Controls of Mechanisms of Salt Diapirism: Experimental Modeling and Natural Examples
Salt diapirs are formed by a combination of passive, active, and reactive mechanisms. Analog modeling shows that the shapes of the diapirs and their flow rates are controlled by the rate of sedimentation, the sediment load, which is dependent on the column height and density, and the thickness of the source layer. Low rates of sedimentation under an optimum sedimentary load result in cylindrical to flared shapes culminating in emergent sheets, developed primarily through passive mechanisms, whereas high rates result in tapered shapes and their eclipse and eventual occlusion. The thickness of the salt source layer controls both the rate of the salt flow and the dimensions of the diapir. A thin source layer results in a narrow diapir which flows at a lower rate and is eventually eclipsed, possibly prior to complete salt evacuation. On the other hand, a thick source layer results in a higher rate of flow, and also results in a wider flared diapir. Variable rates of sedimentation result in changes in diapir shape over time. An initially slow sedimentation 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 results in a decrease in the surface area of the salt/sediment interface, and results in a possible transition from a passive to active mechanism. Continued salt movement requires an optimum balance between the total load and the rate of sedimentation at all times. Although passive diapirism may be the prevalent mechanism for most of the history, active (or reactive) mechanisms play a key role at two main stages: the initial transition from a buried pillows to an emergent diapir, and the continued movement of a mature diapir after its eclipse due to high sedimentation rates. The formation of radial faults is one of the key features suggesting an active mechanism in natural salt diapirs. An understanding of the controls of these mechanisms can be used to improve interpretation of salt-related structures and related traps in areas with poor data quality. A variety of shapes observed in the East Texas basin can be explained due to the factors discussed above. Variations in sedimentation rates are primarily responsible for the geometry of the Bethel and Oakwood domes, whereas the Butler and Palestine domes, also involve the possible effects of a depleted source layer.
AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015