青海南部奇异蚌动物群生态环境与时代的探讨

A PRELIMINARY INVESTIGATION ON PALEOECOLOGICAL AND CHRONOLOGICAL SIGNIFICANCE OF PEREGRINOCONCHA (BIVALVIA) FAUNA FROM SOUTHERN QINGHAI

一、前言奇异蚌(Peregrinoconcha)动物群最早发现于云南景星群。近来又相继在青海南部温泉地区(文世宣,1979),西藏东部八宿地区被发现(陈金华等,1983;文世宣,1985)。迄今仅见于东特提斯的云南、藏东和青南中生代地层产出,是一类地方性色彩很浓的区域性双壳类动物群。但对于这一动物群所代表的地层时代、沉积环境颇多争议。1984-1985年。

Over the past decade the endemic bivalve group——Peregrinoconcha fauna, initially found in western Yunnan, has been studied with regard to their stratigraphical range and paleoecology. Few people, if any, have concerned themselves with a comprehensive investigation. This paper presented here has made an endeavor to clarify the question of whether the fauna should be ranked in the marine, brackish-water or non-marine bivalve group, in terms of stable carbon and oxygen isotopic compositions of fossil shells and clastic-rock grain size analysis on the basis of the new collection from southern Qinghai. To distinguish the paleoecological environmen in which Peregrinoconcha lived, the 26 shell-sam-ples were applied to the analysis of the stable carbon and oxygen isotopic compositions by using the equation Z=a (δ^(13)C+50)+b(δ^(18)O+50), designed by Keith and Weber (1964). According to calculation Z values of all samples could be recognized as four groups that exhibit an adequacy with the principal ecological divisions of these fossil samples (see Text-table 1). Stenohaline brachiopods and epibyssate bivalves such as Camptonectes, Entolium, Radulopecten and 'Chlamys' sp. as well as Gryphaea have higher Z values ranging from 124 to 130. The values around 122 are shared by euryhaline bivalves including endobyssate Isognomon and Pteroperna, as well as Praeexogyra which have been proposed to have lived in a brackish water environment of this area. Furthermore, the mesohaline bivalves such as Neomiodon yanshipingensis (Wen), Proto-cardia hepingxiangensis J. Chen et Lin show the Z values between 117—120. On the other hand, with the lower Z values very close to those of Cuneopsis, a well-known fresh genus, Peregrinoconcha should be regarded as a non-marine bivalve g nus By utilizing Mook's method (Mook, 1970), the author has postulated that the two sets of δ^(13)C and δ^(18)O values of the present samples from the Yangshiping and Qoima Co sections should be considered as from different estuaries in the workingarea; therefore, if the two regressive lines obtained with the least square method converge upwards, the intersection point would mean theδ^(13)C and δ^(18)O values of salinity-35‰; on the other hand, if the two regressive lines converge downwards, the intersection point would represent the δ^(13)C and δ^(18)O values of salinity of fresh water. Generally, the latter mainly occurs in the case of more proportion of brackish water samples in total samples. Text-fig. 2 shows that the Peregrinoconcha is close to the intersection point representing the values of fresh water. In addition, both parameters and probability accumulative curves of the grain size analysis have proved the Kimmeridgian regression in south Qinghai that brought about the environmental change from a protected bay or brackish lagoon through the estuarine area to a fresh lake near the shoreline, and sedimentary data suggest that Peregrinoconcha should live in a non-marine sedimentary environment. The marine Jurassic strata have been known to be well-developed from Bajocian to Kimmeridgian stages in the region as the bivalve assemblages there can be well compared with those of West Europe. The Zhaworong Formation yielding the Peregrinoconcha fauna is in conformable contact with the Shuowa Formation containing marine Kimmeridgian elements such as Myopholas multicostata (Agassiz), Myopholas perticostata (Douville) and Entolium corneolum (Young et Bird). Below the Kimmeridgian Myopholas muhicostata-Entolium corneolium Assemblage there are number of Oxfordian elements such as Radulopecten fibrosus (Sowerby), Gervillella aviculoides (Sowerby), Modiolus bipartitus (Sowerby), Mytilus ungulatus (Young et Bird), Corbicellopsis laevis (Sowerby), Chlamys laevis Wen, as well as some Oxfordian brachiopods such as Septaliphoria aruennensis, Thurmanella acuticosta, Laconosalla trilobatiformis, Pentithyris pelagica and Rutorhynchia jieshanensis. Both bivalve and brachiopod assemblages support this point of view that there is no apparent stratigraphic gap between the horizons of the Middle Jurassic fossil assemblages and the Peregrinoconcha fauna. Consequently, it is unreasonable to say that there are no Upper Jurassic deposits in southern Qinghai. Based on the understanding of the significance of the fossil assemblages studied, the author suggests that the Peregrinoconcha fauna of this area appeared at least after Kimmeridgian, and persisted probably till Early Cretaceous. A description of 7 genera and 8 species is given, with attention paid to the taxonomic revision of certain species.