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LECTURE 7TRIGGERING MECHANISMS OF TURBIDITY CURRENTS

CEE 598, GEOL 593TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS

http://clasticdetritus.com/category/marine-science/

ScrippsandLa Jolla Submarine Canyons, San Diego, California

What causes turbidity currents?

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A PARTIAL LIST OF TRIGGERING MECHANISMS

1. Hyperpycnal flows

2. Surface waves

3. Breaching

4. Double-diffusive phenomenon

5. Transformation from slope failures and submarine debris flows

6. Internal waves

7. Wave-supported sheet turbidity currents

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DENSITY OF FRESH WATERThe density of (sediment-free) water varies with temperature (e.g. C), salinity (e.g. grams/liter) and pressure (~ depth according to the hydrostatic law). Here we consider the effect of temperature and salinity.

Fresh water has a maximum density of 1 ton/m3 at 4 C.

0.95500

0.96000

0.96500

0.97000

0.97500

0.98000

0.98500

0.99000

0.99500

1.00000

0 20 40 60 80 100

Temperature deg C

Den

sity

tons

/m3

Temperature Deg C

Density tons/m3

0 0.9998682 0.9999684 1

10 0.999720 0.998230 0.995740 0.992250 0.988160 0.983270 0.977880 0.971890 0.9653

100 0.9584

Double-click to activate Excel spreadsheet.

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DENSITY OF SEA WATER

The “standard” density of sea water is ~ 1.026 tons/m3. This corresponds to a salinity of 35000 mg/liters (35 grams/liter) and a temperature of 14 C. Increasing temperature (usually) makes water lighter, and increasing salinity makes it heavier.

You can find a calculator of seawater density athttp://www.csgnetwork.com/h2odenscalc.html

The density of seawater can, however, vary considerably. In brackish coastal waters near river mouths, the density can approach that of freshwater, i.e.1.000 tons/m3. In nearly-enclosed seas with high evaporation rates, such as the Red Sea, the salinity can be as high as 40000 mg/liters. Assuming, for example, a water temperature of 20 C, the density is about1.029 tons/m3

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1. PLUNGING AND HYPERPYCNAL FLOWS

Reuss River plunging into Lake Lucerne, Switzerland: flood of summer, 2005

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WHAT IS PLUNGING?WHAT IS A HYPERPYCNAL FLOW?

Plunging is a phenomenon when sediment-laden river flow is heavier than the body of water it flows into. The sediment water immediately sinks, forming a continuous turbidity current.

Plunging is usually associated with muddy flows, whereas (most of the) sand (most of the time) tends to deposit in a delta upstream.

Plunging is very common in lakes and reservoirs. The resulting turbidity currents can emplace sediment over long distances.

Lake MeadDelta of

Colorado River

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PLUNGING IN LAKE MEAD

Image from USBR

Logjam near plunge point

plunge line

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PLUNGING CREATES A HYPERPYCNAL FLOW

Hyper excess, pycnal density. So a hyperpycnal flow is a continuous bottom turbidity created by the excess density of the river flow relative to the ambient standing water due to the presence of sediment.

Coarser sediment deposits in delta (topset and foreset).

A (usually muddy) turbidity current is created by hyperpycnal conditions.

Finer sediment (mud) deposits in bottomset.

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HYPOPYCNAL FLOWS

A hypopycnal flow is a sediment-laden surface plume created when the sediment-laden river water is less dense than the ambient water into which it flows. Sediment gradually rains out from the surface plume (hemipelagic sedimentation) to emplace the bottomset.

Coarser sediment deposits in delta (topset and foreset).

Surface plume of muddy water

Sediment gradually rains out to emplace the bottomset.

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HYPOPYCNAL SURFACE PLUMES ALONG THE ADRIATIC SHORELINE OF ITALY

Hypopycnal plumes along the Adriatic margin of Italy: image from J. Syvitski.

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TURBIDITES IN LAKE MEAD EMPLACED BY PLUNGING TURBIDITY CURRENTS

Twichell et al. (2006)

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TURBIDITES IN LAKE MEAD EMPLACED BY PLUNGING TURBIDITY CURRENTS

Twichell et al. (2006)

Seismic image of deposits in the west end of the Virgin Basin.

