Beds, Bars, Bends, Banks and Basins: Construction of 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 Acknowledgements to: Kyle M. Straub, Anjali Fernandes, Vishal Maharaj, & Jim Buttles National Center for Earth-surface Dynamics, a NSF STC Brunei – Shell Petroleum Shell Exploration and Production
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.
Incompleteness of Models for Deep-water Stratigraphy Based on Concepts of Grade , Accommodation , Submarine Channel X and the Water depth (m) Equilibrium Profile Channel Erosional Turbidity Currents do act on 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.
When are the spatial decelerations & accelerations of a current not reflected in the deposit properties? 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 ). x 1.41 Bathymetry (cm) x 1.41
When are the spatial decelerations & accelerations of a current not reflected in the deposit properties? When the length scale of interest ( L ) is smaller than the sediment-advection length scale ( l ) Non-local patterns of suspended sediment deposition Upstream control. Topographic irregularities in bed are preserved through time. Acoustical Image: Centerline profile through 9 turbidites 1 cm 60 cm Basement
When are the spatial decelerations & accelerations of a current not reflected in the deposit properties? (Hickson et al.,1999) Non-local w r r w α s s c exp x sedimentation ε x t 0 Uh pattern bed (Lamb, McElroy, Rate of bed- Reference Suspended- Down-Flow Kopriva, Shaw elevation change Sediment Concentration Distance & Mohrig, 2010)
Down-flow profiles of turbidites from strongly depositional currents are remarkably insensitive to local irregularities in channel topography. x 1.41 Bathymetry (cm) x 1.41 Channel Centerline Red = Original channel bottom Blue = Top surface of 3 stacked turbidites x 1.41
What happens in the case of eroding turbidity currents? Red = Top surface of 3 stacked turbidites 26 Blue = Surface after passage of 3-Flows 26.5 2 weakly erosional currents EFlow #2 depth, cm 27 27.5 28 28.5 20 40 60 80 100 120 140 160 180 200 Centerline Distance (cm x 1.41) Erosion rate is sensitive to local velocity and turbulence production. Local topography affects erosional patterns. 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 .
What are the properties of stratigraphy produced by strongly depositional versus net-erosional turbidity currents traversing channels of complex form? T1 T2 (A. Fernandes, 2012) T3 Channel Length ~ 4 m
Bend 1 Beds from strongly depositional turbidity currents traversing a sinuous subaqueous channel. Original 1 channel + Bend 2 deposits of 24 turbidity currents Bend 3 3 2 (Straub, Mohrig, McElroy, Buttles & Pirmez, 2008)
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 (A. Fernandes, 2012) Depth = 15cm, Width = 50cm, Side-wall slope = ~20 o Sinuosity = 1.05 Down-channel slope = 7 o
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
Topographic change: Flow 1 – Initial surface (A. Fernandes, 2012) 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 2 - Flow 1 (A. Fernandes, 2012) Deep scours grow (local turbulence production by bed irregularities enhances local erosion) Continued sedimentation at inner banks of bends
Simultaneous development of A A’ erosional inner channel and depositional benches C B B ’ A B C’ A’ B ’ Hot colors = Net Deposition Initial surface Cool colors = Net Erosion C C’ After Flow 1 After Flow 2 After Flow 3 After Flow 4 After Flow 5 (A. Fernandes, 2012)
Implications for Spatial Continuity of Deposits from Strongly Depositional versus Net-Erosional Currents LAP deposited by Channel- net-erosional plugging currents deposit Abreu, Sullivan, Pirmez, & Mohrig (2003)
Lateral Migration of Sinuous Channels Requires the Spatial Variability in Bed-Elevation Change Original Channel form with sediment bed Associated with Net-Erosional Turbidity Currents Flow direction Y (200 = 0.40 m) Sediment Drape Over Entire Channel Form - Initial Condition X (200 = 0.40 m)
Bend Evolution by Net-Erosional Turbidity Currents Original Channel form with sediment bed Channel form after Turbidity Current Y (200 = 0.40 m) Y (200 = 0.40 m) X (200 = 0.40 m) X (200 = 0.40 m)
Lateral Migration of Channel Bends by Net-Erosional Turbidity Currents Net Deposition (cm) Bars on inner banks of bends grew due to cross- channel transport of bedload sediment. Sediment erosion at Y (200 = 0.40 m) outer banks of bends. Net Erosion (cm) Flow direction Reorganization of Sediment Bed by a Single Erosional Turbidity Current X (200 = 0.40 m)
Outer Banks of Turbidity-Current Channels can be Sites of Net Erosion or Net Sedimentation Net Deposition (cm) Deposit Thickness Y (200 = 0.40 m) Net Erosion (cm) X (200 = 0.40 m)
High velocity core near outer banks of bends can produce maximum erosion or deposition on bed of turbidity-current channels Net Deposition (cm) Net Erosion (cm) A A’ B B’ A A’ -62 B B’ Channel Base -63 Depositional Flow1 Depositional Flow7 -64 Rel. Depth [cm] -65 -66 -67 -68 -69 110 115 120 125 130 135 140 (Straub et al., 2008) Cross-stream [cm]
Conceptual model: Building seascapes from a S. China Sea 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 . Two examples from a Quaternary Slope System where depositional patterns of turbidity currents are insensitive to significant changes 1200m water depth in bed slope. 200m water depth 30 km Data provided by (Straub & Mohrig, 2009)
S. China Sea Example 1. 1100 m Insensitivity of depositional pattern to downslope change in the long profile. 300 m Seismic Time Slice 2 km Shale diapers driving anticline growth (Straub & Mohrig, 2009)
Seismic Strike Line C ~200 m Laterally persistent turbidite stratigraphy that thins with distance from the shelf-slope break. D Seismic Strike Line D C ~200 m 2 km Seafloor C.I. = 8m (Straub & Mohrig, 2009)
Example 1. Depositional patterns that are not correlated with spatial change in the long profile. Canyon Axis ( 2 ) Unconfined Slope ( 1 + 3 )/2 600 Water Depth m) 800 1000 1200 2 km Gradient (m/m) 1 2 3 0.1 Downslope 0.05 0 Thickness (m) Deposit 200 100 0 0 2 4 6 8 10 0 2 4 6 8 10 Downslope Distance (km) (Straub & Mohrig, 2009)
Example 2. Channel S. China deposition that is not Sea 1200 m correlated with spatial change in the long Borneo profile. Channel Channel A B A B 200 m Champion Delta (Straub, Mohrig, & Pirmez, 2012)
Example 2. Channel deposition that is not correlated with spatial change in the long profile. Channel head Channel head (Straub, Mohrig, & Pirmez, 2012)
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