CHANGCHENGIAN FAN-DELTAIC DEPOSITS IN THE YANSHAN REGION,NORTH ERN CHINA

A sequence of the coarse-grained terrigenous clastic deposits 100 m and more thick are devel-oped on the Archean to Early Proterozoic crystalline basement in Luanxian in eastern Yanshan re-gion.This sequence has long been examined by the previous workers in comparison with the conti-nental sedimentary strata near the bottom of the Changchengian strata in the Jixian region,and is as-signed to the Changzhougou Formation.Qian Xianglin et al.(1980),on the basis of the systematic study,interpreted the sequence as the Dahongyuian(Late Changchengian)onlap deposits.From that time on,the nature of these coarse-grained elastic deposits and their bearings on the whole sedimen-tary environmental systems in the basin remained uncertain.The results of research obtained by the authors in recent years show that both coarse-grained clastic deposits and overlying carbonate de-posits represent the characteristic Gilbert-type fan-deltaic deposits.The recognition of the fan-deltaic deposits provides the important basis for the limitation of the basin boundary on the southeastern margion of Middle and Late Proterozoic aulacogens in the Yanshan region and for the study of depo-sitional-filling patterns during early extension of the basin.