Tectonic Controls from Turbidity Current Deposition
Tectonic activity creates sub-marine topography that influences the routing of turbidity currents, and sites of localized sediment deposition. In both extensional and compressional settings patterns of syn-kinematic sediment deposition and erosion are determined by the surface expression of the tectonic activity and flow parameters such as velocity and height. In extensional regimes turbidity currents flow down fault scarps and accelerate leading to increased erosion, sediment bypass and extended transport. Extensional fault scarps can also produce barriers to flow, depending on the relative angle and direction of flow to the fault scarp. Emerging toe thrusts, i.e. those developing in the compressional zones of deltaic systems, create barriers to turbidity currents from the continental shelf.
This study uses numerical models to determine the importance of tectonic activity on controlling and changing depositional and erosional patterns. Models of flows on simplified bathymetries show channels initially cutting through emerging thrusts and depositing levee sands. As the surface expression of the thrusting increases, erosion may be insufficient to cut through the barrier. Later flows begin diverting laterally and the depositional pattern changes. Sediment concentration in the flow is a dominant influence on when this switch occurs. Flows with lower sediment concentrations produce more pronounced levee growth and channel aggradation. They are also able to flow over thrust ‘barriers’ of greater height. Applying these observations to an extensional setting in the North Sea, we have run numerical models on restored palaeobathymetries to recreate sediment deposition patterns. The deposition of high quality reservoir sands can be shown to result from flows decelerating across ridges above tilted blocks. Wells further from the ridges do not share these reservoir quality horizons. A combination of different flow input points and parameters have been used to test alternative flow paths in order to recreate depositional patterns that match the wells.
Tectonic activity can be shown to exert a considerable influence on flow path and sediment deposition. How turbidity currents interact with obstacles is controlled by a variety of flow parameters which in nature would be unique for each flow, but are within definable limits. Numerical modelling allows these parameters to be tested to provide a better understanding of the interactions and dominant processes involved.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009