Process

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4 Sediment-Gravity Flows and Their Processes

Definition of Sediment Gravity Flow

How are sediments transported to, and deposited in, deepwater environments? Thisquestion has been the subject of discussion and debate since the 1870s and researched sincethe 1950s (see Shanmugam, 2000, for a comprehensive review). Collectively, the primary pro-cesses that transport sediment into deep water environments are called “sediment gravityflows” (Middleton and Hampton, 1973). Sediment gravity flows range from mass movements(rock falls) and cohesive debris flows at one end of the spectrum to fully turbulent flows at theother end of the spectrum (Fig. 4-1). Although the term “turbidity current” is often used todescribe deep water transport processes, it is only one of several types of flows that transportsediment into deep water, and rework them once deposited.

Unfortunately, transport and depositional processes are only rarely observed or measuredin deep marine and lacustrine environments. Thus, to understand sediment gravity flows, wemust combine data sets from outcrops, subsurface well logs and cores, seismic reflection pro-files, modern sea floor images and samples, laboratory flume experiments, and mathematicalmodels. Fortunately, rapidly evolving imaging, measurement, and computing technologies—mainly driven by petroleum exploration and development—are providing new data and insightsthat are leading to improved understanding of the complexities of sediment gravity flows andtheir deposits.

This chapter summarizes our current state of knowledge by combining key historicalconcepts with more recent observations and analyses. The chapter is organized to discuss theinitiation of sediment gravity flows, followed by the spectrum of processes and resultingdeposits, flow combinations and transformations, and temporal and spatial variations in pro-cesses. Processes of post-depositional reworking of sediment gravity flows on the sea floor arealso discussed in a separate section. Finally, mention is made of allocyclic and autocycle pro-cesses, since vertical stacking patterns are important to interpreting larger scale evolution ofsedimentary sequences and basin fill.

Figure 4-1. Chart listing the different types of sediment gravity flows by their corresponding sed-iment support mechanism. The classification is mainly after Lowe (1982). The figure was pro-vided by D. Pyles (2002).

The Sediment Gravity Flow ContinuumSediment gravity flow Sediment Support Mechanism

turbidity currents

fluidized flow

liquified flow

grain flow

debris flow

slump-slide

creep

rock fall

fluid turbulence

hindered settling

hindered settling

dispersive pressure

matrix strength

matrix strength

matrix strength

Increasin

g sed

imen

t con

centratio

n

Sediment-Gravity Flows and Their Processes

4-112

We want to stress that understanding of sediment gravity flow processes is continuallyevolving as results of new research are presented at technical meetings, research conferencesand in publications. Because of this, the sections in this chapter summarize key points of deep-water processes and deposits and are not meant as exhaustive compilations of the existingliterature.

Generation and Frequency of Occurrence of Sediment Gravity Flows

Upslope sediment gravity flows can be triggered by: (a) seismically-generated slides;(b) instability and slope failure resulting from rapid sedimentation, oversteepening and/orchange in pore pressure; (c) hyperpycnal flow (underflow) produced when dense, high sedi-ment concentration river effluent discharges into the sea, and (d) fine-grained underflowswhich trigger downslope sandy flows (Kneller and Buckee, 2000; Mulder et al., 2001a). Pro-cesses (a) and (b) are collectively termed “ignitive flows,” and (c) and (d) are termed “non-ignitive flows.” Sudden discharge of gas hydrates (clathrates) upward through slope sedimentto the sea floor are also thought to be capable of initiating ignitive flows on submarine slopes.

The 1929 Grand Banks of Newfoundland, Canada earthquake generated an ignitive flowby slope failure (Piper et al., 1988; Cochonat and Piper, 1995; Mulder et al, 1997; Piper et al,1999). Here, a single flow transported sediment for several hundreds of km into the deep Atlan-tic ocean floor. A second example of turbidity current generation by ignitive processes is the1979 failure of part of the Nice, France airport (Piper and Savoye, 1993). The frequencies ofoccurrence of ignitive sediment gravity flows are not recorded, but in seismically-active areas,we may speculate that they could occur on the order of one flow every 10s to 100s of years.

At a smaller, and more recordable time scale are hyperpycnal flows, which are sustainedflows that form at a river mouth during periods of high river discharge and move along the seafloor due to excess density relative to the ambient sea water (Mulder et al., 2003). Hyperpycnalflows can be generated at a frequency of years from rivers with extremely high suspended load(Mulder and Syvitski, 1995; Mulder et al., 2003). Eighty-four percent (84%) of the worlds’ riv-ers can produce hyperpycnal flows and can account for 53% of the world’s oceanic sedimentload. During the time period between 1887 and 1937, 30 submarine cable breaks were recordedin the Congo Submarine Canyon (West Africa), apparently due to turbidity currents generatedduring times of high bed-load discharge from the Congo River (Heezen et al., 1964). On theAmazon Fan, age-dating of a 240 m thick, 20 ka year interval of thin-bedded and laminated,muddy sediment indicate an average occurrence of 1 flow event every 2 years (Pirmez et al.,2000). The Amazon Fan flows also are thought to have been triggered by flood discharges fromthe Amazon River system. In a core collected from the Var submarine canyon, located in thewestern Mediterranean Sea, 13–14 hyperpycnal flow deposits were recorded during the past100 years and 9–10 were recorded during the past 50 years; this represents a frequency ofoccurrence of one hyperpycnal turbidity current every 5–7.5 years (Mulder et al., 2001a). Anintense sediment gravity flow was recorded in the Zaire submarine valley in March 2001. Thisflow demonstrated that even during periods of relatively high sea level, sediment can be trans-ported to deep water, in this case because the Zaire River is connected to the canyon and fanvalley for a length of 760km (Khripounoff et al., 2003).

Sediment Support Mechanisms and Types of Sediment Gravity Flows

Once a sediment gravity flow is generated, a variety of processes may transport the sedi-ment into the deep ocean, or into the deeper parts of lakes. Numerous studies suggest that thenature of a particular flow and its resultant deposit are a function of: (a) gravity, (b) velocityand fluid pressure within the flow in time and space (c) sediment support mechanism within the

Sediment Support Mechanisms and Types of Sediment Gravity Flows

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flow, (d) size frequency distribution, composition, and concentration of particles available fortransport, (e) topography of the seabed over which the flow is transported and deposited, and (f)frictional forces between the seabed and the flow.

Lowe (1982) classified sedment gravity flow types according to flow behavior and sedi-ment support mechanism. Sediment support mechanisms are illustrated in Figure 4-2. Creepand rockfall flow types, listed in Figure 4-1, were not included in Lowe’s (1982) originalclassification.

According to Lowe (1982), the support mechanism that prevails at any given instant dur-ing flow is principally a function of the flow velocity and concentration of sediment within theflow. Fluid turbulence (Newtonian flow) is the process of random motion of fluids within theflow, and dominates under conditions of relatively low sediment concentration. Turbidity cur-rents are characteristic of fully-turbulent flow. Sediment concentration is low enough thatupward-directed turbulence supports particles in suspension during transport.

With increasing sediment concentration, particles begin to settle from the flow, butupward flow of displaced water results in hindered settling of the particles (Fig. 4-2). With fur-ther increases in sediment concentration, dispersive pressure, caused by grain-to-graincollisions, becomes the predominant support mechanism (Fig. 4-2). At even higher sedimentconcentrations, plastic (Bingham) flows with a high matrix strength predominate (Fig. 4-2).The ratio of sediment to fluid is great enough to fully entrain the sediment as it flows in a plas-tic or laminar fashion.

Critical sediment concentrations, which distinguish plastic, fluidal or intermediateflows, are not presently clearly defined. Based on a compilation of published sediment concen-tration values of various flow types, Shanmugam (2000) suggests that the boundary betweenlaminar and turbulent flows occurs at about 20–25% sediment volume within the flow. Recentexperiments by Baas and Best (2001) have suggested that 3–4% clay content in a flow is thecritical concentration distinguishing laminar from turbulent flows. Marr et al. (2001) place thecritical clay concentration in experimental flows at 0.7–5 wt. % when the clay is bentonite and7–25 wt. % when the clay is kaolinite. Numerical modeling has suggested a critical sedimentconcentration of 10% (Pratson et al., 2000).

Figure 4-2. Diagrams illustrating the sediment support mechanisms (after Lowe, 1982). Withincreasing concentration of sediment within a flow, the support mechanism changes from fluidturbulence to hindered settling, to dispersive pressure to matrix strength. The figure was pro-vided by D. Pyles (2002).

Sediment Support Mechanisms

Fluid turbulence

-random motion of fluid in eddies

Hindered settling

-sediment begins to settle out of the flow,

space required for a grain to fall makes

water move upward, providing a lift force

Dispersive pressure

-interaction of grains with one another,

rattling of grains against each other,

happens when shear occurs

Matrix strength

-cohesion, usually provided by fines

Increasin

g sed

imen

t con

centratio

n


4-114

Hyperpycnal flows differ from ignitive flows because the water within the flow is freshrather than denser sea water. To overcome density contrasts and buoyancy effects, suspended

sediment concentrations in excess of 36kg/m3 are required for the flow to sink to the sea floorand move downslope. Sedimentation rates on the order of 1–2 m/100yrs. can be generated byhyperpycnal flows (Mulder et al., 2003).

