hydrogeology of. the mesozoic carbonate platform...

Karst Water Resources (Proceedings of the Ankara - Antalya Symposium, July 1985).

IAHSPubl.no. 161

HYDROGEOLOGY OF. THE MESOZOIC CARBONATE PLATFORM OF APULIA (SOUTH ITALY) AND THE REASONS FOR ITS

DIFFERENT ASPECTS

D. Grassi

Faculty of Engineering Basilicate University, Potenza Italy

T. Tadolini

Faculty of Engineering

University of Bari Italy

Summary

The carbonate platform of Apulia, although it has considerable lithological uniformity and although it has not undergone complex tectonic - paleogeographic event, contains highly diversified hydrogeological environments, some of which are of a very specific nature.

In fact the hydrogeology of the two main morphological - structural units which characterize it, i.e. the Murgian plateau and the Salento lowland, is substantially different although the groundwater belong to one huge aquifer.

The characteristic and variations in hydrogeological environment within the same Mesozoic platform should be seen essentially in relation to the effects produced on the permeability of the carbonatic assise and therefore on the subterranean hydric network by a differentiated tectonic - karstic evolution.

Introduction

Although the Mesozoic carbonate platform of Apulia is characterized by a considerable degree of lithological uniformity and has not been subject to complex tectonic and sedimentary events, it contains hydrogeological environments which differ markedly from each other, some of which are peculiar. In fact, while the waters circulating within the platform itself belong to one huge aquifer, the karstic hydrogeology of the two main morphological - structural units that characterize it is substantially differentiated into the plateau of the Murgia and the lowland of the Salento.

It has been possible to ascertain that this marked dissimilarity in hydrogeology is not due to particular lithological - stratigraphical situations that are intrinsic to the platform. In fact, the platform does not contain impermeable rocks of the clayey type; it is consituted only be very pure detrital and biostromal limestones, dolomitic limestones, and to a lesser degree calcareous dolomite rocks.

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IAHSPubl.no

294 D. Grossi & Tadolini

The circumcribed and sporadic presence within the carbonatic formation of lithofacies of cotidal - lagoon environment, which are only slightly karstifiable or not karstifiable at all, does not account for the hydrogeological peculiarities or dissonances affecting very vast areas and considerable thicknesses of carbonatic rock.

This paper, which will also take into consideration the results of recent researches carried out on the genetic and evolutional processes of karst phenomena in Apulia (3,4,6), aims at identifying the characteristics and the variations of the hydrogeological environment within the same Mesozoic platform; it also aims at demonstrating how this diversity in hydrogeological environment is to be seen essentially in relation to the effects produced on the current karstic aquifer by tectonic - sedimentary evolution and thus by differentiated karstic evolution.

Brief Outline of the Main Morphological - Structural Features of the Platform

The portion of the carbonate platform outcropping today streches continuously from the Ofanto river to the extreme southern tip of Apulia. Only in certain areas it is covered by modest residual outliers of Tertiary and Quaternary deposits, often of low thickness. It constitutes on huge geological body, of several thousand meters in thickness. corresponding almost exclusively to the central part of the original paleogeographic Mesozoic unit. As an autochthonous stratigraphical - structural unit, it took shape when the nearby Apennine chain was already coming to the end of its tectogenetic deformations. From neotectonics it has mainly drawn those orographic elements that distinguish it today.

Having played the role of foreland, the platform has only felt to a limited extent the effects fef Apenninic orogenesis and tectogenesis. It has therefore been subject to major tectonic events and also to complex paleogeographic and sedimentary events. In fact the morphology is flat virtually everywhere and is made uneven essentially by the karstic landscape. The rocky strata are subhorizontal almost everywhere. The most significant tectonic and morphological elements correspond to tensive faults oriented mainly in the WNW-ESE direction and to a lesser extent E-W, NE—SW, N—S. The main folding structures (very bland folds) are also oriented WNW-ESE.

Corresponding to the Murgia area the platform presents a horst structure, which gives rise to a fairly low plateau (maximum height 680 m), stretching in the WNW-ESE direction and pushing out as far as the "Messapic Threshold" (a paleostructure, buried under the Tertiary and Quaternary rocks and developing roughly in accordance with the Taranto -Brindisi alignment). From here the platform continues, up to the extreme tip of the Salento Peninsular, as a lowland.

