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: (02)-978.31.77 /272.07.24 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. Los originales para publicación, catastro, revisión de libros y bibliografias, deben enviarse a la Comisión Editora, previamente de haber seguido las pautas expuestas en las "Instrucciones a los Autores", que aparecen en el retiro de contraportada de este Boletín. Todos los originales y correspondencia deben ser enviados a : Comisión Editora, Sociedad Venezolana de Espeleología Apartado 47.334, Caracas 1041 A, Venezuela. La Comisión Editora está formada por: Editor: Carlos Bosque (SVE, USB). Editores asociados: Pedro Aso (SVE, USB), 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. Sin embargo, los autores son los únicos responsables del contenido de sus artículos. 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