David Mohrig Jackson School of Geosciences, Department of Geological - - PowerPoint PPT Presentation

Beds, Bars, Bends, Banks and Basins: Construction

• f the Seascape and Deep-Marine Strata by

Turbidity Currents David Mohrig

Jackson School of Geosciences, Department of Geological Scineces, The University of Texas at Austin, Austin, TX, USA

National Center for Earth-surface Dynamics, a NSF STC

Brunei – Shell Petroleum Shell Exploration and Production Acknowledgements to: Kyle M. Straub, Anjali Fernandes, Vishal Maharaj, & Jim Buttles

Session Focus: Controls of sediment gravity flow processes on deep marine depositional system architecture My Main Points:

• 1. At the turbidite-system scale, concepts of grade, accommodation, and

“equilibrium long profiles” are misleading. [Relevant to Conceptual Models] REASONING: Sediment accumulations associated with strongly depositional currents are insensitive to spatial changes in surface slope or flow accelerations.

• 2. Erosive & depositional turbidity currents interact differently with local, pre-

existing bed topography. [Relevant to Numerical Models] FINDING: Topographic perturbations act as “hot spots”, focussing erosion. Beds from strongly depositional currents are insensitive to local topography.

• 3. Turbidites may be commonly remobilized as higher density, quasi-laminar
• flows. [Relevant to Numerical Models]

FINDING: Remobilization is driven by the metastable character of original deposits and the stresses applied to their surfaces by overriding currents.

Submarine Channel X

Water depth (m)

Incompleteness of Models for Deep-water Stratigraphy Based on Concepts of Grade, Accommodation, and the Equilibrium Profile

Channel

Erosional Turbidity Currents do act

• n the seafloor as a function of

local bed slope. Accumulation patterns for Strongly Depositional Currents are uniquely insensitive to local bed slope & spatial flow accelerations.

x 1.41 x 1.41

Bathymetry (cm) Familiar Terrestrial Case: Sediment flux follows the local transport capacity → local decelerations & accelerations of the flow drive sedimentation or erosion Depositional Turbidity Current Case: Non-local patterns of suspended sediment deposition  length scale of interest (L) is smaller than the sediment-advection length scale (l). When are the spatial decelerations & accelerations of a current not reflected in the deposit properties?

Acoustical Image: Centerline profile through 9 turbidites

Topographic irregularities in bed are preserved through time.

1 cm 60 cm Basement

When the length scale of interest (L) is smaller than the sediment-advection length scale (l)  Non-local patterns

• f suspended sediment deposition 

Upstream control. When are the spatial decelerations & accelerations of a current not reflected in the deposit properties?

(Hickson et al.,1999)

          x Uh w r c r w t

s x bed α s

ε



 exp

Non-local sedimentation pattern

When are the spatial decelerations & accelerations of a current not reflected in the deposit properties?

(Lamb, McElroy, Kopriva, Shaw & Mohrig, 2010)

Rate of bed- elevation change Reference Suspended- Sediment Concentration Down-Flow Distance

x 1.41 x 1.41 x 1.41

Channel Centerline Bathymetry (cm)

Red = Original channel bottom Blue = Top surface of 3 stacked turbidites

Down-flow profiles of turbidites from strongly depositional currents are remarkably insensitive to local irregularities in channel topography.

20 40 60 80 100 120 140 160 180 200 26 26.5 27 27.5 28 28.5 depth, cm 3-Flows EFlow #2

Erosion rate is sensitive to local velocity and turbulence production. Local topography affects erosional patterns.

What happens in the case of eroding turbidity currents?

Red = Top surface of 3 stacked turbidites Blue = Surface after passage of 2 weakly erosional currents

Centerline Distance (cm x 1.41)

Change in local bed elevation is always the sum of the deposition rate and the erosion rate. It is primarily the erosional component that sets the observed morphodynamic connection between turbidity currents and bed slope.

