Boletín de la Sociedad Venezolana de Espeleología

82 Pages·1999·16.5 MB·Spanish

Caracas, Diciembre 1999
ISSN 0583 - 7731
Dirección de la sede: JUNTA DIRECTIVA (1998-2001)
SOCIEDAD VENEZOLANA DE ESPELEOLOGÍA
Av. Caurimare, Residencias Y oraco, Sótano LE, Presidente: · Rafael Carreño
Colinas de Bello Monte, Caracas.
Vicepresidente: Carlos Bosque
(Reuniones todos los miércoles de 7 a 10 p.m.)
Secretario: Bernardo Urbani
Dirección postal:
Tesorero: Franco Urbani
Sociedad Venezolana de Espeleología
Apartado 47.334, Caracas 1041-A, Venezuela. Vocal: Luis Melo
Teléfono: (02)-74.64.36. Fax: [phone].24
E-mail: [email protected]. ve
carlosb @ usb.ve
rafaelcarreno @ hotmail.com
Los artículos de este Boletín, dependiendo de su contenido, aparecen indizados en las publicaciones indicadas a continuación:
Speleowgical Abstracts de la Unión Internacional de Espeleología; Bibliography and /ndex of Geology publicado por la Geo
logical Society of America; Georef del American Geological Institute; Geo Abstracts de Elsevier, Holanda; Current Geographi
cal Publications de la American Geographical Society; Mineralogical Abstracts, Inglaterra; Bulletin Signalétique, Centre Na
tional de la Recherche Scientifique, Francia; Antropológica, Fundación La Salle, Caracas.
El Boletín de la Sociedad Venezolana de Espeleología se publica anualmente por los miembros de la misma en Caracas,
D.F., Venezuela. El Boletín está abierto a todos aquellos trabajos de interés espeleológico, particularmente de la región neotropical.
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Apartado 47.334, Caracas 1041 A, Venezuela.
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Miguel Angel Perera (SVE, UCV) y Franco Urbani (SVE, UCV). Editores de campo: Franz Scaramelli (Antropoespeleología),
Rafael Carreño y Joris Lagarde (Catastro), Francisco Herrera (Bioespeleología).
La Comisión Editora agradece a los siguientes árbitros que actuaron en este Boletín: Pedro Rivas (IPC), Henry Briceño (Litos),
Carlos Yanes (UCV), Kay Tarble (UCV), Lilliam Arvelo (IVIC), Erika Wagner (IVIC), Sebastián Grande (UCV), Virgil Winkler
(UCV), Nuris Orihuela (UCV), Carlos Bordón (UCV-Maracay), Carlos Tinoco (BCV), Angel Vitoria (LUZ), Manuela Billaudot.
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El Boletín es gratis para todos los miembros de la SVE que se encuentren al día en sus cuotas. El costo de un ejemplar es de
US$ 15 incluyendo los gastos de envío al exterior. Toda información concerniente a suscripciones debe ser solicitada a la Sociedad
Venezolana de Espeleología, Apartado. 47.334, Caracas 1041 A, Venezuela o por fax al (58-2)-978.31.77 y 730.64.36.
Los costos de impresión de este Boletín han sido subvencionados por la Dirección de
f
Información Científica y Tecrwl6gica del Consejo Nacional de Investigaciones Oentí.ficas
Tecnológicas (CONICIT).
Diagramaci6n: Joris Lagarde Foto portada: acceso a la Galería Alí Primera,
Impreso en Gráfica León s.r .l. Sima Aonda (Bo.8.). Ver artículo p. 1
Depósito legal: pp. 196703DF15 (Biblioteca Nacional, Caracas). ISSN 0583-7731 Foto: Joris LAGARDE
GEOESPELEOLOGIA
HYDROGEOLOGY ANO Si0 GEOCHEMISTR Y OF THE AONDA CAVE
2
SYSTEM, AUYAN-TEl'UI, BOLIVAR, VENEZUELA
Marco MEcCIUA1 & Leonardo PicCINI2 durante un evento de inundación (mínimo 0,17 mg/1, máximo 0,48
mg/1). Esto implica que en un evento de esté tipo, el agua enriquecida
1 Societá Speleologica Italiana. Via dei Ramni 24. · en SiO retenida en los depósitos de turba, depresiones y fracturas,
2
00185 Roma, ltaly. [email protected] es arrastrada por el agt14 de lluvia. ·
2 Dipartimento di Scienze della Tena, Universitá di Firénze. El agua ｾ＠ percolaci6n y goteo subterráneo tiene una
Via La Pira 4. 50121 Firenze, Italy. [email protected] concentración de SiO de alrededor de 1 mg/1, pero una sola muestra
2
de agua goteando de una espeleotema de· ópalo dio un valor de 7,1
mg/1, siendo la única· muestra sobresaturada en sílice.
