SALT-WITHDRAWAL BASIN GEOMETRY AS A RECORD OF DIAPIRIC TECTONICS: CARROZA FORMATION, LA POPA BASIN, NUEVO LEON, MEXICO

Fluvial and growth-stratal geometries related to salt diapirism are rarely observed in outcrop, but the La Popa Basin contains superb exposures of syntectonic growth strata that resulted from combined foreland shortening and local withdrawal of salt. Early Cretaceous to early Cenozoic syndepositional salt rise influenced facies distribution and stratigraphic geometry, including positions of sandstone bodies, paleosols, and stratigraphic thickness. In the study area, evacuation of salt from an elongate diapiric wall and coeval Laramide shortening created a fault-like, northwest-trending structure, termed a salt weld. The La Popa weld separates an upthrown northeast block from a down-thrown southwest block. The middle Eocene Carroza Formation was deposited in an asymmetrical, synclinal salt-withdrawal basin on the down-thrown block of the weld. The Carroza contains growth strata with several angular unconformities near the weld and sand-rich fluvial facies near the basin axis. Growth strata thin and steepen toward the weld. The growth-stratal geometries record evolving fold shape during Carroza deposition. Small alluvial fans derived from the elevated diapir intertongue with fluvial-plain deposits of the basin. Overbank deposits near the weld contain well-developed paleosols, which are less mature elsewhere in the small basin. Although formed in a shortening environment, the depositional systems of the salt withdrawal basin are analogous to those of extensional half graben basins. During Carroza deposition, the northwest-trending weld was probably an elongate, topographically elevated diapir analogous to the uplifted footwall of a half graben. Fluvial channels oriented parallel to the wall occupied the adjacent subsiding basin axis. Detritus was also shed directly into the basin from the diapiric source. Sedimentation rates were highest in the basin axis, producing the observed thinning of strata and associated paleosols near the diapir. In extensional half grabens, maximum subsidence-driven accommodation localizes fluvial channels near the toes of footwall-derived alluvial fans. In the salt-withdrawal basin, the location of maximum accommodation in the synclinal hinge was determined by a combination of Laramide fold shape and salt-driven subsidence.