T1 T2 T3

(A. Fernandes, 2012)

Channel Length ~ 4 m

What are the properties of stratigraphy produced by strongly depositional versus net-erosional turbidity currents traversing channels

• f complex

form?

Bend 1 Bend 3 Bend 2

1 3 2

Beds from strongly depositional turbidity currents traversing a sinuous subaqueous channel.

(Straub, Mohrig, McElroy, Buttles & Pirmez, 2008)

Original channel + deposits of 24 turbidity currents

  

Down-flow profiles of turbidites from strongly depositional currents are remarkably insensitive to local irregularities in channel planform.

Non-local pattern of suspended sediment deposition produces a tapered deposit with lateral skewness connected to channel planform.

Centerline

(Straub, Mohrig, McElroy, Buttles & Pirmez, 2008)

Experimental channel with erodible bed

Depth = 15cm, Width = 50cm, Side-wall slope = ~20o Sinuosity = 1.05 Down-channel slope = 7o

(A. Fernandes, 2012)

Erosional current in experimental channel

(A. Fernandes, 2012)

Dyed Current: Notice run-up at the outer banks of bends & flow separation from the inner banks

Bed erosion correlates with the pathway of the high velocity core Deepest scours at outer banks of bends Weak sedimentation at inner banks, within separation zones

Topographic change: Flow 1 – Initial surface

(A. Fernandes, 2012)

Deep scours grow (local turbulence production by bed irregularities enhances local erosion) Continued sedimentation at inner banks of bends

Topographic change: Flow 2 - Flow 1

(A. Fernandes, 2012)

Simultaneous development of erosional inner channel and depositional benches

A A’ B B’ C C’

Initial surface After Flow 1 After Flow 2 After Flow 3 After Flow 4 After Flow 5

B B’ C C’ A A’ Hot colors = Net Deposition Cool colors = Net Erosion

(A. Fernandes, 2012)

Abreu, Sullivan, Pirmez, & Mohrig (2003)

Implications for Spatial Continuity of Deposits from Strongly Depositional versus Net-Erosional Currents

LAP deposited by net-erosional currents Channel- plugging deposit

Lateral Migration of Sinuous Channels Requires the Spatial Variability in Bed-Elevation Change Associated with Net-Erosional Turbidity Currents

Original Channel form with sediment bed

Y (200 = 0.40 m) X (200 = 0.40 m)

Sediment Drape Over Entire Channel Form

• Initial Condition

Flow direction

Original Channel form with sediment bed Channel form after Turbidity Current

Y (200 = 0.40 m) X (200 = 0.40 m) Y (200 = 0.40 m) X (200 = 0.40 m)

Bend Evolution by Net-Erosional Turbidity Currents

Y (200 = 0.40 m) X (200 = 0.40 m)

Net Deposition (cm) Net Erosion (cm) Bars on inner banks of bends grew due to cross- channel transport of bedload sediment. Sediment erosion at

• uter banks of bends.

Lateral Migration of Channel Bends by Net-Erosional Turbidity Currents

Flow direction

Reorganization of Sediment Bed by a Single Erosional Turbidity Current

Y (200 = 0.40 m) X (200 = 0.40 m)

Net Deposition (cm) Net Erosion (cm)

Outer Banks of Turbidity-Current Channels can be Sites of Net Erosion or Net Sedimentation

Deposit Thickness

110 115 120 125 130 135 140

• 69
• 68
• 67
• 66
• 65
• 64
• 63
• 62

Cross-stream [cm]

• Rel. Depth [cm]

Channel Base Depositional Flow1 Depositional Flow7

A A’ B B’ Net Deposition (cm) Net Erosion (cm) A A’ B B’

High velocity core near outer banks of bends can produce maximum erosion or deposition on bed of turbidity-current channels

 

(Straub et al., 2008)

1200m water depth 200m water depth

30 km

• S. China

Sea

Conceptual model: Building seascapes from a combination of sedimentation patterns that are insensitive to spatial change in surface slope and erosion patterns that are sensitive to spatial change in surface slope.

