NEW PROGRESS IN THE RESEARCH OF THE MESOPROTEROZOIC CHANGCHENG SYSTEM (1 800—1 400 Ma) IN THE YANSHAN RANGE, NORTH CHINA
ZHU Shi-xing, HUANG Xue-guang, and SUN Shu-fen( Tianjin Institute of Geology and Mineral Resources, Tianjin, 300170 )
The Yanshan Range in North China is a typical area of the lower Mesoproterozoic Changcheng System. According to the current subdivision, the Changcheng System includes five formations, namely the Changzhougou, Chuanlinggou, Tuanshanzi, Dahongyu and Gaoyuzhuang formations in ascending order. The new progress made in recent years is as follows: 1) There is not only a clear disconformabe contact between the Gaoyuzhuang and underlying Dahongyu Formations, but also two beds of basal (conglome-)rate in the Qinglong and Qianxi counties, eastern Hebei Province (eastern Yanshan Range), which represent the two episodes of the Qinglong Uplift repectively, namely the main one (the lower conglomerate) and the second one (the upper conglomerate). 2) The rocks between two beds of conglomerates are composed mainly of clastic rocks of several meters thick at the ancient continental margin, but carbonate composition and the thickness increases markedly toward the ancient basin, so that it is predominated by dolostones with banded cherty in the Jixian area, which has a thickness of 148.56m and is assigned to the upper member of the Dahongyu Formation previously. Because the strata are restricted by two disconformities, they are proposed to build a new formation that is named the "Madiyu Formation" according to the typical locality in this paper. 3) The Madiyu Formation has not only the basal conglomerate with a large thickness of 22.3 meters, but also contanis the continental deposits( deluvial and fluvial) in its lower part. The basal contact surface, therefore, is a "SBI" type of the depositional sequence unconformity, If we take it as a boundary, the Changcheng System could be subdivided into two homogenous depositional cycles and sequences in the first order, two kinds of sedimentary basins in different nature (i.e. the aulacogen in the lower part and the epicontinental sea in the upper part), and two biota with different compositions. 4) On the basis of the present chronometry data, The age of the boundary between the Madiyu and underlying Dahongyu, or the age of main episode of the Qinglong uplift, is about 1 600 Ma. The boundary age bet-(ween) the Matiyu and the overlying Gaoyuzhuang, namely the age of second episode of the Qinglong Uplift, is about 1500 Ma. The entire Qilong Uplift in the Yanshan Range, therefore, is just a response to the global geotectonic movement (convergence) mainly happened during 1 600—1 500Ma. To sum up, the disconformity between Madiyu and underlying Dahongyu or the main episode of the Qinglong Uplift is a very important geological event or boundary. This paper, therefore, suggests that the Changcheng System may be subdivided into two lithostratigraphic units in group order, namely the Changcheng Group (1 800—1 600Ma) and the Gaoyuzhuang Group(1 600—1 400Ma),and the age of the boundary between two groups is about 1 600Ma. In addition, the Gaoyuzhuang Formation includes four subordinate units at present, that is subformations. Based on the different characters in deposition, the difference in lithological composition, and the stability in regional distribution, this paper also proposes to raise the four subformations to the rank of formation respectively. If these propositions could be accepted, the new subdivision of the Changcheng System could not only reflect the characteristics of the different (evolutional) stages in tectonics, deposition and life of the ancient Yanshan basin better, it will also be more convenient to correlate them to related strata in China and abroad.
【Fund】： 国家自然科学基金(No.40372003);; 中国地质调查局地质调查工作项目(No.1212010511603);; 现代古生物学和地层学国家重点实验室(No.023105)资助项目成果之一。
【CateGory Index】： P534.3
【CateGory Index】： P534.3