Hydrogeological Features of the Carbonate Platform

In the Murgia area the platform has the peculiarity of possessing a permeability which is often decidedly low and always markedly discontinuous especially at depths at which one would expect to find groundwaters. Often this complex of rocks turns out to be practically impermeable even up to a depth of around 500 m below sea level. On the whole

Hydrogeology of the Mesozoic carbonate platform 295

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Fig. 1 — Diagram of specific yeald vs penetration into groundwater illustrating the various degree of permeability of the same karstic aquifer corresponding to the Murgia area, the Salento area and the "Messapic threshold". It can clearly be seen how the data relating to the wells of the Murgia are concentrated in the lower band of the diagram, while those of the Salento are distributed along a band running parallèle with the axis of ordinates. The positions of the wells reported in the diagram can be deduced from fig. 2 .


Fig. 2-Areal distribution of the permeability of the entire Mesozoic karstic aquifer deduced from the values of the relationship specific yeald (Q ) vs penatration (L) into the aquifer of the well. The extent to which the Murgia aquifer (area 1 : Q = 0.01-4 1/s.m; L = 1 + 280 m) diffères from the Salento aquifer (erea 3 : Q 30 + 110 1/s.m; L = 0.5 + 50 m) and the fact that "Messapic thereshold" (area 2 : Q 4 + 3 0 1/s.m; L = 1 + 150 m) presents transitional hydrogeologica! features can be seen clearly.

Fig. 3 — Shape of the piezometric contour lines (1) and of the 1 g/l isohaline (2) of the aquifer. In the dashed areas (3) the groundwater have a salt content greater than 3 g/l-


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Fig. 5 — Shape of the karstic aquifer o the Salento peninsular made evident by the piezometric heads and by the trend of the isohaline relating to the fresh water of the aquifer and to the underlying seawater.


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Fig. 6 — Shape of the karstic aquifer of the Salento peninsular deducible from the piezometric heads and from the trend of the isotherms of the fresh water and the underlying sea water. In white : creataceous limestones and dolomite rocks, representing the aquifer; in dotted line : tertiary - quaternary practically impermeable overlying rocks; s.f. water table; dashed lines isothermes (°C); MA. - Adriatic sea; M.J. — Ionian sea.

Fig. 7 — Isobaths of the karstic aquifer top in the northern Muriga area.


this permeability remains relatively low even within the aquifer (3, 7, 8). Although in a coastal area, this aquifer is essentially of the type that is confined from above, it has an irregular geometrical shape and varies considerably from point to point (as a result of the irregular shape of its top), it possesses hydrodynamic features and, markedly, a relatively low storage capacity. Only in a restricted coastal strip its permeability is high and such as to be comparable with that which predominates in the Salento area. AH this can be clearly deduced from the diagram of Fig. 1 and from the map of Fig. 2, as well as from the values of the piezometric heads and the shape of the piezometric contour lines (Fig. 3). ,;

Fig. 2 highlights the areal distribution of the permeability of the entire platform and thus the extent to which the aquifer of the Murgia is distinct from that of the Salento area; it is also clear as to the way in which the "Messapic Threshold" possesses transitional characteristics. Figures 1 and 2 have been produced bearing in mind the specific yeld as a function of the penetration of the well into the aquifer.

The most noteworthy consequences of this distribution of permeability within the platform are as follows:

a) while in correspondence with the Murgian block the groundwater is forced to flow under pressure and at a great depth below sea level (Fig. 4), in the Salento block the groundwater circulates freely everywhere and at slightly above zero sea level (Figs. 5 and 6);

b) in the Murgia area, even when in the more inland zones the freshwaters float on seawater penetrating the land, the thickness of the aquifer should be 50—60 times that of the Salento area, where the waters float everywhere on the sea - water that penetrates the terra ferma;

c) in the Salento area the aquifer has a precise lenticular shape (Figs. 5 and 6); on the contrary the Murgian aquifer is characterized by a highly irregular geometrical shape, as a result of the irregular trend of the impermeable roof which confines it (Figs. 4 and 7);

d) with respect to those of the Salento aquifer, the hydrodynamic features of the Murgian aquifer are such that: the specific yields are generally 100—300 times less, the aquifer is forced to assume piezometric heights that are even 60—70 times greater than those of the Salento aquifer, and it is more exposed to pullution arising from antropic activity while being much less vulnerable in terms of marine contamination;

e) in the Murgian area the excellent quality of the water (due to the very low saline content) is counterbalanced by the generally modest yields that can be drawn and the high costs resulting from the considerable depth of the aquifer; in the Salento area the relatively low cost of drawing activity is often nullified by the high saline content of the groundwaters (Fig. 3) or by the ease with which the freshwaters are salinized as a result of intensive and prolonged drawing activity.

