The Orinoco turbidite system: Tectonic controls on sea-floor morphology and sedimentation
Résumé
Because of its location in an active margin context, the sand-rich Orinoco turbidite system is controlled morphologically and tectonically by the compressional structures of the Barbados prism, and as a consequence, the sedimentation system does not exhibit a classic fan geometry. The sea-floor geometry between the slope of the front of the Barbados prism and the slope of the Guyana margin induces the convergence of the turbidite channels toward the abyssal plain at the front of the Barbados accretionary prism. Also, whereas in most passive margins the turbidite systems are commonly organized upstream to downstream as canyon, then channel levee, then lobes, here, because of the control by active tectonics, the sedimentary system is organized as channel levee, then canyons, then channelized lobes. In shallow water, landward of the prism, the system has multiple sources with several distributaries, and progressively downward, the channel courses are more complex with frequent convergences or divergences that are emphasized by the effects of the undulating sea-floor morphologies. Erosional processes are almost absent in the upper part of the turbidite system shallower than 1500 m (4921 ft). Erosion along channels develops mostly between 2000 and 4000 m (6562 and 13,123 ft) of water depth, above the compressional structures of the Barbados prism. Incisions show irregular meandering and sinuous courses in the low-relief segments and less sinuous courses where channels incise the structures. Larger incisions (canyons) are 3 km (1.9 mi) wide and 300 m (984 ft) deep. The occurrence of different phases of successive incisions is responsible for the development of morphologically correlative terraces in both flanks of the canyons. This might be the consequence of two mechanisms: the tectonic activity of the deformation front characterized by progressive uplift and thrusting of recent sediments, and the superimposition of the cycles of the Orinoco turbidite system. Piston-core surveys have demonstrated that turbidite sediments moving through the channel and canyon system and deposited in the abyssal plain are mostly coarse sandy deposits covered by recent pelagic planktonic-rich sedimentation, which indicates that sand deposition slowed down during the postglacial sea level rise.