Holocene Shell Bed in Imari City, Saga Prefecture, Kyûshû, Japan

Abstract

The Holocene marine deposits, here called the Imari Shell Bed, was exposed by digging for construction of a water reservoir near the head of Imari Bay, northwestern Kyushu. The marine deposits measure 3.4 meters in thickness and its upper surface is 0.4 meters above the sea level. The deposits consist mainly of silty sediments in which abundant marine molluscan shells and poorly carbonized plant fragments are contained.The molluscan fauna comprised of 22 species (Table 3), including eight of gastropods, one of scaphopods and 13 of pelecypods. All of these species are dwellers of an embayment environment and are known to range from the high tide down to the shallow neritic zone. Among the distinguished species, Anadara granosa bisenensis has been accepted as an index subspecies marking the Holocene transgression phase in Japan. Although no diatom fossils were discovered in the shell bed, pollen and spore fossils could be extracted from the silty sediments, and these are listed in Table 5. The fossil floras points to a warm climate during deposition of the Imari Shell Bed.The radiocarbon dating on the carbonized wood piece occurred from the upper part of the Imari Shell Bed indicates an age of 4430±85 yrs. B.P. (GaK-4486).A relict sea cliff characterized by such erosive features as, notch, pit, cave, crevice and overhang is well preserved along the inner margin of the alluvial plain of the Imari Bay area, and occur up to about four meters above sea level. Honeycomb structures were also recognized in the cliff.From the paleontological and topographical evidences, it is considered that the Imari Shell Bed was deposited during the rising of sea-level up to less than four meters above the present sea level. This marine transgression probably corresponds to the middle stage of the Jomon Period.