广西隆林常么马平组(竹蜓)类化石带——兼论石炭-二叠系分界

FUSULINID ZONES FROM MAPING FORMATION OF CHANGMO, LONGLIN, GUANGXI ON CARBONIFEROUS-PERMIAN BOUNDARY

一、前言广西隆林晚石炭世-早二叠世地层发育,岩相稳定,岩层连续,(竹蜓)类化石极其丰富,是研究我国南方石炭-二叠系界线理想的地区之一。董文兰、龙家荣(1977),黄志勋(1979),夏国英等(1986)先后在这一地区做过工作。1982年底,中国科学院南京地质古生物研究所石炭-二叠系界线工作小组对该地区进行了重点研究,在隆林县卜糯乡常么村附近测制了1条晚石炭世-早二叠世的地层剖面。本文研究的(竹蜓)类化石就是这次采获许多门类化石中的一部分。野外工作中得到吴望始教授等的帮助,成文过程中又受到张遴信教授的指点,蒋汉培磨制薄片,陈毅勇摄影,在此一并致谢。

In the Longlin County, Guangxi, Late Carboniferous and Early Permian marine strata are fully developed, with the Changmo section well exposed. This section is composed of grey and light grey crystalline thick-bedded to massive limestones, extensively exposed in simple structure, with the fusulinid succession appearing quite clear. The Maping Formation of the Changmo section comprises four fusulinid zones in ascending order as follows: 1. Triticites montiparus Zone. With its lower limit marked by the first appearance of Triticites montiparus, this zone ranges from sample levels ABS61 to ABS63 (7.2m in thickness). Its fusulinids are dominated by individuals of Triticites characterized by robust chomata and simple folded septa. 2. Triticites simplex Zone. With the first appearance of Triticites simplex regarded as its lower limit, this zone ranges from sample levels ABS56 to ABS60 (5.6m in thickness). In this zone the genus Triticites is characterized by strong septal fluting and much smaller chomata, while Schwagerina, Rugosofusulina, Mccloudia, Eoparafusulina make their first appearances, but they are fewer in number and in species. 3. Pseudoschwagerina beedei Zone. This zone begins with the first appearance of Pseudoschwagerininae and ranges from sample levels ABS51 to ABS55 (8.1m in thickness), yielding a large and subspherical shell with whorls loosely coiled, together with Pseudoschwagerina, Zellia, Schwagerina, Rugosofusulina, Quasifusulina, Mccloudia and Triticites, among which Triticites is markedly reduced both in species diversity and in abundance. 4. Sphaeroschwagerina sphaerica Zone. With the first appearance of the genus Sphaeroschwagerina regarded as the lower limit, this zone ranges from sample levels ABS48 to ABS50 (4.9m in thickness). In this zone the Schwagerininae are of the more advanced type with larger shells and much stonger septal folds, while Sphaeroschwagerina has a larger and globose shell, with much thinner spirotheca. The Carboniferous-Permian boundary remains one of the boundary problems so far unsolved. For years, palaeontologists from different countries have proposed various schemes for the boundary division based on different fossil groups. This paper makes a few suggestions on the division of this boundary based on fusulinid data from the Changmo section of Longlin. From the Changmo section of Longlin, two distinctive evolutionary stages can be recognized in the fusulinid faunas of the Maping Formation. The first stage occurs between the Triticites simples Zone and the Pseudoschwagerina beedei Zone, corresponding to the lower part of Asselian in USSR, with the reduction of Triticites and the rise of Pseudoschwagerina. In this stage the fusulinids are characterized by a relatively small shell, well-developed chomata and strongly folded septa, such as Triticites montiparus and T. simplex; these species become extinct in the middle Maping Formation in which those forms bearing a large, spherical shell with tightly coiled inner whorls and loose outer whorls are predominant, such as Pseudoschwagerininae, while those forms with thick spirotheca and strongly folded septa are also abundant, including Schwagerina, Rugosofusulina,, Quasifusulina and Psedofusulina. In China, the limestones with Pseudoschwagerina and the underlying strata with abundant Triticites are continuously sequential sediments of the same lithofacies. Since Psedoschwagerina has a large, subspherical and loosely coiled test, it is easy to identify in the field; in addition, due to its rapid evolution, wide geographic distribution, and stratigraphical stability, it is available for global correlation. Therefore, it is here suggested that the first occurrence of Pseudoschwagerina be used to mark the boundary between Carboniferous and Permian. The second stage occurs between the Sphaeroschwagerina sphaerica Zone in the upper part of the Maping Formation and the Pamirina-fauna in the lower part of the Changmo Formation, corresponding to the upper part of Asselian in USSR. In this stage, the Pseudoschwagerinae begin to reduce, while Schwagerinae, Triticites, Rugosofusulina and Quasifusulina are absent; instead, the Pamirina fauna begins to dominate, including Pamirina, Staffella, Biwealla, Darvasites, etc. These fusulinids are characterized by having a small, delicate and tightly coiled test with well-developed chomata. If the Carboniferous-Permian boundary is placed at the beginning of this stage, it is equally appropriate from the view-point of fusulinid faunal change. However, the disadvantage lies in the limited geographical distribution of the Pamirina fauna, making difficulties about global correlation.