Lower Cretaceous Sandstones of the Katsuuragawa Valley, Shikoku

The sandstone petrography is herein treated in detail from various points of view as one of the effectual means to elucidate the Lower Cretaceous sedimentation in the Katsuuragawa valley of eastern Shikoku, where the Lower Cretaceous System is divided into the Tatsukawa, Hanoura, Hoji and Fujikawa Formations in ascending order on the basis of a major cycle of sedimentation. A special attention has been given on the variation in the sandstone properties to make clear the change of the sedimentation through the above four cycles.
It is inferred that the clastic rocks were predominantly derived from older sedimentary, basic volcanic and granitic terrains, with a smaller contribution from a metamorphic terrain. Although these rocks contributed throughout to the Lower Cretaceous deposits of the whole sequence as the source rocks, the vicis-situde of them is manifested by the statistically significant difference in the mineral composition of sand-stones. The most remarkable compositional difference is recognized statistically between the sandstones of the Hanoura and Hoji Formations, and this suggests that a considerably intense crustal movement took place to expose granitic rocks extensively in the interval between the two formations. While, no or a little compositional change between those of other formations suggests that intense movements did not occur too seriously to bring out the vicissitude of the provenance.
Most of the Lower Cretaceous sandstones are characterized by the wacke type with more than 15 percent matrix content, being texturally closely similar to "greywacke". Some others belong to the arenite type with much mature texture. Judging from the gross litho- and bio-facies, they are reasonably referred to a variety of the arenite-wacke associations.