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Speleogenesis and speleothems of the Guacamaya Cave, Auyan Tepui, Venezuela

  • Miles Beyond for ESA
Francesco Sauro
, Joyce Lundberg
, Jo De Waele
, Nicola Tisato
, Ermanno Galli
Department of Biological, Geological and Environmental Sciences, Bologna University, Via Zamboni 67, 40126
Bologna, Italy,,
Associazione di Esplorazioni Geografiche la Venta, Via Priamo Tron 35/F, 31030, Treviso
Department of Geography and Environmental Studies, Carleton University, Ottawa, Ontario, Canada, K1S 5B6
ETH Zurich, Geological Institute, Soneggstrasse 5, 8092 Zurich, Switzerland
Department of Chemical and Geological, University of Modena and Reggio Emilia. Largo S. Eufemia 19, I-41121,
Modena, Italy
In March 2009 the Guacamaya Cave was discovered on the Auyan Tepui. It represents one of the longest caves explored
on this table mountain, the only one known today with a complete horizontal development, comparable with those of the
Brewer Cave System, in the Chimanta Tepui (the world largest sandstone cave). Guacamaya Cave presents peculiar
morphologies, developed along an obvious bed of iron hydroxides and amorphous silica (Banded Iron Formation, BIF)
interposed between hard and massive quartzite banks. In the walls around this layer, a variety of opal speleothems, of unusual
dimensions and shapes, together with gypsum flowers and crusts have been documented.
Here we present a compositional and morphological characterization for the BIF layer and one tufa-like bio-speleothem.
The speleogenetic control exerted by the BIF stratum is discussed, and in relation to collapse morphologies observed on the
surface. An attempt to date the bio-speleothem with the U-Th system shows the difficulties of applying this method to these
silica formations, because of post-depositional alteration in this porous material.
1. Introduction
In the last twenty years many new cave systems have been
discovered in different “tepuis” (table mountains) of the
Guyana Shield (Venezuela and Brazil), formed in the
Precambrian quartzite sandstones of the Roraima Supergroup
(Piccini and Mecchia 2007; Sauro 2009; Aubrecht et al.
2011). The formation of caves and karst features in quartzite
rocks is considered exceptional given the low solubility and
solution rates of quartz (Wray 1997a, 1997b). These recent
explorations have shown that the largest karst systems in
these poorly-soluble siliceous rocks are controlled
predominantly by stratigraphic rather than by tectonic factors.
Many hypotheses were discussed by previous authors
regarding the genesis of these caves, from the weathering
process called “arenisation” (Martini 2000; Piccini and
Mecchia 2009) to hypogenic processes related to
hydrothermal activity (Zawidzki et al. 1976), or even
diagenetic predisposition (Aubrecht et al. 2011), but these
ideas are still in discussion and there is not yet a clear
understanding of the main speleogenetic factors (Sauro et al.
In this work we present a morphological description of the
Guacamaya Cave, the longest horizontal cave explored in the
Auyan Tepui (1.1 km). In addition to the impressive variety
of silica bio-speleothems (Aubrecht et al. 2008a) and
secondary minerals such as gypsum that were documented
in a lateral fossil branch, this cave shows a peculiar
characteristic not documented in other quartzite caves of the
area: a bed of iron hydroxides (Banded Iron Formation)
strictly controls its development.
We performed petrographic studies with thin section and
SEM imaging, EDX and XRD chemical analysis on iron
hydroxide layers, silica bio-speleothems and gypsum. In
addition, following Lundberg et al. (2010), we attempted to
date one of the bio-speleothem by the U-Th system.
2. Geographical and geological setting
The Gran Sabana is a wide 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 main massifs of the Gran
Sabana, named “tepuis”, have the shape of large table
mountains. They are delimited by vertical to overhanging
walls, often more than 1,000 m high. The massifs are
separated from each other by the surrounding lowlands of the
Figure 1. The Auyan Tepuy with caves locations: Guacamaya Cave
(Guaca), Sima Aonda (Aonda), Sistema Auyantepui Noroeste (SAN).
Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
Wonkén planation surface (Briceño and Schubert 1990).
More than 60 tepuis are present in the region and Guacamaya
Cave opens in the northernmost one, the Auyan Tepui
(700 km
, Fig. 1), not far from the Angel Falls, considered
the highest waterfall in the world (975 m).
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.
Guacamaya Cave is developed in the Mataui Formation, the
younger deposits of the Roraima Group, about one and a half
billion years old (Santos et al. 2003). These are quartzitic
sandstones of 600 to 900 metres thick, which form the highest
part of the tepui. These sandstones are made up of quartz
grains, representing well over 90% of the composition, held
together by a cement, also mostly quartz, which gives them
the term “quartz-arenite” (Martini 2000, 2004).
