Catuneanu O. Principles of Sequence Stratigraphy

Подождите немного. Документ загружается.

190 5. SYSTEMS TRACTS

deep-water deposits are of course critical to recognize

on seismic data prior to drilling. The formation of

thrust faults and compressional ridges is caused by

the tendency of mudflows to freeze on deceleration,

due to the high internal strength (cohesiveness) of the

muddy matrix. This ‘plastic’ rheological behavior, in

contrast to the ‘fluidal’ behavior of turbidity currents,

explains why mudflow deposits tend to accumulate in

more proximal areas of the deep-water setting relative

to turbidites (illustrated by the relative location of

frontal lobes/splays in Figs. 5.26 and 5.27). Seismic

examples of deep-water cohesive debris flow (mudflow)

deposits, which summarize their defining characteris-

tics, are provided in Figs. 5.33–5.36. These diagnostic

features include: high erosional relief generated by the

flow (Fig. 5.33); the presence of basal grooves caused

by the drag force of the plastic flow over the seafloor

(Fig. 5.34); internal chaotic/contorted seismic facies that

reflects the plastic behavior of the flow, as the flow

freezes on deceleration rather than allowing sediments

to settle according to weight (Fig. 5.35); and the presence

of internal thrust faults, and associated compressional

ridges at the top of mudflow deposits, which once again

reflect the plastic rheology of the flow (Fig. 5.36).

The late forced regression corresponds to the late

stage of base-level fall at the shoreline, when most, if

not all, of the continental shelf becomes subaerially

exposed (Fig. 5.27). During this stage, the early forced

regressive paleoshoreline sandbodies may loose their

original linear geometry due to prolonged fluvial and

wind degradation (Figs. 4.31C and 5.27). As the shore-

line approaches the shelf edge, the fluvial sediment

starts to be delivered straight to the continental slope,

causing major gravity-flow events. Additional sedi-

ment supply is generated by processes of fluvial

incision upstream. The lack of accommodation for the

fluvial and shoreline systems explains the large volume

of turbidites which accumulates during this time

in the deep-water environment (Figs. 5.14 and 5.27).

Basal grooves

1 km

FIGURE 5.34 Seismic time slice illustrat-

ing grooves at the base of a mudflow

deposit (upper-left side of the image;

Pleistocene, eastern Gulf of Mexico; modi-

fied from Posamentier, 2003; image courtesy

of H.W. Posamentier). These striations are

caused by the drag force imposed by the

plastic flow upon the unconsolidated

substrate. The time slice shows reflection

amplitudes—see Fig. 2.51 for a visualization

of seismic facies along the same surface.

This image is time-transgressive, as the

lower-right side of the slice shows a slightly

older, channelized turbidity system.

Flow direction

FIGURE 5.33 Erosional relief caused by the motion of a cohesive

debris flow (mudflow) over the seafloor (Pleistocene, Gulf of

Mexico; images courtesy of H.W. Posamentier). The dimensions of

the scoured relief shown in this seismic image are as follows:

approximately 30 km in length, approximately 12.5 km maximum

width, and approximately 240 m in depth.

FALLING-STAGE SYSTEMS TRACT 191

Internal

convolute architecture

1 km

Internal

convolute architecture

1 km

FIGURE 5.35 Seismic time slices showing

the internal contorted/convolute architecture

of a debris flow deposit (upper-left side of

images), caused by the tendency of the flow

to freeze on deceleration (Pleistocene, eastern

Gulf of Mexico; modified from Posamentier,

2003; images courtesy of H.W. Posamentier).

The dense and plastic nature of the sedi-

ment/water mixture in mudflows does not

allow sediments to settle from suspension;

instead, the flow stops abruptly as soon as

the slope angle decreases below a critical

threshold of approximately 1°. This explains

the lack of layering of mudflow deposits, as

well as other structural features captured in

Fig. 5.36. The two seismic time slices in this

Figure are slightly above the image shown

in Fig. 5.34.

one km

100 msec

Flow Direction

one km

100 msec

Flow Direction

five km

Compressional ridge

FIGURE 5.36 Compressional structures of mudflow deposits, generated by the tendency of the flow to

freeze on deceleration (Pleistocene, Gulf of Mexico; images courtesy of H.W. Posamentier). Owing to its plastic

rheology, the flow ‘freezes’ as soon as the shear stress becomes less than the internal shear strength of the

sediment/water mixture. The frontal part of the flow freezes first, when reaching areas of lower slope gradient,

acting as a barrier against which the rest of the sediment/water mixture crashes. This causes the formation of

internal thrust faults (see cross sectional views), whose expression at the top of mudflows deposits is repre-

sented by compressional ridges (see plan view).

