SEDIMENTARY CHARACTERISTICS AND SEQUENCE FRAMEWORK OF THE ZHUHAI-ZHUJIANG FORMATION IN THE MIDDLE AREA OF PEARL RIVER MOUTH BASIN

Sequence of Zhuhai Formation in the Pearl River Mouth basin in the north of the South China Sea can be divided into SQ32 and SQ25.5,and Zhujiang Formation can be divided into SQ23.8,SQ21,SQ17.5 and SQ16.5.So,the three frontal sequences contain TST and HST,and the others contain LST,TST and HST.Baiyun movement which was a great tectonic event at the end of Oligocene made the Baiyun sag subside quickly and shelf break shift from south to north,then the sedimentary environment in the area changed from shelf sediments of Zhuhai Formation to shelf and slope sediments of Zhujiang Formation.This event led to the changes in geomorphologic subsidence,sequence boundary and sedimentary environment.Meanwhile,this movement gave different sequence boundary characteristics of the shelf and slope.In the shelf there are erosion and exposition,and in the slope there are incised channels,basal erosion of moundy deep-marine fan and slump deposits.With changes in the sequences,sediments first show large,then small,then large again,and finally small supplies.So in the time of Zhuhai Formation the delta was a mid-shelf delta and had large-scale deposits and subsequently transgression occurred,the supply to Zhujiang Formation turnt poor,and the delta changed to a shelf-edge one in the LST,and then became a mid-shelf one in TST and HST.In the LST period,the delta extended towards slope direction across the shelf break,and gradually turnt into a shelf-edge delta,with its front connecting with canyons and channels out of the shelf break.Lots of sediments transported to slope to from a controlling deep-marine fan.So,sea-level changes,tectonic subsidence and sediment supply are the three main factors.