LATE CENOZOIC OSTRACOD BIOSTRATIGRAPHY OF THE NIHEWAN BASIN,HEBEI PROVINCE

Very abundant Ostracoda fossils were found in the Nihewan Basin from more than twenty Late Cenozoic sections and drilling cores.They contain 60 species in 23 genera and may be subdivided into 5 assemblage zones(in ascending order as follows):Zone Ⅰ Potamocypris plana-Candoniella-Ilyocypris assemblage zoneThis assemblage zone is distributed in the Upper Shixia Formation and a few fossils are found in the Upper Hongyacun Formation and mainly represented by Potamocypris plana Schneider,P.cf.horrida Schneider,Candoniella mirabilis Schneider,C.albicans(Brady),and a few Ilyocypris bradyi Sars,I.gibba(Ramdohr),I.dunschanensis Mandelstam,I.cornae Mandelstam,Candona lantianensis Lee,Candoniella suzini Schneider and Eucypris cf.decurta Schneider.Zone Ⅱ Leucocythere-Ilyocypris-Candoniella assemblage zoneThe assemblage zone is distributed in the Lower Member of the Nihewan Formation and the appearance of genus Leucocythere is its characteristic.The fossils of the assemblage zone consist of 28 species in 11 genera and mainly represented by Leucocythere mirabilis Kaufmann,L.exilitropis Huang,L.buranensis Huang,Ilyocypris spp.,Candoniella spp.,Candona spp.,Heterocypris spp.,Eucypris spp.,Cyclocypris sp.,Cypridopsis sp.,Zonocypris sp.,Darwinula stevensoni(Brady et Robertson)and Tanella opima Chen.Zone Ⅲ Leucocythere-Cytheressa-Limnocythere assemblage zoneThis assemblage zone is distributed in the Middle Member of the Nihewan Formation.It is characterized by the occurrence of Limnocythere flexa Negadaev and mainly represented by 28 species in 11 genera:Leucocythere mirabilis Kaufmann,L.burangensis Huang,Cytherissa lacustris(Sars),Ilyocypris spp.,Candona spp.,Candoniella spp.,Cyclocypris sp.,Eucypris spp.,Heterocypris sp.,Cyprideis sp.and Darwinula stevensoni(Brady et Robertson).Zone Ⅳ Ilyocypris-Limnocythere flexa-Limnocythere dubiosa assemblage zoneThe assemblage zone appered in the Upper Member of the Nihewan Formation.The fossils of the assemblage zone mainly consist of 18 species in 9 genera:Limnocythere flexa Negadaev,L.dubiosa Daday,Ilyocypris gibba(Ramdohr),I.bradyi Sars,I.dunschansis Mandelstam and a few Leucocythere mirabilis Kaufmann,Cyclocypris sp.,Candoniella spp.,Eucypris sp.,Candona sp.,Cytherissa sp.and Cypris.Zone Ⅴ Limnocythere dubiosa-Limnocythere sancti-patricii-Ilyocypris assemblage zoneThe assemblage zone is distributed in the Hutouliang Formation and the Xujiayao Formation.The fossils of this assemblage zone contain 19 species in 8 genera.The thrivingness of Limnocythere dubiosa Daday,L.sancti-potricii Brady et Robertson and L.binoda Huang,and the disappearaness of the Leucocythere are the important characteristics for the assemblage.According to the above-mentioned distribution of Ostracoda fossils,the first assemblage zone and the strata of the Hongyacun Formation and the Shixia Formation should belong to the Pliocene.The second to fourth assemblage zones and the Nihewan Formation belong to the early Pleistocene in age.The fifth assemblage zone belongs to the Mid-Late Pleistocene in age,of wich the Hutouliang Formation,Xujiayao Formation can be ascribed to the Middle and Late Pleistocene respectively.