Sedimentary Environment and Basin Analyses of the Miocene Kumano Group in the Kii Peninsula, Southwest Japan

Abstract

The Lower to Middle Miocene Kumano Group in the southeastern part of the Kii Peninsula makes a thick sedimentary pile, consisting of mudstone and muddy alternation of sandstone and mudstone. A special attention was paid to the genesis of such fine-grained clastic rock piles and the reconstruction of the sedimentary basin. The Kumano Group is lithologically divided into three formations, namely, the Shimosato, the Shikiya and the Mitsuno Formations in ascending order. The sedimentological analyses were made on the former two formations which are predominant in fine-grained clastic materials. First, based on the analyses on the various sedimentary features of muddy alternation and bedded mudstone, such as thickness distribution, mud-sand ratio, bedding type, internal sedimentary structures and combination of sedimentary divisions, it is concluded that most of these fine-grained materials have been transported and deposited by a low-density current and partly by a tractional current. A morphological reconstruction of the Kumano basin was attempted by paleocurrent analysis through various kinds of current marks, cross lamination and grain-orientation and by paleoslope analysis through slump structure and debris flow deposits. As the results, the Kumano basin in the study area which occupies a southern part of the basin is revealed to have located on the southward inclining slope and slope-basin bordered by the upheaval zone related to magmatism to the south. On the other hand, the Kumano Group in the northern part is known to be of the shallow marine origin and that of the central part in between is much thinner in thickness than in the other two areas suggesting a topographic high of this region. Thus the whole Kumano basin is concluded to have comprised topographically the shelf, the shelf edge high, the slope, the slope-basin, and the outer upheaval zone (the outer ridge), from north to south. Such a situation is very similar to that of the present forearc region in Southwest Japan, though it was located more northward than the latter.