A REINTERPRETATION OF DEPOSITIONAL CONTROLS ON THE CRETACEOUS TEPEE BUTTES METHANE SEEP DEPOSITS, COLORADO

Now visible as meters-high limestone mounds exposed above the Pierre Shale, the Tepee Buttes of Colorado record marine communities – dominated by Nymphalucina occidentalis – that thrived on methane seeping along faults of the Campanian Western Interior Seaway. The buttes were first interpreted as fossilized seeps in 1895, and geochemical studies have since identified a methane signature. Past reconstructions have identified a central vertical gas-flow conduit with other facies/faunal associations ringing the core concentrically, defining a cemented chimney rising off the sea floor with dipping flank beds. This study takes a taphonomic approach to reinterpreting original seep morphology. Instead of a single vent-controlled facies distribution, we have found a complex intermixing of facies. In addition, we see evidence for shell bed formation and reworking at the sediment-water interface that may contradict the existence of steeply dipping flank beds.

At outcrop scale, roughly horizontal shell beds consistently appear in the cap-rock of many buttes, with a shallow dip apparently unrelated to present topography. Polished slabs show that reworking at the sediment-water interface appears to be the major control on many sedimentologic and taphonomic features. The presence of bivalve fragments/hash in different facies and the relation of shell-bed geometry to geopetal mud-fill support event-controlled sedimentation and/or current reworking as opposed to gas-release or slope-controlled reworking. Some evidence for brecciation and/or soft-sediment disturbance of sedimentary fabrics by rising methane gas has been seen. No pattern of graded beds has yet been observed, and thus no evidence for explosive gas release and settling, or for avalanche-like slope deposits. Therefore we interpret the shell beds to have been reworked and deposited on a near-horizontal surface. Evidence for current or storm reworking would also place the seeps at an original depth too shallow to allow the presence of methane clathrates, as some interpretations suggest. Our interpretation favors a low-relief seep topography, and further study of the multiple generations of microbially-derived authigenic carbonate in the limestone should help clarify possible effects of early cementation on slope stabilization and seep morphology.