Solution weathering rate and origin of karst landforms and caves in the quartzite of Auyan-tepui (Gran Sabana, Venezuela)

Geomorphology 106 (2009) 15–25

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Solution weathering rate and origin of karst landforms and caves in the quartzite of Auyan-tepui (Gran Sabana, Venezuela) Leonardo Piccini a,b,⁎, Marco Mecchia b a b

Dipartimento di Scienze della Terra, Universitá di Firenze, Italy Associazione Geografica La Venta, Treviso, Italy

a r t i c l e

i n f o

Article history: Received 3 October 2007 Received in revised form 8 July 2008 Accepted 18 September 2008 Available online 7 October 2008 Keywords: Quartzite Solutional weathering SiO2 dissolution Karst geomorphology Speleogenesis Auyan-tepui

a b s t r a c t The paper reports the results of SiO2 analyses in the Aonda Cave system, located on the Auyán-tepui, one of the widest table-mountains of the Gran Sabana (South Venezuela), characterised by karst landforms developed in siliceous rock. Chemical analyses underline the very low concentration of SiO2 of the surface water. Percolation and cave drip waters have a SiO2 concentration of about 1 mg/l. The mean silica load of the cave stream is 184 mg/s, mainly derived from surface solution removal in the allogenic recharge area. In the Aonda Cave system, the mean SiO2 dissolved load is 40 mg/s, in part from surface solution (15%) and mainly from underground processes (85%). The low solubility of SiO2 in slightly acidic water implies the importance of the time factor in the formation of cave systems. With the present dissolution rate, about 10 Ma would be necessary to form the known karst system. This estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years. Furthermore, this age can be taken as a minimum estimate, while, according to the geomorphic evolution of the area, the origin of the Aonda Cave system could be reasonably dated back to at least 20–30 Ma, that is, to the Oligocene. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Karst is usually developed in carbonate rocks or evaporites. Actually, solution processes affect all rock types but, in most of the Earth-surface environments, surface solution forms cannot significantly develop because physical denudation destroys them. Quartzite and quartz-rich sandstones are among the less alterable rocks, yet despite this some areas of the world exhibit complete karst systems as a result of solutional weathering (Wray, 1997a, and refs. therein). The best-developed karst landforms in quartz-rich rocks occur in the quartzitic mesas of the Gran Sabana (Guyana Shield, South America) (White et al., 1966; Urbani and Szczerban, 1974; Galán, 1988; Galán and Lagarde, 1988; Briceño and Schubert, 1990; Piccini, 1995; Doerr, 1999), in south-eastern Brazil (notably Minas Gerais) (Correa Neto, 2000), in central and southern Africa (Martini, 1979, 1985; Aucamp and Swart, 1991; Peyrot, 1997; Martini and Marshall, 2002) and in some areas of Australia (Jennings, 1983). Most of these areas are characterised by subdued relief and a rainy and warm-temperate climate, although quartzose-rock karst landforms are not restricted to such environmental conditions (Wray, 1997a). The low-gradient topography seems to be one of the most important factors for the development of karst in quartz-rich sandstones, because it enhances

⁎ Corresponding author. Dipartimento di Scienze della Terra, Universitá di Firenze, Italy. E-mail address: [email protected]fi.it (L. Piccini). 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.09.019

the role of chemical weathering in respect to mechanical denudation (Martini, 1979; Piccini, 1995). In the Gran Sabana (Venezuela, Guyana and Brazil), spectacular karst landforms occur in the orthoquartzite of the Roraima Group. The karstic character of the landscape is underlined by the fact that the drainage of rainwater is locally through underground pathways, with spectacular resurgences on or at the base of the high cliffs that border the major table-mountains. Presently, a fully exhaustive evolutionary model of such karst landforms is still lacking, and the timescale of development of such impressive subterranean networks is still a matter of debate. Actually, the mesas of Gran Sabana have been affected by weathering from at least the Cretaceous, in a state of almost absolute tectonic quiescence (Briceño and Schubert, 1990), thus the time of formation of the karst landscape and caves could entail several tens of millions of years. In 1993 and 1996 two research missions investigated some areas of the Auyán-tepui, one of the widest table-mountains of the Gran Sabana region, in southern Venezuela. During the two expeditions the team of Italian and Venezuelan cavers explored several caves, among which were the Sima Auyán-tepui Noroeste (depth −370 m, length 2950 m), and the Aonda Cave System (depth 360 m, length 1880 m), two of the major quartzitic caves in the world. A complete topographic and hydrologic survey of the caves was performed, along with field chemical analysis of surface and subsurface waters. On the grounds of the morphological features of the caves, hydrologic measures and analytical results, we are able to estimate the dissolution and weathering rate of rock in different hydrodynamic

