Foreland deformational pattern in the Southern Adriatic Sea

Two major deformation belts occur in the portion of the Adriatic Sea offshore the Gargano Promontory. Although these two belts display similar characters on seismic profiles, they are different in other respects. The NE-SWtrending Tremiti Deformation Belt, located north of the Gargano Promontory, originated during the Plio-Quaternary, while the E-W-trending South Gargano Deformation Belt, located south of the Gargano Promontory, formed in a time span that goes from Eocene to early Pliocene. On the ground of structural and stratigrafic evidence these deformation belts are interpreted as originated by tectonic inversion of Mesozoic extensional faults. This inversion tectonics, of Tertiary age, can be related to the evolution of the fold-and-thrust belts that surround the Adriatic Sea. A moderate seismic activity, recorded around the Tremiti Island, and historical seismological data suggest that the whole of study area is, at present, seismically active. Therefore, this portion of the Adriatic block still represents a preferental site of deformation.


Introduction
This study aims at contributing to the investigation of an area in the Southern Adriatic Sea where the presence of tectonic structures of possible regional significance has been reported and where seismic activity occurred in the last years.The area is located offshore the Gargano Promontory and rests within the so-called <<Adria continental block>> that is the foreland of the Apennines and Dinarides fold-and-thrust belts.Whether Adria belongs or not to the African plate is at present a matter of debate, and some Authors (Favali et al., 1990; Westaway, 1990) favour, mainly on the ground of seismological data, a plate boundary running just across this part of the Adriatic Sea.
Using a data set consisting of exploration wells and seismic profiles, mostly commercial but some collected on purpose, we attempt to define geometries, trend and geologic evolution of the above-mentioned structures and to relate them to the observed seismicity.The results we obtain, although still preliminary, show a picture of complexity so far unreported that we believe may be relevant to interpret the kinematics of the region.

