Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays
Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays
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展示的碳酸盐水泥织物：横穿埃克苏马群岛李斯托金岛附近的巴哈马平台边缘
DOI：
10.1016/0025-3227(93)90086-b
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发表时间：
1993
期刊：
影响因子：
2.9
通讯作者：
Linda S. Rouch
中科院分区：
文献类型：
--
作者：
G. Whittle;C. Kendall;R. Dill;Linda S. Rouch
在Exuma Cays Lee Stocking岛地区发现的固结到易碎的碳酸盐岩包括：(1)礁岩，(2)河道叠层石，(3)浅水硬岩，(4)岛屿边缘的滩岩和(5)更新世基岩。观察到的最常见的水泥织物有：文石纤维，包括针状扇形纤维和方尖粗纤维，粘接滩岩和叠层石；还有一个等径针状纤维边缘，将硬地和叠层石粘合在一起。不太常见的是高镁方解石叶片结构的礁岩和叠层石。存在两种类型的叶片：更常见的高mg或低mg方解石的短粗品种，以及仅在三个海滩岩石样本中发现的高mg方解石的细长品种。文石泥晶包裹层通常包裹着滩岩、硬地和叠层石中的颗粒，但只与纤维水泥相结合。文石外壳胶结在潮汐通道的石灰泥床表面，而高镁方解石隐晶胶结出现在所有岩石类型中。来自该地区丰富的绿藻和蓝绿藻的高镁方解石的钙化藻丝是叠层石中的初级胶结物，是硬地和滩岩中的次级胶结物。低镁方解石等晶石胶结更新世样品，并与更新世地表的大气成岩作用和胶结作用有关。水泥沉淀与来自埃克苏马湾的冷水的路径一致，因为它变暖并失去二氧化碳，并随着涨潮向上移动到Lee Stocking附近的河岸。隐晶水泥是向海方向形成的最常见的水泥，而向平台方向形成的主要是纤维状水泥。根据它们的晶体大小和在陆架边缘的位置，我们认为礁岩隐晶物质是胶结物形成最快的地方，那里的海水相对于碳酸钙更饱和，并且经历了最显著的变暖。升温和脱气过程的速率被认为在潮汐通道中增加，尽管胶结速率被认为由于碳酸钙可用性的减少而略有下降。在平台内部，进一步的升温和脱气被认为会导致水泥沉淀和硬地层的形成，但这些形成的速度可能比边缘的慢，尽管这个速度仍然很高。从潮汐通道到潮间带发生胶结梯度：(1)西诺曼池塘礁，水泥结构表明碳酸钙的可用性降低；(2)西李放养岛，矿物学的变化表明水化学的变化。因此，可以追踪水泥织物和矿物学的序列。泥晶织构出现在更靠海的位置；细的、纤维状的文石纤维更呈泻湖状和堤状；在潮间带和潮上位置有较粗的文石纤维和镁方解石胶结物。这个序列被认为可以追踪水团的演化。
Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock.The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites.Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples.Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface.Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO2and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Norman's Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry.Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.