Gravel fabric characteristics of the Late Cretaceous-Paleogene conglomerate layers in southeast Qiangtang Basin,Tibet,and their geological significance

Based on an analysis of gravel fabric characteristics of conglomerate layers around the Late Cretaceous-Paleogene Amdo microcontinent in southeast Qiangtang Basin of Tibet,this paper elaborates characteristics,origin,source,transport way and tectonic background of the conglomerate in combination with the regional geology.Studies show that conglomerate layers are alluvialproluvial products that have experienced short- middle distance transportation.The gravels have the characteristics of complex composition,different constitutents and good sorting.The paleocurrent of the statistical points possesses the characteristics of a single flow direction,and the condition of hydrodynamic force is relatively stable.The analytical results of the compositional features and orientations of the gravel reveal that the gravels were derived from Amdo microcontient,Bangongnujiang suture zone and their surrounding mountains,indicating the sedimentary features of the piedmont water system formed by alleviation and subsequent proluvial action.In combination with regional geology,the authors hold that in the Late Cretaceous the north of Amdo microcontient experienced uplifting and formed the palaeogeographic pattern which was higher in the northeast and lower in the southwest,and in Paleogene,the study area which preserved the previously palaeogeographic pattern was subjected to integral uplifting.It is thus concluded that the uplifted area of Tibet occurred in the north of Bangongnujiang suture zone in the Late Cretaceous,and Tibet experienced uplifting as a whole in Paleogene.