Stone City Bluff, Texas Formation of barrel concretions around methane seepage pathways in Upper Middle Eocene shelf sediments

Upper Middle Eocene siliciclastic transgressive systems tract sediments at Stone City Bluff, Burleson County, Texas, contain large (up to 8 in by 2 ft; 20 cm by 70 cm) barrel-shaped, vertically-oriented carbonate concretions comparable to pipe concretions associated with methane seeps. Centers of barrel concretions contain 0.4 in (1 cm) diameter micropipes with pyrite walls, indicating sulfate reduction associated with gas escape through sediments. The concretions are septarian with large shrink-age cracks partly filled with calcite. Septarian fracture fills show two episodes of calcite filling. Micropipes and carbonate matrix are split by the septarian fractures. Stable isotopic analyses of the concretions yield very low 13C values, from 29 to 35 Vienna Pee Dee Belemnite (VPDB). The less negative 13C values (-28 to 31 VPDB) of early-formed matrix, along with pyrite formation, indicate formation during sulfate reduction in the presence of carbon derived from oxidized methane and marine dissolved inorganic carbon (DIC), while the more negative values (-32 to 35 VPDB) of late stage septarian fracture fill and micropipe fill suggest methane oxidation in sediments with less dilution by marine DIC. This methane was generated in shallow shelf sediments during the Middle Eocene Climate Optimum.

Formation of barrel concretions around methane seepage pathways in Upper Middle Eocene shelf sediments, Stone City Bluff, Texas Academic Article