Sediment Gravity Flow Processes and Deposits

Historically, the most commonly cited classifications of sediment gravity flow depositsare those of Walker (1978) (Fig. 4-3), Lowe (1982) (Fig. 4-4), Mutti et al. (1999) (Fig. 4-5)and Kneller (1995) (Fig. 4-6). These classifications are discussed in subsequent sections of thischapter. Each author uses different terminology to describe the deposits of approximatelyequivalent flow types. An attempt at comparing the deposits of these flow types is shown inFigure 4-7, although comparisons are not possible for all categories by all authors. In the fol-lowing sections, the deposits associated with the three main flow types—plastic flows,intermediate flows, and fluidal, turbulent flows—are discussed, along with more in-depth dis-cussion of associated sediment gravity flow processes.

Figure 4-3. Chart illustrating the sediment transport processes of sediment gravity flows and resulting depos-its. Sediment support mechanisms (inset) are those illustrated in Figure 4-2; i.e., 1= fluid turbulence, 2 = hin-dered settling, 3 = dispersive pressure and 4 = matrix strength. The vertical axis depicts sedimentconcentration, and the horizontal axis represents time and/or space. According to this diagram, flows can fol-low a number of transport paths until sediments within them are deposited. (Modified fromWalker, 1978).Reprinted with permission of American Association of Petroleum Geologists.

Debris

flow

Pebbly ssts

Conglomerates

Massive

sandstone

Classical

turbiditesTime and/or space

Slump

Slide

Remolding

LiquefactionDebris flow

(Traction)Traction

Fluid

turbulence

Rivers

in flood

High concentration

turbidity current

Grain

interaction

Upward flow

of pore fluids

Low concentration

turbidity current Support mechanisms

Flow initiationMain long-distance

transport processLate stage modifications

Decre

asin

g c

oncentr

ation

1

2

1

1

2

4

3

1

2

3 4

(Traction)


4-115

,

Figure 4-5. Diagram illustrating the deposits of the sediment gravity flow continuum. According to this classifi-cation, flow processes change in the flow direction, so the type of sediment deposited also changes. Cohesivedebris flows (CDF) are precursors to other flow types, such as gravelly high dencity turbidity currents(GHDTC), sandy high density turbidity currents (SHDTC) and low density turbidity currents (LDTC). Result-ing deposits, such as debrites, massive sandstones (F7), and Bouma divisions are explained in the text. (Modi-fied from Mutti et al., 1999). Reprinted with permission of American Association of Petroleum Geologists.

Figure 4-4. Diagram illustrating the sediment grav-ity flow continuum (vertical column on left). Thecontinuum figures reflecting various deposits of thecontinuum (1–13) move from bed 1 through bed 13.Lines without arrows (e.g., 1–2 and 1–3) connectmembers between which there probably exists acontinuous spectrum of flow and deposit types, butwhich are not part of an evolutionary trend of singleflows. Arrows connect members that may be partsof an evolutionary contnuum for individual flows.The transition from disorganized cohesive flows (1and 3), to thick, inversely graded, density-modifiedgrain flows and traction carpets (5) and to turbu-lent, gravelly, high-density turbidity currents (6) isspeculative. (Modified from Lowe, 1982). Reprintedwith permission of Society of Sedimentary Geology(SEPM).

LIQ

UIF

IED

FL

OW

SL

OW

-DE

NS

ITY

[TU

RB

IDIT

Y C

UR

RE

NT

S]

GR

AIN

FL

OW

SC

OH

ES

IVE

FL

OW

S

4

1

6

2

5

3

7

81312

11 10 9

T

HIG

H-D

EN

SIT

Y T

UR

BID

ITY

CU

RR

EN

TS

Tb

Tc

Td

Te

TaS3

Tb

c

Td

Te

=

S3

S2

S1

S1

R3

R3

R3

R2

R2

Tt

Facies F4 Facies F2 Facies F4 Facies F5 Facies F7 Facies F8 Facies F9a

Facies F9b

FT

TCTCFT

Facies F3

WF

LDTCGHDTC

FT

CDF HCF

DepressionMud drape scours Tabular scours

Rip up mudstone clastsFacies F6

SHDTC

Bouma-a Bouma b-e

Flow Direction

TC Traction Carpets

CDF Cohesive debris flows

HCF Hyperconcentrate flow

GHDTCGravelly high-density

turbidity current

SHDTCSandy high-density

turbidity current

LDTCLow-density turbidity

current

WFWavy-laminated facies composed

of poorly sorted gravel and sand

FT Flow transformation

Fluid escape structures


4-116

Figure 4-6. Different types of turbidity current deposits formed under a variety of spatial (accumulative, uni-form and depletive) and temporal (waning, steady and waxing) flow conditions. Bouma divisions are shown bydifferent colors. The horizontal thicknesses of individual deposits are indicators of thicknesses of the beds innature. Arrows point in the downcurrent direction. The nine flow types and their five deposits are explainedin the text (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-7. Chart comparing the common classifications of sediment gravity flow deposits shown inFigures 4-3–4-6. Figures in which symbols and names of deposits are explained are provided beneath author’snames. The various deposits are described in the text.

WAXINGSTEADYWANING

ACCUMULATIVE

UNIFORM

DEPLETIVE

EROSION/

NON-DEPOSITION

Yellow = Ta

Tan = Tb

Orange = Tc

Red = Td/e

direction of transport

Bouma

(Fig.14)

Walker

(Fig.3)

Lowe

(Figs. 4/22)

Mutti

(Fig.5)

Kneller

(Fig.6)

-Ta

-Tb-e

Debris Flow

Clast-supported

Conglomerate

Pebbly Sandstone

Massive Sandstone

Classical Turbidites

Cohesive Flow

[Slurry beds?]

R1-R3

S1-S2

S3

LDTC

F4(CDF)

F4/F7(?)

F5/F7(?)

F8

F9a

Steady

Waxing

Waning


4-117

Debris Flows and Debrites

Because of their high matrix strength, debris flows move in a plastic, laminar, cohesivestate. Their flow properties have been likened to that of wet concrete (Pratson et al., 2000). Debrisflows can be large or small, and can move for long distances down a slope and into a basin.

Debris flows are composed of a shear flow region and a plug flow region (Fig. 4-8). Theshear flow region is located in the lower part of the flow. Shear stresses in the bottom of theflow, generated by its movement, exceed the matrix shear strength and cause the flow to shear.The shearing decreases upward through the flow. The plug flow region is located in the upperpart of the flow at the height at which the shear stress becomes less than the matrix shearstrength (Fig. 4-8). At this height, shearing stops and the flow moves as a plug with uniformvelocity. The uppermost portion of the flow may exhibit a decrease in velocity due to frictionaleffects with overlying ambient sea water.

Laboratory experiments by Mohrig et al. (1998) and Marr et al. (2001) have documentedthe process of “hydroplaning” wherein the basal layer of a debris flow is lubricated by awedge-shaped layer of water forced beneath the flow during downslope movement (Fig. 4-8).This water layer has the effect of deflecting upward the debris from the bed. The effect of basallubrication is that the head of a debris flow moves downslope at a higher velocity than thebody, attenuating and even detaching the head from the body. Other features associated withlaboratory-generated deposits of debris flows include structureless and ungraded grain sizedistribution of the deposit, tension cracks, water-escape structures, compression ridges andimbricate slices (Marr et al., 2001).

Whether or not debris flows have large erosive capabilities is debateable. Posamentier(2003) have documented from shallow-subsurface 3D seismic data grooves with 40–500m ofvertical relief that are associated with mass transport complexes 25–30 km long, indicatingthat erosion by mass movement can occur on the sea floor. On the other hand, Mohrig et al.’s(1998) laboratory experiments, as well as numerical modeling (Pratson et al., 2000), indicatethat debris flows are not capable of eroding the substrate over which they move. In theseexperiments, the volume of sediment within the flow does not increase appreciably betweenwhen the flow starts and when it ends. The dense, mud matrix of the flow also tends to inhibitloss of sediment across its upper surface.

Figure 4-8. Schematic cross section illustratingthe internal flow structure of a debris flow.Velocity of the shear flow region (Us) increasesupward from the base of the bed. Within theplug flow region, velocity (Up) remains con-stant. The symbols hs and hp refer to thicknessof each zone. Z is the vertical dimension orthickness of the flow. Note that the front of theflow is above the base of the bed. S is the angleof the bed relative to the horizontal plane. Awedge of water lubricates this basal zone, givingrise to the process of “hydroplaning,” explainedin the text. (Modified from Pratson et al., 2000).Reprinted with permission of Society of Sedi-mentary Geology (SEPM) and American Asso-ciation of Petroleum Geologists.

Plug Flow Region

Shear FlowRegion

S

x

z

hp

hs

Up

Us


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If the constancy of sediment/water ratio throughout the length of a flow does occur inthe marine environment, as it does during laboratory experiments, then it provides one of themost significant differences between debris flows and turbidity currents. However, the heightof the flow is inversely proportional to its velocity. If the flow velocity decreases, the flow willthicken, and internal sediment concentration will decrease. If this velocity increases, the flowwill thin, and the internal sediment concentration will increase. The flow is driven forward(downslope) by the weight of the flow and is retarded by friction acting on the seabed. Internalfluid pressure causes the flow to spread radially as it travels. The flow will continue downs-lope to a lower gradient on the sea floor where the flow spreads radially and fluid pressure isreduced below the frictional threshold for movement, so forward movement ceases.

