新潟県北蒲原地域の新第三系微化石層序と鍬江暦基底の不整合形態
1998 年 63 巻 4 号 p. 301-314
詳細
1998 年 63 巻 4 号 p. 301-314
An integrated micropaleontological survey (foraminifera, calcareous nannofossil and diatom) of the Kitakanbara area was carried out to reconstruct the geomorphology of the unconformity widely traced at the base of the Kuwae Formation correlated to the Nishiyama Formation. The lower part of the formation which is composed of coarser sediments forms reservoir rock, and therefore a recognition of the paleogeography at the early Nishiyama Age is very important for petroleum exploration in the area.
In the north of the Tainai river, the Kuwae Formation unconformably covers the older formations with large amount of basal conglomerate. However, although the formation has an unconformable relationship with the underlaying formations in the main part of the studied area between the Tainai and the Kaji rivers, the basal congromerate is not developed, which makes it difficult to recognize the unconformity accurately during the field observation. Based on the diatom stratigraphy, it is found that a part of the uppermost Miocene which is correlated to the Rouxia californica Zone and lower part of the Neodenticula kamtschatica Zone, is completely missing in the area. The Kuwae Formation which started to deposit at the late stage of the N. kamtschatica Zone, further eroded the underlaying formations in the northern area which is closer to the channel deposits. Moreover, in the Sasakami Hills located in the most southern part of the investigated area, the Annogawa Formation also correlated to the Nishiyama Formation, unconformably overlies the older formations with pebble to boulder breccias derived from the Thalassionema schraderi Zone and the Sakanaiwa Formation. Foraminifera and calcareous nannofossil biostratigraphies are useful for the identification of biozones of the Kuwae (Annogawa) and Shimoseki (Sakanaiwa) Formations.
By further integrating biostratigraphic analysis, geological and sedimentological interpretation, the predictive distribution of the reservoir rock will be made clear with greater precision.
