Study On Mesozoic Biostratigraphy In Tamtsag Basin, Mongolia

Based on the investigation of the Cretaceous stratigraphic sections in Tamtsag basin, Mongolia and the research history of the Cretaceous in this area and nearby,the Cretaceous petrological stratigraphic sequences in Tamtsag basin are established following the international stratigraphy criterion.Taxonomic research of the pollen and spores in the drillings is undergone systematically,totally 80 species of 62 genera(including 3 new species)are described in the Tamtsag basin.Moreover,4 pollen and spores assemblages and 2 algae assemblages are recognized by their vertical distribution of different palynological taxa in descending order for the Cretaceous of the Tamtsag basin.In order to study the geological time of the pollen and spores assemblages,the biostratigraphic significance of 7 genera of pollen and spores are summarized in detail.The geological ages of every petrological stratigraphic unites are discussed based on the palynological data.The sedimentary strata of the Tamtsag basin can be divided into three groups in ascending order.The Xinganling Group:Middle Jurassic-Lower Cretaceous Tamulangou Formation, Lower Cretaceous Tunbomiao Formation,Lower Cretaceous Nantun Formation(the Tunbomiao and Nantun formations equal horizentally to the Shangkuli Formation at the margin of the basin).The Zhalainor Group:Damoguaihe Formation and Yimin Formation.The Bairhu Group:Upper Cretaceous Qinyuangang Formation,Pliocene Hucashan Formation and Quanternary.Totally 80 species(including 3 new species)of 62 genera are described from the Cretaceous of Tamtsag basin.The new species are Biretisporites punctatus sp.nov., Concentrisporites reticulates sp.nov.and Chasmatosporites contractus sp.nov..There are some pollen and spores in the Cretaceous of the Tamtsag basin,such as Cicatricosisporites,Paleoconifers(Paleoconiferus,Protoconiferus,Paleopinus,Paleopicea, Pseudopicea,Piceites,Protoabies,Pseudopinus,Protopicea,Pseudopodocarpus,Walchites, Pseudowalchia,Protopinus,Protocedrus,Pinites,Protopodocarpus,etc.),Pilosisporites, Triporoletes,Aequitriradites,Appendicisporites,Clavatipollenites etc.which are very important for the division and correlation of the Cretaceous,and are usually taken as index fossils in determining the geological age of the strata.The biostratigraphic significance of these index fossils is particularly summarized.4 pollen and spores assemblages are recognized from the Cretaceous in the Tamtsag basin. They are:1)Bayanhuasporites sp.-Hailarspora sp.- Concentrisporites sp.Ass.,distributed in the Tunbomiao Formation;2)Foraminisporis asymmetricus - Classopollis sp.- Pseudopicea sp.Ass.,distributed in the Nantun Formation;3)Cicatricosisporites australiensis -Matonisporites equiexinus - Protopicea sp.Ass.,distributed in the Damoguaihe Formation;4) Appendicisporites macrohyzus - Pinuspollenites sp.-Retitrcolpites geogensis Ass.,distributed in the Yimin Formation.These 4 pollen and spores assemblages are not only the biostratigraphic criteria for the division and correlation of Cretaceous in the wells of Tamtsag basin,but also the main evidence to ascertain the geological age of every petrological stratigraphic unite.2 algae assemblages are recognized from the Cretaceous in the Tarntsag basin.They are:1) Vesperopsis granulata -Dinogvmniopsis sp.Ass.,distributed in the Damoguaihe Formation of the Tartan depression in Tamtsag basin;2)Tetraedron sp.- Vesperosis fragilis Ass., distributed in the Yimin Formation of the Tanan and the Bayangebi depressions in Tamtsag basin.These 2 algae assemblages are not only the biostratigraphic criteria for the division and correlation of Cretaceous in the wells of Tamtsag basin,but also the main evidence to ascertain the geological age of every petrological stratigraphic unite.Of all kinds of fossils,palynological fossil records of the Tamtsag basin are most abundant and continuous,playing an important role in confirming the geological age of the strata.The results of palynological chronology analysis show that the geological age of the Tunbomiao Formation might be Berriasian-Valanginian,the Nantun Formation might be Valanginian-Hauterivian in age,the geological age of the Damoguaihe Formation might be Hauterivian-Barremian of the Early Cretaceous and the Yimin Formation might be Barremian-Albian.Totally33 plates,2 pictures,6 tables are included in the present thesis.