Introduction

1The growth of a coastal plain is related to the re-distribution by waves and tides of the material brought to the coast by rivers during floods. Coastal areas close to high mountains, characterised by high rainfall and fronting low energy marine – or lake – basins are the most favourable sites for the development of deltas and related coastal plains.

2Human presence and climatic changes also play an important role in the evolution of deltas and coastal plains (Bird, 1993; Douglas et al., 2001). In particular, the Mediterranean basin and its highly populated coastal areas show a long and complex evolution marked by several phases of deforestation, soil erosion and coastal progradation due to the superimposition of climatic changes and the continuous human impact during the last millennia (e.g.: Brückner, 1986; 1997; 1998; Jelgersma, 1988; Brückner and Hoffman, 1992; Provansal, 1993; Boenzi et al., 2000; Amorosi and Milli, 2001). However, the influence of strong earthquakes, which are often accompanied by rapid, vertical coseismic movements and by tsunamis, is generally neglected in the reconstruction of the geomorphological evolution of delta areas and coastal plains. To fill this gap, interdisciplinary research has been carried on the Fortore River coastal plain and on the Lesina Lake area (southern Italy) (Fig. 1), a region strongly affected by several earthquakes during the last two millennia (Tintiet al., 1995; Gianfredaet al., 2001; Mastronuzzi and Sansò, 2002; De Martiniet al., 2003).

1. Geological and geomorphological setting

3The study area is located along the northern coast of the Gargano Promontory. The Gargano area is the most uplifted area of the Apulia region, which is the emerged part of the Adria plate. The latter, in turn, represents the foreland of both the east-verging Apennine and west-verging Dinaric orogens (Channelet al., 1979) (Fig. 1). The Apulian foreland is made up of a Precambrian crystalline basement associated with a continental Permo - Triassic cover. This is overlain by an anhydritic-dolomitic Triassic succession and by a 6000 m thick sequence of Jurassic-Cretaceous carbonate platform sediments. The entire sequence is capped by thin deposits of Neogene and Quaternary age (Ricchettiet al., 1988).

4The distribution of the epicentres of the earthquake occurred in this area shows that the seismic activity clustered along two main tectonic alignments (Fig. 2): the E-W trending Mattinata fault (Billi and Salvini, 2000; Valensiseet al., 2003), and the tectonic alignment running in a SSW-NNE direction along the right side of the Fortore River valley, which extends offshore up to the right-lateral transfer zone of the Tremiti Islands. According to Del Gaudioet al. (2002), the structure of the Fortore River valley is the most likely cause of the major historical earthquakes recorded in the area and of the generation of large tsunamis, which struck the northern coast of the Gargano Promontory with a recurrence period of about one thousand years (Gianfredaet al., 2001).

5The study area is located at the limit between the Mesoadriatic trench to the north, the Gargano Promontory to the east, the northernmost part of the Tavoliere delle Puglie alluvial plain to the south and the Apenninic chain to the west. It comprises three main units which are, from the west to the east:

• the Fortore River coastal plain (Photo 1);

• the Punta delle Pietre Nere head (Photo 2);

• the Lesina Lake coastal barrier (Photo 3).

1.1. The Fortore River coastal plain

6The Fortore River is a typical Mediterranean river marked by considerable discharge during winter, with occasional flooding of the coastal plain, and very small load during summer. At present, the river is characterised by low discharge and solid load due to the presence of several dams built within its drainage basin (Caldaraet al., 1998). The latter is shaped in a Paleogenic clayey complex, in Miocene flysch deposits and in Miocene-Plio-Pleistocene clays, sands and conglomerates (Melidoro, 1971). Morphologically, the uppermost part of the basin shows slopes strongly affected by landslides, whereas the lower part is marked by an asymmetric profile. The gentler-sloping left bank is marked by a low staircase of fluvial terraces.

7The coastal plain is elongated E-W for about 30 km, from Marina di Chieuti (Molise), at the eastern side of the Apenninic Chain, and Torre Mileto head on the Gargano Promontory (Fig. 2). The plain comprises terrigenous sands drifted NW to SE along the Adriatic coast of Italy and mainly discharged by the Fortore and Biferno Rivers. These sandy deposits are arranged in a complex of swamps, lagoons, dune belts and beach sediments, cut by relict river channels characterised by a strong variability in arrangement (Mastronuzziet al., 1989) (Photo 1).

8The plain stretches between +6 m of elevation and the present-day sea level, at the foot of a low cliff whose top constitutes the outer border of the lowermost marine terrace, locally placed between +25 and +10 m above present mean sea-level.

