The Campanian-Maastrichtian Boundary in the Pierre Shale, Cedar Creek Anticline, Montana: Environment of Deposition Based on Geochemical, Sedimentological, and Paleontological Evidence

The Pierre Shale is well exposed on the Cedar Creek Anticline in east-central Montana. It is 65 m thick and consists of silts and clays deposited in the Late Cretaceous Western Interior Seaway. The section spans the Campanian-Maastrichtian boundary and comprises the upper Campanian Baculites eliasi Zone, corresponding to the “Inoceramus” redbirdensis Zone, the lower and upper parts of the lower Maastrichtian B. baculus Zone, corresponding to the Endocostea typica Zone and “Inoceramus” incurvus Zone, respectively, and the overlying lower Maastrichtian B. grandis Zone, corresponding to the Trochoceramus radiosus Zone. The Pierre Shale is richly fossiliferous and contains a diverse molluscan fauna, most of which is preserved in concretions. We reconstruct the environment of deposition of the Pierre Shale on the Cedar Creek Anticline based on an analysis of the mineralogy and geochemistry of the concretions, the grain size of the sediments, and the composition of the fauna. Concretions in the B. eliasi Zone consist of either Fe-Mn or siderite and, combined with the depauperate fauna, suggests that the bottom waters were hypoxic at the time, with intense organic matter diagenesis below the sediment-water interface, driven by reduction of Fe3+, at least in part coupled with anaerobic oxidation of methane (Fe-AOM). The limestone concretions in the B. baculus and B. grandis zones, in contrast, formed in early diagenesis, with organic accumulations developing over tens to hundreds of years. The diverse fauna in these zones, including benthic molluscs and nektobenthic cephalopods, together with the isotopic composition of the concretions, suggests a more oxygenated water column closer to shore. The base of the B. baculus Zone coincides with the appearance of sandstone beds and methane seep deposits. The temperatures derived from the oxygen isotopic composition of the concretions suggest a cooling trend from the late Campanian B. eliasi Zone into the early Maastrichtian B. baculus Zone. The calculated temperatures range from 20.3° C to 26.6° C for samples from the B. eliasi Zone, 16.0° to 20.3° C for samples from the lower part of the B. baculus Zone (including the methane seep deposit, but excluding a concretion from the sandstone bed), and 19.4° C to 19.8° C for samples from the upper part of the B. baculus Zone. 40Ar/39Ar analysis of a sample of a bentonite from the bottom of the B. eliasi Zone yields a weighted mean age of 72.47 ± 0.11 Ma (2σ). In conjunction with the previously published age of a sample of a bentonite from the top of the lower part of the B. baculus Zone (71.96 ± 0.08 Ma (2σ)), they help bracket the age estimate of the Campanian-Maastrichtian boundary. The change in sediments and fauna reflect an eastward migration of the shoreline of the Western Interior Seaway in eastern Montana at this time.