Deep-Water Depositional Systems · Deep-Water Depositional Systems Class reading from Boggs,...
Transcript of Deep-Water Depositional Systems · Deep-Water Depositional Systems Class reading from Boggs,...
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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)
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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)
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(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
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0
2
4
6
8
10
12
14
16
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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?