10:45 〜 12:15
[MIS19-P05] 能登半島北部に分布する飯塚層（中新統中―上部）から産するメタン湧水堆積物
キーワード：冷湧水、メタン、化石群集、化学合成群集
A meter-sized carbonate bodies with densely packed chemosynthetic bivalves (Conchocele) have been reported from middle-late Miocene Iizuka Formation in northern Noto Peninsula, Ishikawa Prefecture. Because of the co-occurrence of chemosynthetic bivalve Conchocele, the bodies would be formed under influence of methane and/or hydrogen sulfide rich fluids. However, the bodies haven’t been investigated in detail. Here, we report lithological and geochemical properties of the carbonate bodies with distribution of the bodies in northern Noto Peninsula.
The carbonate bodies with Conchocele were only distributed in western flank of Iizuka Formation. Among the localities, well-exposed outcrop with carbonate bodies are along Mitsukohama-coast near Wajima City. At least three carbonate bodies are cropping out along the coast. The bodies are mainly composed of creamy-colored dolomicritic diatomaceous mudstone. The bodies also contained fragmented dark gray mudstone cemented by microsparry/dolomicritic high-Mg calcite cements as a minor portion. Carbon isotopic signatures of those lithofacies are around +10 to +20‰ VPDB and -30‰ VPDB, respectively. Crocetane and pentamethylicosane (PMI), known as archaeal or seep characteristic biomarkers, were detected from the bodies.
Dominance of dolomicrite with highly positive carbon isotopic compositions indicate that the carbonate bodies were formed under strong influence of dissolved inorganic carbon generated in methanogenesis zone similar to methane-hydrate-bearing seep site in Joetsu Basin, the Japan Sea.
The carbonate bodies with Conchocele were only distributed in western flank of Iizuka Formation. Among the localities, well-exposed outcrop with carbonate bodies are along Mitsukohama-coast near Wajima City. At least three carbonate bodies are cropping out along the coast. The bodies are mainly composed of creamy-colored dolomicritic diatomaceous mudstone. The bodies also contained fragmented dark gray mudstone cemented by microsparry/dolomicritic high-Mg calcite cements as a minor portion. Carbon isotopic signatures of those lithofacies are around +10 to +20‰ VPDB and -30‰ VPDB, respectively. Crocetane and pentamethylicosane (PMI), known as archaeal or seep characteristic biomarkers, were detected from the bodies.
Dominance of dolomicrite with highly positive carbon isotopic compositions indicate that the carbonate bodies were formed under strong influence of dissolved inorganic carbon generated in methanogenesis zone similar to methane-hydrate-bearing seep site in Joetsu Basin, the Japan Sea.