Micro and molluscan fossils and radiocarbon ages from the Holocene deposits of Moroiso area, southern part of the Miura Peninsula, south-central Japan
Kanie, Y.; Matsuda, T.; Kashima, K.; Matsubara, A.; Matsushima, Y.; Hirata, D.
Science Report of the Yokosuka City Museum 1991(39): 61-75
Molluscan shells, foraminifers and diatoms were collected from three pits excavated on the alluvial plain of the filled inlet, Moroiso, Miura Peninsula. These fossils were analyzed to know the depositional environment of the inlet of the Holocene time. The area was a narrow rocky inlet that had formed by the post-glacial transgression of the Jomon age. The lower part of the deposits is of marine environment and the upper of freshwater environment. The brackish water deposits are intercalated in the lower (western) area of the inlet. The transition from murine to freshwater occurred 1100 yBP in the lower area and betwen 6100 yBP and 1870 yBP in the upper (eastern) area of the inlet, and altitude of marine-freshwater boundary becomes higher progressively toward the inner part of the inlet. The molluscan, foraminifral and diatom assemblages from the lower deposits are mixtures of the sandy-bottom and muddy-bottom species and assemblages of the bay-mouth and the inner bay. They indicate depositional environment of the lower tidal zone to the upper littoral zone in the central and inner part of the bay. Molluscan and diatom assemblages indicate that the uppermost marine deposits in the lower to the central part of the inlet are brackish, and were deposited on the tideland in the inner bay. The near-surface deposits are the freshwater swampy deposit as indicated by the diatom assemblage. The changes in the environment of the inlet are considered as the result of burial of the bay bottom by depositional and crustal uplift associated with earthquakes.