Rendiconti Seminario Facoltà Scienze Università Cagliari Supplemento Vol. 71 Fasc. 2 (2001)

Some examples of karst phenomena in the Sulcis region

JO DE WAELE(*), FRANCO FRAU(**)

Abstract. In this paper is described the field trip which brought almost 50 participants of the WRI-10 to visit the Sulcis region in south-west Sardinia. This excursion led across a great range of geomorphological landscapes and a wide lapse of geological time. The first stop concerned the Riomurtas karst depression near Narcao, with interesting sinkhole collapse phenomena related to withdrawal of the water table by overexploitation of the karstic aquifer. Near Nuxis, a small carbonatic outcrop with interesting karstic microforms was the object of the second stop, whereas the third stop was at the Is Zuddas cave close to Santadi. The visit of this cave permitted the participants not only to admire striking aragonitic helictites, beautiful calcite concretions and various typical cave deposits, but also to see the results of a geo-environmental monitoring of this cave and its fragile environment.

Riassunto. In questo lavoro viene descritto il field trip che ha portato quasi 50 congressisti del WRI-10 a visitare la regione del Sulcis, nella Sardegna sud-occiden- tale. Questa escursione ha interessato un’ampia varietà di paesaggi geomorfologici riferibili a diverse epoche geologiche. Il primo stop ha riguardato la depressione carsica di Riomurtas vicino a Narcao, con interessanti fenomeni di crollo riconducibili all’abbassamento della tavola d’acqua in seguito a sovrasfruttamento dell’acquifero carsico. Vicino a Nuxis, un piccolo affioramento carbonatico con interessanti microforme di erosione carsica ha rappresentato l’oggetto del secondo stop, mentre il terzo stop ha riguardato la grotta di Is Zuddas presso Santadi. La visita della grotta ha dato la possibilità ai congressisti non solo di ammirare le spettacolari eccentriche di aragonite, le belle concrezioni calcitiche e vari tipici depositi di grotta, ma anche di conoscere i sistemi di monitoraggio qui installati per controllare la salute di questo fragile ambiente sotterraneo.

INTRODUCTION Some of the most characteristic landscape-features of the Sulcis area are represented by the dolomitic and limestone facies in which karst phenomena, such as caves, canyons,

(*) Department of Earth Sciences, Via Trentino 51 Ð 09127 Cagliari, Italy. [email protected]. (**) Department of Earth Sciences, Via Trentino 51 Ð 09127 Cagliari, Italy. [email protected]. 126 J. DE WAELE, F. FRAU sinkholes and various microforms, are very well developed. Karst also includes mineralizations, and different lead-zinc and barite mines of the region exploited paleokarst fillings of Tertiary, Permo-Triassic and/or Cambro-Ordovician age. Karst in the Sulcis area is fragmented, being known about 20 carbonatic outcroppings, some of which have important caves and springs. These latter often give rise to travertine deposits. The most interesting features can be seen near the villages of Santadi, Nuxis, Villamassargia, Narcao and Carbonia. This field trip offers the opportunity to visit some features related to karst phenomena in this region. The cover-collapse sinkholes near Narcao are the result of a long-during human impact on the covered karst relief; in fact, overexploitation of the karstic aquifer has caused ravelling which ultimately has resulted in the formation of sinkholes in alluvial deposits covering the buried karst topography. A more directly observable karstic phenomenon is the lapiez surface near Nuxis, where carbonate dissolution by rain water has caused the formation of a complete series of micromorphologies which are typical of exposed compact limestone surfaces. Finally, the Is Zuddas cave near Santadi is one of the most beautiful tourist caverns of Sardinia, and since 10 years it is the subject of a monitoring project aiming at the assessment of the effects of the flow of visitors on its fragile environment. The cave formed since early Pliocene in the slowly dissolving Cambrian dolomite, and nowadays is particularly appreciated thanks to the abundance of aragonite helictites. The complete itinerary is outlined in fig. 1; the numbers refer to the stops planned during the field trip.

GEOLOGY OF THE AREA Many different geological landscapes, mainly constituted of Palaeozoic bedrock covered by Tertiary sediments and volcanic rocks, can be admired during this field trip.

