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Imawarì Yeuta: a new giant cave system in the quartz-sandstones of the Auyan Tepui, Bolivar State, Venezuela

  • Miles Beyond for ESA
Francesco Sauro
, Freddy Vergara
, Antonio De Vivo
, Jo De Waele
Associazione di Esplorazioni Geografiche la Venta, Via Priamo Tron 35/F, 31030, Treviso
Department of Biological, Geological and Environmental Sciences, Bologna University, Via Zamboni 67, 40126
Teraphosa Exploring Team, Puerto Ordaz, Venezuela
In February 2013 a new huge cave system in quartz-sandstones has been discovered in the eastern sector of the Auyan
Tepui, Venezuela by an Italian-Venezuelan team (La Venta Esplorazioni Geografiche and Teraphosa Exploring Team). In
twelve days of exploration over 15 km of passages were surveyed and other 4 km were explored. The system has nine
entrances and hosts three rivers giving rise to three resurgences. The cave passages belong to two levels, an active one in
which the rivers flow with a general and gradual WNW trend, and a inactive level located from 4 to 20 metres higher with
a N-S direction. The active levels are lithologically controlled whith typical pillars, potholes and wide and rounded bedrock
channels with very scarce sediment fill. The inactive level instead hosts large quantities of secondary minerals, mainly
sulphates-phosphates, and very nice and rare opal and silica speleothems. Flowing, drip and standing waters have been
analysed in situ for their Si content and many morphological observations have been carried out in order to try and
understand the speleogenesis of such large quartzite caves. Further geological, mineralogical, geomicrobiological and
biological studies are required in this exceptionally interesting quartzite cave.
1. Introduction
About twenty years ago cavers and karst scientists believed
that speleogenesis of caves in the quartz-sandstones of the
tepui mountains (Bolivar State, Venezuela) was related to
exceptional conditions and only of local importance. In the
80s and 90s the exploration of deep and wide shafts called
“simas” was explained as the result of arenisation processes
along open fractures close to the high cliffs surrounding the
plateaus. In these decades the researches didn’t investigate
the inner sectors of the plateus believed to be less promising
for the discovery of caves.
Since 2000, instead, several horizontal cave systems have
been explored and obviously this required to rediscuss the
speleogenesis of quartzite caves (Aubrecht et al. 2011;
Aubrecht et al. 2013; Sauro et al. 2013). After the
exploration of kilometre long caves in the Roraima and
Chimantha massifs (Galán et al. 2004; Sauro 2009;
Brewer Carìas and Audy 2011), in April 2013 a new giant
cave, named Imawarì Yeuta (the Cave where the Gods
live in Pemon Kamarakoto indian language) was
discovered by an Italian-Venezuelan expedition on the
Auyan Tepui in the Canaima National Park. This
exploration suggests that well developed and extensive
underground drainage systems probably occur below the
surface of most of the tepuis in the Gran Sabana area.
Speleological investigations in the quartz-sandstone
mountains of Venezuela and Brazil seems to be only at
the beginning.
This article shows the main results of the expedition
“Auyan Tepui 2013” and the scientific researches planned
for the future.
2. Geographical and geological settings
The Auyan Tepui is one of the largest table mountains of
the Gran Sabana area (700 km
, Fig. 1), well known for the
presence of the Angel Falls, considered the highest waterfall
in the world (975 m).
The Gran Sabana is a vaste geographical region located in
northern South America, between Venezuela and Brazil,
crossed by several tributaries of Rio Caroní, which in turn
flows into the Orinoco River. The Auyan Tepui has the
shape of a large table mountain delimited by vertical to
overhanging walls, often more than 1,000 m high. In plan
view it looks like a triangle pointing to the south. In the
inner part the Canyon del Diablo separates the north-
western sector from the north-eastern one, while the
southern part is a continuous plateau reaching its highest
elevation at 2,450 m asl.
Figure 1. The Auyan Tepuy from a Landsate image. The bounding
box represent the North-Eastern sector of the tepui.
