Turbidite Sedimentation along Topographically Complex Slopes in Convergent Margins: The Example of the Makran Accretionary Prism (Gulf of Oman, NW Indian Ocean)
Quaternary sediment distribution of the Makran turbidite system (Makran subduction zone, NW Indian Ocean) is investigated through a nearly complete (~ 162,000 km2) reflectivity and bathymetry mapping of the Oman basin, a large set of coring data, and 2D seismic recovered during several cruises leaded by the IFP and SHOM institutes.
The Makran continental slope is composed by numerous accreted ridges associated to intra-slope, piggy-back basins. Our results show that the mechanisms of sediment dispersal in the Makran turbidite system are mostly influenced by the pre-existing and evolving sea floor topography, that vary along-strike with the tectonic regime and the magnitude of sediment flux. Abrupt changes of gradients (up to 20°) along thrust-induced knickpoints generate hydraulic jumps that enhance preferential deposition of the coarse-grained load of the flows, substantial erosion, and by-pass of the finer-grained component. At the deformation front, these processes can lead to the formation of base-of-slope plunge pools that potentially form wide sand reservoirs. Rapid deposition of the coarser material downstream of the hydraulic jump lead to a strong segregation of the sediments, as only the uppermost, finer-grained component of the turbidity flows is transferred to the abyssal plain. Wide plunge pools develop at areas where the sediment flux (i.e. erosion rates) is too low to compensate the deformation rates, leading to non-equilibrium conditions. Inversely, equilibrium conditions (resulting from higher sediment flux and a lesser tectonic control) promote channel formation at the canyon mouths and transfer of coarse-grained material to the trench. Hence, channel formation and trench fill characteristics depend of the equilibrium conditions of the feeder system, related to the balance between erosion rates (i.e. fluvial input, initial sediment load, and slope gradients) and deformation rates. Preliminary simulations with the process-based model CATS (Salles et al., 2007) were performed in order to simulate the turbidity flow paths along the thrust-induced complex slope.
Our results from the Makran turbidite system provide another evidence of how complex submarine slopes may interplay with the sedimentary processes and control the reservoir architecture. It constitutes an excellent modern analogue to deep water sedimentary systems with structurally complex depocenters, in the context of convergent margins.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009