HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS ... - CC-WaterS

CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
1. GEOGRAPHICAL LOCATION
The Lessini Mountains are in the western part of the Venetian Fore-Alps (fig. 1) and develop over
an area of approximately 1500 km 2 . From an administrative point of view, they belong to the
provinces of Verona, Vicenza, and Trento. They are bordered by the Adige Valley to the West, by
the Leogra Valley to the East and N-E, while the Val dei Ronchi separates them from the Pasubio–
Carega Group in the north-western sector. From a geographic point of view, they include the
western Lessini Mountains (from the Adige Valley to the Illasi Valley) and eastern Lessini
Mountains (from the Illasi Valley to the Leogra Valley).
In the territory of the Veneto Region, the Lessini Mountains are subdivided in the Lessini Veronesi
(fig. 2) and Lessini Vicentini, separated by the Mount Calvarina ridge (683 m a.s.l.) included
between the Alpone Valley (VR) and the Chiampo Valley (VI). The maximum altitude reached by
these mountains is 1865 m a.s.l. (with the Cima Trappola).
The “Kater II” project covers the area included between the Adige Valley and the Chiampo Valley,
extending for an overall length of about 980 km 2 , mainly involving the territory of the province of
Verona and only partly that of Vicenza.
As explained in detail later, the western Lessini Mountains essentially consist of carbonate rocks,
whose role is particularly important in the development of karst phenomena, while the eastern
Lessini Mountains are prevalently made of volcanic rocks linked to the tertiary venetian
magmatism: here, the karst morphology is virtually unseen, while the territory is mainly modelled
by gravitational causes. In its turn, the tectonic pattern equally affects the evolution of surface
forms in both sectors.

CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
Figure 1: Geographical location of the Lessini Mountains
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Figure 2: A panoramic view of the Conca dei Parpari
2. CLIMATIC FEATURES
Generally speaking (SORBINI, 1993) the pluviometric regime in the area of the Lessini Mountains is
very similar to that of Fore-Alpine areas, though with a much lower average rain levels. The
variability of total rainfall approaches 22% and reaches 28% in the northern and higher sector of
the basin. On average, the permanence of snow on the ground varies from a few days at low
altitudes to 3–5 months at higher altitudes.
The graphs below show the rainfall values obtained by processing the data provided by ARPAV
regarding the sites in the Lessini territory where continuous measurements concerning the 1992 to
2005 period are available.
Figure 3: Annual rainfall in Fosse di S. Anna d’Alfaedo
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Figure 4: Average monthly rainfall in Fosse di S. Anna d’Alfaedo
The longest and most complete series of the area refers to the Fosse di S. Anna d’Alfaedo station
(954 m a.s.l.), consisting in measurements taken in the 1961-1990 30-year period (figs. 3 and 4).
The rainfall regime of this station seems to be slightly abnormal compared with the typical regime
of Fore-Alpine areas registered by the other stations (fig. 5). In fact, the most rainy month in Fosse
di S. Anna is August, followed by May and June, while the other pluviometric stations detected
more abundant rainfalls in autumn and, in the second place, in spring, except for the Illasi area,
where the second maximum is reached in the summer. All the other stations showed that the less
rainy month of the year is February.
Figure 5: Average monthly rainfall in the meteorological stations of Lessini Mountains
Comparing the annual values obtained by pluviometric stations with comparable data are
available (fig. 6), it comes out that the average annual rainfall ranges from 826 mm to 1413 mm.
The highest values are seen in the mountain areas, and secondarily in the areas at the western
(Dolcè) and southern margins (Montecchia di Corsara) of the Lessini Mountain.
Figure 6: Average annual rainfall in the meteorological stations of Lessini Mountains
As far as temperatures are concerned, average annual values are established around 13° C in the
mid-low Lessini region, and around 9.3 °C in San Bortolo.
Data regarding temperatures (1992-2005) show a thermal regime characterised by maximum
average values in the months of July and August, and minimum values in the months of January and
December (fig. 7).
