常磐堆積盆外側陸棚におけるメタン起源の炭酸塩コンクリーションと化学合成群集 : 茨城県北部中新統高久層群九面層の例 Methane-induced calcareous concretions and chemosynthetic community on an outer shelf of the Joban forearc basin : an example from the Miocene Kokozura Formation, Takaku Group, north of Ibaraki Prefecture, central Japan
Various-sized and -shaped calcareous concretions occur abundantly in the Lower Miocene Kokozura Formation of the Takaku Group which crops out on the Izura coast and its nearby north, Kita-Ibaraki City, north of Ibaraki Prefecture. The Kokozura Formation is mainly composed of intensely bioturbated, very fine-grained muddy sandstone, which may have been deposited on the outer shelf of the Joban forearc basin. The calcareous concretions are divided into the following six types on the basis of their shapes, sizes and modes of occurrence : Type I (large-sized concretions with undulatory surface, 1 to 6m in diameter), Type II (smaller, less than a few decimeters in diameter, and irregular-shaped concretions closely associated with Type I), Type III (pipe-shaped hollow concretions, a few decimeters in diameter and less than 2m in length), Type IV (sheet to lenticular concretions, 10 to 20cm in thickness and a few meters in lateral length), Type V (burrow-fill sand concretions concentrated and connected to each other in very fine-grained sandstone) which grades upward into Type VI (concreted sandstone bed, 2 to 3m in thickness, including abundant burrows). In the lower part of the outcrop, the Types I and II are dominant and commonly associated with Type III, whereas the uppermost part consists of Types V and VI which developed into a huge concreted bed, 2 to 3m thick, over 50m wide and 100m long. ^<13>C-depleted carbon isotopic compositions (-20.9 to -29.2‰ vs PDB) of Types I and II suggests that these concretions were formed under the influence of methane seepage. Molluscan assemblages dominated by Lucinoma acutilineatum are associated with the Type I, II, and III concretions. Most of L. acutileatum shells keep their life position. The mode of fossil occurrence indicates that L. acutileatum formed a chemosynthetic community and used methane seepage through chemosynthetic bacteria for their energy source.