THE GEOLOGICALLY YOUNGEST METHANE SEEP IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

During the Late Cretaceous most of North America was covered in a broad, shallow epicontinental seaway (the Western Interior Seaway) littered with cold-methane seeps. These unique environments were most common during the middle and late Campanian time period and are concentrated in the northern part of the seaway in modern Montana, Wyoming, and South Dakota. We present the first known occurrence of methane seeps in the lower Maastrichtian Baculites baculus ammonite range zone of the Pierre Shale, which represents the geologically youngest occurrence of such deposits in the Western Interior Seaway. The seeps outcrop on the Cedar Creek Anticline in east central Montana. The exposed deposit we focused on (AMNH locality 3911) is a 2.00 m wide and 2.71 m high massive vesicular limestone that contains a rich invertebrate macrofossil fauna. The dominant faunal element is a species of lucinid bivalve (Nymphalucinia occidentalis) which is found alongside additional bivalves, gastropods, scaphitid and other heteromorph ammonites, isolated ammonite jaws, scaphopods, and echinoids. There are a total of 40 identifiable species in the seep deposit, 17 of which are epifaunal, 19 infaunal, and 4 nektic/nektobenthic. Unlike many older Campanian seep deposits, inoceramid bivalves are a rare component of the fauna at this site. Baculitid ammonites are also absent at the seep, and very rare in the non-seep shale deposits at the same stratigraphic horizon. The seep also differs from older examples in the absence of seep-associated concretions (SAC’s) around the main carbonate mass. The combination of vuggy limestone and a fauna with high numbers of winged gastropods, flat epifaunal bivalves, and lack of encrusting organisms indicates a muddy seafloor that likely inhibited low surface area epifauna. The highly vesicular texture of the carbonate mass, absence of SAC’s, and the fragmented taphonomy of the preserved fauna could also indicate rapid burial and a high rate of methane flux at this site. An overall change in lithology from shale to siltstone containing less organic matter may explain the disappearance of methane seeps above this stratigraphic level on a regional scale, as the coarser sediments may not have promoted the canalization of methane emissions.