RESUMEN
Estos datos penniten reali7.ar un cálculo aproximado. de 1a
Hidrogeologla y geoquímica del Sistema kárstico de Sima de
cantidad de sílice disuelto en la platafonna de Aonda. Para ello
A.onda, Auyán-tepui, Bolívar, Venezuela.
utilizamos el caudal de la resurgencia Ali Primera de unos 800 Vs para
El Sistema Sima Aonda se localiza en la parte noroccideot.al del
el 4 de marzo de 1993. La carga de.sílice resulta en unos 184 mg/s,
Auyán-tepui y es el complejo de cuevas desarrollado en rocas
que proviene principalmente del aporte superficial del río Superior,
cuarcíticas mejor conocido del mundo. A pesar de la naturaleza silícea
en el orden de 144 mg/s; mientras que la. sílice disuelta en su trayecto
de la roca, puede considerarse un sistema kárs~co, ya que su origen
en el Sistema Aonda aporta unos 40 mg/s, a su vez compuesto de
se debe principalmente a procesos de disolución, donde el agua de
aproximadamente un 15% de la disolución superficial que ocurre en
escorrentía es drenado fundamentalmente a través de conductos
t\ll'ba, po:zas y cubetas, y un 85 % de los procesos de disolución
subterráneos. Las peculiares fonnas superficiales en Auyán-tepui son
subterráneos
el resultado de la meteoriución quimica. La importancia de la
Palabras claves: Geomorfología, espeleogénesis, meteoriución,
disolución química de las rocas es enfatiuda por las pequeñas formas
cuarcita, karst, Gran Sabana.
superficiales, típicas de terrenos kársticos cÍe calius, como
acanaladuras, cubetas, huecos y otros. El Sistema representa la red
río ABSTRACT
de drenaje subterráneo de un curso superficial, el Superior,
capturado en el extremo Este de la Plataforma de Aonda, que The Aonda Cave. System is located in the NW of the Auyán-tepui,
finalmente resurge en el acantilado exterior del tepuy. · it is the best known cave complex ·d eveloped in siliceous rock in the
· En el período investigado el caudal del río subterráneo varió entre world. Despite the nature of the rock, it can be regarded as a karst
0,2 y 2,5 m3/s. Con estos pocos datos, podemos asumir que durante la . fonn, because its origin is mainly due to dissolution processes. This
temporada de lluvias, las crecidas probablemente excedan System represents the underground drainage netwodc ,of a surface
1ｾ＠ 15 m3/s, mientras que el caudal medio anual puede estar en el orden stream captured through a sinkhole in the flat top of the tepuy and
de 0,5 - 1 m3/s. out-flowing ata resurgence in the peripheral scarp. In the investigated
Las muestras de agua analiudas fueron de varios tipos: lluvia, period, the discharge of this underground stream ranged from a mini
pequeñas po7.a5, corrientes superficiales y subterráneas,· turberas y mum of about 0.2 to a maximum of 2.5 m3/s, but we can assume that
goteos. El pH, temperatura, conductividad eléctrica (CB) fueron during the rainy season floods the discharge probably exceeds 1~15
medidos con equipos portátiles y las concentraciones de SiO se m'/s, while the mean annual discharge should be around 0.5-1 m3/s.
2
midieron con una prueba colorimétrica. . Temperature, pH, elc:ctric conductivity (EC) and SiO concen
2
El pH del agua de lluvia fue siempre ácido, de 3,8 a 6,5. La CE trati.on of water samples from raiA, small ponds, surface or subterra
fue baja (<15,9 µS/cm) y· la SiO no fue detectada. El agua de nean streams, peat deposits and cave drippings have been measured.
2
escorrentía al fluir a través de depósitos de turba se enriquece en The pH of rainwater is always acid, EC is always very low (<15.9
.m ateria orgánica ck-rivada de la descomposición de. la vegetación; el µS/cm), and silica was not detected. The runoffwater, flowing trough
pH varía de 3,6 a 4,5 con una acidez siempre superior a la del agua de . peat deposit gets· enriched with organic matter. The samples show a
lluvia; la CE varia de 12 a 29 µS/cm; la concentración de la SiO es pH ranging from 3.6 to 4.5, the EC ranges from 12 to 29 µS/cm. Silica
muy variable de < 0,01 a 0,43 mg/l y se ck-riva totalmente de2 la concentration ranges from about < 0.01 ｾ＠ 0:43 mg/1.
disolución de la roca, variando probablemente seg6n el tiempo de Chemical data underline the very low concentration of SiO of
2
contacto agua/roca y el grado de evaporación. the surface water. Percolation and cave dripping waters ha1 a SiO
2
Los datos químicos señalan que el agua que procede de 1a· conoentration of about_1 mg/1._S orne water falling ·from a drip-stone
superficie superior del tepuy que fluye de rocas cuarciticas y con corto of opal has a concentration of 7.1 mg/1 SiO, being the only water
2
tianpo de contacto, tiene muy baja concentración de SiO• En las sample over-saturated in silica.
2
cuevas apenas se nota un pequeño incremento, mientras que en The total silica load of the stream is 184 mg/s, mainly derived
el rio Caaao, al pié de la meseta, su concentración es from surface dissoluti.on removal in the upper platfonn. In the Aonda
significativamente mayor ya que cireula por rocas ricas en System, the SiO dissolved, · parUy from surface_d issolution (15 %)
2
feldespatos. and partly from underground processes (85% ), is 40 mg/s.
