Underground Geology and Diatom Thanatocoenoses of the Late Quaternary Sediments in Kurashiki City, Okayama Prefecture, Japan

Core samples from the two boreholes were studied to clarify the sedimentary environment by diatom assemblage analysis: the results are as follows. 1. The Holocene sediments are divided into two parts, namely the sand and the silt facies (Hs Hc). It seems that they are synchronous and heterofacies to each other. The upper part of Hs-facies may be correlative with sediments during and after the small regression of Yayoi belonging to Subatlantic age. And lower and middle parts of Hs-Hc-facies may be correlative with sediments during the Jomon transgression. 2. Beneath the Holocene, the late Pleistocene sediments are developed to fill up dissected and buried valleys. The uppermost Clay (C) may be correlative with the Nanagochi formation in the Tokyo downtown area and the Nanyo formation in Nobi Plain. The sand and gravel (G-1) may be correlative with the Buried terrace gravel-1 in the Tokyo downtown area. The clay underlying the former (Mc) may be correlative with the Tachikawa Loam in South Kanto district. Carbon date of a sample from the lower part of the alternation of sand, silt and clay (Alt. s-c) shows the value of 24, 260+2, 810 -2, 080y. B. P. 3. Some additional discussion on the systematic position of Coscinodiscus temperei, Cymatotheca weissflogii and Tryblioptychus cocconeiformis are made from a viewpoint of morphological features of diatom valves.