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Syndepositional and Early-Post-Depositional Changes in Hybrid Flow Deposits


Hybrid or transitional flows deposits have been described in a wide range of deep-marine basins such as the Agadir Basin (offshore Morocco), Britannia Formation in the North Sea and the ultra-deep-water sub-salt Wilcox Group (USA). Hybrid sediment-gravity currents are thought to be responsible for generating “enigmatic” deposits unrepresented in classic deep-water facies models (i.e. Bouma Sequence). Furthermore, hybrid flow deposits often present a wide range of soft-sediment deformation structures indicating that post-depositional alterations play an important role in changing the characteristics of the original deposit. Here we present results from two sets of experiments that illustrate how syndepositional and early-post-depositional changes to hybrid flow deposits are connected to the initial slurry rheology. The first sets of experiments include two-dimensional (0.3 m wide by 5.5 m long flume) and three-dimensional (4 m wide by 8 m long basin) hybrid flows, that were imaged using high resolution photography, laser-distancing probes, and coring. In addition to these techniques, an ultrasonic transceiver was used to visualize the early, post-depositional changes over several minutes following deposition of a hybrid-flow deposit. Using seismic acquisition and imaging techniques, vertical changes of deposit properties can be inferred. In the second set of experiments, initially homogeneous mixtures of sand, clay and water ranging from low- to high-viscosity were placed in a series of 7.5 l fish tanks in order to explore the degree of dewatering and grain size restructuring that occurs through time. The two sets of experiments show that there is a wide range of initial deposit restructuring in the form of dewatering processes that tend to elutriate the finest sediment and drive a creeping post-flow motion to the deposits.