La concentración de · SiO del agua que circula en el Sistema Key, words: Geomorphology, quartzite, spcleogcnesis,
2
Aonda mostró un incremento a medida que aumentaba el caudal · weathcring, kant, Gran ·S abana.
v,.,,,,1.,,.u, .,;..,,..,
Rnl .'v.,- ni l 1000
INTRODUCTION
In 1993 and 1996 two speleological missions in 62° 40' 62º 20'
the Auyán-tepui region, one of the widest table
mountains in southern Venezuela, were organiz.ed by
the Associazione Esplorazioni Geografiche "La
Venta", with the significant support from Sociedad
Venezolana de Espeleología and Societá
Speleologica Italiana. In 1993 the team of Italian and
Venezuelan cavers explored three different areas. In
that occasion, six !1ew caves were explored: at that
time, one of these the Sima Auyán-tepui Noroeste,
registered in the "Catastro Espeleológico Nacional" • •
6 00
as number Bo.87 (SSI-SVE 1997) was the longest
and deepest cave in the world developed in siliceous
t
rock (depth -370 m, length 2950 m), now exceeded
by Gruta do Centenario in Brazil. A new investiga
N
tion of the Sima Aonda (Bo.8) and the exploration of
new caves in the surrounding area were also carried
out (BERNABEI et aL 1993, BERNABEI 1994). In 1996,
a second mission focused the efforts on the explora
tion of the active undergrow:id network of the Aonda
Cave System. This aim was partially reached by the
exploration of the Alí Primera resurgence, which
represents the western part of the main collector
· (SSI-SVE 1997).
km
·. The Aonda cave system is the best known cave
complex developed in siliceous rock in the world and o 5 10
it is located in the NW of the Auyán-tepui (Fig. 1) . It Fig. l. General sketch map of the Auyán-tepui and location of the Aonda
consists of several caves nót yet completely con platform (AP). .
nected by explored passages; the connectjon exists
from a hydrogeologic point of view. The System represents the
Although we do not know yet the exact pattem of the under
underground drainage network of a stream, the Superior river
ground drainage network, the speleol9gical and hydrogeologic
(Río Superior, Fig. 2), captured through a sinkhole at the east
surveys allow a first hypothesis about the hydrodynamic
border of the Aonda platform. The water flows out from a behavior of the System.
spectacular resurgence in the peripheral scarp of the tepuy.
A first analysis of the water éhemical datá allows to better
Currently 11 caves have been explored in the Aonda plat
understand the development processes of endokarst in
form, but many other deep shafts are waiting to be investigated. siliceous sedimentary rock.
Rio Superior
~tf!!iflfi
Simas del Este
w
E
-,E~~~;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;==---
~¡::
m
..,j. . o 200
Fig. 2. Sketch profile of the Aonda cave system (RAP: Alí Primera resurgence). Modified from SVE (1983).
2 &L Soc. Venezolana EspeL (33) 1999
GEOGRAPHIC AND GEOLOGIC weathering, allowing, in a very long time, the development of
endokarst forms (URBANI 1986, GALÁN 1991, WRA Y 1997).
OVERVIEW
This karst landscape is the result of chemical weathering
The Aonda platform is a small bench, 1.5 km long and about processes. Toe importance of the chemical solution of
1 km wide, located in the northem part of the Auyán-tepui, quartzarenite is well emphasized by small-scale solution
about 1O km NW from Angel Falls (Fig.1 ). Toe geographic co · forms: rills, pans, pits, and small pockets of phyto-corrosion
ordinat es are 6° 02' of N latitude and 62° 36' of W longitude, origin covered by algae. Mechanic-erosive processes are
the elevation is about 1500 m a.s.l. Toe Platform is limited to active toq, but their effect is significant only along the streams,.
the NE ~d SE by a rock-wall of about 100-150 m, to the W by close to the rim of the plateau, and inside the active caves. A
the rim of the plateau, which falls with a drop of about 500 m general evolutionary model of the karst system is accepted
facing the valley of the Aonda river, tributary of the Carrao (URBANI' 1986, GALÁN & LAGARDE 1988) but many details are
· river. still missing, and the time of development of so impressive
From a geologic point of view, the Auyán-tepui belongs to underground network is unknown. These plateaus are affected
the Roraima-Canaima Province, where the silico-clastic rocks by weathering since c;retaceous times, in a state of almost
of the Roraima Group widely outcrop (REm 1974, GH~SH · absolute tectonic qui~scence ·(BRICEÑO & ScHUBERT 1990), thus
1985). Toe sandy formations of this group display continental· the time of formation of caves could entail several millions of
to peri-continental facies, whose age ranges from 2300 - 1800
years.