Stratigraphy The south-western part of Sardinia is characterised by lithologies covering a lapse of time going from Palaeozoic to Quaternary: among these the Palaeozoic sequences are the most interesting. This sedimentary succession starts with the monotonous siltitic and arenaceous sediments of the Bithia Formation (Precambrian? Ð Lower Cambrian), followed by a thick sequence of Cambrian-Lower Ordovician rocks [1]. The Palaeozoic rocks are well described by BECHSTADT & BONI (1996) [2]; a schematic geological section is represented in fig. 2. From a stratigraphic point of view the Cambrian-Lower Ordovician succession in south-west Sardinia is divided in three major Groups: Nebida Group, Gonnesa Group and Iglesias Group. The Nebida Group (Lower Cambrian) is composed of delta and coastal sediments, and is divided in two formations [3]: the Matoppa Formation and the Punta Manna Formation. The former is characterised by sandstones and shales, and is partially heterotopic with the SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 127

Figure 1. Itinerary and stops during the field trip. 128 J. DE WAELE, F. FRAU

Bithia Formation, therefore having an indefinite range. The Punta Manna Formation has a range of about 80 meters and starts with oolithic limestones and calcareous sandstones, followed by sandstones with carbonatic fossiliferous lenses and strata. The Gonnesa Group (Lower Cambrian) is characterised by carbonatic sediments, and is divided in two formations according to the trilobite content: the Santa Barbara Formation and the San Giovanni Formation [3]. This succession starts with grey dolostones with clear sedimentary structures, followed by darker and intensely dolomitised rocks in which no structures or fossils have been found, ending with a thick succession of waxy and intensely karstified limestones. The Iglesias Group (Middle Cambrian-Lower Ordovician) is divided in two formations: the Campo Pisano Formation, constituted of nodular limestones, followed by a thick succession of shales of the Cabitza Formation [3]. The deposition of the Ordovician conglomerates (Puddinga auct.) corresponds to an important tectonic phase (Fase Sarda auct.) that determined a long period of continentality, causing erosion of the Cambro-Ordovician rocks, followed by the deposition of fluvial sediments. During Caradoc-Ashgill upon the conglomerates were deposited coastal and deep-sea sediments. The Tertiary sediments outcropping in the area are clearly separated from the Palaeozoic rocks by an E-W striking fault of regional importance of Oligo-Miocene age. The observed lithologies are represented by the sediments of the Cixerri Formation (Eocene-Lower Oligocene [4]), mainly composed of quartzitic sandstones with conglomeratic, clayey, marly or siltitic lenses and strata deposited in a continental environment. More to the north-west (Carbonia), this formation covers Eocene transitional and continental sediments, containing some coal seams [5]. From the early Tertiary, volcanic activity in the Sulcis area starts with alkaline rocks, occurring as sills which cut the Upper Palaeocene to Lower Eocene sediments [6], with an age between 62.1 and 60.2 Ma [7]. Later on, beginning from 32 Ma, Sardinia was interested by important geodynamic and tectonic events, responsible for the opening of the north-western Mediterranean and connected with the drifting of the Sardinian- Corsican microplate. Due to these phenomena, a new volcanic cycle of prevalently calc- alkaline character, related to the subduction of oceanic lithosphere underneath the Sardinian-Corsican microplate [9] [10], starts, giving rise to many volcanic products also in the Sulcis area. The geological succession ends with Quaternary deposits, such as alluvial sediments, landslide deposits, travertines and coastal sandy sediments (e.g. Thyrrenian, fossil and recent dunes, beachrock).

Tectonics The first tectonic events recognised in this area are related to the Fase Sarda (auct.), in connection with the Caledonian Orogenesis, and are dated Lower-Middle Ordovician. This tectonic phase is represented by E-W structures, characterised by folds of different SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 129

Figure 2. Geological sketch map of south-west Sardinia and schematic stratigraphical section of the Cambro-Ordovician sequence (from [2]). 130 J. DE WAELE, F. FRAU dimensions in relation with the lithologies involved. Later, during the Lower Carboniferous, another tectonic phase, related to the Hercynian Orogenesis, is recognisable. First the tectonic style is similar to the one produced by the Fase Sarda, later it orients perpendicularly (N-S), causing intense cleavage of the shales and a lengthening of the pebbles of the Ordovician conglomerates. The last important tectonic phase that has left a sign in the present geological framework of this area is Oligocene-Lower Miocene in age. In this period the area has been interested by the formation of E-W striking faults, associated with NNE-SSW faults, related to the rifting of the Sardinian-Corsican microplate [1][2][8].

