Ramps, Flats, and Rubble Zones: Case Studies of Deformation beneath Allochthonous Salt in the Flinders Ranges, South Australia

Abstract

<p>The strata adjacent to salt bodies (e.g., diapirs, sheets) serve as significant traps for hydrocarbons in numerous basins throughout the world.  The viability of these traps depends on the hydrological properties of the salt-sediment interface as well as the rocks within 200-300 m of that interface.  Although a variety of studies have described shear zones, rubble zones, gouge zones, drag zones and brecciated zones in rocks adjacent to salt, the exact nature and origin of these zones remains unclear.  Do these zones represent halokinetic deformation or slumps and soft-sediment deformation of suprasalt carapace?  Do their hydrological properties vary with structural position (e.g., subsalt ramps or flats) or other variables (e.g., mudrocks vs. carbonates) that are easily identified and risked?  A limited number of drill data are available to address these questions and because these zones typically occur less than 300 m from the salt-sediment interface, they are rarely amenable to seismic investigation. To resolve this data gap, we use field studies of allochthonous salt exposed in the Flinders Ranges of South Australia, a north-south trending foldbelt in the Adelaide Geosycline. The Neoproterozoic strata and evaporites that make up the Flinders Ranges were deposited during the breakup of the Rodinian supercontinent and later subjected to thin- and thick-skinned deformation during the Delamerian orogeny. The strata around many of the salt structures in this region hosts scapolite, suggesting a metasedimentary environment in excess of 250°C. Uplifted strata and salt structures are tilted to expose an oblique, cross-sectional view of both suprasalt and subsalt strata. For this study, we analyze the spatial variability of deformation beneath an allochthonous salt sheet exposed at a site called Tourmaline Hill, specifically looking at the differences between ramps and flats, and the presence (or lack thereof) of a rubble zone. We use high-resolution sUAS (i.e., drone) imagery to facilitate mesoscopic structural analysis and characterization of fracture orientation, style, timing, mineralization and abundance of features too large to photograph on the ground, but too small to be seen in satellite imagery. Detailed drone images are used to characterize deformation along transects perpendicular to the salt-sediment interface to approximately 200 m away in both the subsalt and suprasalt strata. Fractures are generally nonsystematic and abundant near the salt contact and become systematic and less abundant with distance away from salt. We find there is a change in fracture orientation between suprasalt and subsalt strata. Subsalt ramps feature decameter scale folding with halokinetic growth strata and abundant mineralized fractures suggesting fluid migration (accumulation?), whereas subsalt flats feature strata-bound, decimeter scale folding, suggesting soft sediment deformation of slumped carapace with little to no mineralized fractures. Rubble zones are not always present beneath salt in these field locations, but the style of deformation may be linked to the angle of the salt base.</p>