million years (2.3 - 1.8 Ga) of the granitic basement to the 1.8- The great shafts, named with the'Spanish word simas, are
1.4 Ga of the basaltic dikes and sills that cross the Roraima thé most important landscape features of the Aonda platform
Group (~RicEÑo·et al. 1990). A low-~ metamorphism, with (SVE 1986, SSI-SVE 1997). Their origin is largely due to
quartz-pyrophyllite paragenesis of the shaly beds, is the result
collapse of deep shafts, enlarged by basal erosion fostered by
of the load of a now eroded thickness of almost 3 km of rock underground water _flow (SZCZERBAN & URBANI 1974, PicCINI
(URBANi et al. 1977). 1995). The huge main shaft of Sima Aonda is 362 m deep, 500
Toe 'scarps and the plateau of the Auyán-tepui are formed m long and about 100 m wide, for instance, formed by the join- .
by orthoquartzites to subarkoses with subordinate middle-fine ing of different shafts (SVE 1983), while the total
grained ¡l ithic wackes (Matauí Formation), that rest onto
desniveJation of the System is 383 m.
protoquktzites, arkoses and wackes with beds of chert, lutite According to the evolutionary ideas of SzczBRBAN &
and siltite (Uaimapué Fol'Illation). Toe wide plain at the base URBANI (1974) and PlcCINI (1995), the simas of the Aonda
of the tepuy, where the Carrao river flows, is formed by the
System show different evolutionary stages. Toe Sima Aonda
siltstones and shales of the Kukenán Formation (BRICEÑO
2 (Bo.83, -360) displays an initial stage, where rock-collapse
1985). The flat top of the Aonda bench is caused by the occur
does not yet occur, whereas the Sima Aonda 3 (Bo.84, - 335),
rence of a hard bed of fine ·quartz arenite (Piccoo et al. 1994).
represents a young-middle stage where collapsing of the lower
This hard "cap" lies over a sequence of medium to coarse part of the cave is now in progress. The Sima Aonda and the
quartz arenites, white or ochre in color, with cross-lam.inated
Sima del Este 1 (Bo.27) and Sima del Este 2 (Bo.28) represent
beds. Al>out 80-90 m below the surface a 1 m thick level of the final stage, which follows the full collapse of the
red-white banded shale is found; an X-ray diffraction analysis
underground cavities.
has shown the presence of pyrophyllite (URBANI 1996), chio
rite and tale.
HYDROGEOLOGY OF TIIE AONDA SYSTEM
Toe structural setting of the area is very simple, being the
beds almost perfectly horizontal. Toe main tectonic features
Toe Aonda platform is the best investigated area in the
are sets'of vertical fractures, which cut the platfonn into rect
Auyán-tepui quarzitic massif (GALÁN 1986, 198~, 1991;
angular to rhombic prisms sorne meters wide. In the Aonda
BBRNABEI 1994; SVE 1983~ 1986; SSI-SVE 1997). Currently,
platform the main sets of fractures are oriented NNW-SSE and
although only eleven of the many deep shafts have been
NE-SW.
explored, the hydrogeologic • setting of the System is well
depicted (MEcCHIA et al. 1994). It is commonly accepted that
KARST GEOMORPHOLOGY
many of the caves in the Aonda platform are joined in a single
Despite the siliceous tiature of the rock, the landscape of system but, presently, only two caves, the Sima Aonda and the
the Auyán-tepui plateau shows typical karst solution land Sima del Bloque, are c~nnected through a path accessible to
forms: karren-like forms, stone-forests, dolines,. sinkholes, man (Fig. 2). .
caves and impressive sha1f• ts w1 hich underline the fa• ct that run- In the east of, the Aonda platform, a waterfall, about 100 m,
off waters are mainly drained through subterranean paths. high, falls from the rim of the upper plane, _the Aonda upper ·
The geomorphic -setting Qf the tepuy has been widely de platform at _t he top of the tepuy. It is fed by a river that is ·
scribed by several authors (SZCZERBAN & URBANI 1974, URBANI referred here as ~uperior river (Rfo Superior, Fig. 2), a surface
1986, 1991ab, GALÁN 1988, 1991, GALÁN & LAGARDE 1988, · stream whose básin extends over an area probably larger than
·BRICEÑO & ScHUBERT 1990, 1992, GoRI et al 1993, PicCINI 1O km2. At the base of the waterfall water disappears beneath
1995, DoERR 1999). Most of them agree thatthe development a pile oflarge boulders. After nmning 120 m the river emerges
of a karst landscape has been possible because the environ with a ,waterfall 120 m high nearby Sima del Este 1 (Bo.27).
