|Budget Amount *help
¥1,800,000 (Direct Cost : ¥1,800,000)
Fiscal Year 1997 : ¥100,000 (Direct Cost : ¥100,000)
Fiscal Year 1996 : ¥200,000 (Direct Cost : ¥200,000)
Fiscal Year 1995 : ¥1,500,000 (Direct Cost : ¥1,500,000)
Diatom analyzes were made on exogenetic rock samples collected from the Neogene strata, which contain sea cow fossils and fossil whale bones discovered in Yamagata Prefecture. Dating of the lithostratigraphic unit and reconstruction of the depositional environment were the basic purposes of this investigation, however, precise paleontological resolution could not be obtained because the diatom assemblages have almost gone out. Most of the diatom fossils were completely dissolved by silica diagenesis, making the corroded valves impossible to identify. As other biosiliceous sediments as well as diatom skeletons seem to remain out of the opal-A facies.
Upper Miocene deposits distribute well in the hills on the western margin of Yamagata Basin in Yamagata Prefecture. Because, that their lithofacies are quite similar, that only a few key beds are available and that their lithology remarkably change lateraly, the lithostratogrphic division and the geographical distribution have not been defin
ed properly. Based on the extensive geologic survey, the strata are divided into five formations. Two of those namely, the Hashikami Formation and the Kuzusawa Formation are a contemporaneous heterotopic facies. According to the diatom fossil analyzes under this new lithologic division, the Kuzusawa Formation is correlated to the Denticulopsis dimorpha Zone. In addition, it is possible that the Hashikami Formation may also correspond to the D.dimorpha Zone or to underlying the Thalassiosira yabei Zone.
During the last dicade, Neogene diatom biostratigraphy has been advanced in the North Pacific region. Most of these biozones and datum levels were correlated to the geomagnetic polarity time scales of Berggren et al. (1985) or Cande and Kent (1992). In this study most applicable Neogene diatom zones and events in the middle-to-high latitudes of the Northwest Pacific have been evaluated and recalibrated to the geomagnetic time scales of Cande and Kent (1992,1995). Several planktonic taxa indicating paleotemperature can be selected from diatom assemblages based on their distributional patterns, which are drawn characteristically by fossil frequencies in time and space, and then a diatom temperature index (Td index) reflecting the extent of paleo-isotherm is defined as Td=W+T-C,where W,T and C are frequencies in percentage of warm, temperate and cold water species respectively.