Geologic setting
The area we examine (fig. 1) lies within a portion of relatively undeformed continental crust (Adria) rimmed on three sides by thrust and folded units belonging to a previous passive margin succession.They are: to the west the east-verging Apennines, to the north the southverging Southern Alps, and, to the east the westverging Dinarides (Channel1 et al., 1979).Adria represents the foreland area of these fold-andthrust belts and, as such, it has been deflected in response to the load applied by the adjacent mountain belts (Moretti and Royden, 1988).All along the deformed belt surrounding Adria a marked seimicity is reported (McKenzie, 1972).This seismic activity defines the main plate boundary between Adria and Europe.The connec- tion of Adria with the African plate is favoured by several Authors (Argand, 1924; McKenzie,  1972; Channel1 et al., 1979; D'Argenio and Horvath, 1984; Dewey et al., 1989), while Others (Vanderberg and Zijderveld, 1982; Morelli,  1984; Jongsma et al., 1987; Anderson and Jackson, 1987; Favali et al., 1990; Westaway, 1990)  consider Adria, at present, as a separate microplate.
Lithospheric thickness in this region is in the order of (100 + 110) km (Calcagnile et al., 1982;Mueller and Panza, 1984) while the crust is 30 km thick on average (Morelli et al., 1969;Geiss, 1987), although some thickness variations do occur.These geophysical data, therefore, indicate that we are dealing with a piece of truly continental lithosphere.
Moho depth underneath the Gargano Promontory is estimated to be less than 25 km.This is considerably shallower than the adjacent region where Moho depths are in the order of 35 krn (Geiss, 1987).
Units belonging to the foreland sedimentary cover outcrop in the Gargano Promontory that is located just onshore the study area (Martinis and Pavan, 1967;Cremonini et al., 1971).They are made up essentially by carbonate rocks ranging in age from Jurassic to middle Miocene with a thickness of over 4000 m.Exploration wells have found Triassic evaporites at depths of about 6000 m.The Gargano Promontory appears as a broad east-west-elongated anticline affected by faults trending NW-SE, E-W and, in minor extent, NE-SW.The nature of these fault systems and their cronology are not too well assessed.Apart from the Notes to the Geologic Map, only few papers have been devoted to the study of this Promontory and they differ significantly in their conclusions.Some Authors put more emphasis on sinistral strike-slip tectonics acting along the E-W-trending Mattinata fault system (Funiciello et al., 1988), while Others favour N-S and NE-SW compressional phases (Ortolani and Pagliuca, 1987).They only agree on the fact that the last phase reactivated in extension many of the previous faults and that some component of strike-slip motion occurred along the E-Wtrending fault system.
The Tremiti Islands, located north of the Gargano Promontory, consist of four small islands where a Tertiary succession about 500 m thick outcrops.Sediments range in age from Paleocene to Upper Pleistocene and bedding planes generally dip SE-ward defining a NE-SW-trending monocline.The tectonic evolution of these Islands, according to the Geologic Survey, is as follows (Cremonini et al., 1971).Minor folds, trending NE-SW and WNW-ESE, affected only Paleocene and Eocene sediments while faults with little throw, mainly trending NW-SE but also E-W, cut across sediments older than Middle Pliocene.Plio-Pleistocene sediments record a progressive southeastward tilting of the monocline of about 5'-6'.A micro-and meso-structural study carried out in these islands (Montone and Funiciello, 1989) emphazises the role played by E-W strike-slip faults.In their interpretation the Tremiti Islands represent a pushed-up ridge within a E-W dextral strike-slip system, with the main faults being, however, in an unknown position offshore.
As far as the geology offshore Gargano is concerned, Finetti (1984) in his synthesis of the Adriatic Sea, suggested the presence of two major dextral strike-slip faults, the NE-SWtrending Tremiti fault and the E-W-trending Mattinata fault, located north and south of the Gargano Promontory, respectively.Unfortunately, the detail of these faults is not represented.The area south of Gargano has been further investigated (De'Dominicis and Mazzoldi, 1987;  Colantoni et al., 1990) and an E-W-trending structural high has been observed.The Authors interpret this structure either as a diapir anticline originated during Mesozoic salt tectonics and later reactivated by strike-slip faulting (De'Dominicis and Mazzoldi, 1987), or as a positive flower structure originated during dextral strikeslip movements active until the base of Pliocene (Colantoni et al., 1990).
The E-W trend defined by the Gargano Promontory is also well expressed in the Bouguer gravity anomaly contours (fig.2).The positive anomaly, that reaches 110 mGal over the Promontory, stretches offshore to a considerable extent following the same trend (Finetti and Morelli, 1973).The same applies to the magnetic basement that underneath the Gargano Promontory and its offshore is about 2 km higher than the surrounding regions where it is deeper than 10 km (Cassano et al., 1986).et al. (1992), gravity data from Finetti and Morelli (1973).
Paleomagnetic data collected in Upper Cretaceous sediments of the Gargano Promontory, show a CCW rotation of about 23' with respect to the Africa pole (Channell, 1977;Vandenberg, 1983).However, a rotation of 17' seems more appropriate taking into account conservative data evaluation (Lowrie, 1986).Gargano Promontory and Istria present the same amount of post Late Cretaceous rotation suggesting that this part of Adria behaved as a separate block with respect to Africa.Recent data (Tozzi et al., 1989) show a CW rotation of 9' in Eocene limestones.This would imply an alternate CCW and CW rotation of the Gargano area that, according to the Authors, fits amodel of E-W-trending strike-slip faults and block rotating about vertical axes.However, the limited amount of rotation and the slight worsening of the precision parametres of the Fisher statistics, after the tectonic correction, cast some doubts as to the age of magnetization that in fact has the same direction of the present Earth magnetic field before correction.

