甘肃平凉安国镇白垩系六盘山群的孢粉植物群

PALYNOFLORAS FROM THE LIUPANSHAN GROUP (CRETACEOUS) AT ANGUO TOWN OF PINGLIANG, GANSU

对甘肃—宁夏交界处六盘山群一个露头剖面进行了孢粉分析,从和尚铺组、李洼峡组、马东山组和乃家河组中获得了孢粉化石。这些材料表明,六盘山群的孢粉植物群中白垩纪分子丰度高、分异度大,时代可能为晚Aptian—早、中Albian。孢粉反映当时的气候条件中性偏干,其中和尚铺组和乃家河组比李洼峡组和马东山组要更干一些。根据孢粉资料,六盘山群的和尚铺组至乃家河组的沉积可与盆地主体的志丹群的上部即泾川组对比,三桥组则可能相当于罗汉洞组。陕甘宁盆地西部边缘区缺少相当于志丹群宜君组至环河华池组的早白垩世早期的沉积。

The Liupanshan Group is widely distributed and well exposed in the bordering areas of Shaanxi, Gansu and Ningxia, or the western margin of the Shaanxi Gansu-Ningxia Basin. A variety of spores and pollen are discovered during the present study of a section near Anguo Town, Pingliang, Gansu Province. Fifteen samples are found quite palyniferous from four of the five composing formations, e.g. the Sanqiao, Heshangpu, Liwaxia, Madongshan and Naijiahe formations in ascending order with the basal one, Sanqiao Formation, as an exception. Gymnosperm pollen predominate the palynoassemblages except LD272, 271 and 206, in which fern spores prevail. The most abundant of this group is Classopollis, and others include Cycadopites, Ephedripites and Jugella. Saccate pollen of conifers is common but always in low percentages. Fern spores Cicatricosisporites is abundant and of high diversity, flourishing in a few samples. Schizaeoisporites acts in a similar way of Cicatricosisporites, but usually in lower percentages. Cyathidites and Lygodiumsporites are common in the assemblages. Compared with the assemblages discovered by Liu Zhaosheng （1983）, which only refers to two formations of the Liupanshan Group, e.g. the Liwaxia Formation and the Naijiahe Formation, much more Cretaceous indicators are encountered in our study. They include Foraminisporis, Hsuisporites, Pilosisporites, Plicatella, Wulongspora, Yichangsporites, Coptospora, Jugella, Zhonghuapollis and Tricolpopollenites. Based on these taxa, especially the occurrence of angiosperm pollen Tricolpopollenites, a late Early Cretaceous age, e.g. Early to Middle Albian, is indicated though late Aptian may not be entirely ruled out. The xerophytic Schizaeoisporites, Classopollis, Ephedripites and Jugella occur abundantly and diversely in the palynoassemblages, especially in those of the Heshang-pu and Naijiahe formations. Thus an arid climate is indicated though semiarid or periodic arid may be more likely for the Liwaxia and Madongshan formations. A comparison is made between the Liupanshan Group and the Zhidan Group deposited at the central Shaanxi-Gansu-Ningxia Basin. That is, the Heshangpu, Liwaxia, Madongshan, and Naijiahe formations are correlative to the Jingchuan Formation, and the Sanqiao Formation to the Luohandong Formation. It thus indicates that the early Early Cretaceous sediments are unproductive at the marginal region of the basin.