192 5. SYSTEMS TRACTS

As the sediment entry points get gradually closer to

the shelf edge during the falling stage, the overall verti-

cal profile of the forced regressive submarine fans is

coarsening-upward, with a transition from mudflows

to sandy turbidites (Figs. 4.18, 5.4, 5.11, 5.32, and 5.37).

As a consequence of the processes described above,

the best petroleum plays that may form in relation

to the late stage of forced regression are the sandy

turbidites associated with the deep-water submarine

fans (Figs. 5.14 and 5.27). These reservoirs form the

coarsest part of the basin-floor fans, are topped by the

correlative conformity sensu Hunt and Tucker (1992)

(Figs. 4.18 and 5.5), and their position on 2D seismic

lines may be inferred by mapping the youngest clino-

form with an offlap type of stratal termination up

dip (Figs. 4.17, 5.32, and 5.38). The turbidity currents

LOW

HIGH

FLOW DENSITY

(SEDIMENT / WATER RATIO)

SAND / MUD RATIO

FSST

LST

TST

HDT

LDT

Slope gradients Flow rheology

Steep

(> 1°)

Moderate

(> 0.5°)

Low

(~ 0°)

Plastic flow

Fluidal flow

(c-u)

(f-u)

FIGURE 5.37 Trends of change in the main types of gravity flows that operate in the deep-water environ-

ment during the formation of the falling-stage, lowstand and transgressive systems tracts. Abbreviations:

DF—cohesive debris flows (mudflows); HDT—high-density turbidites; LDT—low-density turbidites; FSST—

falling-stage systems tract; LST—lowstand systems tract; TST—transgressive systems tract; c-u—coarsening-

upward; f-u—fining-upward. The table indicates the minimum slope gradients required by the three main

types of gravity flows to move, as well as the rheology of each flow type. These flow characteristics explain

why mudflows tend to travel shorter distances relative to turbidity currents, and also why low-density

turbidites may reach farther into the basin than the high-density turbidites. The latter is also facilitated by the

lower sand/mud ratio of the low-density turbidity flows, which sustains the formation of levees over larger

distances. See text for details (Chapters 5 and 6).

FIGURE 5.38 Interpreted seismic line showing the location of the best deep-water reservoirs in a sequence

stratigraphic framework (from Catuneanu et al., 2003a; image courtesy of PEMEX). See Fig. 2.65 for the unin-

terpreted seismic line. The offlap type of stratal termination is highly significant for deep-water exploration

because the youngest clinoform associated with offlap (i.e., the correlative conformity sensu Hunt and Tucker,

1992—dashed line in the figure) leads to the top of the coarsest deep-water facies. Abbreviations: FSST—

falling-stage systems tract; LST—lowstand systems tract; TST—transgressive systems tract; HST—highstand

systems tract; SU—subaerial unconformity; RS—transgressive ravinement surface; MRS—maximum regres-

sive surface; MFS—maximum flooding surface.

FALLING-STAGE SYSTEMS TRACT 193

triggered during the late stage of forced regression

tend to have high sand/mud and sediment/water ratios

because of the large amounts of terrigenous sediment

supplied by fluvial systems at the shelf edge (Fig. 5.37).

Due to their high-density nature (high sediment/water

ratio), these turbidity currents tend to be overloaded,

which favours channel aggradation and the construc-

tion of levees on the continental slope as opposed to

canyon incision (Figs. 5.27, 5.39, and 5.40). As levees

are built by the finer-grained sediment fraction of the

flow, they tend to be relatively short for turbidity

currents with a high sand/mud ratio of the initial sedi-

ment mixture (i.e., insufficient amount of mud to

sustain the construction of levees over large distances).

