SHOULDER FORMATION IN THE PARADOX BASIN: A RECORD OF PROGRESSIVE DIAPIR NARROWING AND MINIBASIN EXPANSION (Invited Presentation)

Salt shoulders may be an underappreciated constituent of vertical diapirs, but are key to understanding the kinematics of rising diapirs and adjacent subsiding minibasins. Salt shoulders, are abrupt inward steps of the margins of salt diapirs, and represent narrowing of the diapir. Integrating field mapping, observations, and measured sections, we identified salt shoulders on 6 Paradox Basin diapirs. Previous studies misinterpreted the salt shoulders in the Paradox Basin as roof collapse blocks. Most of the diapirs exhibit long histories of progressive shoulder formation that decrease the areal extent of the diapir over time. The diapirs began rising in the Late Paleozoic and continued until the latest Jurassic. Gypsum, Paradox and Onion Creek salt walls have Permian shoulders on their northeastern sides. Gypsum, Moab, Castle, and Sinbad salt walls have Triassic shoulders. Several diapirs exhibit shoulders that formed progressively from the Late Permian through the Latest Jurassic. Shoulders have shaped much of the stratigraphy and structure at the margins of the Paradox Basin diapirs. Shoulders form an under-recognized component of salt systems and represent deposition on deforming salt bodies. Subsidence and rotation of the shoulder preserves strata deposited on salt diapir caprock. These show syndepositional deformation indicating dissolution or movement of the underlying salt. The shoulders we mapped have previously been interpreted as solution collapse structures.