Seismicity
The Gargano Promontory and its neighbouring area are well known as a seismically active zone.Many earthquakes, such as the 1627,1646 and 1731 events, reached destructive effects.
Moreover, these earthquakes were often followed by many aftershocks and in some cases the seismic sequences lasted over one year (Baratta, 1901).Nevertheless, the seismicity was often mislocated as it was based only on macroseismic information.In fact, macroseismic investigation associates the epicentral area of an earthquake to the most damaged localities, where the damage can be caused by local geological situations.In many cases it is possible to infer that the earthquakes took place offshore but caused damages and site effects inland.For example the 1627 earthquake is classified as XI MCS.Such an intensity was reached only in some small centres of Gargano (San Severe, Apricena, etc.) (Postpischl, 1985a), but historical chronicles about tsunami waves along the northern coast suggest an offshore hypocentral area.A detailed analysis of the Italian seismic catalogues (Postpischl, 1985b;ING, 1990) points out the occurrence of seismic activity offshore Gargano.This activity is located both north and south the Promontory although, on the basis of these data, it is quite impossible to localize it correctly.
On the other hand, the implementation of the Italian Telemetered Seismic Network, of which the Istituto Nazionale di Geofisica is in charge, has allowed a better localization of the lowermagnitude earthquakes occurring in Italy and surroundings.During the last years (1 986-1990) three interesting seismic sequences occurred in the Southern Adriatic Sea and, when properly localized (Console et al., 1989; 1992) (fig.2), they appear associated with the Tremiti fault zone (as defined in Finetti, 1982 andFinetti et al., 1987) and demostrate that this structure is seismically active (Console et cd., 1992).Only the main shock of the 1988 sequence (mi, = 5.3) allowed one to compute a reliable focal mechanism.This mechanism was obtained using a (double couple^ model (using only teleseismic data) and was also supported by the Centroid-Moment Tensor (CMT) method (Dziewonski et  al., 1981).Both solutions show strike-slip component, more evident in the <<double-couple>>, and thrust component, more evident in the CMT.Tha actual fault plane was interpreted as striking ENE-WSW with a sinistral strike-slip movement (Console et al., 1992).

Data set
This study is based on a data set consisting of several exploration wells and of a rather dense grid of seismic profiles.Most of these profiles are commercial but some were collected on purpose during a cruise carried out in 1991 (fig.3).About 1000 km of multichannel seismic reflection profiles were collected offshore the Gargano Promontory onboard of the R/V Bannoch of CNR.The details of acquisition and processing are given in tables I and 11, respectively.Not all of the profiles were processed to the final slip; for some of them only the stacking was performed.
Seismic profiles have been interpreted defining first the unfaulted panels and then, within such panels, identifying reflection terminations and geometric arrangements of reflection packages.
A set of about 15 exploration wells has been used to work out the stratigraphy of the area and, whenever possible, well data have been tied to seismic profiles in order to put stratigraphic constraints to seismic interpretation.
Nine stratigraphic logs are shown in fig. 4 and 9 where they are arranged in two transects cross-  ing the northern and southern parts, respectively, of study area.
For the wells where the stratigraphic record is more complete, water-loaded subsidence curves have been computed using a modified version of the Program BASTA (Friedinger, 1988) that follows Steckler and Watts's (1978) equations.Thickness of dated stratigraphic intervals, porosity-depth relationships for different lithologies and paleowater depths are needed to calculate the water-loaded subsidence.The first piece of data derives directly from well logs, while porositydepth functions were taken from Sclater and Christie (1980) and Schrnoker and Halley (1982).mine and, for this reason, a depth range has been used so as to take uncertainties into account.Paleowater depth estimates are taken from Wood (1982).Airy isostasy has been assumed to describe the lithospheric response to loading.This assumption may be valid for the Mesozoic rifting subsidence but is likely to be not so good an approximation for the successive evolution of the area where some flexural rigidity has been shown to exist (Royden, 1988).
Sea-level variations have not been taken into account because the several sea-level curves presented differ significantly both in time and magnitude of fluctuations, and no one has gained Paleowater depths are more critical to deter-general acceptance.

Interpretation
In a describing and interpreting the data, the study area has been subdivided into two parts separated by the Gargano Promontory.The reason for this is not just purely geographic because, as will be shown below, these two parts appear to have undergone a different geologic evolution, at least from the Tertiary onward.After the two trasted in a unified picture will be presented.

