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.