S. China Sea. Study Region. Brunei. Borneo. H41F – 0838: Constructional canyons built by sheet-like turbidity currents: Observations from offshore Brunei Darussalam. K.M. Straub, St. Anthony Falls Laboratory, University of Minnesota [[email protected]]
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H41F – 0838: Constructional canyons built by sheet-like turbidity currents: Observations from offshore Brunei Darussalam
K.M. Straub, St. Anthony Falls Laboratory, University of Minnesota [[email protected]]
D. Mohrig, University of Texas at Austin [[email protected]]
Submarine canyon formation and deepening are typically attributed to erosional processes. We present data from an industry-grade seismic volume located offshore Brunei Darussalam illustrating how topography typically associated with erosional processes can be produced under conditions of net sediment deposition. This data was generated via subsurface mapping in the vicinity of a shale-cored anticline on the Quaternary continental-slope. Three canyons traverse the structure at right-angles to the crest line with maximum canyon relief of 165 m. Subsurface mapping reveals that the structure is a site of net sediment deposition and defines a background sedimentation pattern that decreases gradually with distance from the shelf-edge. Profiles down canyon axes reveal local minima in deposit thickness over the anticline hinge that are associated with high downstream gradients. Deposition on ridges adjacent to canyons also displays local minima at the anticline hinge, but these minima are not correlated with gradient. A comparison of canyon axis and ridge deposition shows that somewhat higher rates of sedimentation on the ridges resulted in the preservation and growth of the submarine canyons with time. Laterally persistent seismic reflectors and depositional packages suggest that the canyon forming currents were sheet-like flows, extending for many kilometers in the strike direction. The currents drained into canyons as they approached the anticline hinge, leaving only a small supra-canyon fraction available to deposit sediment on the non-channelized zones.
What are the topographic signatures of depositional and erosional environments in the deep marine?
Are all canyon systems net erosional?
1200m water depth
200m water depth
Horizontal Data Resolution = 25 m by 25 m
Vertical Resolution ~ 5 m
Boundary of Swath Profile
SR2 – SR0
SR4 – SR2
Gradient Surface SR0
SR4 – SR0
Additional funding provided by the National Center for Earth-Surface Dynamics, an NSF Science and Technology Center
Support for our research was provided by Brunei Shell Petroleum and Shell International Exploration and Production Inc.
Local Swath Profiles
Canyons traversing the anticline are preserved and grow due to higher rates of sedimentation on non-channelized areas compared to adjacent lows.
Greater relief → Greater Current Height → Greater tb → Less sediment deposition by channelized currents
High gradients on downslope
Terrestrial & Marine
In the terrestrial environment the high density of the transporting fluid, water, relative to the ambient fluid, air, results in flows that are more strongly affected by and confined to local topography when compared against turbidity currents. As a result, terrestrial overbank environments, in regions of relative uplift and canyon formation, are seldom inundated by sediment depositing flows and increases in relief require focused erosion within canyons. In contrast, the low excess density of turbidity currents allows a significant quantity of a significantly large flow to traverse over inter-canyon highs without being funneled into canyons, thereby supplying overbank regions with sediment to counter deposition occurring within canyons.
Evidence for Sheet-Flow Turbidity Currents
Horizons can be traced in the strike direction for 10’s of km and stratigraphy thickens away from axis of anticline, suggesting deposition by laterally extensive sheet flow currents
How Thick were the Sheet Flows?
A minimum estimate for turbidity current thickness in unchannelized regions was calculated using the cross-sectional area of confined flow over the anticline crest. This measurment assumes the current was 10m thicker than the ridge crest of the inter-channel regions. Using this area a minimum current thickness was calculated for the unconfined regions of 30 m.
High gradients on downslope
Canyon Axis: Gradient Deposit
Overbank: Gradient Deposit