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conditions and to infer the rate of development of caves in quartz-rich sandstones. 2. Geological, geomorphological and hydrological setting The Gran Sabana is a province of the Guyana shield, in the Orinoco River basin, which is located within Venezuela, Guyana and Brazil. The major elevations have the shape of wide table-mountains, locally named tepui (a word meaning “mountain”). The tepui are usually delimited by vertical to overhanging walls, often from 400 up to more than 1000 m high. Because of the difficulties in reaching them, many of these plateaus are not yet fully explored; the Auyán-tepui is one of the largest of this region, and consists of a roughly triangular, plateau with an area of about 700 km2 (Fig. 1). The Auyán-tepui, as for most of the Gran Sabana region, is made up mainly of silicate arenites of the Roraima Group (Reid, 1974; Ghosh, 1985; Santos et al., 2003). The arenaceous formations of this stratigraphic group display continental to peri-continental facies, whose ages range from 2.3–1.8 Ga to 1.8–1.4 Ga (Briceño et al., 1990). A low-grade metamorphism, with quartz–pirophyllite paragenesis in the shaly beds, is interpreted as the result of the lithostatic load of a presently eroded thickness of approximately 3 km of rock (Urbani et al., 1977). Metamorphism is responsible for the overgrowth of quartz grains that leads to a saccaroid structure of the orthoquartzite. The plateau and the peripheral scarps consist of orthoquartzites to subarkoses with subordinate middle to fine-grained lithic wackes (Matauí Formation); in the central part of the area a cover of basic volcanic rocks occurs (Fig. 2). Protoquartzites, arkoses and wackes, with beds of chert, lutite and siltite (Uaimapué Formation) commonly form the pediment of the table-mountains. The wide pedemontane plain consists of siltstones and shales of the Kukenan Formation. The summit plain descends from the east, where highest elevations are at more than 2800 m a.s.l., towards the west where the height reduces to 1500–1600 m; the mean elevation is roughly 2000 m. The drainage is mainly centripetal, towards the deeply entrenched valley of Rio Churún that receives a large amount of water from several lateral waterfalls. The highest one is the impressive and

world-renowned “Angel Falls”, with an almost 1000 m single drop, but several other waterfalls are higher than 500 m. Briceño and Schubert (1990) consider the uppermost plain of the plateau as a part of a planation surface named “Auyán-tepui Surface”. The age of this ancient peneplain is not known, because of the lack of any temporal element to date it; the authors argue a Triassic–Jurassic age, that is to say before the separation of South America from Africa. The lower planation surface, a typical “pediplain” with a mean elevation of about 1000 m (King, 1956), represents the major lowland of the Gran Sabana region and is referred to as the “Gondwana Surface” of Brazil and Africa, being Cretaceous or older in age. Between these two major planation surfaces, the Auyán-tepui shows several intermediate plains and benches, which have developed step-like profiles. These surfaces are the result of several cycles of selective erosion, controlled by lithologic changes. Usually, the widest platforms are formed on fine-grained orthoquartzites, capping coarse and more erodible beds. The area studied is a small sector in the NW of the Auyán-tepui. The relief is shaped as an upper plain and minor peripheral bench at lower altitudes. The major flat surfaces are commonly developed on a hard bed of fine quartz–arenite. This hard “cap” overlies a sequence of medium to coarse quartz–arenites, white or pinkish in colour, with cross-laminated beds. The beds are almost perfectly horizontal. The main tectonic features are sets of vertical fractures, which cut the platform into rectangular to rhombic prisms some tens of meters wide (Fig. 3). In the areas investigated the main sets of fractures are NNW– SSE and NE–SW oriented. The Aonda Cave System is one of the largest known caves of the Gran Sabana region, consisting of 11 explored caves, with a total passage development of about 5.6 km; nevertheless, many deep shafts are waiting to be investigated. All these caves belong to a single hydrologic network, although connected with non-accessible passages, which represents the underground drainage network of a stream (Río Superior) captured by some deep fissures at the eastern border of the Aonda bench (Fig. 4). Along the subterranean pathway the stream is visible at the bottom of four vertical caves; the water flows out in the western peripheral scarp, after a linear deep pathway

Fig. 1. Oblique aerial view of the Gran Sabana region, with the typical profile of a tepui rising up from the lowland.

