纹层状细粒碳酸盐岩成因:以贵州习水吼滩剖面下奥陶统为例

纹层是组成层理的基本单位,既可以是低能深水沉积环境的产物,也可能形成于高能环境中。在受风暴影响的区域,微小的泥质颗粒能够在流动过程中发生絮凝形成絮粒并沉积下来形成纹层。贵州习水吼滩剖面下奥陶统桐梓组碳酸盐岩纹层发育,纹层中风暴沉积构造丰富,包括风暴侵蚀面和渠模等。基于细致的野外观察和室内沉积学研究,将在风暴作用影响下形成的纹层划分为5个类型:平行厚纹层、单向交错薄纹层、平行薄纹层、波状厚纹层和水平厚纹层。采用EDS元素面扫描方法,发现亮、暗纹层中都含有铝、硅和镁元素且分布都比较均匀,而暗纹层所含黏土矿物稍多,所以暗纹层中3种元素的含量比亮纹层的稍高。利用开尔文—赫姆霍兹不稳定性解释了亮、暗纹层的形成机制,即絮粒与未发生絮凝的微小颗粒在开尔文—赫姆霍兹不稳定性作用下交替沉积,其中以絮粒为主的纹层更容易发生重结晶形成亮纹层。根据各纹层不同的垂向结合序列划分出3种风暴序列,且从Ⅰ型到Ⅲ型风暴作用逐渐减弱。

Origin of fine-grained laminated carbonate rocks:A case study of the Lower Ordovician at Houtan section in Xishui County,Guizhou Province

Lamination has been viewed as indicative of a low-energy environment,and commonly,relatively deep water.This viewpoint has undergone radical change after recent studies showed that lamination can also be deposited in a high-energy environment. On storm-influenced areas,mud can be transported in bedload as floc aggregate grains,deposition from these turbulent flows produces laminated mud current ripples. Abundant and diverse storm sedimentary structures were found in laminations at Houtan section in Xishui County of Guizhou Province,included storm-erosional structures and gutter casts.Based on the fine sedimentary research both in field and in laboratory,5 laminated types were identified:Thick parallel lamination, unidirectional cross-lamination, thin parallel lamination, thick wavy lamination and thick horizontal lamination. By EDS mapping,three elements of Al, Si and Mg were analyzed in light and dark lamination. These elements are uniformly distributed in light and dark lamination, with a bit more content in dark lamination due to the more clay minerals in it. Kelvin-Helmholtz shear instabilities have been invoked to explain the alternating light-dark lamination.Three types of storm deposit have been recognized. From type Ⅰ to type Ⅲ,storm influence decreased gradually.