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Upper Cretaceous to Cenozoic Reactivation of Pre-existing Salt Bodies and its Impact on Slope Depositional Systems in the Southwest Sable Subbasin, Offshore Nova Scotia

Abstract

Despite the implications for the distribution of potential reservoirs and the integrity of potential hydrocarbon traps, the history of younger (Late Cretaceous to present) salt-related deformation is largely unknown on the Scotian Margin. This study investigates the style and timing of Late Cretaceous to Cenozoic salt rejuvenation on the upper Scotian Slope and related impacts on down-slope (turbidite, mass failure) and cross-slope (contourite) depositional systems. The study area overlaps a boundary between a vertical salt diaipir system in the west and an allochthonous tongue canopy system in the east. Using two 3D surveys covering an area of approximately 5300km2, nine horizons were mapped in order to establish a seismic stratigraphic framework, calibrated to available wells. Salt diapirs in the western study area are generally circular to elliptical in plan view. The presence of radial crestal faults, pinched off stems, folded cover strata above salt crests and associated stratal thinning onto folds are diagnostic signs of rejuvenation. Evidence of rejuvenation is also seen in the eastern study area where cover strata above salt tongues, and in some cases the salt tongues themselves, are folded. The timing of rejuvenation is deduced from onlap patterns, thickness variations above fold crests, timing of faults, and the response of depositional systems (e.g. canyons and mass transport deposits diverting around salt-cored folds). Preliminary results suggest at least two separate rejuvenation events in the Late Paleogene to Neogene followed by an important period of erosion and slope re-grading to yield the present slope character. Contraction of pre-existing salt bodies in the west may be related to a landward shift in the locus of contourite sedimentation, whereas the cause of late folding of salt tongues in the east is still being determined. Present day bathymetry shows little to no expression of salt as a result of a dramatic increase in erosion from sediment-gravity flows, in particular mass transport deposits. This may be linked to the onset of shelf-crossing glaciations.