1

Deep-Water Depositional Systems

Class reading from Boggs, Principles of

Sedimentology and Stratigraphy: p.349 - 364.

In the submarine environment, particularly the continental slope, patterns of sediment erosion and deposition are governed to sediment-gravity flows. Since these currents move through water, gravity drives these flows by acting on the “excess” density of the water+sedimentmixtures; that fraction of the bulk density that exceeds the density of water. Turbidity Currents

(~ <10 % suspended sediment by volume)

Debris Flows(~ 50:50 sedimentand water by volume)

2

A type example of a deepwater-depositional system; the Brazos-Trinity Slope System

(Prather et al., in press)

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Topography, depositional patterns and structural deformation associated with the Brazos-Trinity slope system and its 5 minibasins.

Active structural deformation is the product of motion of the Luann Salt.

Location map and seismic cross-section through the B-T system. (Pirmez et al., in press)

Badalini et al., 2000

Seascape development is often the product of competition

between deforming substrate and sediment-transporting

flows (note spatial change in patterns of deposition and

erosion below).

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Seismic section through shelf-edge delta

delivering sediment to Basin 1 during the

last sea-level lowstand. (Prather et al., in press)

5

(above) Seismic cross section through

Basin 2 deposit. All of this sediment

accumulated during the past 20,000 yrs.

(left) Maps of deposit thickness

Figure (a) = the 6a deposit shown in

cross section above.

Figure (b) = the 6b deposit shown in

cross section above.

Figures (c) & (d) = the 6c deposit shown

in cross section above.

Notice the changes in sedimentation

pattern as deposits fill in the pre-existing

Basin 2 topography.

Basin 2 of the

linked, Brazos-

Trinity Slope

System.

(Prather et al., in press)

(Beaubouef & Friedmann, 2001)

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Low Sediment-Concentration Turbidity Currents

(~ < 1 % suspended sediment by volume)

High Sediment-Concentration Turbidity Currents

(~1 - 10 % suspended sediment by volume)

Debris Flows(~ 50:50 sediment

and water )

Sediment-Gravity Flows

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Initiation and Evolution of Sediment-Charged Submarine

Flows

Hyperpycnal Flow

SlopeFailures

Low-Sediment Concentration

High-Sediment Concentration

Slide and Slump

DebrisFlow

Extensive InternalDeformation

Minor InternalDeformation

TurbidityCurrent

Sediment Gravity Flows

River flows with high suspended-sediment concentration can be

sufficiently dense to plungeunderneath seawater and

continue down-slope

Slope-Failure Triggers:1) Waves and currents of

large storms2) Unusually low tides3) High internal pore

pressures4) Earthquakes5) Tectonically steepened

slopes

There are modern examples of failures in deposits with surface slopes as small as 0.1 degrees.

< 10% > 50%

50 m

2 km

Turbidites: Amazon Fan

Debrites: Bear Island Fan, North Sea

A ‘A

10 km

1600m

50ms

(40m)

(Pirmez et al., 2000)

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Turbidity Current Deposits (= Turbidites):

• Relatively low depositional slopes. • Strong spatial sorting of sediment.

Debris Flow Deposits (= Debrites):

• Relatively flat tops and steep margins.

• Minor spatial sorting of sediment.

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Velocity and Sediment-Concentration Profiles in the Body a

Turbidity Current

• h = Turbidity Current Height,

• c = Suspended Sediment Concentration

• u = Velocity (down-dip direction),

• Cb = Sediment concentration at base,

• Um = Maximum Velocity, • Hf = Front (or Head) Height,

• Uf = Front (or Head) Velocity

• hh = Height of Overhanging Nose

10

0

2

4

6

8

10

12

14

16

18

0.0001 0.001 0.01 0.1Vertically Averaged Volume Concentration of

Suspended Sediment

Av

era

ge

Bo

dy

Ve

loc

ity

(m

/s)

h ~ 10 m

h ~ 50 m

h ~ 150 m

AverageConcentration

AverageDown slopeVelocity

Sediment Conc.Current Velocity

h

h = current thickness

Turbidity-Current Velocities & Thicknesses

Turbidity

Currents can

be erosive

Monterey Fan Channel (offshore CA);W = ‘waterfall’, T = terrace, TB = transverse bedforms.

Masson et al. (1995)

Seafloor topography

(blue = lowest

elevation)

Governing Parameters:• Grain size• Sediment concentration• Current velocity and thickness• Bed slope

Field Examples of Deposits Filling an Ancient Slope Canyons

Outcrop Expression: Gravely Ridge Mbr.,

Stony Creek Fm., CA

• Oblique-cut through a deposit filling an

ancient submarine canyon

• 33 km in length: Up to 300 m thick

• Pebble conglomerate to sandstone

Subsurface Expression: Hackberry Sandstone

• Oligocene deposits of TX and LA Gulf Coast

3-D Seismic1 Km

• High resolution 3-D seismic data set

• Average channel depth: 44m

• Average channel width: 435 m

• Sinuosity: 2.36

FL

OW

625m100 m

300m

100 m

BARS CHANNEL

Predominantly fine-grained, thick, siliciclastic deposits on the

slope and basin floor of the Permian Delaware Basin.

Submarine slope channel fills exposed in dip-oblique section, in

Shumard Canyon in the Guadalupe Mountains.

