Stratigraphy and origin of Upper Cretaceous wedge-top and proximal foredeep deposits in the Mexican foreland basin, east-central Mexico

Abstract

Upper Campanian (?) to lower Maastrichtian strata exposed in the Mexican fold and thrust belt east and south of Cárdenas, San Luis Potosí (SLP), record an abrupt transition from marine foredeep sedimentation to continental deposition on the frontal part of the orogenic wedge. Shale and subordinate fine-grained sandstone of the middle Campanian–Maastrichtian Méndez Formation east of Cárdenas represent medial and distal foredeep deposits of the Mexican foreland basin. Correlative proximal deposits of the Cárdenas Formation near the village of Cárdenas consist of thick shale and fine-grained sandstone turbidite beds intercalated with thickly-bedded breccias composed of transported rudists, and other pelecypods and corals in a mudstone matrix. The fossiliferous beds are interpreted as bioclastic debris-flow deposits that accumulated in deep water of the proximal foredeep and/or wedge-top depozone. The overlying Tabaco Formation consists of conglomerate and lenticular sandstone beds as much 1.6 m thick interbedded with red shale and bioturbated, cross-stratified sandstone beds, representing deposition in fluvial channels and floodplains in a rapidly uplifted wedge-top basin.Maximum depositional age (MDA) values calculated from young grains (n = 3 grains) of the Upper Cretaceous strata yield somewhat older ages than those derived from published and new biostratigraphy data. A sandstone sample from the Méndez Formation yielded an MDA of 73.3 ± 7.5 Ma, imprecise but consistent with a middle to late Campanian biostratigraphic age reported here from an associated Méndez shale sample. Two sandstone samples from the middle part of the Cardenas Formation yielded MDAs ranging from 73.4 ± 1.5 Ma to 72.9 ± 3.7 Ma, slightly older than a reported early Maastrichtian biostratigraphic age based on ammonites and corals, but consistent with a new age range, based on nannofossils, of middle Campanian-early Maastrichtian for strata consisting of mixed Méndez and Cárdenas lithofacies. Five sandstones of the Tabaco Formation yielded an MDA range of 73.9–67.9 Ma (Campanian–Maastrichtian), but with substantial 2σ errors of 3–5 my.Sandstone petrography and detrital zircon U -Pb geochronology indicate that sediment sources were Upper Cretaceous sedimentary strata exposed in the Mesa Central and the frontal part of the Mexican fold-and-thrust belt. Cárdenas and Tabaco sandstones contain dominant extrabasinal carbonate lithic grains, reflecting progressive uplift and significant erosion of Lower Cretaceous platformal carbonate and pre-Campanian foreland basin strata, coupled with a decrease in the magmatic activity of the Mexican Cordilleran arc. We propose that the Cárdenas Formation, formerly interpreted as a carbonate platform deposit on the basis of its abundant coral content, was deposited by sediment-gravity flows sourced from nearby reefal facies that likely developed on growing folds. Deposition of the Cárdenas Formation took place in the proximal foredeep or a wedge-top basin on the frontal part of the fold-thrust belt. Coeval deposition of the Méndez Formation took place in the proximal foredeep. Continental deposits of the Tabaco Formation, which overlies the Cárdenas Formation, record an abrupt shift to continental deposition concomitant with a transition to dominantly thrust-belt sediment sources. The transition in facies and sandstone composition record shoaling of the wedge-top basin during emergence of a frontal thrust sheet. Upper Campanian-lower Maastrichtian strata therefore provide important insight on temporal and topographic evolution of the Mexican orogen in east-central Mexico.