Late Jurassic To Early Cretaceous Palynofloras Of Northern China And Xizang And Their Palaeoenvironmental Significances

China has a wide distribution of the Late Jurassic to Early Cretaceous sediments,including the terrestrial deposits mainly in northern China and the marine to marine-continental transitional deposits representatively in the Xizang area.However,the geological ages,division and correlation of the regional strata of this period are controversial,with different views proposed on the palaeogeography of eastern China during that time.Palaeovegetation,as an effective approach to reconstruct the palaeogeography and palaeoclimate,has not been well studied for the Late Jurassic to Early Cretaceous in China.The northern Hebei and western Liaoning in the northern China and the central and eastern Xizang(Lhasa Block)in the southwest China have been the representative areas of the Upper Jurassic to Lower Cretaceous,providing excellent palynological fossil material for tackling those issues above.In this dissertation,a systematic palynological analysis was conducted on the Late Jurassic to Early Cretaceous strata from seven sections in the northern Hebei and western Liaoning area and the central and eastern Xizang(Lhasa Block).Among the 168 samples treated,71 samples were palyniferous,from which 14211 specimens were recoverd belonging to 97 genera,144 species and some undefined species.Eight palynological assemblages in total were recongnized from the studied sections according to the compositional variations,FADs and LADs of key taxa,and their ages were discussed respectively.The regional Late Jurassic to Early Cretaceous palynological sequence was established.The four palynological assemblages recovered from northern Hebei and western Liaoning area include the Berriasian-middle Valanginian Bisaccates-Fixisporites-Ephedripites(S.)assemblage in the Jinlingsi-Yangshan and Jianchang basins(Cuizhangzi and Nanshimen sections,Tuchengzi Fm.),the middle Valanginian Classopollis-Densoisporites-Cicatricosisporites(Yushuxia section,First Member of the Dabeigou Fm.),middle-late Valanginian Bisaccates(Yushuxia and Xiaying sections,Second and Third members of the Dabeigou Fm.and the lower part of the First Member of the Dadianzi Fm.)and early Hauterivian Aequitriradites-Hekousporites(Xiaying section,the upper part of the First Member of the Dadianzi Fm.)assemblages in the Luanping Basin.Four palynological associations were recognized from the Lhasa Block,including the Late Jurassic Cyathidites-Classopollis(Qubsang section,Duodigou Fm.),Berriasian-Hauterivian Classopollis-Cooksonites(Qubsang section,Linbuzong Fm.)and Classopollis-Dicheiropollis(Wadag section,Duoni Fm.),and late Hauterivian-early Barremian Dicheiropollis-peak associations(Lalong section,Duoni Fm.).Combined analysis of the palynological assemblages of the Tuchengzi Formation in Jinlingsi-Yangshan,Luanping and Jianchang basins shows a gradual decrease of Classopollis upward from the middle-upper part of the First Member of the Tuchengzi Formation.This decrease is accompanied with a flourishing of bisaccate pollen,which finally replaced Classopollis as the most abundant palynofloristic component in the Third Member of this formation.Meanwhile,the Early Cretaceous key taxa Fixisporites and Cicatricosisporites occur in the middle-upper part of the First Member of the Tuchengzi Formation,implying the Jurassic-Cretaceous boundary of northern China should be located within this interval.The Bisaccates-Fixisporites-Ephedripites(S.)association recovered from the Third Member of the Tuchengzi Formation at Nanshimen,Jianchang Basin,is similar to the Bisaccates assemblage of the Dabeigou Formation at Yushuxia in the Luanping Basin.Their geological ages are about the same.In addition,this research obtained a fern-dominated palynological assemblage from the Dadianzi Formation for the first time,which has never been encountered from the lower part of the Yixian Formation yet.A comparison of the Berriasian-Hauterivian palynofloras through eastern China reveals that there is no transitional zone between the south and north palynofloras.Also,there is little differentiation between the west and east.All these together indicate no geographical barriers to the exchange of palynofloras at that time.Therefore,there is no solid palaeovegetational evidence for an "Eastern Plateau" in China during the Early Cretaceous.A comparison of the Late Jurassic to Early Cretaceous palynological assemblages between the Lhasa and Qiangtang blocks shows they share similar palynofloras,implying the two blocks had merged before the Late Jurassic.There is an increase in abundance of Cheirolepidiaceae from the continent marginal Lhasa Block to the interior of Qiangtang Block and Tarim Basin,showing an inland decrease of precipitation.Reconstructions of palaeovegetation and palaeoclimate suggest that the vegetation in the Early Cretaceous Berriasian-Valanginian of northern Hebei and western Liaoning area was mainly composed of cool-adapted and mesophytic Coniferae,which indicates a semi-humid to semi-arid and cool climate.Nonetheless,the vegetation during the sedimentary period of middle to late Valanginian Bisaccates assemblage was more monotonous and less diverse than that of the Berriasian to middle Valanginian Bisaccates-Fixisporites-Ephedripites(S.)assemblage,and the climate of the former occasionally fluctuated in humidity and temperature.During the depositional period of the early Hauterivian Aequitriradites-Hekousporites assemblage,warm-and wet-adapted ferns and bryophytes increased to be the dominant community,showing a hotter and wetter situation than before and a semi-humid and warm to hot climate with occasional weak fluctuation in humidity and temperature.The vegetation of the Lhasa Block during the Late Jurassic to the early Barremian of Early Cretaceous transferred from warm-and wet-adapted ferns dominated to the forests of cheirolepid conifers that favored hot and arid environments.The climate changed from generally semi-humid and warm-hot to arid or semi-arid and warm-hot during that time.