Soft-sediment deformation structures in eolian sandstones: Morphology, genesis, and stratigraphical relationships through time

Soft-sediment deformation structures (SSDS) are widespread in the eolian sedimentary record, with examples commonly associated with other depositional systems. In such contexts, deformation is typically enhanced by sealing horizons related to interdunes or sabkha deposits. The Late Permian Buena Vista Formation of the Paraná Basin contains extensive intervals with SSDS, but these are restricted to eolian dune deposits lacking dry or wet interdune facies or sealing horizons. The deformed horizons record multiple events, with superimposed structures that allow the reconstruction of deformation episodes at the outcrop scale. These distinctive features make the Buena Vista Formation an exceptional case for understanding SSDS in eolian environments and for evaluating the role of driving forces behind their formation. The active SSDS described in this study include sand diapirs, interpenetrative cusps, sand blows, complex recumbent folds, intra-set convolute folds, massive beds, and pockets. Passively generated structures comprise overturned beds, contorted strata, and intra-set syn-deformational faults. Seismic activity is inferred as the primary trigger for deformation, based on several lines of evidence: (i) similarities with other seismically induced structures, (ii) large areal distribution with zonation in structural complexity, (iii) wide lateral continuity along individual outcrops, (iv) stratigraphic recurrence, and (v) the active geotectonic setting during deposition of the unit. This tectonic activity coincides with the accretion of the Patagonia Terrane to southwestern Gondwana. Under these conditions, rapid water-table rise likely generated overpressure, facilitated by air entrapment in the eolian sediments, ultimately enabling widespread soft-sediment deformation.