As a result, levees’ height decreases rapidly down flow

and as the turbidity currents become unconfined the

frontal splays accumulate relatively close to the toe of

the slope, albeit more distally relative to the mudflow

lobes of the previous stage (Figs. 5.4, 5.41, and 5.42;

also, compare Figs. 5.26 and 5.27). Both the channel

fills on the slope and the frontal splay on the basin

floor are sand-prone and hence potentially good reser-

voirs (Fig. 5.42). The leveed channels on the slope may

be confined within older submarine canyons, or may

form in areas between slope canyons. These issues

concerning the processes and products of the deep-

water environment during the various stages of the

base-level cycle are discussed further in Chapter 6.

Besides the submarine fans and the associated feeding

leveed channels, additional exploration potential is

offered by the offlapping shelf-edge deltaic lobes that

prograde the upper part of the continental slope, as

well as by the strike-oriented elongated sandbodies

abandoned on the shelf during the forced regression of

the shoreline (Fig. 5.27). As in the case of the early

forced regression, no fluvial reservoirs are expected to

form during this stage (Fig. 5.14).

The primary risk for the exploration of coastal to

shallow-marine falling-stage reservoirs accumulated

on the continental shelf, is represented by the potential

lack of charge due to the insufficient development of

seal facies on top. Where present, however, the overly-

ing lowstand fluvial floodplain shales and/or fluvial

or marine transgressive shales may seal the detached

shoreline to shoreface sands of the falling-stage

systems tract. Therefore, as with all systems tracts, the

exploration potential of each individual reservoir

needs to be assessed on a case-by-case basis. The deep-

water reservoirs of the falling-stage systems tract, which

arguably form some of the best petroleum plays of

an entire stratigraphic sequence, involve fewer risks

relative to their coastal to shallow-marine counter-

parts, as the deep portion of the basin is generally

most conducive toward the accumulation of source

and seal facies. All underlying and overlying systems

tracts (lowstand, transgressive and highstand) may

provide source rocks and seal facies, respectively, for

the falling-stage turbidite reservoirs.

Coal Resources

The stage of base-level fall is unfavorable for peat

accumulation and subsequent coal development

because accommodation is negative, and the nonma-

rine environment is generally subject to fluvial valley

incision and/or paleosol development in interfluve

areas (Fig. 5.15). Therefore, according to the standard

sequence stratigraphic models (Fig. 4.6), the only

remnant in the rock record of the surface processes

that take place in the nonmarine realm during base-

level fall is the subaerial unconformity. This ‘standard’

view is based on the underlying assumption that fluvial

processes are mainly controlled by base-level changes.

As discussed in Chapters 3 and 4, this process/response

relationship is particularly true for the downstream

FIGURE 5.39 High-sinuosity channel-

ized turbidity system in a continental

slope setting (Late Pleistocene, offshore

Nigeria; modified from Posamentier and

Kolla, 2003; image courtesy of H.W.

Posamentier). Flow direction from right to

left. The formation of such leveed channels

on a continental slope is most probable in

the case of high-density turbidity flows,

which may be overloaded (sediment load >

energy), and hence aggradational, in high-

gradient settings. In contrast, low-density

turbidity currents tend to be entrenched

(underloaded) on continental slopes. The

manifestation of high-density turbidity

flows is most likely during late stages of

forced regression, when terrigenous sedi-

ment supply to the deep-water environ-

ment is highest.

194 5. SYSTEMS TRACTS

reaches of fluvial systems, but it may be significantly

distorted due to the interference of the climatic control.

For example, the decrease in fluvial discharge during

stages of glaciation may lead to fluvial aggradation in

spite of the fall in sea level (Blum, 1990, 1994). Even in

such cases, however, the cold climate is not favorable

for the accumulation of any significant peat deposits.

