Fischer, Mark P.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
This thesis aims to establish and rank the variables that affect the hydrologic behavior of the Willouran Salt Weld, a tertiary salt weld in the Willouran Ranges of South Australia. To address this question I have undertaken three research tasks: 1) determine the composition, texture, kinematics and timing of vein and host rock cements; 2) characterize the fracture network near the weld and create a 1:10,000 scale geologic map of the study area; 3) collect geochemical data constraining the temperature and composition of fluids from which vein and host rock cements precipitated. Structural data indicate a complex evolution of the Willouran Salt Weld. Base of salt geometries show that the Willouran salt canopy developed from the suturing of the Kingston and Breaden Hill Salt Sheets. Complete welding occurred in the southern portion of the structure and correlates well with the distribution of the Tindelpina Shale. Elsewhere, salt evacuation stalled prior to welding, leaving a significant amount of remnant diapiric breccia. Mesoscopic deformation includes a localized zone of deformation that is localized beneath a base of salt ramp, which is potentially related to salt movement. Also, fracture orientations differ substantially on opposite sides of the weld when significant remnant evaporite is present, which suggests that the Callanna Group acted as a mechanical detachment layer. Petrographic, fluid inclusion, and stable isotope data indicate a salt-℗Ưrelated fluid system with the following characteristics: quartz vein cements precipitated from saline, H₂O-CO₂-CH₄-N₂-rich, metamorphic fluid with temperatures likely >200°C and dolomite vein cements precipitated from a saline sedimentary brine or metamorphic fluid with temperatures likely >100°C. Quartz cement precipitated first, followed by later dolomite. Stable isotope data indicate that there was no significant difference in the composition of quartz-rich fluids with structural position. However, there is a significant difference in the isotopic composition of dolomitic fluids with structural position. These results suggest that the weld did not act as a hydrologic barrier early in its history but did act as a barrier later in its history, perhaps in response to mineralization that occluded fracture porosity and permeability or a decrease in fluid pressure in subsalt strata; however, the existence of a regional fluid system may limit our ability to interpret the behavior of the structure early in its history.
Williams, Nicholas James, "Structural evolution and paleohydrology of a tertiary salt weld, Willouran Ranges, South Australia" (2017). Graduate Research Theses & Dissertations. 5078.
viii, 160 pages
Northern Illinois University
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