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Fig. 2. Location map and geological sketch of Auyán-tepui: 1) quarzite of Matuì Formation; 2) basic volcanic rock.

of 1.5 km. The large depression of the Sima Aonda also drains the water of some minor surface streams, which fall directly into it with a drop of more than 350 m. All the waters flow together and emerge from the peripheral scarp, through the remarkable Resurgencia Aonda, about 100 m above the base of the wall (Fig. 5). Hydrological data are few. Rainfall can be estimated on the grounds of 3 years of measurements provided by a weather station, located at 2000 m a.s.l. in the south of Auyán-tepui, from 1980 to 1983 (Galán, 1992). Mean annual precipitation is about 3000 mm, whereas the annual runoff can be evaluated at about 2300 mm (Galán, 1992; Mecchia et al., 1994). The discharge ranges of the main stream flowing through the Aonda Cave can be estimated on the basis of a few and approximate measurements referred to the dry season (February–March): Rio Superior – from a minimum of about 0.1 m3/s to a maximum of 2.5 m3/s; Aonda cave stream – from 0.05–0.1 m3/s to more than 2 m3/s; Resurgencia Aonda – from 0.15 m3/s to 3.5 m3/s (Mecchia et al., 1994; Mecchia and Piccini, 1999). During rainstorms, the stream commonly reaches a flood peak of more than 5–6 m3/s. In the period of our second mission, from 24 February 1996 to 5 March 1996, the weather was characterised by an intense storm on the February 24, in which about 60 mm of rain fell in a few hours. In the whole period, rainfall was 97 mm. The mean discharge of cave stream was roughly evaluated on the basis of daily measures, assuming an approximate peak discharge of 6 m3/s (deduced from the marks left in the caves by the maximum water level). According to these few and

coarse data, the mean discharge of the Aonda stream was, in the period of our fieldwork, of the order of 1.5 m3/s, with an error probably greater than 20%. Since the runoff time is almost certainly less than 24 h, comparing the estimate mean discharge with the rainfall in the period of our second mission, the catchment area of the stream results to 13.4 km2. Assuming a mean runoff of 2300 mm per year (Galán, 1992), the annual average of discharge of the Aonda cave stream should range from 0.78 to 1.17 m3/s. In short, although our researches have concerned the dry season, we believe that hydrological and chemical results are quite representative of the mean conditions. 3. Methods and results Chemical analyses have been undertaken on rain, occasional ponds, peat deposits, surface and subterranean streams, and cave drip water. Temperature, pH, and electrical conductivity (EC) were measured by field portable instruments. SiO2 concentration was measured by a field colorimetric test (Merck – Aquaquant 14410 Silicon), which allows the detection of silica in the concentration range 0.01–0.25 mg/l, with an error less than 20%. The samples with a higher concentration were diluted with distilled water and then analysed. Discharges were obtained measuring repetitively the flow speed by floating objects and calculating the cross section of the stream in well-selected sites, where cross section had a regular form and the flow of water was sufficiently slow and not so turbulent. Mean velocity

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Fig. 3. Deep open fissures enlarged by subsurface weathering and collapse (photo: arch. La Venta). The area is about 100 m wide.

was estimated as 80% of surface velocity. Cross section of the streams were measured with a rigid meter. For this reason, they can be considered only as indicative and are probably affected by an error of about 30%. A detailed report of the analytical results has already been presented in a previous paper (Mecchia and Piccini, 1999); here, we briefly recall the main characteristics of the different kinds of water. Fig. 6 shows the location of sampling sites in the Aonda area, inside and all around the cave system. 3.1. Rainwater The pH of rainwater is usually acidic, ranging from 3.8 to 6.5; the mean pH is 5.1. EC is always very low (1–16 µS/cm), and silica is absent. Such a low EC is due to the long distance from the sea (the natural

source of salty aerosol) and from human activities and industries (source of dust and pollution). A sample of rainwater, after a week of no precipitation, had an EC of 16 µS/cm, whereas the water collected during a storm had a very low EC, under the accuracy range of the instrument (1.3 µS/cm; i.e., pure water). 3.2. Peat and surface water The central part of the Aonda platform is widely covered by brushes, grass-carpets and peat deposits (about 50% of the surface). The peat occurs in the depressions in non-fractured bedrock, with thicknesses usually ranging from 30 cm to 1 m. The water, flowing through the peat, gets enriched with organic matter, derived from the decomposition of vegetation, which is responsible for the characteristic amber colour of the water.

Fig. 4. Sketch profile of the Aonda karst system (RAP = Resurgencia Ali Primera).

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Fig. 5. The resurgence of the cave system, about 400 m below the rim of the peripheral escarpment of the Aonda bench (photo: arch. La Venta).