Ore deposits The Sulcis region, together with the more important Iglesiente massif, is known for its ore deposits in the autochthonous Palaeozoic rocks. These deposits, that have been intensely exploited during the past 150 years, can be subdivided in two main groups: pre- Hercynian (stratabound or stratiform ore bodies) and late- to post-Hercynian (skarn, veins, paleokarsts) deposits. The economic metals generally present in these mineralizations are Pb, Zn, Ag and Ba, sometimes accompanied by F, Cu, Sb, Bi and W. Most of these economic deposits are situated in the Cambrian rocks, concentrated in the stratigraphic range going from the Punta Manna Formation to the Santa Barbara Formation, and subordinately in the San Giovanni Formation. The most important economic mineralizations are of Cambrian age, and consist of stratiform and/or stratabound Pb, Zn and Ba deposits (galena, sphalerite, barite). However, these deposits are not very important in the Sulcis area, whereas the post- Hercynian deposits are relatively abundant. These latter are represented by veins, often related to skarns (e.g. Mont’Ega near Narcao), with sphalerite, chalcopyrite, galena, fluorite and barite in a gangue constituted of quartz, calcite and dolomite, and by paleokarstic Ba-deposits (e.g. Barega, Barbusi) with yellow dolomite, quartz and calcite as gangue minerals. The paleokarst ore deposits generally are related to the Triassic continental period [2].

STOP 1: COVER COLLAPSE SINKHOLES OF RIOMURTAS (NARCAO) Geographical outline Near Narcao, in the locality called Riomurtas, collapses occur since more than ten years. In a wide depression on the contact between Cambrian phyllites and limestones many cover collapse sinkholes have formed and are continuously forming especially after rainy periods. In one of these sinkholes a temporary river disappears. Other cover collapse sinkholes, most of which in fluviatile deposits and close to a small natural channel, occur close to Acquacadda, in the mining area of Sa Marchesa. Here, a minor collapse shows dolomite rock at shallow depth. SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 131

Genesis Most of the cover collapse sinkholes observed in the epikarstic zone are of secondary origin and only few are related to primary collapse due to collapsing of the roof of a karstic void (cave) in the upper part of a karst aquifer (epikarstic zone). Natural collapsing caused by dissolution of limestone is extremely difficult to observe in a Man’s time frame. The most common cover collapse sinkholes are related to the transport of sediments in existing major karren and/or karstic tunnels and are generally triggered by the oscillation of the water table and only occasionally by the collapse of a karstic void at greater depth. The downward movement of sediments into the epikarstic zone is termed ravelling [11]. In general, cover collapse sinkhole formation is enhanced by four major factors: 1) the decreasing of buoyant support of water; 2) the increasing of the gradient of water velocity (and thus ravelling); 3) the water table fluctuations; 4) the induced recharge of the aquifer. It is quite clear that these factors, and thus the major part of the hazards, are correlated to human activities, especially to extensive pumping of aquifers and to modification of the natural runoff patterns (e.g. construction of channels, deviation of water flows) [12]. In fact, the sinkholes have developed close to pumping stations or not far from mines in which pumping of water was performed to enable cultivation of lead-zinc ores. The installation of pumping boreholes creates a forced circulation of subterranean water and solid transport, causing the ravelling and thus enlarging existing voids in loose sediments, especially above karst aquifers. This emptying of the voids, together with the lowering of the water table, enhances ravelling and subterranean collapsing, and during this process, in particular if the embedding sediment is dry and more solid, an air-bubble can be formed above the void. By further ravelling and collapsing in this air-bubble, generally during wet periods, a collapse sinkhole suddenly appears on the surface. In these sinkholes the karstic origin is never really clear, because no carbonatic rocks crop out (fig. 3). The effective collapsing of a sediment package covering a void depends on different factors: the weight of the covering soil and its physical characteristics, the depth of the roof of the void and its width [13]. When the void is deeper than 10 meters collapse will only occur if the dimension of the subterranean chamber is large enough, or after many years of ravelling. Anyhow, most of the sinkholes are less than 5 meters deep, with a presumable depth of the carbonatic substrate (thus the voids) of less than 10 meters. But in cases where collapsing should normally not occur in natural conditions, hazard is caused especially by oscillation of the water table. In fact, the movement of water in an alluvial aquifer causes solid transport and therefore the modification of soil structure, diminishing the solidity of the entire sedimentary cover. The withdrawal of the water table by itself, due to extensive pumping, would not have great consequences, excepting the case in which heavy rains cause the fluctuation of its level in the vertical range between the roof of the void and covering sediments. These movements of water generate subterranean solid transport and loss of cohesion in the soil, causing collapse. These 132 J. DE WAELE, F. FRAU

