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Spatial Variability in the Velocity and Density Structure of Unconfined Turbidity Currents, and the Implications for Reservoir Quality of Submarine Lobe Deposits


Submarine fans represent some of the largest sedimentary bodies on the planet, composed of a series of amalgamated and stacked lobate deposits that result from a rapid loss of flow confinement near the slope to basin floor transition. Numerous outcrop and seismic studies have demonstrated that basin-floor sedimentation is initially constrained to a small portion of the fan that eventually forms a lobe, after which the flow avulses and depositional locus abruptly shifts to a previously inactive region before repeating the process. This lobe switching takes many forms (prograding/retrograding/lateral shifting), resulting in complex stacking styles over a range of spatial scales, highlighting the complexity of the processes governing the morphodynamics of submarine fans. Unlike canyons and leveed channels, currents over lobes lack pronounced channel margins to steer the flow's direction. Accordingly, the rate of lateral flow switching needed for compensational stacking is controlled by bed topography, which is dictated by the velocity and density structure of the current. Indeed, unconfined flows are envisioned as sediment clouds that are characterized by longitudinally oriented filaments of alternating high and low velocity (and shear stress) across the current. These filaments are used to explain subtle lateral variations in topographic expression of sandstone cleanliness in lobe deposits. However, this type of velocity structure has not been documented in sediment gravity currents, and it is unclear how it may alter span wise variations in the current's density structure, which is increasingly recognized as a key parameter controlling depositional characteristics. The present study details the velocity and density structure of experimental sediment gravity currents across a slope to basin floor transition, which mimics the loss of lateral confinement associated with switch between channelized and unchannelized deposits. The flow's velocity and density characteristics are captured using an array of ultrasonic Doppler velocimeters (UDV's) spanning the current (spatial resolution ~0.1 m), allowing for the simultaneous acquisition of velocity and, crucially, density profiles. Results indicate substantial lateral variations in both these parameters, and help to explain lateral variations in deposit thickness and grainsize sorting in lobe deposits.