All the samples have a pH ranging from 3.6 to 4.5, significantly lower than that in the rainwater. Such a low pH is probably due to organic acids (Briceño and Paolini, 1992), but no analytical data are available for this area. The EC ranges from 12 to 29 µS/cm. Silica ranges from minimum detection level (0.01) to 0.43 mg/l, and it is apparently related with the evaporation rate and with the time of water/rock contact, that is to say the time elapsed from the last rain event. Table 1 shows the most representative results of chemical analyses on surface water. 3.3. Stream water The water sampled just upstream of the sinking point of Rio Superior (sampling sites 3 or 4; Fig. 6), has a SiO2 concentration persistently below 0.3 mg/l and is significantly under-saturated. This is due to the fact that the catchment area of the stream consists only of quartzite of the Matauí Formation; furthermore, the flow is quick as demonstrated by the fast fluctuation of discharge in respect to rainfall. Along the subterranean path of the main stream, we observe only a

slight increase of SiO2, while downstream of the Resurgencia Aonda (sample sites 9 and 28) the silica is significantly higher, reaching a concentration of 3.4 mg/l in the water of Rio Carrao. Such a rise of silica is due to the fact that in the pediment and along the plain, the streams flow on feldspathic sandstones, where the rock has been altered through hydrolysis and the waters are also enriched with Na and K (Mecchia and Piccini, 1999). The higher solubility of feldspars and other silicate minerals, in respect to quartz, and the longer time of water–rock contact, explain the increase of silica and electrical conductivity in the Río Carrao. Table 2 shows the SiO2 concentration in the water sampled upstream and downstream of the subterranean path of the cave watercourse. The increase in dissolved silica is significant and is almost entirely due to percolation waters collected along the subterranean path. 3.4. Underground water The SiO2 content of the stream flowing through the Aonda Cave was repeatedly measured at the Resurgencia Ali Primera (sample

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Fig. 6. Location of sampling sites of surface, stream and cave waters.

site 9) under different discharge conditions. Generally we observed a regular decrease of concentration with the growth of discharge (min. 0.17 mg/l–max. 0.48 mg/l, SiO2). Conversely, the dissolved silica load rises with the increase of the discharge. This suggests that during a storm the SiO2 enriched water, trapped in the peat deposit, surface depressions and fissures, is washed away by rain. Percolation and cave drip waters differ from surface and cave stream waters, since they are uncoloured and significantly more enriched in silica, with a typical concentration of about 0.9 mg/l. A sample of water falling on a dripstone of opal had a concentration of 7.1 mg/l SiO2 and was the only sample over-saturated with silica, with respect to its pH (Brady and Walther, 1989). Drip water was collected in two caves, at the depth of 80 and 290 m below the ground level. Although the variability of silica concentration is very large, a positive trend of SiO2 versus depth exists (Fig. 7). In short, the results of chemical analyses allow the following considerations. All the analysed waters are significantly under-saturated with respect to silica, except for one feeding a dripstone. During the low-flow periods, the stream at the sinking point has a discharge ranging from 0.1 to 0.4 m3/s while SiO2 is normally 0.15– 0.19 mg/l. At the resurgence the discharge is almost 0.1 m3/s higher, whereas the dissolved silica ranges from 0.17 to 0.48 mg/l. The increase in discharge is mainly due to contributions from surface waters via tributary caves, whereas the increase of the silica must be due to SiO2 enriched percolation water collected along the cave passages. During a moderate flood event (see Table 2), the increase of discharge in the underground path from 2.5 to about 3.5 m3/s, was

Table 1 Field analyses of water from ponds, peat bogs and solution pans Sample site

Water source

Date

T

m/d/y

°C

14 5 13 6 12

peat peat solution pan solution pan pond

02/29/96 03/04/96 03/03/96 03/04/96 02/20/93

See Fig. 6 for location of sampling.

23.0

pH 4.1 4.3 4.4 4.5 4.4

EC

SiO2

µS/cm

mg/l

27 20 14 14 12

0.19 0.13 0.15 0.02 0.02

Discharge 0.1 l/s 0.01 l/s stagnant stagnant stagnant

accompanied by an increase of dissolved silica (from 0.19 to 0.27 mg/l), which can be explained by a piston-effect on the water trapped in the network of subcutaneous fissures of the epikarst. In short, the analysis of surface and cave waters displays that the silica load of the Aonda cave stream is in part from surface solution (peat, ponds, pans) and partly from subsurface processes. According to the data (Figs. 7 and 8) we can assume that about 15% of the silica load comes from surface weathering while 85% comes from underground solution. These results demonstrate that slow percolation along vertical fissures or vertical movement of water films on cave walls are the only situations where a significant dissolution of quartz can occur. All these measures refer to the dry season, when the discharge of streams is appreciably lower than during the rainy season. We have no data concerning the silica concentration during wet season. Probably, during a period of continuous rain, the dilution effect causes a significant decrease of SiO2 concentration. 4. Karst and cave features Despite the siliceous nature of the rock, the summit plane of Auyán-tepui exhibits solutional landforms typical of karst landscape: Table 2 Field analyses of water from surface and cave streams Sample site 4 4 4 4 9 9 9 9 9 9 9 27 28

Water kind sinking stream sinking stream sinking stream sinking stream cave stream cave stream cave stream cave stream cave stream cave stream cave stream surface stream surface stream

Date

T

m/d/y

°C

03/03/93 03/01/96 03/02/96 03/04/96 03/01/93 03/01/93 03/04/93 02/28/96 02/29/96 03/01/96 03/04/96 03/03/93 03/03/93

17.2

See Fig. 6 for location of sampling.