Exploration and Cave Techniques – oral 2013 ICS Proceedings
From a geological point of view the Gran Sabana is part of
the Guyana Shield. The igneous and ultra-metamorphic
rocks in the northern portion of the shield (Imataca-Bolivar
Province, after González de Juana et al. 1980) have an age
of 3.5 Ga. The silico-clastic rocks (Roraima Group) belong
to the continental-to-pericontinental environment of the
Roraima-Canaima Province (Reid 1974). The age of this
arenaceous group can be inferred only on the basis of the
absolute dating of the granitic basement (2.3–1.8 Ga) and
of the basaltic dykes and sills that cross the upper formation
of the Roraima Group (1.4–1.8 Ga) (Briceño and Schubert
1990; Santos et al. 2003). The Roraima Group was also
intruded by Mesozoic diabases (Hawkes 1966; Teggin et al.
1985). These form thin NE-trending dykes with ages around
200 Ma.
A slight metamorphism, with quartz-pyrophyllite
paragenesis in the more pelitic beds, is the result of the
lithostatic load of almost 3-km-thick sediments now eroded
(Urbani et al. 1977).
3. Brief history of speleological explorations in
the Auyan Tepui
The presence of caves in the Auyan Tepui is reported
already during one of the first ascensions of the mountain,
the expedition of Felix Cardona Puig and the italian
geologist Alfonso Vinci in December of 1946 (Vinci, 1957).
The cave explored and described by Vinci was never found
again (Merlak, 2010).
The first speleological expedition, with helicopter support,
was realized in 1983 by the Sociedad Venezolana de
Espeleología, with the main aim to explore the Sima Aonda
(Fig. 1), previously known thanks to aerial surveys in the
NW sector of the massif. With its 362 m of depth this
collapse was considered the world deepest quartzite cave
until 1993, even if it consists of an elongated depression
and not a true shaft.
Most of the expeditions carried out in the years after were
basically concentrated in this area where the SVE explored
several other deep “simas”, such as the Sima Auyantepuy
Norte and the Sima Aonda Este 2.
In 1992 an Italian expedition organized by four caving clubs
(CAI SEM Milano, Castellanza, Laveno and Cividale del
Friuli) works on the upper Aonda platform and explores
several caves summing up to 1,700 m of development. The
same year La Venta carries out a first aerial recognition in
the Aonda area in order to organize a future expedition. The
year after a big expedition organized by La Venta works in
three different areas of the same western sector: in Camp
Aonda, the team descends Sima Aonda and explores the
active horizontal system at its bottom (Ali Primera Cave)
and other deep shafts of the platform; in Camp 1, slightly
NE, Sima Churun (Sima Auyantepuy Norte 2) is explored,
and in Camp 2, moved further W, Sima Auyantepuy
Noroeste is discovered, becoming the deepest and longest
quartzite cave in the world at that time (3 km, -370).
In 1996 La Venta organizes another expedition in the Aonda
area, exploring several other simas and connecting Sima del
Bloque to Ali Primera, realizing a cave system 352 m deep
and about 2 km long. In 2010 a short prospection to the
south-western plateau allowed to find and explore the
Cueva Guacamaya (1.1 km), the first horizontal cave found
in the Auyan Tepui, presenting peculiar morphologies
similar to those described in the cave systems of the
Chimantha massif.
Before the expedition of April 2013 a total of about 10 km
of cave passages were explored on Auyan Tepui, but
exclusively in the western part of the massif. The southern
and eastern sectors were completely unknown from a
speleological point of view.
4. The expedition Auyan Tepui 2013
The joint Italian-Venezuelan expedition on Auyan Tepui
took place in March 2013. The expedition was organized
by La Venta Esplorazioni Geografiche together with the
Teraphosa Exploring Team from Puerto Ordaz city. Ten
cavers (7 from Italy and 3 from Venezuela) and two park
rangers from InParques took part to the expedition. The
main aim was to explore the southern and eastern sector of
the mountain, with several unknown entrances located
during previous flights by the helicopter pilot Raul Arias.
