扬子区后Llandovery世(志留纪)胞石的发现及其意义

DISCOVERY OF POST LLANDOVERY CHITINOZOANS IN THE YANGTZE REGION AND THEIR SIGNIFICANCE

坟头组时代争论长期不能统一的原因之一，在于各家对坟头组涵盖范围理解不一。坟头组有广义和狭义之分：以侯家塘组的顶为底、以秀山动物群的最高层位为顶的坟头组概念，为狭义坟头组。以侯家塘组、茅山组之间岩层的总和作为坟头组的概念，为广义坟头组。本文建议使用狭义坟头组概念，以求定义统一。坟头组之上、茅山组之下还存在寨山组和新河庄组（新组）。宁镇山脉志留系存在倒序、脱顶、丢层等问题，建议将坟头组、寨山组的标准剖面选在安徽铜陵寨山剖面；新河庄组的标准剖面选在安徽宣城新河庄剖面。根据胞石带的研究，坟头组顶界穿时，其时代为Ｔｅｌｙｃｈｉａｎ晚期至Ｗｅｎｌｏｃｋ世早期。寨山组底界穿时，顶界时代为Ｌｕｄｌｏｗ世末期；新河庄组的时代为Ｐｒｉｄｏｌｉ世早、中期；茅山组的时代推测仅限于Ｐｒｉｄｏｌｉ世晚期。本文还对“管状砂岩”的时代、秀山动物群回归的时代、红层与冰川的关系、海平面升降、分统建阶及古地理再造等问题作了讨论。

This paper traces the development of the definition of the Fentou Formation from Pan Jian's original propsal in 1962 to modern usage. The formation was indroduced by Pan Jian by separation of Grabau's (1924) Kaochiapien Formation into the lower Kaochiapien Bed and the upper Fentou Bed, which is now termed the Fentou Formation. The definition of the Fentou Formation, however, has been used ambiguously in the literature. Three different usages exist. Usage 1 took the last occurrence of the Xiushan Fauna as the top of the Fentou Formation s.s. Usage 2, a prevailing usage during the past 36 years, placed a 9-17m thick interval between the beds bearig the Xiushan Fauna and the basal red beds of the “overlying”(actually underlying) Maoshan Formation as its top. The supporters of usage 2 contended that the continuous infra formational contact and its stratigraphic position close to beds bearing the Xiushan Fauna make a Latest Llandovery age certain. Usage 3 extended the scope of the formation which crops out outside the Nanjing Hill into the whole bedrocks having a maximum thickness of about 400m, between the Fentou Formation s.s. and the Maoshan Formation and assigned the top of the Fentou Formation s.l. to the post Llandovery based on the presence of Geraspis rara Pan et Chen (fish).Misinterpretation of the type Fentou Formation in the Nanjing Hill as the lateral equivalent of the Late Telychian Xiushan Formation in SW China resulted in long standing confusion over the reginal correlation, palaeogeographic construction and geological evolution of the Yangtze Region and was corrected after the work of Geng and others, who during 1989-1998 established the existance of a big missing at the clastic section of that formation between its top 15m thick interval, previously known as the upper Fentou Formation and now called the top Zhaishan Formation, and the “underlying” (actually overlying Fentou Formation s.s. which was originally termed the Middle Fentou Formation. The Xiushan Fauna ( Salopinella Coronocephalus Sichuanoceras ), which has long been considered as Latest Llandovery, is presently shown to may pass up into Early Wenlock age on the basis of a chitinozoan biozonal control (the solida chitinozoan biozone). The top Zhaishan Formation in the Nanjing Hill yields Latest Ludlow sinica biozonal chitinozoans. Therefore, bedrocks of the Late Sheinwoodian-Middle Lufordian age were missed out by a layerslip thrust. It should be noted that the inverted Silurian strata were involved in a nappe as detached units in the Nanjing Hill, where an intra and inter formational missing is fairly common since a multi occurrence of layerslip thrusts drived by the Mesozoic Indosinian Orogeny. In this circumstance we suggest that the top of the Fentou Formation s.s. be intuitively delineated just above the last occurrence of the Xiushan Fauna and that two stratigraphic units be added on lithological gounds between the Fentou Formation s.s. and the Maoshan Formation, from the base upwards, viz., the Zhaishan Formation and the Xinhezhuan Formation. The former contains chitinozoans of the solida and lycoperdoides biozones of Wenlock age and of the vesicularis, rarispinosa, elongata, philipi , and sinica biozones of Ludlow age. The latter does chitinozoans of the kosovensis and thyrae biozones of Pridoli age (Geng et al., 1997; also see fig.2 in Chinese text). In this paper we present the view that marine Red bed 1, represented by the Rongxi Formation of turriculatus crispus biozone time (Geng, 1990), and marine Red bed 2, represented by the basal Xiaoxiyu Formation of Early spiralis grandis biozone time, are thought to have direct relation to withdrawal of epicontinental seas resulted from growth of the continental ice sheet in South America (Grahn & Caputo, 1992). Sea level dropped, by estimate, by 25-30m in the Yangtze Sea at that time. After th