Channel deposits <2km from the shelf edge.

10m

Brushy Canyon Fm. Channels

Brushy Canyon Fm. Channels

Turbidity Currents can be Strongly

Depositional

Ross Fm. Sullivan et al. (2000)

Brushy Canyon Fm.,

west TX

Governing Parameters:• Grain size• Sediment concentration• Current velocity and thickness• Bed slope

Stratigraphic Consequence of Suspended Sediment Columns that are

Thick Relative to Relief on Existing Bottom Topography

Mean grain size of deposit

1000 2000 3000 4000 5000 6000 7000

Distance (m)

Ele

vati

on

(m

) mm

mm

mm

- 0.7°

2.1°

0.2 mm

0.1 mm

Mean Grain Size

Multiple size fractions

Uo = variable

Ho = constant

Co = constant (1 %)

15 stacked turbidites

20 cm

(Hickson et al.,1999)

Example:

Draping deposits of

interslope basins

1600m

50ms

(40m)

(Pirmez et al., 2000)

Late Pleistocene

Amazon Fan (200km

from shelf break, at

3000m water depth)

(Mohrig & Buttles, 2007)

Overbank and in-channel

sedimentation are sub-

equal for relatively thick

turbidity currents.

current #10

current

#1Control of current thickness on sedimentation pattern

0.76 0.82

#1

#10

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Rule of Thumb:Largest clasts that can

to moved (rolled), travel at < 1/4 the average turbidity-

current velocity.

Fully suspended particles travel at the

turbidity current velocity.

Mode of Grain Transport by Turbidity Currents

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Coarsest-Grained Turbidites

Bedloaddeposits

Mixedsuspended-load/bedloaddeposits

Bedding Surface

Bedding Surface

• Pronounced normal grading

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Pure Suspension Deposits: The Bouma Succession

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Bouma Subdivisions

Complete BoumaSuccession

IncompleteSuccessions:Repetitive BoumaSubdivisions

Td

Tc

Tb

Tc

Ta

Td

Tb

Te

Tc

Tb

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All Debrites (regardless of composition):

1) structureless matrix;

2) disorganized ‘floating’ clasts (large grains

and

organics);

3) shear banding

Key Sedimentary Structures

Fabric of Submarine Debrites (debris-flow deposits)

Base of muddy debrite

Low

permeability,

sandy debrite

High permeability,

sandy debrite

Critical difference in subaerial and

subaqueous environments is the

density of the ambient fluid.

water/air 800

Enhanced Mobility of Subaqueous Debris-

Flows, Slumps, & Slides

Key consequence of density

difference is substantial change in

magnitude of reactive stresses.

u2

Necessary Hydroplaning

condition characterized

using the Densimetric

Froude

Number:1

)(

2

hg

u

dd

Fr

Hydroplaning and basal

lubrication can lead to

higher front velocities and

longer run-out distances.

Finneidfjord

The extension of a debris flow that produces

outrunner blocks is evidence for

hydroplaning during transport.

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Questions You Should be Able to Answer1. What is a sediment-gravity flow?

2. What are the differences between a turbidity current and a subaqueous debris flow?

3. What defines the continental shelf – continental slope break?

4. What is a shelf-edge or shefl-margin delta?

5. What is a minibasin?

6. Sedimentation in the minibasins of the Brazos – Trinity Slope System has been

occurring for how long?

7. What is the largest river system completely contained within the geographic

boundaries of the State of Texas?

8. What is causing the minibasins of the Brazos-Trinity Slope System to deform?

9. How is a hyperpycnal flow different than a turbidity current?

10. What are the differences between a debris flow, a slump, and a slide?

11. What are 5 mechanisms for triggering slope failures?

12. What are 2 mechanisms for triggering turbidity currents?

13. Name two properties of turbidity-current deposits (turbidites).

14. Name three properties of debris-flow deposits (debrites).

15, What is the shape of the velocity profile for a turbidity current and what is the

vertical position of the maximum down-slope velocity?

16. What is the shape of the concentration profile for suspended sediment within a

turbidity current?

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Questions You Should be Able to Answer17. How thick can turbidity currents be?

18. What are two parameters that control turbidity current velocity?

19. What are 5 parameters that control whether turbidity currents erode from or deposit

sediment on the bed?

20. The Oligocene Hackberry Sandstone was deposited in what depositional environment?

21. What kind of deposit is associated with turbidity currents that are very thick relative to

relief on existing bottom topography?

22. How thick must turbidity currents be relative to their guiding channels in order for the

thickness of levee deposits to equal 80 % of affiliated channel-bottom deposits?

23. What is the coarsest sediment moved by turbidity currents and how is it moved?

24. What is the coarsest sediment fully suspended by turbidity currents?

25. How fast are grains travelling as bedload moving relative to grains traveling in suspension?

26. What is and what causes normal grading?

27. What is a Bouma succession or sequence? What are its 5 sub-divisions and how are they

related to changes in current velocity and deposition rate?

28. What are 3 key sedimentary structures associated with submarine debrites?

29. Why can submarine debris flows, slides and slumps hydroplane? Why does the

Densimetric Froude number characterize the condition for hydroplaning?

30. Why are outrunner blocks considered evidence for the occurrence of hydroplaning?