PLIOCENE SHELLS FROM THE SHIODANI AND AKUMI OIL-FIELDS.

Abstract

The stratigraphy of the Shiodani Oil-Field, Niigata Prefecture, was fully investigated by Mr. I. Matsuzawa of the Imperial Geological Survey of Japan, according to whom the geological sequence of this area, enumerated from below, is as follows:
Tertiary
Lower Series……Alternation of Sandstone and Shale.
(Tuff Sandstone.)
Middle Series……Gray Shale and Sandy Shale. 450m.
Upper Series……Shaly Sandstone and Sandstone. 300m.
Uppermost Series……Sand, Gravel and Clay.
Quaternary
Terrace Deposits and Alluvium.
Although fossil shells are rather rare in this field, we find many well preserved shells in the “Sandy Shale”, especially in the sand lenses of this horizon. The total faunal list is given in Table I. of the Japanese paper. A glance at the table is sufficient to show that this fauna is a mixed one, that is to say, a fauna of the sand facies is mixed with that of mud. The fauna, no doubt, is of cold current (Oyashio) type.
The species that have been determined number only 32; the number of those that are not known to be living now amount to 14, which is 44 percent of the entire fauna. Needless to say, the number of fossils is too small to allow any determination of the geological age of this fauna. But we find some important fossil species, such as Clavatula cfr. dainichiensis (Yokoyama), Anadara trilineata amicula (Yokoyama), Mercenaria yolcoyamai Makiyama, Turritella (Haustator) fortilirata saishuensis Yokoyama, etc.-all essential components of the so-called Omma fauna of Kaga Province, which fauna is considered as a typical Pliocene fauna of the cold current type. Therefore, the fossil fauna of this field may be said to be of Pliocene (lower to middle) age.