Figure 7: Average maximum monthly temperature in the meteorological stations of Lessini Mountains
One anomalous finding is the value of average minimum temperatures in the months of January,
February and December in the Montecchia di Crosara station (fig. 8), which is placed at 50 m
above sea level, but in spite of this shows lower temperatures in these months even compared to the
San Bortolo station that lies at an altitude of 936 m. This anomaly, which may be attributed to local
thermal inversion phenomena, is also highlighted by the frequent presence of fog in the area.
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Figura 8: Average minimum monthly temperature in the meteorological stations of Lessini Mountains
3. GEOMORPHOLOGY
3.1 Morphology
From a morphological point of view, the Lessini Mountains complex consists of an inclined plateau
sloping SW for about 5°. Its southern and central parts are deeply carved by a thick series of
parallel valleys generally positioned in NNE-SSW (to the West) and NNW-SSE (to the East)
directions. These valleys, which are called “vaj” or “progno” by the locals, are initially narrow,
but then widen up considerably as long as they proceed southward. Their rectilinear course shows
a clear tectonic influence: the Lessini valleys, in fact, are set on tectonic discontinuities.
The long and narrow ridges that lead off the wide northern plateau (fig. 9) plunge below fluvialalluvial
areas when they reach the plain and then re-emerge locally in the form of low isolated
hills.
Figure 9: View from a ridge summit near Conca dei Parpari
The summit tableland of the Lessini Mountains extends for about 60 km 2 and is entirely covered
with grazing land.
The area examined (the central-western Lessini) may be in turn divided into different areas
depending on their altitude and different evolution of the local morphology:
• a basal strip, between the Adige riverbed and 900 m a.s.l., where valley incisions extend
upwards, until they become large hilly areas (fig. 10);
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• an intermediate strip, between 900 and 1200 m a.s.l., characterised by narrow ridges and
narrow valleys (locally named “vaj”), with many scattered human settlements (fig. 11);
• a summit strip or “upper Lessini”, between 1200 and 1800 m a.s.l., with extensive grazing
land from which impressive rocky peaks emerge. In the North, the surface slopes down
towards the Trentino Region, with marked slopes and steep cliffs. This area is the site that
presents the most interesting karst phenomena (fig. 12).
Figure 10: The wide valley-bottom of Vajo di Squaranto nearby Montorio
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Figure 11: The village of Lesi (Bosco Chiesanuova)
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Figure 12: Karst phenomena in the upper Vajo di Squaranto
3.2 Karstification
The karst phenomenon has not given to the Lessini Mountains area the classic karstic features
(dolines, polje), even though the total absence of surface water already informs us about the
magnitude of the underground phenomenon. The hypogean forms are extensively and largely
spread, as proven by the 825 caves known until today, some of which are extremely important.
SAURO (1973) describes karstification in the Lessini Mountains both as “fluvial karst”, for the clear
predominance of river morphologies, and as “tectonic karst”, for the significant impact of tectonics
on the development of karst and, consequently, onto the entire landscape.
The role played in karstification and in the hydrogeological system of the Lessini Mountains by
overburden (landslide deposits, moraines, debris, eluvial and colluvial deposits) is very important,
as it represents epikarst water reservoirs that are slowly released into the underlying karstfied
rocks.
Acting on the several rock formations of the area, karstification has developed different
morphologies that vary based on their lithology; for example, the so-called “rock cities” (fig. 13)
are a typical phenomenon of Rosso Ammonitico, while dells and dry valleys are more typically
found on Biancone, on Oolite di S. Vigilio and Calcari Grigi (figs. 14 and 15). Dolines are
prevalently formed by the contact between Biancone and the underlying jurassic limestones, while
rocks, which are more favourable to the development of caves, are Rosso Ammonitico, Oolite di S.
Vigilio and Calcari Grigi.
