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首页> 外文期刊>Earth Surface Processes and Landforms: The journal of the British Geomorphological Research Group >Distribution, petrology and mode of development of silcretes (sarsens and puddingstones) on the eastern South Downs, UK
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Distribution, petrology and mode of development of silcretes (sarsens and puddingstones) on the eastern South Downs, UK

机译：英国南唐斯东部的硅藻土（sarsens和布丁石）的分布，岩石学和发育方式

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This paper presents the first intensive integrated field and laboratory study of an area of typical silcrete occurrence in the UK. and enables parallels to be drawn with in situ silcretes in the neighbouring Paris Basin and other parts of northern Europe. Silcrete distribution in the eastern South Downs is localized and discontinuous, with occurrence principally as dislocated boulders on the Chalk. These boulders mainly occur between the Cuckmere and Goldstone valleys, with greatest concentrations around Stanmer, Falmer, Rottingdean, and in the Goldstone valley. Many occur at higher levels in the landscape. on or near interfluves usually peripheral to outliers of Palaeocene sediments and in close association with Clay-with-Flints. At lower levels they occur on valley floors and on the coastal plain within Quaternary sediments. Boulders commonly have a-axis dimensions of 0.5-2 m and thicknesses of 0.3-0-6 m. Silcretes in higher positions typically exhibit angular tabular or prismatic shapes whilst those in derived settings are more rounded, suggesting weathering and erosion during transport from a localized high level silcrete lens (or lenses). Three varieties of silcrete have been identified. Pale grey 'saccharoid sarsens' are most widespread, and exhibit a simple grain-supported (GS-) fabric with predominantly sand-sized quartzose sediment cemented by optically continuous quartz overgrowths and minor micro- and cryptocrystalline silica. Brown 'hard sarsens' occur as smaller blocks, mainly in the Seaford Head area. These also exhibit a GS-fabric but with more fine sediment and a greater variety of cement types. Textural and geochemical evidence suggests there is gradation between the saccharoid and hard varieties of sarsen. Conglomeratic 'puddingstone' is common in the Goldstone valley but rare elsewhere, and shows closest similarity to saccharoid sarsen in texture and cementation. Silcrete micromorphology is universally simple and uniform, with fabrics suggestive of groundwater or drainage-line silicification. The uniformity of sediments within all three types of silcrete indicates a common host, suggested to be the Upnor Formation of the Lambeth Group (Palaeocene). Deposition of the host sediment appears to have been followed by illuviation of clay-rich material via the primary fabric and along cracks to form geopetal caps, drapes and vein-like structures. During silicification this finer material has been replaced by less well-ordered silica whilst optically continuous quartz overgrowths characterize 'purer' areas. The silcrete was also affected by late-stage influxes of iron oxides, clay minerals and occasionally calcite. On the basis of distributional and other evidence it is suggested that silicification occurred during the Neogene or Quaternary, in association with acid leaching of Lambeth Group sediments probably under temperate conditions. The timing of silcrete formation in the eastern South Downs is thus much later than proposed for other parts of the UK. Copyright (C) 2004 John Wiley Sons, Ltd.
机译：本文介绍了英国一个典型的silcrete发生区域的首次集约化现场研究和实验室研究。并能够与邻近的巴黎盆地和北欧其他地区的现浇混凝土进行平行绘制。西丘陵东部的Silcrete分布是局部且不连续的，主要发生在Chalk上的巨石错位。这些巨石主要分布在Cuckmere和Goldstone谷之间，在Stanmer，Falmer，Rottingdean和Goldstone谷附近最为集中。许多发生在景观的较高级别。通常在古新世沉积物离群值外围或与火石粘土紧密相关的边界上或附近。在较低的水平，它们发生在第四纪沉积物内的谷底和沿海平原上。巨石的a轴尺寸通常为0.5-2 m，厚度为0.3-0-6 m。处于较高位置的silcrete通常表现为角板状或棱柱形，而处于衍生位置的silcrete则更圆滑，这表明从局部高水平silcrete透镜运输时，风化和侵蚀。已经确定了三种silcrete。浅灰色的“糖状肌”（Saccharoid sarsens）分布最广，呈现出一种简单的颗粒支撑（GS-）织物，其中主要是沙子大小的石英糖沉积物，其由光学连续的石英过度生长和少量的微晶和隐晶硅石胶结而成。棕色“硬皮尖刺”以较小的块状出现，主要发生在西福德角地区。这些也表现出GS织物，但沉积物更细，水泥种类更多。结构和地球化学证据表明，糖浆和萨森硬变种之间存在一定的等级关系。砾岩化的“布丁石”在戈德斯通山谷中很常见，而在其他地方则很少见，并且在质地和胶结作用方面显示出与糖状沙森最接近的相似性。 Silcrete的微观形貌普遍简单且均匀，其织物暗示地下水或排水管线硅化。在所有三种类型的silcrete中的沉积物均匀性表明是一个共同的宿主，建议是Lambeth组（古新世）的Upnor组。沉积宿主沉积物之后似乎是通过初级织物和沿裂缝对富含粘土的材料进行了辐照，形成了地幔盖，悬垂物和脉状结构。在硅化过程中，这种较细的材料已被排列较差的二氧化硅所代替，而光学连续的石英过度生长则代表了“较纯净”的区域。晚期氧化铁，粘土矿物和偶尔的方解石的涌入也影响了混凝土。根据分布和其他证据，建议硅化发生在新近纪或第四纪，可能与温带条件下的兰贝斯群沉积物发生酸浸有关。因此，南唐斯东部的硅质岩形成时间要比英国其他地区提议的时间晚得多。版权所有（C）2004 John Wiley Sons，Ltd.

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