Constructional Canyons Built by Sheet-like Turbidity Currents: Observations from Offshore

Formation and deepening of submarine canyons are typically attributed to erosion. We present data from an industry-grade seismic volume located offshore Brunei Darussalam illustrating how topography typically associated with erosion can be produced under conditions of net sediment deposition. These data were generated via subsurface mapping in the vicinity of a shale-cored anticline on the Quaternary continental slope. The shale ridge is located 20 km down slope from the present-day continental shelf edge in , 900 m of water. Its crest line runs for 18 km and is oriented perpendicular to the regional 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 several 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. This interpretation is consistent with minima in ridge deposition being correlated with maximum canyon relief. The currents drained into canyons as they approached the anticline hinge, leaving only a small supra-canyon fraction available to deposit sediment on the nonchannelized zones. We use the cross-sectional area of the confined flow over the anticline crest to estimate a minimum thickness of 20 m for the sheet-like currents as they approached the anticline.