Zone North of Gargano
Fig. 4. Summary stratigraphy from coast to offshore north of the Gargano Pronlontory.Note the The major feature in this Part is a NE-SWexpansion of the Mesozoic section and the reduction trending structure that is well expressed in the of the Paleogene section that occurs passing from bathymetry (fig.3).The Tremiti Islands belong Stella to Famoso.For further discussion, see text.plete, more expanded and made up by deeper-passive margins (Wooler et al., 1992).From 50 water sediments.Following the Messinian evap-to 30 Ma minor pulses of uplift and subsidence orites are the fine-grained clastic sediments of the are observed while at about 5 Ma the onset of the Apennine foredeep that thin northeastward.The Apennine foredeep is clearly marked.tectonic subsidence curves (fig.6a)) show a typi-Three seismic lines have been selected to ilcal passive margin trend from 220 to 50 Ma.On lustrate the principal aspects of this zone (fig.5). the contrary of what commonly reported in the The dating of horizons and seismic units have literature (Bernoulli and Jenkyns, 1974; Winterer been assessed by means of well ties whenever and Bosellini, 1981) the main rifting episode is possible.The westernmost line (B-427) is far observed in Late Triassic, before the carbonate away from the TDB and shows, at a depth of platform drowning occurred.A comparable about 3 s Two Way Traveltime (TWT), apackage early-to mid-Triassic stretching episode results of reflections onlapping onto a tilted horizon.from a study carried out along the Tethyan From well ties the onlapping package is at- Fig. 6b).Observed water-loaded tectonic subsidence curves based on well data from the area south of the Gargano Promontory.tributed to Mesozoic pelagic sediments while the foredeep sediments display an onlapping unit, at underlying horizon represents the top ofplatform the base, followed by a thick packet of northcarbonates tilted by extensional block faulting.eastward prograding clinoforms.Line PG-16 and Above the southwestward tilted unconformity PG-5 cross each other on top of the TDB and Ã ‡MÃ that marks the base of the Plio-Quaternary, illustrate its general folded appearance.The TDB formation Belt (SGDB).On the contrary of what happens for the TDB, the SGDB has no bathymetric expression but does result as a relative high in the base Plio-Quaternary isochron map (fig.8).As discussed below, the SGDB displays no substantial activity during the Plio-Quaternary.
Wells are located on either side of the structure as well as on its top.A N-S transect, perpendicularly crossing the structure is shown in fig.9.As seen in the area north of Gargano, the Mesozoic is characterised by the presence of Triassic and early Jurassic platform carbonates overlain by pelagic sediments.This transition from platform to basin sediments never occurred in the near-coast well Jolly.Interestingly, the major depocentre of Mesozoic pelagic sediments occurs in the well Gondola that is located right on the top of the SGDB.These sediments have been relatively uplifted with respect to the adjacent coeval sediments.Tertiary sediments are noticeably absent over the SGDB while they occur in two depocentres on either side of the structure.Paleocene and Eocene sediments have never been encountered in any well of this area.Tectonic subsidence curves (fig.6b)) show Late Triassic-Early Jurassic rifting and ensuing smooth thermal subsidence interrupted, between 35 and 15 Ma, by slight uplift followed by a subsidence pulse centred at about 20 Ma.Another subsidence acceleration occurs around 5  Ma and is related to the south Dinarides foredeep.on the seismics and its relationship with the thick Of the three lines in fig. 10 two (D-436 and Mesozoic pelagic succession reported in the well PG-22) cross the SGDB, while the third one (F-8) is not clear.However, on the southern flank of is located on the easternmost continuation of this this broad asymmetric fold a seismic unit with belt where its trend changes substantially.Line parallel reflections onlapping at its base and tmn-D-436 crosses the structure where it is about to cated at its top by the Ã ‡M unconformity is well die out westward.The SGDB is very narrow and displayed.Such a unit likely represents the to the south of it there is a wedge-shaped unit with Oligo-Miocene sequence and rests on the Mesoonlapping reflections at its base.The lower part zoic pelagic sediments. of this unit represents Mesozoic pelagic sedi- The onlap of the Plio-Quaternary sediments ments onlapping the tilted top of platform carbo-onto the Ã ‡M reflection is remarkable and is in nates, while the upper part, bounded on top by the agreement with what observed in the previous Ã ‡M reflector and onlapping at its base, is profile.Moving eastward, the SGDB changes its thought to represent the Oligo-Miocene se-trend (fig.8).Work in this sector is still in proquence.In reality, as all of the wells are located gress, but there is good evidence of contraction onto relative structural highs, it is not given to as indicated by the fault-bend-fold geometry disknow if the lack of Paleocene and Eocene is real played in Line F-8 where the folded ~M Ã horizon all over the area or if it represents a biasedpicture. is onlapped by Plio-Quaternary sediments.In Line PG-22 the SGDB is wider and is better Also in this area, as in the north Gargano one, developed.The well Gondola is located afew km Mesozoic rifting favoured the accumulation of to the east of this line but, unfortunately, the great thicknesses of shallow-water carbonates internal geometry of the structure is not resolved and led, eventually, to the breakdown of carbo- nate platforms and to the deposition of pelagic sediments within half-grabens.Paleocene and Eocene sediments are not reported in exploration wells although they may occur in undrilled structural lows.The observed onlap of the Oligo-Miocene sediments onto the tilted Mesozoic units indicates that some tectonic activity occurred sometime during Paleocene and Eocene.This tectonic episode originated the SGDB which was later reactived in Messinian time, as documented by the tilted Oligo-Miocene sediments and by the onlapping Plio-Quaternary strata.Unlike the TDB, where most of the deformation took place during the Plio-Quaternary, the SGDB apparently was not active in that time interval.The broadly folded reflectors of the SGDB, its position above a Mesozoic extensional fault (D-436) and the presence at its core of a depocentre of Mesozoic pelagic sediments (well Gondola) suggest that this structure might be related to tectonic inversion of a previous extensional fault.

