Erosional%20Development%20of%20Submarine%20Canyons%20in%20Continental%20Slopes

1
Morphologic modelling of continental slopes Neil
C. Mitchell Cardiff University
Canyon erosion Diffusive topography
USA Atlantic coast
Southern Santa Cruz Mountains, CA
2
Tributaries converge smoothly in elevation at
confluences (Playfairs Law) - implies erosion
by many erosive flows on adjacent channels.
Mitchell, Am. J. Science 04.
3
Channels are concave upwards. Inverse relation
between channel gradient and contributing area,
with similar graph slopes to some bedrock-eroding
rivers.
In bedrock-eroding rivers, concavity arises from
discharge compensated by gradient. How does
concavity arise in submarine canyons?
Taiwanese rivers (Whipple)
Slope-confined linear canyons
Mitchell, Marine Geol 05.
4
Flows originate from slope failures in canyon
walls.Frequency size of sedimentary flows
increases downslope, so time-averaged erosion
rate increases with canyon area.
Interpretation
a
Plan
Profile
Mitchell, Am. J. Science 04.
5
Slope-heading and shelf-heading canyons have
similar concavity
-gt During low-stand conditions, all canyons
supplied by hemipelagic material. (Otherwise,
shelf-edge supply only would have eroded back
shelf-heading canyons differently.)
6
Knickpoints in tectonically active slopes
anticline
channel
Knickpoint diffusion (S. Barbados prism).
Transport-limited erosion?
Knickpoint migration (Alaska).
Detachment-limited erosion?
Mitchell, Bull GSA, 2006.
7
Diffusive-like terrain ?H/?t K ?2H/?x2?
USA Atlantic
Diffusive
Threshold
Mitchell Huthnance, in review.
8
Origin of diffusion?
?H/?t K ?2H/?x2 Derived by combining Sediment
transport flux proportional to gradient Q -K
?H/?x with ?H/?t -?Q/?x (continuity) Effect
of gravity on bedload produces down-slope flux
proportional to gradient. Kltugt2 to Kltugt3
Flume experiments
Mitchell Huthnance, in review.
9
Atlantic USA
Upper slope - a sandy clinoform
rollover. Predictable from how currents decline
with depth and bedload models.
Forward simulation
Gradient
Mitchell Huthnance, in review.