Figure 3. Schematic example of formation of cover collapse sinkhole (Guardia Su Merti, Iglesias): 1. Cambrian limestones; 2. Hard sandstones (Cixerri Formation, Eocene); 3. Loose sediments (Cixerri Formation, Eocene); 4. Quaternary deposits (Holocene); 5. Karstic voids. SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 133 phenomena are not the result of sudden and extraordinary events (heavy rains), but reflect a long-during state of water table oscillation and thus subterranean erosion, causing collapse even several years after the beginning of pumping.

Forecasting Most cases of cover collapse sinkhole formation are related to human activities, especially exploitation of subsurface and subterranean aquifers, and occur in covered karst regions. The historical analysis of these phenomena together with the geological knowledge of the region allow to determine the most probable areas where subsidence might occur in the future. Cover collapse sinkholes form in areas of high infiltration rates, where downward erosion of covering sediment into pre-existing karstic voids is enhanced. This means that collapses prevalently occur in low areas (valleys and low plains) covering the epikarstic zone, often close to streamlets where high water flows occur during heavy rains. The presence of pumping stations or drainage systems can trigger collapsing, while high construction density favours concentrated infiltration and turbulent flow. Construction and settlement should be avoided in the areas that already have been interested by collapsing phenomena in order to prevent human losses and material damage in the future.

STOP 2: KARST GEOMORPHOLOGY AND MICROFORMS AT NUXIS Geographical outline Near the little village of Nuxis the karst morphologies of the Is Ollargius area, composed of oolithic limestones of the Santa Barbara Formation (Gonnesa Group, Lower Cambrian), can be seen [2]. These relatively pure limestones have been dissolved by rain waters creating solution flutes, cockled and crinkled surfaces, kamenitze and little caves.

Description of the karst phenomena In the Sulcis region it is relatively rare to find beautiful and well developed karstic micro- and macroforms. These latter are scarcely present because of the exiguous extension of the carbonatic outcrops; only in some places can be seen sinkholes (Narcao). For what concerns the microforms their formation depends on different factors. The carbonatic rocks have to be rather pure, with appreciable solubility, and disposed in not too thin and only weakly fractured beds. Micrites are more readily dissolved than sparites, and limestones host more microforms than dolomites. A too closely fractured rock or vertical stratification conveys water too quickly downwards, giving no time for the formation of karstic dissolution morphologies. Near Nuxis, not far from a couple of barite quarries, a nice surface of scarcely fractured oolithic limestone of the Santa Barbara Formation crops out and shows an 134 J. DE WAELE, F. FRAU

Figure 4. Solution flutes, cockled and crinkled surfaces, and kamenitze (from [14]). SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 135 interesting variety of karstic dissolution morphologies such as solution flutes, cockled and crinkled surfaces, kamenitze and little caves. The solution flutes are composed of tiny parallel, 1 cm deep, 1 cm wide and up to 50 cm long canals separated from each other by sharp ridges. These solution flutes are well developed on surfaces with inclination between 20° and 70°, and are formed by corrosion action of downwards streaming rain water in the early instants of water-rock interaction, before the drops get saturated with calcium carbonate. The cockled and crinkled surfaces are intensely corroded rock outcrops formed by phytokarstic processes. The cavities of these surfaces have dimensions of 1 cm and are separated by tiny diaphrams, all covered by a black phytokarstic film. The kamenitze, or potholes, are elliptic rounded decimetric water reservoirs with a flat horizontal bottom covered by a black phytokarstic film [14]. In fig. 4 is presented a scheme with all these microkarstic features.

STOP 3: THE IS ZUDDAS CAVE General information Discovered in the late 1960’s by speleologists of Santadi, the cave was thoroughly explored by the Gruppo Ricerche Speleologiche ÇE.A. MartelÈ of Carbonia and by the Clan Speleologico Iglesiente some years later. The cave has its entrance near the little village of Su Benatzu, in the territory of Santadi, in the carbonatic Cambrian hill of Monte Meana. Its extraordinary richness in concretions of various types has determined the visit of many people that came to collect the stalactites, and only after the closure with a fence this destruction came to an end. In 1985 the cave was opened to public by a local cooperative, becoming an important example of sustainable development in this part of Sardinia afflicted by unemployment since the closure of many mines.