Deposits of debris flows, often termed “debrites,” may be composed of mud, mixtures ofmud and sand, or mixtures of mud, sand, and gravel arranged in a disorganized or randommanner (Fig. 4-9). The high matrix strength is sufficient to hold gravel-size clasts within theflow and resulting deposit. Internally, debris flows may exhibit random orientation of clasts orthere may be some imbricate orientation of larger clasts. Because of the high matrix strength,sedimentary structures resulting from fluidal movement are lacking. Slide and deformationstructures may be present.

Figure 4-9. Outcrop photograph of a poorly sorted debrite bed. Note the large boulders are sup-ported within a dense, fine-grained matrix. California, U.S.A.


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Intermediate Flows and Their Deposits

Lowe (1982) defined three types of intermediate flows on the basis of sediment supportmechanism: grain flows, liquified flows and fluidized flows (Fig. 4-1). Grain flows (Fig. 4-4)are dispersions of particles maintained within a current solely by dispersive pressure arisingfrom grain-to-grain collisions (Fig.4-2). This process implies a relatively high concentration ofsediment within the flow. Lowe (1982) claims that grain flows can exist only on slopesapproaching the angle of repose of subaqueous sand (18–28 degrees). In deep water, grainflows would form as thin beds of avalanche foresets on dune slipfaces (Fig. 4-4).

In liquified and fluidized flows (Figs. 4-1 and 4-4), pore fluids are forced upward duringsediment transport, as particles settle toward the base of the flow. Upward-rising fluid causesthe particles to remain in suspension. Deposits from liquified and fluidized flows may exhibitfluid escape structures (e.g., dishes and vertical pipes) (Fig.4-10). Such structures form whenvertically-escaping water creates a cavity within the flow, causing internal, localized collapse.Sandstones that do not exhibit fluid escape structures are more likely to reflect grain-to-graincollisions (dispersive pressure) during initial deposition, without forceful escape of fluids.

Pratson et al. (2000) and Marr et al. (2001) claim that with > 10% sediment concentra-tion in a flow, grains will be constantly in mutual contact. Grain flows, liquified flows, andfluidized flows probably fall within this category (>10% concentration), but are not consideredtrue debris flows in the sense of Lowe (1982).

Figure 4-10. Photograph of dish structures in a sandstone bed. Stratigraphic top is toward the top of thephoto. Formation of dish structures is discussed in the text. The formation which contains the dish structuresis unknown. California, U.S.A.


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Subsequent to Lowe’s (1982) classification of flow types, Lowe and Guy (2000) recog-nized an unusual deposit that they term a “slurry bed,” interpreted to form from a watery flowtransitional between a turbidity current and a debris flow. Such beds contain 10–35% detritalmud matrix, are enriched in water-escape structures, and are grain-supported. Lowe and Guy(2000) interpret the slurry beds to have originated as low density turbidity currents containingan abundance of flocculated or ripped-up, sand-size, cohesive mud particles that behave in ahydrodynamically similar manner to more rigid silt- and sand-size quartz and feldspar grains.As the flow wanes, the mud particles settle toward the base of the flow. There, the grainsabrade against more rigid quartz and/or feldspar grains and disaggregate into their componentsilt- and clay-size particles, giving rise to a mud-rich basal flow. The increased mud contentnear the base of the flow increases viscosity and cohesion and suppresses near-bed turbulence,thus creating a quasi-laminar basal flow, and the resulting slurry bed. Fluid escape structuresare common in slurry beds, owing to the upward flow of water as the density and viscosity inthe basal layer increase during particle breakup. Slurry beds have been recognized by Loweand Guy (2000) in the Lower Cretaceous Britannia Formation of the North Sea, the Pennsylva-nian Jackfork Group of Arkansas, and the Cretaceous Great Valley Group of California.

Turbidity Currents and Turbidites

Processes

A turbidity current is a sediment gravity flow with fluidal (i.e., Newtonian) rheology inwhich sediment is held in suspension by fluid turbulence (Figs. 4-11, 4-12). A turbidity currentcontains a head, body, and tail. The head may erode the sea floor and entrain sediment backinto the body (Kneller and Buckee, 2000). Owing to viscosity differences between the turbid-ity current and overlying ambient sea water, a series of billows, called “Kelvin-Helmholtz”instabilities (Allen, 1985), form at the frictional interface of the fluids (Figs. 4-11, 4-12).

Laboratory experiments have indicated that once a turbidity current is initiated, its inter-nal sediment distribution and velocity structure are quite complex (Kneller and Buckee, 2000).A vertical velocity profile through an experimental turbidity current is shown in Figure 4-13,along with a variety of experimental sediment concentration profiles. The velocity of the flowis low near the base owing to frictional forces with the sea bed. Flow velocity reaches a maxi-mum at some distance above the bed where the flow is fully turbulent, then decreases upward.Sediment concentration profiles vary according to the absolute sediment concentration and thenear-bed processes of erosion and/or transport. Experiments measuring vertical grain size dis-tributions show that fine-grained particles are distributed uniformly vertically through theflow, whereas coarser-sized grains diminish in abundance upward from the base of the flow(Garcia, 1994) (Fig. 4-13). Thus, within a flow containing a range of coarse- to fine-grainedparticles, the base of the flow may be more poorly sorted than the overlying parts. Conversely,well-developed slurry flows should exhibit greater mud concentrations near their base (Loweand Guy, 2000).

Numerical modeling by Pratson et al. (2000) suggests that turbidity currents movedownslope by the combined influence of the weight of the current (controlled by sedimentconcentration) and internal fluid pressures that are counterbalanced by frictional forcesbetween the seabed and the current. The body may have velocities that are greater than thoseof the head, so that with distance, the head may expand in thickness (Fig. 4-12) as the bodyattempts to outrun the head (Talling et al., 2001).


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Figure 4-11. Schematic cross section of a high-density turbidity current. Arrows point to the flow directionswithin the current. Vertical flow velocity profiles are shown in shaded black. The flow is size-graded, withcoarsest grains at the base of the bed. The head of this flow is thicker than the body. The billows at the top ofthe flow are termed “Kelvin-Helmolz Billows.” (Modified from Postma et al., 1988). Reprinted with permis-sion of Elsevier Publishing Co.

Figure 4-12. Photograph of a cross section of a turbidity current generated in a flume tank. The head and bodyof the flow are clearly shown. The sediment box from which the sediment was dropped through a removablebottom is in the upper right of the picture. Note that the head is thicker then the body, even after only a shortdistance of transport. Photograph is from the flume tank of the St. Anthony Falls Laboratory, University ofMinnesota.

AMBIENT WATER HIGHER-DENSITY

TURBULENT SUSPENSIONLOWER-DENSITY

TURBULENT SUSPENSION

Velocity profile

at 2-seconds distance

from the flow head

Velocity profile

at 1-second distance

from the flow head

Suction due to

strong pressure

gradient in the

head of the flow

Turbulent suspension clouds

"thrusted" backwards

LAMINAR

INERTIA-FLOW

Slope = 25o

10

cm

Velocity of

the head is

108 cm/s

Pebble "tracers"

limited in this interval

(data in FIG. 3)


4-122

Figure 4-13. Four graphs illustrating velocity (solid lines) and associated sediment concentration (dashedlines) profiles for a variety of experimental turbidity currents: (a) two layer concentration model with a con-stant concentration lower interval and an upper region of sediment detrained from the head of the flow; (b) asmooth concentration profile, characteristic of low concentration, weakly depositional flows; (c) a steppedconcentration profile observed in erosional flows; (d) a distribution observed in turbidity currents in whichcoarse material is concentrated towards the lower part of the flow and the fine-grained material is evenly dis-tributed thoughout the flow (Modified from Kneller and Buckee, 2000). Reprinted with permission of Interna-tional Association of Sedimentologists.

Turbidity currents are capable of eroding the substrate. Thus, sediment can be continu-ally entrained into the head of the flow, even as sediment is deposited from within the bodyand tail. Also, Kelvin-Helmoltz billows may transport sediment from the head of the flow backinto the body. Ambient water can also be entrained within the flow, acting to dilute the sedi-ment concentration while, at the same time, increasing the overall thickness of the flow.

Thus, the actual sediment concentration can decrease and increase in a non-systematicfashion along the length of a flow, as well as vertically at any one position within the flow. If theamount of new sediment entrained by erosion is less than the amount lost through deposition,then the turbidity current eventually ceases. But, if the amount of new sediment entrained isgreater than the amount lost through deposition, the flow gains momentum, and further erodesas it moves downslope. However, if sediment concentration becomes too high, turbulent flowcan be suppressed and a different sediment support mechanism becomes operative (Fig. 4-2).

