ABSTRACT

Surface expression of two salt domes and a thrust fault in the La Popa basin of Northern Mexico near Monterrey, Nuevo Leon suggest a complex history of interrelated salt tectonics, Late Cretaceous clastic and carbonate sedimentation, and thrust fault growth.

Detailed stratigraphic analysis of clastic sediments and shallow marine carbonates of the Difunta Group (Campanian and Maestrichtian) near the two salt domes indicate that delta front sandstones thin and pinch out towards the salt domes. Successively higher sandstones lap higher and farther onto the domes and probably covered the domes completely during Late Cretaceous time, though present day erosion precludes the certainty of complete coverage. During high sea level stands, algal and rudistid carbonate banks formed on the flanks of the domes and may also have covered the domes completely. Facies irregularities in the enclosing sedimentary rocks clearly indicate that the salt domes were active during Late Cretaceous time.

Though hidden under the allochthonous block of the La Popa fault, a major northwest-southeast trending thrust fault, the presence of a third dome is implied by the occurrence of evaporites on the fault plane and by the presence of similar, though smaller, faults associated with the two exposed domes. The La Papa fault may be thought of as an out-of-the-syncline, southwest verging thrust fault with the overthrust block acting as a trap door for evaporites derived from the salt dome.

The largest carbonate bank in the basin, the 250-meter (800-foot) thick La Popa Limestone Lentil, occurs only on the upthrown side of this fault. The downthrown side is comprised of an unusually thick [approximately 4.600-meter (15,000-foot)] sequence of clastic rocks with an atypically high sand/shale ratio. This combination indicates that, during high sea level stands, carbonate banks grew on shoal areas that were present on the overthrust side of the fault. During low sea level stands, deltaic and shallow marine clastic sedimentation became concentrated in low areas associated with the down thrown side of the fault.