Summary
Two major deformation belts occur offshore the Gargano Promontory with different trends and different geologic evolution.The TDB, to the north, has a marked bathymetric expression and a NE-SW trend.The development of this structure occurred essentially during the Plio-Quaternary and its activity lasted until very recent.The E-W-trending SGDB, on the other hand, lacks bathymetric expression.The onset of this structure is difficult to constrain, although it occurred sometime within the Paleocene-Eocene time span.Assuming that this structure is due to inversion tectonics, an Eocene onset is more likely because it would be coeval with a major tectonic pulse in the Dinarides.A second activation of the SGDB occurred during the Messinian.This seems to be the main event responsible for the deformation of the easternmost part of the structure whose trend swings from E-W to NE-SW.

SUMMARY LITHOSTRATIGRAPHY AND
Although the Gargano Promontory and its surroundines area are sites of seismicitv both Only a few papers have been devoted to this task and the results are sometime controversial; for 1984; ~olantoni et al., 1990) and as strike-slip sinistral in the onshore (Funiciello et al., 1988).Our study, that concerns only the offshore, allows one to define the timing of activity of the two major deformation belts present to the north and to the south of the Gargano Promontory.A summary of the deformational events occurred in these two zones, and described in greater length above, is illustrated in fig.11.Is is worth to point out that the two deformation belts show a different time of activity.The main deformational episode affected the TDB during Plio-Quaternary while in the SGDB it occurred about the Eocene.Given the position of the two deformation belts, it appears reasonable to link these episodes to the evolution of the adjacent chains.In particular, the SGDB seems to have recorded the onset of the Dinaric foredeep, while the deformation in TDB appears to be related to the evolution of the Plio-Quaternary Apenninic foredeep.It is likely that the Eocene episode was recorded, to a minor extent, also in the northern zone where indications of tectonic activity have been observed.In our opinion, this Eocene event marks the transition from an evolution controlled by extensional tectonics to one where contraction is the dominant motif.
As far as the structural style is concerned, the two belts show similar characters.They both appear as broad and unfaulted folds when looking at the upper sedimentary units.As pointed out above, stratigraphic and structural data suggest that these folds may have originated by tectonic inversion of Mesozoic extensional faults.Although other Authors (Finetti, 1984;Colantoni et ul., 1990) interpret the TDB and the SGDB essentially as strike-slip features, we believe that there is no need to call upon transcurrent motion to explain the structures observed.While the arguments used to support strike-slip tectonics (great length of the SGDB and one change in the sense of separation; Colantoni et al., 1990) are not unambiguous, we fail to recognize other characters considered typical of strike-slip tectonics.For example, en-echelon structures related to the main fault zone are absent, and, although the SGDB is rather curvilinear (fig.8), no evidence of structures connected to restraining and releasing bend has been observed.However, a strikeslip component of motion cannot be ruled out after this, still preliminary, interpretation.
A moderate seismic activity offshore Gargano has been recorded in the last few years andmostly in connection with the TDB.Small earthquakes occurred also south of Gargano, but their magnitude is too small to allow areliable location.Data concerning historical earthquakes indicate that seismic activity occurred both north and south of Gargano and, therefore, both areas can be considered at present seismically active.The link with the structures observed on seismic profile is neverthelesses far from clear.In fact, although the TDB has been actively deformed until recent time, the SGDB ended its activity by the early Pliocene.It is worth to point out that the seismicity of the Adriatic Promontory occurs in a portion of this continental block where a great deal of structural deformation is also observed.It seems therefore that the Gargano Promontory and its offshore represent an area of foreland where deformation preferentially concentrated since the emplacement of the surrounding foldand-thrust belts.The large gravimetric anomaly present above the Gargano Promontory and roughly elongated E-W (fig.2) appears to be related to a perturbing mass located at a depth of about ( 3 5 ~4 0 ) krn, i.e. within the lithospheric mantle.The horizontal component of the gravity field of a rotating Earth acting on this mass anomaly can increase the local stress field (Gasperini et ul., in preparation) and this may also contribute to the recent seismicity.
Looking at the whole of the Adriatic block (fig.12), 2 appears that the foredeep system is asymmetric and that the axis of the peripheral bulge presents a major kink.The Gargano Promontory is located within this zone of distorsion where the stress propagating from the surrounding chains is likely to be amplified.This fact, couoled with the occurrence of inherited Mesozoic weakness zones, may have favoured the tectonic inversion that we envisage as the main L deformational feature of the area.

