Sub Salt Origin of Exotic Blocks in Piercement Domes Reveals the Probability of Oligo-Miocene Salt in the Gulf of Mexico Region

ABSTRACT The occurrence of exotic blocks of igneous and sedimentary rocks in piercement salt dome cap rock is a common phenomenon around the world. Although salt piercements in many basins are known to have pierced through great thicknesses of sedimentary overburden, fragments of the sediments traversed do not appear to have been incorporated in cap rock. Jurassic, Cretaceous or Tertiary material has not been found in salt piercements of the Gulf Coast interior salt basins nor have post Cambrian sediments been found amongst Hormuz cap rock detritus of the numerous Persian and Arabian Gulf piercements. Evidence from salt piercements in the Persian Gulf area suggests that insoluble cap rock material is derived either from rocks interbedded with the parent evaporite or from sub-salt formations as a result of glacier-like plucking from the salt sub-crop during flow toward the diapiric exit. In basins where bedded salt is too deep for penetration by the drill the age of exotic material in piercement cap rock often has been used as an age indicator of the salt deposit when, in fact, the exotics probably represent pre-salt formations often lying unconformably below the evaporite deposit. Evidence from the Salina basin in southeastern Mexico suggests that salt structures may be cored by Oligocene rather than Jurassic evaporite and, likewise, the salt in the Moron basin diapirs, northeast Cuba may be Oligo-Miocene rather than Jurassic as is currently supposed. The possibility that Oligo-Miocene salt may core many of the offshore Texas and Louisiana structures is suggested both by diapiric immaturity of many of the structures and by the presence of Oligo-Miocene sediments in Belle Isle and Eugene Island piercements of Louisiana.