Abstract:

A great amount of fossil ostracodas are well preserved in eastern China。In this paper,the writer wishes to deal mainly with the characteristics of the ostracoda assemblages of the Cretaceous-Tertiary oil-gas bearing seriesand to discuss their geological significances.1.The assemblage charactristics and evolution stages of the ostracodaThere are a great amount of ostracoda in the related series.Eighty percentof about a thousand of the fossils are new species.According to my point ofview,the evolution of ostracodas found from different formations can be subdi-vided into three stages,namely:early and middle Cretaceous,late Cretaceous-early Palaeogene and middle-late Palaeogene.In the early Cretaceous,after theJurasslc volcanism had just terminated,there existed a comparative stablesedimentary environment in eastern China.Except for Fuxin coal basin,in themore coarser clastic sediments of many other regions,only a little amount offossil were found.On the contrary,the middle Cretaceous,with a stablesedimentary Conditions,is widely distributed in Songliao basin,and a greatnumber of ostracoda have been found.The main assemblage is chiefly Cypridea-Ilyocyprimorpha-Limnocypridea.The associated genera and speciesare Timiriasevia,Lycopterocypris,Mongolianella,Harbinia,Kaitunia,Zizipho-cypris.The middle Cretaceous is the major oil-bearing series of the Daq-ing Oil Field.From late Cretaceous to early Palaeogene,the main assemblageof ostracoda is characterized by Cristocypridea-Cypridea-Candona,associa-ted with Candoniella,Cyclocypris,Cyprinotus,Eucypris,Limnocythere,Me-tacypris.In the assemblage,Cristocypridea is clearly marked by the beak atthe front terminal of the shell.It existed even in a short interval of depo-sition,but was widespread and more stable,and so is considered to be astandard genus in the continental facies.The assemblage is usually found inthe corresponding strata in many other basins too,for instance,from theupper Fulongquen formation to Mingshui formation in Songliao basin,Wang-shi group in Bohai Gulf basin,Paomagang formation in Jianghan basin,fromthe upper Fenshuiao formation to the lower Yuanjiang formation in Dongtingbasin,Daijiaping formation in Hengyang basin and etc.It shows that upto late Cretaceous,a more stable sedimentary environment in vast regionsand a transitional assemblage of ostracodas from late Cretaceous to earlyPalaeogene was widely existed.After the last phace of Yenshan move-ment,the area of Songliao basin in Tertiary became much smaller,the sedimen-tary conditions was unstable,and the ostracoda was declined and found a littleonly,such as Candona,Cyprinotus,Candoniella and Ilyocypris.Middle-late Palaeogene formations of great thickness were deposited in Bohai Gulf,Subaiand Jianghan basins respectively.In these isolated basins of different sedi-mentary conditions had been preserved different assemblages of ostracoda called types of Bohai Gulf,Subai and Jianghan correspondently.Type Bohai Gulfconsists of Huabeinia,Camarocypris,Phacocypris,Chinocythere and Dong-yingia.Some of them have a big individual(2mm)and a variety of delicateornaments in there shell surface.They are associated by Candona,Cyprino-tus,Eucypris,Cyclocypris,Limnocythere etc.Type Subai consists of Sinocyp-ris,Ilyocypris,Eucypris,Limnocythere,Cyprisnotus,Candoniella,Caspiolla,Candona etc.Type Jianghan consists of Limnocythere,Eucypris,Cyprinotus,Cypridopsis,Cypris,Ilyocypris,Cyclocypris,Paracandona,Candoniella,Candona,etc.2.The geological significance of the ostracodaThe growth and evolution of the ostracoda are closely related to thetectonic movement and thus reflect the tectonic environment during the depo-sition.For instance,in late Cretaceous,the deposits in eastern China tend-ed to be in comparative stable sedimentary conditions and in more extensiveand flat basins,thus occured the Cristocypridea-Cypridea-Candona assem-blage.But in the Palaeogene,the three different types of assemblages of ostracoda said above,were developed in different basins which were iso-lated each other by the Yangshan movement.Besides,the major oil-ga