Some Characteristics of Sedimentation of the Yinmin Formation

The Yinmin Formation is a unit of the Middle Proterozoic Kunyang Group distributed in Yunnan and Sichuan provinces, and is considered to be the as source bed of the wellknown Dongchuan type copper deposits. It is lithologically composed of breccia and or conglomerate, sandstone, siltstone, slate, and some dolomite at the upper part. The well-developed arkose and the distinctive purple colour reveals that the Yinmin Formation is a continental red bed which was formed in rift environment, and was the lowest unit of a transgressive series of rift sedimentation. Different kinds of breccia (and conglomerate) are developed in the Yinmin Formation which is. a special feature reflecting the rift background and later structural deformation. The present paper gives a preliminary classification of these breccia and describes some of them, with an emphasis on the process of formation of the syngenetic breccia. It is concluded that the Yinmin Formation as a whole is of terrestrial facies but not a marine facies nor a flysch.