西北地区潮水盆地侏罗系青土井群轮藻化石

潮水盆地青土井群为一套含煤碎屑岩，在盆地东部潮参l井青土井群三段发育大量以Aclistochara占优势的轮藻化石群，其组合特征显示浓厚的晚侏罗世色彩。青土井群一、二段轮藻化石稀少，结合地层层序，推测其地质时代为早侏罗世。围岩沉积特征表明本井轮藻化石群为浅水生物群。

JURASSIC CHAROPHYTES FROM THE QINGTUJING GROUP IN THE CHAOSHUI BASIN, NW CHINA

The present paper deals with the charophytes including 3 genera 19 species found from the Jurassic Qingtujing Group (mainly from its third member)of borehole Chaocan-l in east Chaoshui Basin.The charophyte fossils from the third member of the Qingtujing Group can be called the Aclistochara platyglobata-A.subglobara Assemblage,which is dominated by A.platyglobata and A.subglobara sp.nov.in association with A.abshirica,A.brevis,A.jiangyouensis,A.nuguishanensis, A.obovata A.stellerides,Obtusochara maedleri, and Porochara tarimensis,etc.One of the dominating elements, A.platyglobata, was mainly found from the Late Jurassic before. It was first reported from the lower part of the Datonghe Formation in Minhe ,Qinghai (Hao et al.,1983).Afterwards,it was found from the Lower Cretaceous in Longxian,Shaanxi (Li Z.,1988) and Wuji, Hebei(Li H., 1989),and the Upper Jurassic Qigu Formation in the Turpan Basin. The new species Aclistochara subglobata is similar to A. platyglobata and A. bransoni, the latter has been found from the Upper Jurassic Morrison Formation in middle western America and the Middle Jurassic to Early Cretaceous in China. For the other elements of this assemblage,mostof them such as A.abshirica,A.brevis A.nuguishanensis,A.jiangyouensis,A.microturbinata,A.stellerides and A.yunnanensis are often seen from the Middle to Late Jurassic continental deposits in China. A few such as A.suwanensis and A.plana have been reported mainly from Middle Jurassic,and a few such as A.aff.mundula and Porochara tarimensis were yielded only from the Upper Jurassic.Therefore,the age of this fossil assemblage should be Middle-Late Jurassic,more probably the late Middle Jurassic or early Late Jurassic.Only a few individuals of Aclistochara suwanensis, A. major and A. brevis have been found from the cutting samples of the first member and the second member of Qingtujing Group.They may be the pioneers of the charophyte assemblage in the third member,with an Early to Middle Jurassic age. But, the possibility that some of these fossils may be caved from the overlying formation can not be ruled out,for all of them were found from cuttings.According to the sedimentary features of the rock samples yielding charophytes,the charophyte flora from the Qingtujing Group might have been living in a shallow water environment, and the shape of Charophyte fossils seems affected by their living condition.