SUBMARINE CARBONATE DIAGENESIS IN A FOSSIL METHANE-METABOLIZING COMMUNITY: CAMPANIAN COQUINOID LIMESTONE IN THE PIERRE SHALE “TEEPEE BUTTES”, WESTERN INTERIOR SEAWAY, PUEBLO REGION, COLORADO, U.S.A

Conical and flat-topped hillocks dotting the shale plains near Pueblo are erosional remnants armoured by irregular limestone lenses. Dense lucinid bivalve assemblages and carbonate cements with stable carbon isotope signatures indicative of mineral precipitation in the presence of methane are characteristic of the lenses. Coquinoid limestones comprise pelletal packstones and grainstones, mostly articulated lucinid shells, and fibrous botryoidal, anhedral blocky, euhedral blocky and lublinite-like cements. Lucinid shells typically are preserved as steinkerns, where shells have dissolved and refilled by blocky euhedral cement; less commonly, they may be partially replaced by quartzine and lutecite preserving microstructure. Lublinite-like cements are intergrown with kaolinite, postdate botryoidal and anhedral blocky cements, but predate blocky euhedral ones. Meniscus lublinite-like cement bridges span blocky euhedral cement filled cavities. Microprobe analyses indicate that pelletal carbonates and botryoidal and anhedral blocky cements are magnesium rich, whereas lublinite cements are iron and magnesium rich, and blocky euhedral cements are iron rich. d¹³C‰ and d¹⁸O‰ PDB stable isotope ratios of shells and cements are, respectively: baculitid 2.4 and -4.6, botryoidal –44.8 to –46.3 and -2.1 to –4.0, pelletal –33.7 to-39.9 and -5.2 to –9.7, non-pelletal micrite –31.5 to –40.8 and -9.8 to –11.5, anhedral blocky –28.4 to –31.9 and -7.8 to –10.7, lublinite-like –12.6 to –15.7 and -11.3 to –13.2, and euhedral –15.1 to –30.4 and -11 to –13.6. Submarine diagenesis proceeded with the oxidation of methane in open exchange with overlying seawater to restricted conditions below the seafloor with incongruent dissolution of aragonite and partial dissolution of magnesian calcites in the zone of sulphate reduction and subsequent precipitation of kaolinite and iron calcites.