【摘要】 <正> “鸟眼”(Birdseyes)是古代碳酸盐岩地层中一种常见的沉积构造现象。有些人把鸟眼构造作为潮上干燥环境的典型标志。对于“鸟眼”的定义及其成因,Deelman(1972)认为,鸟眼构造是沉积期间或刚刚沉积之后由物理的、化学的、生物的作用形成的一种(原生的和准同生的)沉积构造。只有确定了其成因方式之后才能进一步论述其形成环境。也就是说,不能把次生作用(如成岩后的溶解作用)形成的孔洞称作鸟眼,也不能一见鸟眼构造就认为是潮上环境。笔者赞同这种观点。本文仅对我国震旦系和白垩系-第三系碳酸盐岩中的鸟眼构造特征、成因及古环境意义作初步探讨。更多还原

【Abstract】 Birdseye structure is a sedimentary structure formed by physical, chemical and biological processes. Based on the studies of the carbonate rocks of the Upper Sinian in southern China and the Upper Cretaceous-Lower Tertiary in southern Xin-jing and northern Jiangsu, this Paper sums up six birdseye structural categories: isolate, isolate-like, irregular, tabular-and-striped, vermiform, and discontinuously laminated.The isolate-type birdseye structure in mudstone and wackestone was formed by the aggregation of gases produced by decomposition of organic matter in the sediments. The birdseye pores of this type are characteristic of isolated spots of mm level in distribution, filling sparry dolomite or calcite and normally lacking rim cement and geotropic sediments. In packstone and grainstone it might result from the aggregation of gases or and air bubbles sealed by capillarity when drying sediments were flooded. The former might occur in various environments from subtidal zone to intertidal zone and supratidal marsh, but the latter mostly developped in the inter-tidal zone and flooded supratidal marsh.The isolate-like-type birdseye pores, usually in wackestone, exhibit laterally elongate patches which are roughly parallel to stratification and better link up with each other, and in which there are commonly geotropic sediments of vadose silts.This type of birdseye pores might result from the air bubbles sealed by means of capillarity under the condition alternating drying with wetting. The surfaces of geotropic sediments in the birdseye pore spaces appear often to be at the same level which might roughly represent groundw-ater level at that time.Irregular type and tabular-and-striped type of birdseye structures, which developed commonly in various cryptalgal carbonate rocks, were formed by non-calcified growth of blue-green algae. The host rock with irregular birdseyes appear to be distinctly characteristic of abundant peloids and clots and of the irregular birdseye pores linking up each other very well which occupy relatively high percentage in volume so that the rocks exhibit poorly supporting state in thin section. They develop best in both of lower intertidal zone and seaward edge of supratidal marsh (Monty, 1976; Monty and Hardie, 1976), and sometimes they can occur in the upper subtidal zone ( Kinsman and Park, 1976 ) .Vermiform-type birdseye structure was caused clearly by drying and shrinking and usually developed in the carbonate muds and laminated cryptalgal carbonate sediments of intertidal or supratidal zone.Discontinuously laminated type of birdseye structure was mostly formed by evaporation, partly by penecontemporaneous leaching process. This type of birds-eye structure in laminated cryptalgal carbonate rocks could occur in the intertidal zone and supratidal marsh, but that in laminated non-cryptalgal carbonate rock ( mostly dolostone ) could be found only in the supratidal zone.It is considered that the aggregation of gases, noncalcified growth of blue-green algae, drying and shrinking, and sealed air bubbles are the four main origins forming birdseye structures. In the laboratory, two patterns of birdseye structures caused by the aggregation of gases and sealed air bubbles have been obtained.The sample of recent unconsolidated sediments collected from the intertidal zone of Shanya Bay, Hainan Island, resulted in two layers in colour, brownish yellow layer in upper 1/3 of the height of the sample in the bottle and brownish grey layer in lower 2/3, after two to three months. Isolate-type birdseye pores in millimetre size ( the largest up to over 1 cm ) took place in the lower because of the aggregation of gases produced by decomposition of organic matter in this layer (Plate 1-5 a and 5 b, Fig. 1 ). In addition, when the samples,skeletal fragments from the reef flats of Xisha Islands, were set into the bottles and then water was slowly poured into it till the samples were covered at all, numerous air bubbles occurred and were kept in intergrain pore spaces, aad might later aggregate to form更多还原