Middle Jurassic Strata And Environment In Longjiang Basin Of The Middle Da Hinggan Mountains

Longjiang Basin is located in the border area between eastern Inner Mongolia and western Heilongjiang in the middle Da Hinggan Mountains,NE China,yielding the Middle Jurassic coal-bearing strata,which consists of conglomerate,sand,and volcanic clastic rock with sedimentary rock and coal seams deposits.The Jurassic strata of this Basin have been poor in study before,and argued in geological age and formation naming(the Wanbao Formation)of the coal-bearing strata,for a long time.Based on stratigraphic,palaeontological,geochronological,and geochemical studies,this paper has firstly and synthetically studied the Middle Jurassic flora(including sporopollen and petrified wood)of this basin,in which the author established 5 new species,compared this coal-bearing strata and their flora with the well-known coeval strata and floras at home and abroad,and confirmed the flora of the Longjiang Basin belonging to the Early Assemblage of Coniopteris – Phoenicopsis Flora in China,aged in the early Middle Jurassic.Meanwhile,the author has obtained the isotopic dating results presented mainly by 166.2±1.3Ma,which also evidenced the Middle Jurassic age for the Longjiang coal-bearing strata.In addition,the author analyzed the characters of the sedimentary source,environment,climate and tectonic background of the Basin in the Middle Jurassic,and explained the rationality on using the Wanbao Formation naming in the Longjiang Basin.In the study of the plant mega-fossils,the author firstly discovered the Jurassic flora of the Longjiang Basin composed of 21 genera and 34 species,with 5 new species,including the fossil wood.The flora is dominated by ferns(ca.38%)and conifers(ca.21%),associated with cycads(including cycadalean and bennettitalean,for ca.17%),czekanowskialean(ca.17%)and ginkgoalean(ca.7%).In the ferns,the dicksoniaceous pollen in situ and 2 new species were found for the first time.Besides,the coniferous wood fossils with 3 new species also were reported.Palynologically,29 genera and 35 species of palynomorphs were found and named as Cyclogranisporites-Alisporites assemblage for the first time.The gymnosperm pollen are mainly characterized by the bisaccate pollen of conifers and monosulcate pollen of cycads and ginkgoales.Paleoclimatically,the flora indicates a warm-temperate to temperate climate with seasonal changes in the Longjiang Basin during the Middle Jurassic,where might be in a low mountain area in environment.Geochronologically,the author obtained the zircon U-Pb dating results of the Wanbao Formation of the Basin,including 166.2±1.3Ma,165.2±1.7Ma,164.0±1.3Ma and 162.1±1.6Ma,and the clastic rock zircon U-Pb date showing the youngest peak in the 4 groups as 167.5±2.5 Ma.All the above-mentioned ages supplemented the evidence of the Middle Jurassic age for the Wanbao Formation in the Longjiang Basin.Paleoenvironmentally,this paper comprehensively and synthetically studied the Longjiang Basin in sedimentary environment and climate in the Middle Jurassic.The study revealed that the sedimentary environment was probably a mountain fan delta-shallow lake basin formed in intermittent stages of volcanic eruptions.The climate might be a warm temperate to temperate with seasonal changes.In sum,this paper has firstly confirmed the existence of the Middle Jurassic strata in the Longjiang Basin of the middle Da-Hinggan Mountain region,firstly discovered the Middle Jurassic flora and obtained the zircon U-Pb dating results on the Wanbao Formation of this Basin.All the new discoveries would extremely improve the stratigraphic study of Jurassic strata in the middle Da Hinggan Mountain region,and put forward widening the prospects for exploration of coal,oil and other mineral resources in the western marginal areas of the Songliao Basin.In addition,the present study would promote the study of the Jurassic phytogeography in northern China.