Even with this complex flow behavior, or perhaps because of it, a turbidity current cantravel for long distances at high velocities. Single-event turbidity currents have been docu-mented, which have transported sediment several hundred kilometers from their source. The bestdocumented of these, the 1929 Grand Banks of Newfoundland turbidity current, is calculated tohave traveled at the following velocities: approximately 20 m/sec at a distance of 300 km fromthe earthquake epicenter, 14 m/sec at a distance of 500 km from the epicenter, and 11 m/sec at adistance of 600 km from the epicenter. This single flow traveled over 600km in 13 hours (Uchupiand Austin, 1979) at a velocity sufficient enough to transport particles up to 3 cm in diameter insuspension. In a different flow, inferred flow velocities of the Nice airport turbidity currentreached 30 m/sec., forming deep submarine scours and transport of cobbles and boulders as bedload, and coarse sand as suspended load (Piper and Savoye, 1993). On the Amazon Fan, individ-ual flows are estimated over periods of several hours to days, at velocities of 1–3 m/sec (Pirmezet al., 2000). The Zaire submarine valley flow was documented at an average velocity (integratedover 1 hour) of 1.2m/sec (with higher instantaneous velocities) 150m above the sea floor, andcoarse sand and plant debris were collected 40m above the floor (Khripounoff et al., 2003).

The areal extent and volume of single-event turbidity current deposits can also be quitelarge. For example, the Holocene Black Shell turbidite on the Hatteras abyssal plain covers anarea of approximately 25,000 sq. km. and comprises a minimum volume of 100 cu. km(Elmore et al., 1979).

Normalized velocity and density/concentration

No

rma

lize

d h

eig

ht b ca

Density/concentration

Velocity

d

Fine sediment

Coarse sediment


4-123

The Bouma Sequence

The Bouma Sequence (Bouma, 1962) has long been considered to be the fundamentalsand/mud deposit from a turbidity current (Fig. 4-14). Bouma (1962) defined this sequence asthe product of continuous deposition from a turbidity current. A complete Bouma sequenceconsists of a grain-size fining-upward succession of (a) massive or normally size-graded, sandyBouma Ta division; (b) parallel laminated, sandy Bouma Tb division; (c) ripple/climbing-ripplelaminated/convoluted, sandy Bouma Tc division; (d) parallel laminated to massive, silty BoumaTd division; and (e) silt-clay, often microfaunal-rich Bouma Te division (Fig. 4-14). Some out-crop examples of Bouma divisions are provided in Figures 4-15, 4-16 and 4-17.

The upward decrease in grain size and change in sedimentary structures are a result ofgradually waning flow velocities, ultimately leading to deposition of progressively finer-grained sediment under progressively lower flow regime conditions. The Bouma Ta division isthought to be deposited rapidly, directly from suspension. The Bouma Tb and Tc divisions arethe product of traction of grains along the sea bed under upper (Tb) and lower (Tc) flowregime conditions. The Bouma Td division is deposited by suspension from the tail of a turbid-ity current. The Bouma Te division probably is a mixture of fine-grained sediment from boththe tail of the current and slow settling of pelagic grains. Mixtures of shallow- and deep-marine microfauna in the Te division are indicative of a turbidity current process, as is size-grading of silt- and clay-sized particles. Turbidite mudstones and shales also exhibit a charac-teristic suite of waning-flow sedimentary structures and textures (Fig. 4-18).

Laboratory experiments in which relatively dilute concentrations of sand were mixedwith varying amounts of clay and water simulated development of some of the common fea-tures of Bouma divisions and provided insight into when these features form (Marr et al.,2001). For example, grain-size grading was found to occur only during flow deceleration,rather than during downslope movement of the flow (Marr et al., 2001). Water escape struc-tures also formed soon after the flow came to rest, and continued for several minutesafterward. Hydroplaning was not observed to occur in the dilute flows.

Figure 4-14. Sediment grain size, struc-tures, divisions of a complete Boumasequence, and sediment transportmechanisms. The different divisions—from Bouma Ta to Bouma Te—areexplained in the text. (Modified fromJordan et al., 1993). Reprinted withpermission of American Association ofPetroleum Geologists.

SUSPENSION

MIXED

TRACTION

SUSPENSION

Te

Td

Tc

Tb

Ta


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Figure 4-15. Photograph of beds illustratingBouma Ta, Tb, and Tc divisons in outcrop. Theupward gradational decrease in grain size isnoted by a “smoothing” of the outcrop surface.Location of the outcrop is unknown.

Figure 4-16. Photograph ofbeds illustrating BoumaTa-Te divisions in outcrop.Note the normal size grad-ing as evidenced by thedecrease in size and abun-dance of coarse (white)particles upward throughthe Ta division. Name ofthe formation which con-tains this rock is unknown.Newfoundland, Canada.


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Figure 4-17. Photograph of Bouma Tb-Tc division in outcrop. The upward change from parallel laminated toripple (including climbing-ripple) bedding is a result of decrease in traction current flow from upper to lowerflow regime velocities. Upper Miocene Mt. Messenger Formation, New Zealand.

Figure 4-18. Succession of sedimen-tary textures and structures in fine-grained turbidites Grain size andsedimentary structures change sys-tematically upward much as they doin sandy turbidity current deposits.In this diagram, the Bouma Tc andTd divisions are the same asdescribed in Figure 4-14. However,the Bouma Td division is dividedinto T1 and T2 subdivisions, and theBouma Te division is divided intosix subdivisions (T3-T8) based uponvariations in grain size and small-scale sedimentary structures. (Mod-ified from Stow and Shanmugam,1980). Reprinted with permission ofElsevier Publishing Co.

GR

AD

ED

LA

MIN

AT

ED

MU

D

GRADED MUD

WISPY.CONVOLUTE LAMINAE

INDISTINCT LAMINAE

THIN REGULAR LAMINAE

CONVOLUTE LAMINAE

BO

UM

A (1

96

2)

E

D

C

B

A

MICRO-BIOTURBATED MUD

THIN IRREGULAR LAMINAElow-amplitude climbing ripples

BASAL (LENTICULAR) LAMINAFading ripples, micro-cross and parallel

lamination, sharp scoured load-cast base

70

60

50

40

30

20

10

SCALE

mm

T0

T1

T2

T3

T4

T5

T6

T7

T8

+silt lenses

UNGRADED MUD+silt pseudonodules


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Turbidity Current Classification Based Upon States of Flow

The implication of the Bouma Sequence is that turbidity currents will diminish in veloc-ity over time and in the downcurrent direction, giving rise to deposition of progressively finer-grained Bouma divisions, both vertically and laterally. Although this must be true where Boumadivisions are present in their normal strataigraphic position (Fig. 4-14), Kneller and Branney(1995) have challenged the general assumption of progressively waning flow on the groundsthat there is no real reason to believe that all turbidity currents in the deep ocean behave in thismanner. Kneller (1995) proposed that flows may wax, wane, or remain constant both over timeand distance (Fig. 4-19). Over time, at any one place on the sea floor, a flow can wax (increasein velocity), wane (decrease in velocity), or remain steady (constant velocity). Over distancealong the sea floor, a flow can become accumulative (increase in velocity), depletive (decreasein velocity), or remain uniform. Based upon these temporal and spatial variations in flow veloc-ity, Kneller (1995) classified flow types into nine possible combinations (Fig. 4-6). Sedimentconcentration does not directly enter into this classification scheme. Thus, different Boumadivisions can occur both vertically within a stratigraphic sequence or spatially along a deposi-tional profile depending upon the temporal and spatial variations in flow velocity.

The topographic relief and gradient of the seafloor are thought to play major roles inmodifying the velocity of a flow as it travels down the depositional profile. Accumulativeflows (Figs. 4-6, 4-19, 4-20) might form with a downcurrent increase in slope gradient ordowncurrent convergence a of flow through a restriction on the sea floor. Uniform flows(Figs. 4- 6, 4-19, 4-20) might form over a floor with a progressively slight decrease in down-current gradient. Depletive flows (Figs. 4-6, 4-19, 4-20) might form with a downcurrentdecrease in slope gradient or downcurrent divergence of flow as the flow moves beyond a con-striction on the sea floor and becomes unconfined.

Also, when a sediment gravity flow meets a seafloor obstacle, the flow can either par-tially or completely override the obstacle, be deflected around the side of the obstacle, beconfined or ponded by the obstacle, or reverse flow direction and flow back down the obstacle(Fig. 4-21) (Kneller and Buckee, 2000). Which of these processes dominates is a function ofthe velocity and density of the current, the flow stratification within the current, and thedimensions of the obstacle.

Figure 4-19. Graphs of time (t) vs. velocity (u) and dis-tance (x) vs. velocity (u), showing different types offlow under a variety of conditions. In the upper dia-gram, flow velocity first waxes, then wanes, thenbecomes steady over time. In the lower diagram, flowvelocity first increases (accumulative), then decreases(depletive), then becomes uniform with downcurrentdistance. (Modified from Kneller, 1995). Reprintedwith permission of The Geological Society of London.

u

u

depletive

accumulative

NON-UNIFORM UNIFORM

x

STEADYUNSTEADY

waning

waxing

t

u=velocity

t = time

x= distance


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Figure 4-20. Diagram illustrating some causes of spatially depletive (downcurrent decrease in flow velocity) andspatially accumulative (downcurrent increase in flow velocity) sediment gravity flows. The upper diagram, (a) isa plan view of depletive flow resulting from divergent flow on the sea floor as the flow becomes unconfined, and(b) is a cross section view showing downcurrent decrease in flow velocity due to reduction in slope gradient. Inthe lower diagram, (c) is a plan view illustrating accumulative flow resulting from convergence of flow on the seafloor , and (d) illustrates accumulative flow resulting from a downcurrent increase in the slope gradient of thesea floor. (Modified from Kneller, 1995). Reprinted with permission of The Geological Society of London.