The expedition was based in Kavak and a helicopter
supported the installation of the exploration camps on the
plateau. A total of 12 days of exploration activity was
carried out on the mountain. A first group reached an
entrance which opens on the eastern wall of the mountain,
suspended nearly a thousand metres above the surrounding
plain. Unfortunately the impressive opening revealed only
a short gallery about sixty meters length. The same day
another group descended a big collapse, named Sima del
Viento (Fig. 2), apparently close to the bottom. After several
hours of research, finally a narrow passage in between big
boulders led to an impressive active gallery. In the days
after a new camp was installed on the border of the collapse,
allowing a continuous work of exploration, survey,
documentation and scientific researches. After only four
days about 5 km were surveyed, following two main rivers
(Fig. 3). The exploration led quickly to two further
entrances, named Mundo Perdido and Grieta de Los
Guacharos. In the meantime the structure of the cave
suggested the presence of another river to the north-west,
in direction of a giant collapse doline. The supposed river
was finally reached from inside the system through a
labyrinthic network of fossil galleries. An underground
Figure 2. The large collapse of Sima del Viento represent the main
entrance of the Imawarì Yeuta Cave. Photo by F. Sauro.
Exploration and Cave Techniques – oral 2013 ICS Proceedings
camp of three days permitted to survey about 10.5 km of
new galleries, with huge rooms (the biggest 270 metres long
and 150 metres wide), complex labyrinths and other three
new entrances. Finally the total surveyed length reaches
15,450 metres, thanks to the use of the Polish CaveSniper
instrument ( About other 4 kilometres
were explored but not surveyed because of lack of time.
Imawarì Yeuta represents one of the world’s longest
quartzite caves explored until now, consisting in a unique
cave not divided by valleys, collapses or “grietas”. The
structure of the cave is quite complex but the main
hydrological routes draining the plateau are already well
clear (Fig. 4). Only the network of no more active branches
needs to be better explored and documented.
The expedition recovered all the wastes produced and also
all the faeces from the cave and the surface of the tepui to
avoid microbial contamination. The exploration was carried
out using always the same trail to limit our impact on the
vegetation going to the cave and on the fragile cave floor,
in particular in the fossil branches.
5. The cave Imawarì Yeuta
The new cave consists of three hydrologically independent
collectors (Fig. 5), two of which coming from the big
sinkhole of Sima del Viento, while the other one derives
from the catchment area of a large collapse doline to the
north, about five hundred metres wide, and of a nearby
smaller sinkhole. Here a stream falls into the cave with a
waterfall about 90 metres high. During our explorations,
carried out during a particularly dry period, the first two
streams had a minimum discharge of about 20 l/s, while the
main river reaches a minimum of 100 l/s. From the signs
left by water on the walls it is clear that this last river can
probably reach several cubic metres per second during
floods. The direction of drainage is in general from ESE to
WNW following the dip of the sandstone beds. A labyrinth
network of inactive galleries, developed along an evident
bed, interconnects the different rivers. This “open” bed can
reach impressive width (more than 300 metres in some
sectors) creating huge environments where the ceiling is
supported only by random pillars. This situation causes
large collapse zones with chaos of fallen boulders. In some
cases the fossil galleries show palaeo-phreatic rounded
morphologies and are in general almost perpendicular to the
actual vadose drainage.
One of the most impressive peculiarities of the cave is the
presence of widespread crystallizations of gypsum, opal,
and other secondary minerals (probably alunite, sanjuanite
and rossiantonite). The gypsum occurs in form of acicular
crystals, flower-like forms, crusts and desert roses covering
thousands of square metres of the cave floor. Also deposits
of iron hydroxides are present, in form of stalagmites up to
5 metres high, flowstones, rimstone dams, and coralloids.
Anemolites, helictites and stalactites and other speleothems
of opal and amorphous silica are present (Figs 6–7).
These formations are extremely fragile and the exploration-
documentation of the cave must be carried out with double
flagged trails in order to avoid unnecessary damages.
