Sedimentation in Matsushima Bay, Miyagi Prefecture, Northeast Japan

Matsushima Bay is sheltered from the Pacific Ocean by small islands situated in the bay mouth area. The bay is about one to three meters deep and the floor is rather flat. Tidal currents flow in and out of the bay through several narrow channels situated between the islands during the ebbing and flowing tides, and one river and two canals drain into the bay. Twenty-five surface and five cored sediment samples were obtained in and around Matsushima Bay in February and March, 1991, and were examined in order to reveal depositional processes in the bay. Dark olive-colored muddy sediments mostly cover the bottom surface of Matsushima Bay, while sandy sediments dominate outside the bay. Muddy sediments are composed mainly of dusty materials in the bay, whereas they consist of volcanic glass shards and diatom fragments outside the bay. Sandy sediments consist chiefly of pumice, quartz, plagioclase, volcanic glass shards and a small amount of heavy minerals in the bay, but contain a considerable amount of heavy minerals outside the bay. The composition of sandy sediments suggests that they were derived mainly from Neogene strata exposed along the coastal cliff of the bay. Cored sediments are composed mainly of dark olive-colored mud with intercalations of sand layers, except for a core obtained in a channel in the east of the bay, which consists of massive sand. Biogenic disturbance is developed in the mud, and parallel laminations areidentified in the sand layers. The sand layers occupying the lower part of cores 1 and 4 were formed about 30 years ago as judged from depositional rates of these cores, and the sand lavers are inferred to be deposits induced by the Chile Earthquake Tsunami in 1960