Hydrogeological Processes in Karst Terranes (Proceedings of the Antalya Symposium and Field Seminar, October 1990). _ IAHS Publ. no. 207, 1993. 107 FIRST RESULTS FROM THE MONITORING SYSTEM OF THE KARSTIC COMPLEX "GROTTE DI FRASASSI-GROTTA GRANDE DEL VENTO" (CENTRAL APENNINES, ITALY) W. (V. U.) DRAGONI Earth Sciences Department, Perugia University, Piazza dell'Université, 06100 Perugia, Italy A. VERDACCHI Consorzio Frasassi, 60040 Genga (Ancona), Italy ABSTRACT The karst complex of "Grotte di Frasassi-Grotta Grande del Vento" is located in the gorge cut by the River Sentino in the anticline of Mt. Valmontagnana, about 50 km from the town of Ancona (central Italy). In order to manage the caves in a rational way, and to get new information about the karstic processes at the cave "Grotta Grande del Vento", a computerized monitoring system was installed for temperature, humidity, rain, percolation and air velocity, inside and outside the cave complex. The first data collected suggest the following preliminary results: (a) Normally the flow of groundwater is towards the Sentino River. During floods this flow is reversed. The effect of the waters mixing can increase karstic dissolution. This hypothesis seems to be confirmed by the greater dimensions of the cavities close to the Sentino River, (b) As expected there is a close correlation between air flow through the cave and the temperature difference inside and outside the cave. However the data seem to show that in some zones of the caves the air flow is mainly controlled by the processes of condensation-evaporation, (c) The condensation phenomena probably play an important role in the karstic evolution of the system, (d) An initial estimation of the groundwater draining into the Sentino River along the Frasassi Gorge has been made (about 50 1/s); according to the Maillet equation the depletion constant of the river is 2.8 X 10"2 day"1, that of the aquifer is around 8.4 x 10"2 day"1. INTRODUCTION In central Italy, about 50 km from Ancona, in the Frasassi Gorge, one of the most spectacular karst complexes in Europe has developed. The largest underground system in the area is that known as "Grotta Grande del Vento" (Great Cave of the Wind), the explored part of which extends for over 15 km. The Grotta Grande del Vento system is economically quite important to the area: since being opened to tourists in 1974, approximately 400 000 have visited the cave each year. The part open to tourists is run by a public agency, the "Consorzio Frasassi", which has set up a Scientific Committee with the task of studying local karst processes and how they might be altered by the steady flow of tourists in the cave. For example the visitors alter the C02 content in the air of the cave and algae tend to grow in artificially lit areas, spoiling the best speleothems. The Committee is composed of small teams of scientists from various fields and is divided into three different sections: the first one deals with the chemical, physical and geological aspects of the problem (Physical Section), the second one is the Bio-Speleological Section; the third one is the Dating Section, with the task of dating the speleothems using isotopic methods. In early November 1989 a continuous real-time monitoring system was installed for monitoring the physical data most relevant to the aims of the Scientific Committee. Up to now priority has been given to temperature, rainfall, percolation inside the caves, 108 W. (V. U.) Dragoni & A. Verdacchi atmospheric pressure, humidity, air velocity and carbon dioxide content in the air. We expect these data will make an important contribution towards the understanding of some of the phenomena taking place in a karst environment. In this paper the monitoring system will be briefly described, and some of the preliminary results inferred from the data obtained will be discussed. GEOLOGICAL AND GEOMORPHIC FEATURES Figure 1 gives a simplified geological sketch of the area where the Frasassi Gorge is located. Here the geological situation has been synthesized. The reference list includes many entries which give information on the Gorge. The Grotta Grande del Vento cave system has developed entirely within the Mt. Valmontagnana anticline core, which is situated on the western side of the Umbria- Marche ridge. This anticline has an Apennine trend (northwest-southeast) with an Adriatic virgence (northwest). Towards the west it joins with the Camerino syncline, whereas towards the east it crosses the southern edge of the Pergola syncline. , tssl 5 Do • 7 Ho • '9 HlO 11 12 —1— 13 0 14 O 15 • 16 Fig. 1 - Geological sketch of the Frasassi area. 1: Calcare Massiccio; 2: Bugarone and heteropic formations.; 3: Maiolica ; 4: Marne a Fucoidi; 5: Scaglia Bianca e Rosata; 6: Scaglia Cinerea e Variegata; 7: Bisciaro; 8: Schlier; 9: alluvial deposits; 10: debris and landslides; 11: fault; 12: uncertain fault; 13: anticline axis; 14: syncline axis; 15: major sulphatic spring; 16: beginning and end of the Frasassi Gorge. The monitoring system of the "Grotte di Frasassi-Grotta Grande del Vento" 109 The thickness of the sediments making up the Mt. Valmontagnana anticline is less than that of the adjacent synclines, due to settling in low depth conditions over a "structurally lifted block" existing during the entire Jurassic period. This brought about two different sequences in the sedimentation area: the Mt. Valmontagnana sequence (condensed sequence), and that of the adjacent synclines (complete sequence). Triassic evaporitic formations ("Formazione di Burano"?), which do not outcrop in the Frasassi area, should be present below both sequences. Both sequences begin with the limestone formation of "Calcare Massiccio" (Hettangian-Sinemurian) which is made up of beds ranging in thickness from about 1 m to 10 m. This formation has a total thickness of between 500 and 700 m. The uppermost portion of the Calcare Massiccio has about 80 m of typical oolitic facies (Colacicchi & Pialli, 1974). On a large scale the Calcare Massiccio is the most permeable formation in the area. Below the degree of permeability of the geological formations is given with respect to the permeability of the Calcare Massiccio. In the condensed sequence, from Lias to Tithonian, the deposition of the "Bugarone" formation took place, after the Calcare Massiccio, in conditions of a "structurally lifted block". This formation is composed of thin beds of limestone nodules, reaches a maximum thickness of 50 m (Centamore et al., 1975), and, on the whole, can be considered as having medium permeability. The other sequence, deposited in pelagic basins, is heteropic with the Bugarone, about 450 m thick, and composed of limestone, marly-limestone, and cherty-limestone (from Sinemurian p.p. to Lower Tithonian), with an overall medium-low permeability. From the Tithonian on, both sequences continue with the typical Umbria-Marche sequence: — "Maiolica" (Lower Tithonian p.p.-Aptian), made up of bedded limestone, with medium-high permeability; — "Marne a Fucoidi" (Aptian p.p.-Cenomanian), consisting of marls, with very low permeability; — "Scaglia bianca e rosata" (Middle Cenomanian-Middle Eocene p.p.), made up of limestone and marly-limestone in the upper portion, with medium-high permeability; — "Scaglia variegata e Cinerea" (Middle Eocene p.p.- Oligocène), made up of marls, marly-limestone, with low permeability; — "Bisciaro" (Lower Miocene), made up of marls and silty-clays, with very low permeability; — "Schlier" (Lower Miocene), made up of marls, silty-marls and clays, with very low permeability. Although there is a certain amount of tectonic activity in the Frasassi area at the present time (Forti & Postpischl, 1979; Centamore et al., 1978), the features of the zone are essentially due to past tectonic stages; in particular, conjugate strike-slip systems were most likely formed in the late Pliocene, the dextral faults striking NNW-SSE and the sinistral faults northeast-southwest. The latter are definitely prevalent (Menichetti, 1988; Cocchioni et al., 1988). The north-south system of inverse faults existed before the strike-slip systems. The Sentino stream flows through the northern part of the Mt. Valmontagnana anticline, cutting deeply into the Calcare Massiccio core (Fig. 1). The latter is the formation which is most affected by karst phenomena, although they also occur in the Maiolica and in the Bugarone (Coltorti & Galdenzi, 1982). On the basis of the morphology of Frasassi Gorge, it appears that the deepening of the Sentino River into the Mesozoic formations underwent a progressive migration towards the north. 110 W. (V. U. ) Dragoni & A. Verdacchi Fig. 2 - Map of the karstic system "Grotta Grande del Vento-Grotta del Fiume". The dimensions of the cavities increase close to the river; the main directions are parallel to the main fault systems. The karst phenomena, enhanced by some primary porosity in the Calcare Massiccio, developed accordingly to pre-existing tectonic features, such as faults and joints (Figs 2 and 3). At the Frasassi Gorge at least four main karst plains can be recognized (Cattuto & Passeri, 1972; Cattuto, 1976); according to a more recent study there seem to be eight plains (Antinori, 1979). These plains are often matched by particular external morphologic features such as terraces, orographic benches, etc. This shows that the formation of the gorge was characterized by stasis periods. Two types of groundwater are present in the area. One has a bicarbonate chemical composition, originating essentially from in loco infiltration; it floats in places on a water with sulphate-chloride composition. This second water, which acquires its chemical composition from contact with the Triassic evaporitic formations, belongs to a confined Fig. 3 - Map of the karstic system "Grotta di Frasassi-Mezzogiorno". The cave is located in the Frasassi Gorge, at the same altitude the upper levels of Grotta Grande del Vento. Note the parallelism of the main directions of the cavities and how the dimensions of the cavities increase close to the paleo rivers.