Geocronología de circones detríticos en capas del Jurásico Inferior de las áreas de la Sierra de Catorce y El Alamito en el estado de San Luis Potosí

Abstract

This report describes probable Lower Jurassic marine strata cropping out in the Sierra de Catorce, San Luis Potosí and compares them with confirmed Toarcian layers near El Alamito, west of Rioverde, San Luis Potosí. An initial report of Lower Jurassic strata in the Sierra de Catorce, based on the presence of the ammonites Vermiceras sp. and Arnioceras cf. Abjectum Fucini n. subsp., has remained in doubt since 1956 because of uncertainty regarding the exact location of the outcrop and a lack of subsequent reports of fossils. Detrital zircon geochronology by the Laser Ablation Multicollector ICPMS method on two medium- to coarse-grained litharenites from the above localities indicates: 1) Maximum Late Triassic-Early Jurassic depositional ages based on young zircon grains in the samples; 2) three primary sources of detrital zircons that included Grenvillian basement (~900–1200 Ma), Pan-African basement rocks (~500–700 Ma) and the Permo-Triassic magmatic arc (~245–280 Ma); 3) uncommon (n=2) Jurassic zircons in the El Alamito sample, in combination with interlayered mafic rocks in the Sierra de Catorce succession, likely record the Jurassic volcanic activity. Petrographic studies, including point counts performed to interpret the origin of the detrital components of the rocks, indicate a recycled orogen and continental block provenance. On the basis of our studies, we conclude that both successions, marine facies in El Alamito (?-Toarcian - pre-Callovian age) and marginal marine to deltaic facies in the Sierra de Catorce (post-Norian to pre-Bajocian?), are probably not comparable in their precise age but are interpreted as deposits situated on the Pacific margin of Mexico during Early Jurassic time.