Study on Sporo-pollen Assemblage and Coal -accumulating Environment of the Middle Jurassic Yan'an Formatiom in Yuling Region, Shaanxi Province

Abstract: Ordos Basin is one of the largest coal-bearing basin in China. In this paper. 103 species of spores and pollen grains referred to 60 genera, form the Yan'an Formation, the Middle Jurassic coal series of ZK1534 dril Ihole in Yuling region of this basin, have been studied. In the sporo - pollen assemblage of the formation, the percentage of pteridophyta spores amounts to 60. 4%, the pollen grains of gymnospermae to 39. 5%. Among the spores, Cyathididtes and Delloidospora are predominant (33. 1%). Osmundacidites (3. 0%), Duplexisporites (3.2%), Converrucosisporites (2.7%), Neoraistrickia (2.2%), Cibotiumspora (1. 6%), and Lycopdiumsporites (0. 6%) are comparatively abundant. Among the pollen grains, Cycadopitts (26. 7%) is the most important, the rest pollen grains only have small amount. By studing sporo -pollen assemblage, in terms of actuopaleontology and the ecoenvironment of the affiliated plants, the main spores and pollen (over 1 % on average) could be divided into three ecotypes: (a) Hu-midophytes, representative dispersed sporo -pollen are Lycopodiumsportes, Neoraistridia, Calamospora. Osmundacidites, Duplexisporites and Steresporites. (b) Humidomesophytes, are Delloidospora, Cyathidites, Ci- botiumspora and Converrucosisporites. (c) Arid - mesophytes, are Cyca dopites and Disaccites. Besides this, the percentage of each ecotype and diversity (by) D=N(N -1)/∑《sub》（-0^s Ni (ni -1) of sporo -pollen are calculated. The ecotype and diversity can be used as main ecoelements to reflecte the palaeoclimate. The former might be the main result of humidity changes and the later might reflect the variation of humidity and temperature. When the humidity and temperature are more suitable to the plant comunity developing and its species flourishing, the diversity might be maximum, and if the circumstance is contrary to above, the diversity might decrease. Morever, the sedimentary environment is considered. All these studing results show that the palaeoclimate of the Middle Jurassic period (Yanan stage) could be divided into three comparative dry periods, they are, the first member, the lower third member and the upper fouth membert and three comparative hot periods: the first to the lower second member, the lower third member and the upper fouth member; During middle -upper third member sediment period, the palaeoclimate is appropriate, palynoflora is fluorishing and delta plain facies is extensively distributed, those make the No. 3 coalbed (in the top of the third member) have a great thickness, wide distribution and a good industry value.