Characteristics and origin of bioclastic sediments of subtidal sandridges, off Matsuyama, Ehime Prefecture, southwestern Japan

Abstract

Bioclasts comprise considerable parts of seafloor sediments of shallow marine tidal sandridges in the Itsuki Nada Sea, off Matsuyama City, western part of the Seto Inland Sea, Japan. Characteristics of the bioclasts and patterns of their distribution are analysed in order to detect their sources of supply.
The bioclasts in the area studied are significantly concentrated on the sandridges, and are composed mostly of barnacle's plates and molluscan shells. The former mostly consists of disarticulated and abraded plates of a large barnacle Megabalanus rosa, and is abundant in the western part of the so-called Oozu Sandridge. Its content among the bottom sediment becomes higher toward the caldron which has been formed by tidal erosion and is situated upstream of the flood tidal current. In addition, the Oozu Sandridge is covered with sand- to granule-sized clastics and there is no rockground that can provide substrate for such barnacles to attach. This suggests that the barnacle's plates were produced in the caldron where rocky substrate exposed, and transported to the sandridge by flood tidal currents. On the other hand, the latter is dominated by disarticulated shells of a byssally attached semi-infaunal, muddy sand-dweller of Modiolus comptus. Although most of the shells are strongly abraded, well-preserved ones having periostracum are found from two areas: East and southwest of the Oozu Sandridge, where muddy sand bottoms, generally preferred by the species, are well-developed. These are interpreted as the potential sources of the bivalve shells.
It is thus considered that the bioclasts apparently concentrated on the sandridges are not to have been produced there, but transported and gathered to the sandridges from various surrounding environments due to tidal currents.