From a structural point of view folds are absent, except for
some wide-curvature folds at a very large scale. The bedding
in the proximity of the cave is slightly inclined toward the
east. Some sets of mainly vertical fractures cut the plateaus,
creating a regular network of quadrangular prisms. Important
faults have not been observed, at any scale.
2.1. Morphological description
The main branch of Cueva Guacamaya is a hydrologic tunnel
about 350 metres long. A permanent stream with a discharge
of some litres per seconds crosses the cave from the highest
entrance (Higher Entrance in Fig. 2) to the resurgence (Lower
Entrance in Fig. 2). About one hundred meters from the lower
entrance, a lateral fossil branch develops to the south for 700
metres ending in a boulder choke close to the surface (Tramo
de los Opales). The passage is of significant size, more than
30 metres wide and about 15 high in some sectors, with a
great collapse room (Salon Roberto Campano) at the
intersection of the two branches.
It is evident that the cave is part of a more extended system,
now dissected and open to the surface: the valleys upstream
and downstream of the cave represent the unroofed
continuation of the main gallery, showing the same
lithological control (Fig. 3). To the east a wide and complex
area of tilted and fallen boulders is probably related to the
collapse of the gallery in a more fractured sector of the
Figure 3. Lower valley entrance and the uroofed continuation of
the cave.
Figure 2. Plan, profile and cross sections of Guacamaya Cave.
Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
plateau. In general the relict cave is situated at shallow depth
from the surface, only 10–5 metres in some places.
Both the active and the fossil branches are developed along
a layer of iron hydroxides with minor amorphous silica,
similar to typical Banded Iron Formations, from some
decimetre to a metre thick. This control is evident in the cross
sections of the galleries showing an elliptical or keyhole
profile. Original small phreatic conduits entrenched by
vadose canyons are recognisable.
2.2. Silica speleothems and gypsum
The cave shows a wide variety of silica and opal speleothems,
in particular in the “Tramo de los Opales” (Fig. 4). These
formations are concentrated nearby the iron hydroxide layers,
preferentially where the gallery cross sections are reduced,
with increasing air flow. This branch of the cave is in fact
characterised by a strong wind between the two main
entrances and the highest final boulder choke open to the
surface. Speleothems of different forms and dimensions were
observed: coralloid speleothems like “dolls”, tufa-like
“champignons” (Fig. 4B), “kidneys” and all the morpho-
types described by Aubrecht et al. (2008a). More complex
composite formations completely cover the cave walls for
several square metres (which we have called “clouds”,
Fig. 4D). A spectacular wind-guided coralloid stalactite 1
metre long was also documented (Fig. 4C).
In general, biologically mediated speleothems such as
champignons grow mainly on sharp edges of quarzite blocks
or emergent quartz veins (in relief due to differential
weathering, resembling boxwork) and they show the
tendency to growth toward the centre of the conduit.
Only few speleothems were sampled for this study, in
particular a “champignon”-type bio-stromatolite, which we
attempted to date.
Gypsum was commonly found in the dry gallery of “Tramo
de los Opales”. Two different preferential mineralization sites
were observed: on the floor, in the form of acicular crystals
around weathered blocks of quartzite fallen from the cave
ceiling; or as overgrowth on the surface of the amorphous
silica bio-speleothems.
3. Methods
For X-Ray Diffraction analyses (XRD) iron-hydroxide bed,
silica biospeleothems and gypsum samples were ground to
an ultrafine powder in an agate mortar and lightly pressed in
a plastic sample holder. XRD patterns were recorded with a
Philips PW 1050/25 and a PANalytical X’Pert PRO
Diffractometer (experimental conditions 40 Kv and 20 mA
tube, CuKα Ni filtered radiation λ = 1.5418 Å) at the
Department of Geology of Modena-Reggio Emilia
University. In order to better identify iron hydroxides we
employed Raman spectroscopy on a fresh cut surface of
sample GC3. The Raman spectrometer is an Horiba Jobin
YVON – LABRAM HR, with spot size ~2 µm, hole 300 µm,
slit 300 µm, 10× optical objective and 632.81 nm wavelength
(i.e. red light).
For X ray fluorescence chemical analysis of the banded iron
formation the sample was finely ground and analyzed in an
Axios PANalitical spectrometer equipped with 5 diffraction
crystals in the ETHZ Geologic Institute facilities (Zurich).
