30 Sediments, Carbonates (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

DEPOSITIONAL MODEL FOR THE TEPEE BUTTES METHANE SEEPS - ZONED AND CONICAL, OR FLAT AND PATCHY?

Paper number: 30-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Tepee Buttes are carbonate mounds in the Cretaceous Pierre Shale, located in the Western Interior Seaway, from Colorado to South Dakota. Isotopic and stratigraphic data suggest that the Tepee Buttes formed in ancient, shallow-water methane seeps. The interval of seep formation began 78.7 Ma and ended 69.1 Ma. The Tepee Buttes have been reconstructed as both low-relief carbonate features on the sea floor, surrounded by the siliclastic Pierre Shale, and conical mounds rising up to 5 m above the seafloor with a central methane vent, much as they look today. Field observations suggest that the Tepee Buttes mounds consist of carbonate beds ~ 50-100 cm thick, and that the emergent conical shape of the modern Tepee Buttes results from erosion. Our reconstruction of the Tepee Buttes as low-relief features on the seafloor is supported by observations of authigenic carbonate at modern methane seeps, which precipitates a few centimeters under the sediment surface, forming low ledges (~ 20-50 cm thick) on the seafloor.

We observed four facies in most Tepee Buttes mounds: dense lucinid limestone, vuggy limestone with few lucinids, brecciated limestone, and ropy-textured limestone (referred to as the ‘bioturbated’ or tube-worm facies by some previous workers). Some mounds have a micrite facies, and many have carbonate nodules. These facies had a patchy distribution on mounds rather than forming organized zones around a central vent. The ropy-textured facies appears to result from weathering rather than from bioturbation, or fossilization of tube worms, and may have formed in active methane vents where sediments were too disturbed to support macroinvertebrates. The lack of biotic and facies zonation and the presence of layered carbonates supports the low-relief, patchy depositional model for the Tepee Buttes. The paragentic sequence of Tepee Buttes mounds began with the formation of pelloidal micrite with fossils, followed by generations of sparry and botryoidal calcite cements. Relatively late in the sequence, ferroan calcite replaced the lucinid shells and filled vugs. Silica replacement of lucinid shells occurred last in the paragenetic sequence. Isotopic data suggest that all non-ferroan calcites formed early and show the influence of methane.