The Provincialization Position and the Paleoclimate of the Early Cretaceous Microflora in the Minhe Basin,Gansu Province

Abstract: This paper deals with the spores and pollen from the Upper Subformation of the Hekou Formation of Lower Cretaceous in Xigu District of Lanzhou, Gansu Province,China. The sporopollen assemblage of the Upper Subformation has been established,and it was named as PiceaepoTlenites-Cicatricostsporiles-TricolPoyollenites. The assemblage is characterized by the slight dominance of Gymnospermous pollen,its percentage amounts to 56. 5%. The Pteridophyta spores amounts to 42. 2%,and the Angiospermous pollen merely to 1. 4%. The Lygodiaceae of Pteridophyta is the most important family in the assemblage. The genus Cicatricosisparites is predominant. Lygodzumspurites and Trillobosprites are common. The spores of Selaginellaceae are important,too. Among them,Densoisporites is predominant,and the others are Aequitriradiles and so on. Schizaeaisparites of Schizaeaceae is of higher percentage,consisting of many species. The other families are low in the percentage,such as Cyathidiies, DelioidosPora, Lycopodiumsparites etc.. The Pinaceae pollen of the Gymnospermae is predominant. The most abundant genus is Picesepollenites,and Abietinaepollenites is next to it. The others in the family are Pinusyollenites etc.. The genus Classnpollis of Cheirolepidiaceae amounts to 11. 9%. The appering genera in other families of the Gymnospemae include Podocarpidites,EPhedripites,Rugubivesiculites,Cauialasporiles, Jiaohepolllis etc.,and the percentage of each one is low. Triodpopollenites is the only genus in the Angiospermae. In the sporo-pollen assemblage of the Upper Subformation,the first subassemblage includes old types and disaccate pollen of Gymnospermae more than the second one,and some microfossils of Dinoflagellate and Acritarch appear. But the percentages of Classapollis and Cycadopites are lower. By analysing the periods of the important genera and species in the assemblage and comparing with the known assemblages in various districts, the author cosidered that the geologicage of the assemblage is the late phase of Early Cretaceous,that is Aptian-Albian stages. So far the stratum of the Upper Subformation is the uppermost one of the Lower Cretaceous which has been found in the Minhe Basin. Based on the studies above,the paleoclimatope of the botanic community in the late phase of Early Cretaceous has been discussed according to actuopaleontolog. It is suggested that the Minhe Basin situated in the tropics or subtropics in the Early Cretaceous, where the plants were luxuriant. The climate should be humid in the early stage for the first subassemblage,and the climate should be comparatively arid in the late stage for the second one. Furthermore,the author referred plenty of material on Early Cretaceous palynology of. China,tried to interpret the paleobotanic provincialization and thought that the Minhe Basin was situated in the Transitional Zone of the Southern Region in Early Cretaceous on the traditional biogeographic provincialization. On the numerical taxonomic provincialization of Early Cretaceous palynology,China can be divided into two regions:the North Region and the South Region, which can further be divided into some provinces. The Minhe Basin is in the Northwest-Northeast Province of the North Region.