中国南方的二叠-三叠系界线

海相三叠系广泛发育于中国南方,含有丰富的动物化石:瓣鳃类、菊石、腹足类及介形虫等。在此区内,海相二叠系也极为发育,有许多二叠-三叠系连续沉积剖面。因此,二叠-三叠系的界限剖面,可以在许多地点见到。 二叠-三叠系的界限日益受到国际地层学界的注意。

ON THE PERMIAN-TRIASSIC BOUNDARY IN SOUTHERN CHINA

The Permian-Triassic Boundary profiles described in this paper were studied by the authors from Zunyi district, northern Guizhou, and other profiles surveyed by other colleagues from Zhejiang and Jiangxi prov are also cited here for the purpose of fully understanding the general feature of the Permian-Triassic boundary in southern China.The marine Triassic formation of Zunyi district, northern Guizhou is one of the well-developed marine Triassic in southwestern China, it is composed of carbonates and elastics with total thickness up to serveral hundred meters. In this district, the Triassic formation is underlain by upper Permian Changxing formation composed of ’ dark grey thick-bedded or massive limestones with a lot of cherty nodules. The stra-tigraphical boundary between the two formations is very distinct with continuous succession of sedimentation, thus providing fortunate conditions for studying Permian-Triassic boundary.From the lowermost Triassic at Zunyi district, a rich collection of bivalves with predominately Claraia wangi, Cl. griesbachi, Oxytoma scythicum and Bakevellia costata has been found, but no ammonites occur; through percentage calculation of the number of the total individuals of each species, we have noted that the fauna is composed predominately of Claraia wangi, about 50%. Since Claraia griesbachi is usually associated with Otoceras, Ophiceras in Himalaya, the horizon with Claraia wangi at the basal part of Lower Triassic in South China may be equivalent to Lower Griesba-chian or Otoceras woodwardi zone.The occurrence and its world distribution of Claraia wangi, Cl. griesbachi assemblage have important significance in defining the Permian-Triassic boundary. According to present information available, the distribution of Claraia wangi is confined in southern China and the northern part of Vietnam, although this species has been reported in Transcaucas, the occurrence of this species is yet to be substantiated. The species of Claraia griesbachi is distributed in Himalaya, South China, North Vietnam and Malaya. Oxytoma scythicum occurs only in Southern China, no Oxytoma has been found from other regions of the world, thus it could be assumed that South China may be regarded as a special faunal realm during early Early Triassic, and that this assemblage may be regarded as the most primary fauna-assemblage, of which Claraia wangi was spread eastward and westward into Tethys and Arctic-Pacific region respectively, and eventually evolved into Claraia clarai, Cl. aurita, having a worldwide distribution during the later time.The Permian-Triassic mixed fauna have been found at many localities in South China, of which the typical Permian brachiopod Crurithyris usually associates with Early Triassic bivalves. It seems true that the typical Permian brachiopod might survive into Early Triassic, therefore we suggest that the first occurrence of Claraia wangi in basal part of Lower Triassic can be regarded as the beginning of the lowermost Triassic, even though it contains Permian BraChiopods.