Because of the uncommonly high iron content, samples had
to be diluted with pure silica in order to compare them with
Figure 4. Different morphotypes of biologically mediated silica speleothems. A) tufa-like speleothem; B) champinons agglomerate;
C) Giant corralloid wind-guided stalactite; D) “clouds” formations on the wall; (Photos by F. Sauro).
Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
standards. Thus, a variety of powder-pills with different
percentages of sample and pure silica were analysed.
For Sem images we used a FEI Quanta 200F SEM equipped
with an EDAX Pegasus detector for Energy Dispersive
X-ray Spectroscopy mapping (ETH, Zurich).
For the dating attempt of the silica bio-speleothem the
methods described by Lundberg et al. (2010) were applied.
Small samples (~0.1 g) were taken from two different layers
under binocular microscope avoiding the opalized
laminations and the vuggy dark-colored material. Samples
were ultrasonically-cleaned, spiked with
tracer, dissolved in a mixture of concentrated HNO
and HF
over 48 h on a hot plate, dried, and then taken up in 7 N
. U and Th were isolated on anion exchange columns
(Dowex AG1-X 200–400 mesh). Measurement of U and Th
isotopic ratios was made by thermal ionization mass
spectrometry (TIMS) using the Triton thermal ionization
mass spectrometer at the Isotope Geochemistry and
Geochronology Research Facility, Carleton University,
Ottawa. Ontario. The analyses were accompanied by the
processing of uraninite in secular equilibrium to ensure
accurate spike calibration and fractionation correction.
Activity ratios were calculated using half lives from Cheng
et al. (2000).
4. Results and discussion
4.1. Banded Iron Formations, their geochemistry and
their control on speleogenesis
X-Ray diffraction and Raman analyses show that the Banded
Iron Formation is composed mainly of goethite and hematite,
together with minor amorphous silica (Fig. 5C). In thin
section and SEM images it presents a thin laminations, with
nodules and goethite agglomerates. Also rare remnants of
siderite and dolomite were identified by EDX punctual
chemical analysis, both typical minerals of BIFs (Klein
WD-XRF Fluorescence analyses show that the BIF layer is
composed of 76% of iron with only 18% silica, with minor
phosphorus (2%). Phosphorus is also typical of Banded Iron
Formation because it tends to be absorbed into iron oxides
(Bjerrum and Canfield 2002). Aluminium is below the 2 %,
with potassium and calcium below the 0.01%.
Of the minor elements, Zn and Cu are present, together with
relatively high U and Th concentrations (respectively 96 and
24.5 ppm).
Aubrecht et al. (2011) described similar iron hydroxide
deposits in the Churì Tepui as by-product of laterization (clay
minerals turned to Fe-hydroxides by weathering).
In the case of Guacamaya Cave the strata consists in a
continuous meter thick layer in between the quartz-sandstone
beds. Its structure finely laminated (fig. 5C), its high content
of iron (more than 70%) and minor amorphous silica are in
full agreement with the definition of James (1954) and
Trendall and Morris (1983) for Precambrian banded iron
formations. The depletion of Al and the lack of a typical
latheritic profile suggest that this strata represents a true BIF
formation deposited in a shallow marine environment and
not a by-product of laterization. Alternatively, this layer could
represent a only partially preserved paleo-soil (iron enriched
part of the profile), related to weathering and temporary
emersion just during the Precambrian (Gutzmer and Beukes
1998; Beukes et al. 2002).
In some place the iron hydroxide bed is folded and stretched,
clearly because of shear related to the lithostatic load and the
stronger rigidity of the overlying beds of quartz-sandstones.
In places, where the layer is stretched it is possible to observe
the weathered iron hydroxides flowing out from the strata
forming massive brownish flowstone, similar to other
goethite speleothems described in other quartzite caves of the
Sarisariñama and Chimanta tepuis (Zawidzki et al. 1976;
Aubrecht et al. 2011).
Banded Iron Formations are well documented in other areas
of South America (Dorr 1973) but were never described
before in the Mataui Formation, probably because they are
present only locally and because they are much easier
weathered than the quartz-arenites.