Geology The system has formed in the calcareous dolomitic sequence of the Santa Barbara Formation [3] (Arcu Biasterria Member), composed of tidal and supratidal carbonates with stromatolitic laminated fabrics (previously called ÇDolomia rigataÈ) and a coarse crystalline upper part (known as ÇDolomia grigiaÈ) deposited in an arid climate. In this thick sequence some rare oolithic limestone beds occur and thick oolites are found at the top in the Is Ollastus Member (San Giovanni Formation). All these rocks belong to the Gonnesa Group of Lower Cambrian age [2].

Cave deposits The cave is among the most beautiful tourist attractions of the Island thanks to its marvellous and unique eccentric concretions. It is characterised by a succession of rooms 136 J. DE WAELE, F. FRAU connected by narrow natural tunnels, sometimes artificially enlarged to make the passage of tourists easier. From a geologic point of view the cave represents a fossil karst system with the typical forms of phreatic water flow. The tunnels and the corridors are generated along tectonic directories, and where these cross each other formed big rooms [15] [16]. The fossilisation of the cave has determined the formation of a wide variety of both physical and chemical deposits. The concretional forms vary from big calcite flowstones to stalactites, stalagmites and fragile eccentrics, unique for their abundance, form and dimensions. Concretioning is related to the well-known karstic process which can be summarised in the following chemical reaction:

↔ 2+ Ð CaCO3 + CO2 + H2O Ca + 2HCO3

Generally the meteoric water, passing through a more or less developed soil, is enriched in CO2 and is therefore able to dissolve more CaCO3 than the quantities normally dissolved by surface waters in equilibrium with atmospheric CO2. When the percolating water reaches a subterranean cave it will depose CaCO3 in the form of calcite or aragonite.

In fact, air in the cave, even though about 10 times richer in CO2 than open air, is still at least 30 times poorer in CO2 compared to the concentration of dissolved CO2 in the percolating water. This means that a percolation drop that settles on the roof of a cave will immediately release CO2 in order to reach its equilibrium with air in the cave. This process causes the precipitation of CaCO3, either in form of calcite or aragonite, according to the reaction:

2+ Ð ↔
 Ca + 2HCO3 CO2 + H2O + CaCO3

This mechanism is especially efficient in the summer period, when the bacterial activity in the soils, and thus the quantity of CO2 in the infiltrating water, is highest, while during the winter period this process is much slower. According to this concretioning process, many forms of chemical deposits are present in the Is Zuddas cave, mostly related to more humid periods of the past, since only few concretions are dripping regularly nowadays. These concretions are stalactites, stalagmites, columns, flowstones, draperies, coralloids, crusts, frostwork, helictites, moonmilk, gours (rimstone dams) and cave pearls [17] [18] (fig. 5). Stalactites in Is Zuddas cave can be slender but are generally thick and massive, hanging vertically from the ceilings and reaching lengths of a couple of meters. Most of them have a central canal and are made of calcite, showing colours ranging from pure white to yellow and reddish brown, depending on different impurities such as organic substances (e.g. humic acids) and metals (e.g. Fe-oxides). Below some stalactites and other dripping points on the ceiling, stalagmites can form. SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 137

Figure 5. Types of concretions recognisable at the Is Zuddas Cave (modified from [17]). 138 J. DE WAELE, F. FRAU