Conclusions
The area offshore the Gargano Promontory has been investigated using multichannel seismic reflection profiles and exploration wells.Two major deformation belts have been observed north and south Gargano, the NE-SW-trending TDB and the E-W-trending SGDB, respectively.
Although these zones were known already in the literature, our study points out that their evolution was more complex than previously mentioned.The TDB is essentially of Plio-Quaternary age, while the deformation of the SGDB began during the Eocene and was completed by the early Pliocene.Despite previous interpretations regarding these structures as due to strikeslip faulting, we propose that tectonic inversion of Mesozoic extensional faults can equally explain the observed geologic features and also fits the regional geologic setting of a foreland surrounded by fold-and-thrust belts.The stress propagating from the adjacent mountain chains is believed to have caused the tectonic inversion: the timings of deformation in the chains and in the Garganic area are, in fact, almost coincidental.
The area is at present seismically active suggesting that it still represents a preferential site of deformation within the Adriatic continental block.

[Fig. 1 .
Fig. 1.Summary geological sketch of the central Mediterranean region.Bold outline represents the study area that surrounds the Gargano Promontory.

Fig. 2 .
Fig. 2.Bouguer gravimetric anomaly field and earthquake epicentres of the Gargano Promontory and surrounding areas showing the + 100 mGal anomaly located on the promontory and the focal mechanism of the 5.3 magnitude earthquake that occurred in 1988 midway between the Italian and Yugoslavian coasts.Seismicity data fromConsole et al. (1992), gravity data fromFinetti and Morelli (1973).

Fig. 3 .
Fig. 3. Grid of seismic lines utilised in the present study (dotted: commercial; continuous: PG91 cruise) and location of public domain exploration wells.Bold lines indicate seismic sections illustrated in fig. 5 and 10. ~a t h ~r n e t r ~f r o r n IBCM dataset.

Fig. 5 .
Fig.5.Selection of seismic lines north of the Gargano Promontory showing the general stratigraphic features of the area (Line B-427) and the geometry of the main structural feature of this zone, the TDB (Lines PG-16 and PG-5).For discussion, see text.

Fig. 8 .
Fig.8.Base Pliocene isochron map of the sector south of the Gargano Promontory.The SGDB can be followed from close to the Gargano Promontory, eastwards, where it changes direction and shows up more markedly.Deepening towards the north-east reflects the proximity of the foredeep related to the southern Dinarides thrust belt.

Fig. 9 .
Fig.9.Summary stratigraphy along a N-S transect south of the Gargano Promontory.Note that Gondola, located on the SGDB, shows the most expanded Mesozoic pelagic sequence, notwithstanding the probable erosion of further section since Plio-Quaternary elastics rest directly onto lower Cretaceous pelagic limestones (about 110 Ma of missing section).

Fig. 10 .
Fig. 10.Seismic sections showing the structural characters of the SGDB passing from west (Line D-436) to east (Lines PG-22 and F-8).For discussion, see text.

Fig. 12 .
Fig. 12. Sketch map of the Adriatic foreland with an outline of the flexural bulge resulting from the double load of the Apennines and Dinarides fold and thrust belts.

Table I .
Acquisition parameters.

Table 11 ,
Processing sequence.tothis structure that is hereafter referred to as Trerniti Deformation Belt (TDB).Wells are unevenly distributed in this part (fig. ("') Time from zero water bottom.(")Time from base Plio-Quaternary.