The role of this bed in guiding the speleogenesis is
particularly evident in the Guacamaya Cave, and also in the
nearby higher platforms of the Auyan Tepui, where intensive
collapses show a predominantly stratigraphical distribution
(headwall retreat by basal erosion). It probably represents
what is commonly defined as an “inception horizon” in
classical karst (Lowe 1992), due to peculiar chemical or
rheological characteristics. Many of the hypotheses about
speleogenesis in this region are consistent with the
petrographic properties and behaviour of this bed. This could
include regions of enhanced primary porosity, or even a
seepage water alkalinisation effect on the contact with the
quartz-sandstone beds, with increasing solubility of quartz
and therefore arenization (Martini 2000). Micro-crystalline
iron hydroxides in peculiar conditions are also more soluble
than silica (Schwertmann 1991) and this factor could enhance
Figure 5. The GC3 sample of the Banded Iron Formation bed:
A–B) location of sampling in the Tramo de los Opales; C) thin
layers of iron hydroxides in thin section; D) SEM image of iron
hydroxides aggomerates in the not laminated part; E) Raman
spectrum of GC3 compared with the spectrum of goethite and
Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
weathering. Further research is needed to better understand
the main speleogenetic factors enhancing the opening of
voids along this peculiar layer.
4.2. Silica biospeleothems
The tufa-like silica bio-speleothem (Fig. 6) is shown to
consist of two main textures: tubular casts and peloids
(Fig.7). Quartz grains, derived from weathering and
arenization of the cave walls, are also common. The inner
layers are opalized, while the outer whitish layers are softer
and preserve the original structure. The opalization is related
to hydration processes, and therefore could be related to
longer time of fluids seeping (and therefore with age) or with
peculiar events The same opalization of the inner layers was
observed also in the samples from Chimanta tepui described
by Aubrecht et al (2008a) and Lundberg et al. (2010),
suggesting that this process could be driven by regional
climatic factors.
The two layers sampled for U-Th dating (GCo and GCi in
Fig. 6) show high but different contents of uranium: 4.33 ppm
in the inner layer, 0.86 in the outer one. This disparity
suggests a potential contamination of U rich particles of dust,
probably related to the BIF layer that shows really high
concentrations of this element. Particles of dust are
impossible to observe in thin section or with an optical
microscope, but are evident in SEM images. Therefore is not
possible to exclude them during the sample preparation.
The isotopic ratios resulted from TIMS analysis are shown
in table 1. The
U versus
U, plotted in Fig. 8.
The ingrowth curves for initial
U ratios are those from
dated speleothem of the Chimanta tepui published in
Lundberg et al (2010). Figure 8 shows that both samples from
GC1 lie outside of the dating envelope, and therefore cannot
be dated by this method. This is due probably to depletion in
U in comparison with the original ratio, related to leaching,
or contamination from other sources of uranium (dust
particles). The porous nature of this material suggests that,
in spite of the low solubility of silica, the system probably
has been open for uranium migration.
With respect to the genesis of this silica bio-speleothem,
some observation can be given (Fig. 9). All these speleothems
grow on sharp edges, prominent veins of quartz and in
proximity of the iron hydroxides layers. A control in their
formation by air flow and condensation-evaporation
processes is evident also in the anisotropy of the inner layers.
These observations not only support the idea of Aubrecht et
al. (2008b) that the major source of silica is related to a small-
particle aerosol driven by air flows, but also suggest that
silica dissolved by condensation waters could be an important
source collected on quartz veins and prominent edges of the
cave walls. The changing equilibriums between these two
sources (growth driven more by aerosol or more by
condensation water) in wetter and dryer periods could be the
cause of the opalized layers and more porous external ones.
Figure 6. The “champignons” bio-speleothem GC1.
Figure 7. Tubular cast structure of the outer layers in GC1.
Figure 8. Isotopic ratios from sample GC1 inner(i) and outer (o)
layers. The lines labelled “3.6”,”3.3”, and “3.0” are ingrowth
curves for initial UU ratios, using ratios from Lundberg et al 2010.
Both samples lie outide of the dating envelope (graph was drawn
using Ludwig 2000, Isoplot).
Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
The presence of gypsum crystals on the speleothem surface
and in trace in the inner layers suggests that evaporation
processes were active during the formation of the speleothem
and in its final growing stage.
5. Conclusions
Guacamaya Cave shows peculiar characters not described
before in other quartz-sandstone caves of the Gran Sabana
region. The presence of a BIF bed controlling the
speleogenesis is evident, and the same layer is probably
related to headwall retreat by basal erosion of the higher
platform of Auyan Tepui. This layer show a composition
typical of primary banded iron formation related to marine
sedimentation or, alternatively, of a Precambrian paleo-soil.
The cave shows an exceptional variety of biologically
mediated silica speleothems of different morphologies and
sizes. Their formation is most likely related to aerosol sources
of silica and condensation-evaporation processes. Post-
depositional alteration, and the possible presence of
contaminant dust, limits the ability to date these samples with
the U/Th system.
