古淀江潟の予察的検討

A glassy fine volcanic ash layer and a diatomite bed were discovered to intercalate in the Holocene sediments from several boring cores in the Yodoe Plain，Tottori Prefecture．The volcanic ash layer，about 0.2cm thick，intercalates in organic mudstone．It is rich in bubble-type volcanic glasses，containing subordinate pumice-type volcanic glasses and rare phenocrysts of hyperthene and augite．The glasses are mostly brown in color．On the basis of EPMA analysis，these are safely idetified to the Akahoya Tephra which eruptied from the Kikai Island，South Kyushu，6,300y．B．P．The deatomite bed，the horzon of which is 2m below the surface，is mostly formed of Fregilaria，and is associated with the other freshwater spcies．The existence of the diatomite bed strongly suggest an abrupt enviromental change from brackish to freshwater，in 1,000 to 2,000y．B．P．The Paloe-Yodoe Lagoon has been imagined from mrchaeological studies，and the above discoveries are important to reconstruct an ancient lagoon and environmental changes．