長野県の中部中新統下部別所層に挟在するメタン湧水性石灰岩（穴沢石灰岩）のボーリングコア試料の岩相記載

抄録

The Anazawa Limestone is one of the largest methane-seep carbonate mounds intercalated in massive siltstone of the lower Middle MioceneBessho Formation, Northern Fossa Magna Region, central Japan. In August 2007, boring core samples were obtained in order to make clear the subbottom structure of the Anazawa Limestone. The drilling direction (N57°W, 35°S) is normal to the bedding, and the core samples were recovered continuously from the outcrop surface to 50.5 m in core depth. This is the first data report on its lithological succession and fossil occurrences in the core samples. The Anazawa Limestone is mainly composed of muddy micrite yielding abundant chemosynthetic bivalve fossils, such as vesicomyids (Adulomya uchimuraensis) and bathymodiolins (“Bathymodiolus” akanudaensis). In the core succession, the muddy micrite was intermittently confirmed in the upper part of the core (above 20.8 m in core depth), intercalating about 1-m thick beds of calcareous siltstone and massive siltstone. Chemosynthetic bivalve shells commonly occur from both micrite and siltstone in the upper part of the core. Almost all of the muddy micrite and calcareous siltstone are characterized by auto brecciated or fractured fabrics, which contain intraformational breccias of variably cemented micrite-intraclasts exhibiting both brittle and ductile deformation. Below the depth of 20.8 m grey massive siltstone is continuously distributed, but above 39.5 m core depth the siltstone contains abundant irregular-shaped small calcareous granules, less than 1 cm in diameter.