宁强组时代的再认识

<正> 陕西宁强一带的志留系出露整齐,既有产笔石和其它浮游生物的页岩或泥岩层,又有富产珊瑚、层孔虫、海百合、腕足动物等底栖固着生物的碳酸盐岩。两种相型地层呈指状交叉或互成夹层,是扬子地台上富有沉积特色的志留系出露地区。1967年,陈旭、刘第墉,李蔚秾、王俊庚与俞昌民以及四川石油管理局南充地质调查处的同志,将本区的志留系划分为:下志留统龙马溪组、崔家沟组、王家湾组、中志留统杨坡湾组及宁强组。龙马溪组的下伏地层为晚奥陶世五峰页岩。宁强组与上覆地层早二叠世梁山组呈平行不整合接

Review of the age of the Ningqiang Formation

The Silurian strata best exposed in Ningqiang, Southern Shaanxi are characterized by the well developed shale and siltstone containing graptolites and other pelagic organisms and intercalated or interfingering with limestone yielding abundant and diverse benthon such as corals, brachiopods, bryozoans, crinoids, stromatoporids, etc. Recent finding of the index graptolites Monoclimacis griestoniensis, oktavites tullbergi, O. tullbergi spiraloides from the mudstone or shale and siltstone immediately above and below the limestone in the Ningqiang Formation formerly referred to the Middle Silurian suggests correlation with the Monoclimacis griestoniensis Zone to M. crenulata Zone referable to the Telychian Stage. It proves that the Silurian sequence in this region subdivided into the Lungmachi Fm. Cuijiakou Fm., Wangjiawan Fm., Yangpowan Fm. and Ningqiang Fm. in ascending order can only be referred to the Llandovery series completely correlating with the Rhuddanian, Aeronian and Telychian Stages. Comparing the trilobites, brachiopods, nautiloids and some graptolites from the Ningqiang Formation with those in the Xiushan Formation in S. E. Sichuan, it is believed that these formations can be correlated with each other approximately and there is no deposition after Llandovery time in most parts of the Yangtze platform, probably due to the Latest Llandoverian global eustatic fall of the sea level.