新潟県中部•北部地域の含油新第三系の層序

新潟県中越•下越地方の石油地質学的研究(そのI)

抄録

Owing to the recent advancements in technology of geophysical exploration and drilling, many geological data concerning to the deep seated formations have been accumulated. Recent studies about surface and subsurface geology and about planktonic foraminiferal fossils have made clear the lower and middle Miocene stratigraphy. The writer proposes, in this paper, to divide the Miocene to lower Pleistocene formations into 7 stages in this district.
Stage I-Mikawa stage (lower to middle Miocene)
Formations included in this stage are the Aikawa group (Sado Island), Matsukawa group (Kita-Uonuma County) and Jonai group (Minami-Uonuma County). The formations consist mainly of pyroclastic rocks (lower part of the "green tuff") including propyrite and contain Aniai-type flora in the intercalating non-marine-shallow marine sediments.
Stage II-Nanatani stage (middle Miocene)
Lithology of the formations of this stage is composed of various kinds of pyroclastic rocks (upper part of the "green tuff"), coarse elastic sediments which may have been deposited in the shallow marine and fine clastic sediments of rather deep marine origin.
This stage belongs to Globorotalia fohsi zone and Hopkinsina morimachiensis - Gyroidina orbicularis zone. Fossils such as Daijima-type flora, Miogypsina - Operculina fauna, Acid - Potamid fauna and Pectinid fauna characterize this stage.
According to planktonic foraminiferal zonation newly established by Japex's paleontologists, the Iwafune group, the Tsugawa formation and the Nanatani formation, which had been considered to be deposited in this order and to be unrelated stages, have been concluded to constitute only one stage. However, in Akita region, two stages are recognized in the age corresponding to the
stage II of this province. To solve this discrepancy, more detailed studies should be carried out.
Stage III-Teradomari stage (lower part of the upper Miocene)
Main lithology is black mudstone being deposited under stagnant water. Benthonic foraminiferal fauna is mostly composed of arenaceous forms belonging to Spirosigmoilinella compressa-Martinottiella communis zone.
In the area west of the Nagaoka plain, alternations of sandstone and mudstone are predominant and some planktonic foraminifers with calcareous Benthonic forms are found.
Volcanic rocks mainly of andesite and dacite are found in places. Submarine eruption and
intrusion of basalt are highly characteristic in this stage.
Stage IV-Shiiya stage (upper Miocene)
Lithology is alternation of sandstone/mudstone and mudstone. The former is called Shiiya alternation and the latter, being grey mudstone, called Araya mudstone.
Benthonic foraminifers are characterized by coexistence of arenaceous and calcareous forms of Miliammina echigoensis zone. Planktonic foraminiferal zone is "Orbulinna" universa - Globigerina woodi zone which contains Globigerina pachyderma (dextral) and Globorotalia inflata.
It is noted that the volcanic activity subsided during this stage as compared with the previous stages. Andesitic and dacitic pyroclastic rocks are found in some places.
Stage V-Nishiyama stage (lower Pliocene)
Characteristic lithology is greenish grey mudstone named the Nishiyama mudstone. Frequently, especsally in the lower part of this stage, alternation of sandstone and mudstone, so called the Hamatsuda alternation, is found.
The middle and the lower parts of this stage are distinguished by Globigerina pachyderma (dextral)-Globorotalia in flata (s.l.) zone, in which Globorotalia in flata group appears in two zones. Coiling direction of fossil Globigerina pachyderma changes from dextral to sinistral at the middle of this stage. The upper part of this stage belongs to Globigerina pachyderma (sinistral) - Globigerina gninqueloba zone.