FROM SEAFLOOR TO SUBSURFACE: COMPARISON OF COLD SEEP CARBONATES FROM THE TEPEE BUTTES (CRETACEOUS) AND THE STONE CITY BLUFF (EOCENE)

Colds seeps create a unique ecosystem and geological setting through the interaction of microbes and methane seeping from the subsurface. Much of the focus of cold seep research has been on the surface expression of seeps; the subsurface plumbing is poorly understood. The Tepee Buttes and the Stone City Bluff barrel concretions allow examination of the surface and subsurface features of seeps.

The Cretaceous Tepee Buttes are a well-known cold seep system formed in the Western Interior Seaway. They are faunally dense, carbonate mounds preserved in the Pierre Shale from Montana to New Mexico and formed on the forebulge of the Laramide Orogeny as methane migrated from the subsurface. The methane is the basis of the ecosystem with the chemosynthetic organisms forming the base of the food chain while promoting carbonate production. The Tepee Buttes are dominated by chemosynthetic lucinid bivalves and peloidal textures indicating a thriving bacterial mat. Very negative stable carbon isotopes show preservation of early-formed fabrics, while less negative carbon values and more negative oxygen values reflect diagenetic trends seen in later cements and iron rich carbonates.

The Stone City Bluff on the Brazos River, Burleson County, TX preserves unique carbonate concretions that may represent the subsurface plumbing of a seep system that formed in an Upper Eocene shallow shelf siliciclastic transgressive systems tract. These barrel concretions range up to 20 cm in diameter and 70 cm in length, and in situ concretions were found with the elongate axis extending vertically through the sediments. These concretions are septarian in nature and preserve a small, central pyrite-lined micropipe that extends the length of the barrel. This micropipe is the hypothesized pathway through which methane saturated fluids traveled. The concretions exhibit negative carbon stable isotopic values consistent with methane; with early formed matrix exhibiting less negative values, and later septarian fracture and micropipe fill have more negative values.

No evidence has been seen for subsurface plumbing in the Tepee Buttes, and no faunally dense carbonate mounds have been reported for the Stone City Bluffs. By looking at the two seep systems together, the full morphology of a seep system can be explored, from the subsurface to the seafloor.