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Figure 13: The “Rock City” near Camposilvano (Velo Veronese)
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Figure 14: Dry valley (Conca dei Parpari)
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Figure 15: Small dolines on the floor of a dry valley (Branchetto Pass)
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The most common among hypogean forms are vertical development caves, such as, for example, the
Spluga della Preta (near the Corno d’Aquilio), a drop of 877 m to create the third deepest cave of
the Veneto Region and a space extent of 4518 m to give the fourth largest cave of Veneto.
In addition to Spluga della Preta, the main caves in the area examined are Spurga delle Cadene
(Dolcè) and the Lesi Abyss (Bosco Chiesanuova), with space developments of 1200 m and 472 m,
respectively. More well-known karst morphologies are Covolo di Camposilvano (fig. 16), an
imposing 83 m deep collapse dolines, and the Veja Bridge, near S. Anna d’Alfaedo, a hypogean
complex consisting of a natural rock bridge that represents the non-collapsed portion of the vault of
the initial chamber.
More spectacular typical karst forms of the Lessini Mountains can be admired in the Sphinx Valley,
in the vicinity of Camposilvano (fig. 17).
Figure 16: Covolo di Camposilvano
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Figure 17: Sphinx Valley, near Camposilvano
3.2.1 The Sphinx Valley
The Sphinx Valley area is recognised as a natural monument representing a karst landscape named
“rock city” (figs. 13 and 17). The evolution of karst erosion combined with other physical-chemical
crumbling processes have enlarged the pre-existing vertical discontinuities and karst fissures, thus
creating karren and corridors in Rosso Ammonitico layers. This generated special stone blocks,
either isolated or in groups, with typical parallelepipedon (fig. 18) or mushroom-like forms (fig.
19). The typical rock "mushrooms" have a "hat" of Rosso Ammonitico, that is more resistant to
erosion, and "stalk" of the oolitic calcarenite belonging to the S. Vigilio Group, with a greater
susceptibility to erosion.
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Figure 18: Rock city (Sphinx Valley – Camposilvano)
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Figure 19: The “Mushroom” (Sphinx Valley – Camposilvano)
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3.2.2 Covolo di Camposilvano
Covolo di Camposilvano (fig. 16) is the most interesting example of collapse doline found in the
Lessini Mountains. It is a spectacular and very peculiar karst cave generated by the collapse of a
great doline set on Biancone, Rosso Ammonitico and oolitic limestones of S. Vigilio Group. The
entrance of the cave is at an altitude of 1204 m, the global drop is 83 m, the diameter is 60 m and
the global development ranges for 130 m.
Globally, the Covolo is a set of karst caves consisting of many rooms that form a complex system
originated by the karst processes connected with water circulation between layers.
The main cavern sometimes works as a “trap” for cold air: the considerable temperature difference
from external air may sometimes result in ice formations in spring, which may even last throughout
the year, or in the development of condensation mists.
Not far from the karst cave lies a small local museum (fig. 20) preserving interesting fossil remains
and pre-historic findings that have been found nearby, including both mineral and fossils, either
local or from other places. In particular, the Covolo area, which was already settled by humans in
the Neolithic age, is an important deposit of Quaternary fossils, with bone finds from holocenic
Cervus sp. and Bos taurus.
Figure 20: Shark vertebrae and ammonite fossils at the entrance into the Museum of Camposilvano
4 GEOLOGY
4.1 Stratigraphical succession
The Lessini Mountains mainly consist of carbonate sedimentary rocks dating back to the Mesozoic
and Tertiary periods. Cretaceous-Jurassic lithotypes crop out in the northern sector, while the
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southern sector is occupied by Eocene lithotypes. The lithology here (fig. 21) essentially consists of
dolomite rock, dolomitic limestone, limestone and marly limestone, but Eocene volcanic rock also
crops out, mainly in the Alpone Valley.
Here are the formations of the stratigraphic series of the Lessini Mountains: Dolomia Principale,
Calcari Grigi, Oolite di S. Vigilio, Rosso Ammonitico, Biancone, Scaglia Rossa, eocenic limestone.