Figure 4-21. Schematic diagram of possible flow paths (arrows) for sediment gravity flows when they encoun-ter a sea floor obstacle. Flows can partially or completely override the obstacle, can divert around the obstacleor flow partway up the obstacle then reverse flow. (Modified from Kneller and McCaffrey, 1999). Reprintedwith permission of Society of Sedimentary Geology (SEPM).

divergent flow

DEPLETIVE FLOW

ACCUMLATIVE FLOW

convergent flow

decrease in slope

increase in slope

flow path


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Low- and High-Density Turbidity Current Deposits

Lowe’s (1982) classification of “low-” and “high-” density turbidity currents, differenti-ated on the basis of concentration of sediments in the flow, is also widely cited (Fig. 4-4). Heapplies the designation S to sediment deposited from high-density turbidity currents (Fig. 4-22).

Lowe (1982) further subdivided S sediments (Fig. 4-22). His S1 division is depositedfrom traction currents, thus exhibiting traction structures, mainly planar laminations and crossstratification. The S2 division contains stacked, thin, inversely-graded beds deposited from bedload. Grain collisions predominate during rapid sedimentation, thus suppressing turbulence.The S3 division is deposited during high sediment fallout rates. Deposits are massive to nor-mally graded, and may exhibit fluid escape structures.

Low-density turbidity currents contain dilute concentrations of clay, silt, and fine- tomedium-grained, sand-size particles (Lowe, 1982) (Fig. 4-4). Low-density turbidity currentshave been defined as containing 1–23% sediment by volume and high-density turbidity cur-rents have been defined as containing 6–44% sediment by volume (Shanmugam, 1996). Theoverlap in concentrations according to these definitions points to the present lack of clearly-defined criteria to define these types of flows on the basis of sediment concentration.

Figure 4-22. Vertical profile of sediment grain size and sedimentary structures illustrating high-to low-density turbidity current deposits using the terminology of Lowe (1982). S and R designa-tions, and processes, are explained in the text and in Figure 4-4. Reprinted with permission ofSociety of Sedimentary Geology (SEPM).

Gra

ve

l

Traction

Traction Carpet

Suspension

Traction Carpet

Suspension

Sa

nd

an

d F

ine

Gra

ve

l

S3

S2

S1

R3

R2

S3

S3

S3

S3

S2

S2

S1

S3

Tt


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Massive (Structureless) Sandstones

Because the origin of massive (structureless) sandstones has generated considerable debatein the literature, they are discussed separately in this chapter. Massive sandstones are the mostcommon type of sediment gravity flow sandstone observed in outcrops and cores (Fig. 4-23)(Kneller,1995). Both Walker (1978) and Lowe (1982) differentiated massive sandstones fromgraded Bouma Ta sandstones. Walker (1978) differentiated massive sandstone from the BoumaTa division (Fig. 4-3) by (a) the common presence of fluid escape structures, (b) fewer associatedshale interbeds, (c) an increase in erosionally-based and irregularly-bedded sandstone, (d) coarsergrain size relative to associated sandstones, and (e) sandstone beds that are thicker than associ-ated beds. Lowe (1982) referred to massive (S3) sandstones as fluidized or liquefied flowdeposits, depending upon the presence or absence of fluid escape structures (Fig. 4-4).

Shanmugam (1996,1997, 2000) has argued that massive sandstones are not turbiditycurrent deposits, but are the product of deposition from plastic or laminar flows. He uses theterm “sandy debrite” for a massive sandstone, and claims that a mud matrix as low as 1% issufficient to provide cohesive strength to a flow. As mentioned previously, experimental workby Marr et al. (2001) support this interpretation if the clay matrix is composed of bentonite andthe water content is 25–40%. More clay is required if the mineral is not bentonite. Marr et al(2001) stress that their results apply only to laboratory-scale, and not field-scale flows.

Figure 4-23. Core of unconsolidated sand (lightcolor) and lithified shale (darker color) from LongBeach Unit, Wilmington oil field, California (Slatt etal., 1993). Individual sand beds are massive, struc-tureless, and of uniform grain-size from base to top.Scale is in 0.1 ft. increments. Reprinted with permis-sion of the Society of Sedimentary Geology (SEPM)


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Shanmugam’s argument is at least partially based on semantics. He claims that a BoumaTa bed must be size-graded to qualify as a turbidity current deposit, even though Bouma’s (1962)definition alludes to the fact that some Ta beds are size-graded and others are not. But, if there isnot a range of particle sizes in the original flow—such as might occur from a pre-sorted sand—then size-graded beds cannot be deposited from the turbulent flow. Shanmugam (2000) furtherclaims that the presence of shale clasts within a sandstone argues for cohesive, rather than turbu-lent flow, yet he claims that debris flows do not have the capability to erode the sea floor.Because many shale clasts found within deep water sequences appear similar to underlying shalebeds, erosion of the muddy substrate by the flow must have occurred to generate the clasts.

Kneller’s (1995) classification shows that massive sand of uniform grain size will be depos-ited under “steady state” flow (over time) in combination with “depletive flow” (over distance)(Fig. 4-6). Even though grain size and bed thickness decrease in the downcurrent direction, thedeposit remains massive and of uniform grain size at any one depositional site on the sea floor.

It is reasonable to expect relatively steady flow of a turbidity current over time on the seafloor. For example, steady flow has been suggested for a period of at least 2 hours for theGrand Banks turbidity current (Piper et al. 1988). This should be sufficient time to deposit amassive sand on the sea floor. The common occurrence of ungraded beds in the rock recordsuggests steady state flow is common.

Hyperpycnal Flows and Hyperpycnites

Hyperpycnal flows have been a topic of considerable discussion and interest at confer-ences during the past few years. According to Mulder et al., (2003), the importance ofhyperpycnal floods as a sediment transport process in deep water has been underestimated formany years. The following discussion summarizes current knowledge about hyperpycnalflows and their deposits.

Mulder et al. (2003) differentiate turbidity currents that are generated by ignitive trans-formation of a submarine slide into a turbulent flow and those that are generated by non-ignitive conditions, such as from continuity of hyperpycnal discharge of a river during floodstage (Fig. 4-24) Because the fluid in such flows is fresh water, the density contrast betweenfresh water and sea water is such that a very high suspended sediment concentration isrequired for the flow to sink or plunge to the sea floor (Fig. 4-24). Mulder et al. (2003) placethe critical sediment concentration at 36–43kg sediment/m3 fluid. Variations within this rangeare due to the variations in temperature and salinity of sea water at the river mouth.

Sediment concentrations measured from two sediment traps on the Zaire submarine fanvalley (one in the channel and one on the levee 18km away), were 3.28kg sediment/m2 of traparea/day above the channel and a peak of 11kg sediment/m2/day at the levee site (Khripounoff etal., 2003).. Sediment in the channel trap consisted of silt and large plant remains with finergrained siliciclastic particles and 464 mg organic carbon/m2/day found in the levee sediment trap.

Hyperpycnal flows can be relatively long lived. During a major flood that lasted forthree days in 1994, the Var River in France generated a hyperpycnal turbidity current thatlasted for 18 hours (Mulder et al., 2003). The Var River transported 11–14 times the river’snormal particle load, which ultimately was deposited in the deep ocean. The total duration ofthe sediment gravity flow in the Zaire submare valley is estimated to have been 10 days, with a3 day delay between the channel and the levee station 18km away. Flow thickness exceededthe 150m depth of the channel floor beneath its levee crest, allowing sediment to overflow(Khripounoff et al., 2003).


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After a hyperpycnal flow debouches from the river mouth into marine water, it plunges tothe sea floor and moves downslope. If sediment is lost from the flow by deposition, the densityof the remaining flow can decrease to the extent that the flow detaches from the sea floor andrides within the ambient water column at an appropriate depth (Fig. 4-24). This fact, due to theinitial fresh water nature of the internal fluid, provides a fundamental difference between normalmarine turbidity currents and hyperpycnal turbidity currents. Mulder et al. (2003) state thathyperpycnal flows with relatively low sediment concentrations and density are the low-densityturbidity currents of Lowe (1982). In contrast, turbidity currents generated by ignitive processes,in which the internal fluid is sea water, are the high-density turbidity currents of Lowe (1982).

The typical hyperpycnite deposit consists, from the base upward, of a coarsening-upwardsuccesssion overlain by a fining-upward succession of beds (Fig. 4-25). The lower succession isdeposited during rising flood stage when the flow is waxing and river discharge is high at theriver mouth (Figs. 4-6, 4-19, 4-25). After the period of peak discharge, the flow wanes(Figs. 4-6, 4-19, 4-25) and the upper succession is deposited. The stratigraphic continuity of asingle hyperpycnite bed can be disrupted by an erosional or bypass surface if the waxing-stageflow reaches a threshold velocity capable of eroding the underlying rising-stage deposit(Fig. 4-25). Grain size of a typical hyperpycnite increases vertically from silt to fine sand, andthen back to silt. Climbing ripples are a common sedimentary structure. Land-derived organicmaterial also is present in hyperpycnites. A hyperpycnite bed is shown in Figure 4-26.

