Preliminary Strontium isotope stratigraphy of the Jurassic Minas Viejas Formation, México

Abstract

The Minas Viejas Formation consist of shallow marine evaporite and carbonate rocks, with subordinated siliciclastic, volcanic and volcanogenic beds. This stratigraphic unit mark the beginning of the marine transgression during the middle Jurassic. It overlies conformably and transitionally onto continental siliciclastic syn-rift deposits of the La Joya Formation, likely of Bajocian or pre-Bajocian age. The paleontological content used to determine Minas Viejas stratigraphic position occur in carbonate beds from the middle member of Los Cuervos (e.g. Cross, 2012), to the top of the formation, and indicate an Oxfordian to Kimmeridgian age (e.g. Zell et al., 2015). Strontium isotope (87Sr/86Sr) ratio data from calcite obtained from shells and whole rock samples from the basal, middle and top carbonate intervals of the Minas Viejas Formation, suggest a reappraisal for an age older than Callovian. Whole rock calcite samples from the basal carbonate, the Tranquitas member, indicate a Bajocian age (≈169 Ma). The middle carbonate interval, the Los Cuervos member, yield ages ranging from the Bathonian to Callovian (≈ 168 to 165 Ma). Overlying Los Cuervos member and interbedded with gypsum and limestone beds occurs volcanic and volcaniclastic deposits of the La Primavera Member, which youngest zircons yield a U-Pb age of 164.8 ± 1.7 and 163.3 ± 2.0 Ma (Callovian-Oxfordian; Cross, 2012). Fossiliferous carbonate in the Loma Larga member provide a Kimmeridgian age (153.85 + 1.3/-1.1). The top of the Minas Viejas Formation, marked by the El Potosí member suggest deposition occurred during late Kimmeridgian or early Tithonian (151.15 + 1.15/-1.3); this age range is assigned based on the error range displayed by the data and because of its stratigraphic position underlying La Caja Formation of the Kimmeridgian (Idoceras zone). Based on the obtained Strontium isotope data, the Minas Viejas Bajocian basal members (e.g. La Nieve and Tranquitas) appear to be older than initially considered. This suggest that just a segment of the evaporite and carbonate succession of the Minas Viejas Formation (Los Cuervos member) may be coeval with the younger Callovian Louann salt (e.g. Pindell et al., 2019) in eastern Gulf of Mexico. Additional strontium data form evaporite (gypsum, anhydrite and halite) of the whole succession in Minas Viejas Formation, particularly form the basal la Nieve member, is required to better constrain age and timing of marine transgression during the middle Jurassic post-rift deposits in the Gulf of Mexico.

AAPG Datapages/Search and Discovery Article #90369 © 2020 AAPG Hedberg Conference, Geology and Hydrocarbon Potential of the Circum-Gulf of Mexico Pre-salt Section, Mexico City, Mexico, February 4-6, 2020