Placer Deposits

Surface processes during the falling leg of the base-

level cycle are generally considered to be most condu-

cive to the formation of placer deposits. According to

standard sequence stratigraphic models, erosional

processes are expected to affect both nonmarine and

wave-dominated shallow-marine environments during

forced regression, resulting in the formation of subaer-

ial unconformities and regressive surfaces of marine

erosion, respectively (Fig. 3.27). These two types of

unconformities have a different geographic distribu-

tion within the basin, even though they may partly

Thick levees

FIGURE 5.40 Turbidite leveed channel in an upper slope conti-

nental setting (Late Pleistocene, De Soto Canyon area, Gulf of

Mexico; images courtesy of H.W. Posamentier). As seen on the seis-

mic isochron map, channel sinuosity is significantly higher than

levee sinuosity. The 2D seismic line (A – A’) reveals the wedge-

shaped geometry of levee deposits. The presence of levees indicates

a high-density turbidity flow, whose sediment/water ratio is high

enough to allow aggradation even on the steep-gradient continental

slope. Such leveed channels are most likely to form during late stages

of forced regression, when terrigenous sediment supply to the deep-

water environment is highest. This massive sediment influx over-

comes the high energy of the flow on the steep-gradient continental

slope, and leads to the aggradation of the channelized turbidity

system. Levees are important depositional elements that keep the

flow confined during the motion of the sediment/water mixture

within an aggrading turbidity system. As levees are built by the finer-

grained sediment fractions of the sediment/water mixture, the flow

collapses when the system runs out of mud, leading to the formation

of frontal splays at the end of leveed channels (Fig. 5.41). Due to the

high sand/mud ratio that commonly characterizes high-density

turbidity flows, the frontal splays of such flows tend to be found in

proximal locations, close to the toe of the continental slope (i.e., insuf-

ficient amount of mud to sustain the formation of levees over large

distances). The location of frontal splays may thus provide an addi-

tional criterion to evaluate the type of turbidity flows (high- vs. low-

density) at the time of sedimentation. For scale, the leveed channel on

the seismic isochron map is approximately 1.8 km wide.

leveed channel

transition from

leveed channel

to frontal splay

frontal splay

6 km

FIGURE 5.41 Leveed channel to frontal splay transition within an

aggrading turbidity system (Late Pleistocene, De Soto Canyon area,

Gulf of Mexico; image courtesy of H.W. Posamentier). The presence

of aggrading leveed channels on the continental slope indicates an

overloaded (sediment load > flow energy) high-density turbidity

flow. The transition from leveed channel to frontal splay takes place

where the levees’ height decreases below a critical threshold that is

required to maintain the flow confined. This transition is placed

more proximally in the case of high-density (relative to low-density)

turbidity flows, due to the lower mud content of the former (see also

Fig. 5.40). For scale, the leveed channel in the upper part of the

image is 1.8 km wide.

FALLING-STAGE SYSTEMS TRACT 195

overlap in the region of the paleoshoreline (Fig. 3.27),

so their differentiation is important for the design of

exploration and production strategies.

Subaerial unconformities are the most important

placer-forming stratigraphic surfaces, because of the

extended period of time that is available for the

erosion and reworking of the underlying highstand

deposits. In this case, erosion is linked to fluvial and

wind degradation processes, and takes place during

the entire stage of base-level fall at the shoreline. The

reworking of late highstand fluvial deposits is parti-

cularly prone to the development of significant lag

deposits, because of the amalgamated nature of fluvial

channels that accumulate under very low accommoda-

tion conditions. The concentration of ten 10-cm-thick

individual channel lags, for example, may lead to

the formation of a 1 m thick lag (placer) deposit. The

Zandpan conglomerate that caps the Vaal Reef in the

Witwatersrand Basin is one of the many examples of

placers associated with subaerial unconformities

(Catuneanu and Biddulph, 2001; Fig. 5.43).

Wave scouring within shoreface to inner shelf envi-

ronments during the same stage of forced regression

may lead to the formation of other lag deposits, this

time associated with shallow-marine facies. The

Upper Vaal reef, up to 0.8 m thick, which is one of the

5 km

B’

A’

Frontal splays

Basin

floor

Continental

slope

FIGURE 5.42 Deep-water turbid-

ity systems, comprising sand-domi-

nated frontal splays on the basin

floor and their associated sand-filled

submarine channels on the continen-

tal slope (from Catuneanu et al.,

2003a; image courtesy of PEMEX).

Channel aggradation on the conti-

nental slope and the location of

frontal splays close to the toe of the

slope are indicative of high-density

(overloaded, and with a high

sand/mud ratio) turbidity currents,

which characterize late stages of

forced regression. These features

provide important criteria to differ-

entiate the high-density turbidity

flows of the falling-stage systems

tract from the low-density turbidity

flows (entrenched channels on slope,

and frontal splays located deeper

into the basin) of the lowstand and

early transgressive systems tracts.