19.8

pH 4.4 4.2 4.2 4.3 3.9 4.5 4.4 4.2 4.2 4.2 4.3 4.3 4.7

EC

SiO2

µS/cm

mg/l

m3/s

18 21 21 21 21 16 19 20 21 21 20 22 10

0.19 0.19 0.16 0.15 0.48 0.38 0.23 0.19 0.19 0.17 0.21 0.27 0.76

2.50 0.40 0.40 0.10 0.15 0.30 0.80 0.65 0.80 0.50 0.20 3.50 N30

Discharge

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Fig. 7. SiO2 concentration versus depth of surface and cave waters.

karren-like forms, stone-forests, sinkholes, caves and impressive shafts. The runoff waters are also drained through subterranean routes, mainly in the peripheral zones of the plateau. The geomorphic setting of the Auyan-tepui has been widely described by several authors (Urbani, 1986; Galán and Lagarde 1988; Briceño and Schubert 1990; Urbani, 1991; Piccini, 1995); all of them agree that this landscape is due to chemical weathering processes. The importance of the chemical solution of quartzite is well emphasised by small-scale solution forms: rills, pans, pits, and small pockets covered by algae (Fig. 9). Mechanical erosion is active too but, due to the low-gradient topography, its effect is relevant only close to the rim of the plateau and, particularly, along the streams inside the active caves. In other words, the energy of runoff waters is generally greater in the subterranean drainage systems than in the superficial ones.

On the basis of aerial surveys, performed by helicopter on a large part of the Auyan-tepui, major sinkholes and large shafts occur only in the peripheral part of the plateau or near the secondary scarps of intermediate plains. This suggests that stress release processes control the development of caves: usually, the higher the scarps, the larger the distance of caves from the rim. The formation of a peripheral shaft triggers a further mechanic release of surrounding rock, allowing, under particular conditions, a progressive inward retreat of open fractures and cave formation. This is probably the case of the Aonda platform, where we find large shafts up to 700 m from the rim of the platform. The initial stage of the formation of caves is believed to be mainly by solution processes along vertical joints and piping on bedding planes (Martini, 1979; Galán and Lagarde, 1988). Subsequently, the enlargement is mainly by collapse processes and erosion (Piccini,

Fig. 8. Synoptic sketch of mean silica concentration of different kinds of water in the Aonda cave system.

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5.2. Weathering processes

Fig. 9. Small-scale solution ponds in the quarzite (dimension of photo is about 1.5 m).

1995). The narrow fissures, which represent the initial stages of the shaft formation, are usually found relatively far from the boundaries of the plateau, while the large chasms, which we found trending towards the peripheral scarps, represent the last evolution stages. The largest shafts, named with the Spanish word sima, are the major landscape features of the Aonda platform and their dimensions are sometimes enormous: the “Sima Aonda”, for example, is 360 m deep, 400 m long and about 100 m wide. The origin of simas is mainly due to the collapse of active deep passages, enlarged by lateral erosion until their dimensions are such to cause the collapse of the overlying rock. Afterwards, running water erodes the collapsed blocks mechanically. The void so formed extends progressively upward; when it reaches the surface, a large and deep sinkhole is formed. The largest and deepest chasms have a huge chaos of blocks at the bottom, which can fill a large part of the original void. The morphology of underground passages is simpler than in carbonate rock; their shape is usually controlled by the bedding planes and by the joints, as we can particularly see in the collapse chambers but also in the erosional conduits (Fig. 10). All the explored caves show a pattern deeply controlled by the joint sets orientation. As the evolution proceeds, simas extend in the direction of the main fracture, joining together in a rectangular network of pseudocanyons that open towards the external cliff of the plateau. The progressive enlargement of canyons leads to the formation of quadrangular towers. When these towers collapse, they give origin to an impressive chaotic array of huge rock blocks. This long process causes the progressive retreat of the peripheral and secondary scarps.