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Sample Age (ka)
U 238
230 Th /
2s Error
230 Th /
2s Error
234U /
2s Error
GC1 Inner layer Not datable 4.33 1.6009 0.00354 1.0927 0.0022 0.6825 0.0007
GC1 Outer layer Not datable 0.86 1.3124 0.0086 1.0339 0.0033 0.7878 0.0049
Table 1. Concentrations of 238U in the sample GC1 and isotopic ratios for different isotopes of U and Th.
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weathered, the quartz vein remain in relief and became a
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Karst and Caves in Other Rocks, Pseudokarst – oral 2013 ICS Proceedings
... Cell colours are used to distinguish the different water classes, as in Fig. 7 (see legend and extremely low EC (5 μS cm −1 ), if compared to the others. Under the drip, the leaking out of iron-hydroxide deposits typically builds up mound-like, soft, reddish accumulations, which have been described for most of the caves explored in the tepuis as "barro rojo" (Aubrecht et al., 2012;Sauro et al., 2013a;Mecchia et al., 2014;Sauro, 2014). Unfortunately, logistic reasons did not allow collecting enough water to perform chemical analysis for major and minor elements. ...
... This peculiar speleothem occurs in all the caves explored in the tepuis, and it was first documented in Sarisariñama by the Polish-Venezuelan expedition of 1976 (Zawidzki et al., 1976). These deposits consists mainly of goethite (Urbani, 1996;Sauro et al., 2013a), and can form big mounds or stalagmites as those documented in Sima Menor and Sima de la Lluvia (Fig. 9). ...
... Samples of water dripping from fractures encrusted with red mud, very similar to sample SW4, were also collected in the Roraima tepui (sample R18 in Cueva Ojo de Cristal) and in Auyán Tepui (sample A4Y in Imawarì Yeuta, Mecchia et al., 2014). The cause of such a high iron concentration in this type of slow infiltration drippings is still debated: Aubrecht et al. (2008) proposed an origin from lateritic soils, whereas Sauro et al. (2013a) argued for a potential source in iron hydroxide primary strata documented within the Matauí Formation, like the sample SR3 collected from an interlayer in Sima Yadanaima Ewutu ( Table 2). The presence of high contents of dissolved silica and iron in waters characterized by extremely low contents of other ions like Na and Ca and very low EC, favours the second hypothesis, suggesting that other processes like hydrolysis or weathering of clays only have a secondary role. ...
Quartz sandstone of the Sarisariñama massif in Venezuela hosts the world biggest collapse dolines in quartz-rich lithologies, with volumes up to some millions of cubic meters. Due to extremely complex logistics required to reach the massif, the genesis of these depressions and of the underlying caves has never been studied in detail. The lack of field campaigns and extended data has fostered a decade-long scientific debate on whether their origin was due to epigenic or hypogenic processes. This study integrates petrological, structural and hydrochemical observations, including analyses of silica concentration, pH, conductivity of surface and cave waters (EC), to investigate the speleogenetic processes acting underground. Petrographic and compositional analyses of the host rock (Matauí Formation) show that in the Sarisariñama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides. Compared to surface waters, subsurface infiltration along vertical fractures and fault planes show enrichment in silica, higher pH and lower EC, confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities. The weathering of the clay and iron hydroxide interlayers guides the speleogenesis, weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata. The initial void, created by piping of the loose sand released by quartz sandstone weathering, can migrate upwards by means of roof and wall breakdown; this chain of events eventually triggers a collapse at the surface, which generates a circular or squared sinkhole. The weathering acts mainly along the dominant fracture networks, showing a clear guidance by regional tectonics. These speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas, suggesting a primary role of long-term epigenic chemical weathering and mechanical erosion guided by joints, weak clay and iron hydroxide interlayers, followed by subsequent massive collapses.
... Genetically similar caves, but which evolved along iron hydroxide beds, were described also in Venezuela by Sauro et al. (2013b). They supported the idea that, as is commonly accepted in carbonates, the presence of "inception horizons" could also be crucial for the initiation of cave formation in quartz sandstones (Lowe, 1992;Filipponi et al., 2009). ...
... Sauro et al. (2014b) were the first to investigate a clear example of a hypogenic cave in quartzite, but not the processes leading to the formation of such voids, when they studied the Corona 'e Sa Craba Cave in the Mediterranean island of Sardinia (Fig. 17). This cave holds the distinction of representing the world's first hypogenic quartzite cave where the speleogenetic mechanisms have been reconstructed in detail using a variety of modern methods (including SEM petrographic Iron in the solution around the IH increase Si solubility, thus increasing arenization Szczerban et al. (1977) and Sauro et al. (2013b) Water advection or diffusion along the inception horizon ...
