20世纪70至80年代,几乎所有关于江汉盆地跑马岗组和渔洋组的古生物文献都将其时代确定为晚白垩世。李伟同在深入地研究了跑马岗组标准剖面上的轮藻化石后提出跑马岗组大部分属于古新统。取自层位与跑马岗组相当的渔洋组上段上部的岩芯样品经分析后获得了丰富的孢粉化石。这一组合总的面貌与王大宁等获得的晚白垩世渔洋组的第三组合区别明显,而与古近纪的新沟嘴组的第四组合相似处很多。在该孢粉植物群中,常见于当地的晚白垩世分子,如希指蕨孢属、克拉梭粉属、江汉粉属、刺参粉属、鹰粉属等没有或极少见到。而在本组合常见的南岭粉属、五边粉属、漆树粉属等多出现在我国南方古新世地层中。因此,其时代很有可能为古新世。如果是这样,渔洋组上部有可能跨越了白垩系和古近系界线。从当前孢粉植物群含有很多亚热带成分以及麻黄粉属以及胡颓子粉属、忍冬粉属等看,它反映了江汉盆地当时偏干旱的亚热带气候。

The type section of the Paomagang Formation is located at Dangyang in the western part of the Jianghan Basin, Central China. Geological age of the Paomagang Formation was assigned to Late Cretaceous by most palaeontologists based on their palaeontological studies. According to his detailed study on the charophyte fossils from the type section, Li (1985; 1998) suggests a Paleocene age for the greater part of the Paomagang Formation and believes the Paomagang section of the Jianghan Basin spans the Cretaceous/Paleogene boundary. Pollen fossils are the key stratigraphic tool in defining the terrestrial Cretaceous/Paleogene boundary in North America. Up to date few fossil pollen grains were obtained from the type section of the formation. The Upper Member of Yuyang Formation is the contemporaneous subsurface sediments of the Paomagang Formation. Its geological age is considered as the same as that of the Paomagang Formation. To ascertain the potential Cretaceous/Paleogene boundary in the basin we processed some drilling core samples for pollen from the Upper Member of the Yuyang Formation. Most of the processed samples do not contain significant amount of fossils except those from Well Qu 3 at the Qujiawan area of the basin.The fossil pollen flora from Well Qu 3 favours Li's argument of potential Cretaceous/Paleogene boundary in the Jianghan Basin. The palynoflora obtained from the Upper Member of the Yuyang Formation is dominated by angiosperm pollen, amounting to about 80% of the total. Gymnosperm pollen occupies some 14% of the total. Ephedripites constitutes the main body of the gymnosperm pollen. Percentage of spores is low (6%), about half of which is Pteridaceoisporites. The palynoflora contains abundant Quercoidites, Ulmoideipites, Salixipollenites, Rhoipites, Sapindaceidites, Nanlingpollis, Rutaceoipollis, Pentapollenites, Tricolporopollenites, with very a few of Cretaceous pollen types. Comparing the pollen sequences of the Jianghan Basin (Wang and Zhao, 1980b) and Sanshui Basin of Guangdong (Song et al., 1986), the present pollen palynoflora bears much more resemblance to the Early Paleogene assemblages, rather than the Late Cretaceous ones. On the other hand, many common members of this pollen flora, e. g. Nanlingpollis, Pentapollenites, Rutaceoipollis, Quercoidites microhenrici, Q. henrici frequently appear in the Paleogene, especially Paleocene pollen floras of southern China. While those Cretaceous types of South and Central China are individual or absent, i. e. Classopollis, Sapindaceidites projectus, Schizaeoisporites, Rugu- bivesiculites , Callistopollenites, Jianghanpollis, Cranwellia, Morinoipollenites, Consoliduspollenites. Therefore, this palynoflora probably belongs to Paleocene in geological age. In other words, the Upper Member of Yuyang Formation probably spans the Cretaceous/Paleogene boundary and contains some Paleocene sediments in its upper part.

P618.130.2 P534.612

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