The slight metamorphism of the rock and the re-crystallisation of quartz grains have produced a mosaic structure, where the original shape of grains is no longer recognizable. This process facilitates the disintegration of the rock because it is not necessary to remove all the cement (probably more than 30% in the old arenite), but only a part of the overgrowth quartz along the boundaries of grains (Martini, 1982; Doerr, 1999). This process has been called “arenisation” by Martini (1979) and, according to microscopic observations, the rock is almost completely disintegrated and removed by flowing water when the dissolution has removed about 15–20% of the quartz (Fig. 11). As amorphous silica cement is only 2–4%, its removal does not produce any disintegration of the rock, thus the chemical weathering of quartzite necessarily involves a relevant part of the grains (Chalcraft and Pye, 1984). The arenisation process acts mainly underground, along small fractures and bedding planes, where a film of water always wets the rock. Furthermore, along narrow fissures, the low velocity of the water allows a greater time of reaction between water and rock. Conversely, on the surfaces of rock exposed to meteoric weathering, the alternation of wet and dry conditions leads to the capillary movement of pore-water and the deposition of a thin and hard crust of silica cement and iron oxides, which preserves the rock from further alteration. As a result, on the flat free surface and along the walls of canyons and wide shafts, the arenisation process cannot act significantly and the rock is extraordinarily hard. Inside the caves, the inter-granular dissolution of quartz leads to the formation of fine sand that is easily removed by running waters. Rock softening along joints also causes the fall of blocks inside the larger cavities due to the progressive weakening of the rock mass.

5. Surface and subsurface weathering of quartzite 5.1. Petrographic features of quartzite Pouylleau and Seurin (1985) provide a chemical analysis of unweathered quartzite of the Roraima Group, obtaining the following results: quartz = 94.67%, combined SiO2 = 3.38%, Al2O3 = 0.78%, MgO = 0.25%, Fe2O3 = 0.16%, CaO = 0.14%, TiO2 = 0.06%, K2O = 0.01%, insoluble elements = 0.78 %. Ghosh (1985) reports the mineralogical composition of 21 samples of quartzose sandstone of Roraima; SiO2 is always more than 93% with a mean percentage of about 99%. The remaining part consists of feldspars; siliceous matrix ranges from 0.2 to 8%. No mineralogical analysis exists concerning the sandstones of the Auyan-tepui, but an examination with a petrographic microscope reveals the presence of quartz as almost the only component. Amorphous silica usually ranges from 2 to 4%; porosity of unweathered quartzite is commonly less than 1% (Rigamonti, 1995; Doerr, 1999).

Fig. 10. A joint-controlled active conduit in the Sima Auyán-tepui Noroeste (photo: arch. La Venta).

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Table 3 Main geomorphic conditions and erosion processes of quartzite in the tepui Condition Low gradient surfaces of bare rock

Humidity

Weathering Weathering Dominant process rate process

Wet and dry Formation of silica hard-crusts Rock under soil Always wet Moderate and vegetation softening Vertical rocky surfaces Occasionally Moderate on walls and wide shafts dry softening Deep fissures and caves Always wet Strong softening

Negligible

Negligible erosion

Medium Slow

Solution weathering Rock falls

Medium– high

Running water erosion

These different SiO2 concentrations produce a chemical potential that allows the removal of silica from the rock without significant movement of water. This process also explains the higher SiO2 concentration of cave water dripping from small fissures. Table 3 summarises the main geomorphic conditions and weathering processes that affect the quartzite. 6. Discussion

Fig. 11. Schematic steps from diagenesis to weathering of quarzite: A) uncemented quarzitic sand, B) arenite cemented by amorphous silica, C) re-crystallized quartzite due to grains over-growth, D) weathered quartzite.

Along the cave stream, where the rock is continuously eroded, the quartzite has an intermediate hardness. The thickness of weathered rock on the cave walls, far from the entrance and where the rock is always wet, occasionally exceeds 25– 30 cm. This softening process could be explained assuming that SiO2 is reworked towards the external rock surface, due to the chemical diffusion along the water filled porosity (Fig. 12). Actually, while the external film of water is continuously renewed, also by condensation processes, and thus under-saturated, the water inside the intergranular porosity should be at saturation point.

Fig. 12. Dissolution process of quarzite (arenisation) due to chemical diffusion of silica along inter-granular porosity. The graph shows qualitatively the SiO2 concentration change in pore waters.

