PALEOECOLOGY AND TOPOGRAPHIC EXPRESSION OF A CRETACEOUS COLD SEEP: A TAPHONOMIC ANALYSIS OF THE TEPEE BUTTE LIMESTONE

The Tepee Buttes of the Campanian Pierre Shale exposed south of Colorado Springs, CO were produced by hydrocarbon seepage along front range faults below the Western Interior Seaway. This interpretation was first suggested by Gilbert and Gulliver in 1895 and has been further demonstrated through geochemical analysis and faunal studies. The Tepee Buttes have been reconstructed as having a central vertical conduit for methane flow recognized by clam-rich facies. Other facies/faunal associations were then interpreted to ring the central core in a roughly concentric pattern. The seafloor exposure of the buttes has been interpreted as a cemented chimney rising off the sea floor with dipping flank beds. Our work on a wide distribution of buttes reinterprets the depositional history of the mounds by recognizing horizontal layers deposited near the sediment/water interface as the dominant pattern. A purely vent-driven faunal distribution was not seen, and the paleotopography of the mounds is in question. Exposed limestone at the top of the buttes usually has a dense accumulation of articulated and disarticulated Nymphalucina occidentalis. Modern lucinids at hydrocarbon seeps harbor chemosymbionts, so it is likely that N. occidentalis had a similar life habit.

In order to reconstruct the seep habitats and topographic expression, we identified distinct lithologic and taphonomic facies in the exposures. The butte limestones are characterized by several textures based on the taphonomic characteristics of the clams. These include: 1) densely packed articulated clams often in irregular clumps or masses, sometimes in horizontal layers, 2) matrix-dominated lucinid layers with mixed articulated and disarticulated layers, and 3) a distarticulated, sometimes-imbricated shell hash. Reworked clam beds indicate the existence of clams at the sediment/water interface. Nearly all clam-rich exposures showed some evidence of bedding. Based on our observations, the chimney reconstruction is less likely. Low relief mounds subject to frequent disturbance and redistribution of skeletal remains may be a more plausible reconstruction for the Tepee Butte seeps.