Sedimentary Belts in Lagoon of Kapingamarangi Atoll
Sedimentary Belts in Lagoon of Kapingamarangi Atoll
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卡平加马兰吉环礁泻湖沉积带
DOI：
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发表时间：
1959
期刊：
影响因子：
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通讯作者：
E. Leopold
中科院分区：
文献类型：
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作者：
E. D. Mckee;J. Chronic;E. Leopold
在Kapingamarangi环礁泻湖进行的系统性底部取样表明，那里积累的沉积物有六种主要类型，每种类型都有一定的深度限制，并在很大程度上限制在一定的深度范围内。这些类型的沉积物形成相带，环绕着大约6乘8英里的泻湖的最深部分（240英尺），并在较小的规模上，围绕着泻湖中的每个斑块礁延伸。相带从礁向内向更深处由以下物质组成：（1）主要由有孔虫形成的中等粒度碳酸钙砂（Amphistegina madagascariensis和Marginopora vertebralis），（2）主要由软体动物碎片形成的细粒碳酸钙砂，（3）生长在附近灌木丛中的分支珊瑚的碎片，（4）Halimeda属藻类节段的堆积，（5）碳酸钙砂，主要由不同于浅水的有孔虫物种（Amphistegina lessonii）组成，和（6）碳酸钙泥。力学分析表明，在浅水区，沉积物随着深度的增加而逐渐变细，在波浪和潮汐的作用下，从有孔虫砂到贝壳砂出现了一个分级泻湖。然而，在分支珊瑚带下方，深度大于约90英尺，三个内相带的沉积物几乎没有混合。在强大的洋流从南面进入泻湖的地方，贝壳和珊瑚碎片形成的淤泥形成了一个舌状物，向泻湖前方伸出约一英里，扰乱了通常平行于外围珊瑚礁的相带模式。在Kapingamarangi环礁的大部分沉积物中有孔虫的丰度允许定量研究它们与水深和相带的关系。五个大型种是沉积物中重要的有孔虫组分。两种大量出现在浅沃茨，直到外围礁的顶部，三种出现在100至200英尺深的沃茨。环礁上的其他有孔虫要小得多，分布更稀疏，作为岩石形成者的重要性要小得多;在大多数地方，它们的总体积不到样品体积的1%。六个不同相的样品有孔虫含量各不相同。有孔虫的存在，沉积物的类型，水深和波浪的数量之间的关系是明显的。对泻湖底部沉积物中微体化石的定量分析表明：（1）硅藻和海绵骨针在泻湖中心相对较深、平静的水域沉积物中聚集最多，而在邻近泻湖海滩的沉积物中则很少;（2）微有孔虫在泻湖的所有主要沉积相中均有发育，但在水很浅或很深的海底数量特别多;在深水区集中的部分原因可能是环境引起的其他较大形式的矮化;（3）沉积物中的花粉和孢子都是本地来源的，它们的极度稀少可能反映了本地来源植物的数量有限;（4）沉积物中甲藻孢囊的数量随深度增加而增加，但在150英尺以下的浅水沉积物中没有发现，这可能与成藻的水深分层有关。可能的因素控制生物在每个相带中所代表的是在控制动物群在不同深度的底部，湍流和温度的类型的变化，和光可以确定藻类Halimeda的下限。其他可能具有重要意义的因素，如化学成分、pH值、盐度和密度，在所代表的所有深度似乎都是相似的，可能不是重要的控制因素。在一些古代沉积矿床中，特别是在晚古生代的岩石中，可识别出与Kapingamarangi类似的深度控制相带或测深带。几个古生代系统的某些珊瑚礁被认为在许多方面与外围珊瑚礁相似，其他方面与该环礁的斑块珊瑚礁相似。
Systematic bottom sampling in the lagoon of Kapingamarangi Atoll indicates that sediments accumulating there are of six principal types, each characteristic of and largely restricted to definite depth limits. These types of sediment form facies belts that encircle the deepest parts (240 feet) of the approximately 6-by-8-mile lagoon and, on a smaller scale, extend around each of the patch reefs in the lagoon. The facies belts, from the reef inward toward greater depths, are composed of the following materials: (1) medium-grained calcium carbonate sand formed largely of Foraminifera (Amphistegina madagascariensis and Marginopora vertebralis), (2) fine-grained calcium carbonate sand formed largely of fragments of mollusks, (3) debris from branch corals growing in thickets ne rby, (4) accumulations of algal segments of the genus Halimeda, (5) calcium carbonate sand composed largely of a Foraminifera species (Amphistegina lessonii) different from those in shallow water, and (6) calcium carbonate mud. Mechanical analyses indicate that in areas of shallow water, sediments are progressively finer with depth and a gradation lagoonward occurs from foraminiferal sand into shell sand, the effect of waves and tides. Below the belt of branch corals, at depths greater than about 90 feet, however, sediments of the three inner facies belts show virtually no mixing. Where strong currents from the sea enter the lagoon on its southern side, a silt formed from shell and coral fragments has developed a tongue that projects lagoonward for about a mile and disturbs the pattern of facies belts that normally parallel the peripheral reef. The abundance of Foraminifera in most of the sediments at Kapingamarangi Atoll permits a quantitative study of their relation to depth of water and to facies belts. Five large species are important foraminiferal constituents of the sediments. Two occur abundantly in shallow waters up to the crest of the peripheral reef, and three occur in waters from 100 to 200 feet in depth. Other Foraminifera from the atoll are much smaller, occur more sparsely, and are much less important as rock formers; collectively they form in most places less than one per cent of the volume of a sample. Foraminiferal content of samples from each of the six different facies is distinctive. Relations are apparent between the Foraminifera present, the type of sediment, the depth of water, and the amount of wave a tion. Quantitative analysis of microfossils in bottom sediments of the lagoon shows that (1) diatoms and sponge spicules are accumulating in greatest numbers in sediments of the relatively deep, quiet water of the center, and are rare in sediments adjacent to the lagoon beaches; (2) microforaminifers are present in all the major sediment facies of the lagoon, but are particularly numerous on bottoms where water is either very shallow or very deep; concentration in deep areas probably is, in part, the result of environmentally induced dwarfing of otherwise larger forms; (3) pollen and spores in the sediments are locally derived, and their extreme scarcity probably reflects the limited number of End_Page 501------------------------------ local source plants; (4) dinoflagellate cysts in the sediments increase in number with depth, and were not found in sediments from water shallower than 150 feet; bathymetric stratification of the adult planktonic forms may be responsible. Possible factors controlling organisms represented in each facies belt are variations in type of bottom, turbulence, and temperature which control the faunas at various depths, and light may determine the lower limit of the algal Halimeda. Other factors of possible significance such as chemical composition, pH, salinity, and density seem to be similar at all depths represented and probably are not important controls. Depth-controlled facies belts, or bathymetric zones, comparable with those at Kapingamarangi, are recognized in some ancient sedimentary deposits, especially in rocks of late Paleozoic age. Certain reefs of several Paleozoic systems are believed to be similar in many respects to the peripheral reef and others to the patch reefs of this atoll.