196 5. SYSTEMS TRACTS

1 m

WSW ENE

500 m

Vertical exaggeration: 175 times

WSW mining area:

MB3 quartzite

Zandpan Marker

Upper Vaal Facies

Stilfontein quartzite

Stilfontein conglomerate

Witkop and Grootdraai channels

MB5 quartzites

ENE mining area:

MB3 quartzite

Zandpan Marker

MB4 quartzite

GV Bosch conglomerate

Zaaiplaats channels

MB5 quartzites

LST 1

FSST

HST

TST

LST 2

MB 3

MB 5

Stilfontein

MB 4

G.V. Bosch

Zandpan

Upper Vaal

Witkop and Grootdraai

Zaaiplaats

Johannesburg

Witwatersrand

Basin

Central Rand Group

LEGEND

West Rand Group

Archaean granitoids

Mining areas

0 50 100 km

South Africa

cross-sections

below

FIGURE 5.43 Cross sectional profile showing the position of gold placers (‘reefs’: Zandpan, Upper Vaal,

B.V. Bosch and basal Stilfontein) in a sequence stratigraphic framework (modified from Catuneanu and

Biddulph, 2001). See map for the location of the cross-section. Depositional environments: deltaic (MB 5,

Upper Vaal); fluvial (Witkop, Grootdraai, Zaaiplaats, Zandpan, MB 3); and transgressive shallow-marine

(Stilfontein, G.V. Bosch, MB 4). Abbreviations: LST—lowstand systems tract; TST—transgressive systems

tract; HST—highstand systems tract; FSST—falling-stage systems tract.

important gold placers in the Witwatersrand Basin,

has been identified as such, and is associated with a

regressive surface of marine erosion (Fig. 5.43).

Facies relationships across these unconformable

surfaces are critical to establish their nature and

sequence stratigraphic significance (Fig. 4.9), which in

turn are important to evaluate the geographic distribu-

tion of the associated placers, as well as to predict

changes in grades and placer quality along dip. In the

case of subaerial unconformities, the level of reworking,

and implicitly the textural maturity of the lag deposit,

is proportional to the amount of base-level fall and

downcutting. During this process, the finer sediment

fractions are removed, allowing for a concentration of

coarser clasts. As the amount of fluvial incision in

response to base-level fall changes along dip, commonly

decreasing in an upstream direction, the best reef qua-

lity tends to be found adjacent to the paleoshoreline.

Such a reef not only loses quality upstream, but it also

thins until it eventually disappears beyond the area

of influence of base-level changes. Depending on the

distance between paleoshoreline and the proximal rim

of the basin, such placers may not have a physical

expression along the basin margins, and may be missed

if exploration is solely based on mapping basin margin

unconformities. Similarly, shallow-marine forced regres-

sive placers that overlie regressive surfaces of marine

erosion, develop only offshore relative to the paleoshore-

line at the onset of base-level fall, and may be missed

where exploration is based solely on mapping basin

margin unconformities.

The shoreline is therefore a central element in the

exploration for placer deposits, because it limits the

lateral development of all placer types. The subaerial

unconformity-related placers may be found only land-

ward relative to the end-of-fall paleoshoreline, whereas

the regressive surface of marine erosion can form

only seaward from the onset-of-fall paleoshoreline.

Consequently, a successful exploration program must

include paleogeographic reconstructions for succes-

sive time steps, upon which reliable sequence strati-

graphic models may be built.

LOWSTAND SYSTEMS TRACT

Definition and Stacking Patterns

The lowstand systems tract, when defined as

restricted to all sedimentary deposits accumulated

during the stage of early-rise normal regression (sensu

Hunt and Tucker, 1992), is bounded by the subaerial

unconformity and its marine correlative conformity at

the base, and by the maximum regressive surface at

the top (Figs. 4.6, 5.4, and 5.5). Where the continental

shelf is still partly submerged at the onset of base-level

rise, following forced regression, the basal composite

boundary of the lowstand systems tract may also

include the youngest portion of the regressive surface

of marine erosion (Fig. 5.6; also see Fig. 4.23, and the

discussion in Chapter 4). The lowstand systems tract

forms during the early stage of base-level rise when

the rate of rise is outpaced by the sedimentation

rate (case of normal regression; Figs. 4.5 and 4.6).

