FACIES, BIOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF LOWER CRETACEOUS CARBONATE ROCKS OF THE MURAL FORMATION, CERRO PIMAS SECTION, SONORA, MÉXICO

The Lower Cretaceous Mural Formation is well exposed in the northern part of Sonora, Mexico and has similar stratigraphic characteristics and is correlative with similar rocks exposed in southern Arizona and New Mexico in the United States of America. These sedimentary rocks have been divided into six members viz. i) Cerro La Ceja (CLC), ii) Tuape Shale (TS), iii) Los Coyotes (LC), iv) Cerro La Puerta (CLP), v) Cerro La Espina (CLE) and vi) Mesa Quemada (MQ). The Cerro Pimas section is composed of limestone lithofacies ranging from bioclastic wackestone to boundstones whose biota is characterized by low diversity. The occurrence of prominent age-diagnostic benthic foraminifer species Orbitolina (Mesorbitolina) cf. texana, Paracoskinolina sunnilandensis, Nezzazata isabellae and Arenobulimina macfadyeni suggest Late Aptian to Early Albian age.The biota of the Mural Formation in the Cerro Pimas section is consistent with the published Late Aptian to Early Albian age.

Limestones of the Mural Formation have negative to positive δ¹³C values (–4.63 to +2.6‰) and highly depleted δ¹⁸O values varying from –12.74 to –8.34‰. The absence of correlation between δ¹³C and δ¹⁸O values supports a primary marine origin for the δ¹³C values of these limestones. The carbon-isotopic curve of the Cerro Pimas section is compared with published curves of similar age that show well-defined δ¹³C segments (C9 – C15). In the Lower Albian Los Coyotes Member, the presence of OAE1b is indicated by an increase followed by a decrease in δ¹³C values, allowing correlation with the Kilian Event. The middle part of the Los Coyotes Member show significant negative carbon isotope excursion associated with the globally recognizable Early Albian Paquier event. Moreover, another significant negative carbon-isotope shift is observed in the upper part of the Los Coyotes Member, which can be correlated with the Leenhardt Event. The occurrences of Kilian, Paquier and Leenhardt Events (OAE 1b cluster) at the Cerro Pimas section confirm the global nature of these Early Albian disturbances in the carbon cycle.