H41F – 0838: Constructional canyons built by sheet-like turbidity currents: Observations from offshore Brunei Darussala

1. 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 [kmstraub@umn.edu] D. Mohrig, University of Texas at Austin [mohrig@mail.utexas.edu] 140N, 1080E 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? 00N, 1180E 00N, 1080E 1200m water depth 200m water depth Horizontal Data Resolution = 25 m by 25 m Vertical Resolution ~ 5 m 30 km Seismic Data Regional Morphology Canyon 1 Canyon 3 Canyon 2 4 km 4 km 1100 m ~100 m ~100 m SR 0 B’ B SR 1 SR 2 SR 3 A’ A SR 4 B A A’ B’ Map Trends Surface SR4 Surface SR0 Shelf-edge 300 m Boundary of Swath Profile Swath Profiles Focused Uplift Focused Uplift SR2 – SR0 SR4 – SR2 • Diapirism of mobile shale formed system of shelf-edge parallel growth structures • A system of canyons dissects growth structures • Highest surface gradients on swath profiles found down slope of growth structure hinge-lines Gradient Surface SR0 SR4 – SR0 Observations • Study region is net depositional on intermediate and long time scales • Laterally persistent stratigraphy that thins over anticline • 1st order control on deposition is distance down-slope from shelf-edge • Highest surface gradients are along canyon axes and downslope limb of shale ridges 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.

2. Long Profiles Local Swath Profiles Upstream migrating Knickpoint • Upslope and downslope of growth structure deposit thickness decreases quasi-linearly with distance from shelf-edge • Increase in deposit thickness in region with highest downslope gradient • Deposit thickness between canyons is fairly uniform Evolution of Canyon Relief 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 Anticline limb Terrestrial & Marine Terrestrial System Brunei System 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. Google Earth Evidence for Sheet-Flow Turbidity Currents Sheet Flows 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 Anticline limb Conclusions • Laterally extensive stratigraphy that thins over anticline suggest deposition from sheet flow turbidity currents • High gradients are correlated with decreased sedimentation in studied submarine canyons • Deposition on non-channelized surfaces is inversely correlated with local canyon relief • Constructional submarine canyons are the products of higher rates of sedimentation on non-channelized areas compared to adjacent channels = Canyon Axis: Gradient Deposit Thickness = Overbank: Gradient Deposit Thickness