mental conditions have restricted the effects of mechanical Water is lost again in the floor of the shaft, and flows out in the
BoL'Soc. Venezolana l:speL (33) 1999
basal tunnel of the Sima del Este 2 (Bo.28) (SVE 1983, 1986; The large depression of Sima Aonda drains also the water
GALÁN 1986). In this cave the stream is accessible for about of the small surface streams, which fall in after a drop of more
250 m, along a narrow rectilinear canyon, until it flows into a than 320 m. Such waters are collected and emerge from the
small lake. peripheral scarp of the Auyán-tepui, through the spectacular
The exploration of the Sima del Bloque in 1996 (SSI-SVE Aonda resurgence, about 100 m above the foot of the wall. We
1997), has allowed to know another important segment of the presume that beneath the chaotic accumulation of rock-blocks
underground stream which connects· the sinkhole with the at the bottom of the Sima Aonda, a non-fractured bedrock
resurgence in the Sima Aonda. In this unknown path the stream collects the water from other undetectable underground
does not seem to change sigÍlificantly its discbarge. But, streams, coming · from the "Sima del Sur" ar~ and from the
lacking simultaneous measurement of discharge, we cannot Sima Aonda 3 area (Fig. 3). The confluence of another hypo
rule out the presence of tributarles coming from different parts thetical collector from the north, where unexplored simas and
of the platform or, conversely, the loss of water towards sinkholes e:xist, is probable because no other springs are vis
another unknown resurgence. ible at the base of the western scarp of Aonda platform. This
At the bottom of the main shaft of the Sima del Bloque could explain the increase in discharge displayed by the stream
(-316 m), the water emerges from narrow submerged fissures. fro..m Alí Primera resurgence to the Aonda resurgence (Fig. 3).
From here, the subterranean stream flows towards NNW along The discharge of streams can be estimated on the ground
a large canyon (Galería de la Cascada), which features long of the few measurements of Venezuelan and Italian cavers.
sand banks in the middle part. Along ali the pathway, it These measurements refer only to the dry season. The authors
receives only a tributary from the left. After about 700 m, the report the following discharges: Superior river - from a míni
stream falls out near the bottom of Sima Aonda, forming a mum of about 200 1/s to a maximum of25001/s (MEcCHIA et al
waterfall named Alí Primera resurgence (Fig. 2 and 3). 1994); Alí Primera resurgence - from 50-100 1/s to more than
In 1996 a tracing test was perfonned through the input of 20001/s (SVE 1983). In 1996, after two days of rain, a signifi
800 g of fluorescein dye in the river just _beneath the waterfall cant increasing of discharge of the Superior river and of Alí
of Rio Superior. Toe charcoal captors were placed in the Primera resurgence had been observed. Both probably reached
Resurgencia Ali Primera. About six hours later the water of a discharge of more than 5-6 m3 /s. In the underground stream
the resurgence seemed to be slightly green, but the analysed of the Sima Aonda, hints of flooding were found 4-5 m above
captor gave an uncertain result. Probably the high concentra the usual water level. According to these few data, we can
tion of dissolved organic matter did not allow the captors to assume that during the rain-season floods the discharge
absorb a sufficient quantity of dye to be revealed by field probably exceeds 10-15 m3/s, while the mean annual
analysis. discharge should be about 0.5-1 rrr/s.
...
camp
0
stream
o
cave stream
A
dripping water
PLATAFORMA o
AONDA pond
peat
Resurgencia
Ali Primera solution pan
[I3J*
i((2Q}
SimaAonda 3
500m
Fig. 3. Details of the Aonda platform with location of sampling stations of water.
Sketch modified from SVE (1983) and SSI-SVE (1997).
4 BoL Soc. Venezolana EspeL (33) 1999
SILICA GEOCHEMISTRY OF SURFACE AND UNDERGROUND WATER
During the 1993 and 1996 field researches, severa} water EC is always very low (1.3 - 15.9 µS/cm), and silica was not
samples, collected in the NW part of the summit plateau of the detected. Such a low EC is due to the long distance from the
Auyán-tepui (Fig. 3), and along the Carrao river valley had sea (the natural source of salty ae~osol) and from human
been analyzed. Toé samples concemed differe~t kinds of wa acti.vities and industries (sources of dust and pollution).
ter: rain, ponds, surface or subterranean streams, peat deposits A relationship between EC and rainfall seems to exist (Fig.
and cave drippings. 4). Namely, the first sample of rain, after a week of no rain,
Temperature, pH, and electric conductivity (EC) were mea has an anomalous EC = 15.9 µS/cm, whereas the water of the
sured with field portable instruments. Si02 concentration was more intensive rain has a very low EC = 1.3 µS/cm (i.e., pure
analyzed as soon as possible using a colorimetric test water). With the increasing of EC the pH fall. This behavior is
Aquaquant 1441 O Silicon by Merck, because of its low con typical of rain water and it indicates a storage of sulfur and
centration (<l mg/1) the measurements most be carried out no nitrate oxides in the atmosphere, which increase the acidity of
later than 8 hours after sampling (UNESCO-WHO 1978). Toe rainwater.
test-kit allows the analysis in the concentration range 0.01-0.25 Only in one case, the pH of rainwater shows a total corre
mg/1, with an error probably lower than 20%. The samples with spondence with the theoretical equilibrium value of pH: 5.6.
a higher concentration were analyzed by dilution with distilled One sample has a value of pH: 6.5, while three samples show
water. The reacti.on of the analytic tests depends on the tem middle acid values of pH: 3.8, 4.5 and 4.9; the mean pH results
perature: below 20ºC we obtain a concentrati.on pro~sively 5.1. Such high variation of pH is normal, because every rain
lower than the real one. The low EC of water is a further war event has a different evolution and the chemical composition
ranty that the method is sufficiently selective to ntle out in of rain can change, in time and space, also during a same rain
terference by other chemical elements. event.