Basaltic volcanic rocks (such as basalts, hyaloclastite and tuffs) crop out only in some parts of the
territory, prevalently in the eastern sector.
Figure 21: Lessini Mountains lithologies visible in the walls of a building
The most ancient formation is the Dolomia Principale, which consists of bioclastic calcarenites,
biomicrites, and stromatolites. The overall power of 900 m is almost entirely visible only on the
Adige and Ronchi valley slopes, while only the highest portion of the units emerges from the upper
Illasi Valley.
Jurassic units are represented by Calcari Grigi, Oolite di S. Vigilio and Rosso Ammonitico (fig.
22), with an overall 400–450 m thickness. From a lithological point of view, oolitic calcarenites are
frequently found, together with biostromal limestones, lumachelle limestone, encrinitic calcarenites,
and marly flinty limestones, while the most recent terms are micritic limestones. These formations
make up the main frame of the Lessini ridges and form the slopes of the deeply-embedded, and often
vertically walled, valleys.
Rosso Ammonitico has peculiar morphological features: it generates both the summit frames of the
ridges, as well as the shelves along the slopes and the plateaus of the northern tableland.
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Figure 22: A quarry near Camposilvano, with Oolite di S. Vigilio (below) and Rosso Ammonitico (above)
Stratigraphically overlying jurassic terms are the Biancone and Scaglia Rossa formations, lower
and upper Cretaceous in age, respectively. They consist of thickly-stratified fine-grained limestones,
whose global thickness ranges from about 200 m in the western sector to over 400 m in the eastern
sector. They come in the form of wide strips, scarcely steep along the slopes and rounded ridges at
the top of the medium-high Lessini Mountains.
The top member of the Biancone, which corresponds to the Cenomanian (fig. 29), plays a
significant role for its important hydrogeological implications: they are thickly-stratified marly
limestones and bituminous marls, whose thickness ranges from 50 to 80 m in the area of the Lessini
Mountains. In particular, the passage to the Turonian is marked by 65–130 cm of black and yellow
argillites and siltites. This formation is intensely fractured in the Lessini Mountains due to the
presence of many faults, and plays a considerably important role from a hydrogeological point of
view (see section 5).
Subsequently, during the Paleogene, basalt-like volcanic rock was laid in the Alpone-Agno graben
which took the form of small-grain, often stratified, breccias. The same magmatic cycle also
includes dyke bodies and eruption vents of breccia (necks), which intruded into sedimentary
formations all over the area of the Lessini Mountains (fig. 23).
Units of the Eocene (Scaglia Cinerea, Calcari Nummulitici and Marne di Priabona) crop out along
ridge tops and, in the case of Calcari Nummulitici, originate a marked karst morphology with
scattered groups of dolines. Eocene limestone are also found in the grabens of the area of Bosco
Chiesanuova.
Finally, the most recent terms of stratigraphic succession are found in the vicinity of Verona,
represented by calcarenites and sandstone from the mid Miocene.
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Figure 23: Columnar basalts in San Giovanni Ilarione
4.2 Structural layout
From a tectonic point of view, the Lessini Mountains have gone through extensional phases during
the ancient Mesozoic and Tertiary, a compressive phase in the Neogene and a southward tilting
phase in the Pliocene-Quaternary.
The faults originated in the Mesozoic have variable directions between North and NNE. They are
synsedimentary faults that formed at the margin of the Trentino platform, while this migrated
eastward in the Jurassic.
The faults originated during the initial phases of the Tertiary have a NNW direction, but can also
be found in the NNE directions, where they represent pre-existing faults later reactivated as leftlateral
strike-slip faults (ZAMPIERI, 1995 and 2000). It is in this phase that the most significant
feature of the Lessini Mountains was established: the Alpone-Agno graben (localised in the eastern
portion of the area considered in this report), a structure linked to the Paleogenic extension (fig.
