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SPELEOGENESIS AND SPELEOTHEMS OF THE GUACAMAYA CAVE,
AUYAN TEPUI, VENEZUELA
Francesco Sauro
1,2
, Joyce Lundberg
3
, Jo De Waele
1,2
, Nicola Tisato
4
, Ermanno Galli
5
1
Department of Biological, Geological and Environmental Sciences, Bologna University, Via Zamboni 67, 40126
Bologna, Italy, cescosauro@gmail.com, jo.dewaele@unibo.it
2
Associazione di Esplorazioni Geografiche la Venta, Via Priamo Tron 35/F, 31030, Treviso
3
Department of Geography and Environmental Studies, Carleton University, Ottawa, Ontario, Canada, K1S 5B6
4
ETH Zurich, Geological Institute, Soneggstrasse 5, 8092 Zurich, Switzerland
5
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.
2012b).
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).
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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
2
, 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.
1977).
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.
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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).
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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
233
U–
236
U–
229
Th
tracer, dissolved in a mixture of concentrated HNO
3
and HF
over 48 h on a hot plate, dried, and then taken up in 7 N
HNO
3
. 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
2005).
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
hematite.
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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
230
Th/
238
U versus
234
U/
238
U, plotted in Fig. 8.
The ingrowth curves for initial
234
U/
238
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
238
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).
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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
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microg/g
230 Th /
234U
2s Error
230 Th /
238U
2s Error
234U /
238U
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
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