黔中三叠系台地边缘相碳酸盐岩中的玛瑙纹构造及其成因

 本文通过一条剖面的重点解剖,研究了广泛分布于上扬子台地边缘相碳酸盐岩中的玛瑙纹构造的成因。研究表明,这是一种早期的成岩组构,组成成分为纤状方解石和粒状方解石。沉淀介质为溶解了部分地表藻类的淡水同孔隙和裂隙海水发生不同程度混合的混合水溶液。被玛瑙纹充填的围岩裂隙主要由岩溶坍塌作用造成,但也有牵引拉伸裂隙和暴露收缩裂隙的叠加。由于介质溶液以地下潜水方式侧向运动,因而随潜水面波动形成了间隔的多层状含玛瑙纹岩层。

AGATE-LIKE STRUCTURE AND ITS GENESIS IN TRIASSIC PLATFORM.MARGINAL CARBONATE ROCKS IN THE MIDDLE PART OF GUIZHOU PROVINCE

This lagate,consists of fibrous calcite ande,agate-like structure has been studied in detail in this paper.The structure is widely dispersed in the carbonate rocks of Ladinian Stage in southern marginal belt of Upper Yangtze platform.The structure was formed in early diagenetic stage.It appears in various shapes,such as veined,petted,orbicular wrapping or pisolitic shapes.The rock formation containing the structure occurs in the forms of poly-layers. Thickness of a single layer is 1.6 to 9.2 meters. This lagate,consists of fibrous calcite and granular calcite.As compared with country rock and other related rocks,Ca and Sr content in the calcites is high and Mg and Mn content is low in chemical composition.δ¹⁸O and δ¹³C value in them is lower than that in country rocks,but is higher than that in modern fresh water.These cheracteristics indicate that the ‘agate’-forming solution is neither normal sea water nor fresher water.It might be a kind of mixed solution.Its sources might be atmospheric water which have dissoved some amount of algae on the surface of sediments and sea water which had filled in pores and cracks of sediments (or rocks). The cracks filled with‘agate’in country rocks were mainly formed as a result of lixiviation-collapse and replacement,but some of the cracks might be formed by traction-tension or exposure-shrinkage. The characteristics of occurrence and distribution of the structure indicate that vadose pattern of the mixed water solution took mainly the way of phreatic water.The place of the phreatic water level might be controlled by sea level in a period.