广东省泥盆纪藻叠层石及其形成环境

Devonian Algal Stromatolites and Their Environments in Guangdong Province

摘要:

自从Ginsburg (1955)及Logan等(1961,1964)研究了巴哈马、佛罗里达和西澳大利亚的现代藻叠层石以来,已经建立了用藻叠层石的形态和构造来解释现代或古代海洋沉积环境的基础。在我国,报道前寒武纪叠层石的文章较多,上古生代则不多见。作者自1980年以来对广东泥盆纪碳酸盐沉积中的藻叠层石做了一些研究,现根据所得资料进行报道。

Abstract:

Devonian algal stromatolites occur in the areas of Lechang and Yangchun, Guangdong Province.The algal stromatolites may be elassified into four types: rippled, quaquaversal, cylindrical and spher ical(or oncolite).which may correspond to the LLH, SH-V and SS types, respectively, as proposed by Logan et al (1964). The rippled stromatolites may be subdivided into two types: mega-rippled and micro rippled. The former, which show a wide range of shapes, 10 to 40 cm in wave length and 5 to 20 cm in wave height, consist chiefly of micritic limestone, whereas the latter, about 1 cm both in wave length and in wave height, consist of lime dolomite or dolomitic limestone. The quaquaversal stromatolites, generally 10 to 60 cm in diameter and 10 to 40 cm in height, in elude flat- or sharp-crested and symmetric or asymmettric forms. In some cases, several small quaqua versal or cylindrical stromatolites may grow upon the larger ones (i. e. tubercular fork). The laminae are wavy or flat. Sometimes, the algal laminae are seen to intergrow with Stromalolora. There are two kinds of the occurrences for the quaquaversal stromatolites., some are associated with therippled stro matolites in the upper rhythmic layers of dolostone and stromatolitic limestone, others with cylindrical stromatolites in the rhythmic layers of micritic and stromatolitic limestones. The stromatolites are fre quently scoured and cut on their edges, filled with the stromatolitic fragments and bioclasts. It has also been found that the mud cracks within the quaquaversal stromatolites tend to be filled with sparry grainstone in the Fankou Pb-Zn deposit. The cylindrical stromatolites are generally 10 to 40 cm, occasionally up to 80 cm in diameter,and more than 70 cm in height. According to the forms, they may be subdivided into three types:cone, cylindrical and tower. In the Lechang area, the cylindrical stromatoilitic limestones constitute sedimentary rhythms regularly with bioclast containing micritic limestone, whereas near Chunwan in Yangchun, short-cylindrical stromatolites occur in the upper rhythmic layers composed of dolostone and limestone. The microstructure of the stromatolites exhibits the alternation of dark- and light coloured laminae, 0.5 to 2 mm in thickness. The poorly-defined laminae tend to bend concentrically in the forms of waves or microwaves. The light-coloured laminae consist of microspar or sparry calcite and contain more quartz silt, whereas the dark ones are made up of micritic calcite and contain more organic matter and less quartz silt. Sometimes the poorly-preserved algal filaments may be observed.The cylindrical stromatolites often intergrow with Stromatolora. The laminae of ,Stromatopora may make up one-fourth of the volume of the cylinders. The laminae of algae and Stromatlopra are often equally spaced. Morphologically, the spherical stromatolites (oncolite) may be subdivided into two types: regular and irregular. The irregular oncolites vary in size from several millimeters to several centimeters. Both the shapes and convolute laminae are irregular, and the cores are usually made up of the larger bio clasts. The individuals formed in different stages are often heaped together. The well-preserved fila ments of Girvanella may also be recognized in the dark-coloured laminae of oncolites. By and large, the irregular oncolites don't form a single bed by themselves but are commonly associated with rippled and quaquaversal stromatolites or all kinds of grainstones. The regular oncolites are commonly uniform in size, 2 to 4 cm in diameter, round or elliptic.The cores are relatively small, consisting of bioclasts of lime pellets. The concentric convolute laminae are distributed regularly and densely. The micritic limestones containing regular oncolites are frequent ly interbedded with styliolina and siliceous concretion-containing micritic limestones. Sedimentary facies indicators shown in Tables 1 and 2 are combined one another in three types of sedimentary rhythmites which represent different sedimentary environments, respectively: The dolostone-stromatolitic limestone rhythmite is a kind of incomplete rhythmite with scour sur face. Micro-rippled stromatolitic dolostones occupy the lower, part of the rhythmite, indicating the supratidal environment, whereas mega-rippled stromatolites, and part of quaquaversal stromatoiites comstitute the upper part of it, indicating the intertidal environment. The micrite-stromatolitic micrite rhythmite is a kind of incomplete rhythmite without scour sur face.The cylindrical stromatolites and part of quaquaversal stromatolites are developed in the upper part of thd rhythmites. The intergrowth of stromatolites and Stromatopora and the existence of coral fossils suggest the subtidal environment. Since modern cylindrical stromatolites occur principally in the intertidal zone, the alternation of Stromatopora and algal laminae may repesent the alternation of the subtidal and intertidal environments. The siliceous concretion-containing micrite and oncolitic micrite rhythmite is a kind of complete rhythmite in which regular spherical stromatolites occur, indicating the deeper-water environment, i.e.the shallow shelf environment. In general, the irregular spherical stromatolites do not form a single layer by themselves. The as sociated sedimentary facies indicators show a shallow-water origin, or even exposed to the surface. It is clear that the stromatolites were formed in the intertidal zone. Algal stromatolites were widely spread in the shore zone in Lechang and Yangchun areas during the early and middle Late Devonian times: At that time, .the Lechang area was then in a semi closed sea basin or a lagoon. The rippled an.d quaquaversal stromatolites occurred mainly in the northeastern part of it, suggesting the supratidal and intertidal environments. The cylindrical stromatolites occurred principally near Lechang and decreased gradually southwestwards and at last they were replaced by regular spherical stromatolites.It is suggested that the lower part of the intertidal zone near Lechang passed southwestwards into the centre of a deeper-water basin. On the other hand, the chunwan area in Yangchun was then in a semi-closed gulf which was near to the land in the northeast. The algal stromatolites were developed only on the carbonate platforms on the margins of the gulf basin and wedged out rapidly southeastwards into a deeper-water basin. During the Middle and Late Devomian times, the arid and semi-arid climates might intermittently prevail over the present-day Guandong Province. The salinity of sea water might become higher locally,or increased in the gradient coastwards, similar to that of the modern Shark Bay in West Australia where the cylindrical stromatolites are devloped.