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Controls of the Geometry and Evolution of Salt Diapirs: Experimental Models and Applications

Abstract

Analog modeling of salt diapirs was conducted to understand the controls of diapirs shapes and flow rates using garnet and silica sand to simulate sediments and silicone gel to simulate salt. Results show that the key factors are the rate of sedimentation, the sediment load, which is dependent on the column height and the sediment density, and the thickness of the salt layer. Sedimentation rate plays a dual role in the movement of salt. It provides the load, which is the main driving force for the salt movement, but can also restrain the movement of salt. Low rates of sedimentation result in the formation of a cylindrical diapir that eventually develops a flared shape, whereas a high rate of sedimentation results in eclipse and occlusion of the diapir after initial movement. Sediments with low density require a lower rate of sedimentation to compensate for the decreased load for the diapir to grow. Continued salt movement requires an optimum balance between the total load and the rate of sedimentation at all times. Variable rates of sedimentation result in changes in diapir shape over time. An initially slow rate may result in cylindrical and flared shapes of increasing diameter. An increase in the sedimentation rate may result in initial tapering followed by eclipse of the diapir, or continued movement after reduction in the diapir diameter. Tapering enables an increase in the salt velocity by decreasing its surface area, and results in a possible transition from passive to active diapirism. The thickness of the salt source controls both the rate of the salt flow and the dimensions of the diapir. A thick source layer results in a higher rate of flow, and also results in a wider diapir and flare, whereas a thin source layer results in a narrower diapir which flows at a lower rate and is eventually eclipsed. The results provide a guideline to better understand the evolutionary history of diapirs which can be used for analyzing natural structures. Features observed in natural salt diapirs show similarities to the experimental results, so that their growth histories can be predicted by studying the sediment shapes. Investigation in the East Texas Basin using seismic data and well tops suggest that the evolution of the Butler, Palestine and Oakwood domes is related to the nature and rate of sediments deposited, as suggested by the experimental models.