Data provided by

(Straub & Mohrig, 2009)

Two examples from a Quaternary Slope System where depositional patterns of turbidity currents are insensitive to significant changes in bed slope.

1100 m 300 m

• S. China

Sea

2 km

Shale diapers driving anticline growth

Seismic Time Slice

Example 1. Insensitivity of depositional pattern to downslope change in the long profile.

(Straub & Mohrig, 2009)

~200 m

Seismic Strike Line C

~200 m

Seismic Strike Line D

Seafloor C.I. = 8m

2 km

C D

Laterally persistent turbidite stratigraphy that thins with distance from the shelf-slope break.

(Straub & Mohrig, 2009)

Downslope Distance (km)

0 2 4 6 8 10 0 2 4 6 8 10 600 800 1000 1200 0.1 0.05 200 100 Deposit Thickness (m) Downslope Gradient (m/m) Water Depth m) 2 km

1 2 3

Canyon Axis (2) Unconfined Slope (1+3)/2

(Straub & Mohrig, 2009)

Example 1. Depositional patterns that are not correlated with spatial change in the long profile.

1200 m 200 m

Champion Delta

Borneo

• S. China

Sea

Channel A Channel B A B

(Straub, Mohrig, & Pirmez, 2012)

Example 2. Channel deposition that is not correlated with spatial change in the long profile.

(Straub, Mohrig, & Pirmez, 2012)

Channel head Channel head

Example 2. Channel deposition that is not correlated with spatial change in the long profile.

0.1 0.05

Channel Gradient (m/m)

(Straub, Mohrig, & Pirmez, 2012)

Channel A

Comparing Thickness

• f Channel Bottom

Deposit to Magnitude

• f Channel-Bed Slope

0.1 0.05

Channel Gradient (m/m)

(Straub, Mohrig, & Pirmez, 2012)

Channel B

Comparing Thickness

• f Channel Bottom

Deposit to Magnitude

• f Channel-Bed Slope

High Discharge: Distributed Entrance Low Discharge: Distributed Entrance High Discharge: Channel Entrance Low Discharge: Channel Entrance

Sedimentation patterns in experimental 3-D minibasin(s) associated with strongly depositional Turbidity currents

(Buttles, Maharaj, & Mohrig, in prep)

Elevation (mm) Downslope Distance (mm)

Flow Direction 1200 1100 1000 900 800 700 600 500 400 300 200

• 1250
• 1300
• 1350
• 1400
• 1450

(Buttles, Maharaj, & Mohrig, in prep)

Fill pattern in an experimental minibasin following 18 depositional episodes:

Input currents ~ constant through time

Thickness maps for individual depositional episodes

Draping Turbidite Turbidite + Remobilized Deposit Turbidite + Remobilized Deposit

Deposit Thickness (mm)

(Buttles, Maharaj, & Mohrig, in prep)

Resedimentation of turbidites dominates the later stage of basin filling. If remobilized sediment is a sandy turbidite, the resulting deposit is a structureless sand with increased topographic confinement.

(Buttles, Maharaj, & Mohrig, in prep)

Session Focus: Controls of sediment gravity flow processes on deep marine depositional system architecture My Main Points:

• 1. At the turbidite-system scale, concepts of grade, accommodation, and

“equilibrium long profiles” are misleading. [Relevant to Conceptual Models] REASONING: Sediment accumulations associated with strongly depositional currents are insensitive to spatial changes in surface slope or flow accelerations.

• 2. Erosive & depositional turbidity currents interact differently with local, pre-

existing bed topography. [Relevant to Numerical Models] FINDING: Topographic perturbations act as “hot spots”, focussing erosion. Beds from strongly depositional currents are insensitive to local topography.

• 3. Turbidites may be commonly remobilized as higher density, quasi-laminar
• flows. [Relevant to Numerical Models]

FINDING: Remobilization is driven by the metastable character of original deposits and the stresses applied to their surfaces by overriding currents.