微化石層序からみた新潟地域における石油根源岩の時空分布
1995 年 60 巻 1 号 p. 76-86
詳細
1995 年 60 巻 1 号 p. 76-86
We describe in detail the paleoenvironment of the Niigata sedimentary basin related to the distribution of the petroleum source rocks based on the analysis of the foraminifera) assemblages in the latest Nanatani, Teradomari, Shiiya and early Nishiyama ages. The Teradomari stage is divided into five zones named LT 1 to LT 4 and UT based on the characteristics of the formaniferal assemblages. These are:
LT 1: mixed assemblage of calcareous benthic foraminifera (Globobulimina spp. and Uvigerina spp.) and arenaceous foraminifera (Haplophragmoides spp.)
LT 2: arenaceous foraminifera assemblage characterized by the abundant occurrences of Dorothia spp. and Haplophragmoides spp.
LT 3: Cassidulina-Gyroidinoides assemblage mixed with arenaceous foraminifera (Haplophragmoides spp.)
LT 4: Dorothia spp.-Haplophragmoides spp. assemblage UT: Spirosigmoilinella compressa, Haplophragmoides spp., and Martinottiella communis assemblage.
Both LT 1 and LT 3 zones are also characterized by the relatively abundant planktonic foraminifera. The assemblages of LT 2 and LT 4 zones, in which arenaceous foraminifera occur abundantly, indicate that the paleosea of the Early Teradomari age was anoxic environment. Furthermore, on the basis of distribution pattern of Nanatani stage, LT 1 to LT 4 zones, and Upper Teradomari stage, the paleogeographies of the sedimentary basin of the Niigata area in the middle to late Miocene are reconstructed as follows. The paleo-ocean in the Nanatani age was widely present in the Niigata area. The sedimentary basin of the earliest Teradomari age (LT 1-2) was shifted to the limited area in the western part of the Niigata area, such as Kubiki, Nishiyama-Chuo oil field, Minami-Nagaoka, and Minami-Aga areas, in contrast with both the Naka-Kanbara and eastern part of the Niigata areas where no sediment was deposited. On the basis of the paleogeography in the earliest Teradomari age, it is inferred that the organic rich sediments were deposited in the Nishiyama-Chuo oil field to Minami-Nagaoka areas near the edge of sedimentary basin trending northwest to southeast where productivity of phytoplankton was high due to upwelling.
