Biostratigraphy And Paleoecology Of The Early Cretaceous Ostracods From Luanping Basin,northern Hebei,China

The Lower Cretaceous Dabeigou-Xiguayuan formations in Luanping Basin,northern Hebei Province,China,is characterized by continuous sedimentation and relatively complete preservation,as well as yielding the famous Jehol Biota,including abundant ostracods.These advantages make the Luanping Basin an ideal area to study the Early Cretaceous non-marine strata and the early-middle evolution of the Jehol Biota.Ostracods are small in size,abundant in number,fast in evolution and easy to be preserved as fossils.They play an important role in the division and correlation of non-marine Mesozoic strata,and are also one of the proxies for the exploration of paleoenvironment.Previous studies on ostracods in Luanping Basin have been done a lot,and acquired series of achievements.However,due to the overestimation of the species diversity of ostracods and the different understanding of some ostracods'taxonomic units,the research on ostracods' taxonomy,biostratigraphy and paleoecology has brought certain controversy and difficulties.Based on the detailed lithostratigraphic correlation and the latest research progress of ostracods,this thesis aims to carry out the systematic taxonomic analysis and stratigraphic study of the early Cretaceous ostracods in Luanping Basin.On the above results,the ostracod biostratigraphic correlation between basins is discussed.Paleoecology and its paleoenvironmental implication of the ostracods from Luanping Basin have also been discussed,by analyzing the habits of the present relatives of the ostracods fossils,as well as using other methods,such as sedimentary,taphonomy.The main progress of this paper includes the following aspects:The field description of 6 typical sections of Dabeigou-Xiguayuan formations of Luanping Basin is sorted out in detail.The division and correlation of the strata in each section of Luanping Basin are established by comparison of the lithological characteristic,isotopic dating and biological fauna.The ostracods from the Lower Cretaceous Dabeigou to Xiguayuan formation of the Luanping Basin are systematically summarized and identified,with 15 genera and 44 species are recognized.The ostracods of the Dabeigou,Dadianzi and Xiguayuan formations can be concluded as the Luanpingella-Eoparacypris-Ocrocypris,Cypridea-Timiriasevia-Daurina,and Cypridea-Limnocypridea-Lycopterocypris assemblage zones,respectively.In addition,the ostracod assemblage zone of the Dadianzi Formation can be further divided into 4 subzones,including the Cypridea dabeigouensis,Cypridea luanpingensis,Cypridea sulcata,Cypridea pangi subzones,which corresponding to from Member 1 to 4,respectively.Research progress of the biostratigraphy of other taxa,chronostratigraphy and magnetostratigraphy of the Early Cretaceous strata from Luanping Basin has been summarized.The related contents of the Jibei stage have been revised,based on the new progress the research.It is considered that the earliest occurrence age of Cypridea in North China is?130 Ma,younger than the J/K boundary age.The Dabeigou and Dadianzi formations yields two different ostracod faunas,i.e.,the non-Cypridea fauna and the Cypridea fauna,which exist in relatively continuous deposits.It is likely that some species from non-Cypridea fauna,such as Eoparacypris and/or Daurina of the Dabeigou Formation,evolved to Cypridea.The transition from the early Jehol Biota to middle Jehol Biota,including the occurrence and rapid prosperity of Cypridea in the Dadianzi formation,as well as the large-size dominant ostracods change to small-size dominant ostracods,is likely to respond to the increasing paleotemperature from the deposition period of Dabeigou Formation to the deposition period of Dadianzi Formation.Based on the analysis of the paleoecology of the ostracods,combining with the sedimentology and other proxies fossils results,the paleoenvironment during the early Cretaceous of the Luanping Basin is discussed.