9According to De Martini et al. (2003), sandy layers ascribed to three different tsunamis have been detected in the uppermost part of the coastal plain sequence. The youngest one is referred to the 1627 AD strong earthquake whereas the lowermost sandy layer would have been deposited during the interval 3630-3350 yr BC.

1.2. Punta delle Pietre Nere head

10The Punta delle Pietre Nere head separates the Fortore River coastal plain from the sandy coastal barrier of the Lesina Lake (Photo 2). At this head, igneous rocks, Upper Triassic black limestones and deformed gypsum deposits crop out in a limited area. According to Cotecchia and Canitano (1954) these rocks rose through the Jurassic-Cretaceous sedimentary sequence as a result of diapirism; however, other interpretations suggest a genetic process of tectonic squeezing (Guerricchio, 1983) and wedge expulsion by tectonic compression (Ortolani and Pagliuca, 1987). According to Bigazziet al. (1996) it is likely that the Triassic carbonate-evaporite sequence at the Punta delle Pietre Nere head was squeezed and pushed upwards by tectonic events occurred in late Pliocene-early Pleistocene times. The Punta delle Pietre Nere head is also marked by the presence of a Holocene bioherm characterised by calcareous tubes of polychaete annelids, calcified Serpulid worms, sponges, Vermetus sp. and coralline algae and marked by large globular colonies of Cladocora caespitosa (Linneo) in living position. U/Th and radiocarbon age determinations fix its age at 8.8+0.1/-0.2 Ka and 5.9±110 conv.Ka respectively. The detailed study of this bioherm indicates that this particular area has been affected during the Holocene by an average uplift rate of 1.5 mm/yr. Moreover, younger biogenic encrustations detected on the bioherm surface indicate that the general uplift of the area would have occurred in accordance with a «seismic cycle» characterised by a slow coastal subsidence, rapidly increasing shortly before a major earthquake, followed by coseismic vertical displacements with amplitude > 0.5 m (Mastronuzzi and Sansò, 2002).

1.3. The Lesina Lake coastal barrier

11The Lesina Lake is a coastal lake (sensuEmery and Stevenson, 1957) completely isolated from the Adriatic Sea by a continuous sandy coastal barrier. The barrier is elongated parallel to the shoreline with a length of 22 km and it is characterized by a mean elevation of about 3 m above present MSL, reaching a maximum elevation of about 8 m at Gravaglione locality. Its width ranges from 1400 m at its western tip, to a minimum of 350 m to the east. The lake is characterised by a maximum depth of about 1.9 m and by brackish waters fed by a number of fresh water springs. Two artificial channels connect the lake with the Adriatic Sea. The coastal barrier is characterized by the presence of four wide washover fans (Photo 3). Geomorphological analyses and radiocarbon age determinations suggest that they would be the result of three distinct tsunamis which struck the barrier about 2430 years BP, in the year 493 AD and the 30th of July 1627 AD, respectively (Gianfredaet al., 2001).

12The sedimentary sequence in the Lesina Lake area has been recently revealed by the digging of a channel at the bottom of the lake to facilitate navigation. The sequence is composed of yellow sands with a eurialine fauna marked by abundant Cerastoderma sp.. This level is capped by a 10 cm -thick duricrust marked by mud crack, overlain by two metres of loose, dark silts and clays.

2. Geomorphological features

13The detailed geomorphological survey of the coastal area constrained by several radiocarbon age determinations, allowed the identification of six morphological units (namely Unit A to F), each of them representing a main phase of coastline progradation, and generally bounded by erosive surfaces (Fig. 3). According to Gianfredaet al. (2001) the main morphological discontinuities recognisable within the Lesina Lake coastal barrier can be ascribed to the action of large tsunamis, which would separate A/B and C/D units and, to minor extent, E/F. However, further minor discontinuities have been detected in the Fortore River coastal plain (discontinuites B/C and D/E) (tab. 2). Since this area is more sensitive to fluvial dynamics, they probably record changes in the Fortore River load or in the local wave energy. Moreover, geomorphological analyses suggest the occurrence of a number of shifts in the Fortore River thalweg along the coastal plain. In particular, some relict river branches running parallel to the shoreline and in a W-E direction have been recognised (Fiume Morto, Acquarotta). However, the collated data are still very few and do not allow, at present, the integration of these changes in the course of the river with the general evolution of the area.

2.1. Morphological unit A

14It constitutes the innermost part of the Fortore River plain (Photo 1; Fig. 3) and it stretches between +6 m and +3 m above present MSL with a mean width of about 1.5 km. Unit A inner margin runs in a WNW-ESE direction at the base of a cliff whose top constitutes the outer border of the lowermost marine terrace, which is locally made up of partly cemented conglomerates. Unit A is crossed at the innermost part by a small river course which flows eastward up to the Acquarotta area.

