This study combines field structural analysis with thin‐section petrography, U‐Pb dating, and strontium, carbon and oxygen isotopic analysis of calcite fracture fills to constrain the evolution of the 2‐5 km scale paleofluid system around the faulted, plunging fold nose comprising the southern termination of the Gypsum Valley salt wall in the Paradox Basin, U.S.A. Brittle deformation in this region began with the formation of a down‐to‐the‐northeast, counter‐regional fault and then progressed into jointing and faulting in a radial pattern, followed by jointing in a concentric pattern. Coupled with increases in fracture abundance toward the faults, multiple stages of mineralization suggest that the faults served as efficient and long‐lived conduits for vertical fluid migration. Although fracture cement textures and calcite colour are variable throughout the area, the distribution of these characteristics does not correlate with fracture orientation, relative age, stratigraphic or structural position. Irrespective of the type of calcite comprising the fracture cements, δ13C values average near −7‰ (VPDB), whereas δ18O values cluster into groups whose averages are roughly 6‰ apart, with the more negative grouping stratigraphically restricted to fracture cements in Jurassic rocks. The stratigraphic segregation of δ18O values suggests the paleofluid system contained two distinct paleofluids, a more recent one comprised of meteoric waters and an older one comprising brine that originated in Pennsylvanian strata. 87Sr/86Sr ratios in fracture‐filling calcite cements indicate that the older fluid underwent fluid‐rock interaction with Permian strata and that this evolved fluid migrated upwards along the faults until the Triassic or Jurassic. Thereafter, fluid migrating along the faults was more meteoric and appears to have migrated downward along the faults, where it interacted with Permian strata. Consistent U‐Pb dates from carbonates precipitated from the older fluid suggest this stage of the paleofluid system was active around 240 Ma. Local burial history models and published temperatures for fracture cements elsewhere in the basin suggest the younger stage of the paleofluid system occurred during the Latest Cretaceous to Oligocene. This study highlights the spatial and temporal complexity of fluid systems in the vicinity of salt structures and emphasises the need to interpret them through careful integration of high resolution stratigraphic and structural data in the context of evolving salt tectonics.

<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>

<p>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.</p>

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