FIRST PALEOGENE COLD SEEPS FROM MEXICO: ANOMALOUS LIMESTONE DEPOSITS IN THE LATE OLIGOCENE PART OF THE EL CIEN FORMATION, NEAR EL CIEN, BAJA CALIFORNIA SUR

Anomalous, authigenic limestone deposits occur within otherwise stratified to massive silty sandstones in the transgressive, lower part of the marine Late Oligocene San Juan Member (El Cien Formation), north of the town of El Cien, Baja California Sur, Mexico. These limestones take the form of erosionally resistant crusts, ‘chimneys,’ and low-relief mounds, similar to carbonate deposits that are typically found associated with modern hydrocarbon cold seeps off the coast of Oregon, California, New Zealand, and elsewhere. The structures we report from Mexico preserve evidence of bioturbation and conduits formed by fluid flow, and many deposits contain laminated calcitic crusts and veins, and rarely, vugs with calcite cement. These limestones were likely precipitated from seeping hydrocarbon-rich fluids, via the microbial oxidation of methane. The source of the fluids was the unconformably underlying marine Middle to Late Eocene part of the Tepetate Formation. The limestone deposits occur about five meters stratigraphically above the contact between the two formations. Preliminary isotopic analyses of the carbonates yielded d¹³C values ranging from -14.1 to -9.1‰ PDB and d¹⁸O values ranging from -4.1 to -1.5‰ PDB. Molluscan fossils are relatively sparse within the limestones, and some, especially Turritella sp., appear to be reworked. The combination of the regional historical geology, stratigraphy, lithology, invertebrate and vertebrate fossils, and sedimentary structures indicate that these seeps were in a moderately deep-water, outer shelf depositional environment.