15This unit is marked by a number of dune ridges with irregular spacing. The innermost and highest one, reaches a maximum elevation of about +22 m (Colle d’Arena dune belt), and formed at the foot of a relict cliff.  It is characterised by loose terrigenous, well sorted fine sands, with a faunal assemblage of pulmonate gastropods represented by Helix sp.(sample Arena 1, 4340 ± 80 years BP; Tab.1), and rare Pomatia sp. The dune belt runs eastwards forming the innermost part of the sandy coastal barrier that closes the Lesina Lake. In locality Gravaglione, it reaches the maximum elevation of about +8 m above present MSL; and the fauna is characterised by Pomatia sp and Rumina sp., which yielded an age of 4450 ± 40 years BP (sample Grava 1 in Tab.1).

16Within the dune complex laminated sands, gently sloping seawards, are alternated with thin layers of small pebbles and bivalves shells. An association of marine bivalve shells and cuttlebones in the latter deposits provides an excellent supralittoral-floatsam indicator (Laborel pers. comm.). Samples were collected near the seaward edge of this unit, just below the contact between the beach deposits and the related aeolian sands, at about +2 m. In particular, a marine bivalve Lima sp. (sample Arena 2) yielded a conventional 14C age of 3030 ± 190 years BP (Tab.1).

17Unit A forms a continuous belt from Marina di Chieuti, to the west, up to Torre Mileto, to the east (Fig. 2). On the contrary, the younger morphological units developed in two separate parts either side of the Punta delle Pietre Nere head (Photo 2).

2.2. Morphological unit B

18A small morphological step running roughly parallel to the present-day shoreline divides this unit from the older one (i.e. Unit A). Unit B is about 500 m wide and is located 2 m above the present sea level. The inner part is marked by a small river course flowing eastward to Torre Fortore; the outer part is marked by a number of straight dune ridges with E-W direction (Photo 1; Fig. 3).

19Within the Lesina coastal barrier, this unit closes the San Andrea washover fan apex and therefore formed soon after the occurrence of the tsunami that produced it (Gianfredaet al., 2001). Gastropod specimens collected within the first dune ridge, formed after the tsunami suggest that the development of this unit started at about 2430 ± 40 years BP (sample Andrea1 in Tab. 1).

2.3. Morphological unit C

20Unit C marks the Fortore delta area (Photo 1; Fig. 3); it has a maximum width of 300 m and is separated from the older Unit B by a sharp dune ridge. Dune ridges within Unit C are not straight like the older ones but they are arcuate to indicate the development of a small cuspidate delta. This unit is closed seaward by a continuous dune ridge. Within the Lesina coastal barrier this unit merges with the former one and no morphological discontinuity can be recognised.

2.4. Morphological unit D

21Unit D is recognizable at the Fortore River delta and is characterised by a maximum width of 400 m (Photo 1; Fig. 3). The spatial distribution of the dune ridge outlines a cuspidate delta. A sharp, erosive contact separates this unit from the older one (i.e. Unit C). A sample of pulmonate gastropods (Helix sp.) collected near the Foce Vecchia (sample Foce Vecchia) locality indicates an age of 1590 ±190 years BP (Tab. 1).

22Unit D is easily recognisable on the seaward part of the Lesina Lake coastal barrier since it has a sharp morphological contact with the older morphological unit (Photo 3; 4). The first ridge of this unit closes the apex of the Foce Cauto washover fan produced by the strong tsunami that damaged the barrier also near Torre Scampamorte. Radiocarbon age determinations performed on Pomatia sp. collected within this first ridge (sample Cauto 1) indicate a conventional age of about 1550 ± 50 yr BP (Tab.1).

2.5. Morphological unit E

23It constitutes a large cuspidate delta still recognizable on the 1950 aerial photograph survey (Photo 1; Fig. 3). At present, this unit is preserved only on the western flank of the Fortore delta having been completely eroded only during the last 50 years between the Fortore River mouth and the Punta delle Pietre Nere head (Tessariet al., 2003). Within the Lesina costal barrier this unit merges with the older one (Photo 3 et 4).

2.6. Morphological unit F

24This last unit is a narrow beach that is only recognisable on the 1950 aerial photographs in the Fortore delta area (Photo 1; Fig. 3). At present it has been completely eroded.

