Sedimentological Study on the Early Jurassic Shallow Marine Facies in Southwest Japan and the Comparison with Daedong Supergroup in South Korea

Abstract

Sedimentological studies, especially petrographic analyses, are carried out on the Early Jurassic Yamaoku, Higuchi, and Toyora Groups in the Inner Side of Southwest Japan and the Daedong Supergroup in South Korea. Based on this study, the sedimentary environment and the relation to tectonism of Southwest Japan and South Korea are discussed. The Yamaoku Formation is represented by shallow marine sediments of regressive cycle judging from grain-size analyses. Remarkable differences of geologic structure and sediment composition between the Yamaoku and the succeeding early Cretaceous terrestrial beds indicate a middle to late Jurassic crustal movement correlated to the Daebo movement in South Korea. On the other hand, the geologic structure and sediment composition of the Toyora Group indicate that the Triassic movement was stronger than the post-Toyora crustal movement. This Triassic movement corresponds to the Songrim movement in South Korea. The Daedong Supergroup is a product of the intermontain basin. The characteristic grain-size distribution of sandstones is confirmed to be well comparable to that of the Pleistocene lacustrine-delta sands of the Kobiwako Group around Lake Biwa. Almost all the sandstones of the Daedong Supergroup and the late Paleozoic-middle Triassic Pyeongan Supergroup are highly quartzose assignable to quartz sandstone, while all of the sandstones in Southwest Japan belong to lithic or feldspathic wacke or arenite. Judging from the absence of K-feldspar in the sandstones and common occurrence of orthoquartzite clasts in the conglomerate in South Korea, the Sinian orthoquartzite was once widely distributed around the Ogcheon belt and the present Sea of Japan region. Abundant occurrence of sand-grains and gravels of acidic volcanic rocks as well as interbeds of acidic tuffs in the Yamaoku Formation suggest the presence of acidic volcanic mountains in the provenance. The Inner Side of Southwest Japan most probably occupied a convergent zone along the margin of Asian continent during the Jurassic time and the Jurassic basin in South Korea was in a backarc basin region. The middle to late Jurassic tectonic movement resulted in an upheaval of all over the Inner Side of Southwest Japan and South Korea may be related to the southward shifting of the convergent zone.