6. Researches and scientific interests
One of the main objectives of the 2013 expedition to Auyan
Tepui was scientific research, especially concerning
speleogenesis on quartzite caves, in the framework of a
PhD thesis by one of the authors (FS). InParques granted
us the autorisation to carry out chemical analyses on
waters, and to make geomorphological observations at the
surface and inside the discovered caves. Water samples
have been taken in underground streams, of drip waters, in
still-standing pools and lakes, and of surface waters. The
very low contents of Si and SiO
in the sampled waters
required quick in situ measurements (UNESCO-WHO
1978; Mecchia and Piccini, 1999; Piccini and Mecchia,
2009) with an Aquaquant © 114410 Silicon by Merck, able
to analyse in a concentration range
of 0.01–0.25 mg/l, with
an error lower than 20%. Analyses were carried out within
24 hours from sampling. Samples with a higher
concentration of Si were analysed by dilution with distilled
water. Temperature, pH, and electric conductivity (EC)
were measured by a HI 991300 Hannah Instruments ©
field portable instrument.
Figure 3. Gallery along the “Rio de los Venezuelanos”. Photo
V. Crobu.
Figure 4. Preliminary sketch of the cave plan obtained using the
Cave Sniper instrument.
Exploration and Cave Techniques – oral 2013 ICS Proceedings
Running waters have a pH ranging between 3.1 and 5.9.
Still standing waters in cave pools and lakes can reach
neutrality and high SiO
values 7–8 mg L
. The highest
content was documented for an about one hundred
cubic metres pool of standing water with a value of
8.6 mg/l. This confirms that running waters (streams) are
always undersaturated with respect to Si because of their
fast passage through the system while percolation waters
are generally much higher in content confirming that a
dissolution process in fractures and cave walls is effective.
These results support the arenisation model of
speleogenesis in quartz-sandstone (Martini 2000; Sauro et
al. 2013), even though also processes of hydrolysis and
laterisation are possible where sandstones also contain
silicates as suggested by Aubrecht et al. (2011). For
morphological and speleogenetic studies we measured size
and bearings of over 100 pillars in the cave, together with
fractures and bedding planes in order to check the theory of
“pillar flow” proposed by Aubrecht et al. (2011). The results
of these studies will be presented soon.
Further researches are planned for the near future also
regarding the secondary minerals deposits present in the cave.
7. Conclusions
Imawarì Yeuta represents one of the largest cave systems in
quartz sandstone in the world. The discovery of this cave
demonstrates that speleogenesis is widespread in the tepuis
of the Gran Sabana region suggesting that many other cave
systems are waiting to be discovered and documented.
The scientific interest of these caves is very high, ranging
from the processes of weathering that lead to the cave
formation, to the exceptional secondary minerals,
speleothems, cave fauna and geomicrobiological
interactions. In particular this last topic will certainly need
more attention in the future in order to better understand all
the processes interacting in the weathering and secondary
mineral deposition processes. For this reason La Venta is
going to organize a new expedition to this cave, hoping to
achieve all the permissions from the Venezuelan Ministery
of Environment for geological, biological and
geomicrobiological sampling, involving Venezuelan cavers
and scientists interested in this project.
The fragility of this cave will require a protocol of
protection similar to those applied in many other caves in
the world (for example Lechuguilla Cave), where the
visits must be carried out only for documentation and
scientific purposes, following restricted trails and
recovering all artificial and human waste in order to
minimize the impact.
The following persons took part in the expedition: Virgilio
Abreu, Raul Arias, Alfredo Brunetti, Carla Corongiu,
Vittorio Crobu, Antonio De Vivo, Jo De Waele, Fulvio
Iorio, David Izquierdo, Jesus Lira, Francesco Sauro, Freddy
Vergara, Jesus Vergara, and the helicopter pilot Julio
Figure 5. Sixty meter wide gallery along the “Collector de
Noroeste”. Photo V. Crobu.
Figure 6. Coralloids of amorphous silica growing on the cave
floor. Photo V. Crobu.
Figure 7. Rounded silica speleothems in the ceiling of the inactive
network. Photo V. Crobu.
Exploration and Cave Techniques – oral 2013 ICS Proceedings
The expedition was possible thanks to the permission for
speleological explorations granted by the Director General
Sectorial of InParques Ing. Carlos Cova and by the sponsors
Geotec SPA, Raul Helicopteros and by the following
technical partners, Dolomite, Intermatica, Ferrino,
Amphibious, De Walt, Allemano Metrology, Chelab,
Scurion, GTLine, New Foods, Bialetti, MountainHouse.