... Rare examples of interconnected rounded chambers due to arenization and formation of alcoves due to negative feedback between stress and erosion Cueva Autana (Galán, 1982), quartzite caves in Rio Grande do Sul (Frank et al., 2015) Ramiform O R Equal control of stratigraphic and structural patterns, with evolution of collapse rooms at joint intersections Sima de la Lluvia (Zawidzki et al., 1976), Guacamaya Cave (Sauro et al., 2013b) Passages Shafts A C Mainly elongated along release fractures Aonda (Piccini and Mecchia, 2009), Guy Collet (Epis and Ayub, 2010), Chimanimani (Aucamp and Swart, 1991) Rectangular cross section galleries C A Galleries evolved by lateral and vertical erosion of arenized strata Sima de La Lluvia (Zawidzki et al., 1976), Caverna Aroe-Jari (Hardt et al., 2013), Akopan Dal Cin Cave , Berlin Cave (Martini, 1987) Canyons C F Straight corridors along joints Aonda (Piccini and Mecchia, 2009), Walemouth Cave (Grimes, 2007) Tubes A C Much smaller in size than in classical karst, frequently of phreatic origin, or vadose with rounded sections due to negative feedback between stress and erosion Cueva Autana (Colveé, 1973), Queensland small caves (Wray, 2009;Grimes et al., 2009b) Perched tubes F O Occasional and usually elliptical over less permeable layers (lithology controlled) ...
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.
... detailed survey and sampling of weathered and unweathered quartzsandstones. The data from Imawarì Yeuta were then compared with further morphological observations and sample analyses from other caves developed in the Mataui Formation: the Roraima Sur System in the Roraima Tepui (Galán et al., 2004;Aubrecht et al., 2012), the Akopan-Dal Cin System in the Chimanta massif Sauro, 2009), Guacamaya cave in the western sector of Auyan Tepui (Sauro et al., 2013d) and the deep crevice networks of the Aonda and Auyan Tepui Noroeste systems (Piccini and Mecchia, 2009). ...
... A labyrinth network of inactive galleries, developed along a distinctive stratigraphic position, interconnects the different rivers. The guiding stratum is situated some metres above the actual stream levels and, where preserved, is often characterised by the presence of a layer of iron hydroxides laminated with amorphous silica, resembling Banded Iron Formation described in other caves of the area (Guacamaya cave; Sauro et al., 2013d). The voids formed along this bed can reach impressive widths (more than 300 m in some sectors) creating huge flat environments where the ceiling is supported only by relict pillars and wide columns (Fig. 3b). ...
... Guacamaya cave represents the first horizontal cave discovered in Auyan Tepui in 2009 (Sauro, 2009;Sauro et al., 2013d;Fig. 1c). ...
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).
... Also, Wray (1999) considered the layered structure of silica speleothems a sign of deposition from rapidly evaporating solutions. The effect of air flow in the speleothem formation in the Venezuelan tepuis caves is documented by speleothem concentration along sharp edges on cave walls and the presence of wind-guided forms (Sauro et al., 2013;Wray & Sauro, 2017), a situation parallel to that in the Elbe River Canyon. ...
... Harsh et al., 2002). A more the surface of silica-dominated speleothems (Sauro et al., 2013(Sauro et al., , 2014aLundberg et al., 2018). It crystallizes from pore waters on cliff faces during alternating episodes of wetting and drying . ...
A variety of speleothems are present in crevice and boulder caves developed in Cretaceous sandstones of the Elbe River Canyon in northern Czech Republic. A set of complementary instrumental mineralogical methods was applied to characterize the speleothems and cave dripwaters, including X-ray powder diffraction, scanning electron microscopy and microanalysis, Raman spectroscopy and optical emission spectrometry. Four morphological types were distinguished and characterized in terms of their mineral and chemical composition: 1, rusty brown mud-dominated coatings with micro-gours, composed of a mixture of clay minerals; 2, white “chalky” coatings (moonmilk) composed of calcite with minor gypsum; 3, cauliflower-shaped coralloids composed of calcite and silica in a layered structure, with gypsum layers in apical parts; 4, knob coralloids, dark gray-brown with smooth surfaces and distinctly layered structures, composed of silica (quartz, opal-A) and Si–Al phases (kaolinite) and including phosphate-rich laminae (sasaite, vashegyite, taranakite). Only modest microbial mediation of silica precipitation was observed in cauliflower-shaped coralloids while no clear signs are present in knob coralloids despite organic enrichment in the topmost layer. White “chalky” coatings and cauliflower-shaped coralloids precipitated from weakly acidic Ca-, Mg- and sulphate-rich deeper sandstone percolates. These forms are probably still active, much like the micro-gours, produced by particulate clay deposition. Formation of knob coralloids combined clay deposition and the dominant silica precipitation from pore waters similar to the present shallow acidic percolates under changing climatic conditions, probably in the Pleistocene. It was favored by specific rock lithology (quartzose sandstone with kaolinite admixture), which explains the scarcity of similar forms in sandstone caves. Concentration of knob coralloids along protruding vertical edges and the presence of wind-guided forms suggests that silica precipitation was driven by evaporation under a constant air flow.
