The origin of the carbonate mud constituting Paleozoic mid-shelf and deeper-water mud mounds has long been debated. Mississippian mud mounds and their laterally adjacent coeval inter-mound strata in the Lake Valley Formation are well exposed along cliff faces in south-central New Mexico. The mounds originated on erosional surfaces on a gently sloping ramp in 110 to 250 m water depth.

Carbonate muds that constitute the inter-mound strata, mud mounds, and syndepositional neptunian dikes within the mounds are low-Mg calcite microspars with similar petrographic, cathodoluminescent, and fluorescent characteristics. Carbon and oxygen stable isotope analyses demonstrate that the microspars constituting the inter-mound, mound, and neptunian dikes have average δ13C values of 2.6‰ (σ = ±0.2), 3.4‰ (σ = ±0.6), and 4.8‰ (σ = ±0.3) PDB, respectively. Additionally, well preserved brachiopods from the inter-mound strata and from Muleshoe mud mound have δ13C values markedly different from each other but similar to their surrounding microspars, i.e., brachiopod [Xmacr]'s = 2.7‰ and 3.9‰ PDB, respectively. The stable-isotope distributions within each environment, similarity of values between microspar and brachiopod data from the same depositional environments, as well as agreement of δ13C values with other investigations of coeval strata support the conclusion that the measured δ13C values are essentially primary values.

It is envisioned that 13C-enriched fluids sourced by bacterial fermentation in the subjacent strata rose into the mounds, some through neptunian dikes, and influenced the precipitation of the microspars in the mounds and dikes. Near the sediment-water interface, these fluids mixed with normal marine waters, thus precipitating the microspars, or their precursors, which have high δ13C values that characterize the mud mounds. The inter-mound microspars and inter-mound brachiopods have the lowest δ13C values and are in agreement with worldwide coeval marine calcites, that is, the inter-mound microspars formed under normal marine conditions. The high δ13C values measured in all of the mound-related samples indicate that all mounds received a contribution of 13C-enriched waters. This relationship indicates that microspar precipitation in the mounds was associated with seepage of methanogenic fluids from the underlying strata.

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