25Within the Lesina coastal barrier this unit closes the Casino La Torre and Foce Schiapparo washover fans and the most recent break in the former dune ridges near Torre Scampamorte. These features were produced by the strong tsunami related to the 30th of July 1627 AD earthquake (Gianfredaet al., 2001).

3. Settlements dynamics since Neolithic Age

26The hundreds of prehistorical and historical sites discovered in the Tavoliere area, a region stretching roughly between the Fortore River, Foggia town and the Ofanto River, allow the characterisation of the dynamics of the settlements since the Neolithic (Fig. 4). These dynamics have been compared with the occurrence of environmental crisis (i.e. mainly arid climatic conditions), which affected Southern Europe during the Holocene as pointed out by several authors (i.e. Jalutet al., 2000; Magnyet al., 2002). In the Tavoliere area, in particular, the occurrence of severe arid conditions in the Middle Neolithic Age and in the Middle Bronze Age has been recognized on the base of geological and archaeological evidence (Alloccaet al., 2000; Caldara, 2002).

27The Tavoliere Plain was densely colonized during the Ancient Neolithic Age, more precisely in the period spanning between 5500 cal. years BC and the last centuries of the 6^th millennium BC (Gravina, 1999). The plain was gradually abandoned until about 4700 cal. years BC (Scaloria Bassa phase) because of a long arid phase locally represented by a thick calcrete horizon, as recognised in the Lesina Lake sedimentary sequence. At the southern margin of Gargano Promontory, in the Siponto area, coastal lagoons developed in overall sabhka conditions with associated gypsum deposits (Boenziet al., 2001).

28Improvements in the overall climatic conditions during the Late Neolithic - Eneolithic and Early Bronze Age promoted the increase in the number of sites especially along the Fortore River valley and around the Lesina Lake (i.e. from the Fortore River mouth to Torre Mileto). This period of dense settlement distribution spanned from 3700-3600 years BC to 2500-2400 years BC and continued in the Early Bronze Age until 1800-1700 years BC. Settlements were placed along a former coastline at the foot of the Colle D’Arena- Cornone cliff and around the wide Lesina Bay, which, in this period, was progressively closed by the development of a sandy coastal barrier.

29A new arid phase occurred during the Middle Bronze Age between 1800-1700 years BC and 1400-1300 years BC (corresponding to the arid phase A.4 recognised in the Mediterranean area by Jalutet al. (2000) and to the arid period 3 of Magny (2002)). It caused a new dramatic decrease in the number of settlements, which clustered along the Fortore River valley, on the Appenninic Chain and on the Gargano Promontory. However, along the Lesina Lake the sites already occupied during the Eneolithic were still present during this period. In the Recent and Late Bronze Age, corresponding to the periods between the 13^th and 12^th centuries BC and the 11^th and 10^th centuries BC, respectively, settlements in the Fortore River coastal plain and around the Lesina Lake area were very few (Gravina, 1995). Only the sites of Rivolta, Brecciara, Lesina and Torre Mileto and a number of farms can be ascribed to the Iron Age (9^th to 5^th century BC). The latter became more frequent in the Late Republican Roman Age (from the 4^th-3^th to the 1^st century BC) and in the Imperial period (from the 2^st to the 2^sd century AD). During this last phase the Lesina coastal barrier was colonised as testified by occurence of the Porcareccia and Cauto villas.

30Finally, sites placed at Foce San Andrea locality, on the Lesina coastal barrier, near the Lesina railway station, at Lesina Village and Ripalta can be ascribed to the Late Roman Age. These last two sites were inhabited until the 6^th-7^th centuries AD (Gravina, 1995; 1996).

4. Discussion

31The available data set collated in the area of the Fortore River plain and the Lesina coastal barrier suggests a complex evolution during the Late Holocene influenced mainly by earthquakes, and related coseismic movements, tsunamis and, subordinately, by climatic changes and anthropic influence (Tab.2).

32The following model for the Fortore River coastal plain and the Lesina Lake coastal barrier evolution can be reconstructed as follows.

4.1. From the maximum postglacial transgression (about 6500 cal. years BP) to the strong earthquake of ca. 664 BC

33During the Middle Holocene, about 6500 cal. years BP, the postglacial sea-level rise reached its relative maximum position at about 8.5 m above present-day sea level, shaping a cliff wtih a W-E direction. Based on the available glacioeustatic sea-level curves and the present elevation of this coastline an uplift rate of this area of about 1.5 mm/year can be inferred (Mastronuzzi & Sansò, 2002). The Fortore River was marked during this phase by a negligible solid load so that only a narrow beach formed at the cliff foot. Few kilometers offshore, at about 5-10 m water depth, a bioherm with Cladocora coespitosa developed, whereas in the area of the Lesina Lake an open lagoon promoted most likely the deposition of sands with Cerastoderma sp.

