华南泥盆纪埃姆斯期腕足类、双壳类群落、竹节石生物相及生态地层

EMSIAN(EARLY DEVONIAN)BRACHIOPOD AND BIVALVE COMMUNITIES,TENTACULITID BIOFACIES AND ECOSTRATIGRAPHY IN SOUTH CHINA

摘要:

近年来,地层学这一古老的地学领域,发生了很大的变化,出现了许多新的分支学科,如层序地层学、旋回地层学、生态地层学,以及综合地层学等等。这些地层学的分支学科,虽然都是研究沉积地层的,但它们的学科理论、研究途径、工作方法等都大相径庭。例如生态地层学,就是以古生物群落生态和生物相为基础,由古生态学和生物地层学相互渗透而发展起来的。由于生态地层学特别强调解释有机界和生存环境之间的关系,因而特别为岩相古地理学、沉积地质学的同行所注目。为了较为系统的、有具体研究实例的介绍这一新兴学科,是我们刊出“华南泥盆纪埃姆斯期腕足类、双壳类群落、竹节石生物相及生态地层学”专辑的原因之一。

Abstract:

Emsian(Early Devonian)palaeogeographic framework and sedimentary facies zonation have changed in response to the gradual expansion of tranagression in South China. From dehiscens zone through perbonus zone, inversrus zone and serotinus zone to patulus zone, the littoral clastic facies zone reduced its range rapidly, followed by the expansion of the platform facies region. The platformmargin facies zone or slope facies zone occurred during the late stage of the development of the dehiscens zone, displaying an elongated pattern intermediate between platform facies and basin facies. Till the late Emsian (from_serotinus^inversus zone to patulus zone), abundant patch reefs or reef knolls were scattered in the platform-margin facies or slope facies.
34 brachiopod-dominated communities occupy the distinctive benthic assemblage zones in different time intervals from dehiscens zone to patnlus zone. These benthic assemblage zones may correspond approximately to the certain sedimentary facies regions. There is a close relationship between the different communities in the same benthic assemblage zone in different time intervals, indicating that these communities may belong to the same community groups. About 50 percent of the abovementioned 34 communities may be assigned to BA1-BA3, suggesting that the inner shelf corresponding to the platform and littoral facies regions may be the most ideal habitat for the benthic brachiopod communities. In addition, 37. 7 percent of the communities with low diversity and high abundance, the products from the restricted environments, may be ascribed to BA1-BA5; the identification of their ecotopes should be made not only with the aid of the diversity and abundance but also in combination with other parameters. There is a notable similarity in age and environment between many Emsian brachiopod communities in South China and those from the other parts of the world. In particular, some deep-water communities may be interpreted as the counterparts from abroad. On the other hand, the brachiopod communities are. evidently local in South China. It is clear, from the analysis of the communities, that there once occurred in South China two large-scale transgressions(i.e. perbonus zone and serotinus zone—patulus zone)and a large-scale regression(i.e. inversus zone)resulting in the extinction of Rostrospirifer toskinensis fauna.
Six bivalve communities have revealed the variations in water-depth gradients from the littoral zone to offshore zone. These communities are mainly restrained by the nature of sea floor, salinity and oxygen content. The open shelf with silty substrate and normal salinity and oxygen content is believed to be the most ideal habitat for the bivalve communities. The research shows that the explanation that“the enrichment of brachiopods and depletion of nutriments and oxygen in waters result in the inexistence of bivalves”appear to be inplansible. In fact, the nature of the substrate and salinity are considered as the leading factors controlling the growth and decline framework showing the distribution of brachiopods and bivalves. There are great differences in diversity, abundance and morphological forms of genera and species between the bivalve communities from the Longmen Mountain area, Sichuan and those from Guangxi, suggestive of greater differences in the nature of sea water, rates of transgression or regression and climatic conditions in the above-mentioned regions which are ascribed to the distinct biogeographic provinces. In the Lower Devonian Yilan Formation in Nandan, Guangxi, there occur plentiful bivalve and brachiopod fossils which are assigned to the products from the oxygen-enriched environments rather than from the oxygen-depleted ones.
The Emsian tentaculitids in South China may be assigned to three biofacies(Facies T,D and H)and seven subfacies, all of which have specific environmental significance. Facies T correspond roughly to the positions of BA2—BA3, indicative of the oxygenated zone; Facies D to the positions of BA4—BA5, characteristic of oxygen-deficient zone, and Facies H is transitional. The planktonic Dacryocornarid does not belong to the pelagic organisms, therefore is inadvisable to be used as the indicator characteristic of the bathyal-abyssal facies. Homoctenida recognized mainly in the platformmargin slope or interplatformal trench facies zones with the same depths is a transitional type or a benthic swimming type whose living habits are more similar to those of Tentaculitida, and whose distribution ranges are much narrower than those of Dacryoconarida. Styliolina occur in substantial amounts in the siliceous rocks, suggesting a close relation to the restricted environments and the chemistry of sea water.