Constructional canyons built by sheet-like turbidity

lameubiquityMécanique

21 févr. 2014 (il y a 3 années et 4 mois)

75 vue(s)

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
]

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. China

Sea

Study

Region

Brunei

Horizontal Data Resolution
= 25 m by 25 m

Vertical Resolution ~ 5 m

0
0
N, 118
0
E

0
0
N, 108
0
E

14
0
N, 108
0
E

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

Are all canyon systems net
erosional?

1100 m

300 m

Regional Morphology

4 km

4 km

~100 m

~100 m

SR 0

SR 1

SR 2

SR 3

SR 4

A

A’

B

B’

B

B’

A

A’

Boundary of
Swath Profile

Swath Profiles

Surface SR0

Surface SR4

SR4


SR2

SR2


SR0

SR4


SR0

Gradient Surface SR0

Seismic Data

Map Trends

Canyon 1

Canyon 2

Canyon 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 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.

Upstream migrating

Knickpoint

High gradients on downslope

Anticline limb

High gradients on downslope

Anticline limb

Canyon Axis: Gradient Deposit



Thickness

Overbank: Gradient Deposit



Thickness

=

=

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
t
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