APPLICATION OF MICROPALEONTOLOGY TO THE SEQUENCE STRATIGRAPHIC STUDIES OF LATE CENOZOIC IN THE ZHUJIANG RIVER MOUTH BASIN
Oin Guoquan(Science and Technology ResearCh Center,CONHE,Guangzhou,510240,China)
Based on the analytic data of micropaleontology in wells,this paper discusses the se-quence stratigraphy of Late Cenozoic marine strata in the Zhujiang River Mouth Basin inthe following five aspects:(1)Biozone,bio-events and age.Paleontologic method is currently a convenient,ac-curate and economic measure for determining ages,According to the vertical distributionof bio-events of planktonic foraminifera and nannoplankton in wells, the ages of LateCenozoic biozones and bio-events in the basin are preliminarily determined by age datinginformation of the bio-events and in reference to the age assigned to the biozones by pre-vious paleontologists. In comparison with the ages of planktonic foraminifera biozones inthe global eustatic chart drawn by Haq et al.(1987),there are some variations in thisbasin.The main changes are:the age of N17/N16 is modified from 7.2Ma to 8. 6Ma,N15/N14 from 10.2Ma to 11. 6Ma,N14/N13 from 11. 0Ma to 12.7Ma,N13/N12 from12.0Ma to 13.4Ma,N5/N4 from 23. 0Ma to 21. 0Ma and N4B/N4A from 25.2Ma to22.2Ma etc.The ages of nannofossil zones are modified correspondingly.(2)Determination of a 3rd order sequence boundary and a maximum flooding sur-face(MFS).The 3rd order sequence division has been made for the whole interval fromOligocene to Quaternary in the wells of BY7-1-1,PY24-1-1 and LF13-1-1 as well as theengineering well of LF 13-1 based on the partially quantitative data of foraminifera,mainly the abundance and percentage curves of planktonic foraminifera larger than0.25mm,and in reference to the nannofossil and seismic data,In comparison to Neogene3rd order sequences of Haq et al.(1988),the transgressive sequence TB1.1 has not yetbeen found,and the MFS age of sequence TB3.1 is corrected from 9.2Ma to 10.0Ma(3)Drawing of relative coastal onlap and sea level changes charts. These charts aredrawn by using the analysis data of foraminifera and the geographic distribution ofplanktonic biozones, The 2nd order sequence is obviously not coordinative with that ofHaq et al.Is(1988)global coastal onlap and eustatic charts.(4)Correlation of sequence stratigraphy to other stratigraphy. By studying the se-quence stratigraphy in the wells,the chronologic correlation chart between planktonicforaminifera and nannofossil bio-events,biozone and the 3rd order sequence has beencompiled,and the composite diagram of biostratigraphy,lithostratigraphy,seismicstratigraphy and sequence stratigraphy is also drawn up.(5)Neogene evolution history.Because of the spreading of South China Sea in 32-17 Ma,transgression took place to the north of SCS ,after 27. 5Ma,sea water began toencroach Zhu 2 Depression and spread over it in early Early Miocene(24. 8Ma).Thistransgression was only limited to the south of the Central Uplift Belt,and Zhu 1 Depression to its north was still mainly continental and occasionally transitional environments.After 21.0Ma,another transgression took place,sea water encroached the Panyu LowUplift and came into Zhu 1 Depression,As the Dongsha Massif was higher than sea lev-el,the eastern part of Zhu 1 Depression formed estuarine environment.With the end ofSCS spreading,the oceanic crust became cold and subsided rapidly,high water stand oc-curred in 17.5-15. 5Ma(TB2.2-2.3),sea water submerged gradually the whole CentralUplift BeIt and formed open sea environment.It was followed by the occurrence of threemajor sea level cycles:Middle Miocene(13. 8-12.5Ma) lowstand(TB2.5)might be relat-ed to the formation of Antarctic ice sheets:Late Miocene(8.26.3Ma)lowstand(T83.2)was affected by Himalayan Movement,and the regression that began at the end ofPliocene(after 2.4Ma)was related to the growth of Arctic ice sheets and the continentalglaciers.