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The Temporal and Lateral Facies Variability and Internal Geometries of Syn-Rift Salt: Insights From the Early Mesozoic Orpheus Rift Basin, Offshore Eastern Canada


Many salt deposits have significant facies variability (e.g., the salt deposits of offshore Brazil and onshore Netherlands), which can influence the salt behavior and deformation patterns in a salt-rich basin. We have examined the distribution, temporal and lateral facies variability, and internal structures of the Late Triassic / Early Jurassic Argo Formation of the Orpheus rift basin, offshore eastern Canada, using well and a dense grid of 2D seismic-reflection data. Our work shows that the relay ramps of the border-fault system influenced the lateral facies variability, rheology, and internal structures of the syn-rift evaporite deposit. Well data from the western Orpheus rift basin indicate that the Argo Formation consists of a lower, older massive evaporite unit that underlies an upper, younger evaporate unit consisting of interbedded evaporites–shales–siltstones. Seismically, the lower massive Argo unit is transparent with few internal reflections. In contrast, the upper Argo unit has two distinct seismic facies. Facies A, present near relay ramps of the border-fault system, consists of numerous parallel, continuous, and moderately deformed internal reflections. Facies B, present far from relay ramps, has fewer internal reflections than Facies A, and these reflections are less continuous and more intensely deformed. We propose that, during early rifting, relay ramps connected the footwall of the border-fault system with the deep parts of the rift basin, providing pathways for clastic sediments to enter the salt-rich Orpheus basin. As a result, the sedimentation patterns of the upper Argo unit varied from interbedded evaporites and shales near the relay ramps to predominantly evaporites in the middle of the basin. Furthermore, the two facies behaved differently in response to subsequent phases of deformation (e.g., differential loading during and after rifting). Detached folds and faults with reverse separation are common in Facies A. In contrast, Facies B behaved more ductilely like the massive evaporites of the lower Argo Formation. The massive evaporites and Facies B formed salt walls and columns, enclosing complex overturned and tight folds of shale lenses.