24) and accompanied by volcanism. At the same time, a complicated system of normal NNE and
NNW faults was activated in the central-western Lessini Mountains.
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Figure 24: Tectonic sketch of the Lessini Mountains (ZAMPIERI, 2000)
In the course of the Neogene, the ongoing compressive phase in the Southern Alps only slightly
involved the Lessini Mountains by producing the Corno d’Aquilio and Marana thrusts (CORSI &
GATTO, 1964; ZAMPIERI, 1991). The latter inverted the northern portion of the Alpone-Agno graben
(ZAMPIERI, 1995). During this phase, the pre-existing faults were reactivated as strike-slip faults.
During the Pliocene, the southward tilt phase begun (ZANFERRARI et al., 1982), which later evolved
in the SW direction due to the thrust caused by the NE migration of the Apennine foredeep
(DOGLIONI, 1993).
Evidence of neotectonics in the Lessini Mountains have been recognised by SAURO (1978), ZAMPIERI
(2000), and SAURO & ZAMPIERI (2001). The latter, in particular, have recognised some slopes
originated by extensional tectonics that have reactivated fault planes dating back to the early
Tertiary period in the Orsara area (upper Pantena Valley) and Scandole (Vajo dell’Anguilla).
4.2.1 Tectonic structures in the central-western Lessini Mountains
This sector of the Lessini Mountains has been studied in detail by ZAMPIERI (2000) and as
summarized below, it is characterised by a rather complex structural configuration created by the
existence of two main systems of faults. The NNE-directed system is located in the central part,
while a NNW-oriented systems is found in the northern and central-eastern areas.
The area of interference between the two systems, which is localised among Bosco Chiesanuova,
Velo Veronese and Cerro Veronese, corresponds to a lowered rhomboidal structure within which
igneous rock intrusions and dolomitisation of the pre-existing rocky bodies took place.
The western margin of this structure is bordered by the fault of Bosco Chiesanuova, whose length
globally reaches ten kilometres, but which consists of three segments whose interconnection is
maintained by relay ramps.
This structure includes two minor grabens (Scardon and Mount Purghestal) with basalt and Eocene
calcareous rocks on their bottom. In particular, the basin of Mount Purghestal shows Eocene
calcareous blocks layers converging toward the centre of the graben, which leads to assume that
the structure has appeared after the collapse in the roof of the magma caldera situated along a
lateral ramp of a fault in the NNE-oriented system (volcanic-tectonic basin).
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Another volcanic-tectonic basin is that of Mount Purga, near Velo Veronese. If we look at the map,
Mount Purga seems to be surrounded by normal faults and by basalt dykes from the Paleogene all
around. Consequently, this is another case where the opening of fractures along the connection
ramps between the various fault segments has led to the development of a rhomboidal basin where
a magma chamber has settled at low depth. Subsequently, the collapse of the roof of this chamber
seems to have formed the subcircular collapse basin of Mount Purga.
5 HYDROGRAPHIC FEATURES AND HYDROGEOLOGY
The major valleys furrowing the area of the Lessini Mountains are generally oriented N-S and
widen considerably while they converge toward the River Adige. The shape of these valleys, which
are rather deep, is the result of the combination of fluvial, karst, glacial and tectonic processes still
active today (SAURO, 1978; ZAMPIERI, 2000; SAURO and ZAMPIERI, 2001).
The drainage network is well developed, but inactive; this reflects the importance of the karst
phenomenon in this area. As a consequence, torrents have transitory flow rates determined only by
significant rainfall events.
Frequent specimens of valleys suspended above the main valleys are seen, whose origin can be
attributed to the rapid incision of valley floors in connection with recent tectonic uplifting
movements in the Lessini Mountains area, but also with the progressive karstification of the
network of dry valleys (SAURO, in SORBINI 1993).
Four drainage basins can be identified:
• Alpone – Tramigna;
• Vajo di Squaranto;
• Progno di Valpantena;
• Progno di Fumane.
