COLD-SEEP SEAFLOOR AND NEAR-SEAFLOOR DIAGENESIS INCORPORATING THE LUCINID BIVALVE NYMPHALUCINA SP., TEPEE BUTTES, NEAR PUEBLO, COLORADO, U.S.A

In the methanogenic, cold-seep limestones of the Tepee Buttes coquinoid Nymphalucina sp. bivalve fossils are ubiquitous, occurring as natural casts and exocast steinkerns of clear, blocky, ferroan calcite spar. Natural moulds developed from mixtures of carbonate particles and cements: botryoidal fibrous, HiMg calcite; neomorphosed Hi Mg calcite; pellet grainstones and wackestones; and clear, blocky and serrated-dendritic rhombic ferroan calcites. By contrast, the shells of modern lucinids are aragonitic. In the instance of Nymphalucina sp., shells also appear to have been aragonitic, but were dissolved incongruently to be subsequently cast with clear, blocky ferroan calcite. Significantly, dissolution and precipitation of blocky, ferroan calcite was accompanied by partial and simultaneous silica replacement and cementation of shell walls, leaving shell wall remnants floating in the blocky, ferroan calcite cement.

d¹³C & d¹⁸O stable isotope ratios (in ‰) of these cements are, respectively, botryoidal, fibrous, HiMg calcite –41 to –44 & -2 to -4; serrated-dendritic rhombic ferroan calcite –10 to –12 & -11 to –14; clear, blocky, ferroan calcite –15 to –43 & -12 to –14; and neomorphosed HiMg calcite -24 to -40 & -0.3 to -11. In addition, microprobe analyses of these cements have different average MgO, FeO and MnO mass percentages. They are, respectively, 2.9%, 0.08% & 0.04% (botryoidal, fibrous, HiMg calcite); 0.3%, 1.1% & 0.7% (serrated-dendritic rhombic ferroan calcite); 0.4%, 1.5% and 0.8% (clear, blocky, ferroan calcites); and 0.9%, 0.7% and 0.3% (neomorphosed HiMg calcite).

Previous interpretations using d¹⁸O stable isotope ratios of the ferroan calcites have noted that they are either products of late stage diagenesis, or of little late stage diagenetic overprinting. We concur with the second opinion despite the extensive present-day subaerial exposure. We further propose that the depleted d¹⁸O stable isotope record reflects early, seafloor and near-seafloor diagenesis where marine fluids interacted with cold, d¹⁸O depleted fluids migrating from below. This observation establishes that carbonate mineral phases depleted in d¹⁸O can precipitate in marine settings at or near the seafloor during early marine diagenesis.