丹波地帯の古生界(その2) : 京都府北桑田郡京北町南部の古生層(団研特集)

Abstract

In this report, the writers have dealt with the stratigraphy, litho-facies, and geological structures of the Palaeozoic formations, that develop at the southern part of Keihoku-cho, which is adjacent to the district described in the Part I. The sedimentary structures of the sandstone layers and lithology of conglomerates of the upper most formation have been described. The sequence of the Palaeozoic formations is as follows (in descending order): 1. sandstone formation 1750 m k. chert formation 210 m j. shale formation 640 m i. chert formation 480 m h. schalstein formation 290-390 m g. shale-sandstone formation 460 m+ The thickness of the entire formations measured in this district is 3850 m, and the total thickness of the Palaeozoic formations (from the lowermost a. formation in the former report to the uppermost 1. formation in this paper) attains up to about 8000 m. The greater part of the formations in this district is regarded to be Middle Permian in age. The formations form as a whole a synclinal structure plunging to the west. The low angle thrust reported by FUJIWARA and SAWADA (1940) (the "Kumogahata thrust") must be denied so far as this district is concerned. In the 1. sandstone formation, many strata possess the character of turbidite, and some ill-preserved sole marks (flute cast, loaded flute cast and groove cast) were found in the formation. Examples of internal structures of the formation are shown in Figures (3, 4). A few muddy conglomerate beds were found in the 1. sandstone formation. Pebbles of the beds are ill-sorted and round-subround in general. There are andesite, dacite, liparite, quartz-porphyry, quartz diorite, granite, gneiss, shale, sandstone, chert, and others in the pebbles of conglomerates. These pebbles clearly show that there had been an upheaval of lands in the Honshu geosyncline, that had been composed of acidic and intermediate volcanic rocks covering palaeozoic sedimentaries, and granitic rocks had intruded into these rocks, Besides, a few orthoquartzitic pebbles were discovered in the conglomerate. The occurrence of Monotis (Entomonotis) ocotica, found in a block of rock on a river-bed in this district, was noted, which indicates the extent of the palaeogeography of the Triassic period in southwest Japan.