34This episode ended during a very arid period, between 4800 and 4600 years BC, which caused the area to be abandoned and produced the development of a thick duricrust.

35Subsequentely, the first progradation of the Fortore River coastal plain (morphological Unit A) and the development of the high dune belt of Colle d’Arena and Gravaglione started at about 3000 years BC. The coastal barrier which closes the Lesina Lake most likely also formed in this phase.

36The coastal plain was mainly built by littoral drift, which produced ridges straight and parallel to the cliff foot at the back, so that a low solid load for the Fortore River can be inferred for this period. The lowest course of the Fortore River is parallel to the coastline flowing in an eastward direction to the Punta delle Pietre Nere area.

37The number of sites increased during the Eneolithic age and the Early Bronze Age with numerous settlements placed along the top of the Middle Holocene cliff and on the small hills around the southern side of the Lesina Lake. A new, shorter arid period occurred in the Middle Bronze Age, from 1700 to 1300 years BC, and caused a decrease in the number of sites, which also remained in the following Recent and Late Bronze Age.

38A strong seismic event ended this first phase at about 664 years BC. It most likely produced the coseismic uplift of the area to the west of the Fortore River as well as the shaping of the cliff presently at +3 m on the outer part of morphological Unit A. The landward margin of the Punta delle Pietre Nere head was uplifted above sea level becoming, from this moment onward a prominent head, whereas subsidence occurred to the east, in the Lesina Lake area. The coastal barrier was struck by a tsunami, which produced extensive erosion and the development of the Sant’Andrea washover fan.

4.2. From the strong earthquake of 664 BC to the 493 AD earthquake

39The strong earthquake of the year 664 BC was followed by a phase of recovery. During this phase the narrow morphological unit B, mostly due to the littoral drift, developed. The Fortore River was characterised by a small solid load and flowed to the east, into the Adriatic Sea, near Torre Fortore. A new set of dune ridges developed on the Lesina Lake coastal barrier, closing the apex of the Sant’Andrea washover fan.

40During the Republican Roman Period the Fortore River straightened its course and started to build up a small cuspidate delta (morphological Unit C), most likely in response to an increase in its solid load due to intense deforestation since 400 BC (Boenziet al., 2000). During the Imperial Roman period the coastal barrier was sparsly colonised by small farms.

4.3. From the 493 AD earthquake to the 1087/1223 AD eathquake

41In 493 AD, a strong earthquake, reported in local legends, produced a tsunami responsible for the development of the Foce Cauto washover fan and the partial erosion of the Lesina Lake coastal barrier (Gianfredaet al., 2001). Severe erosion most likely also affected the Fortore River cuspidate delta.

42The following recovery phase formed the morphological Unit D, represented by a new cuspidate delta at the Fortore River mouth and, on the Lesina Lake coastal barrier, by a sequence of dune ridges that closed the Foce Cauto washover fan apex.

43Eventually, a strong earthquake occurred at the beginning of the second millennium (1087 or 1223 AD) which caused a new coseismic uplift of the Punta delle Pietre Nere head and the severe erosion of the Fortore River delta.

4.4. The last millennium

44During the Little Ice Age (1300 -1800 AD) the area of Punta delle Pietre Nere head and of the Lesina Village were affected by slow subsidence. The Fortore River was characterised by a remarkable increase in its solid load, as testified by the construction of a wide cuspidate delta (morphological Unit E).

45The 30th of July 1627 earthquake, produced the coseismic uplift of the Punta delle Pietre Nere head. This was preceded by a century of rapid subsidence and a severe episode of erosion of the Fortore delta and along the Lesina coastal barrier, where two washover fans, were formed by a large tsunami.

46Finally, the last phase of progradation (morphological Unit F), during the 19^th and the 20^th centuries, has been replaced during the last 50 years by a phase of strong erosion of the Fortore River delta due to the construction of numerous dams within the Fortore River drainage basin.

Conclusion

47This paper is an effort to integrate the available geological, climatic, radiometric, archaelogical and historical data to form an evolutive model of the area. However, we are aware that the proposed model is not definitive but it could represent a useful tool for the planning of the future research.

48The data set suggests that the morphological evolution of this coastal region has been deeply influenced by strong earhquakes and rapid, relative sea level changes. The latter are represented mainly by coseismic vertical movements and tsunamis which produced rapid and dramatic morphological changes. Climatic changes and anthropogenic pressure played a subordinate role in the evolution of the area, although still capable of remarkable modifications of the Fortore river dynamics and of coastal morphology.