鄂尔多斯东北部中侏罗世孢粉化石及古气候意义

Sporopollen assemblage from the Middle Jurassic of the northeastern Ordos basin, Inner Mongolia, and their paleoclimatic implications

鄂尔多斯盆地东北部内蒙古伊金霍洛旗乌兰木伦地区是我国重要的能源矿产地,含煤岩系赋存在侏罗系延安组中,延安组岩石组合以灰白色砂岩、灰色粉砂岩、灰色泥岩为主,夹多层煤层;延安组上覆直罗组以灰色、灰绿色砂岩、灰色粉砂岩、紫红色泥岩为主。通过对乌兰木伦地区武家塔露天煤矿剖面延安组和直罗组孢粉化石分析,在延安组中识别出Cyathidites-Lycopodiumsporites-Neoraistrickia-Cycadopites孢粉组合带,以桫椤科、石松科孢子繁盛,紫萁科孢子和单沟/原始沟类、松柏类两气囊花粉发育,卷柏科孢子及柏科花粉较常见为主要特征,时代为中侏罗世早期(阿林期—巴柔期)。根据延安组孢粉母体植物的生态环境分析,本区在这一沉积时期的气候属于温暖湿润的亚热带—温带型气候。直罗组孢粉化石较少,主要见Cyathidites minor,Osmundacidites parvus,Concentrisporites fragilis,Cycadopites spp.等。综合地层岩性、孢粉组合及古植被分析,延安组到直罗组沉积时期总体上呈现从温暖湿润的亚热带—温带型向半干旱—干旱炎热型的气候变化过程。

The Wulanmulun area in Ejin Horo Banner,Inner Mongolia,which is located in the northeast of Ordos basin,is an important energy base in China.The Jurassic Yan'an Formation is the most important coal-bearing strata in the area.The rock assemblage is mainly gray-white sandstone,gray siltstone,gray mudstone,and multilayer coal seams.The Yan'an Formation is overlain by Zhiluo Formation.The rock assemblage of Zhiluo Formation is mainly gray,gray-green sandstone,gray siltstone and purplish red mudstone.Based on the analysis of sporopollen fossils of Yan'an Formation and Zhiluo Formation in the profile of Wujiata coalmine in Wulanmulun area,the sporopollen assemblage zone ofCyathidites-Lycopodiumsporites-Neoraistrickia-Cycadopitesis identified in the Yan'an Formation.In this assemblage,Cyatheaceae and Lycopodiaceae spores are abundant,Osmundaceae spores,Monocolpates/Praecolpates and coniferous Disaccate pollen are developed,and Selaginellaceae spores and Cupressaceae pollen are common.This indicates that the age of Yan'an Formation is the early Middle Jurassic(Aalenian-Bajocian period).The palynological record confirms that a warm and humid subtropical/temperate climate in the Wulanmulun area during the Yan'an Formation deposition period.There are few sporopollen fossils found in Zhiluo Formation which mainly includeCyathidites minor,Osmundacidites parvus,Concentrisporites fragilis,Cycadopitesspp,etc.Based on the analysis of stratigraphic lithology,sporopollen assemblage and palaeovegetation,the sedimentary period from Yan'an Formation to Zhiluo Formation generally presented a climate change process from warm and humid subtropical/temperate type to semi-arid/arid and hot type.