The Auyán-tepui is one of the best-studied quartzitic massifs in the world, at least from a speleological point of view. Many of the explored caves have morphological features that involve the action of different geomorphic agents under particular environmental conditions. The peculiar surface landforms are mainly the result of chemical weathering processes. The solution of quartz along joints has a very important role in the initial stage of cave origin, whereas in the last stages the widening is mainly by erosion and collapse, forming big shafts and sinkholes (simas). This circumstance, along with the existence of subterranean drainage, leads to the definition of the landscape of the tepui as a karstic terrain, as already proposed by Urbani (1986, 1991) and more recently by Wray (1997b). The development of a karst-type landscape has been possible because the environmental conditions have limited the effects of mechanic weathering, allowing, over a very long time, the development of solution forms. Actually, these plateaus have been subject to weathering from, at least, the end of the Cretaceous in a state of almost absolute tectonic quiescence and with a very low morphologic gradient (Urbani, 1986; Galán, 1988; Doerr, 1999). On the surface of the plateau, we see mainly landforms due to selective denudation. The small-scale ones are controlled by lithology, while the large-scale ones are controlled by tectonic features. The recurrent change from dry to wet conditions, leads to the formation of a hard crust of silica that preserves the rock from further weathering. In these conditions the denudation rate is extremely low, allowing the development of endokarst forms through a slow process of subsurface weathering and piping (Martini, 1982). As in limestone karst, caves can form only if an embryonic pathway exists, allowing the movement of water along fissures. Successively, conduits are formed when piping moves the weathered rock away along bedding planes to springs. This process can develop only close to a scarp, and it is triggered by joint release and gravitational deformation of the peripheral part of the plateau. Major caves occur only in the marginal zone of a tepui, whereas in the central area of the plateau, minor caves can develop only close to secondary scarps. In the initial stage, joint release and gravitative deformation can involve only the volume of rocks close to the rim of the plateau and can roughly extend a distance from the rim of the same order of magnitude as the relief of the scarps. Because the process of cave formation is very slow, it can proceed only when scarp retreat is much slower than cave development. All this implies that there are some crucial morphological and climatic conditions

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for the development of caves in the quartzite of Gran Sabana: such conditions allow us to infer some chronological constraint for the origin of these caves. The landscape of Gran sabana displays a morphology that is typical of arid or semi-arid zones: plateau, mesa, vertical scarps and erosional pediments (King, 1956). According to some authors (Briceño and Schubert, 1990) the area was moulded during a period of arid climate that can be referred to the severe continental conditions that occurred before the separation of South America from Africa (Triassic–Jurassic). An arid or semiarid climate probably persisted even during the first phase of continental rifting. Regional uplift and the rearrangement of river drainage furnished the gradient for the formation of canyons and the downcutting of main rivers (Potter, 1997). In these environmental frameworks, mechanical processes override chemical weathering and solution forms cannot develop. These morpho-climatic conditions probably persisted until the Late Cretaceous. Karst systems began to form when the climate became sufficiently humid to allow a significant role for chemical weathering and when the morphological setting was not so different from the present one. Dissolution and deep arenisation acted mainly along release joints close to the rim of the plateau. Cave formation could begin only when the downcutting of rivers allowed the development of scarps, and when the climate became sufficiently humid to allow a significant action of chemical weathering (Fig. 13). This situation probably did not occur before the Early Tertiary. The lack of intermediate sub-horizontal cave levels, from the surface to present active levels, seems to suggest that cave development did not start during the downcutting phase of the river network. Furthermore, the position of peripheral springs is controlled mainly by lithologic factors, suggesting that the base level was already situated at its present elevation. A minimal age for the origin of the studied cave system can be speculated by comparing cave surveys and dissolution rates obtained from chemical analysis of surface and cave waters. Our few analyses allow only a rough computation of mean SiO2 load, however it seems that only small variations of silica concentration occur. The mean SiO2 close to the resurgence of the cave system was 0.27 mg/l, with a discharge ranging from 150 to 800 l/s, whereas the SiO2 concentration at the sink point was usually 0.19 mg/l. The difference is 0.08 mg/l and we have supposed that 85% of it is due to subsurface solution processes. Assuming, for simplicity, a mean

