云南湖泊的碳酸盐沉积

CARBONATE SEDIMENTS IN LAKES OF YUNNAN, CHINA

云南湖泊多属中等矿化度的淡水湖泊,碳酸盐沉积物处于缓慢的积累过程。就其外形和成分可分为微粒碳酸盐、颗粒碳酸盐、生物贝壳碳酸盐和陆源碎屑碳酸盐。颗粒碳酸盐多分布在特定环境的湖区,集合粒成因与微生物的生物化学沉积作用有关;球状粒为微晶方解石组成,由藻群落及其代谢物形成的微环境对方解石缓慢沉淀有利;鲕状粒的形成可能与湖泊不定期干涸引起镁、钙比值变化及方解石、文石交替沉淀有关。微粒方解石是碳酸盐沉积的主要矿物形式,局部湖区形成的以颗粒碳酸盐为主的堆积体是云南湖泊早期碳酸盐沉积作用的主要特点。

The lakes in Yunnan have a complicated evolutionary history under the action of the intensive Neu-tectonism and regional physical conditions. As a result, the carbonate sediments are accumulated slowly in the lakes. According to their appearence and origin, three kinds of the carbonate sediments can be distinguished: 1. Organic shell carbonate which are mainly from gastropoda and Lamellibranchia; 2. Micro-grained carbonate, including aragonite needles or radial aragonite aggregates, crypto-crystalline calcite lumps or monocrystalline aggregates and dolomite; 3. Cleargrained carbonate of lumps, pellets and ooids distributed in definite environmental areas of the lakes. The lumps are composed of fine elastics cemented by crypto-crystalline carbonate and are considered being the result of the biochemical sedimentation of microorganism on the basis of total nitrogen analysis. The pellets are small ellipoids consisting of micro-crystal calcite formed in an area with abundant algae. Calcite was converted into pellets due to activity of denitrifying becteria in a micro-environment produced by algae metabolism. Ooids have some laminae which express the alternation of calcite growing radially with the aragonite growing in a tangental direction around its core. Its origin is likely related to temporary dryness of lakes and hence a change in Mg/Ca ratio, displaying alternated precipitation of calcite with aragonite.There are seldom large carbonate-forming organism like those in marine. Therefore, carbonate sedimentation in lakes is mainly subject to the micro-grained carbonate, but it is also possible for clear grained carbonate to occur in some areas of the lakes, causing the complication of the carbonate sedimentation in lakes. By comparison of characters of the carbonate sedimentation in Chenghai lake of recent 100 years, the general regularity about the lacustrine carbonate sedimentation is obtained.