SEDIMENTARY CHARACTERISTICS AND SEQUENCE FRAMEWORK OF THE ZHUHAI-ZHUJIANG FORMATION IN THE MIDDLE AREA OF PEARL RIVER MOUTH BASIN
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摘要: 南海北部珠江口盆地珠海组分为SQ32、SQ25.5两个三级层序,珠江组划分为SQ23.8、SQ21、SQ17.5和SQ16.5四个三级层序,其中前3个层序分为水进域、高位域,后3个层序分为低位域、水进域和高位域。珠江组初期的白云运动使珠海组的浅水陆架环境变更为珠江组的陆架-陆坡沉积环境,造就了古地貌的更迭、层序界面的形成和沉积相的突变。层序界面在陆架主要为粗粒沉积物形成的侵蚀界面、浅水暴露界面,而陆坡为下切水道、丘状扇体底部侵蚀界面和滑塌层。层序的更迭使古珠江三角洲物源供给强度呈现强-弱-强-弱的特点,即珠海组时期的三角洲为陆架三角洲,沉积规模大;之后珠江组物源变弱,珠江组各层序低位域形成陆架边缘三角洲,水进域、高位域演化为陆架三角洲。低位期的三角洲前缘带越过坡折带沉积,并与陆架外的峡谷水道相连,三角洲进积、加积的碎屑沉积物向上陆坡内的白云凹陷内沉积,形成深水扇。因此,相对海平面变化、构造升降和沉积物供给强度控制着层序结构和沉积体展布。Abstract: 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.
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