These concretions show no central canal and their thickness and shape is a measure of dripping flow rate during formation. Fast dripping from a low ceiling causes large stalagmites, while slow dripping from the same spot will cause tall and thin stalagmites. Where the ceiling is high all stalagmites formed will be relatively large. Stalagmites uniform in diameter indicate constancy of dripping over time. Where stalagmites grow together with their feeding stalactites, columns or pillars are formed. All columns are aligned along ceiling joints where the greatest amount of water drips into the cave. Another very frequent speleothem is subaerial flowstone, composed of a succession of thin calcite layers deposited by a film of water flowing over a cave floor. At Is Zuddas these concretions become active only after heavy rainfall, and have deposited in the past under much wetter conditions. Draperies, also named curtains, are scarf-like speleothems that hang from inclined cave ceilings, and can be seen in several places at Is Zuddas. They are composite flowstone-speleothem deposits that originate from droplets flowing down an inclined bedrock surface: at the end of the drapery the droplets fall to the floor forming a dripstone. Where the draperies are thin the coloured banding can be clearly observed, but normally the thickness of the curtains does not permit light to pass through. Along several walls of the cave, especially in places of appreciable air flow, seepage, capillarity and condensation cause the formation of water films that deposit popcorn-like subspherical concretions called coralloids. Of the same origin are the calcite crusts, formed by seepage along a homogeneously porous cave wall. Near the aragonite helictites, another curious kind of concretion occurs, composed of huntite, CaMg3(CO3)4, characterised by a soft, pasty white material when wet. This speleothem, called moonmilk, seems to have an alteration origin because it ÇgrowsÈ on aragonite, but probably it also precipitates directly from Mg-rich seeping water, never creating macrocrystals but remaining microcrystalline and wet. In different parts of the cave can be admired gours (also called rimstone dams), constituted of barriers of calcite obstructing the laminar water flow and delimiting shallow waterpools. Downstream, these rimstone dams gradually evolve towards normal flowstone deposits, and cave pearls can often be found inside the pools. Cave pearls are concentrically banded and contain generally a foreign grain in their centre. They are normally non cemented to the floor owing to the occasional dripping of water into the pool which disturbs and prevents cementation. But the most remarkable concretions of the cave are the aragonitic helictites and frostworks, mainly localised in the Eccentrics Hall. Helictites are contorted speleothems that seem to grow in random directions, and are called also eccentrics or vermiforms. At Is Zuddas there are filiform, beaded and vermiform helictites. The first are extremely thin hair-like aragonite speleothems, the second form bushes of smaller helictites and the last are worm-like white concretions growing in any direction. These concretions form in an aerated cave environment with very little air circulation, constantly high humidity and SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 139 variations of temperature of less than 1°C. This enables the existence of Çpermanent hanging dropletsÈ. The percolating water, by seeping and capillarity, comes out of the porous bedrock (or concretion) and deposits a thin carbonate film around the pore (due to the loss in CO2). In this way the helictite starts growing and changes direction any time the central canal gets clogged or simply by rotation of the new crystals getting formed. By increasing the amount of water dripping from the end of the helictite it can turn into a soda straw concretion (fine and hollow stalactite). Frostwork could be defined a special type of helictite: it is composed of bushes of aragonite needles (acicular crystals) that form from a subaerial capillary film on a porous and finely textured substrate such as fine clayey sand. In general, the formation of aragonite instead of calcite seems to be favoured by high Mg concentrations in water. Besides the numerous concretions, another curious formation can be seen at the Is Zuddas cave: the so-called boxwork, characterised by thin calcitic and quartzitic blades protruding from the cave walls and ceilings. The calcitic boxwork is probably of secondary origin, because it does not extend deeply into the bedrock. In contrast, the quartzitic boxwork has formed from primary veins that are less readily dissolved than the dolomitic bedrock, remaining thus in relief. During the karstic evolution many gravely and sandy sediments have also been deposited at different levels, and some of these are covered by a limestone pavement, showing variations in the water table and different phases of deposition and erosion [19].

Environmental monitoring The first air temperature measurements in the Is Zuddas cave date back to 1990, year in which 5 stations were considered (Outside, Medusa Hall, Organ Room, Theatre Hall and Eccentrics Hall) [16][20]. During the first year, measurements were taken using thermometers having a 0.5°C scale; from 1991 these instruments were replaced with more accurate thermometers (0.1°C scale). Over ten years about 100 daily average temperatures have been recorded, and these data have been processed by CIGNA & SULAS [21]. From these studies it is clear that the seasonal variations have a very small influence on the cave air temperature, ranging between 0.34 °C in the Medusa Hall and 0.08 °C in both the Theatre Hall and the Eccentrics Hall. Furthermore these measurements have shown no considerable influence of visitors on cave climate over the past 10 years, and it can therefore be assumed that visitors’ capacity of Is Zuddas is higher than the present 50,000 persons per year [21]. In the framework of a Regional Project [22], in July 1999 some researchers of the Laboratory of Environmental Geology of the Department of Earth Sciences of the University of Cagliari have installed a monitoring system composed of three independent instruments provided by Digital Analog Systems (DAS), composed of a Data logger, that allows the transmission of data to a personal computer for subsequent elaboration, and two sensors each. Two instruments are characterised by two temperature sensors, while the third Data logger has a temperature/relative humidity sensor connected. The location 140 J. DE WAELE, F. FRAU

Figure 6. The monitoring system of the Is Zuddas Cave (Santadi) (modified from [23]). of the instruments and sensors is shown in Fig. 6. In August 1999 measurements were taken every minute and the average value was stored every 5 minutes. From the middle of September average measurements were stored every hour (24 values/day) [23] [24].