... Silica dissolution occurs at the micro-scale, as vug porosity ( Figure 9A), and at the macroscale, as irregular caverns, both within the silicified host rock. The caverns, up to 60 cm in size, are formed at the intersection between discontinuity surfaces such as bedding planes and fractures ( Figure 9B), confirming that such discontinuities act as important pathways for fluids causing preferential dissolution [102,103]. Thus, most silica precipitating within fractures is provided by the surrounding sandstones and conglomerates. At the micro-scale, dissolution preferentially affects the sandstone matrix constituted by opal and clays, feldspar and mica grains, and syntaxial feldspar overgrowths ( Figure 9A,C). ...
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Near-surface diagenesis has been studied in the Langhian siliciclastic rocks of the Montjuïc Hill (Barcelona Plain) by means of petrographical (optical and cathodoluminescence) and geochemical (electron microprobe, δ18O, δ13C, δ34S and 87Sr/86Sr) analyses. In the hill, these rocks are affected by strong silicification, but the same unit remains non-silicified at depth. The results reveal that fracturing took place after lithification and during uplift. Fracture cementation is clearly controlled by the previous diagenesis of the host rock. In non-silicified areas, cementation is dominated by calcite, which precipitated from meteoric waters. In silicified areas, fractures show multiepisodic cementation produced firstly by barite and secondly by silica, following the sequence opal, lussatite, chalcedony, and quartz. Barite precipitated only in fractures from the mixing of upflowing seawater and percolating meteoric fluids. The presence of silica stalactites, illuviation, and geopetal structures, and δ18O values indicate that silica precipitation occurred in the vadose regime from low-temperature percolating meteoric fluids, probably during a glacial period. Moreover, the presence of alunite suggests that silica cement formed under acidic conditions. Karst features (vugs and caverns), formed by arenisation, reveal that silica was derived from the dissolution of surrounding silicified host rocks.
... hosting some of the most beautiful silica speleothems described in the tepuis ( Sauro et al., 2013). The passage is more than 30 m wide and about 15 m high in some sectors and is developed along a layer of iron hydroxides with minor amorphous silica, similar to Banded Iron Formations, from some decimeters to a meter thick. ...
A review of sandstone and quartzite caves of South America
... Analyses performed also in the more arkosic beds, show low concentrations in K and Na, and almost no Ca and Mg, suggesting that the content of feldspar in these lithotypes is below 1-4%. Iron is present in the form of iron hydroxides such as goethite and limonite (determined by raman analyses, see Sauro et al., 2013c). ...
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The discovery of significant cave and karst landscapes formed in quartzites and sandstones in South America, Africa and Australia has led to a debate among scientists over the definitions of karst and the processes forming karst in quartzites. In the past these caves were listed under the ambiguous definition of 'pseudokarst' landforms. It is now generally agreed that the chemical dissolution of silica within massive quartzite or sandstone units plays a significant role in the development of certain types of quartzite caves and the term syngenetic karst may better describe non-carbonate landscapes where dissolution and sediment transportation by erosion processes both play major roles in karst development. The recent discovery of towers formed within Precambrian orthoquarzite rock adjacent to Tertiary basalt on the edge of the Borradaile Plains in northern Tasmania poses questions regarding the processes of quartzite dissolution and karst development in silica rich rocks in an area that has had a subalpine or glacial climate for much of the Quaternary. It is suggested that the overlying basalt has been stripped from around the towers by Quaternary erosion and the caves have formed by arenisation induced by acidic upland soils.