Figure 4-24. Schematic cross section of maintypes of flows debouching from a river mouth.Hyperpycnal flows are denser than ambientwater, and flow along the sea bed. Hypopycnalflows are less dense than ambient water, so rideon the sea water surface. Interflows are at someintermediate density. and flow within the ambi-ent water. Lofting occurs when the density of ahyperpycnal flow decreases due to loss of sedi-ment within the flow by deposition, and the flowrises into the ambient sea water. (Modified fromMutti et al., 2003). Reprinted with permission ofElsevier Publishing Co.

Interflow Hypopycnal Lofting

Low-density hyperpycnal flow

Figure 4-25. Graph illustrating flows with differ-ent discharge through time, and the representa-tive lithofacies. 1. Low magnitude floodgenerates a normally-graded bed. 2. Low magni-tude flood which generates a complete inverse-to-normally graded hyperpycnite due to waxing,then waning flood stages. 3. Mid-magnitudeflood generates a complete sequence which iscoarser grained than the example in 2. 4. High-magnitude flood generates an inversely-gradedbed from waxing flow, followed by an erosion orbypass surface due to peak flood flow velocitysufficient to erode the substrate, followed by thewaning flow, normally-graded bed. (Modifiedfrom Mulder et al., 2001b). Reprinted with per-mission of Springer-Verlag Publishing Co.

LEGENDErosional contact

Sharp contact

Horizontal lamination

Climbing ripples

Ripple cross lamination

Time

Critical discharge for the formation

of a hyperpycnal turbidity current1

23

4

Discharge

progressive

deposition of

type 4 bed

1

Cl fs ms cs fsa msa

2 3 4


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Whether the graded beds deposited by these flows should be termed hyperpycnites, hyper-

pycnal turbidites, or turbidites is subject to debate (Mutti et al, 2002). Mutti et al (2003) suggest

that the stratigraphic succession of beds deposited from hyperpycnal flows should be termed

“mixed depositional systems” to emphasize their transitional character between truly basinal,

ignitive turbidites and delta-fed, turbidite-like deposits derived from rivers in flood stage.

Modern hyperpycnites have been documented for a distance of 700km downcurrent

from linked river- canyon-fan systems in the central Japan Sea in water depths to 3350m

(Nakajima, 2006). Two 4m long cores contain alternating turbiditic silt and hemipelagic mud

beds. Some of the silt beds exhibit a typical fining-upward grain size trend. However, other

beds on the order of 5-10cm thick exhibit a distinct coarsening-upward trend with mean grain

size in the range 25-40 microns, capped by an erosional surface, then overlain by 5-10cm of

beds which decrease in grain size upward in the range 5-25microns. It is postulated that the

duration of these flows was on the order of several days to 3-4 weeks in order to travel 750km,

and that they could have maintained the density required for them to hug the sea floor by

entraining sea water and eroding sediment into the flow. Estimated velocities were on the

order of 0.3m/sec, and they have an estimated frequency of occurrence of 70 years.

Siltstones with similar characteristics have been documented for the Cretaceous Dad

Sandstone member of the Lewis Shale leveed-channel deposits (Chapter 6 and Chapter 7)

(Soyinka and Slatt, 2004; Soyinka, 2005). Laser grain size analyses of individual laminae and

thin beds reveal systematic changes in mean grain size within the silt size range (Fig. 4-27A).

In some intervals the change from coarsening- to fining-upward trend is separated by a subtle

erosional surface. This same surface has been documented in the modern hyperpycnites from

the Japan Sea, mentioned above (Fig. 4-27B). A major delta system is known to have fed the

Dad Sandstone slope and basinal facies (Pyles and Slatt, 2007), so the likelihood of hyperpyc-

nal processes and deposits is high.

Figure 4-26. Ouctrop photo ofa hyperpycnite of the typeshown in 4 of Figure 4-25. Anerosional or bypass surfacecaps an inversely-graded bed,and is overlain by a normally-graded bed. Marnoso-arena-cea Formation, ApennineMountains, Italy.


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Figure 4-27. (A) Polished slab of siltstone and mudstone outcrop interval analyzed for grain sizedistribution by laser grain size analysis (Soyinka, 2005); coin for scale. The graph to the right ofthe slab is a plot of the modal grain size of individual samples analyzed over the length of the slab.Ha = basal unit; Hb = top unit; HL = sandy horizontal laminae; GC = gradational contact; SC =sharp contact; SBC = sharp basal contact. Coarsening-upward and fining-upward trends inmodal size are noted. An erosional surface is shown in the thin-section photomicrograph.(B) Average grain size of samples over a 10+cm interval of siltstones and mudstones from thefloor of the Japan Sea; these beds and laminae are interpreted as hyperpycnites (Nakajima,2006). EC = erosional surface denoting the vertical change in average grain size from coarsening-upward to fining-upward. For comparative purposes, the vertical scales are the same in A and B.

Gravel Deposits from Turbidity Currents

Gravelly deposits of turbidity currents are not as common as the sandier and muddiertypes described above. One reason is the general lack of pebbles and coarser grains withinshallower water areas from which sediment gravity flows originate. Suitable source areas usu-ally are confined to tectonically active basins with narrow shelves, and relatively highsedimentation rates (Reading and Richards, 1994). Although it might be surmised that suchlarge grains would only be found in proximal deep water settings, gravel has been cored in theyoungest channel of the Mississippi Fan a distance of approximately 220 km from the presentshelf edge (Stelting et al., 1985).

Walker (1978) defined a series of gravelly, sediment gravity flow deposits on the basisof the abundance of pebbles and coarser grains and their sedimentary structures and stratifica-tion style (Figs. 4-3, 4-28). One type, pebbly sandstone (Fig. 4-3), contains dispersed orconcentrated pebbles within a sandstone matrix. In outcrop, alternating pebble-rich and peb-ble-poor beds (Fig. 4-29) and trough or planar-tabular cross beds are the most characteristicinternal sedimentary structures. Normal size grading (Fig. 4-30), large sole marks, lenticular-ity, and scoured bases of beds (Fig. 4-31) are also common features.

0 10 20 30 40 0

185

190

195

Grain size (um)

hemipelagite

De

pth

(cm

)

EC

hemipelagite

05

00

90

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Grain size (um)

Depth (mm)

ModeHb Fall

Hb Fall

Ha Rise

Ha Rise

Peak

Peak

SBC

SC

SBC

GC

Hemipelagites

HL

HL

Coarsening upward

Fining upward

A

B


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Figure 4-28. Diagram illustrating the sedimentary features of the spectrum of clast-supported conglomerateswith suggested downcurrent positions. (Modified from Walker, 1978). Reprinted with permission of AmericanAssociation of Petroleum Geologists.

Figure 4-29. Photograph of a stratified, pebbly sandstone to sandy conglomerate. Lower Pennsylvanian Jack-fork Group, Arkansas, U.S.A.

GRADED-

STRATIFIED GRADED-BED

INVERSE-TO-

NORMALLY

GRADED

NO INVERSE

GRADING,

STRAT.,

CROSS-STRAT.,

IMBRICATED

DISORGANIZED-

BED

NO INVERSE

GRADING,

NO STRAT.,

IMBRICATED

NO STRAT.,

IMBRICATED

NO GRADING,

NO INVERSE

GRADING,

NO STRAT.,

IMBRIC. RARE

THESE THREE MODELS SHOWN IN SUGGESTED

RELATIVE POSITIONS DOWNCURRENT


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Figure 4-30. Photograph of a graded, pebbly sandstone in outcrop. Top is toward the upper left corner. LowerPennsylvanian Jackfork Group, Arkansas, U.S.A.

Figure 4-31. Photograph of a series of amalgamated sandstone beds. The middle bed has been eroded, and theresulting scour (arrow) has been filled by a normally-graded, pebbly sandstone. Top is toward the upper rightcorner. Lower Pennsylvanian Jackfork Group, Arkansas, U.S.A.


4-136

Walker (1978) classified other types of clast-supported conglomerates on the basis oftype of grading (normal or inverse), presence or absence of stratification, and presence orabsence of imbrication (Fig. 4-32). Based upon theoretical considerations, Walker definedgravelly deposits ranging from (upcurrent): (a) inverse to normally graded, imbricated con-glomerates; to (b) normally graded, non-imbricated conglomerates; to (c) graded-stratified,imbricated conglomerates (downcurrent) (Fig. 4-28). Walker (1978) also recognized a fourthclass, composed of disorganized, non-stratified, and non-imbricated conglomerates. All ofthese conglomerates tend to be lenticular, with scoured bases.

According to Lowe (1982), most coarse gravel is probably transported near the bedwithin a highly concentrated traction carpet, and in suspension in the lower part of a turbulentflow.Intergranular dispersive pressure maintains coarser grains in suspension, whereas finergrains filter to the sea bed, thus forming traction carpets near the bed. The coarser grains even-tually fall from suspension to the bed, resulting in an inversely graded bed (Fig. 4-33).However, Legros (2002) has suggested that size segregation by the kinetic sieving process and/or by progressive aggradation of increasingly coarse-grained particles on the sea bed are morelikely causes of inverse grading than is the maintainence of coarse grains within the flow bydispersive pressure.Lowe (1982) applies the designations R1 for coarse gravel with tractionstructures, R2 for inversely- graded gravel layers, and R3 for normally graded gravel layers(Fig. 4-22).

