Venezuelan sandstone caves: a new view on their genesis, hydrogeology and speleothems

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

Venezuelan tepuis: their caves and biota

From the Preface: The world at the top of the tepuis of Venezuela is amazing. Ever since Sir Arthur Conan Doyle wrote The Lost World , the enigma of the South American steep and isolated table-mountains have attracted many people. Everyone in our research team dreamt about exploring these blank spaces on the map . However, despite this great attraction, scientific literature concerning tepuis has remained rather scarce. We are therefore proud to present this scientific monograph on tepuis, which is considered to be only the second issue of its kind, following the work of Huber (1992). This volume summarizes the main scientific results of expeditions to these tepuis between 2002 and 2011. The major research described in this monograph is dedicated to the great caves discovered in Roraima and Churí tepuis. The geological research was accompanied by biological research on cave and surface fauna, with a special focus on malacofauna, herpetofauna and insects. Although some of the data from this research has been previously published in scientific articles, it did not always obtain the justifiable space to present all gathered documentation and to elucidate all relevant scientific problems from a greater perspective. It is therefore our great pleasure to present this monograph containing detailed information on all research currently performed by our interdisciplinary research team. This is accompanied by a large number of fascinating photographs and several informative maps and diagrams. The greatest benefits to be gained from this combined monograph compared to short scientific publications is that more space is available to discuss currently unresolved problems, to ponder new intriguing questions and to envisage future necessary research. It is our sincere hope that all our readers will appreciate the unique information presented in this manner, and we hope you will really enjoy this monograph and find interesting topics for your research. Open Access See Extended description for more information

Morphological and mineralogical characterization of speleothems from the Chimalacatepec lava tube system, Central Mexico

Siliceous, opal-A speleothems of the Chimalacatepec lava tube system in central Mexico are characterized here for the first time. Morphologically, they can be classified into cylindrical and planar, and display a wide array of shapes, inner textures, and locations within the lava tube. All speleothems analyzed here are composed of opal-A, and their pores are filled with calcite and monohydrocalcite. Microscopic examination reveals a variety of microbial-looking, silicified filaments and cell casts embedded within the micro lamination of the structures. The abundance of biofilms in the Chimalacatepec lava tube may share similarities with other volcanic caves elsewhere. The direct presence of such bimorphs in the microstructure of the speleothems suggests the biological mediation of these structures. Potential mechanisms include nucleation and sorption of silica on extracellular polymeric substances in the biofilms that, along with the SiO2 saturation in the water film and evaporative effects, result in the formation of a speleothem. That said, the presence of microbes in these and other cave systems, or their inevitable interactions with the mineral phase of the speleothems, should not be surprising. In view of this, these structures can be most accurately described as biospeleothems. This study contributes to our understanding of the diversity of such structures in these types of cave systems and our ability to recognize the presence of microbes in these