摘要
贵州及邻区的中奥陶世地层主要为一套碳酸盐岩沉积,在这套沉积的上部,相当于Sin-oceras chinensis大量出现到三叶虫Nankinolithos开始出现一段时限内沉积的岩石(宝塔组)发育着一种特殊的构造类型,就其形状描述,不含成因意义的叫法,称为“马蹄纹”,而带有成因及沉积环境意义者称为“龟裂纹”。近年来,这种特殊的沉积构造已引起一些研究者的注意,开始探讨其成因和沉积环境意义,但至今有关宝塔灰岩中马蹄纹(或龟裂纹)的成因问题仍是众说纷云,归纳起来,有以下几种观点:
At present, there are different views on the structure and origin of the 'Polygonal viens' , also called 'horse-hoofed viens', of Baota limestone in Middle Ordovician in Guizhou and its adjacent regions. According to the author's field observations and laboratory studies of some thin sections, some facts worth paying attention to are as follows:
Ⅰ. A kind of iron-bearing non-skeletal oncolites has been found by the author in Baota limestone in Eastern Guizhou and Northeastern Guizhou,which was formed by bioclastics mainly in an iron material-attracting shallow water environment with blue-green algae, such as bivalve, brachiopod, trilobite, bryozoan and porifera.
Ⅱ. The stromatolite can be seen in Baota limestone, whose algae laminae have bird-eyes in their small holes, show geopetal structure features and catch elastics of bryozoan and echinoderm, etc.
Ⅲ. Bioclastics in Baota limestone sections are abundant, with oolites in them, and the bioclastics were mainly in a shallow water environment.
Ⅳ. From chemical analyses of the elements of Baota limestone, it can be easily seen that the content of Fe is higher than that of Mn and the contents of V and Cr are also higher; showing that the environment was a shallow water one.
V.In the bioclastic mud crystal limestone of the Baota Formation, a kind of pellets can be often seen under the microscope, which is in false platy and columnar crystals remaining after gypsum was replaced by calcite, showing that the water body of the sedimentary environment at the time was shallow, the climate was hot and the evaporation was great.
As stated above, the author considers that the horse-hoofed viens of Baota limestone can't have all been mud cracks under the exposure of supratital soil, nor the sediments under the environment where the water depth was beyond or clearly beyond the light-penetrating zone. So far as Baota limestone in Guizhou and its adjacent regions concerned, it was a sedimentation under a shallow or very shallow water environment, or sometimes under an exposure environment.
出处
《贵州地质》
1989年第1期35-38,22,共5页
Guizhou Geology