Figure 4-32. Photograph showing imbrication in ashale-clast conglomerate stacked between two tansandstone beds. Rock hammer is oriented approxi-mately parallel to the orientation of the brown shaleclasts. Upper Cretaceous Dad Sandstone Member,Lewis Shale, Wyoming, U.S.A.

Figure 4-33. Diagram illustrating the sedimentary processesof formation of inversely graded, traction carpet deposit. (A)Basal part of a high density flow shows development of alower, inversely graded zone due to intergranular dispersivepressure. (B) Fallout of grains from suspension increases theclast concentration in the basal layer and results in formationof a traction carpet in which grains are supported by disper-sive pressure. (C) Continued fallout from suspensionincreases the density of grains in the traction carpet andcauses freezing in the upper part of the carpet. D) Final freez-ing of traction carpet results in formation of a new inverselygraded basal layer above the deposit (Modified from Lowe,1982). Reprinted with permission of Society of SedimentaryGeology (SEPM).

A B

C D


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Flow Combinations and Transformations

The various classification schemes discussed above all imply that flows can vary tempo-rally and spatially in a predictable, downcurrent continuum, giving rise to different sedimentgravity flow deposits along a single bed (Figs. 4-3–4-6). However, direct evidence of flowtransformations within a single bed are relatively rare because most outcrops are of insufficientlength and orientation with respect to bedding to trace a single bed laterally for a long distance.Some single bed flow transformations have been reported by Baruffini et al. (1994) and Drink-water and Pickering (2001). Al-Siyabi (1998) documented a transformation between a slurrybed and a massive sandstone in a single bed. Over a lateral distance of 5m (15ft.), this bedchanges from one with a mud content of 9–20% and an abundance of fluid escape pipes, to amassive sandstone with a mud content of 4–7% and without fluid escape structures. Whetherthe slurry bed is upcurrent or downcurrent of the massive sandstone is not known, as no pale-ocurrent indicators are present on the bed.

Remacha and Fernandez (2003) state that individual beds within the Eocene HechoGroup (south-central Pyrenees, Spain) can be traced and correlated for ten’s of km in thedowncurrent direction, where they grade from coarse-grained channel fill, to finer-grainedchannel-lobe transition deposits, to sheetlike lobes, and, finally, into basin plain deposits. Cor-relations of individual beds is possible because of the presence of numerous key marker bedsthat can be traced for these distances. Numbers of beds between markers, as well as theirstacking patterns and facies characteristics, provide the means for correlation. In this manner,>50% of lobe beds (individual beds >9cm thick) have been traced downcurrent to form thin-bedded basin-plain facies (individual beds <9cm thick). There is only a decrease of 2.5 thinbeds/km over these distances. The basin plain facies comprise classical Bouma turbidites(Fig. 4-5) (F8 and F9 beds of Mutti et al., 1999).

Laboratory experiments support the observations of downcurrent flow transformations.A flume experiment by Hampton (1972) provided a conceptual model for the evolution of aturbidity current from a debris flow. Marr et al. (2001) and Mohrig and Marr (2003) have gen-erated turbidity currents from parent debris flows in laboratory flume experiments. Twoprocesses that generate the turbidity currents from debris flows are: (1) grain-to-grain erosionof sediment from the surface of a debris flow and its subsequent ejection into the overlyingwater column, and (2) turbulent mixing of ambient water in front of a debris flow into its head,and subsequent dilution to form a turbidity current. In the first case, erosion of sediment fromthe head of the debris flow generates an overlying, less dense turbidity current that outruns thedebris flow and continues advancing down slope. The resultant deposit is a thin turbidite bedon top of, and in front of the debrite. Whether the first or second process dominates in an indi-vidual flow depends upon whether dynamic stresses at the head of the flow are of sufficientmagnitude to overcome the effective yield strength of the parent debris flow. If these stressesare sufficiently large, then ambient water in front of the flow can be entrained into the head ofthe flow, promoting transformation to a less dense turbidity current. If dynamic stresses areinsufficient to overcome yield strength, then grain-to–grain erosion from the leading edge ofthe flow occurs, and eroded grains inject into the overlying water column to generate a turbid-ity current. The velocity at which the head is moving downslope appears to be a main factor inwhich of the two processes dominates.

Contrary to the examples presented above, Pratson et al. (2000) claim that it is not possi-ble for debris flows to transform into turbidity currents. Their experimental work indicates thatat a sediment concentration of > 10% , the flow is sufficiently cohesive to inhibit the exchangeof water and sediment across the surface. Thus, the sediment concentration remains constant.


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The presence of more than one gravity flow type within a single flow, documentedexperimentally by Marr et al. (2001) and Mohrig and Marr (2003), had been suggested ear- lierby others. Sanders (1965) interpreted the Bouma sequence as the deposit of two different kindsof flow: a basal “flowing grain layer” and an overlying turbulent flow to which he restrictedthe term “turbidity current” (Mutti et al., 1999). Allen (1985) and Postma et al. (1988) consid-ered debris flows and low- and high-density turbidity currents as parts of a single sedimentgravity flow (Fig. 4-11). The most recent model (Wagerle, 2001) places debris flows at thebase of the flow where sediment concentrations are >50% (Fig. 4-34). The body and head ofthe flow contain 20–50% sediment concentration (high-density flow) in turbulent suspen-sion,whereas the low-density wake of the flow contains <20% sediment, also in turbulent sus-pension, and consists of the finest-grained sediment fraction. Sedimentary particles withineach part of the flow are deposited at a different time and in a different position down the dep-ositionalaxis.

The process by which sediment gravity flows split apart from an original single flow istermed “flow stripping.” Flow stripping is the process of separation of the components of asediment gravity flow as it travels within a sinuous channel (Piper and Normark, 1983), and isthought to be unique to sinuous submarine channels (Peakall et al., 2000; Posamentier, 2001).This process occurs along outside bends of sinuous channels, where turbidity current flowsaccelerate, similar to flow in fluvial channels. Sediment gravity flows of high velocity areunable to negotiate the bend, resulting in breaching of levees and deposition of sediment assplays on the outside of the levee, immediately downcurrent from the bend (Fig. 4-35). Flowvelocity would normally be insufficient for all sediment to overtop the levee bend, so that thecoarser-grained fraction of the flow remains within the channel while the finer-grained sedi-ment is stripped out and transported to an extra-channel or levee location.

Mutti et al., (1999) refer to such flows as “bipartite gravity flows,” which include abasal, fast-moving, relatively coarse granular layer and an overlying turbulent suspension offiner-sized grains. Mutti et al. (1999) claims that the basal layer corresponds to the high-den-sity turbidity current of Lowe (1982), in contrast to a low-density turbidity current whichprobably originated directly as a turbulent flow. One direcct line of evidence for bipartite grav-ity flows is the bidirectionality of paleocurrent indicators within an apparently single flow(Fig. 4-36). According to Mutti et al. (2002), these current indicators result from the lower,denser part of a bipartite flow moving in one direction within a channel, while the upper, lessdense part of the flow splits along the channel margin, and then recombines with the denserpart of the flow further downcurrent.

Figure 4-34. Conceptual model (cross section) for a single sediment gravity flow composed of debris flow, high-density, and low-density turbidity current components. (Modified from Wagerle, 2001). Reprinted with per-mission of R. Wagerle.

Low-Density Flow

High-Density Flow

Debris Flow

The Parts of a Gravity Current

BILLOWS

sediment concentration

(Vol. %)

W = wake

H = head

B = body

Fine Grained Fraction

20% upper concentration threshold

Medium Grained Fraction

50% lower concentration threshold

Coarse Grained Fraction

W

BH

0%

De

cre

asin

g E

ne

rgy

De

cre

asin

g G

rain

Siz

e

De

cre

asin

g C

on

ce

ntr

atio

n


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Figure 4-35. Schematic illustration of a turbidity current in a channel and the process of flow stripping, wherethe upper part of the turbidity current flow overtops the outer levee at a bend in the channel. The lower,coarser-grained part of the flow remains in the channel. (Modified from Peakall et al., 2000). Reprinted withpermission of Society of Sedimentary Geology (SEPM) and American Association of Petroleum Geologists.

Figure 4-36. Photograph showing two different orientations of paleocurrent directions in a single bed. Thebase of the sandstone exhibits flute casts which are oriented obliquely to the viewer. The upper part of the bedexhibits climbing ripples oriented at approximate right angles to the viewer. Marnoso-arenacea Formation,Apennine Mountains, Italy.


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According to Kneller’s (1995) classification, temporal variations in flow can result in avariety of flow transformations and resulting deposits in one position on the sea floor(Fig. 4-6). One example of a set of alternating Bouma Tb-Tc beds, interpreted to represent thedeposit of a single fluctuating flow, is shown in Figure 4-37. An unusual deposit has beenobserved in some outcrops of deep water strata. The deposit consists of a lower, massive ordewatered sandstone capped by a sandstone bed with shale and sandstone clasts, and withcomplex soft sediment-deformed and/or undulatory structures (Fig. 4-38). Haughton et al.(2001) refer to such beds as a “linked” sediment couplet; possible origins for the upper part ofthis couplet include: (a) vestige of a debris flow that only partially transformed to a turbidite,and (b) bulking of a lagged tail of a sandy turbidity current and overriding of the flow. Anotherpossibility is flow stripping, whereby the lower and upper parts of the flow split in differentdirections, then recombined further in the downcurrent direction (Mutti et al., 2002).

