Abstract
A bed containing large ammonoids (mostly Mesopuzosia yubarensis, 40-60 cm in diameter) and an underlying bed containing huge calcareous concretions in the middle part of the Obisagawa Member (lower Coniacian), Ashizawa Formation, Futaba Group, are exposed in the Iwaki City Ammonite Center. This study investigated these beds to reconstruct their formation on the basis of sedimentary facies and taphonomy, as well as geochemical analyses of the concretions, including major elements, mineral components, and carbon and oxygen isotopic compositions. Numerous shells of M. yubarensis lacking soft tissues may have been transported into the littoral region through postmortem drift from the initial offshore habitat. Subsequently, these shells were partially destroyed (especially body chambers and umbilical whorls), sorted in size, accumulated, and transported offshore by a series of storm waves and storm-induced currents. Finally, the remains were scattered on mounds of hummocky cross-stratification and rapidly buried within amalgamated hummocky cross-stratified very fine sandstone on the offshore side of a lower-shoreface sedimentary environment. Abundant calcareous concretions of 15-194 cm in diameter and with prolate to oblate spheroid shapes are densely and uniformly distributed under the ammonoid bed. Considering the spatial positioning of the ammonoid shells and concretions within the beds, and isotopic values of δ13C = −6‰ to −1‰ and δ18O = −11‰ to −6‰ for 21 concretion samples, the concretions are interpreted to have begun to form in association with the decomposition of organic matter that had accumulated under the influence of storm waves and storm-induced currents, and carcasses of prolific meiobenthos organisms within shallow substrata. Subsequently, the concretions were buried more deeply and enlarged through further filling of calcium carbonate involving bicarbonate ions generated by the methanogenetic decomposition of organic matter.
