--> Response of Unconfined Turbidity Currents to Complex Bathymetry in Deepwater Fold and Thrust Belts

AAPG ACE 2018

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Response of Unconfined Turbidity Currents to Complex Bathymetry in Deepwater Fold and Thrust Belts

Abstract

Diverse seafloor bathymetry observed within deepwater fold and thrust belts can have a major effect on the behaviour of turbidity currents and resulting deepwater depositional systems, causing complex reservoir geometry and heterogeneity. We use 3D numerical models of turbidity currents and structural deformation to study unconfined turbidity current behaviour on seafloor bathymetry typical for deepwater fold and thrust belts. Parameters of both hydraulic and topographic domains are varied based on a range of natural case studies.

Results show unconfined turbidity currents being confined and reflected, either partially or completely, by the fold-controlled seafloor bathymetry. The reflected flow rejuvenates the turbidity current, and can take many forms based on the current velocity, flow density and dip of the upslope fold limb. Folds with significant seafloor relief and along-strike length create a thick low-density reflected reverse overflow immediately after the turbidity current encounters the fold. Coinciding alongside this flow reversal, a thinner high-density back-squeezed reverse underflow re-feeds sediment to the parental turbidity current, increasing its duration and run-out distance. Sand is deposited upslope of the fold, as well as along the gently-dipping upslope limb. Folds with lower seafloor relief and a smaller along-strike length also generate a thick reflected reverse overflow but develop a thin back-squeezed reverse underflow that forms only after the flow-feed is discontinued. Sand is deposited upslope of the fold and to a minor extent along the steeply-dipping downslope limb, but preferably deposits along the upslope limb. Folds segmented along strike also display similar sediment depozones but trap a reduced amount of sediment due to flow bypass. Therefore, sand distribution in fold and thrust belts is largely controlled by the surface geometry of folds and high net growth tend to occur along large folds. These findings complement flume tank studies and deepwater seismic geomorphological studies, and provide new insights into the effect folds have on the flow hydraulics and potential reservoir architectures of unconfined turbidity currents.