勝浦川盆地西部の下部白亜系について

Cretaceous System in the Western Part of the Katsuragawa Basin, Tokushima Prefecture, Shikoku, Japan

NUMANO Kyoichiro

NAKANO Mitsuo

廣島大學地學研究報告

Geological report of the Hiroshima University

The Mesozoic rocks are well known and scattered in the narrow areas of the so-called Chichibu terrain in the Outer Zone of Southwest Japan, and they have been studied by a lots of geologists and palaeontologists because of abundant occurrences of fossils and their complicated structures.
The Cretaceous strata containing a number of animal and plant fossils are somewhat widely developed in this area located at about 20 Km to the southwest of Tokushima-city, and have been surveyed by many authors, such as YEHARA. (1924), YABE and NAGAO (1927), and others. On account of the complicated geologic structure, their oppinions for the Cretaceous are somewhat disagree with one another.
The stratigraphy of the Cretaceous system surveyed by us is as follows:
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The Cretaceous formation lies unconformably on the folded Palaeozoic group in the central part, but it is fault contact in the northern and southern wings. It is divided into two portions, namely cast and west blocks, by a NNE-SSW fault in the center. Generally speaking, in the west block the Cretaceous forms a homoclinal structure dipping northward, though a small syncline appears in the northern part. On the other hand, an axis of anticline and syncline are recognized in the east part and the Fujikawa formation developed in the northern side is shown as a graben.
From the Fujikawa formation, Desmoceras kossmati was collected. This fossils must indicate to the Infra-Gyliakian, i. e. lowest Cenomanian, in age.
It is interesting to see that a lots of Trigoniids were collected from the upper member of the Hoji formation and that this member can be subdivided into two parts, i. e. lower and upper or Nipponitrigonia- and Pterotrigonia-beds, by the faunal and lithological characters. In many cases, Nipponitrigonia is associated with Ostrea living in brackish water and it occurs in coarse-grained sandstone and conglomerate. On the other hand, Pterotrigonia is collected from the medium-grained sandstone layers and it is accompanied with marine Pentacrinus, Glycymeris and others. Therefore, Nipponitrigonia might be inhabited in brackish water of shallow places where the current agitation is somewhat strong. While, Pterotrigonia was possibly lived and liked to the marine and calm water of somewhat off-shore.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676