Petrography of the Cretaceous Sandstones in the Monobegawa Valley, Shikoku

MIYAMOTO Takami

NAKAI Isao

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

The Cretaceous System of the Monobegawa valley, Kochi Prefecture, Shikoku, is distributed in a narrow belt within the Paleozoic Chichibu terrain, forming a synclinorium. On the basis of a major cycle of sedimentation, it is divided into the Ryoseki (Kochian), Yunoki (Aritan), Hibihara-Hagino (Mi-yakoan), Nagase (Gyliakian) and Kajisako (Urakawan) Formations in ascending order. The stratigraphy is described in this paper in some detail, and besides the sandstone petrography is carried out to comprehend the mode of the Cretaceous sedimentation definitely.
The Cretaceous sandstones mostly show a moderately sorted, normally to positively skewed and meso-to lepto-kurtic type in textural pattern. Besides, most of them are characterized by the feldspathic and lithic wackes and several specimens belong to the feldspathic arenite, based on OKADA's (1971) classification scheme. Judging from the gross lithological features, the Cretaceous sandstones of this basin are referred to a variety of the arenite-wacke associations defined by KRUMBEIN and SLOSS (1963), although resembling the greywackes texturally.
The contents in the major and accessory minerals of sandstones and the pebbles of conglomerates suggest that the provenance was composed mainly of older sedimentary, basic volcanic, granitic and metamorphic rocks and they functionated throughout the Cretaceous Period. However, based on the stratigraphical changes of mineralogical composition, which are confirmed by the statistical test, it is concluded that some transition of the provenance took place through the five cycles of sedimentation; the role of the granitic rocks as the source became increased gradually from the Ryoseki through the Yunoki to the Hibihara Formation in the Lower Cretaceous, and then decreased again in the Nagase and Kajisako Formations in the Upper Cretaceous. The mode of this vicissitude seems to be well reflected on the volumetrical change of the feldspar content.
From the comparison between the Monobegawa basin and other basins in the Chichibu terrain such as Katsuuragawa and Yatsushiro, it is clarified that there is a close similarity in characteristics of the sandstones and modes of the sedimentation as well as in gross litho- and bio-facies, if excepting minor differences.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718