PETROLOGY AND PETROGRAPHY OF THE TEPEE BUTTES (CRETACEOUS) METHANE-SEEP CARBONATES

The Tepee Buttes (~76 Ma) are ancient methane seep deposits of the Cretaceous Interior Seaway. They formed from methane-rich fluid seeping out of the underlying Pierre Shale and Niobrara formations during early Laramide faulting. Similar to modern seeps, these carbonates most likely formed from geochemical changes brought on by methanotropic bacteria (oxidation of methane) and sulfate reducing bacteria.

The Tepee Buttes carbonates consist of six different lithofacies: (I) vuggy intrapelsparite with complex cements and articulated and disarticulated clams; (II) dense coquina of articulated lucinid clams in vuggy intrapelsparite; (III) clams in micrite; (IV) limestone concretions; (V) micrite without shells; and (VI) vuggy peloidal intraclastic microbialite. 106 buttes were mapped for lithofacies near Colorado Springs, CO. Contrary to earlier studies that suggest a concentric arrangement of lithofacies around a central vent core, we found evidence of horizontal bedding of limestone separated by thin shale layers.

Samples were analyzed for petrographic fabrics and stable isotopes to determine a series of primary to late stage diagenetic features recording fluid diagenesis. Petrofabrics include pelmicrite, intrapelmicrite and intrapelsparite, and fringing, isopachous, botryodal and late stage spar cements. Lithofacies I consists of intrapelsparite and intrapelmicrite with fringing, botryodal and isopachous cements that fill in void spaces. II and III consist of sparitized shells filled in with spar or micrite. III has pelmicrite and II has intrapelsparite and voids with a thin (0.1mm) fringe cement and thick (1.25mm) botryoids. IV and V consist of a homogenous gray micrite. VI contains intrapelmicrite, intrapelsparite and pelsparite.

?13 C isotope ratios for the microbial peloids range from -36.77 to -10.50 ‰ PDB. The primary micrite with siliciclastic grains range from -38.58 to -12.07. Values for the intraclasts within the primary micrite are -35.86 to -7.33. The primary cements include yellow calcite fringe with ?13 C values of -39.28 to -36.98, botryoidal and isopachous cements with ?13 C values of -48.73 to -34.95. During the latest stages, blocky calcite spar cement filled in the remaining void space and preserves a ?13 C ratio of -34.31 to -12.07.