摘要
作名从生物学和岩石学角度研究巴哈马安德罗斯岛西北部潮上淡水沼泽区采集的现代叠层石,获以下几点认识:1.藻碳酸盐沉积物既与不同类型的藻类有关,又与不同环境有牵连;2.碳酸盐叠层石日生长层在成岩期间会被强烈改造;3.一些蓝绿藻能富集镁,形成高镁方解石,较易于白云石化;4.粘捕碳酸盐颗粒和原地沉淀碳酸盐晶体都是叠层石的重要成因;5.构成叠层石的藻类的降解作用在叠层石堆积过程中不间断地进行,所以藻碳酸盐岩中的这些生物难于保存为化石。
The modern stromatolitic material studied in this paper was collected from supratidal freshwater marshs in NW Andros Island,Bahamas by Cao R.in 1983.It was here that the phenomenon of ecological relationship between laminated stromatolites and soft algal heads is quite clear(Fig.2).Vertical sections in stromatolites show evidently three types of coloured laminae i.e.(1)dark green"living algae"laminae consisting of fresh filaments of Scytonema and a few carbonates;(2)brown-yellow calcified preliminarily"dead algae"laminae in which preserved only calcified tubular sheaths and lacked fresh filaments;(3)white peloidal laminae in between dead algae laminae(Fig.3).In the upper living algae laminae,the rhythms built by filaments of Scytonema range regularly,possibly representing daily growth layers.But in lower living algae laminae,the filaments range disorderly.In dead algae laminae,it is difficult to decipher daily growth layer owing to degration and calcification.On the inner wall of calcified tubular sheath in these laminae,cycloidal scaly carbonate crystal layers showing"growth line"structure are observed under SEM(Pl.Ⅰ-4a—b).This phenomenon is interpreted as that the carbonates precipitated or secreted around the filaments of Scytonema is bound to follow growth of filaments.Logan et al.(1964)claimed that algal stromatolites are laminated structures composed of particulate sand,silt,and clay-size sediment,which have been formed by the trapping and binding of detrital sediment particles by an algal film.The interpretation that"trap sediment"mechanism was the only mode of formation of stromatolites can not satisfactorily explain modern stromatolites in Bahamas.The present data suggest that the major mode of formation of stromatolites in Bahamas was in situ precipitation of carbonate in the algal mats,with only a few of aragonite needles traped in the laminae of stromatolites.In the Bahamian stromatolites there is obvious difference in mineralogy between the living algal laminae and the dead alga laminae.Aragonite is dominant in the living algae laminae,but the dead algae laminae are almost entirely composed of high-Mg calcite which is similar to the mineral component of the skeletal fragments of the coralline alga and echinoids from the tadal flat of Sanya Bay,Hainan Island S.China(Fig.4,5).The peloid laminae intercalated in the dead algae laminae,with fabric similar to the pellet sediment on the margin of the levee toward channel,are made up mostly of two kinds of peloids,one consisting of aragonite mud<1μm in size and another having abundant endolithic algae and aragonite crystals>1μm.These peloid laminae were deposited clearly by spring or storm tide.The algal mat exposed aerially on the margin of the supratidal freshwater marsh has the laminae composed of alternating aragonite and high-Mg calcite.The former deposited by algal trapping and binding,but the latter precipitated by algal biochemical processe or evaporation.In the X-ray diffraction trace of the algal mat there is a major peak(2θ=31.75°)near that of dolomite.The crystals of this mineral can be seen in the SEM microphotographs,and they can not be stained with alizarin red and relatively resist the erosion of dilute chloric acid on the polished slate.It is considered that they might be prodolomite.In the microphotograph it can be seen that there are a number of these mineral crystals each with a pore in the centre in the form consistent with the outline of the crystal.It is suggested that these pores resulted probably from dissolution of high-Mg calcite which originally was the crystal nuclei for prodolomite precipitation.From above,the writers have got some thoughts as follows:(1)various kinds of algae and environments are involved in algal carbonate sediments;(2)the daily growth layers of carbonate stromatolites would be strongly altered during diagenesis;(3)some of blue-green algae are able to concentrate Mg to form high-Mg calcite which then could be dolomitized more readily;(4)both trapping and binding carbonate particles and in situ precipitation of carbonate crystals are important in the formation of stromatolites;(5)the degradation of the algae building up the stromatolites can take place very early when they are just buried by new living algae laminae,so that these algae are difficult to be preserved as fossils in algal carbonate rocks.
作者
曹瑞骥
薛耀松
Cao Ruiji;Xue Yaosong(Nanjing Institute of Geology and Palaeontology,Academia Sinica)
出处
《地质学报》
1985年第3期203-212,274,共11页
Acta Geologica Sinica
