Subtropical carbonates in a temperate realm; modern sediments on the Southwest Australian shelf
Subtropical carbonates in a temperate realm; modern sediments on the Southwest Australian shelf
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DOI：
10.2110/jsr.69.1297
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发表时间：
1999-11
影响因子：
2
通讯作者：
N. James;L. Collins;Y. Bone;P. Hallock
中科院分区：
文献类型：
--
作者：
N. James;L. Collins;Y. Bone;P. Hallock
位于西印度洋的澳大利亚西南大陆边缘（南纬19°-22°）是冷水和温水碳酸盐区的过渡地带。它包括南部刚形成边缘、平顶、陡峭前缘的罗特内斯特陆架、鲨鱼湾附近亚热带饥饿的卡那封坡道和北部的宁加洛岸礁。边缘受到向极地流动、温暖、营养贫乏的露纹洋流的强烈影响，该洋流促进整体下降流和强烈的夏季吹向赤道的风，从而产生局部季节性上升流。罗特内斯特大陆架北部结构静止，可容纳空间最少，其特点是繁茂的海草和大型植物生长在珊瑚覆盖的硬质地面上，并扎根于富含珊瑚藻和较大的共生有孔虫的沉积物中，以及丰富的冷水元素，如苔藓虫、软体动物和小型有孔虫。哈利梅达的钙化程度很差，对沉积物没有贡献。偶黄藻珊瑚虽然存在，但并不常见。这种沉积模式在地质记录中有许多类似物，尤其是新生代早期。早期边缘是形态复杂的线性山脊系统，被解释为埋藏的中更新世堡礁或海滩沙丘复合体。山脊复合体的北部被豪特曼·阿布洛霍斯礁台所覆盖。深部、外陆架和上坡的亚光沉积物，受季节性上升流影响，是典型的冷水、分选较差、以苔藓虫为主的沉积物，富含小有孔虫和海绵骨针。结构更加活跃的卡那封坡道的内部范围从陡峭的风成岩悬崖到鲨鱼湾的高盐环境再到宁加洛岸礁。中坡沉积物，尤其是鲨鱼湾附近的沉积物，大部分是残留或搁浅的、以有孔虫为主的沙子，并含有丰富的镁方解石胶结的内碎屑。这些沉积物堆积在相对贫瘠的海底，可能代表了由鲨鱼湾产生的盐水下降和间歇性侵入坡道而导致的碳酸盐产量减弱。外坡道要么是浮游有孔虫沙，由强大的底部水流分类，要么是针状泥。这种沉积模式应该是地质历史时期大洋盆地西部边缘的典型特征，当时环全球赤道环流活跃，赤道门户狭窄，而超大陆时代则在类似环境下盛行冷水碳酸盐沉积和上升流。此外，由于低纬度信风在冰川时期减弱，冷水上升流带向赤道移动到热带地区，这些地区的温带和亚热带沉积模式在地层记录中应该以 100 ky 尺度振荡。
The southwestern continental margin of Australia (19 degrees S-22 degrees S) in the western Indian Ocean is transitional between cool-water and warm-water carbonate realms. It comprises the incipiently rimmed, flat-topped, steep-fronted Rottnest Shelf in the south, the uniform subtropical starved Carnarvon Ramp off Shark Bay, and the Ningaloo fringing reef in the north. The margin is strongly influenced by the poleward-flowing, warm, nutrient-poor Leeuwin Current, which promotes overall downwelling and strong summer equatorward-blowing winds, which generate local seasonal upwelling. The structurally quiescent northern part of the Rottnest Shelf, with minimum accommodation, is characterized by luxuriant stands of sea-grasses and macrophytes growing on coralline-encrusted hardgrounds and rooted in sediments rich in coralline algae and larger, symbiont-bearing foraminifers together with abundant cool-water elements such as bryozoans, molluscs, and small foraminifers. Halimeda is poorly calcified and does not contribute to the sediment. Azooxanthellate corals, although present, are not common. Such a sediment pattern has many analogs in the geologic record, especially the early Cenozoic. The incipient rim is a morphologically complex linear ridge system interpreted to be a buried mid-Pleistocene barrier reef or beach-dune complex. The northern part of the ridge complex is capped by the Houtman Abrolhos reef platforms. Subphotic sediments on the deep, outer shelf and upper slope, affected by seasonal upwelling, are typical cool-water, poorly sorted, bryozoan-dominated deposits rich in small foraminifers and sponge spicules. The inner part of the more structurally active Carnarvon Ramp ranges from steep eolianite cliffs to hypersaline environments of Shark Bay to the Ningaloo fringing reef. Mid-ramp sediment, especially off Shark Bay, is mostly relict or stranded and foraminifer-dominated sand with abundant Mg-calcite-cemented intraclasts. These sediments, accumulating on a relatively barren seafloor, likely represent attenuated carbonate production brought about by downwelling and episodic incursions of saline, Shark Bay-derived waters onto the ramp. The outer ramp is either planktic foraminiferal sand, sorted by strong bottom currents, or spiculitic mud. Such sedimentation patterns should typify the western margins of large ocean basins during those periods in geologic history when circumglobal equatorial circulation was active, and equatorial gateways narrow, in contrast to eras of supercontinents, when cool-water carbonate sedimentation and upwelling prevailed in similar settings. Furthermore, because low-latitude trade winds relax during glacial epochs and the cool-water, upwelling belt moves equatorward into otherwise tropical realms, temperate and subtropical sedimentation patterns in these regions should oscillate at the 100 ky scale in the stratigraphic record.