H41F – 0838: Constructional canyons built by sheet-like turbidity currents: Observations from offshore Brunei DarussalamK.M. Straub, St. Anthony Falls Laboratory, University of Minnesota [kmstraub@umn.edu]D. Mohrig, University of Texas at Austin [mohrig@mail.utexas.edu]

Support for our research was provided by Brunei Shell Petroleum and Shell International Exploration and Production Inc.

Additional funding provided by the National Center for Earth-Surface Dynamics, an NSF Science and Technology Center

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.

200m water depth

1200m water depth

30 km

Borneo

S. ChinaSea

StudyRegion

Brunei

Horizontal Data Resolution = 25 m by 25 mVertical Resolution ~ 5 m

00N, 1180E00N, 1080E

140N, 1080E

What are the topographic signatures of depositional and erosional environments in the deep marine?

Are all canyon systems net erosional?

1100 m

300 m

Shelf-edge

Regional Morphology4 km 4 km~100 m ~100 mSR 0

SR 1SR 2

SR 3SR 4A A’

B B’

B B’A A’

Boundary of Swath Profile

Swath Profiles

Surface SR0Surface SR0 Surface SR4Surface SR4

SR4 – SR2SR4 – SR2 SR2 – SR0SR2 – SR0

SR4 – SR0SR4 – SR0 Gradient Surface SR0Gradient Surface SR0

Seismic Data

Map Trends

Canyon 1Canyon 2Canyon 3

Focused Uplift

Focused Uplift

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

•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

Long Profiles

Google Earth

Local Swath Profiles

Evolution of Canyon Relief

Evidence for Sheet-Flow Turbidity Currents

1) Laterally extensive stratigraphy that thins over anticline suggest deposition from sheet flow turbidity currents

2) High gradients are correlated with decreased sedimentation in studied submarine canyons

3) Deposition on non-channelized surfaces is inversely correlated with local canyon relief

4) Constructional submarine canyons are the products of higher rates of sedimentation on non-channelized areas compared to adjacent channels

Conclusions

Sheet FlowsHorizons 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.

Upstream migratingKnickpoint

High gradients on downslope Anticline limb

High gradients on downslope Anticline limb

Canyon Axis: Gradient Deposit Thickness

Overbank: Gradient Deposit Thickness

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=

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.

Terrestrial & Marine Terrestrial System Brunei System

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 b → Less sediment deposition by channelized currents

•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