CHARACTERISTICS OF THE TURBIDITE AND VOLCANIC-SEDIMENTARY ASSEMBLAGES OF CAMBRIAN-MIDDLE ORDOVICIAN IN THE XICHANG—JING AREA OF BEISHAN

In 1983-1984, The authors found the clastic turbidite of the deep water continental source and the pelagic deposits in the several-thousand-meter clastic rock of Cambrian-middle Ordovician in Xichangjing of Ejinaqi, Inner Mongolia. This area belongs to the Caledonian folded belt with the gabbro and ultrabasic rocks intruding into the Cambrian-Ordoviciaa System in the late period of the Caledonian cycle, and with magma activities of neutral or acid rocks, such as plagiogra-nite and quartz diorite of the Hercynian cycle. The Cambrian deposits in this area are mainly turbidite, 1504 metres in thickness and composed of siliceous rock, gravel-bearing graywacke, feldspar-quartz graywacke and a little of slate; the lower Ordovician deposits are also turbidite, 1320 metres in thickness and contain interbedding of feldspar-quartz graywacke and slate; whereas the middle Ordovician deposits of more than 1579 metres. The south side of this zone is an ancient continental folded basement of Precambrian; the north one is an area dispersed over by ophiolite suite and residual ancient ocean crust of the end of middle Ordovician-late Ordovician and disintegrated by a series of overthrust faults. Based on the field investigation of the authors, it is found that early Cambrian-early Ordovician in the study area is a principle developmental period of turbidity ourrent and the characteristics of turbidites in different era are as fol-lowst l.The lower Cambrian turbidite: It is 979 metres in thickness and its rhythmit-es are composed mainly of feldspar-quartz graywack and silty slate, in which the graywacke content is over 3/4. There are three groups of the Bouma sequence, from bottom to top they are: 1)ABD, ABCD, ABD, 2)ABCD, ABCD, AD, 3)AD, AD, AD. The Bouma sequence in the lower part of the turbidite is mainly Section A and lacks Section E, However, Section A in the middle part of the turbidite is thinner, generally speaking, the Bouma sequence in this part develops well and the oblique bedding of the Section C shows that the sediment is transported from south to north and the folded basement of Precambrian in the southern part is the material source area. The Bouma sequence of the upper turbidite develops only Sectoin A and C, indicating the turbidite is near the source area. 2.The lower Ordovician turbidite: It is 1300 metres in thickness including more than handred metres of deepwater sediments interbedded with the turbidite. Every section of the Bouma sequence is well developed with tongue-like and finger-like scouring floormolds. There are four groups of the Bouma sequence form bottom to top in this turbi-dite: 1)AD, AD, AD, 2 ) ACD, ACD, 3 ) AD, ABCD, 4 ) ABCD. In the Section C of the upper part of the turbidite apperars a sliding layer, indicating that there is a slope in the sedimentary environment, so that a gravity sliding motion under the water develops in the Section C and then a long tail-current period occurs and the thicker Section D is formed. The volcanic-sedimentary assemblages can be divided into five groups, they are. 1 ) the sedimentary assemblage of continental margin sea of the bottom of Lower Cambrian; 2 ) The semi-deepsea sedimentary assemblage of middle and upper Cambrian; 3 ) The deepsea sedimentary assemblage of the upper part of Lower Ordovi-cian; 4 ) The rift-margin volcanic assemblage of the lower part of middle Ordovi-cian; 5 ) The pelagic deposit assemblage of the upper part of middle Ordovician. Based on the characteristics of the turbidite and the comprehensive study on the volcanic-sedimentary assemblages,it is found that the sedimentary environment of Cambrian in the study area successively underwent continental margin sea, continental slope, continental rise and finally semi-deepsea. In early Ordovician, the eontinental slope environment appeared once again. The turbidites discussed in this paper are the results of the turbidite sedimentation on continental slope and continental rise. However, the sedimentary environment of middle Ordovician is the period from as active volcano of rift t