THE ORIGIN AND ENVIRONMENTAL SIGNIFICANCE OF BIRDSEYE STRUCTURES

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 liangsu, this Paper sums up sit 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 anti grainstone it might result from the aggrega-tion of gases or and air bubbles sealed by capillarity when drying sediments were flooded .The former might occur in various environments f rom 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一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 groundwater 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一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 supratdal marsh(Monty, 1976,Monty and Hardie, 1976)，and sometimes they can occur in the upper subtidal zone (Kinsman and Park, 1976).Vermiform一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一green algae. drying and shrinking, and sealed air bubbles are the four main origins forming birdseye structures. In the laboratory. two patterns of birdseye struc-tures caused by the aggregation of gases and sealed air bubbles have been obtained.The sample of recent unconsolidated sediments collected from the incertidal 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一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 1一5 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, and might later aggregate to form isolate一like type of birdseye structure.From mentioned above, birdseye structures may be formed by various modes of origin and in various environments, i.e.from subtidal zone to supratidal marsh.Therefore, it is inexact to consider all birdseye structures as a symbol indicating supratidal or iniertidal environment.We can give the birdseye structure a more correct paleoenvironmental interpretation only after the mode of origin has been defined and petrological characteristics have been studied.