A big thanks to Ortensia Berti and to the community of
Kavak, to Felipe Campisi and his Robinson, to Karina
Ratzevicius of Raul Helicopteros, to the Hotel Gran Sabana
and to Elements Adventure for the logistic support.
The following institutions gave their patronage:
Ambassador of the Republic Bolivarian of Venezuela in
Italy Julian Isaias Rodriguez Diaz, Foundation Dolomiti
Unesco, Italian Speleological Society, Central Commission
for Speleology of CAI, CONI Veneto, Italian Institute of
Aubrecht R, Lánczos T, Gregor M, Schlögl J, Šmída B, Brewer-
Carías Ch, Vlcek L, 2011. Sandstone caves on Venezuelan
tepuis: Return to pseudokarst? Geomorphology, 132, 351–365.
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Charles Brewer-Carías (Ed.), Caracas, 290.
Briceño HO, Schubert C, 1990. Geomorphology of the Gran
Sabana, Guyana Shield, Southeastern Venezuela.
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Galán C, Herrera FF, Carreño R, 2004. Geomorfología e
hidrología del Sistema Roraima Sur, Venezuela, la mayor
cavidad del mundo en cuarcitas: 10,8 km. Boletin Sociedad
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Venezuela y de sus cuencas petrolífera. Edic. Foninvés,
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Exploration and Cave Techniques – oral 2013 ICS Proceedings
... About thirty years ago cavers and karst scientists believed that speleogenesis of caves in quartz-sandstones was related to exceptional conditions and only of local importance because of the extremely low solubility and dissolution rate of quartz (Tricart, 1972;Wray, 1993Wray, , 1997aWray, , 2013. Since 2000, several huge horizontal cave systems have been explored in the "tepui" table mountains of Venezuela (Fig. 1), showing a wide variety of morphologies and dimensions that compare well with the most developed cave systems in classic carbonate karst (Aubrecht et al., 2012;Sauro et al., 2013b). ...
... All these theories are still under discussion and there is not yet a clear understanding of the main speleogenetic and morphogenetic processes involved. Sauro et al. (2013bSauro et al. ( , 2013c suggested that quartz-sandstone could be weathered according to the "arenisation" model (Martini, 2000) and that the degree of fracturing, the mineralogical composition and the petrographical features (grain size, sorting, depositional structures, primary porosity), and not the diagenetic degree of the quartzsandstone as suggested by Aubrecht et al. (2011), probably control the intensity of the weathering processes along the strata. ...
... The main discussion has focused on the presence of some typical morphologies, like funnel-shaped pillars and pendants. These features are considered by Aubrecht et al. (2013) as channels of a descending silica-bearing hardening diagenetic fluid flow, while for Sauro et al. (2013b) they represent secondary forms due to arenisation and erosion processes along fracture networks. ...
A detailed petrographic, structural and morphometric investigation of different types of caves carved in the quartz-sandstones of the “tepui” table mountains in Venezuela has allowed identification of the main speleogenetic factors guiding cave pattern development and the formation of particular features commonly found in these caves, such as funnel-shaped pillars, pendants and floor bumps. Samples of fresh and weathered quartz-sandstone of the Mataui Formation (Roraima Supergroup) were characterised through WDS dispersive X-Ray chemical analyses, picnometer measurements, EDAX analyses, SEM and thin-section microscopy. In all the caves two compositionally different strata were identified: almost pure quartz-sandstones, with content of silica over 95 % and high primary porosity (around 4 %), and phyllosilicate-rich quartz-sandstone, with contents of aluminium over 10 % and low primary porosity (lower than 0.5 %). Phyllosilicates are mainly pyrophyllite and kaolinite. SEM images on weathered samples showed clear evidence of dissolution on quartz grains to different degrees of development, depending on the alteration state of the samples. Grain boundary dissolution increases the rock porosity and gradually releases the quartz grains, suggesting that arenisation is a widespread and effective weathering process in these caves. The primary porosity and the degree of fracturing of the quartz-sandstone beds are the main factors controlling the intensity and distribution of the arenisation process. Weathering along iron hydroxide or silt layers, which represent inception horizons, or a strata-bounded fracture network, predisposes the formation of horizontal caves in specific stratigraphic positions. The loose sands produced by arenisation are removed by piping processes, gradually creating anastomosing open-fracture systems and forming braided mazes, geometric networks or main conduit patterns, depending on the local lithological and structural guidance on the weathering process. This study demonstrates that all the typical morphologies documented in these quartz-sandstone caves can be explained as a result of arenisation, which is guided by layers with particular petrographic characteristics (primary porosity, content of phyllosilicates and iron hydroxides), and different degrees of fracturing (strata-bounded fractures or continuous dilational joints).
