BACTERIAL FOSSILS FROM CRETACEOUS METHANE-SEEP CARBONATES

It is widely accepted that the production of authigenic carbonates at methane seeps is facilitated by bacterial mediation. Supportive geochemical and petrographic data are found in a variety of modern and ancient seep deposits. The benthic microbial communities responsible for the formation of the carbonate are known from active deposits but have remained largely elusive in the rock record (e.g., Miocene of Italy).

Putative bacterial sheaths from Cretaceous (~76 Ma) cold-water methane seep carbonates of Colorado (Tepee Buttes) have been recognized through transmitted light and scanning electron microscopy. The Tepee Buttes formed where methane-rich waters seeped out at the seafloor along early Laramide faults. Like modern methane-carbonates, these mounds probably formed by physiological processes driven at the seafloor by methylotrophic and S-bacteria.

The carbonates are composed of a complex variety of fabrics. Near the vent, the limestone is dominated by peloidal packstone and grainstone with abundant micritic intraclasts. Fragments and complete articulated shells of Nymphalucina are common. Primary cements occur as calcitized botryoids and isopachous rims. Later stage blocky spar fills the voids. Previous researchers demonstrated that the primary carbonates have d¹³C values of –40 to –45 ppm compared to the later stage cements with values of –12 ppm. Dissolution rinds are common in the cements. Pyrite and bitumens are found in patches throughout the primary carbonate.

The bacteria are recognized as sheaths and cocci replaced by sulfides or as microcrystalline external and internal molds. Morphologically, the bacteria occur as agglomerates of cocci and straight and curved rods (approximately 1 µm in diameter). Larger sheaths (7 µm diameter, 42 µm long) are also present as well as ‘mucilage’ between crystals. Bacteria are primarily found in authigenic carbonate peloids (0.5 mm diameter) that also contain framboidal pyrite. Some of the bacteria served as nucleation sites for botryoidal cements. Bacteria have not been found in the cements.