Sedimentology and Basin Analysis of the Paleogene Muro Group in the Kii Peninsula, Southwest Japan

Abstract

The Oligocene to lower Miocene Muro group in the Kii Peninsula represents the later stage of the Shimanto geosyncline which was widely extended along the Pacific coast of Southwest Japan from late Jurassic to earliest Miocene time. The Muro group is composed mainly of flysch-type alternations of sandstone and mudstone, intercalating conglomerates, thick-bedded sandstones and conglomeratic mudstones. These sequences are well exposed in the southern coast area in the peninsula. Based on the analyses of the sedimentary features such as grading, stratification and clast fabric, and field occurrence as well, the conglomerates are referred to have been transported and deposited by grain flow and turbidity current combined. Thick-bedded sandstones are considered to have been transported and deposited by turbidity current. It is safely concluded that conglomeratic mudstones such as pebbly mudstone and angular fragment-bearing mudstone were transported and deposited by debris flow. All of these coarse sediments were deposited in proximal suit of deposition. There are observed several channel-scours and -fills in the upper formation of the Muro group, which indicate the submarine fan deposition of the formation. Paleocurrent analysis based on sole marks and clast fabrics shows that the provenances existed not only to the north, but also to the south of the basin. General trends of the channels and paleocurrents in channel fillings indicate the distributary channel system developed on the submarine fan in the southern side of the basin. The existence of the Kuroshio Paleoland estimated to the south of the Shimanto terrain by the Kishu Shimanto Research Group is strongly supported by the present study.