... To date, more than four hundred quartzite and quartz sandstone caves have been explored and documented, mostly in South America, Africa, Australia and India. Forty of them exceed 1 km of passage length (Table 5; Appendix A), while the two longest caves have horizontal lengths of 18.7 km (Imawarì Yeuta cave) (Sauro et al., 2013a;Sauro et al., 2013d) and 10.8 km (Roraima Sur cave system) (Galán et al., 2004a). Recently (2017) a 12.3 km long cave (Krem Puri) in quartzsandstones has been explored also in the Megalaya region of India, but limited information are available so far (pers. ...
... (Aubrecht et al., 2012) and in impressive amounts in the Imawarì Cave System(Sauro et al., 2013a). Here gypsum occurs as acicular crystals, Different kinds of biologically mediated stromatolite-like silica speleothems. ...
Quartz is considered one of the less soluble minerals of the Earth’s crust, and thus hardly affected by chemical weathering. Despite this, since more than forty years, it is clear that the formation of caves and peculiar solutional weathering dominated landforms in quartz-rich lithologies is common and shares several similarities to the well-known karstic ones in carbonate rocks. In the last thirty years great strides have been made in furthering our knowledge of the distribution of these forms around the world, and the geochemical processes involved. These studies have clearly shown that solutional weathering is a fundamental process, acting through interganular dissolution of quartz increasing the rock porosity and decreasing the rock strength to erosion. This process has been described in the concepts of ‘arenization’ and ‘phantomization’ and the widespread evidences of the fundamental role of quartz solution in landform genesis has even developed to the extent of several geomorphologists reassessing the definition of the term ‘karst’, and its application to these peculiar lithologies. Nonetheless the process is complicated by several factors, related both to environmental conditions (water chemistry and availability) as well as to the compositional and textural characters of the lithology (presence of clays, iron hydroxides, carbonate cement, etc.). All these aspects have to be taken carefully in consideration in order to understand if solution is a dominant or accessory process in the landscape evolution. In this review the state of knowledge on the relevant chemical processes, weathering mechanisms, and speleogenesis involved in the surface and underground karstification, and clear examples of quartz solution and solutional landforms from different world locations, are outlined and discussed.
... Unfortunately, the original site of discovery in the San Juan province can no longer be found (Colombo et al., 2011) and therefore a detailed comparison is impossible. However, in the last two years, sanjuanite was discovered not only in the Akopan-Dal Cin System (this study) but also in two other caves of the tepui mountains: the Muchimuk cave system (Aubrecht et al., 2012) and, in large amounts, in the recently discovered Imawar ı Yeuta Cave in the Auyan Tepui (Sauro et al., 2013c). This widespread presence in the quartz-sandstone caves from different tepuis suggests that the formation of gypsum is driven by the same process at all of these sites. ...
... Surface irregularities, and in particular prominent features, will result in air turbulence, causing increasing particle collisions with the walls and therefore greater deposition at specific sites. This process can account for the accumulation of gypsum on sharp ledges and protuberances of the cave walls, and also for the formation of air-moulded biospeleothems (Sauro et al., 2013c;Fig. 7B). ...
Gypsum (CaSO4 center dot 2H(2)O), alunite (KAl3(SO4)(2)(OH)(6)), and rare phosphate-sulphate sanjuanite Al-2(PO4)(SO4)(OH)(9)(H2O) and rossiantonite (Al-3(PO4)(SO4) 2(OH)(2)(H2O)(14)) have recently been identified as secondary mineral deposits in different quartz-sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz-sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho-sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz-sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non-sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H2S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co-occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz-sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.
