A preliminary report on Paleozoic sandstoes and slates of the Northern Kitakami Zone

Abstract

Permian sandstones and slates in the Northern Kitakami Zone, a part of the Kitakami mountains, were petrographically and chemically studied. The Permian rocks are geosynclinal deposits which are composed mainly of slates, sandstones, cherts, and contain a small amount of limestones and basaltic tuffs.
Sandstones are mostly medium-grained, and belong to graywacke though their matrix content is rather small and the graded bedding is obscure. Clastic minerals and rock fragments are fresh and poorly sorted. The sandstones are rich in clastic plagioclase (up to 35% in amount) and lithic volcanic rocks (up to 27%), and poor in potassium feldspar (0.5-10%). From their amount and petrographical characters, it is considered that for provenance of the sandstones felsic volcanic and granitic rocks are important, and chemical weathering of the parent rocks were not so proceeded. The peculiarly high content of Na₂O (3.88-5.59%) is due to the predominance of fresh plagioclase which is present not only as clastic grains but also in the lithic fragments of volcanic rocks.
Some sandstones are very calcareous and are intercalated with strongly ill-sorted sandstones. Such calcareous ones may be a turbidite.
Slates range in grain grade from claystone to siltstone, rarely to finegrained sandstone. The chemical composition of the ordinary slates resembles to the average composition of the world. Some are cherty slates whose contents of CaO, K₂O, P₂O₅ and C are somewhat lower than those of the ordinary slates.
The sandstones (excluding calcareous ones) and slates (excluding cherty ones) are compared with those of the Kiso mountains in Central Japan and the Wellington district in New Zealand. The difference of lithology and chemistry among them can be explained by the difference in the geology and grade of chemical decomposition in their source areas.