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Fig. 8 — Thermal logs along the water column of the wells. The preferential water levels can be seen clearly.

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Fig. 9 — Preferential water levels made evident by the salt logs correlated with the salt contents of the water drawn (in the diagram the levels are shown by a gray band).


In particular it should be observed that in the Salento area permeability allows the groundwaters to circulate essentially freely (under pressure where the covering sediments, predominantly Tertiary, reach heights close to or below sea level) and to float everywhere on the underlying sea - water. The permeability together with the peninsular shape of the region makes the piezometric heights of the aquifer always low (2.5—3 m s.I.m. at most) and its thickness limited (Figs. 5 and 6). As a result of this the groundwaters flow towards the Adriatic Sea and the Ionian Sea, almost radially and with piezometric gradients being at most in the order of 0.2—0.3 %< (Fig. 3).On the contrary, in the Murgian area the same aquifer has an essentially and markedly artesian, character. Only along the coastal strip, and occasionally also in isolated points further inland, do the waters flow freely. The subterranean hydric flow is forced to take place almost everywhere at considerable depths: 200—400 m below sea level, in some cases even about and more than 500 m below sea level (Figs. 4 and 7).

Moreover the flow frequently takes place by means of markedly preferential water levels which have their own specific hydraulic heads and chemical - physical characteristics (Figs. 8 and 9).

Moreover, this means that the groundwater has high hydraulic heads1(Fig. 3) both in the more inland areas (150—200 m above sea level) and in the areas situated a few kilometers from the sea (even 20 m above sea level), and that it also flows under considerable and extremely variable piezometric gradients (1.5—10 %o). The degree of mobility of the groundwaters is almost always low; the isotopic contents of the groundwaters and the filtration velocity data provided by the radioactive tracers are in agreement in indicating that the residence times in the aquifer of the waters themselves are often very long: even several decades. As a result of the predominant piezometric heads, the thickness of the aquifer must be considerable. In fact, even in cases where, in the more inland areas of the Murgia, the aquifer floats on sea - water of continental intrusion, the beginning of the transition zone ought to be found at a maximum depth of 5,000—6,000 m below sea level. This hydrogeological environment is confirmed by the data provided by the thermal and saline logs, carried out with continuity up to a depth of around 900 m.

In fact from Figs. 3 and 10 it can be observed that:

— there is perfect agreement between the shape of the piezometric contour lines and the shape of the isohaline 1 g/l and of the isotherms of the groundwaters (these latter express the areal distribution of the temperature according to horizontal sections traced at zero level and at -200 m and —400 m below s.l.;

— there is perfect agreement between the shape of the piezometric contour lines, the isotherms and the isohalines of the groundwaters and the morpho-structural shape of the region. All this is governed by the main tectonic directions of the region (WNW-ESE, E-W, NE-SW);

— this agreement is more marked in the Murgia area (where the permeability is lower) than in the Salento area (where the permeability is higher and the aquifer is drained by the sea on three sides);


- the Salento aquifer drains the groundwaters of the Murgia aquifer (Figs. 3 and 10)

— the transition from the hydrogeoiogicai environment of the Murgia to that of the Salento area occurs through the structure of the "Messapic Threshold", which in

hydrogeoiogicai terms is identified with a wide band averaging 12 km in width. Besides a marked and more or less sudden loss of hydraulic head and therefore a considerable reduction in the thickness of the aquifer, the sudden thange in the Murgia environment corresponding with the "Messapic Threshold" is characterized by the sudden accentuation of the phenomenon of contamination of the groundwaters by the seawater (Fig. 3).

Tectonic-Sedimentary Events that have Principally Conditoned the Evolution of the Subterranean Water Network

The diversity in hydrogeoiogicai environment mentioned above depends on the different way in which the portion of the platform corresponding to the Murgia area of today and the portion corresponding to the Salento area of today have reacted, starting from the Paleogenic era, to the effects of paleotectonics and of neotectonics. In fact, it is from the Eocene era, when the platform underwent an initial partial loosening process through faults, that the two blocks began to evolve seperately in tectonic - sedimentary - pale-ogeographic and therefore karstic - hydrogeoiogicai terms.