Figure 4-37. Photograph ofalternating planar laminated(P) Bouma Tb and ripplebedded (R) Bouma Tc beds inoutcrop, suggesting surgeflow (alternating higher andlower velocity flow) duringdeposition. Upper CretaceousDad Sandstone Member,Lewis Shale, Wyoming,U.S.A.

Figure 4-38. Outcrop photo-graph of a “linked couplet”(Haughton et al., 2001). Thelower part of this bed is evenlybedded, but the upper parthas been contorted and thebeds have been detached. Thescale is in 0.1 ft. increments.Lower Pennsylvanian Jack-fork Group sandstone.

Post-Depositional Reworking of Sediment Gravity Flows

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Post-Depositional Reworking of Sediment Gravity Flows

Reworking of deep marine sands on the ocean floor by a variety of currents is well docu-mented (Heezen et al., 1966; Bouma and Hollister, 1973; Stow and Lovell, 1979; Stow andHolbrook, 1984, Shanmugam 1997). In this section we discuss the two major types of currents,large-scale contour currents and smaller-scale, local currents. Contour currents, which are major,unidirectional oceanic currents that parallel slope contours (geostrophic currents), have beeninvoked for large-scale reworking of turbidite deposits into “contourite drifts” (Mutti, 1992).The internal stratigraphy of contourites has been discussed extensively by Stow and Lovell(1979) and Stow and Holbrook (1984), who point to the difficulty in differentiating contouritesfrom fine-grained turbidites in the rock record owing to similar characteristics. Reworking ofsands by more localized, unidirectional bottom currents can winnow mud and improve sorting(Fig. 4-39), thus improving potential reservoir quality when buried to depth. As shown in Figure4-39 if reworking is too prolonged or intense, the resulting sand beds may become very thin andisolated, thus reducing their reservoir potential. An example of reworked Bouma Tb-Tc beds isshown in Figure 4-40; in this example, reworking is noted by a sharp top to an asymmetric ripplethat is steep-sided in the opposite direction to primary cross-stratification.

In addition to unidirectional currents, internal waves and tidal currents within confinedareas such as submarine canyons (Shepard et al., 1969; Bouma and Hollister, 1973) canrework sediments and provide bi-directional ripples in sandstones. Complex ripples have beennoted on the modern deep sea floor (Fig. 4-41) as well as in deepwater sandstones (Fig. 4-42).

Figure 4-39. Schematic illustrations showing post-depositional processes by which a complete

Bouma sequence (Fig. 4-14) might be progressively winnowed and reworked by ocean bottom

currents (after Stanley, 1993). Reprinted with permission of Elsevier Publishing Co.

D

CB

A

D

CB

A

CB

A

B

A

A A

A

1 2 3 4 5

6 7 8 9 10


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Figure 4-40. Outcrop photograph of a bottom-currentreworked Bouma Tb-Tc bed. Note the sharp top andreverse orientation of the steep side of the ripple toprelative to the direction of internal cross-stratification.Precambrian turbidite, Newfoundland, Canada.

Figure 4-41. Photograph oflunate- and lingoid-shaped rip-ples from the Scotian Sea floorat 4,010 m water depth, south-east of Terra del Fuego, south-ern Atlantic Ocean. (Reineckand Singh, 1980, Fig. 664).Reprinted with permission ofElsevier Publishing Co.

Allocyclic vs. Autocyclic Processes and Vertical Stacking Patterns

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Figure 4-42. Outcrop photograph of ripples on a sandstone bedding plane surface. The ripplesare similar in appearance to those shown in Figure 4-41. Upper Cretaceous Dad Sandstone Mem-ber, Lewis Shale, Wyoming, U.S.A.

Allocyclic vs. Autocyclic Processes and Vertical Stacking Patterns

The issue of transport and deposition of sediment gravity flows is significant to thelarger considerations of interpreting allocyclic (extrabasinal) and autocyclic (intrabasinal) pro-cesses from vertical stacking patterns observed in outcrop or core. Vertical successionsrepresent the product of localized deposition at one position on the sea floor.

At the single bed scale, a progressive upward decrease in sediment grain size (waningflow deposits of Fig. 4-6) implies decrease in depositional energy over time at one location.Conversely, a progressive upward increase in grain size (waxing-flow deposits of Fig. 4-6)implies an increase in depositional energy over time.

At the larger scale, a succession of sharp-based, amalgamated, massive or pebbly sand-stones in sharp contact with underlying finer-grained Bouma Tb-d beds might be interepretedas recording a rapid drop in relative sea level and resulting seaward shift in the axis of deposi-tion (allocyclic process) (Mutti et al., 1994).

However, without other criteria, an equally plausible explanation for this vertical suc-cession might be a lateral shift in the depositional axis, with higher energy flows beingdeposited atop lower energy flows without any change in relative sea level (autocyclic pro-cess). In this case, successive sedimentation events in one area would provide bathymetricrelief over time. When the relief becomes too high for sediment gravity flows to override, theflows are diverted to the bathymetrically lower, adjacent sea bed (Fig. 4-21). This style of dep-osition, which does not require a change in relative sea level, is termed “compensationbedding” (Mutti and Sonnino,1981). Compensation bedding occurs at all scales, from majorsediment buildups (Walker, 1978, his Fig. 18), to thinner successions of beds (Jordan et al.,


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1993, their Fig. 69). This principle that the same vertical association of strata can be achievedeither by allocyclic or autocyclic processes is shown in Figure 4-43. This principle dictates thatcaution should be used when making three-dimensional interpretation of processes from one-or two dimensional data (logs, cores, outcrops, and 2D seismic).

Figure 4-43. Schematic cross sections of depositional intervals with differing grain size and howtheir vertical stacking arrangement (area in box) would be identical through either (A) landwardshift (allocyclic) or (B) lateral shift (autocyclic) in depositional axis between deposition of the lowerand upper intervals. (Modified from Al-Siyabi, 1998).

Summary: Lessons learned

Our understanding of sediment gravity flow processes continues to evolve at a rapid rateowing to a resurgence of research into their hydrocarbon-bearing deposits. Key points pre-sented in this chapter are summarized below.

1. Sediment gravity flows can be initiated in a number of ways, from seismically-inducedslides to flood stage discharge of rivers into the ocean. Such flows can occur with a fre-quency of years to 100s of years between event, depending upon the type of flow.

2. Critical factors that contol the type of flow and deposit include gravity, sediment con-centration and flow velocity over time and space, These factors are related to sedimentsource area and type, climate, tectonics, and topography of the sea floor.

3. End member gravity flows, which are most common in the rock record, are cohesivedebris flows and fluidal turbidity current flows. Intermediate flow types between truedebris flows and turbidity currents also exist. A combination of observations of rocks androck sequences, measurements in modern sedimentary environments, laboratory flumeexperiments, and computer modeling have illustrated the complexity of sediment gravity

(A)

(Landward) (Basinward)

(Basinward)

(B)

Coarser

grained

Finer grained

Lateral shift

References

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flows. Contrary to historical perspective, flows can wax, wane, or remain steady in bothtime and space, giving rise to complex sets of deposits, both laterally and vertically.

4. For descriptive, as well as some genetic classification, the Bouma sequence remains thefundamental deposit from a turbidity current. Different terms sometimes have beenapplied to divisions of the Bouma sequence by different authors. Other types of sedimentgravity flows that do not conform to this classification include muddy sandstones, slurrybeds, and massive sandstones (though the latter is sometimes classed as a Bouma Tabed). Hyperpycnites (deposits from hyperpycnal flows) can be described by Bouma divi-sions even though their vertical stratigraphy may differ from that of a classic BoumaSequence.

5. Turbidity currents generated from hyperpycnal flows are probably of greater signifi-cance than previously recognized, particularly off of major deltas. A distinctive coarsen-ing-upward, followed by fining upward vertical sequence distinguishes hyperpycnitesfrom other types of sediment gravity flows.

6. Downcurrent flow transformations of one type of sediment gravity flow to another typeof flow undoubtedly occur in nature. Deposits that document flow transformations aredifficult to find in outcrop, though rare outcrops have been identified. Flow transforma-tions have been generated in flume tanks and by experimental modeling.

7. Once deposited on the sea floor, sediment gravity flows can be subjected to reworking bylong-lived bottom currents as well as by internal waves and tidal currents (in the shal-lower portions of submarine canyons). Reworking can winnow the finest-grained fractionfrom sands, and sort them, to give rise to excellent potential reservoir rock upon burial.

8. Vertical stacking patterns of sediment gravity flows are important for interpreting thelarger-scale filling history of a basin. A similar vertical sequence of sediment gravityflows can be produced by processes related to fluctuating base level (allocyclic pro-cesses) or fluctuations in depositional processes and sites without a change in base level(autocyclic processes). These differences should be considered when interpreting thedepositional history of a sequence of sediment gravity flow deposits.

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