湖北西部建始地区志留系秀山组顶部地层新观察

ON THE UPPERMOST XIUSHAN FORMATION(LLANDOVERY,SILURIAN)AT JIANSHI AREA,HUBEI PROVINCE

在湖北省建始县北部细沙坝剖面志留系最上部发现腕足动物及几丁类化石。腕足动物组合以华美瓦尔达贝(Valdaria lauta)为典型分子,属于秀山动物群第二组合带;同时根据几丁类的组合带延限,综合分析确定其时代为特列奇中期,相当于秀山组上段的中部。鉴于与典型纱帽组在岩性、时代的明显差别,鄂西地区的来凤、咸丰、鹤峰、宣恩、恩施、建始、巴东等地的这套以往称作纱帽组的地层改称为秀山组为宜。建始细沙坝与桐梓韩家店、石阡雷家屯、秀山溶溪、宣恩高罗、宜昌大中坝等五地志留系对比说明,上扬子区各地志留系兰多维列统的顶界是参差不齐的,其中秀山的相关地层顶界最高,桐梓和宜昌的最低。上述情况反映当时海域沉积受广西运动的直接影响,导致地壳上升,从而海水退尽,岩层差异剥蚀强烈,此系华夏古陆向西北方向显著扩张的结果。

Brachiopods and chitinozoans were discovered in the uppermost strata of Silurian at the Xishaba section in Jianshi area of Hubei Province.Rhe brachiopod assemblage is dominated by Valdaria lauta,which is a typical component of the second assemblage of Xiushan Fauna.Based on chitinozoans,the strata are interpreted to be middle Telychian in age,and they are considered to be the middle Xiushan Formation.Upper Llandovery strata in western Hubei Province are traditionally labeled as Shamao Formation.However,these strata are distinct in lithology and age from those at the type section of the Shamao Formation,thus the uppermost strata of the Llandovery Laifeng,Xianfeng,Hefeng,Xuanen,Enshi,Jianshi and Badong of western Hubei Province are here reassigned to Xiushan Formation.Our study of Silurian biostratigraphy in Xishaba of Jianshi,Hanjiadian of Tongzi,Leijiatun of Shiqian,Rongxi of Xiushan,Gaoluo of Xuanen and Dazhongba of Yichang,suggests that the top boundary of Llandovery strata in the upper Yangtze Region is diachronous,with the highest one occurring at Xiushan and the lowest at Tongzi and Yichang.This is probably related with a tectonic uplift driven by the Kwangsian movement that induced differential weathering among different areas,and also by the northwestern expansion of the Cathaysian Oldland.