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首页> 外文期刊>Earth Surface Processes and Landforms: The journal of the British Geomorphological Research Group >The nature of calcareous deposits along pan margins in eastern central Namibia
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The nature of calcareous deposits along pan margins in eastern central Namibia

机译：纳米比亚东部中部泛缘钙质矿床的性质

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In a region along the western margin of the Kalahari in eastern Namibia and western Botswana, many pan basins have mainly calcareous deposits along part of their margins. These are typically loned by low vertical scarps. In Namibia, these pans are mainly located in dry river beds. The petrographical study of these deposits demonstrates that they consist of lacustrine sediments that have to a varying extent been affected by early-diagenetic processes and by the formation of late-diagenetic features. The original composition of the deposits ranges highly calcareous sediments, typically with ostracod, diatom and charophyte remains, to entirely non-calcareous diatomites. The deposits generally show an upward increase in total carbonate content, which is mainly a synsedimentary feature. The early-diagenetic processes that affected the deposits include the formation of orthic siliceous nodules. At a later stage, secondary calcite enrichment occurred, leaving only the silica-impregnated sections unaffected. This enrichment partly accounts for the upward increase in total carbonate content in some profiles and often resulted in the development of a highly calcareous surface horizon. Sepiolite and amorphous silica that are part of the groundmass of the deposits may also partly have formed at this stage. Late-diagenetic features include various forms of secondary calcite and silica. This study of pan basins in eastern central Namibia indicates that a lacustrine rather than purely pedogenic origin should also be considered for calcareous deposits that commonly occur along pan margins in other parts of southern Africa.
机译：在纳米比亚东部和博茨瓦纳西部卡拉哈里西部边缘的一个地区，许多盆盆地的部分边缘主要是钙质矿床。这些通常被低垂直陡角所包围。在纳米比亚，这些平底锅主要位于干燥的河床中。这些沉积物的岩石学研究表明，它们由湖相沉积物组成，这些沉积物在不同程度上受到早期成岩作用和后期成岩特征的影响。沉积物的原始组成范围是高度钙质的沉积物，通常有原螯虾，硅藻和硅藻土残留物，到完全无钙质的硅藻土。沉积物通常显示总碳酸盐含量上升，这主要是沉积特征。影响沉积物的早期成岩作用包括正硅质结核的形成。在稍后的阶段，发生了二次方解石富集，只留下了二氧化硅浸渍的部分不受影响。这种富集部分地解释了某些剖面中总碳酸盐含量的增加，并且经常导致高度钙质地表的发育。在该阶段也可能部分形成了海泡石和无定形二氧化硅，它们是沉积物的基础。晚成岩特征包括次方解石和二氧化硅的各种形式。这项对纳米比亚中部东部盆地盆地的研究表明，对于南部非洲其他地区通常沿盆地边缘发生的钙质矿床，也应考虑使用湖相而不是单纯的成岩作用。

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