牛场组——上扬子区南部志留系兰多维列统的一个新地层单位

NIUCHANG FORMATION, A NEW LITHOSTRATIGRAPHIC UNIT OF LLANDOVERY (SILURIAN) FROM THE UPPER YANGTZE REGION

志留纪初的扬子海底域大都被缺氧水占据 ,黑色笔石页岩 (称龙马溪组 )广泛发育 ,底栖生物难以存活 ;但表层水域仍然充氧 ,是笔石等生物的生活场所。在上扬子海南缘的近岸、浅水区 (贵州北部 )繁衍了腕足动物及三叶虫等壳相生物 ,笔石稀少或缺失。相关地层长期被笼统地归于龙马溪组而没有其他名称。现以贵州湄潭牛场高滩剖面为依据 ,将该地原归“龙马溪组”的地层新建为牛场组 ,按全国地层委员会的最新要求 ,描述命名剖面 (地理位置、岩性特征、生物组合、顶、底界线、厚度、横向延展、与上覆、下伏地层的接触关系等 ) ,讨论它与相关地层的区别与对比等。牛场组底部因发现笔石而确定了地层位置 ;其顶界因尚未发现可资肯定时代的化石 ,只是根据区域对比推测其时代归属。研究结果表明 ,牛场组代表兰多维列 (L landovery)世早—中期、黔中古陆以北近岸、充氧、浅水的非笔石相地层 ,在黔北遵义、湄潭、思南一带发育 ,这一适宜于底栖生物生存的正常浅海环境为大灭绝后壳相生物的复苏与辐射提供了基本条件。与牛场组相当的地层包括标准龙马溪组下部的一部分和标准香树园组的下部 ,前者以黑色笔石页岩为特征 ,后者以瘤状泥灰岩、泥质灰岩为主。

The Niuchang Formation was introduced and briefly reported by Rong Jia-yu et al. (2003) based on the Rhuddanian—lower Aeronian section at Gaotan, 2km northwest of Niuchang, 6km south of Huangjiaba, west of Meitan County Town, northern Guizhou. A detailed report of this lithostratigraphic unit is given in this paper. It includes the location of the type section, etymology, lithology, thickness, biotic assemblage, stratigraphic relation to the overlying and underlying strata, its extention, and correlation. The Niuchang Formation is characterized by near-shore, shallow water, clastic sediments rich in shelly fossils (chiefly brachiopods and common trilobites). This formation has been usually called Lungmachi Formation. However, the latter is marked by black, siliceous shale with abundant graptolites, whereas the Niuchang Formation is lithologically different from the Lungmachi Formation and yields rare graptolites, although it can be correlated with parts of the latter (Rhuddanian—lower Aeronian). The Niuchang Formation is underlain by the Pagoda Formation (Caradoc, Upper Ordovician) with a parallel unconformity. According to Chen Xu, the base of the Niuchang Formation yields Monograptus sp., Lagarograptus cf. acinaces (Tornquist), Glyptograptus cf. srratus (Elles et Wood), Rivagraptus insectiformis (Nicholson), indicating a level slightly higher than Cystograptus vesiculosus Biozone, and probably belonging to Monograptus acinaces Biozone(later Rhuddanian). This new unit is overlain by the Shihniulan Formation which can be correlated with the mid-upper Aeronian rocks elsewhere in northern Guizhou in terms of regional stratigraphic correlation. The most abundant fossils in the Niuchang Formation at the type locality are brachiopods. There occur Athyrisinoides(=Kritorhynchia) fengkangensis (Wang),Levenea qianbeiensis (Rong et al.),Merciella striata (Rong et al.,), Strophomena modesta Rong et al., Eostropheodonta sp., Fardenia sp., Zygospiraella duboisi (Verneuil), Eospirifer sp., and a few others, associated with trilobites (Encrinuroides sp. and Dicranopeltis sp.). This shelly association may be assigned to the inner Benthic Assemblage 3 position. Wide distribution of the Lungmachi Formation with graptolite facies in the Upper Yangtze Region suggests that bottom of the basin was occupied by anoxia water in the earlier Silurian when global transgression took place. However, development of shelly fossils in Rhuddanian—early Aeronian in northern Guizhou indicates that the upper water column of the basin was oxygenated and abundant brachiopods and trilobites may have inhabited. Thus the Niuchang Formation represents a shelly facies suggesting near-shore, oxygen water environmental condition in Zunyi, Meitan and Si'nan, northern Guizhou.