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首页> 外文期刊>Geologia Croatica: a journal of the Institute of Geology Zagreb and Croatian Geological Society >Venezuelan sandstone caves: a new view on their genesis, hydrogeology and speleothems
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Venezuelan sandstone caves: a new view on their genesis, hydrogeology and speleothems

机译：委内瑞拉砂岩洞穴：它们的成因，水文地质学和洞穴发育的新观点

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Caves in arenites of the Roraima Group in Venezuela have been explored on the Chimantá and Roraima plateaus (tepuis). Geological and geomorphological research showed that the most feasible method of caves genesis was the winnowing and erosion of unlithified or poorly lithified arenites. The unlithified arenitic beds were isolated by wellcemented overlying and underlying rocks. There is a sharp contrast between these well-lithified rocks and the loose sands which form the poorly lithified to unlithified beds. They are only penetrated by well lithifi ed pillars originated by vertical finger flow of the diagenetic fluids from the overlying beds. Such finger flow is only typical for loose sands and soils where there is a sharp difference in hydraulic conductivity. The pillars exhibit no signs of further dissolution. The caves form when the flowing water accesses the poorly lithified beds through clefts. Collapse of several superimposed winnowed-horizons can create huge subterranean spaces. Futher upward propagation of the collapses can lead to large collapse zones which are commonly observed on the tepuis. Dissolution is also present but it probably plays neither a trigger role, nor a volumetrically important role in the cave-forming processes. The strongest dissolution/reprecipitation agent is condensed air moisture which is most likely the main agent contributing to growth of siliceous speleothems. As such, it can be active only after, but not before the cave is created. Siliceous speleothems are mostly microbialites except for some normal stalactites, cobweb stalactites and flowstones which are formed inorganically. They consist of two main types: 1. fine-laminated columnar stromatolite formed by silicified filamentous microbes (either heterotrophic filamentous bacteria or cyanobacteria) and 2. a porous peloidal stromatolite formed by Nostoc-type cyanobacteria. The initial stages of encrusted shrubs and mats of microbes were observed, too, but the surrounding arenitic substrate was intact. This is strong evidence for the microbial mediation of silica precipitation.
机译：在奇曼塔（Chimantá）和罗赖马高原（tepuis）上，委内瑞拉罗赖马集团（Roraima Group）的拱廊中的洞穴已经探明。地质和地貌研究表明，洞穴成因最可行的方法是对未固结或弱固结的孔隙进行风吹扫和侵蚀。未堆积的砂岩层被上覆和下伏的岩石隔离。这些岩化程度高的岩石与疏松的沙子形成了鲜明的对比，后者形成了岩性差的岩性到非岩性的岩床。它们仅被由上覆岩床中成岩流体的垂直指流引起的井状石柱穿透。这种指流仅适用于水力传导率存在明显差异的松散的沙子和土壤。柱子没有进一步溶解的迹象。当流动的水通过裂缝进入贫瘠的石质层时，形成洞穴。多个叠加的风化地平线的崩溃会产生巨大的地下空间。塌陷的进一步向上传播会导致大型塌陷区，这通常在对虾中观察到。也存在溶出作用，但它可能在溶洞形成过程中既不起到触发作用，也不在体积上起重要作用。最强的溶解/再沉淀剂是冷凝的空气湿气，这很可能是导致硅质脾生菌丝生长的主要因素。因此，它只能在创建洞穴之后但不能在洞穴创建之前激活。硅质蛇纹石主要是微辉石，除了一些正常的钟乳石，蜘蛛网钟乳石和以无机形成的流石。它们由两种主要类型组成：1.由硅化的丝状微生物（异养丝状细菌或蓝细菌）形成的细层柱状叠层石，以及2. Nostoc型蓝细菌形成的多孔倍体叠层石。也观察到了灌木丛和微生物垫的初始阶段，但周围的底物完好无损。这是微生物沉淀二氧化硅沉淀的有力证据。

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