北海道東部および勇払沖における中新統の石灰質ナンノ化石層序と珪藻化石層序

抄録

Calcareous nannofossil shows sporadic occurrence from the Miocene formations in Hokkaido, possibly due to its nature province mainly in the pelagic ocean in a low latitude. Despite the rare occurrence, it is significant data to assign the fossil zones and designate the accurate geologic age for the deeper formations under the ground where the index fossil diatoms for the Miocene are dissolved by the diagenesis for the silica mineral phase transition. This report indicates the useful calcareous nannofossil biostratigraphy of the Miocene distributing deeper portion of subsurface in eastern to southern Hokkaido.

Calcareous nannofossil assemblage interpreted as being in CN3 (Okada and Bukry, 1980), with NPD3A (Yanagisawa and Akiba, 1998) diatom assemblage is found from the Well-A located in the Nemuro area, eastern Hokkaido. The calcareous nannofossil assemblage is characterized by associating Helicosphaera scissura but missing Sphenolithus heteromorphus and Discoaster species.

Calcareous nannofossil Catinaster coalitus ranging for the CN6-8a zones occurs from the outcrops of the Chokubetsu Formation in the Ishiizawa section in the Atsunai area, 130km southwest direction from Nemuro. Its occurrence from Hokkaido is the first discovery. The same samples are assigned to NPD5C of diatom zone. The occurrence shows the province of C. coalitus expanded to the Atsunai area during early Late Miocene.

The boundary of Major interval F and E defined by Young（1998）where coccolith size of genus Reticulofenestra drastically becomes smaller to upwards is recognized in the Well-B situated offshore Yufutsu. Discoaster quinqueramus is found from the interval E of the Well-B and the outcrops of the Nina Formation in the Biratori area（Okada, 1981）.

The occurrences during the CN3-4, CN6-8a, Major interval F and E and CN9 are considered to relate with the intermittent warm water current reaching Hokkaido for a short period in Miocene.