湖南张家界地区志留纪晚期地层新见兼论小溪组的时代

就华南板块而言,志留纪晚期(Ludlow世—Pridoli世)近岸浅海、非笔石相地层多分布在边缘海湾处,是否在扬子地台内部发育,以往尚无确凿记载。依据来自湘西北张家界—桑植地区新近发现的微体化石(主要指植物碎片),确认志留纪晚期地层在扬子地台内部确实存在。对原小溪组(又称小溪峪组)进行厘定,将它"一分为二":上部仍称小溪组,命名地在桑植瑞塔铺,发育最好的是张家界温塘剖面,岩性为黄绿色碎屑岩,富含大型虫管遗迹化石,时代为Ludlow晚期—Pridoli早期;下部称迴星哨组,归于Llandovery统Telychian阶上部。小溪组与迴星哨组之间为假整合接触,缺失Telychian末期、Wenlock世和Ludlow世早期地层。由此得出结论:在"扬子上升"后,扬子地台内部曾被志留纪晚期的海水侵漫;对其他地区迴星哨组与上覆地层的研究需要细化。

ON THE LATE SILURIAN STRATIGRAPHY OF THE ZHANGJIAJIE AREA,HUNAN PROVINCE,WITH A DISCUSSION ON AGE OF THE XIAOXI FORMATION

The Upper Silurian mainly occurs in marginal marine embayments of the South China paleoplate,and its records in the interior of the Yangtze platform are few.Based on newly discovered microfossils(including phytodebris microfossils)from the Xiaoxi Formation in the Zhangjiajie-Sangzhi area,northwestern Hunan Province,the occurrence of Ludlow-Pridoli strata in the interior of the Yangtze Platform is verified.The original Xiaoxi Formation(also known as the Xiaoxiyu Formation)is subdivided into two parts:the upper part being the Xiaoxi Formation(sensu stricto),and the lower part being the Huixingshao Formation.The redefined Xiaoxi Formation was originally named after strata in the Ruitabu area,Sangzhi,but is best developed in the Wentang area,Zhangjiajie.It consists of yellow-green clastic sediments with plenty of trace fossils.The Huixingshao Formation is commonly referred to as the upper red beds with an age of late Telychian,Llandovery.The latest Telychian,Wenlock and early Ludlow is represented by a parallel unconformity between the Xiaoxi and Huixingshao formations.It is suggested that,after the "Yangtze Uplift",the Yangtze Platform was flooded by shallow seawater in the Late Silurian.