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.
- Geological Society of America