ABSTRACT

In 1960, Trusheim described three evolutionary stages of salt movement (pillow, diapir, and postdiapir) recorded in adjacent sediments by three types of peripheral sinks (primary, secondary, and tertiary). Assuming that overburden rocks were viscous, Trusheim interpreted such evolutionary stages as typical of buoyant Rayleigh-Taylor instabilities. We use mechanical reasoning and physical models to reinterpret Trusheim's observations, assuming a brittle overburden. Results suggest that nonpiercing salt pillows do not represent immature diapirs but form as contractional buckle folds or as salt swells triggered by differential sediment loading. Extension also forms salt pillows, but there, the roof of the pillowlike structure is breached early by normal faults. Subsidence of the overburden below the regional datum forms peripheral sinks in between the diapirs. The diapir stage, during which strata thicken against the diapir, corresponds to subsidence of diapir flanks driven by late extension. During the postdiapir stage, the diapir crest rises above the regional datum while the adjacent depocenters neither subside nor rise. This late stage corresponds to tectonics-or gravity-driven contraction that rejuvenates dormant diapirs.