As far as hydrogrology is concerned, the permeability of rock masses should be attributed to both
karst and the thick fracturation of lithotypes, where the latter phenomenon considerably increases
the secondary permeability of formations notoriously considered as scarcely permeable, such as,
for example, Biancone and Scaglia Rossa. For this reason, formations in this area have been
assigned an average permeability due to fracturation, because the fractures pervading these rocks
have such a continuity as to establish communication between the relevant aquifers and the main
underlying aquifer. However, this absorption is so slow that the water has the time to establish a
superficial circulation. Only locally suspended aquifers feeding low discharge springs have been
detected.
Rosso Ammonitico is accounted for a different hydrogeological behaviour, with water penetrating
inside it following a high number of vertical flow directions (SAURO, 1973).
The underground runoff of water is very fast in the areas where Oolite di S. Vigilio and Calcari
Grigi formations crop out, as very large caves are found here, which contribute to determine a
substantially horizontal flow (PASA, 1954).
The lithotypes found in the Lessini Mountains have been grouped under three main hydrogeological
categories:
• an eastern group characterised by volcanic lithotypes, with low discharge springs;
• a central-northern group with a predominance of jurassic–cretacic limestones, with karsttype
springs;
• a southern group characterised by cretacic and eocenic limestones and volcanic rocks: this
sector corresponds to the northern limit of the alluvial plain of the River Adige; springs are
of karst-type here and have limited flow rates.
The springs of the Lessini Mountains have variable discharges, closely depending on the local
hydrogeological conditions. Permanence times may range from a few months for the areas with a
primary porosity, up to a few days or a few hours for karst aquifers.
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There is a rather high number of springs at an altitude, which are characterised by temporary flows
and generally reduced discharges. These springs are closely dependent on meteoric events and are
essentially connected with localised epikarst systems having limited-size drainage basins.
The most important springs are located in the Fraselle Valley (which have been captured with
collection works below the river-bed), the Ossenigo springs (located in a small side valley on the
rocks hanging above the Adige Valley, and the Montorio springs.
5.1 The Montorio springs
The Montorio springs (figs. 25 and 26) are a very interesting natural phenomenon from a scientific
point of view, due to both their hydro-structural arrangement and to their remarkable total
discharge (5 m 3 /s). The gravelly-sandy alluvia of the Montorio area have four adjacent springs
connected to a buried karst structure (30-40 m deep). These springs have been studied for a long
time, since 1889; in particular, there is an extensive study published in 1993 by the Town Museum
of Natural History of Verona. These springs consist in four small lakes located in Montorio, 60 m
a.s.l., which are directly supplied by karst channels placed in the Calcari Grigi formation.
Figure 25: Small sand volcanoes at the water emergence of Spring Fontanon in Montorio
The springs are located at the outlet of the Vajo di Squaranto, which looks like a deep river karst
canyon elongated in the N-S direction for an overall length of approximately 30 km and about 3-4
km wide. In particular, the flow rates of the four springs have been studied from June 1988 to
March 1993 (ANTONELLI in SORBINI, 1993), during which period annual average rainfalls onto the
catchment basin considered were of 1080 mm, a value close to the 30-year average.
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The mean discharge of the springs, concerning the surface flow volumes, is about 5 m 3 /s. So, we
may assume that a non-negligible amount of this discharge is lost in valley floor alluvia. The
minimum discharge measured after a long and special period of drought was 1 m 3 /s, while the
maximum discharge was 11 m 3 /s (ANTONELLI in SORBINI, 1993).
Figura 26: The Spring Fontanon in Montorio
The discharge of these springs is characterised by a high synchronism and is closely connected with
the rainfall regime of the mountain area. Flood peaks are registered within 24-48 hours after the
main rainfall events and are related with channel karst and well developed circuits. The storage
volume estimated with the depletion curve method has been found to be 6107 m 3 .