In particular the events that have principally caused this differentiated evolution of the subterranean water network can be summarized as resulting from be following circumstances:

— both during the Tertiary and the Quaternary eras the paleogeographic (and therefore the paleohydrogeological)conditions of the Murgia and the Salento appear to have been substantially different;

— the postcalabrian uplifting, which was responsible for the withdrawing of the pleistocenic sea, took place to differing extents and in different ways in each of the two blocks.

In particular, during the entire Tertiary era the portion of the platform corresponding to the Murgia area today remained constantly above sea level, often taking on the shape of a huge island. On the contrary, the portion corresponding to the Salento area today underwent several sedimentary and paleogeographic events caused by repeated and ever wider tectonic subsidences. In fact, while the (Eocene - Oligocène) paleogenic transgression involved only a small margin of the current Penninsula, on the other hand (Miocene and Lower Pliocene) neogenic transgressions led to the submerging of up to 70 % of today's Salento area.

Only the plio - pleistocenic transgression, the widest that the entire platform has ever been affected by, also involved small parts of today's Murgia area. Thus in the Lower Pleistocene, while the Salento area was completely submerged, most of today's Murgia area


Fig.10— Shape of the isotherms of the groundwater in the Murgia area and in the area near the "Messapic Threshold", traced at 0 m, —200 m and —400 m .


remained above sea level in the form of two large islands. Hence the carbonatic platform corresponding to these two islands has remained constantly above sea level from the end of the Mesozoic up to present day.

As regards the postcalabrian uplifting, the way in which it took place is of primary importance; in a differential form, both among small and among large carbonate blocks; through stopping phases and brief transgressive episodes, varying from zone to zone in terms of number and size.

The most noteworthy results have been that:

- the Murgia block underwent greater uplifting than the Salento block; the maximum uplifting values measured for the most inland areas of the Murgia are in the order of 500 m (with respect to the sea level of today), while those measured for the Salento block are in the order of 100—150 meters;

- within the Murgia block, differential movements took place among the neighbouring elementary blocks, sometimes in the order of several dozens of meters, while in the Salento block such movements rarely exceeded then meters.

Evolution of the Subterranean Hydro - Karstic Network

These events have considerably affected: the permeability features of the carbonate mass and the optimum positioning of the current aquifer along the vertical of the Mesozoic sequence. This can clearly be deduced bearing in mind that, because one is dealing with a hydric aquifer floating on sea water penetrating in terra ferma, there is a close link between: the position of the aquifer (along the vertical of the rocky formation) and sea level; the thickness of the aquifer, the hydraulic head and the permeability of the aquifer; the geometrical shape of the aquifer and the geographical shape of the region.

Thus, as regards the Salento area, the influence of the tectonic events on the subterranean water network may be summarized as follows :

a) the repeated vertical variations in sea level (and therefore in the aquifer) have encouraged genetic - evolutional processes of karstic analization and vascularization of the entire hydric networks (5);

b) the lowland and peninsular nature which the Salento area has always possessed has meant that the area has never had an aquifer of any great depth and that it has always been particularly sensitive, in an immediate and determining way, even to relatively minor movements between the sea and the terra ferma. Moreover, the peninsular shape of the area has stimulated draining of an almost radial kind;

c) the relatively modest size of the repeated movements of subsidence and of uplifting has meant that the respective migrations upwards and downwards of the karstic aquifer were carried out in an equally low thickness of rocky assise which was always roughly the same. Thus in its evolutional history, the aquifer of the Salento


area has done nothing other than follow pathways that had previously been traced, ali of which has led to a higher degree of vascularization of the hydric - karstic pathways;

d) since one is dealing with a lowland (three quarters of the Salento area today has an average height of 70 m, with maximum heigths generally not even reaching 100 m), the uplifting of 100—130 m, which the sea level has undergone during the last 15,000 years, has greatly favoured the current hydrogeological environment. In fact, it has placed the aquifer once again in the highest and most permeable part of the carbonate mass, at a depth that is not very different from the one it occupied in the past.

