REDEFINITION OF THE EARLY PLEISTOCENE QANGZÊ FORMATION AND SEQUENCE STRATIGRAPHIC DIVISION IN THE ZANDA BASIN, NGARI, TIBET

Based on the contact relationships,sedimentary cycles,sedimentary systems and lithofacies and lithologic characteristics of the stratigraphic section,the fluvio-lacustrine sediments of the early Pleistocene Qangzê Formation in the Zanda basin are divided into 2 sedimentary facies and 3 lithologic members.The first and second members of the Qangzê Formation consist of a sequence of coarse clastic rocks of conglomerate with sandstone,belonging to typical alluvial(diluvial) fan deposits.The third member consists of alternating beds of sandstone and conglomerate,belonging to glaciofluvial and paraglacial deposits.ESR and paleomagnetic dating indicates that the age of the fluvio-lacustrine sediments of the Qangzê Formation in the Zanda basin is earliest Pleistocene(2.681.36 Ma).The Zanda basin was a syngenetic downfaulted basin,which only experienced the alluvial systems tract and transgressive systems tract in the process of its reactivation in the late stage.The alluvial systems tract was formed in the early and middle stages of development of the reactivated stratigraphic sequence in the early part of the early Pleistocene,while the transgressive systems tract was formed in the late stage of development of the reactivated stratigraphic sequence in the early part of the early Pleistocene.