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RELATION BETWEEN SEDIMENT CONCENTRATION IN MG/L AND WATER DENSITY

Let X denote the concentration in mg/l. Where f equals the density of the sediment-laden flow, rw is the density of the river water without sediment and s denotes the density of sediment. The volume concentration C is given as

65.11R,1RX10x1C

f

s

f

6

The density of the sediment-laden flow is thus

)RC1(rwf

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HOW EASY IS IT TO CREATE PLUNGING IN FRESHWATER?

Suppose the lake has a temperature of 10 C (l = 0.9997 t/m3). We consider two cases: the river water has a temperature of 10 C (l = 0.9997 t/m3) and 20 C (rw = 0.9982 t/m3). In the latter case there is a temperature barrier to plunging). The minimum concentration is for plunging can be calculated with the spreadsheet. (Double-click to activate.) The minimum concentrations are 0 mg/l and 2413 mg/l.

As shown on the next slide, even a concentration of 2413 mg/l during floods is not uncommon in mountain streams.

lrwf )RC1(

1R

1R10x1X

1R1C

rw

lf

6

rw

l

The minimum condition for plunging is:

Critical condition forhyperpycnal flow

rw 0.9982 t/m3

l 0.9997 t/m3

C 0.0009X 2413.4 mg/liter

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SUSPENDED SEDIMENT CONCENTRATION IN RIVERS

Sediment concentrations are based on mean flows rather than flood flows. From USGS website.

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Suspended Sediment Concentration Minnesota River Mankato

1

10

100

1000

10000

1 10 100 1000 10000

Q (m3/s)

X m

g/lit

erSUSPENDED SEDIMENT CONCENTRATION IN THE

MINNESOTA RIVER

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CAN PLUNGING AND HYPERPYCNAL FLOWS OCCUR IN THE OCEAN?

Let f equals the density of the sediment-laden flow, rw is the density of the river water without sediment and sea denotes the density of sea water. The minimum concentration for plunging into seawater is:

searwf )RC1(

1R

1R10x1X

1R1C

rw

seaf

6

rw

seal

Critical condition forhyperpycnal flow

rw 1 t/m3

sea 1.026 t/m3

C 0.0158X 41758 mg/liter

If rw = 1 ton/m3 and sea = 1.026 t/m3, C must be at least 0.0158, and X must be at least ~ 40,000 mg/l in order to get plunging.

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THIS CONDITION IS RARELY MET IN RIVERS

Sediment concentrations are based on mean flows rather than flood flows. From USGS website.

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Mississippi River Tarbert Landing

0

500

1000

1500

2000

2500

3000

01-Oct-1949

27-Jun-1952

24-Mar-1955

18-Dec-1957

13-Sep-1960

10-Jun-1963

06-Mar-1966

30-Nov-1968

27-Aug-1971

23-May-1974

Day

Susp

ende

d Se

dim

ent

Con

cent

ratio

n m

g/lit

erAND IT IS ESPECIALLY RARELY MET IN LARGE,

LOWLAND RIVERS FLOWING INTO PASSIVE MARGINS

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Mississippi Submarine Fanmeandering channel

on fan

AND SO HYPERPYCNAL FLOWS ARE NOT LIKELY TO BE RESPONSIBLE FOR THE EMPLACEMENT OF MOST

LARGE SUBMARINE FANS ON PASSIVE MARGINS.

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AND YET SOME RIVERS SOMETIMES DO FORM HYPERPYCNAL FLOWS WHEN THEY REACH THE SEA

Plunging of the Yellow River into the Bohai Sea, China

From International Journal of Sediment Research

And when they do they can move enormous amounts of sediment into the sea.

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NOW WHERE MIGHT RIVERS FLOWING INTO THE SEA HAVE SUCH

HIGH CONCENTRATIONS OF

SUSPENDED SEDIMENT?

Active margins undergoing rapid uplift!

Erosion rates in mm/year for Taiwan.

Image courtesy C. Stark

16 of the rivers listed by Mulder and Syvitski (1995) that go hyperpycnal at least once per 100 years are in Taiwan

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A DOCUMENTED CASE OF HYPERPYCNAL FLOW TO THE OCEAN

The Eel River in Northern California carries a very high sediment load. It is estimated to become hyperpycnal once every 10 years.

