SEDIMENT PROVENANCE AND STRATIGRAPHY OF UPPER CRETACEOUS PROXIMAL FOREDEEP AND WEDGETOP DEPOSITS OF THE MEXICAN FORELAND BASIN, EASTERN MEXICO

Lower(?) Maastrichtian strata exposed in the Mexican fold and thrust belt east of Río Verde, San Luis Potosí (SLP), record an abrupt transition from marine foredeep sedimentation to continental deposition on the frontal part of the orogenic wedge. Proximal foredeep deposits of the Cárdenas Formation near Cárdenas, SLP, consist of thick shale and fine-grained turbidite beds intercalated with thick-bedded breccias composed of transported rudists, pelecypods and corals in a mudstone matrix. The fossiliferous beds are interpreted as bioclastic debris-flow deposits that accumulated in deep water. 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. U-Pb ages of detrital zircon (DZ) grains were analyzed for two samples of each stratigraphic unit by LA-ICPMS (n = 207 total grains). Two sandstone samples from the middle part of the Cardenas Formation yielded weighted mean maximum depositional ages (MDAs) of 75 ± 1 Ma (MSWD = 1, n = 3) and 75 ± 1.5 Ma (MSWD = 1.7, n = 5), both slightly older than an early Maastrichtian biostratigraphic age based on ammonites and corals. A sandstone of the Tabaco Formation yielded an MDA of 73 ± 4 Ma (MSWD= 1.9, n= 3). DZ U-Pb ages of both units are dominated by inferred sources in the Early Cretaceous Alisitos arc (26-28%) and Late Cretaceous Mexican Cordilleran arc (30%). Proterozoic (15%), Cambrian-Silurian (4-7%) and Middle Jurassic (4-7%) ages are less prominent. Triassic and Late Jurassic grains are only present in Tabaco samples. Minor age components likely were recycled from older strata of the orogenic wedge, including the Mineral de Pozos sandstone and Upper Cretaceous turbidites of the Mesa Central. Cárdenas sandstones contain dominant volcanic lithic grains and subordinate carbonate extrabasinal (CE) grains, whereas quartzolithic sandstones of the Tabaco Formation contain dominant CE grains, indicating increased contribution from the fold-thrust belt. The transition from deep marine to continental facies concomitant with a shift to dominantly thrust-belt sediment sources suggests an abrupt shift from proximal foredeep to wedgetop deposition at the front of the orogenic wedge.