The Manmo Group and the Akiyoshi Orogeny in Central and North Manchuria and its Adjacence

Abstract

The stratigraphic sequences of the Triassic and older rocks in Central and North Manchuria and the surrounding areas are here summarized and classified into the following four groups.
1. The Pre-Sinian basement complex
2. The Infra-Manmo group from Sinian to Silurian
3. The Lower Manmo group from Middle Silurian to Carboniferous
4. The Upper Manmo group from Permian to Middle Triassic.
While our knowledge on the basement complex and the Infra-Manmo group is still fragmentary, it is known now that the Manmo group is divisible into the lower and upper parts respectively recording the subsidence of the Mongolian geosyncline and its orogenic disturbances.
In the geosynclinal period the transgression reached the inundation phase in the Middle Devonian or probably in the Givetian age, and renewed in the early Carboniferous after a regression, but later the northern side of the geosyncline emerged extensively.
As indicated by the fusulinid limestones and radiolarian slates as well as spilite and keratophyre the geosynclinal axis has extended in the Permo-Carboniferous period from the middle Amur valley to the border between Inner and outer Mongolia through the central part of the Great Khingan range.
The orogenic disturbances in the Permo-Triassic period are clearly shown by angular discordances, thick boulder conglomerate and fanglomerate deposits, variation of bio-and litho-facies and great geographic changes.
In the early and middle Permian ages were the fusulinaceous and non-fusulinaceous facies in the middle and southern parts of the terrain which beared close affinities to the Indo-Pacific and Russian faunas.
In the middle Permian times the northern sea invaded into North Mongolia through Transbaikalia. The Kazanian fauna there shows intimate relationship to the faunas of Siberia and the Urals. This sea with the boreal fauna was probably detached from the south sea of Mongolia with the austral fauna by a land barrier which was produced by the early-middle Permian disturbance.
The seas retreated from the vast terrain of Mongolia, Transbaikalia, the upper Amur valley and almost all Manchuria during the latter part of the Permian period where the upper Permian naiad beds and the Permo-Triassic plant beds were deposited.
The marine formations of the Upper Permian and Lower and Middle Triassic ages are known from the Lesser Khingan range, the middle Amur valley and South Primoria. It is said that the stratigraphic break between the two systems is small or none in the first and second areas.
In southeast Manchuria and northeast Korea plant beds are intercalated in the Permian formation. The land flora of Kaishantun on the Manchurian side belongs to the Cathaysian flora, but the floras on the Korean side as well as in South Primoria to the Kuznetsk flora.
In Manchuria the Manmo group with the late Upper Permian Lepidolina zone at the top is strongly folded, the crustal deformation having been accompanied by the granitic intrusion. As the result most of the Manmo group became roof-pendants on the great Mongolian granitic batholith. Furthermore, the so-called Chungchao block existed on the south side of the geosyncline during the Palaeozoic era, but these two megatectonic units were fused by the emplacement of the Permo-Triassic granite.
After the Ladino-Carnic Akiyoshi orogeny the Mongolian orogenic zone began to be destructed. The Amur-Okhotsk subgeosyncline was produced by the destruction of the northern part and the Carnic and Noric seas ingressed as far as the Mongolian frontier of Transbaikalia. The neritic sediments accumulated in this embayment attained 5000 m in thickness.