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Quantifying the Spatio-Temporal Evolution of a Channel Complex from Offshore Angola: Testing the Equilibrium Profile Hypothesis

 

Strachan, Lorna J.1, Chris Leppard2, Rob L. Gawthorpe3 (1) The University of Manchester, Manchester, United Kingdom (2) Norsk Hydro, Bergen, Norway (3) University of Manchester, Manchester, United Kingdom

 

The idea that turbidite slope channels have a propensity towards a graded equilibrium profile has been successfully applied to many submarine channels. The equilibrium profile hypothesis - which predicts that longitudinal thalweg depth profiles will adjust to gravity-base-level through erosion, by-pass and deposition - seems to fit well with observed gross channel characteristics in tectonically quiescent and active settings.

 

This presentation uses subsurface 3D seismic data from offshore Angola to quantify the spatio-temporal evolution of a channel complex from a salt-dominated margin, in an attempt to test the equilibrium profile hypothesis in this setting.

 

Detailed analysis of downslope channel complex trends reveals: 1. an irregularly shaped longitudinal depth profile, characterized by an erosive thalweg trace; 2. a mismatch between channel complex incision and gradient whereby incision reaches a local minimum on the steepest slope segments; 3. a decrease in cross-sectional area downslope; 4. an increase in width downslope; and 5. a decrease in thickness downslope, with local perturbations close to intra-slope lows.

 

Iso-proportional slice attribute maps taken through the channel complex reveal rapidly changing planform shapes, characterized by multiple, sinuous thalwegs and cut-offs interpreted as resulting from avulsion, re-incision and abandonment.

 

These characteristics suggest that the channel complex was in a state of disequilibrium throughout its existence. A spatio-temporal evolutionary model for the channel complex has been developed reflecting this state. We conclude that variations in turbidity current flow properties and gravity-base-level varied more rapidly with time than the depositional-erosional equilibrium response resulting in a near constant state of disequilibrium.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California