CARBON AND OXYGEN ISOTOPIC RECORDS OF THE LOWER CRETACEOUS MURAL LIMESTONE, RANCHO BUFALO SECTION, SONORA, MEXICO: IMPLICATIONS FOR OCEANIC ANOXIC EVENTS

The Lower Cretaceous Mural Limestone is well exposed in the Rancho Bufalo area of northern part of Sonora. The Mural Limestone is part of Bisbee Group that comprised of four Formations; Glance Conglomerate, Morita Formation, Mural Limestone and Cintura Formation. The Mural Limestone represents the maximum marine transgression registered in the Bisbee Basin in the late Aptian- early Albian on an epicontinental sea. The Mural Limestone in Rancho Bufalo area has been divided into 8 members: Fronteras (F), Rancho Bufalo (RB), Cerro La Ceja (CLC), Tuape Shale (TS), Los Coyotes (LC), Cerro La Puerta (CLP), Cerro La Espina (CLE) and Mesa Quemada. The petrographic study reveals that the lithofacies ranging from bioclastic wackstone to grainstone, where the biota is of benthic foraminifera and molluscs. The carbonates of the Mural Limestone have negative to positive δ¹³C values (-4.84 to +3.19‰) and δ¹⁸O values that range from -9.99 to -2.79‰. The variations in carbon isotope values (~ 8‰) are larger than those found in other studies (several European sections) of the Upper Aptian-Albian intervals. However, these values are comparable with our earlier study undertaken at Cerro Pimas section of the Mural Limestone. The lower part of the Los Coyotes member shows an increase followed by a decrease and a series of excursions and incursions in δ¹³C values, suggesting a correlation with an oceanic anoxic event (OAE 1b).