Widespread venting of methane-rich fluids in Late Cretaceous (Campanian) submarine springs (Tepee Buttes), Western Interior seaway, U.S.A.

From New Mexico to Montana, Campanian shales contain abundant submarine spring and seep deposits aligned along early Laramide basement faults. From spring conduit to sea floor, characteristic lithofacies are (a) a vuggy pelletoid micrite core to 30 m height, 5 m width; (b) lucinoid bivalve coquina in pelletoid micrite or grainstone; (c) carbonate slump breccias in shale; (d) concretion-bearing shale; and (e) organic-rich shale. Stable isotope analyses of early, zoned, marine core rock cements suggest (a) an oxidized methane source for the carbonate carbon, (b) venting of methane-charged fluids along active fracture zones, and (c) methanogenesis in older and coeval organic carbon rich muds. Six consistently zoned foraminifer and macrofaunal communities suggest a strong environmental stress gradient over a few metres from spring vents to the adjacent sea floor. These methane springs and their biotas were dynamic and episodically active over a 1.25 m.y. time span.