Some water samples were taken to ltaly for laboratory In the period February 23rd - March 7th 1996 a total pre
analysis of Ca, Mg, Li, Mn, Na and K, by atomic absorption cipitation of 60-70 mm was measured, almost ali concentrated
spectrophotometry. in a single event occurred in the February 24, when one sample
of rain was analyzed obtaining pH: 5.5 and EC: 6.3 µS/cm
Rainwater
Table l. Field analysis of rain water (Aonda platform)
The Auyán-tepui meteorological station data shows an av
date pH EC SiO2
erage monthly rainfall of 65 mm in February and 73 mm in
(m/d/y) µS/cm mg/1
March (GALÁN, 1992). During the dry season of February
02/20/93 4.55 15.9 <0.01
March 1993, rain was probably greater than the average, so 1
02/21/93 4.95 5.7 -
after a first period with only sorne middle-intensity rains, on 02/22/93 3.80 2.1 -
March 1-2, a storm yielded more than 30 mm of water. 02/24/93 6.48 4.3 <0.01
Rainwater was sampled on seven occasions and the mea 03/01/93 5.55 1.3 '1 <0.01
sured pH, EC and silica concentration are reported on Table l. 03/05/93 3.80 4.7 -
1
The pH of rainwater is generally acid, ranging from 3.8 to 6.5. 02/24/96 5.55 6.3 <0.01
-r--------------------------------
18 20
16 18
._ec .,_RAINFALL
14 16
e 14 _
12 E
ｾ＠ 12 ,5.
a 10
..J
10 ...1
M a 1ｾ＠
:e
c. 6 ._pH 8
8
4
4
2
2
1
1 I 1
o o
1 1 1 1 T T
17 18 19 20 21 22 23 24 25 26 27 28 1 2 3 4 5
FEBRUARY-MARCH1993
Fig. 4. Rainfall, electrical conductivity (EC) and pH of rainwater, in the period February-March 1993.
BoL Soc. Venezolana EspeL (33) 1999 5
Peat and surface water Table 2. Field analyses of water from ponds, peat bogs and dissolu
The central part of the Aonda pJatform is widely tion pans. See Fig. 3 and 10 for sample location.ANW: Auyán-tepui
covered by brushes, grass-carpets and peat deposits in Noroeste.
about 50% of the surface. Toe peat deposits occur in the station watertype date T pH EC SiO2 discharge
depressions on non fractured bedrock, with a thickness ºC µS/cm mg/1 1/s
ranging from 30 cm to 2 m. In 1993 and 1996 no 14 peat 02/29/96 4.11 26.9 0.19 0.1
researches concerning peat and vegetation were 5 peat 03/04/96 4.32 19.9 0.13 0.01
carried on, but we can assume that the situation on the 13 solution pan 03/03/96 4.45 13.8 0.15 sta~ant
1
Auyán-tepui could be similar to that described 13 solution pan 03/03/96 4.39 15.9 0.13 0.006
6 solution pan 03/04/96 4.52 14.1 0.02 stagnant
by BRICEÑO & SCHUBERT (1992), BRICEÑO &
12 pond 02/20/93 23.0 4.44- 12.1 0.02 stagnant
PAOUNI (1992) and BARRETO (1992) for the Chimantá
12 pond 02/24/96 4.20 23.4 , >0.01 stagnant
tepui. 1 ANW pond 02/21/93 18.3 3.7 26.9 0.43 stagnant
Runoff water, flowing through peat deposit gets en ANW pond 02/25/93 18.1 3.6 28.9 0.43 stagnant
riched with organic m.atter derived from decomposition
of vegetation. The organic solution load is responsible for the The analyses of ali the samples collected during 1993 and
charactéristic amber color of water on the surface and inside 1996 (Table 3 and 4), display a typical pattem with the m.axi
the caves, with the exception of cave dripping waters that are mum of EC in correspondence of pH: 4 (Fig. 7), the same reJa
completely transparent. tionship can be observed in 1he cave dripping water (Fig. 8).
Nine samples of stagnant waters were collected: 4 samples Figures 7 and 8 show the concentration of Si0 • The pH shows
2
in sm.all ponds, 2 samples from peat deposits and 3 samples an opposite pattem with respect EC, with a high correlation
from dissolution pans. Chemical analysis are reported in Table coefficient <R:z: 0.82 for stream water, 8i= 0.96 for cave drip-.
2. Ali the samples show a pH ranging from 3.6 to 4.5, with an ping water). Comparing the data of the water sampled on the
acidity sensibly higher than rainwater. The EC ranges from 12 tepuy, and the relation be~een SiO dissolution rate and pH,
2
to 29 µS/cm. Silica concentration is very variable, ranging after the experimental data of BENNETI et al (1988), KNAuss
from about O.CH to 0.43 mg/1. SiO derives entirely from the & WoLERY (1988) and BRADY & WALTHER (1989) (Fig. 9), we
2
dissolution of rock with a concentration that is related with the observe many analogies, being the lowest concentration of
time of water/rock contact and with the evaporation rate. SiO and 1he lowest quartz dissolution rate in correspondence
2
of the "zero point charge" (ZPC) of quartz (about pH: 3).