... Another set of analyses was carried out on the waters of Imawarì Yeuta cave, located in the NE sector of Auyan Tepui (Fig. 6). This cave probably represents the longest quartz-sandstone cave known in the world, with about 20 km of explored passages (15.5 km already surveyed) (Sauro et al., 2013b). With the exception of three underground collectors draining two sinkholes, most of these galleries are no longer hydrologically active, but host wide standing pools fed by drip and seepage waters. ...
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Chemical mobility of crystalline and amorphous SiO2 plays a fundamental role in several geochemical and biological processes, with silicate minerals being the most abundant components of the Earth’s crust. Although the oldest evidences of life on Earth are fossilized in microcrystalline silica deposits, little is known about the functional role that bacteria can exert on silica mobility at non-thermal and neutral pH conditions. Here, a microbial influence on silica mobilization event occurring in the Earth’s largest orthoquartzite cave is described. Transition from the pristine orthoquartzite to amorphous silica opaline precipitates in the form of stromatolite-like structures is documented through mineralogical, microscopic and geochemical analyses showing an increase of metals and other bioessential elements accompanied by permineralized bacterial cells and ultrastructures. Illumina sequencing of the 16S rRNA gene describes the bacterial diversity characterizing the consecutive amorphization steps to provide clues on the biogeochemical factors playing a role in the silica solubilization and precipitation processes. These results show that both quartz weathering and silica mobility are affected by chemotrophic bacterial communities, providing insights for the understanding of the silica cycle in the subsurface.
Conference Paper
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Nel febbraio del 2013 un gruppo di speleologi italiani e venezuelani (La Venta Esplorazioni Geografiche e Teraphosa Exploring Team) ha scoperto un nuovo grande sistema carsico nelle quarzo-areniti del settore nordorientale dell’Auyan Tepui, In 12 giorni di esplorazione oltre 15 km di gallerie sono state topografate, mentre almeno altri 4 km restano da rilevare. Il sistema ha nove ingressi ed ospita tre fiumi sotterranei. Le gallerie si sviluppano su due livelli ben distinti, uno attivo in cui scorrono i fiumi con una direzione generale ONO, ed un livello fossile situato da 4 a 20 metri più in alto con una generale direzione N-S. I livelli attivi sono controllati litologicamente e contengono dei pilastri tipici, marmitte e canali in roccia larghi e sinuosi con scarsi depositi sedimentari. I livelli fossili invece ospitano una grande quantità di minerali secondari, tra cui soprattutto solfatifosfati, e rarissime concrezioni di opale e silice. Acque correnti, gocciolanti e stagnanti sono state analizzate in situ per il loro contenuto in silice ed alcune osservazioni morfologiche hanno consentito di trarre delle conclusioni riguardo alla speleogenesi di questi grandi sistemi carsici in quarzite. Ulteriori studi geologici, mineralogici, geomicrobiologici e biologici saranno necessari in questa grotta eccezionale.
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In February 2009 La Venta exploring team organized a quick expedition to the Chimanta massif, one of the quarzitic rock Tepuis situated in the Bolivar region, in the south-east of Venezuela. The activity has been focused on the exploration of some important springs located on the Akopan Tepui’s walls. The obtained results show the speleological interests of this area and generally of all the quarzitic mountains belonging to the Guyana Shield.
Venezuelan table mountains (tepuis) host the largest arenite caves in the world. The most frequently used explanation of their origin so far was the “arenization” theory, involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water. New research in the two largest arenite cave systems – Churi-Tepui System in Chimanta Massif and Ojos de Cristal System in Roraima Tepui showed that quartz dissolution plays only a minor role in their speleogenesis. Arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown, including also loose sands and sandstones. Speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites. Only the so-called “finger-flow” pillars testify to confined diagenetic fluids which flowed in narrow channels, leaving the surrounding arenite uncemented. Another factor which influenced the cave-forming processes by about 30% was lateritization. It affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas, feldspars and clay minerals, turning then to laterite (“Barro Rojo”). The main prerequisite to rank caves among karst phenomena is dissolution. As the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role, these arenitic caves may be ranked as karst.
