珠江口盆地晚始新世浮游有孔虫的发现及其地质意义

THE DISCOVERIES OF THE LATE EOCENE PLANKTONIC FORAMINIFERA INTHE PEARL RIVER MOUTH BASIN AND THEIR GEOLOGICAL IMPLICATIONS

根据首次在南海珠江口盆地最南端的ＢＹ７－１－１井中发现的晚始新世和早、中渐新世浮游有孔虫化石组合，认为恩平组的地质时代为晚始新世—早渐新世。

The planktonic foraminifera zones of late Eocene to Late Pliocene were established mainly on the basis of the last appearance datum (LAD) of some key taxa of planktonic foraminifera from the well BY7 1 1 in the Pearl River Mouth Basin,South China Sea.The standard zonation schemes of Blow(1969),Bolli and Saunders(1985) were adopted to correlate with our zones. Enping Formation is defined as the strata between seismic reflection surface T 7 and T 8 in seismic section,that means the strata between 3101m and 3455m in the well BY7 1 1.The age of Enping Formation has been a matter of debate with various views of Eocene,Oligocene,Eocene Oligocene and Oligocene early Miocene. The planktonic foraminifera evidences indicate that Enping Formation is chronologically of Late Eocene early Oligocene. Globigerina praeturitilina appearing lastly in the zone P 17 was discovered at and below 3282m,and Globigerina tapuriensis appearing lastly in the zone P 19 was recognized at 3117m in the well BY7 1 1.In low latitude area,the former only appears in late Eocene,and the latter only occurs in early Oligocene(Bolli and Saunders,1985).The view of late Eocene early Oligocene for Enping Formation was also supported by the occurrence of calcareous nannofossils and dinoflagellate in the well BY7 1 1. Lophodolithus acutus ,which only occurs in Eocene strata,was discovered at 3282m in the well BY7 1 1.Studies by Mao Shaozhi(1993) demonstrate that the lowest horizon in which fossil dinoflagellate occurs is at 3441m in the well BY7 1 1,and the fossils below 3231m were mainly Eocene taxa.The results mentioned above indicate that the lowest marine strata in this region is of Late Eocene. The late Eocene early to middle Oligocene planktonic foraminifera assemblages in the well BY7 1 1 indicate that the lowest marine strata in the Pearl River Mouth Basin might be dated back to late Eocene.This discovery provides important microfossil evidence for the classification and correlation of Tertiary strata in the northern part of South China Sea,especially for the reconstruction of early paleoceanographic history in this region.As for the existence of middle Eocene marine strata,some progressive researching works remain to be done.