Suñace and underground stream water
The relationships between pH and EC of the runoff water Silica concentration in the Aonda and Carrao rivers
are presented in Fig. 5 and Fig. 6. The former concerns the Fig. 1Ó shows water sampling location along the Aonda and
Superior river (d ata from BELLOMO et al 1994 and GoRI et al. Carrao rivers from Auyán-tepui to Canaima, their analysis are
1993), the latter refers to the stream of Sima Auyán-tepui Norte presented in Table 3 and displayed in Fig. 11. The discharge is
2 at the north side of the Auyán-tepui studied in 1993. In both, an estímate from the superficial flow rate. _
growing the EC (always <20 µS/cm), the pH falls. The subterranean stream that flows through the Aonda
18 5.0
.,
4.8
17 pH
. JI, 4.6
.
16
- 4.4
.. -· . . .. ..
15 •
E 4.2
ª
() .
-u::zi . 14 - . . 4.o
....
o
w 13 3.8
. 3.6
12
.,
EC 3.4
1 ,
11
Rio Plataforma Aonda Superior 3.2
10 3.0
15 16 17 18 19 20 21 22 23 24 25 26 27
'AUGUST 1992
Fig. 5. Variations of electrical conductivity (EC) and pH in the Superior river (station 3, Fig. 3) (BELLOMo et al. 1994: 36).
6 BoL Soc. Venezolana·&peL (33) 1999
18 5.0 Table 3. Field aaalyses of water from rivers and cave streams.
---. See Figs. 3 and I O for sample location.
17 4.8 · Norte 2, ANW: Auván-teoui N
station date time T pH EC SiO2 discharge
4.6
16 •·•. EC (m/d/y) ºC i•S/cm mg/1 1/s
e · 4.4 31 02/23/96 9:00 5.12 9.9 2.12 ?
15 30 02/23/96 9:30 5.13 16.8 2.65 ?
4.2
-!:! 29 02/23/96 ll:00 4.96 9.2 3.44 ?
-
'g_ 14 4.0 i 28 03/03/93 11:00 4.74 9.6 0.76 >3000
27 03/03/93 I0:30 19,8 4.26 22.6 0.27 3500
fd 13 •• -·•· ·•-. -•- ·- pH 3.8 99 0033//0011//9933 1156::0300 34..9533 2116..44 00..4388 310500
-. ... -. --------- -- -----------· 3.6 9 03/04/93 17:00 4.39 19.3 0.23 800
12 .: 9 02/28/96 14:00 4.21 10.5 0.19 650
9 02/29/96 14:00 4.24 21.2 0.19 800
3.4
9 03/01/96 12:00 4.22 21.4 0.17 500
11
3.2 9 03/04/96 15:00 4.29 19.8 0.21 200
8 03/06/96 14:00 4.30 21.5 0.25 80
10 3.0. 4 03/03/93 10:00 17.2 4.39 18.2 0.19 2~00
21 22 23 24 25 26 4 03/01/96 12:00 4.22 21.3 0.19· 400?
4 03/02/96 16:00 4.22 21.1 0.16 400?
FEBRUARY 1993 4 03/04/96 13:00 4.28 21.2 0.15 100
7 03/02/96 12:00 4.21 22.0 0.15 60
3 03/04/96 14:00 4.28 21.4 0.17 100
Fig. 6. V ariations of electrical conductivity (EC) and pH in the stream of Sima Auyán-tepui Norte 2.
10 02/20/93 10:00 19.4 3.64 18.0 0.27 0.1
10 02/22/93 14:00 20.4 4.18 24.3 0.27 0.1
r
IO 02/24/93 14:00 4.27 23.1 0.40 l
35] 10 02/24/96 17:00 4.24 22.6 0.19 0.1
10 02/25/96 11:00 4.16 26.5 0.18 60?
11 02/27/93 14:00 4.66 11.7 0.92 2
• 3.5 - AN 02/20/93 18:00 17.2 3.7 13.4 0.21 10
30 :::¡ AN 02/21/93 14:00 19.1 4.4 17.0 0.21 4
t - ••• • S102 3.0 Oü; AANN 0022//2212//9933 1108::0300 1178..28 34..46 1167..04 0.21 · 146
r,.... 25 , • .e5., AANN 0022//2232//9933 1140::0000 1178..85 33..66 1177..13 0.21 s1o6
E ____ ｾ＠ 2.5 AN 02/24/93 L0:00 17.4 3.6 16.9 0.24 50
ｾ＠ $c(i;j ) 20 ..~ r, ,- _..,-- ｾ＠ ~--.•...·.,.. ... . .. 2.0 ¡oz:: AANN 0022//2245//9933 1188::0000 1177..78 33..65 1177..15 · 1s6
ｾ＠ o e( ANW 02/28/93 17.l 3.6 26.6
Se::," w . , • 1.5 a1z:-: AANNWW 0022//2288//9933 1177..24 33..67 2178..12
g 15 • ., wo
1 ' ' ., 1.0 z
o Table 4. Laboratory analyses ofwater.
