ABSTRACT

Strata exposed in the La Popa Basin of northern Mexico range in age from Late Jurassic through middle Eocene and record the evolution of the region currently lying in the foreland of the Sierra Madre orogen from extensional to contractional tectonics. The facies distribution and geometry of the strata demonstrate that salt diapirism was active at least from late Aptian-middle Eocene. Jurassic rocks include evaporites of the Oxfordian-Kimmeridgian Minas Viejas Formation, which is exposed only in diapiric bodies in the basin. The evaporites contain three types of blocks transported to their present levels during diapirism: (1) mafic to intermediate metaigneous rocks with latest Jurassic ⁴⁰Ar/³⁹Ar cooling ages (146 Ma); (2) laminated and nodular gypsum; and (3) Kimmeridgian limestone (Zuloaga Limestone). This Upper Jurassic lithic assemblage represents different parts of a formerly thick (at least 1000 m) stratigraphic section deposited in an extensional, or pull-apart, basin. Post-Jurassic rocks of the basin range from Early Cretaceous to middle Eocene and record both carbonate and siliciclastic deposition in dominant deltaic, shallow-marine, and tidal settings, as well as subordinate basinal and coastal-plain environments. This exposed section is at least 6400 m thick. The Lower Cretaceous section is thin (~100 m) and locally consists of carbonate biostromes deposited on bathymetric highs adjacent to a diapiric salt wall. These carbonates are late Aptian-late Albian in age and much thinner than their regional correlatives. The lower part of the Upper Cretaceous is represented by the Indidura and Parras Formations, the former a basinal carbonate, the latter a prodeltaic or basinal shale that underlies the Difunta Group, a constructional continental-margin clastic wedge or embankment that filled the La Popa Basin. The Difunta Group in the La Popa Basin spans the Maestrichtian-middle Eocene. Lenticular carbonate beds as much as 350 m thick are interbedded with mud-rock intervals of the Difunta Group and represent deposition on bathymetric highs created by rising salt bodies.