Carbonate Bank and Wave-Built Platform Sedimentation, Edel Province, Shark Bay, Western Australia
Carbonate Bank and Wave-Built Platform Sedimentation, Edel Province, Shark Bay, Western Australia
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碳酸盐岩滩和波浪建造平台沉积，埃德尔省，鲨鱼湾，西澳大利亚
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发表时间：
1974
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通讯作者：
J. F. Read
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作者：
J. F. Read
银行和波浪建造的平台沉积，表面上相似，正在西澳大利亚鲨鱼湾埃德尔省的入海口形成。骨骼砾石的边缘和障碍体是由岸面上海草(独脚藻)群落的骨骼物质积累而成的。波浪建造的平台和岩床是由更新世沉积物侵蚀的颗粒堆积而成的。被侵蚀的颗粒主要是骨骼碎片，本文称之为“岩屑”。岩屑是从早期沉积旋回的松散或半固结的沉积物中重新加工而成的完整或碎裂的骨骼颗粒。河岸为生物基质楔形或透镜状，顶部平坦，边缘坡度大于或等于5°。楔形体形成为与入海口海岸接壤的边缘堤岸，而透镜状物体则是作为浅窗槛延伸穿过入海口的障碍体。银行沉积物厚达6米，在周围海底上方延伸2-5米。大片的岸面在2米或更深的水深中，稀疏地被海草(独脚藻)群落覆盖。典型的岸边沉积物是骨骼砂砾岩，其中含有丰富的软体动物、有孔虫和来自附生生物群和受保护底栖生物的珊瑚藻类颗粒。岸滩沉积物中的少量基质反映了稀疏的海草覆盖和叶挡板的有限作用。海草下的沉积物通过海草根茎和根网的结合作用而稳定。堤岸上的海草稳定了巨型海藻，有助于维持堤岸免受侵蚀潮流的影响。楔形或片状的岩屑砂砾石体形成为波浪建造的平台，与入海口海岸接壤；透镜状的岩屑砂砾石体作为浅的窗台延伸到入海口。砂砾岩体向海边缘厚度为0-3m。它们顶部平坦，保护区的边缘坡度从5°到30°以上。波浪建造的平台和基岩的表面居住着软体动物群落，这些群落只向沉积物提供少量的骨骼材料。海草一般不存在，沉积作用一直由物理过程主导。沉积物主要是由圆形砂级软体和珊瑚藻类碎屑、碎石、碎屑石英颗粒和少量居民区的骨料组成的岩屑颗粒。砂砾石体位于近海地区的骨架盖层和怪石之上(0-4.5m厚)，并在更新世石灰岩中切割出的潮汐阶地上向陆地尖灭。在局部地区，骨骼砂砾岩岸横向倾斜成波浪建造的岩屑砾岩台地，随着浅滩和海草的去除，它们也被岩屑砂砾石覆盖。在潮滩上，岩屑砾岩被一层薄薄(0.5米厚)的颗粒砾岩和内部角砾岩所覆盖。海浪和海流一直是形成堤岸和波浪建造平台的重要因素。从波浪建造的平台和堤岸表面输送的物质与就地的骨骼材料一起聚集在斜坡边缘，导致砂砾石体作为倾斜的前积层在骨架堆积岩和怪石上向前推进。堤坝是由对岸的边缘堤岸建造和合并而成的；其他的堤岸是由波浪建造的平台生长而成的，有些是由于更新世表面的“高点”被淹没而形成的。通过沉积在航道入海口的海底扇子上，基床迅速外延。进积砂砾石体与下伏沉积物的接触大多以挖洞有机体S形成的斑驳构造为标志，地质记录中的骨灰岩岩滩可能是在居民区产生大量骨性碳酸盐而不起到挡板作用的地方形成的。然而，在居民社区只提供少量骨骼材料的地区，从较老的地层中侵蚀出来的骨骼物质可以通过积累波浪建造的平台沉积形成。对岩性的识别有助于区分岸滩和波浪造地灰岩。结束_第1页
Bank and wave-built platform deposits, which are superficially similar, are forming in inlets in the Edel province, Shark Bay, Western Australia. Fringing and barrier banks of skeletal grainstone have formed by accumulation of skeletal material from a seagrass (Cymodocea) community on bank surfaces. Wave-built platforms and sills have formed by accumulation of grains eroded from Pleistocene sediments. The eroded grains are mainly skeletal fragments, termed "lithoskels" in this paper. Lithoskels are whole or fragmented skeletal grains reworked from unconsolidated or semiconsolidated sediments of an earlier sedimentary cycle. The banks are biostromal wedge-shaped or lenticular bodies which have flat tops and marginal slopes of 5° or more. Wedge-shaped bodies form as fringing banks bordering inlet shores, whereas lenticular bodies are barrier banks which extend across inlets as shallow sills. Bank deposits are as thick as 6 m and they extend 2-5 m above the surrounding seafloor. Large areas of bank surfaces are in water depths of 2 m or less and are sparsely covered by a seagrass (Cymodocea) community. Typical bank sediments are skeletal grainstones which contain abundant grains of mollusks, foraminifers, and coralline algae from the epibiota and sheltered benthos. Small percentages of matrix in bank sediments reflect the sparse seagrass cover and the limited effect of the leaf baffle. Sediments beneath seagrasses are stabilized by the binding action of the seagrass rhizomes and root mesh. Seagrasses on barrier banks stabilize megaripples and help to maintain the relief of the banks against erosive tidal currents. Wedge-shaped or sheetlike bodies of lithoskel grainstone form as wave-built platforms bordering inlet shores; lens-shaped bodies of lithoskel grainstone extend across inlets as shallow sills. The grainstone bodies are 0-3 m thick at seaward margins. They are flat topped and have marginal slopes ranging from 5° to more than 30° in protected areas. Surfaces of wave-built platforms and sills are inhabited by molluscan communities that supply only small amounts of skeletal material to sediments. Seagrasses are generally absent, and sedimentation has been dominated by physical processes. Sediments are mainly lithoskel grainstones composed of rounded sandsize lithoskels of molluscan and coralline algal fragments, lithoclasts, detrital quartz grains, and small amounts of skeletal mat rial from resident communities. The grainstone bodies overlie skeletal packstone and wackestone (0-4.5 m thick) in offshore areas and pinch out landward on tidal terraces cut into Pleistocene limestone. Locally, banks of skeletal grainstone grade laterally into wave-built platforms of lithoskel grainstone, and they also are overlain by lithoskel grainstones following shoaling and removal of seagrasses. In tidal flats, lithoskel grainstones are overlain conformably by a thin sheet (0.5 m thick) of pellet grainstone and intraclast breccia. Waves and currents have been important agents in the formation of banks and wave-built platforms. Material transported from surfaces of wave-built platforms and banks accumulates on sloping margins along with in situ skeletal material, causing the grainstone bodies to prograde as inclined foreset beds over skeletal packstones and wackestones. Barrier banks have formed by out-building and merging of fringing banks from opposing shores; other sills have formed by growth of wave-built platforms, and some are due to drowning of "highs" in the Pleistocene surface. Rapid outbuilding of sills takes place by deposition on submarine fans at mouths of channels. Most contacts between prograding grainstone bodies and underlying sediments are marked by mottled structure formed by burrowing organis s. Skeletal-grainstone banks in the geologic record have probably formed where resident communities produced large quantities of skeletal carbonate but did not act as baffles. However, wave-built platform deposits can form by accumulation of skeletal material eroded from older formations in areas where resident communities have supplied only small amounts of skeletal material. Recognition of lithoskels may help in distinguishing bank and wave-built platform limestones. End_Page 1--------------------------