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HYPERPYCNAL EVENT ON THE EEL RIVER

A hyperpycnal event was recorded and documented in the flood of 1995.

From Imran and Syvitski (2000)

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2. TURBIDITY CURRENTS IN CANYONS GENERATED BY COASTAL SURFACE WAVE ACTION

One of the first field measurements of turbidity currents in the ocean was performed in Scripps Submarine Canyon off San Diego, California (Inman et al., 1976).

The turbidity currents were generated by wave action, which was in turn driven by a winter storm.

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LITTORAL DRIFT

The canyon is not located near a river mouth. The sediment (fine sand) is delivered from river mouths to the head of the canyon by littoral drift, mostly during the summer. The sediment piles up at the head of the canyon.

Littoral drift is an along-coast flow of sediment driven by incident waves that are not parallel to the shore.

Incident waves Reflected waves

Alongshore sediment transport

shore

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STORMS, INCIDENT WAVES, AND EDGE WAVES

Incoming waves from storms oscillate in the cross-shore direction. These can generate trapped edge waves, which are standing nearshore waves that oscillate in the along-shore direction.

The antinodes of these edge waves locate themselves at depressions, e.g. canyon heads.

A

shore

Incident waves

edge waves

A’

A A’

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THESE EDGE WAVES STIR UP SEDIMENT AT THE CANYON HEAD

The sediment-laden seawater so created is heavier than the ambient sediment-free seawater.

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AND THE RESULT IS A DOWN-CANYON TURBIDITY CURRENT

The current so created is sustained as long as the edge waves are present and there is sediment available in the canyon head (hours, or even days). A storm immediately subsequent to one that generated a turbidity current often creates no turbidity current, because there is no longer any sediment available.

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THESE TURBIDITY CURRENTS CAN BE QUITE STRONG!

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Breaching is the sustained, slow failure of a near-vertical subaqueous face of slightly overdensified clean, fine sand by spalling of sediment from the face.

4. TURBIDITY CURRENTS CREATED BY BREACHING

See van den Berg et al. (2002); Mastbergen and van den Berg (2003).

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BREACHING HAS BEEN USED BY THE DREDGING INDUSTY IN THE NETHERLANDS TO INSTIGATE SELF-SUSTAINED

REMOVAL BY MEANS OF SPALLING TO A TURBIDITY CURRENT

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A SLOW BREACH FAILURE LAUNCHES A BOAT

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BREACHING IN THE LABORATORY (courtesy T. Muto)

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WHY THE NEAR-VERTICAL FACE?

As a particle starts to fail off a modestly overdensified face of clean, fine sand, the negative pore pressure created by the gap prevents it from failing catastrophically, and instead leads to slow grain-by-grain spalling.

negative pore pressure pulls particle back as it tries to fail

K = hydraulic conductivity of sand [L/T]p = porosity of sand [1]cb = retreat speed of breach [L/T]Eb = volume erosion rate/surface area of sediment [L/T]

k)1(25c)1(E,k25c pbpbb

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GENERATION OF THE TURBIDITY CURRENT

vertical breach face slowly retreats

as sediment spalls off grain-by-grain

breaching generates a quasi-continuous turbidity current

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TURBIDITY CURRENT GENERATED BY BREACHING IN THE LABORATORY

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SOME TURBIDITY CURRENTS IN THE MONTEREY SUBMARINE CANYON APPEAR TO BE GENERATED BY

BREACHING

Xu, J. P., M. A. Noble, and L. K. Rosenfeld (2004)

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• Sustained event: lasted 5 - 8 hours

• Max. velocity ~ 1.9 m/s

• Thicker downcanyon?

• Not caused by storm or hyperpycnal flow (failure of dredge spoil by breaching?)

A DOCUMENTED TURBIDITY CURRENT IN THE MONTEREY SUBMARINE CANYON

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4. TURBIDITY CURRENTS GENERATED BY DOUBLE DIFFUSION

Double-diffusion phenomena were discovered in the context of the ocean.

