107-19 Petrography, Diagenetic Textures, and Stable Isotope Analysis of the Boleo Gypsum Member, Baja California Sur, Mexico

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

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The Gulf of California and the Baja California microplate represent one of the best examples of oblique rifting occurring along an active continental margin. This process commonly forms marginal seas and continental slivers that break away from their parent continent. Stratigraphic records documenting the transition from early rifting to seafloor spreading are often buried beneath thick sedimentary sequences, making them difficult to access. Nonetheless, evaporitic sulfates serve as critical indicators for understanding the chemical evolution of seawater and hydrothermal fluids. Their dissolution and diagenetic alteration can provide insights into ore-forming processes, microbial sulfate reduction, and the temperature conditions associated with hydrothermal mineralization. Samples from drillhole DDB10, collected from the Boleo Formation in Baja California Sur, Mexico, were analyzed petrographically and isotopically to understand their diagenetic evolution and burial history. Thin section analysis reveals multiple generations of sulfate minerals, including early selenitic gypsum partially transformed into anhydrite through dehydration during burial, followed by partial rehydration textures such as fibrous gypsum veins and void-filling overgrowths. These textures reflect fluctuating diagenetic conditions influenced by burial depth, temperature, and later fluid migration. Stable isotope data from sulfate phases yield average values of δ¹⁸O ≈ 7.4‰ and δ³⁴S ≈ 23.2‰, suggesting partial isotopic resetting during burial diagenesis while still retaining a broadly marine sulfate signature. The relatively enriched sulfur values are consistent with a marine evaporitic source, whereas the oxygen values indicate recrystallization under moderate temperature, possibly a closed-system condition. The combined petrographic and isotopic evidence supports a diagenetic pathway involving burial-induced dehydration to anhydrite, followed by later, localized gypsum rehydration associated with uplift or fluid circulation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10604

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