海成層の上限 (marine limit) の認定
1982 年 21 巻 3 号 p. 195-201
詳細
1982 年 21 巻 3 号 p. 195-201
This paper deals with the method to determine the upper limit of Holocene marine facies. The example for the Holocene sediments is given by Tamatsu Bed developed in the mouth of the Akashi River, Hyogo Prefecture. The Tamatsu Bed consists chiefly of unconsolidated sand (lower), marine clay (middle) and silt (upper), and the age being from 9, 000y. B. P. to 2, 000y. B. P.
We have examined the Tamatsu Bed on marine molluscan fossils, marine diatom assemblages, trace fossils of marine animals and lithofacies especially formation of pyrite crystals. The altitude of the upper limit above the present sea level is as follows:
Marine molluscan fossils +0.5m
Marine diatom assemblages +2.2m
Trace fossils of marine animals +1.6m
Pyrite crystals formation +2.2m
Among them diatom assemblages, especially Terpsinoe americana which inhabits at the intertidal zone of the mouth of a river may be the best indicator of sea level. It is noticed that the fine crystals of pyrite formed on the surface of the outcrop show the same upper limit as that of diatom assemblage.
