Testing numerical models of subsalt deformation through field observations: Case studies from the Flinders Ranges, South Australia

The viability of hydrocarbon traps beneath allochthonous salt depends in part on the lithology, architecture, and geometry of stratigraphic units near the salt-sediment interface, as well as the hydrological properties of these units. All of these characteristics are intimately associated with the sedimentological and halokinetic processes that operate during salt sheet emplacement.  Key among these processes are the slumping of suprasalt carapace and the deformation of units overridden by the salt.  Although kinematic and conceptual models demonstrate how rates of sedimentation and salt advance work together to influence the geometry of the base salt-sediment interface and the stratigraphic truncations against it, they cannot be used to reliably predict the location, style and extent of subsalt deformation or overridden slumps.  Numerical models that have been used to examine the evolution of salt-sediment systems predict that rocks within 1-2 km of the subsalt-sediment interface should be intensely deformed, but do not incorporate slumping and provide no criteria by which to distinguish between subsalt disturbed zones that were created by halokinetic, ductile shear, and those that were created by slumping or other soft sediment deformation. In this study, we analyze deformation patterns present in the subsalt of three allochthonous salt sheets exposed in the Flinders Ranges of South Australia. Although these structures initiated in the Neoproterozoic, later regional-scale tilting and folding during the Delamerian Orogeny created an oblique, cross-sectional map view that allows for the detailed characterization of near-salt deformation at a scale of meters to hundreds of meters. We use a combination of field mapping and 2-3 cm/pixel resolution drone imagery to conduct mesoscopic structural analysis that characterizes the orientation, dimensions, relative timing, mineralization, spatial distribution, and abundance of deformation features (e.g., joints, veins, cleavage, faults, deformation bands, folds) in the subsalt strata exposed at each field site.  Data were collected along transects that begin at the salt-sediment interface and extend through 50-300 m of subsalt strata.  Two sites are situated in subsalt flats, whereas the third occupies a subsalt ramp.  Deformation beneath the flats appears to correlate to the thickness of the overlying salt sheet.  Where the preserved salt sheet thickness is < 200 m there is little to no mesoscopic deformation.  Where the salt sheet is > 1 km thick, strata are brecciated near the salt-sediment interface, brittle fractures are abundant, and layer-parallel shear zones and mineralized fractures decrease in abundance downward in the stratigraphic section. Deformation at the site with the discordant strata is more diverse and includes meter-scale faults, meter- to decameter-scale folds, abundant brittle fractures and localized brecciation.  These features are typically concentrated within 50 m of the salt-sediment interface and thereafter occur at abundances that are similar to those in strata that are > 100 m away.  Our results suggest that existing numerical models overestimate the amount and stratigraphic extent of deformation beneath allochthonous salt sheets.  Continued field study of near salt deformation will help to constrain future models and provide criteria to distinguish halokinetic and soft sediment deformation.