Plant Composition And Paleoenvironmental Analysis Of Middle Jurassic Flora In Huating, Gansu Province

With the development of the times, a series of serious problems about environmental deterioration force people to pay more attentions. And the fossil plants can provide material for climate change research in the geological history because a lot of paleoenvironmental information had been recorded by the physiological process. Therefore, studying on fossil flora composition and characteristics to explore plant diversity and the evolution of ecosystems in the geological history has important scientific significances for understanding the future trend of the global environment and ecosystems.The fossils studied in this paper were collected from the Yan’an Formation in Huating, Gansu Province, China. Based on a detailed analysis of macro and micro characteristics, 27 genera including 59 species were identified. Ginkgopsida（40.7%） and Filicopsida（25.4%） were the most abundant compositions in this flora, Coniferopsida（17.0%） occupied for some proportion, and Sphenoposida（8.5%） and Cycadopsida（5.1%） were also found. On the basis of fossil compositions and compared with some similar flora, the geological age of the present fossil horizons of the Yan’an Formation in Huating should belong to the Aalenian to Bajocian（Middle Jurassic）.Thirteen Middle Jurassic floras in North China were analysised by the cluster analysis method to subdivided the Northern China province. The results show that the Northern China province in the Middle Jurassic should be divided into two geographic units: I. Eastern Gansu,Central Inner Mongolia, Shanxi, Henan and Beijing; II. Northwest–northeast aeras in China. This results of are consistent with the area tectonic setting and climate characteristics. The branch biogeographical analysis is consistents with the result of cluster analysis, which shows that the flora in Huating is closer to the flora in Yima of Henan than to flora in Yaojie of Gansu.The results of the burial characteristic analysis shows the plant fossils in Huating belong to sub–situ or in situ burial. The morphological functional analysis and the geographical distribution of the plant fossils indicate that the Huating flora might live under a humid and warm climate condition. Flora composition shows this flora has a vertical structure differentiation. In addition,two species of Ginkgopsida were selected to estimate the paleo–CO₂ concentration of the MiddleJurassic in Huating by stomatal ratio method. And the result is 1242±60 ppmv.