,... \ o
10 • - • ·-t See FiJ-.ts. 3 and I O for samole I · ·
ｾ＠ '- EC STREAMS 0.5 Station date time Ca Mg . Li Mn Na K
ｾ＠ -- (m/d/v) m211 -mll/1 m2/I mg/1 mg/1 ml?/1
5 o.o 31 02/23/96 9:00 0.6 0.16 0.16 o 0.45 0.25
"ｾ＠""" 30 02/23/96 9:30 o 0.12 0.02 o 0.29 0.13
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 o
29 02/23/96 11:00 0.1 0.12 0.03 0.38 0.19
o o o o
pH 9 03/01/96 12:00 0.04 0.01
4 03/01/96 12:00 0.9 0.02 0.01 o 0.17 0.03
22 03/03/96 16:00 0.1 o 0.03 o O.IS 0.02
Fig. 7. Relationship between electrical conductivity (EC) (rhombuses) and dissolved Si0
2
concentration (squares) vs. pH, in surface and cave stream waters.
...J
35
·8
•
30 ' Si02 7 :-:.
.. . o
- ... . " .ｾ＠ -.. 6 ü;
25 • .. .. e,
E .. . , ' 5 .§.
u()i 20 EC ..., • ' ' '' oz
$ ' 4 ¡::
u '
w • ｾ＠
15 • 3 zｾ＠
• w
u
• 2 oz
10 o
DRIPPING WATER -1
5 o
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
pH
Fig. 8. Relationship between electrical conductivity (EC) (rhombuses) and dissolved SiO
2
concentration (squares) vs. pH, in cave dripping water.
-11
KNAUSS & WOLERY (1988) , •• ., ••
- -12 70ºC _.,···
DISSOLUTION RA TE OF QUARTZ _,,., B.RADY & WALT HER
ｾ＠ • .. -·,_., (1989)
E -13 , _. . ,··' 11 ....... ···eo0c
ｾ＠ . .. . -··-.. ,.. ...1.1 - ··· •. .-·
-O -14 ____ •• -·· ............. BENNETT et al. (1988)
E ....
-------.----·-.. -.... --.--· · ....... -· •.•- ··11·"' .~· / 25ºC
ｾ＠ ~---
-15 -·-···--··-
ｾ＠ !:::.:.-.-.-..=--·--··-·-·•-·-·-".. . -~
·---('--;:---
g» -16 ----... -------1-----------_---~ -------------·
....-- BRAOY & WAJ.. THER (1989)
..J ·------------·-•- - _.- 25ºC
-17
-18 l-----+---1----t-----t--~---,.---+----,.---+----,---+---t---;---~
o 2 3 4 5 6 7 8 9 10 11 12 13 · 14
pH
Fig. 9. Comparison of different dissolution rate of quartz vs. pH (experimental results) .
.
station date T pU EC Si01 de1,th Oischárge
(m/d/y) •e JtS/cm mg/1 · m
15 03/01/96 6.35 7.10 -300 1 drop/2s
16 02/23/93 17.4 3.67 13.4 1.60 -300 0.61/min
17 02/27/93 4.56 14.1 0.92 -300 quick <.lripping
18 02/27/93 4.86 10.2 1.26 -300 Stagnanl water
19 02/27fJ_3 4.89 9.9 1.18 -300 Sta211anl water
River drainagc F ebruary-March measured River dislance 20a 02/25/96 4.09 27.2 0.15 -80 c¡uick drippin2
slation areas mean discharge discharge from Canaima 20b 03/06/96 4.08 32.8 0.21 -80 quick dripping
km2 m1/s m1/s km 21a 02/26/96 16.77 4.47 16.0 0.34 -80. 3-4 drops/s
3 ? 2.5 57 21b 02/06(96 16.7 7 0.32 -80 1 drop/3s
9 ? 0.8 56 22a 03/03/96 4.13 28.4 0.35 -80 0.1-0.2 1/s
22a 03/05/96 4.13 29.1 0.35 -80 0.0~5 1/s
27 ? 3.5 55
22b 03/03/96 4.15 28.0 0.60 -80 1 drop/4s
28 35 0.63 30 SS 22c 03/05/96 4.36 21.0 0.90 -80 1 <.lrop/60s
29 7340 137 55 22d 03/05/96 4.10 29.5 0.35 -80 1 drnp/3s
30 7430 139 25 22c 03/05/96 4.12 JO.O 0.35 -80 1 drop/1s
31 7500 140 o 22f 03/06/96 4.23 24.0 0.74 -80 1 drop/15s
Table S. Estimated drainage areas and discharges Table 6. Field analyses of water dripping from cave walls and
at river stations. See Fig. 1O for sample location. ceiling. See Fig. 3 for sample location.
8 BoL Soc. Venezolana EspeL (33) 1999