舞鶴地帶に分布する二畳系および三畳系の砂岩の2・3の性質,とくにmaturityの問題について

Abstract

In the Maizuru Zone develope the Permian Maizuru Group, the Lower Middle Triassic Yakuno Group, and the Carnian Nabae Group. Studies on the sandstones of these groups seem to throw light on the geohistory of this zone. In this paper, the writer first describes the general characters of the sandstones, based chiefly on the observations under the microscope, and at the same time on field observations. Some compositional and textural properties of the sandstones have been examined, rather in detail; especially from the viewpoint of their maturity. Generally speaking, the sandstones of the Maizuru Group are the most immature ; next come those of the Yakuno Group ; and the sandstones of the N_2 formation of the Nabae Group are mostly mature. It is supposed that the sediments of the Maizuru Group were supplied mainly from the large exposure of andesite, effusive 〜 hypabyssal acid rocks, and shale etc., judging from the fact that the fragments of these rocks are very abundant in the sandstones of this group, especially in the coarser grained ones. It has been suggested further that these rockfragments were not decomposed into fine-gained sands, but were changed directly into finer clayey detrital matters, and joined with other clayey materials which has been produced by weathering at their provenance. These clayey detritals were not separated from the sand grains, but deposited together with the sands in the same places. The turbidity current theory seems to account well for such a mechanism of deposition. The Yakuno Group grew to its maturity chiefly by selective breaking and selective transportion of felspar grains. As quartz fragments are endurable against destruction, they must have been deposited almost keeping their original sizes. As the result of the selective transportation, felspar grains 0.1-0.2mm in size were accumulated in certa in places, while, at the same time, quartz grains were concentrated in other places. Hence, if the term "maturity" is to be used, the felspar-rich finer sandstone and the quartz-rich coarser sandstone should be considered to be in the same degree of maturity. Such are matters of no small concern also in the studies on the sandstones of the Maizure Group. Decomposition of rock fragments and production of detrital clay must have occurred also in the course of sedimentation of the Yakuno Group. But, in this case the removal of the clay followed ; and, the sandstones of this group have little detrital clay matrix. Removal of clay, like that of felspathic fine sands, is due to different modes of motions of sediment grains of various sizes, under the same velocity of the stream. The sandstones of the N_2 formation have very distinct characteristics from those of the other groups. Removal of clay has been sufficient. The compositional properties of these sandstones, rich both in quartz and in felspar, may be due not only to their maturity, but also to the nature of their provenance. From the abovestated facts is follows that, for the examination of the maturity of sandstones, we should consider both their compositional and textural properties and especially their mutual relation. Selective breaking and selective transportation of various components of sandstones are the two factors of the most importance in study on sandstone maturity.