Consequently, depositional processes and stacking

patterns are dominated by low-rate aggradation and

progradation across the entire sedimentary basin.

As accommodation is made available by the rising

base level, this ‘lowstand wedge’ is generally expected

to include the entire suite of depositional systems,

from fluvial to coastal, shallow-marine and deep-

marine (Fig. 5.44).

Lowstand deposits typically consist of the coarsest

sediment fraction of both nonmarine and shallow-

marine sections, i.e., the lower part of a fining-upward

profile in nonmarine strata, and the upper part of an

upward-coarsening profile in a shallow-marine succes-

sion (Fig. 4.6). Sediment mass balance calculations

indicate, however, that the grading trends observed

within shallow-marine successions do not correlate

with the grading trends that characterize the age-

equivalent deep-water deposits (Fig. 5.11). Thus, pref-

erential trapping of the coarser sediment fractions

within aggrading fluvial and coastal to shallow-

marine systems starting at the onset of base-level rise,

reduces not only the net amount of sand supplied to

the deep-water environment, but also the sand/mud

ratio of the sediment load transported by turbidity

currents. As a result, the lowstand sediments of the

basin-floor submarine fan complex are overall finer-

grained relative to the underlying late forced regres-

sive deposits (Fig. 5.5). The maximum grain size of the

sediment transported by gravity flows during the

lowstand normal regression is also expected to decrease

with time, due to the gradual lowering in fluvial slope

gradients and related competence following the onset

of base-level rise (Fig. 5.11). Consequently, in contrast

to the high-density turbidity currents of the late stage

of forced regression (Fig. 5.27), the deep-water portion

of the lowstand systems tract is dominated by low-

density turbidites (Fig. 5.44). The transition from high-

density to low-density turbidites at the onset of

base-level rise is illustrated in Fig. 5.37. Due to their

lower sediment/water ratio, the low-density turbidity

currents tend to be underloaded on the continental

slope (high energy relative to sediment load), where

channel entrenchment rather than aggradation is often

recorded (Figs. 5.44 and 5.45). Beyond the toe of the

LOWSTAND SYSTEMS TRACT 197

198 5. SYSTEMS TRACTS

Sea level

prograding

shoreface/delta front

entrenched

channels

leveed

channels

frontal

splays

Unincised braidplain

- fluvial aggradation (mainly channel fills)

- delta plain aggradation (topset)

- delta front progradation

- dominant gravity flows: low-density turbidites

shoreline and shoreface sands

longshore currents

Lowstand normal regression: depositional processes and products

fall riserise

Not to scal

highstand

prism

fluvial aggradation and onlap

open

shoreline

Key features:

Backfilled

incised valley

system

deep-water sands

FIGURE 5.44 Depositional processes and products of the lowstand systems tract (modified from

Catuneanu, 2003). In contrast to the falling-stage systems tract (Figs. 5.26 and 5.27), the sediment of this stage

of early-rise normal regression is more evenly distributed between the fluvial, coastal, and deep-water

systems. Sand is present in amalgamated fluvial channel fills, beach, and delta front systems, as well as in

submarine fans. The ‘lowstand prism’ gradually expands landward via fluvial aggradation and onlap.

Aggradation on the continental shelf in fluvial to shallow-marine environments reduces the amount of sedi-

ment supply to the deep basin, and hence the turbidity currents of this stage are dominantly of low-density

type, being underloaded (entrenched) on the continental slope and aggradational only on the low-gradient

basin floor where the energy of the flow drops below the threshold of balance with the sediment load. The

top of all early-rise normal regressive deposits is marked by the maximum regressive surface.

6 km

Channel ‘X’

FIGURE 5.45 Entrenched turbidite channels on a continental slope (Late Pleistocene, De Soto Canyon area,

Gulf of Mexico; images courtesy of H.W. Posamentier). Channel entrenchment indicates flow energy in excess

of sediment load, which is most likely to occur in the case of low-density turbidity currents. Such currents

commonly characterize the early stages of base-level rise (lowstand normal regression and early transgres-

sion). For scale, channel ‘X’ is approximately 1.8 km wide. Note that channel entrenchment on the continen-

tal slope (lowstand normal regression—early transgression) occurs after the aggradation of late forced

regressive leveed channels. Consequently, relict levees may be preserved adjacent to the entrenched channels.

