Formation processes of shell concentrations in the Lower Cretaceous estuarine sediments of the Okurodani Formation, Tetori Group, Gifu Prefecture, central Japan

Abstract

The formation processes of shell concentrations in estuarine environments are complicated by the interplay between the original biogenic and overprinting sedimentary processes. In order to better understand this interplay, the formation processes of shell concentrations composed exclusively of monospecific Myrene tetoriensis (“corbiculoid” bivalve) occurring in the Lower Cretaceous estuarine deposits of the Okurodani Formation of the Tetori Group in central Japan were considered within in the context of their mode of life. In addition, the size–frequency distribution of shells was discussed in relation to the formation processes of shell concentrations. As a result, the autochthonous occurrence of M. tetoriensis clarified a gregarious mode of life in the muddy sediments of a central estuary. In contrast, the post-mortem exhumation, accumulation, and degree of transport of dead shells by hydrodynamic processes were reflected in various occurrence modes of allochthonous shell concentrations. The size distribution patterns of these shell concentrations suggested that the primary size composition of autochthonous living and dead assemblages could have been retained in the absence of sufficient overprinting, even through reworking and transport. This was probably possible because of the high production of dead shells as compared to the relative effect of sedimentary processes.