Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east‐central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well‐preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non‐seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non‐seep sites.

中文翻译：

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美国西部内陆上白垩统（马斯特里赫特）的甲烷渗流沉积物的古生态分析

甲烷渗漏是美国晚白垩世西部内陆航道的共同特征。我们首次记录了蒙大拿州中东部的雪松溪背斜上皮埃尔页岩中甲烷渗漏沉积物的发生。渗水存款发生在最下部杆菊石珽区（在Endocostea typica区域），对应于最低的马斯特里赫特地区。因此，它们是Western Interior Seaway中描述的最年轻的渗透物。我们对单个渗流矿床进行了详细的动物区系分析，并对渗碳碳酸盐和软体动物壳材料进行了地球化学研究，以确定古环境条件。保存完好的软体动物壳材料的氧同位素分析表明，水温在19至27°C之间，而渗碳碳酸盐的碳同位素值枯竭则指示了甲烷的厌氧氧化。渗流沉积物的形态表明甲烷到沉积物-水界面的强烈对流。与附近同时发生的非潜伏地点进行比较后发现，两种环境中都发生了类似的生物群，西洋参（Nymphalucina occidentalis）。底物似乎是渗透位点和非渗透位点的多样性和古生态成分的主要控制因素。