Late Holocene Sedimentary Environments at the Nakanotani Site in Toyooka Basin (Hyogo Prefecture) and the Upper Limit of Marine Facies

Abstract

Diatom assemblages in sediments and sedimentary sulfur contents were analyzed to clarify sedimentary environments during the late Holocene and to determine the upper limit of marine facies at the Nakanotani site in Toyooka Basin, Hyogo Prefecture. Three paleoenvironmental phases were recognized and deductions were made as to the sedimentary environments as follows. (1) Phase I (below the horizon of + 0.30m): Crassostrea bed and other molluskan fossils were found at the horizons between 0m and + 0.30m. Crassostrea gigas obtained from the horizon of ca. + 0.30m showed the 14C age of 3,110 =t 150 yrs BP. The appearance of marine diatoms such as Nitzschia granulata and relatively high sulfur contents indicate that sedimentation took place under marine littoral conditions. (2) Phase II (the horizons between + 0.30m and + 0.70m): High sulfur contents in peat sediments indicate that sedimentation may have occurred under the influence of sea water such as in a salt marsh. A brackish-water diatom Navicula pusilla appeared characteristically at the horizons between + 0.50m and + 0.70m. (3) Phase III (above the horizon of + 0.70m): Sulfur was not found in this phase. Although the appearance of a freshwater diatom Amphora ovalis var. affinis above the horizon of + 1.0 m suggests increasing wetness within the sedimentation basin, sedimentation is considered to have taken place under semiterrestrial conditions such as in a flood plain. Based on the molluskan fossils, diatom assemblages and sulfur contents in sediments, the upper limit of marine facies at this site can be regarded to be the horizons between + 0.30m and + 0.70m.