The paper reports the results of SiO2 analyses in the Aonda Cave system, located on the Auyán-tepui, one of the widest table-mountains of the Gran Sabana (South Venezuela), characterised by karst landforms developed in siliceous rock. Chemical analyses underline the very low concentration of SiO2 of the surface water. Percolation and cave drip waters have a SiO2 concentration of about 1 mg/l. The mean silica load of the cave stream is 184 mg/s, mainly derived from surface solution removal in the allogenic recharge area. In the Aonda Cave system, the mean SiO2 dissolved load is 40 mg/s, in part from surface solution (15%) and mainly from underground processes (85%). The low solubility of SiO2 in slightly acidic water implies the importance of the time factor in the formation of cave systems. With the present dissolution rate, about 10 Ma would be necessary to form the known karst system. This estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years. Furthermore, this age can be taken as a minimum estimate, while, according to the geomorphic evolution of the area, the origin of the Aonda Cave system could be reasonably dated back to at least 20–30 Ma, that is, to the Oligocene.
In the recent work of Aubrecht et al. (2011) the presence of "unlithified or poorly-lithified beds" of sands in the quartz-sandstone stratigraphic succession is proposed as a key factor for speleogenesis in the Venezuelan tepuis. In this comment we observe that in the cited work the geologic history of the region, in terms of sedimentation environment, diagenesis and low grade burial metamorphism, has not been considered. Furthermore, the peculiar "pillar flow" columns that Aubrecht et al. describe as a proof of the unlithification are lacking in many other different cave systems in the same area. Four critical points are discussed: the burial metamorphism of the Mataui Formation, the significance of the Schmidt Hammer measurements, the cave morphologies and the role of SiO 2 dissolution. Finally we suggest that weathering, in its wider significance, is probably the triggering process in speleogenesis, and there is no need to invoke a differential diagenesis of the sandstone beds.
Venezuelan table mountains (tepuis) host the largest arenite caves in the world. The most frequently used explanation of their origin so far was the “arenization” theory, involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water. New research in the two largest arenite cave systems – Churi-Tepui System in Chimanta Massif and Ojos de Cristal System in Roraima Tepui showed that quartz dissolution plays only a minor role in their speleogenesis. Arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown, including also loose sands and sandstones. Speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites. Only the so-called “finger-flow” pillars testify to confined diagenetic fluids which flowed in narrow channels, leaving the surrounding arenite uncemented. Another factor which influenced the cave-forming processes by about 30% was lateritization. It affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas, feldspars and clay minerals, turning then to laterite (“Barro Rojo”). The main prerequisite to rank caves among karst phenomena is dissolution. As the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role, these arenitic caves may be ranked as karst.
The Guiana shield is a metamorphic, crystalline basement unconformably overlain by horizontal sediments of the Roraima Formation. During the Precambrian era, the basement and overlying sediments were intruded by sills, inclined sheets, and dykes of dolerite. The magma was tholeiitic. The Tumatumari-Kopinang intrusion consists of a dyke which rises steeply through the basement and then passes into a sill intruded along the plane of unconformity beneath the Roraima Formation. This unconformity sill, in turn, ascends into a higher sill within the Roraima Formation. In the highest part of the intrusion hornblende-granophyre occurs, but silica-poor ferrodolerite is found in the dyke. Field, petrological, and chemical evidence demonstrates that the granophyre and the ferrodolerite are late differentiates of the same parent magma. Despite contrasting mineral and chemical compositions, the trend of differentiation in both cases was toward absolute enrichment in iron. The granophyre represents moderate, and the ferrodolerite strong, fractionation of the parent magma. Fractionation was accomplished by the gravitational settling of the early-formed minerals and strengthened by the circulation of the magma by convection. Evidence from the Tumatumari-Kopinang intrusion suggests that differentiation of tholeiitic magma always results in iron enrichment and is unlikely to be affected by variations in oxygen partial pressure or in the initial composition of the magma. The presence of water may influence the nature of the mineral phases that crystallize, but does not alter the trend toward iron enrichment.