Geophysical tests and the drilling of well for potable-irrigation water supply have shown that the
depth of the groundwater table in alluvia varies from 20 to 30 m in the northern sector of the valley,
from 15 to 10 m in the central area, and is reduced to 1–5 m in the southern section. The thickness
of alluvial materials filling the valley is rapidly increased southward, reaching over 100 m at the
outlet into the plain (ZAMBRANO, in SORBINI, 1993). The rocky substrate near Montorio is at least
30-40 m below the ponds.
The catchment basin upstream the Montorio springs extends for approximately 100 km 2 and the
highest altitude reached is 1865 m (Cima Trappola). The hydrogeological basin of the springs has
been estimated to be of about 200 km 2 .
The Montorio springs have not been captured and the water, which was used by mills in the past
(fig. 27), now flow out feeding the Torrent Fibbio.
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Figure 27: Mill in the center of Montorio
In the 1988-1990 period, some samples of spring water and valley floor wells have been tested
(SORBINI, 1993). The quality of water was proved to be good: none of the per parameters examined
reached or exceeded the maximum admissible concentration for drinking use and nitrate levels
were among the lowest observed in the Verona area. The chemical-physical tests carried out
showed the prevalence of calcium bicarbonate, among salts, which proves that the water flows
inside calcareous rocks. The mean temperature is around 11 °C, PH is 7.6.
Microbiological tests showed a temporary faecal contamination, with an increase in these levels
close to the first flood events immediately following periods of drought.
The quality of water in the karst reservoir was only partially altered by the high number of
potentially polluting activities existing in the Lessini area, particularly those related to the
settlements: about 10,000 inhabitants in the mountain area (there is a remarkable increase in the
number of inhabitants for tourist purposes in the summer and during Christmas holidays), farming
activities (about 30,000 heads of cattle, 60,000 swine and a few million chicken), and animal
breeding waste spilling. Furthermore, some cattle is taken up to the mountains for pasture in the
more than hundred ”malghe” of the Upper Lessini during the summer.
6 HYDROGEOLOGICAL MAP
6.1 Choices and criteria
In order to create a hydrogeological map, first of all we have collected geologic and hydrogeologic
data regarding the area to be represented. Then, based on these data, we have identified the
“hydrogeologic units”, consisting in geometrically contiguous groups of rocks (lithotypes) having
similar permeability features (fig. 28). The work was carried out following the guide-lines issued by
the National Geological Service (today APAT). The lithotypes cropping out have been grouped in
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CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
units having homogeneous hydrogeological features and distinguished on the basis of their degree
of permeability.
Figure 28: Schematic hydrogeological sections of the Lessini Mountains. Keys – pale blu: calcareous-dolomitic Unit,
pale green: marly-calcareous unit, dark green: marly-clayey Unit, olive green: calcareous Unit, brown: volcanic Unit,
orange: colluvial and eluvial Deposit, red: main tectonic structures.
In figure 28 is shown the stratigraphic relation among the hydrogeological units; note the
difference between western and eastern Lessini, due to litho-stratigraphic variation in the two
areas.
The data contained in the basic geological map have been compared and integrated with other
thematic maps available at different scales and reprocessed to eventually obtain the
Hydrogeological Map here attached.
The “Geological Map of the Natural Park of Lessinia” (scale 1:40,000) has been selected among
the basic geological maps available and adopted, to be then integrated with the Sheets of the
Geological Map of Italy no. 35 “Riva del Garda”, no. 36 “Schio”, no. 48 “Peschiera del Garda”,
no. 49 “Verona” (scale 1:100,000). The Hydrogeological Map has then been drawn adding the
typical elements of the hydrogeology of the area (springs, wells, karst cavities, hydrography,
tectonics).
The map representation scale (1:50,000) was determined by the need to provide a good general
view of the different subjects represented with respect to the extent of the area (980 km 2 ).
