The Influence of Relay Ramps from Syntectonic Turbidite Sedimentation: From Experimental Study Combining Analog Experiments and Numerical Flow Simulations

Athmer, Wiebke ¹; Groenenberg, Remco M.¹; Luthi, Stefan M.¹; Donselaar, Marinus E.¹; Sokoutis, Dimitrios ²; Willingshofer, Ernst ^2
1 Department of Geotechnology, Delft University of Technology, Delft, Netherlands.
² Department of Tectonics, VU University Amsterdam, Amsterdam, Netherlands.

The interaction of faults in rift settings is considered one of the major tectonic factors controlling the deposition and distribution of syn-rift sediments. In particular relay ramps are thought to influence the transport of sediments into the basin, e.g. as pathways in deltaic or fluvial systems such as reported in the East African Rift. The impact of relay ramps on submarine turbidity currents, however, is only poorly understood, but of great relevance as turbidite systems are a main exploration target at many passive continental margins.

The study combines physical scale experiments with numerical models in order to analyze the effect of relay ramps on turbidite flow and deposition. The analog experiments are performed in a sandbox using a brittle-ductile setup with sand and silicone, underlain by a plastic sheet which is steadily pulled during the experiment. As soon as a relay ramp develops, the laboratory experiment is stopped and the topography photographed and digitally scanned with a 3D-video laser. The scan is then transformed into a digital elevation model and used as initial topography in a numerical model in which turbidity currents are simulated under an oblique inflow direction to the relay ramp. In order to mimic syn-tectonic sedimentation the deposits obtained from the numerical simulation are physically added to the sandbox model before resumed extension. This procedure is repeated several times to test the influence of relay ramps on sedimentation during different stages of rifting. After the end of the laboratory experiment serial cuts are taken to visualize the internal structure of the syn-tectonic deposits.

Relay ramps are found to behave only partly as pathway for turbidity currents if the flow enters the ramp obliquely. Most of the sediment, however, is shed above the inner normal fault directly into the basin. On the upper part of the relay ramp some deposition of sediment can take place and result in the creation of small levees. Consequently, a shift in the sediment path can occur during the next turbidite event and, possibly, leads to an enhanced funneling of turbidity currents on the relay ramp. But the generation of channels on relay ramps with depths comparable to fluvial or deltaic settings, in which relay ramps are found to behave as pathway for sedimentary flows, is less likely in deep-marine rift basins and hence the influence of relay ramps as pathway for turbidity currents is regarded as minor.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009