As regards the Murgia area, the Influence of paleotectonics on the current hydric network is to be considered as being fairly minor (3,4,6,9). It has only slightly of moderately fractured the rocky mass and has allowed the karstic process to take place essentially on the surface and up to modest depths. Moreover the very long (at least 65 M.Y.) and uninterrupted continental phase that most of the area has gone through has favoured the occlusion of many of the tectonic joints by means of recrystallized calcite and terra rossa (3,4,6).

The influence of neotectonics on the current hydrogeological environment is to be considered as being on the whole negative, for the following reasons:

a) both the plio - pleistocenic subsidence movements and the subsequent uplifting movements occurred in differential forms, both among the large blocks and among the elementary blocks. This means that the current vertical position of the original most highly fractured, or rather karstified, rocky strata (belonging to the old hydric network) varies considerably from one point to another, as a result of the different size and/or of the direction of the vertical movements affecting the blocks which contain them. It follows that, in direct lateral contact, rocky strata with varying types and degrees of permeability may be found today (9);

b) because of the differential nature of the movement, the old Tertiary aquifer today is considerably split and dislocated at different heights : part of it is positioned in such a way as to contribute in forming the present percolation zone (corresponding to the medium - upper heigths) while part of it still lies on the sea bed;

c) because of the degree of post - calabrian uplifting, in the last 700—800 thousand years the subterranean hydrography has changed markedly, as it has been forced to migrate do depths never before reached. Therefore most of the current aquifer occupies, along the vertical of the carbonatic assise, a position that was never occupied by the original aquifer or by that of the Pleistocene (even more superficial than the previous one); the hydric network there is still young, also bearing in mind that it is only in the last 5,000 years (flandrian transgression) that it has taken up the position that it occupies today.

Finally, it should also be pointed out that the differential movement that has taken place between the Salento block and the Murgia block, through the "Messapic Threshold",


has led to the lateral contact of rocky strata which are markedly different in terms of intensity of fracturing and of karstification; it has put the Salento area in the position of draining the waters of the Murgia aquifer.

REFERENCES

Cotecchia V., Grassi D., Tadolini T. 1974. Groundwater pollution and hydrogeological features of the karst aquifer in Apulia (Southern Italy). Geol. Appl. e idrogeol., vol. IX.

Cotecchia V., Tadolini T., Tulipano L. 1978. Groundwater temperature in the Murgia karst aquifer (Puglia - Southern Italy). Internaz. Symp. on KarstydroL, Budapest, Vol. II,

Grassi D. 1973. Fondamentali aspetti dell'idrogeologia carsica della Murgia'(Puglia) conparticolare riferimento ai versante adriatico. Geol. Appl. e Idrogeol., Vol. VIII.

Grassi D.,i 1974. U carsismo della Murgia (Puglia) e sua influenza sull'idrogeologia della regione. Geol. Appl. e Idrogeol., Vol. IX.

Grassi D. 1983. Difformita di ambiente idrogeologico promossa in seno alia piattaforma carbonatica appula da un'evoluzione tettonico - carsica differenziata. Geol. Appl. e Idrogeol., VOL XVIII - parte I.

Grassi D., Romanazzi L, Salvemini A., Spilotro G. 1982. Grado di evoluzione e ciclicate del fenomeno carsico in Puglia, in rapporte aiî'evoluzione tettonica. Atti del II Simp. Int. sulla "Utilizzazione della area carsiche", Bari.

Grassi D., Tadolini T. 1974. L'acquifero carsico della Murgia nord - occidentale (Puglia). Geol. Appl. e Idrogeol., Vol. IX.

Grassi D., Tradolini T., Tazioli S., Tulipano L. 1977. Ricerche sull'anisotropia dei caratteri idrogeologici della rocce carbonatiche mesozoiche della Murgia nord - occidentale. Geol. App. e Idrogeol., Vol. XII, p. I.

Grassi, D., Tulipano L. 1983. Connessioni tra assetto morfo - strutturale della Murgia (Puglia) e caratteri idrogeologici della falda profonda verificati anche mediante I'analisi della temperature della acque sotteranee. Geol. Appl. e Idrogeol., vol. XVIII, p. I.

Tadolini T., Tulipano L. .1977. Identification by means of discharge tests of water - beraing layers in fracturated and karstic aquifers through the analysis of the chemico - physical properties of pumped' waters. Symp. on Hydrodinamic diffusion and dispersion in porpous media, Pavia.

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