Coupling between sedimentation and salt deformation

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2022-08-18

Authors

Liu, Xinggang Christopher

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Abstract

My dissertation focuses on how sediment-transporting flows (e.g., turbidity currents) couple with deformation of earth materials (e.g., rock salt) to produce a morphodynamic system responsible for generating submarine landscapes and particularly minibasins. Turbidity-current sediments deposited on top of salt layers can drive salt flow by applying differential loads. The flow of salt substratum can produce different styles of surface topography, modifying the sediment dispersal systems that fuel the continental margin sedimentation. These linked systems consist of many nonlinearly interacting elements so that their evolutions cannot easily be predicted. In order to better understand this emergent complexity. I designed experiments to allowed turbidity currents, their deposits, and the structural deformation associated with substratum flow to freely coevolve in three dimensions. These experiments were specifically designed to capture the full morphodynamic behavior of submarine systems possessing a mobile substratum. First, I investigated the role of original salt thickness in controlling patterns of subsidence/uplift and sedimentation. Two experiments with different substrate thickness were perturbed by similar loads and pressure-gradients tied to turbidity-current sedimentation. Over thick salt, turbidite loading drove salt flow that formed a classic minibasin. Basin subsidence/uplift were accurately estimated using only turbidite thickness and the principle of isostasy. Sediment trapping efficiency increased as relief of the minibasin grew. In contrast, turbidite loading over thin salt was never isostatically compensated and formed a deposit with positive topography that was segmented by faults. Increasing displacements on these faults produced horsts and grabens that grew over time. Predicting this pattern of salt deformation required using not only local turbidite thickness, but also the gradient and curvature in turbidite thickness, both of which were found to exhort increasing control on salt deformation as the buried salt layer thinned. Second, I investigated how different styles of structural deformation influenced subsequent patterns of sedimentation and then future patterns of subsidence. In the thin-salt case, growth of a reactive diapir and continuous segmentation of the turbidite cover led to development of a network of structural channels that laterally confined flows, guiding basinward transport by subsequent currents. Over thick salt, the continuous subsidence of produced a minibasin that increased in relief with each sedimentation event. Eventually the walls became steep enough to fail and removal of this sediment induced salt breaching. Coalescence of two salt sheets fully encased the minibasin and formed a structural channel along its suture. This structural channel facilitated the bypassing of sediments to a downdip region where a second minibasin began to form. Both styles of structural deformation formed ‘structural’ channel(s) that laterally confined the turbidity currents, increasing the sediment bypassing fraction, extending the field of deformation, and producing new salt-influenced topography at a basinward position. Third, I investigated the effect of spatially varying salt thickness on morphodynamic evolution of submarine basins. In the laboratory, two experiments with different base-salt configurations were designed to isolate the control of variable salt thickness on basin evolution. The first experiment was conducted over a uniform salt layer and the second experiment had a step in salt thickness. Salt flow in both cases was solely driven by turbidity-current sedimentation with the same initial conditions. A simple step in salt thickness generated multiple minibasins with migrating depo-centers that changed depositional patterns over time. The structural and stratigraphic complexity that emerged exhibited properties that are commonly ascribed to gravity gliding over a regionally tilted basement and point out a potential difficulty in separating the signatures of regional structural deformation from internally generated signals. The feedbacks between sedimentation, substratum flow, and evolving topography need to be considered at all times if accurate reconstructions of basin evolution are to be achieved. In sum, my experimental designs incorporated a necessary amount of complexity that produced geologically realistic structural and stratigraphic geometries and captured the dynamic processes governing pattern formation in time and space. My results demonstrate how salt-sedimentation interactions can generate a wide range of structures in absence of regional salt tectonics, requiring only the internal dynamics that define the morphodynamic evolution of submarine basins in salt provinces

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