Analysis Of Palynomorph Assemblage And Palaeoclimate In The Middle Part Of Yixian Formation,Yixian Basin In Western Liaoning Province

The Yixian Formation of western Liaoning Province is well known for yielding a large number of high quality and rare Jehol Biota fossils from the lower Cretaceous.The lithology of the Yixian Formation is a basic-mid basic-neutral-mid acidic series resulting from volcanic eruption of continental rock.The intermittent eruptions formed six lacustrine sedimentary layers,which are,in ascending order,the Laogonggou layer,Yenangou layer,Zhuanchengzi layer,Dakangbao layer,Zhujiagou layer,Jingangshan layer,and the Huanghuashan layer.Among them,the Zhuanchengzi layer and Dakangbao layer are the main fossil-producing layers and hence the significant targets for research.According to the theory of sedimentology,stratigraphy,paleontology as guidance,we collected,processed,identified,and analyzed the spores and pollen samples from the Zhuanchengzi Bed in the Yingwoshan area and Dakangbao Bed in the Wangjiagou area of the Yixian Formation,western Liaoning.As a result,we confirm a palynomorph assemblage of Cicatricosisporites-Protoconiferus.The pollen was primarily from gymnosperms,dominated especially by conifer pollen.Pteridophyte spores were less common and some questionable angiosperm pollen occurred occasionally.Among the Zhuanchengzi Bed,we confirm 18 genus and 24 species of pteridophyte spores,28 genus and 37 species of gymnosperms pollen,and 3 genus of angiosperm pollen.Among the Dakangbao Bed,we confirm 16 genus and 22 species of pteridophyte spores,and 27 genus and 36 species of gymnosperms pollen.According to the analysis of the characteristics of palynomorph assemblage,we make the a comparison with palynomorph genus and species in Hauterivian to Barremian of the Erlian Basin in Inner Mongolia and palynomorph genus and species in late Valanginian to Barremian of Northern Siberia area in Rassia.Then we make a conclusion that the age of the palynomorph assemblage is dated as the Hauterivian-Barremian stage,the Early Cretaceous.Combining with the spores and pollen,bivalves,plant,Ostracoda fossil and isotopic dating data comprehensive analysis,the geological dating of the Yixian formation is likely to hauterivian-early Aptian.Presumably,the geological dating in the middle part of the Yixian formation is possible for Hauterivian-Barremian.At the same time,we preliminaryly discuss on age of the Jurassic/Cretaceous boundary.If the geological dating of it is 145Ma in accordance with the international stratigraphic chart,and we find that the geological dating of the Yixian Formation is in middle and late Early Cretaceous.The boundary must under the Yixian formation,and maybe in the Tuchengzi Formation.More fossil evidence is expected to find in the Zhangjiakou Formation or Houcheng Formation,northern Hebei province.The study applies the concept of Palynological Vegetation based on palynological spectra and the paleoecological characteristics of palynological taxa for the first time.Palynological vegetation type,climatic zone type,and humidity type are divided quantitatively for the Zhuanchengzi Bed in the Yixian Formation of western Liaoning.We then obtained the evolutionary trends.The results showed that the overall climate was warm and humid during the deposition period of the Zhuanchengzi Bed in the Yixian Formation.Palynological vegetation types are various and include coniferous forest,deciduous broadleaf forest,evergreen broad-leaved forest,grass,and shrubs.The local temperature changed from warm to much warmer and from a semi-humid to humid climate.And there was seasonal climate change.The overall climate was warm,semi-arid and semi-humid during the deposition period of the Zhuanchengzi Bed in the Yixian Formation.The trend is that the climate is dry after the volcanic phase,but the trend of the whole environment is still warm with the coexistence of various vegetation types.The local temperature change trend is from warm to much warmer,and the climate is from humid to semi-arid and semi-humid.It is worth noting that the dominant vegetation types of coniferous forest and its 50%proportion approximately,the coexistence of various vegetation types and the background of warm climate environment in the middle part of the Yixian Formation.They are indicative of the importance of vertical zonation in vegetation.And vertical zonation of vegetation seems to suggest that the contrast of the surrounding topography may have been quite large during the deposition of the Yixian Formation in western Liaoning.