FAUNAL ANALYSIS AND PALEOECOLOGY OF THE CRETACEOUS TEPEE BUTTE LIMESTONES, CENTRAL COLORADO

The Cretaceous Tepee Butte limestones formed at the bottom of the Western Interior Seaway and are exposed today as resistant conical mounds that rise above exposures of the Pierre Shale. These buttes offer the most widespread geographic distribution of Cretaceous fossil seeps in the world. The butte limestone is characterized by a dense accumulation of lucinid clams along with a small number of other associated species. Modern hydrocarbon seeps are commonly populated by a low diversity fauna with high dominance of a single species. In the case of the Tepee Buttes the dominant species is Nymphalucina occidentalis. N. occidentalis most likely harbored chemosymbionts that thrived on the methane, similar to modern lucinids that are common at hydrocarbon seeps.

The life habits and distribution of the fauna help to reconstruct the paleoecology and topographic expression of the seep systems on the sea floor. We measured diversity and abundance across 14 buttes from four separate butte clusters spanning approximately 75 km between Fountain and Boone, CO. In addition to N. occidentalis, other taxa include several species of inoceramid, two species of gastropod, baculites, and at least three species of heteromorphic ammonites including Solenoceras sp. and Didymoceras spp.

The buttes exhibit a general north to south increase in faunal abundance most noted in the ammonoid species. There are a number of factors that may control this trend including nutrient supply, oxygenation, and current direction, as well as the role of taphonomic processes. Faunal trends also differ among six established lithofacies; three display higher faunal diversity and abundance. The distribution and abundance of species helps to reconstruct the paleoecology within the facies. An abundance of articulated clams indicates that they are in place near the sediment-water interface, although not necessarily in life position. The presence of gastropods and inoceramids suggests the availability of substrates upon which to graze and cement. Abundant ammonites, in higher densities than the surrounding shale, may show the presence of a desirable food source in and around the seeps. These observations help to reinterpret the paleoecology and topographic expression of the Tepee Buttes within the Western Interior Seaway.