合肥盆地安参1井前侏罗纪基底地层的孢粉组合特征与时代归属

Characteristics and Age Assignment of the Sporo-Pollen Assemblages of Pre-Jurassic Basement Strata in Well Ancan-1,Hefei Basin

大别山北缘的安参1井是合肥盆地最深的一口参数井,钻遇前侏罗纪基底地层达千米以上,为一套不含煤层(线)或灰岩以泥岩为主的地层,与周缘华北陆块和大别造山带等构造单元的地层对比困难。因缺少宏观化石记录和明确的定年依据,其时代归属一直存在争议,制约了对这一套特殊地层所反映的地质信息的进一步挖掘。为进一步明确该套特殊地层的地质时代,本项研究对安参1井4160~5152m井段大量岩心和岩屑样品进行了微古化石分析,在其中5块样品中发现了保存良好的孢粉化石647粒,共计23属51种及部分未定种。根据对孢粉化石的类型、属种和含量的分析,将其自下而上划分为两个孢粉化石组合:Ⅰ.Densosporites reynoldburgensisLaevigatosporites perminutus;Ⅱ.Triquitrites-Macrotorispora media。孢粉组合Ⅰ中蕨类植物孢子以平均含量87.67%占绝对优势,代表分子有Laevigatosporites perminutus,Lycospora rotunda,Patellisporites meishanesis等,裸子植物花粉均为单囊粉属Florinites;孢粉组合Ⅱ中,蕨类植物孢子平均含量降低至53.78%,其中蕨类孢子Lycospora rotunda消失,Leiotriletes adnatus,Triquitrites,Crassipora等含量下降,并有特征分子大一头沉孢属Macrotorispora出现,裸子植物花粉含量和丰富度明显增加,以柯达粉属Cordaitina为主。经过与华北其他地区孢粉组合特征的对比分析,上述两个孢粉组合分别与华北陆块晚古生代二叠系太原组和上石盒子组具有明确的相似性,与早期和中晚期华夏植物群面貌相一致,反映了裸子植物逐渐繁盛、属种越来越丰富的过程,孢粉组合Ⅰ和Ⅱ的地质时代分别为晚石炭世晚期至早二叠世早期和晚二叠世,为安参1井前侏罗纪基底存在石炭—二叠系提供了直接证据,也为合肥盆地沉积环境和古气候分析提供了新资料。

Well Ancan-1 is the deepest parameter well in the Hefei basin,northern margin of the Dabie orogenic belt.The primary lithology of the drilled Pre-Jurassic basement strata,with a thickness of more than 1000 m,consists predominantly of dark mudstone,without coal streak,coal seam or limestone,which makes it difficult to compare with the strata of North China craton and the Dabie orogen belt.For lack of megafossils record and definite dating data,the age assignment of basin basement has been in dispute,which restricts deep understanding of the geological information related to this particular formation.In order to further clarify the depositional age of the peculiar strata,a large number of cores and debris samples in well An'can-1 (4160~5152m) were studied,and 23 genera,51 species and 647 sporo-pollen fossils have been identified.According to the analysis of genera,content change and vertical variation of sporo-pollen fossils,this study classified the sporo-pollen into two assemblages as follows (in a descending order):assemblage Ⅰ,Densosporites reynoldburgensis-Laevigatosporites perminutus,and assemblage Ⅱ,Triquitrites-Macrotorispora media.In sporo-pollen assemblage Ⅰ,fern spores are predominant,accounting for 87.67% on average,including Laevigatosporites perminutus,Lycospora rotunda,Patellisporites meishanesis and others.Gymnospermous pollen grains are completely Florinites.In assemblage Ⅱ,the average content of fern spores decreased to 53.7%.Among them,the contents of some spores (such as Leiotriletes adnatus,Triquitrites and Crassipora) decline,with the Lycospora rotunda even disappeared.In addition,there existed Macrotorispora as typical element.The content and richness of gymnospermous pollen,esepically Cordaitina,increased significantly.Comparison analysis of these assemblages with those in other areas in North China shows that the two sporo-pollen assemblages mentioned above are similar to the sporo-pollen in the Taiyuan and the Upper Shihezi formations of late Paleozoic in the north of China,respectively,and are consistent with Early Cathaysia flora and Middle-Late Cathaysia Flora respectively.These features reflect a process of prosperity and enrichment of gymnospermae.The geological time of the two sporo-pollen assemblages should be the late Late Carboniferous to early Early Permian and the Late Permian,respectively.This study provides not only direct evidence for the existence of the Carboniferous-Permian in Pre-Jurassic basement strata in Well Ancan 1,but also new materials for sedimentary environment and paleoclimate analysis of the Hefei Basin.