Contrasting styles of Laramide folding across the west-central margin of the Cretaceous Valles-San Luis Potosí carbonate platform, Mexico

Abstract

Along the west-central limit between the Central Mexico Mesozoic Basin and the Valles-San Luis Potosí carbonate platform exist marked variations in style and intensity of Laramide folding (Late Cretaceous-Early Tertiary). These variations are attributed to: 1) prominent inter- and intra-formational changes in lithology and bedding thickness, and 2) overall thickness contrast between the Cretaceous sedimentary sequences accumulated in the basin and the carbonate platform. Tight, NE-verging folds with fold widths in the order of meters to tens of meters formed in those sites where only thinly bedded, slightly argillaceous, basin calcareous turbidites were deposited. Folding is disharmonic in other localities where thicker beds of slide breccias, made of clasts derived from the nearby carbonate platform, are interlayered with the basin turbidites. Breccia sheets controlled the formation of mesostructures, with fold widths in the order of hundreds of meters, while second order folds, with fold widths in the order of meters, were formed in the thinner bedded argillaceous limestones.

Thick strata of shallow water carbonates, deposited in and around isolated patch reefs in the platform interior, formed open (fold widths in the order of several kilometers), almost symmetrical folds with near vertical axial planes. Despite significant facies changes in the limestone, associated with the presence of patch reefs, no variations in the folding style are related with them.

The occurrence of anhydrite strata (Guaxcamá Formation) under isolated portions of the carbonate platform interior also played an important role in the folding style, causing the tectonically induced accumulation of evaporite near the cores of some folds and/or formation of diapirs. This phenomenon modified the structures and caused intense fracturing in the central portion of the Sierra de Guadalcázar. In few places, anhydrite was injected along fractures in the limestone. Meteoric water infiltration and dissolution of the anhydrite developed intense karsticity and caused the formation of an extensive collapse breccia. Division of the Sierra de Guadalcázar into several domains suggests that the structural anomalies in respect to its immediate surroundings are concentrated in an area where anhydrites are exposed. Furthermore, the emplacement of the mid-Tertiary Cerro de San Cristóbal intrusive, a tin-bearing subvolcanic granite, did not caused doming by forceful injection at the core of the Sierra de Guadalcázar.