These entrenched channels on the continental slope tend to become aggrading leveed channels on the basin

floor (see text for details).

LOWSTAND SYSTEMS TRACT 199

continental slope, on the basin floor, the lowstand

turbidity currents may become overloaded as energy

drops in response to decreasing seafloor gradients. As

a result, the basin-floor setting is likely to record

aggradation of leveed channels during the stage of

lowstand normal regression (Posamentier and Kolla,

2003; Figs. 5.44 and 5.46–5.48). Such leveed channels

are expected to develop further into the basin relative

to the underlying leveed channels of the falling-stage

systems tract because the muddier sediment of the

low-density turbidity currents sustains the formation

of levees over larger distances (compare the location of

frontal splays in Figs. 5.27 and 5.44). More details on

the depositional trends recorded in the deep-water

environment follow below, as well as in Chapter 6.

Coastal aggradation during lowstand normal regres-

sion (i.e., relative increase in coastal elevation; Fig. 5.44)

triggers a decrease in slope gradient in the downstream

portion of fluvial systems (Fig. 5.6), which induces a

lowering with time in fluvial energy and an overall

upward decrease in grain size (Figs. 4.6 and 5.11).

These decreases in fluvial energy and the grain size of

the riverborne sediment that is made available at the

shelf edge, explain the decrease in the maximum grain

size of the sediment transported by gravity flows to

the deep-water environment during the lowstand

stage, as discussed above. The increase with time in

the rate of base-level rise also contributes to the over-

all fining-upward fluvial profile, as it creates more

accommodation for floodplain deposition and

increases the ratio between floodplain and channel

sedimentation (Fig. 5.11). Lowstand fluvial deposits

typically accumulate on an uneven, immature topog-

raphy, and contribute to the peneplanation of a nonma-

rine landscape sculptured by differential erosion

during base-level fall (Fig. 5.12). Due to topographic

irregularities at the stratigraphic level of the subaerial

unconformity, the nonmarine portion of the lowstand

systems tract may display a discontinuous geometry, with

significant changes in thickness along dip and strike.

Typical examples of lowstand fluvial deposits

include amalgamated channel fills (low accommodation

systems) overlying subaerial unconformities (Fig. 5.49),

which may accumulate within incised valleys (forming

the entire valley fill or only the lower portion thereof;

Figs. 5.12 and 5.25) or across areas formerly occupied

by unincised, bypass falling-stage rivers (Fig. 5.44).

The accumulation of lowstand fluvial sediments

• Channel belt slope = c. 0.32°

• Channel thalweg slope = c. 0.07°

• Channel sinuosity = 4.88

5 km

FIGURE 5.46 Aggrading turbidite leveed channel on a basin floor

(Late Pleistocene, De Soto Canyon area, Gulf of Mexico; modified from

Posamentier, 2003; image courtesy of H.W. Posamentier). Well devel-

oped basin-floor leveed channels are typical of low-density turbidity

flows, whose mud content is high enough to sustain the formation

of levees over large distances. Such basin-floor leveed channels are

commonly age-equivalent with entrenched channels on the continen-

tal slope (Fig. 5.45). The change in the character of syndepositional

processes from erosional on the slope to aggradational on the basin

floor relates to changes in slope gradients and associated energy of the

flow. Low-density turbidity currents tend to be underloaded on the

continental slope (insufficient sediment load relative to energy), but

they become overloaded on the basin floor where the energy of the

flow decreases significantly. This is in contrast with the high-density

turbidity currents, whose larger sediment load allows them to aggrade

even on the steeper-gradient continental slope (Figs. 5.39–5.42).

channel fill

levees

slump scars

FIGURE 5.47 Depositional elements of a low-density turbidite

leveed-channel system on a basin floor (details from Fig. 5.46;

images courtesy of H.W. Posamentier). The raised appearance, with

a convex-up top, of the channel fill is caused by postdepositional

differential compaction. Levees are better developed along the outer

channel bends, and their inner margins are characterized by the

presence of scoop-shaped slump scars. The relief of the channel belt

above the adjacent basin plain is approximately 65 m. The channel

fill is approximately 625 m wide.