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CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
Figure 29: Stratigraphic relationship among hydrogeological units outcropping in Lessini Mountains
6.2 Hydrogeological units
Six hydrogeological rock units have been defined, with permeability related to fracturation and
karstification. The loos materials are divided into four classes, depending on their permeability
(related to porosity):
Calcareous-dolomitic unit: it includes dolomites and dolomitic limestones, which generally consist
of massif beds or have an undistinguished stratification. Permeability here is related to fracturation
and karstification, and is locally remarkable. The complex has a thickness of several hundreds of
metres: altitude infiltrating water is propagated underground through the system of karst channels
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CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
and gets to the springs situated at the base in extremely rapid times. Karst springs have discharges
that can reach even a few cubic metres per second, as in the Montorio case.
Marly-calcareous unit: it includes limestones ranging from mildly clayish to marly with
intercalations of marl and shale. Usually, it is thickly stratified and densely fractured, which
generally gives a medium level of permeability. Suspended water flows are found locally, whose
extension and thickness is limited (at different altitudes) and which supply a high number of small
springs.
Marly-clayey unit: it includes thickly stratified marly limestones with important clayish and
organic-clayish intercalations. Permeability (related to fracturation) is low.
Calcareous unit: it includes marly limestones, compact limestones and nummulitic limestones with
macroforaminifers and lignitic intercalations. Permeability, related to karstification and porosity
(nummulitic limestones), is generally high.
Volcanic unit: it includes volcanoclastic rocks, sometimes stratified, breccias, hyaloclastites and
massive or altered basalt lava rocks. Permeability is generally low, locally variable depending on
the degree of clayey alteration.
Marly unit: it includes marls to more or less laminated marls. Permeability is low and locally
related to the degree of fracturation and fissuration.
Eluvial and colluvial deposits: these are eterometric deposits with abundant clay matrix and
coarse skeleton: alteration and degradation blankets of volcanic and sedimentary rocks, fillings of
the main karst depressions and colluvial deposits at the foot of hill slopes and in the areas between
valleys. Permeability is generally very low.
Debris and alluvial deposits: these are valley floor alluvial deposits, alluvial fans, colluvia and
landslide deposits, consisting of elements with a largely variable grain size (from big blocks to
gravelly, sandy and/or muddy materials), characterised by a generally high permeability. Locally,
permeability may be reduced in the presence of cemented levels or very fine grain material.
Colluvial and glacial deposits: these generally consist of prevalently fine-grain materials derived
from the alteration of the bedrock and from heterometric accumulations with abundant silty matrix
of glacial origin. Permeability is generally very medium or low.
Muddy-clayey alluvial deposits: they include high-heterometry deposits ranging from big blocks to
fine or very fine-grain materials with a muddy-silty matrix, generally loose, sometimes cemented.
Permeability is generally low or very low.
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CARTA IDROGEOLOGICA DEI MONTI LESSINI - HYDROGEOLOGICAL MAP OF LESSINI MOUNTAINS
CONTENT
1. GEOGRAPHICAL LOCATION............................................................................................................................ 1
2. CLIMATIC FEATURES........................................................................................................................................ 3
3. GEOMORPHOLOGY........................................................................................................................................... 5
3.1 Morphology................................................................................................................................................. 5
3.2 Karstification .............................................................................................................................................. 8
3.2.1 Valle delle Sfingi or the Sphinx Valley...................................................................................................... 13
3.2.2 Covolo di Camposilvano........................................................................................................................... 16
4 GEOLOGY ......................................................................................................................................................... 16
4.1 Stratigraphical succession........................................................................................................................ 16
4.2 Structural layout ....................................................................................................................................... 19
4.2.1 Tectonic structures in the central-western Lessini Mountains....................................................................... 20
5 HYDROGRAPHIC FEATURES AND HYDROGEOLOGY................................................................................. 21
5.1 The Montorio springs................................................................................................................................22
6 HYDROGEOLOGICAL MAP............................................................................................................................. 24
6.1 Choices and criteria.................................................................................................................................. 24
6.2 Hydrogeological units............................................................................................................................... 26
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