MICROBIAL FOSSIL RECORD FROM THE TEPEE BUTTES (UPPER CRETACEOUS, COLORADO)

The Tepee Buttes (Campanian, Colorado) are a series of anomalous carbonate mounds formed within the siliciclastic Pierre Shale. Petrographic and scanning electron microscopy has demonstrated the preservation of bacterial body fossils and authigenic sulfides. The role of methane has been confirmed via low carbon isotope ratios and preserved bitumen in voids in the primary carbonate phases. A several-kilometer square area of buttes near Colorado Springs was mapped and sampled for micropaleontology and stable isotope analysis. Seven buttes were sampled for carbon and oxygen isotopes and contain a similar pattern of lowest d¹³C values for the primary microbial pelmicrite (as low as –49.5‰ PDB) and botryoidal cements (as low as –48.7‰ PDB) and highest values for the late-stage calcite spar (as high as –11.3‰ PDB). Other fabrics, including fringing cements, fall between these end values. Interestingly, although each butte showed the same increase in values, the absolute values were offset between buttes. Microbial fossils occur at a variety of scales. Microbialite is found as meter-scale thrombolitic zones and centimeter-scale stromatolitic crusts lining voids that are inferred to be the sites of ancient methane seepage. Petrographic fabrics suggestive of microbialite include peloids (0.1-1 mm diameter) and crusts of authigenic micrite (several mm thick). Primary evidence from SEM/EDS comprises µm-scale molds and casts of biofilms (0.1 µm thick by 1 to 5 µm wide sheets, filaments as narrow as 0.2 µm wide) containing pinnate bacteria (0.3 µm in diameter and 1 to 1.5 µm long), sheaths (2 to 4 µm in diameter), coccoids (0.5-1 µm diameter, up to 40 per cluster), and the presence of framboidal pyrite (6 to 8 µm diameter). The fossils are found primarily at the interface between primary carbonate fabrics (e.g., pelmicrite) and first-stage cements, suggesting a sharp physico-chemical gradient. These results are in agreement with studies of other ancient and modern seeps and suggest a morphological conservatism throughout geologic time.