Foraminifera And Seawater Incursions Events Of The Late Cretaceous Songliao Basin

Songliao basin is the ideal location for study of the Cretaceous continentalclimatic and environmental, because of that developed a complete set of lacustrinesedimentary strata in Late Cretaceous. In the Early-Middle Late Cretaceous, SongliaoBasin development has reach its period of peak. During this period, the lake waterswide, biological prosperous, Qingshankou and Nenjiang formations occurred in twolarge-scale lake transgressive event, and the long-standing debate for it was theexistence of seawater intrusion event at the same period. Based on the data ofCCSD-SK-1boreholes, and other drilling data, we engaged in foraminiferabiostratigraphy and ancient ecological researches, moreover, combined with otherdata to discusses the transgressive events in Songliao basin.This paper is the further study that based on the foraminifera fossil was initiallyreported in CCSD-SK-1(south) boreholes. Recently, we identified a total o f4speciesof11genera of foraminifera, and which had9undetermined species, inCCSD-SK-1(north) boreholes and CCSD-SK-1(south) boreholes, these foraminiferafossil including planktonic foraminifera Archaeoglobigerina blowi,Archaeoglobigerina cretacea, Hedbergella flandrini., Globotruncanella sp. andGuembelitria sp.; benthic calcareous foraminifera Gavlinella sp., Anomalinoides sp.,Pullenia sp.; benthic agglutinated foraminifera Haplophragmoides sp., Karrerulinahokkaidoana，Clavulinoides sp. and Conorboides sp. These foraminifera are similarto the West Pacific foraminifera animal groups, especially in planktonic foraminiferaalmost exists widely in the Cretaceous marine strata. In addition, we found three layerof foraminifera fossil in J79boreholes, and one of them may belong to the planktonicforaminifera Hedbergella sp., provides new information for Songliao Basin.The foraminifera fossils in Songliao Basin includ planktonic foraminifers, thebenthic calcareous and agglutinated ones. Their quantities and forms are bothrelatively rich, and they are similar to the west Pacific foraminiferal fauna in thisperiod, which shows that these foraminifers were taken into the Songliao Basin within the seawater intrusion. Therefore the fossils are evidence for the existence of thetransgression events of Songliao Basin. There are eight foraminifera distributionlayers in CCSD-SK-1boreholes, between which exists faults. It can be inferred thatthere were at least eight intermittent seawater intrusion events in the sedimentaryperiod of the one-two Members in Nenjiang formation.Compositing the foraminifera fossils and other evidences, it is comprehensivelydiscussed about the transgression events of Songliao Basin in NenJiang formation. Itincludes that the Songliao Basin was affected by intermittent seawater intrusionduring the period from the bottom of bed one to the top of bed two in NenJiang group,which brought the relatively rich foraminifers and other marine biological community.The seawater invaded with intermittent and the scale was probably not big, so theancient environment may be half salt water lake environment influenced by seainvasion.