Evaporite diapirs in the La Popa basin, Nuevo León, Mexico
Reconnaissance mapping and stratigraphic work in the La Popa basin of Nuevo Leon, northern Mexico, has revealed two large structural features with cores of gypsum and anhydrite at the surface. These features are interpreted to be the surface expression of evaporite diapirs. They are exposed in folded sedimentary rocks belonging to the Potrerillos Formation of the Difunta Group of Maastrichtian age (Late Cretaceous). Sandstone units within the Potrerillos Formation pinch out toward the piercement features. Carbonate lentils composed of rudistid reefs and associated carbonate clastics flank the features and pinch out away from the features into marine shale members of the Potrerillos Formation. Most carbonate lentils occur in the immediate vicinity of the evaporite features and are interpreted to be genetically related to the diapirs. Syndiapiric sedimentary-facies patterns can be documented over 4,000 vertical feet (1,200 m) of section within the Potrerillos Formation. The large vertical extent of facies peculiarities near the piercement features indicates that the features were active for an extended period corresponding to at least two regressive-transgressive cycles during Maastrichtian time. A radial drainage pattern away from the northern feature indicates that the features are probably active today. A limestone inclusion from within one of the features has been dated as Kimmeridgian in age (Zuloaga or Smackover equivalent) and suggests that the evaporites are probably derived from the Jurassic Minas Viejas Formation. It is not known at present whether salt underlies the surface crust of gypsum and anhydrite. At the time that the first syndiapiric sediments of the Potrerillos Formation were deposited, the Minas Viejas Formation was buried at an estimated depth of 17,000 ft (5,200 m). The thick pre-Laramide cover, the pre-Laramide age of at least some of the syndiapiric sedimentary features, and the location of stocks away from major anticlines indicate that the emplacement of the diapirs is not related to Laramide compression. Geostatic loading is considered to be the most important mechanism for emplacement of the domes.