As noted in a previous slide, (sediment-free) oceanic water varies in both salinity and temperature. A higher salinity makes water heavier. A higher temperature makes water lighter. In the ocean, the density of water is controlled by both factors.

In oceanic waters, heat and salt can be fluxed by both convection and molecular diffusion.

Here we are interested in flux by molecular diffusion. Diffusion fluxes a quantity from a zone of high concentration to low concentration. Consider any quantity per unit volume b (e.g. heat in joules per unit volume or salt in grams per unit volume. We further assume that this quantity decreases in the x direction.

Diffusive flux transports a quantity from high concentration to low concentration, or in this case in the positive x direction.

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WHAT IS DOUBLE DIFFUSION? contd.

Thus any quantity diffuses down its spatial gradient.

Now let denote temperature and s denote salinity.

0F0xb

x,bd

Let Fbd,x denote the diffusive flux of quantity b (quantity crossing face/time/face area) in the x direction. If b decreases in x, then Fbd > 0

b

x

Fbd,x

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WHAT IS DOUBLE DIFFUSION? contd.

where w is the density of the water, cp is the specific heat of water at constant pressure (e.g. no. of joules required to raise 1 kg by 1 C) and Dh and Ds denotes the kinematic diffusivity of heat and salt (e.g. m2/s).

Typical values of cp, Dh and Ds are 4.18 x 103,1.45 x 10-7 m2/s and1.35 x 10-9 m2/s

The point to note here is:

This means that in a relative sense, salt diffuses much less rapidly than heat.

We denote the temperature as and the salinity as s. The diffusive flux of heat and salt in the x-direction are denoted as Fhd, x and Fsd,x. These terms are given as

xsDF,

xDcF sx,sdhpwx,hd

1D/D hs

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THE FIRST CASE OF DOUBLE DIFFUSION: HENRY STOMMELS PERPETUAL SALT FOUNTAIN

Consider an aluminum tube that extends vertically from the deep ocean to the surface. We assume that warm, saline water at the surface overlies cold, less saline water at depth. The water is stably stratified, i.e. the deep water is denser than the surface water.

But suppose we start a vertical flow in the pipe. The flow will sustain itself creating a “perpetual” salt fountain!

Why? As the flow in the pipe rises, heat diffuses across the pipe walls, warming the pipe water to the temperature of the seawater surrounding it.

But salt cannot diffuse in due to the walls. So the water from depth arrives at the surface with the same temperature as the surface water, but a lower salinity. Being lighter than seawater, it fountains upward!

cold, less salty

Warm, more salty

Warm, less salty

Heat diffuses in, but not salt

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DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS

Consider hypopycnal river freshwater entering the sea. For simplicity, we assume that both are at the same temperature. The coarse sediment deposits to form a delta, and the fine sediment, i.e. mud, forms a surface plume.

fresh, sediment-laden water

surface plume

saline water

delta

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DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS contd.

The kinematic diffusivity of mud, which is governed by Brownian motion, is far less that the kinematic diffusivity of salt.

Consider a blob of mud-laden, fresh water at the interface.

fresh, sediment-laden water

surface plume

saline water

delta

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DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY CURRENTS contd.

Salt can diffuse into the blob much faster than sediment diffuses out. As a result, the blob can get heavier than the surrounding saltwater and sink.Parsons and Garcia (2000)

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THE BLOBS CAN JOIN TOGETHER TO CREATE A CONTINUOUS BOTTOM TURBIDITY CURRENT

Note: this mechanism is as yet unverified in the field.

fresh, sediment-laden water

surface plume

saline water

delta

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5. TRANSFORMATION FROM SLOPE FAILURES AND SUBMARINE DEBRIS FLOWS

Submarine landslides and debris flows can generate turbidity currents as sediment is entrained into suspension from their heads (and also their bodies). The landslide/debris flow can come to rest, but the turbidity current can run out much father distances.

slide scar

antecedent seafloor profile

landslide/debris flow(comes to rest)

turbidity current (can go much farther before coming to rest)

