Interpreting the nature of the Aulet and Adons diapirs from sedimentologic and stratigraphic analysis of flanking minibasin strata, Spanish Pyrenees, Catalunya, Spain

Abstract

ABSTRACT The Aulet and Adons diapirs are exposures of Triassic Keuper evaporites in the Ribagorça Basin in the south-central Pyrenees. The diapirs have been alternatively interpreted from mapped structural relationships as either passive salt diapirs or extensional salt rollers. Correspondingly, the associated diapir-flanking minibasins have been interpreted as either salt-withdrawal or extensional-rollover minibasins, respectively. New field mapping, stratigraphic sections, petrographic analysis, correlation diagrams, and drone photography characterize the depositional facies and stratal architecture of the flanking Sopeira, Sant Gervàs, and Faiada minibasins (upper Albian to lower Santonian synrift to postrift strata), which in turn, constrains the origin and evolution of each salt body and associated minibasins. Each minibasin displays unique facies patterns and stratal thicknesses that reflect depositional systems evolving in response to spatially and temporally variable extension, salt evacuation, and passive diapirism during the Pyrenean rift and postrift phases. The Sopeira minibasin developed in the late Albian with significant localized subsidence, but it remains inconclusive if the bounding Aulet diapir originated as a passive diapir or a salt roller. The Llastarri fault zone, previously interpreted as a salt weld, separates the Sopeira minibasin from the primarily extensional Sant Gervàs minibasin, and is reinterpreted here as a remnant salt ridge, as it lacks evidence for passive diapirism. The Sant Gervàs minibasin remained relatively uplifted until the middle to late Cenomanian, along with the Faiada minibasin. Evidence for passive diapirism in the Faiada minibasin, including diapir-derived detritus and composite halokinetic sequences, indicate salt evacuation into the bounding Adons passive diapir. Integration of detailed sedimentologic and stratigraphic analyses with interpretations of basin formation and structural development provides better resolution of the earlier phases of gravity-driven extension and loading-driven salt movement of the Aulet and Adons diapirs; these insights help constrain structural interpretations and reconstructions of Pyrenean shortening and megaflap development in the Ribagorça Basin. Sedimentological and stratigraphic evidence for or against passive diapirism need to be integrated into structural interpretations, especially when precursor salt structures are obscured by subsequent contraction. This well constrained basin framework demonstrates the effects of inherited extensional structures and passive diapirism on Pyrenean shortening and megaflap rotation.