AAPG Foundation 2019 Grants-in-Aid Projects

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Contrasting Styles of Deformation Beneath Allochthonous Salt: Case Studies from the Flinders Ranges of South Australia


Risking traps near salt often depends on resolving structural geometry and deformation in areas within 300 m of the salt-sediment interface. This region can also be the site of significant overpressure, creating drilling hazards that are challenging to predict and mitigate. Despite improvements in seismic processing, the near-salt region continues to be difficult to image, making field studies of exposed salt structures exceedingly important to fill this data gap. As a salt body forms, its shape and the architecture of adjacent sedimentary rocks depend on the rates of sedimentation and salt advance, leading to a variety of salt body geometries (e.g., diapirs, allochthonous sheets). Beneath allochthonous salt sheets, bedding-parallel segments of the salt-sediment interface (flats) record times when the salt advance rate is greater than the sedimentation rate, whereas segments that cut steeply upward (ramps) record times when sedimentation rate is greater than salt advance rate. As hypothesized by Williams et al. (2019), deformation near allochthonous salt sheets should vary with structural position (supra- vs. subsalt and ramps vs. flats) and with the time when the deformation occurred. This study will test this hypothesis and the predictions of numerical models (e.g., Nikolinakou et al., 2017; Li and Fischer, unpublished) by documenting deformation patterns beneath allochthonous salt. This study will examine two sites (Tourmaline Hill and Loch Ness; Rowan et al., 2019) in the Flinders Ranges of South Australia, where Neoproterozoic allochthonous salt is exposed in both ramp and flat structural positions. Regional tilting of strata during the Delamerian orogeny provides oblique, cross sectional views of both supra- and subsalt strata. Field and laboratory studies of these regions will be used to document near-salt deformation patterns and correlate these patterns to map-scale structural geometries and salt tectonic styles. We will complete four specific tasks to achieve this goal. (1) Create 1:6,000 scale maps of well-exposed ramps and flats at each area. (2) Conduct detailed mesoscopic structural analysis aimed at characterizing the deformation features in each different structural position. (3) Collect high-resolution sUAS imagery to map deformation features. Features will be traced in georeferenced orthophotomosaics of drone imagery to quantify and analyze deformation using tools in ArcGIS. (4) Collect samples of mineralized fractures and host rocks to be examined in thin section to assess mineralizing paleofluids that may have been involved in subsalt deformation.