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THE GRAND BANKS SUBMARINE LANDSLIDE/TURBIDITY CURRENT

An earthquake in 1929 generated the Grand Banks failure, which produced a huge submarine landslide that devolved into a debris flow, and then into a turbidity current that ran long distances. The layer of sand deposited by the turbidity current covered a surface the size of Quebec. Maximum turbidity current velocities may have been as high as 18 m/s, as evidenced from the timing of transatlantic submarine cable breaks. This event served to catalyze interest in turbidity currents as a submarine process for the redistribution of sediment (e.g. Heezen and Ewing, 1952; Kuenen, 1952).

http://earthnet-geonet.ca/communities/earthquake_e.php

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EXPERIMENTAL EVIDENCE FOR TURBIDITY CURRENTS GENERATED BY SUBMARINE DEBRIS FLOWS

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6. TRIGGERING BY INTERNAL WAVES ON THE CONTINENTAL SHELF

Surface gravity waves travel on at the ocean-air interface, and break at the shoreline. The ocean-air interface is a zone of sharp density contrast.

Another, more diffuse interface with a much smaller density difference is the oceanic thermocline. It is a zone of relatively rapid density increase with depth, from well-mixed, warmer surface water to less-mixed, colder water at depth.

http://upload.wikimedia.org/wikipedia/en/8/82/Thermocline.jpg

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INTERNAL WAVES BREAKING ON THE CONTINENTAL SLOPE

Internal waves can form and propagate along the thermocline. It has been hypothesized that internal waves breaking on the continental slope could be responsible for the generation of turbidity currents (D. Cacchione).

http://pof.aip.org/pof/gallery/2006-Koseff.jsp

shelf

slope

rise

turbidity current

53http://physoce.mlml.calstate.edu/PDFs/McPhee-Shaw_oceanmixingconf.pdf

A VIEW OF BREAKING INTERNAL WAVES IN THE LABORATORY

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7. WAVE-SUPPORTED SHEET TURBIDITY CURRENTS THAT DIE ON (AND CONSTRUCT) THE CONTINENTAL SLOPE

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WAVE-SUPPORTED SHEET TURBIDITY CURRENTS

wave-current boundary layer on shelf edge net-depositional turbidity

current on slope

turbidity current becomes extremely thick and dilute as it

decelerates

56landcontinental

shelfcontinental

slope sea

sediment supply

wave orbital amplitude

suspended sediment concentration profile

waves

below wave base

above wave base

WAVE-SUPPORTED SHEET TURBIDITY CURRENTS CAN BE GENERATED BY WAVE-CURRENT BOUNDARY

LAYERS ON THE CONTINENTAL SHELF

Wave-supported turbidity current

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SUCH CURRENTS BUILD OUT A CLINOFORM

wave-current boundary layer on shelf edge net-depositional turbidity

current on slope

turbidity current becomes extremely thick and dilute as it

decelerates

Migrating clinoform

581 km10 ms

Late Holocene Adriatic clinoform

Wave resuspension

Diluted muddyturbidity currents

THE CLINOFORM THUS BUILDS OUTWARD, EXTENDING THE SHELF

Slide courtesy F. Trincardi

Your first seismic image!

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GANGES-BRAHMAPUTRA DELTA

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GANGES-BRAHMAPUTRA CLINOFORM: KUEHL ET AL. (1997)

Topset deposit = deposit on shelf

Foreset deposit = deposit on slope

Bottomset deposit = deposit on slope base/rise

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References to collectMulder and Syvitski (1995)Parsons and Garcia (2000)Imran and Syvitski (2000)Van den Berg et al. (2002)Mastbergen et al. (2003)TrincardiKuehlXu, J. P., M. A. Noble, and L. K. Rosenfeld (2004), In-situ measurements of velocity

structure within turbidity currents, Geophys. Res. Lett., 31, L09311, doi:10.1029/2004GL019718.

Kuenen, P. H. (1952) Estimated size of the Grand Banks [Newfoundland] turbidity current, American Journal of Science, 250, 874-884.

Heezen, B.C., and Ewing, M. (1952) Turbidity currents and submarine slumps and the 1929 Grand Banks earthquake, American Journal of Science, 250, 849-873.

Mohrig, D. and Marr, J. G. (2003) Constraining the efficiency of turbidity current generation from submarine debris flows and slides using laboratory experiments, Marine and Petroleum Geology, 20(6-8), 883-899.