In the near future a CO2-Data logger will be installed in the lowest part of the cave, in the Tunnel, at a level of approximately 107 m a.s.l., almost 16 meters lower than the entrance, in order to see if CO2 accumulation takes place. Later on this sensor will be placed in the Eccentrics Hall, Organ Room and other places. The short time of continuous monitoring does not enable an exhaustive analysis of the cave environment on the long term, but the detailed measurements have shown some SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 141

Figure 7. Daily diagram showing visitors’ flux and temperature variation (from [23]). 142 J. DE WAELE, F. FRAU interesting short time trends in different stations. The analysis of daily diagrams showing visitors’ flux versus air temperature shows variations of temperature of maximum 0.4 °C immediately after the arrival of the visiting groups. The temperature falls back to the original value in a lapse of time that varies between 30 and 120 minutes (fig. 7). The seasonal trend of cave temperature can be seen on a longer interval of time (7 months). The minima vary from 15.5°C in the Erosion Hall, where the influence of the outside temperature is much more evident, to 15.9°C on the floor and 16.0°C at a height of 2 meters in the Organ Room, and 16.1°C in the Eccentrics Hall. Maxima reach 16.8°C in the Organ Room (sensor 2 meters above a lamp) and 16.7°C in the Eccentrics Hall. Average temperature variation between August and February is 0.2°C for the most internal environments (Organ Room and the Eccentrics Hall), and 0.4°C for the Erosion Hall sensor. The internal temperature starts decreasing at the end of October and reaches its minimum not before March in the Erosion Hall station, much more influenced by the outside temperature, while in the deeper parts of the cave (Organ Room) this minimum is reached in the early days of February (fig. 8). The increase of cave air temperature in Summer is further enhanced by the visitors and lighting system that input an additional flux of energy; this can easily be observed in fig. 9. One of the Data loggers has been placed in the Crib site of the Organ Room, close to a lamp. One sensor was put on the ground at a distance of 5 meters, out of the range of the beam of light, the other was fixed at a height of 2 meters, in front of the spotlight (fig. 10). In this site the difference in temperature between the two sensors reached 0.5°C. In both cases, anyhow, after the visitors left and the lights were switched off, the temperature fell back to the original values in less than 3 hours (fig. 11).

Cave dwelling fauna The Is Zuddas cave has not been studied in detail for what concerns cave dwelling fauna. However, the Trichopterus Micropterna fissa and the fly Bolitophila cinerea were recognised [15]. DI STEFANO [25] observed the spider Meta bourneti, Opilionids, Psochoptera, Thicks and Diplopods of the Genus Callipus. BORDONI [26] determined the stafilinid beetle Conosoma testaceum, while MUCEDDA et al. [27] revealed the presence of a nursery colony of Rhinolophus hipposideros close to the entrance during summertime.

ACKNOWLEDGEMENTS Many thanks to the two referees for their useful suggestions and comments. The Authors also wish to thank the Cooperativa Monte Meana of Santadi for their collaboration in all our studies in the Is Zuddas cave. Most data contained in this paper have been collected in the framework of the Researches of the Laboratory of Environmental Geology (responsible Prof. Felice Di Gregorio). SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 143

ture (from

Figure 8. Annual Internal temperature variation measured by the sensor in the Organ Room in comparison with the Outside tempera [24]). 144 J. DE WAELE, F. FRAU

Figure 9. Influence of visitors and light system on cave air temperature (from [23]). SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 145

Figure 10. Location of one of the Data loggers and connected Sensors nearby a lamp in the Organ Room (from [23]). 146 J. DE WAELE, F. FRAU

Figure 11. Influence of lamps on cave air temperature (from [23]). SOME EXAMPLES OF KARST PHENOMENA IN THE SULCIS REGION 147

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