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Seven silica biospeleothems from Cueva Charles Brewer, Chimantá Plateau, Venezuela have been successfully U-Th dated despite very low U and high detrital Th concentrations. Growth rates are low, between ~100 to ~800 µm/ka, and are greater closer to water level. Dates in unaltered material are in good stratigraphic order, but secondary silicification may compromise the U-Th system, yielding unreliable sequences of ages. Detritally-enriched layers correlate with global climate cycles of the Late Quaternary, in particular the cooler, drier phases of MIS 5d, 5a, and 4. SEM studies indicate that the peloidal material is made up of silica nano-particles assembled to form hollow tubules ~1 µm in diameter. Secondary silicification inside and outside the tubules fills most of the pores. Barite crystals are deposited close to the silicified core material. Chemical analyses suggest that the white peloidal material is deposited by stream-generated foam, while the silicified material and dark core material are closer to rock composition.
South America is well endowed with quartzite outcrops and caves, especially in southern Venezuela and along eastern Brazil. Quartzite caves may display branchwork, anastomotic and network patterns, similar to carbonate caves, showing strong geological control associated with layers less resistant to erosion. Vertical caves close to scarps develop along unloading joints and may be at least in part tectonic features. Cave initiation involves loosening of quartz grain by intergranular dissolution or chemical removal of silica-rich matrix, kaolinite, or mica. Mechanical removal of quartz residue may enlarge cave passages once there is an opened evacuation route. Microbial mediation may play a role, especially in generating unique stromatolitic speleothems. Slow rates of dissolution, low content of silica in the groundwater, and the old age of the bedrock favor a long-term evolution for quartzite speleogenesis, perhaps orders of magnitude slower than in carbonate settings.
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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.
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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 well-cemented 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 lithified 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-laminatedcolumnar stromatolite formedby 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 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 sedimentary iron-formations of Precambrian age in the Lake Superior region can be divided on the basis of the dominant original iron mineral into four principal facies: sulfide, carbonate, oxide, and silicate. As chemical sediments, these rocks reflect certain aspects of the chemistry of the depositional environments. The major control, at least for the sulfide, carbonate, and oxide types, probably was the oxidation potential. The evidence indicates that deposition took place in restricted basins, which were separated from the open sea by thresholds that inhibited free circulation and permitted development of abnormalities in oxidation potential and water composition. The sporadic distribution of metamorphism and of later oxidation permits description of the primary facies on the basis of unoxidized, essentially unmetamorphosed material. The sulfide facies is represented by black slates in which pyrite may make up as much as 40 percent of the rock. The free-carbon content of these rocks typically ranges from 5 to 15 percent, indicating that ultra-stagnant conditions prevailed during deposition. Locally, the pyritic rocks contain layers of iron-rich carbonate. The carbonate facies consists, in its purer form, of interbedded iron-rich carbonate and chert. It is a product of an environment in which oxygen concentration was sufficiently high to destroy most of the organic material but not high enough to permit formation of ferric compounds. The oxide facies is found as two principal types, one characterized by magnetite and the other by hematite. Both minerals appear to be of primary origin. The magnetite-banded rock is one of the dominant lithologies in the region; it consists typically of magnetite interlayered with chert, carbonate, or iron silicate, or combinations of the three. Its mineralogy and association suggest origin under weakly oxidizing to moderately reducing conditions, but the mode of precipitation of magnetite is not clearly understood. The hematite-banded rocks consist of finely crystalline hematite interlayered with chert or jasper. Oolitic structure is common. This facies doubtless accumulated in a strongly oxidizing, probably near-shore, environment similar to that in which younger hema-titic ironstones such as the Clinton oolite were deposited. The silicate facies contains one or more of the hydrous ferrous silicates (greenalite, minnesotaite, stilpnomelane, chlorite) as a major constituent. Granule structure, similar to that of glauconite, is typical of some varieties; others are nongranular and finely laminated. The most common association of the silicate rocks is with either carbonate- or magnetite-bearing rocks, which suggests that the optimum conditions for deposition ranged from slightly oxidizing to slightly reducing. The relationship between the iron-rich rocks and volcanism, stressed by many authors, is considered by the writer to be structural, not chemical: in the Lake Superior region both iron-deposition and volcanism are believed to be related to geosynclinal development during Huronian time. In Michigan, the lower Huronian rocks are iron-poor quartzite and dolomite-typical "stable-shelf" deposits; much of the upper Huronian consists of iron-poor graywacke and slate with associated volcanic rocks -a typical "geosynclinal" assemblage. Thus the iron-rich beds of the middle Huronian and lower part of the upper Huronian were deposited during a transitional stage in structural history. The major environmental requirement for deposition of iron-formation is the closed or restricted basin; this requirement coincides in time with what would be a normal stage in evolution of the geosyncline: namely, structural development of offshore buckles or swells that subsequently develop into island arcs characterized by volcanism.