The effect of relay ramps on sediment routes and deposition: A review

A better understanding of the interplay between rift basin evolution and sediment transport paths can improve the success rate of locating hydrocarbon reservoirs at passive rift margins. This paper reviews the current knowledge and suggests future research directions. Relay ramps at extensional basin margins can form drainage entry points if tectonic activity exceeds sedimentation and incision rates, leading to a diversion of sedimentary flow paths towards the ramp. During base level lowstands, channels and canyons may incise into the relay ramp and provide flow paths from the basin margin into the basin. Their orientation and geometry mainly develops as a response to faulting and fracturing, and their activity is influenced by base level fluctuations. Flow constraints such as channels parallel to the ramp axis direct flows to the foot of the relay ramp where sediment accumulates in response to the basin topography. In subaqueous settings, however, turbidity currents are likely to spill at least partly over channel levees and flow down the fault slope into the basin, depositing its load adjacent to the en-echelon boundary faults. Channels and canyons with oblique and perpendicular orientations to the boundary faults can funnel flows down the hanging wall fault onto the basin floor, by-passing the relay ramp. The prevalent basinward tilt of relay ramps can direct unconstrained subaqueous gravity flows also directly into the basin. In subaerial settings, the duration of channel activity in relation to relay ramp evolution strongly depends on the ratio between flow incision rates and tectonic uplift. Drainage direction on the footwall may revert if footwall uplift exceeds incision rates, and the feeding of former depocentres terminates. In the course of rift margin development relay ramp bounding faults may link, causing the breaching of relay ramps and eventually their burial. The effect of continued rifting on ramp remnants and associated syn-rift deposits, however, is not sufficiently known and needs further investigation. (C) 2011 Elsevier B.V. All rights reserved.