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Geologic and Microstructural Evidence of Differential Salt Movement at Weeks Island Salt Dome, Iberia Parish, Louisiana
An improved understanding of the tectonic behavior of salt can be obtained when microstructural data from the salt are used in conjunction with other geologic information including isopach/isochore maps of the sediments surrounding the salt. Weeks Island is an active Gulf Coast salt diapir where sedimentation was often influenced by contemporaneous faulting and episodes of near surface salt movement. Isopach/isochore mapping can be used to analyze the interaction of the salt mass with the surrounding sediments, giving important clues about the growth history of the salt structure.
A series of isopach/isochore maps constructed for the shallow flank sediments down to the middle-Miocene "Tex W" indicates that the Weeks Island salt diapir has grown episodically during approximately the last 11.5 million years as two major salt spine complexes.
Subgrain formation dominates microstructural development during steady-state flow of salt and it has been shown by Carter, et al. (1993) that subgrain diameter is inversely proportional to stress alone where:
D(µm) = 214 -1.15 (MPa)
Estimated differential stress levels recorded by subgrain size in the salt at Weeks Island vary from 0.87 to 2.33 MPa. Comparison of the microstructure data with other geologic information indicates that geologic significance can be attached to this variation in stress. At Weeks Island, the more actively moving salt is interpreted to be indicated by higher stress levels and smaller subgrains, which are generally associated with higher surface topographic relief and fluid-rich, coarse-grained, recrystallized salt.
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