The transgression of an erg — sedimentation and reworking/soft-sediment deformation of aeolian facies: the Cretaceous Troncoso Member, Neuquén Basin, Argentina

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Abstract The Cretaceous Troncoso Inferior Member of the Huitrín Formation comprises fluvial and aeolian facies that form a drying-upwards succession within the Neuquén Basin. The basal fluvial sandstones were deposited as braided river deposits and lie unconformably on top of either the deep-marine Agrio Formation or, locally, the shallow-marine Chorreado Member (Huitrín Formation.). In places, the fluvial sandstones are interbedded with remnants of aeolian deposits recording an arid environment and ephemeral flows. In the study area the upper section is predominantly aeolian and was controlled by northerly winds with both linear and transverse dune types being deposited. The depositional system was rapidly flooded and dune topography (relief ranging between 2 and 35 m) was preserved on its top surface. In addition to dune topography, the Troncoso dunes also show evidence of reworking and in situ soft-sediment deformation related to the flooding. The principal aim of this paper is to document the soft-sediment deformation and preservation of topography associated with the flooding of the dune field. Within the soft sediment deformed and reworked sediments at the top of the Troncoso Inferior Member spatial and temporal relationships indicate that they formed in a specific sequence. Initially, water-escape processes created convolutedly folded and dish structures that were concentrated in areas of slightly higher preserved dune topography. Secondly, the convolutedly folded and dish facies were eroded and reworked by wave undercutting and migrating three-dimensional dunes in a shallow-marine environment. This subaqueous reworking resulted in an interbedded massive and cross-stratified unit. With further deepening of the water, the topography became stabilized and the uppermost part of the interval (0.1–0.3 m) was reworked by waves across most of the basin. In the topographic lows between dunes, liquefaction-induced sediment gravity flows deposited massive-flatlaminated facies. The reworked and soft-sediment deformed aeolian dune topography is overlain by the evaporites of the Troncoso Superior Member. The distribution of flood-related facies and the amount of preserved dune topography (2–35 m) indicates that the transgression must have been rapid but of low energy.