The origin of muddy sand sediments associated with mud volcanism in the Horonobe area of northern Hokkaido, Japan

The origin of muddy sand and gas in muddy sand sediments in the Horonobe area of northern Hokkaido, Japan, was investigated by analyzing the mineralogical and chemical compositions of the sediments and the chemical/isotopic compositions of the gas. X‐ray fluorescence and X‐ray diffraction analyses indicate that chemically, the muddy sand is derived from a mixing of components from the Hakobuchi and overlying formations, and that the characteristic mineral of the muddy sand is heulandite, which, in the study area, has been detected only in the Hakobuchi Formation. These results suggest that the sediments ascended from depths of at least 2200–2400 m. The δ 13 C CH4 values and the methane/(ethane + propane) ratios of the gas indicate that the primary origin of the methane is by thermogenic decomposition of coal‐bearing beds in the Haboro or Hakobuchi formations, or further deep sources. This study provides new data on processes of onshore mud volcanism in Japan, and contributes to an understanding of processes of subsurface mass transport in regions of mud‐volcanic activity.