FRACTURE-CONTROLLED PALEOHYDROLOGY OF AN ALLOCHTHONOUS SALT WELD, WILLOURAN RANGES, SOUTH AUSTRALIA

The Willouran Ranges expose Neoproterozoic minibasins affected by Callanna Group halokinesis. The Burra and Delusion minibasins represent a delicate balance between sediment supply and passive rise of salt bodies. When the rate of salt rise exceeded the rate of sedimentation, salt breakout occurred, resulting in the formation of an allocthonous salt sheet above the Burra basin. Further deposition caused the evacuation of this salt sheet, forming an approximately 7 km long, complete to incomplete, tertiary weld. The region subsequently underwent contraction during the Delamerian Orogeny, further altering salt geometries.

This research aims to establish and rank the variables that affect the hydrologic behavior of a tertiary salt weld. To address this question we 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.

Preliminary petrographic, fluid inclusion, stable isotopic, and fracture network data indicate a salt-related fluid system with the following characteristics. Vein cements precipitated from saline, H₂O-CO₂-CH₄-N₂ rich, Delamerian orogenic fluids with temperatures >150˚C. Quartz cement precipitated first, followed by dolomite. Veins with apertures up to 2 m and lengths exceeding 10 m are common above the weld, and rare below it. Fracture orientations differ substantially on opposite sides of the weld. These results suggest that strata on opposite sides of the weld experienced different deformational histories, and that the weld served as a persistent hydrological barrier from the time of salt evacuation through the Delamerian orogeny, a period lasting as long as 300 million years.