discharge of 1000 l/s, the SiO2 load would result in 68 mg/s, i.e. 2.14 tons (about 0.8 m3 of rock) per year. Another approximate computation can be made estimating the direct infiltration on the karst system. On the ground of speleological surveys and on the increase of discharge from sinking and resurgence points, the area of autogenic recharge is approximately 1.4 km2. Infiltration is about 100% of effective rain, thanks to the dense net of open fractures and sinkholes. Thus, mean infiltration is about 3.2 × 106 m3 per year, that is to say about 100 l/s. Percolation water has a mean SiO2 concentration of 0.9 mg/l, 85% of which comes from underground dissolution. In other words, annual dissolution removal from the Aonda platform results in roughly 2.4 tons per year, which is not so different from the previously calculated value. Such concentrations of stream and percolation waters refer to dry season and to discharges that are up to half of the estimated average. Lacking analysis during high-flow season, we can cautiously assume that above a discharge of 0.5 m3/s there is no increase of solute load. So we can suppose a dissolution rate of 1.2 tons per year (i.e. about 0.45 m3 of rock) as more probable. The real volume of the Aonda cave system cannot be calculated, due to the incomplete exploration of its cave passages. Most of the volume is anyway that of large simas, a large part of which has been already surveyed. The volume of explored major sinkholes is about 12 × 106 m3, whereas minor explored passages have a total volume of 1.3 × 106 m3. Many other shafts have not yet been explored, but they are of minor dimensions. The increase of silica in the cave system is due also to dissolution in the epikarst zone, whose volume is really difficult to evaluate. Aerial photos show a net of fissures enlarged by weathering that covers about 5% of the area. Typical depths of fissures range from 20 to 60 m. Taking a mean depth of 40 m, the volume of the net of fissures can be estimated at 3 × 106 m3. In short, we believe that the total volume of caves and epikarst fissures should be around 25 × 106 m3. Assuming that arenisation implies dissolution of 20% of the rock, 5 × 106 m3 of rock must be chemically destroyed and washed away as solute load. With the present estimated dissolution rate (0.45 m3 per year), 11 Ma would be necessary to form the present karst system of the Aonda platform. The same results also allow an estimation of surface denudation rate, with solute load due to surface weathering being 0.08 m3 per year. Assuming that arenisation acts on the surface too, the denudation rate results to 0.4 m3 per year, that is to say 0.28 mm/ka.

Fig. 13. Geomorphic evolution stages of the Aonda cave system in respect to table-mountain development in the hypothesis that most of the base level fall happens in the first stage of river downcutting. Ages are only speculative.

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7. Conclusions Many authors accept the possibility that caves in the quarzite of Roraima can be extraordinarily old, but estimations have been hypothetical and often speculative. The importance of the time factor is suggested by the low solubility of SiO2 in slightly acidic water. Mechanical–erosive processes are active too, but only along the streams, mainly near the border of the plateau, and inside the active caves. Cave exploration and chemical analysis of surface and cave waters provide a first set of data that can allow a tentative estimation of the age of the caves. As already stated, comparing the hypothetical volume of the Aonda cave system with the mean SiO2 load increase between the sinkhole and the resurgence of the cave, about 11 Ma are needed to form the present cave system. This age refers to the stage with underground passages subject to running waters, while the inception stage, guided by slow dissolution along stratigraphic planes and joints and successive piping, could be much older. Furthermore, this estimation can be accepted as relatively significant only if we assume that climate has been stable in the last tens of million years. Otherwise we think that this age can be taken only as a minimal estimation because: (i) the volume of the cave system can be significantly greater than that calculated through the known passages, (ii) the mean discharge through the cave must have been increasing during the development of caves, (iii) it is difficult to imagine rainfall significantly greater than now, whereas arid stages certainly occurred during the Quaternary at least. Taking into account all these hypotheses, the origin of the Aonda Cave system could be reasonably dated back to ca. 20–30 Ma, that is to say during the Oligocene. In conclusion, although silica and calcite dissolution rates differ by two orders of magnitude, the time for cave formation in quartzite seems to be only 10 times greater than in classical karst. Acknowledgements We are indebted to the Sociedad Venezolana de Espeleologia, and particularly to Prof. Franco Urbani, for the significant organizational and scientific support during the scientific missions and for the useful discussions. We thank our friends of the Associazione “La Venta” for technical and logistical help. Jo De Waele, Stefan Doerr, Russell Drysdale, and two anonymous referees greatly improved the draft of the paper. The research has been partly supported by a financial contribution of the Associazione Geografica “La Venta”. References Aucamp, J.P., Swart, D.P.R., 1991. The underground movement in Zimbabwe. Bulletin of South African Speleological Association 32, 79–91. Brady, P.V., Walther, J.V., 1989. Controls on silicate dissolution rates in neutral and basic pH solutions at 25 °C. Geochimica et Cosmochimica Acta 53, 2823–2830. Briceño, H.O., Paolini, J., 1992. Aspectos geoquimicos del macizo del Chimantá. In: Huber O. (Ed.), El Macizo del Chimantá – Escudo della Guayana, Venezuela – Un ensayo ecologico tepuyano. O. Todtman Editors, Caracas, pp. 75–88. Briceño, H.O., Schubert, C., 1990. Geomorphology of the Gran Sabana, Guayana Shield, Southeastern Venezuela. Geomorphology 3, 125–141. Briceño, H.O., Schubert, C., Paolini, J., 1990. Table-mountain geology and surficial geochemistry: Chimantá Massif, Venezuelan Guayana Shield. Journal of South American Earth Science 3, 179–194.

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