-water clinoform height in a

seaward direction in a shelf-type setting, in response to base-level

fall (modified from Posamentier and Morris, 2000). This trend is

particularly representative of river-dominated deltaic settings,

where clinoforms are steeper than the wave equilibrium profile, and

as a result no wave scouring affects the lower shoreface to inner

shelf environments. This is the reason why no regressive surfaces of

marine erosion are shown in the diagram.

FALLING-STAGE SYSTEMS TRACT 189

Economic Potential

Petroleum Plays

The formation and distribution of reservoir facies

may be markedly different between the stratigraphically

lower and upper portions of the falling-stage systems

tract. For this reason, the discussion below focuses on

the distinct processes and products of the early vs. late

stages of forced regression (Figs. 5.26 and 5.27).

The early forced regression corresponds to the early

stage of base-level fall at the shoreline, when a signifi-

cant portion of the continental shelf is still submerged

(Fig. 5.26). The shoreline trajectory is defined by progra-

dation and offlap, accompanied by fluvial erosion or

bypass upstream. The lack of aggradation in delta plain

or coastal plain environments prevents the formation

of regressive coastal topsets. Under these circum-

stances, the prograding shoreface or delta front deposits

are truncated by the subaerial unconformity (Fig. 3.27).

Elongated and downstepping beach-upper shoreface

sandbodies may be abandoned on the subaerially

exposed shelf as the base level falls (Fig. 5.26). These

paleoshoreline sands are generally thin (range of meters)

and ‘detached’ (i.e., separated by gaps; Figs. 5.28–5.30).

The degree of detachment depends on the interplay of

sediment supply and the rates of base-level fall (Fig.

3.33; Posamentier and Morris, 2000). During early fall,

the shoreline is still far from the shelf edge, so no river-

borne sand is delivered directly to the continental slope.

However, lowering of the storm wave base causes

instability on the outer shelf, which triggers gravity-

flow processes into the deep-water environment. These

gravity flows include mainly the fine-grained sediment

accumulated on the outer shelf-upper slope area during

the previous highstand (late rise) normal regression, as

well as during the earliest phases of forced regression.

The reservoirs of the early forced regression stage

are mainly represented by the offlapping and downstep-

ping paleoshoreline and shoreface sands abandoned

on the shelf during the fall in base level (Figs. 5.14 and

5.26). These sand bodies, even though generally thin

and detached, may have very good lateral extent along

strike (Fig. 5.26), and may be significant enough to be

seen on amplitude extraction maps of 3D seismic data

(Fig. 5.28). No fluvial reservoirs are expected to develop

during this stage, as, according to standard sequence

stratigraphic models, the nonmarine portion of the

basin is subject to bypass or downcutting processes

(Figs. 5.11, 5.14, and 5.26). The lower (early fall) portion

of the deep-water submarine fans displays a poor

reservoir quality, due to the low sand/mud ratio, and

is usually represented on seismic data by transparent

and/or chaotic seismic facies (e.g., transparent facies

A in Fig. 5.32). The plastic behavior of these mud-rich

cohesive debris flows (mudflows in Fig. 5.26) confers

on them additional characteristics that may be

observed on 2D and 3D seismic data, including thrust

faults and associated compressional ridges, as well as

striations and grooves at the base (Posamentier and

Kolla, 2003). Such diagnostic features of mud-rich

Offlap

0 1 km

BSFR

MRS

FIGURE 5.32 Uninterpreted and

interpreted seismic line showing the

contrast in facies between mudflow

deposits (facies A—early forced

regressive) and turbidites (facies B—

late forced regressive) in a deep-water

setting (from Catuneanu et al., 2003a;

image courtesy of PEMEX). Note that

the top of the coarser-grained facies

of the submarine fan is marked by

the extension within the basin of the

youngest clinoform associated with

offlap (i.e., the correlative conformity

sensu Hunt and Tucker, 1992). In this

example, the maximum regressive

surface downlaps the correlative

conformity, and hence no significant

lowstand normal regressive deposits

are present above the late forced

regressive turbidites. Abbreviations:

BSFR—basal surface of forced regres-

sion; SU—subaerial unconformity;

CC—correlative conformity (sensu

Hunt and Tucker, 1992), MRS—maxi-

mum regressive surface.