The Cala Gonone fan system is made of a series of smaller telescopic and coalescent fans developed along the inner valley from the saddle between Mt. Tuui and M. Bardia to Cala Gonone village (Fig. 2). Remant of such fans can be observed at sites (T, Y, W, Z) (Fig. 2a). Lower fans affecting the village and costal area in more recent times were/are fed by ripid streams from steep coastal hills (Fig. 2b). Two well defined and well exposed ones are the Stadium (S) and Gustui (G) alluvial fans, which also contributed to the formation of the larger Palmasera coastal alluvial fan system (P) (Fig. 2). Minor sediment contribution to the apron system is also from northern Mt Irveri valley (Fig. 2b). The entire system is now crossed by the ephemeral, steep mountain torrents that from the north are: Rio Irveri (ir), Rio Ischirtiore (is) Rio Sos Dollores (sd) (Fig. 2b). The system could be ideally subdivided in three parts all experiencing different depositional models.
The uppermost part of the system develops in the valley between Mt Tului (915 m) and Mt Bardia (882 m). It has a length of approximately 500 m and width of 500 − 700 m (Fig. 2a). lt is made of slope deposits occurring in several lateral, small, steep-sided head-valleys developed along the mountain flanks (Fig. 4a). Locally they resemble grèze litée (ebulis ordonnès of Ozer and Ulzega, 1981) and consist of lenses of greyish orange (10 YR 7/4) to moderate yellowish brown (10YR5/4), fine carbonate pebble to granule conglomerates (breccias) (Fig. 4b, c). They generally occur in openwork framework, frequently showing inverse (coarsening upward) grading (Fig. 4d), alternating with thinner lenses of poorly sorted, matrix rich (primarily sand with minor silt, and traces of dust as coating) pebbly to granule deposits, and few, thin, residual lenses of laminated coarse-grained sandstone (Fig. 4b, c). Occasional isolated boulders to large cobbles are present (Fig. 4d). The layering consists primarily of thin elongated lenses dipping locally up to 30–32o, generally 20–25o. Small cut-and-fill structures are locally present (Fig. 4b); rare large gullies occur filled with a variety of slope deposits (Fig. 4c).
Near the apex, significant unconformities (diverging attitude of strata, or locally large gullies filed by mass flow deposits) indicate switching of sites of deposition (Fig. 4c). Non-cohesive and cohesive debrisflow (df), grain flow (gf), overland sheet-wash/flood (sf) and canalized flow (cf) are the recurrent depositional features (Fig. 4b-d). Some of the resulting deposits show imbrication clusters in section sub-parallel to the slope and some finer-grained gravel layers show well developed fining upward trend (tied to df/gf) (Fig. 4b). In the saddle west of Mt Irveri closer to the steep calcareous hill, bouldery rockfalls (rf) prevail (Fig. 4e).
The lower, central part of the Cala Gonone system includes the Stadium (S) and Gustui (G) alluvial fans (Fig. 2). They have length of approximately 500 m and width of 500 − 700 m, and variable slope ranging from 20° at the apex to 2° in the low parts. They mostly consist of poorly cemented carbonate pebble, with some cobbles and rare small size boulders and variable amounts of sandy, muddy matrix. The clasts are angular primarily in the upper part of the fans to subangular lower down. The sorting varies from poor to moderately well-sorted in different layers, which indicates variable transport agents. Both fans are now covered in the upper part by bushy vegetation, and the lower part is progressively anthropized by the enlarging village of Cala Gonone (Fig. 2b).
Stadium fan (S)
The sediments of the Stadium alluvial fan are exposed in two long outcrop sections cut sub-longitudinally (S1, 2 locations) and diagonally (S3) to the fan slope (Fig. 2). Closer to the hills the longitudinal section S1 shows two uncomformably superimposed units with slightly different sediment characteristics and dip of the beds (Fig. 5a). The lower unit consists of steeper (~ 10–12o) thin, lenticular beds of very poorly sorted, sandy, muddy pebble to locally cobble, alternating with lenses of sand to granule deposits with disseminated fine grained pebbles and faintly lamination (Fig. 5a). The upper unit consists of gently sloping (~ 2–3o in this exposure) thin, lenticular, muddy, sandy pebble layers alternating with thin sandier lenses and drapes (Fig. 5a).
The lower unit were affected mostly by overland fluid flows, sheetflood (sf) and possibly watery debris flows (cf). The upper unit shows sandy conglomeratic horizon likely deposited primarily by sheet flood (sf) and canalized floods (cf) (Fig. 5a).
Gustui fan (G)
The deposits of the Gustui alluvial fan are exposed in sand pits in its middle-upper part (G1, G2) and more extensively along quasi-transversal and diagonal road cuts in its downslope part (G3 to G6; Fig. 2b). At G2 locality three major stratigraphic units may be identified: (i) the lower unit consists of basal steeply inclined thick lenses of small boulder, cobble to pebble openwork conglomerate overlain by massive pebbly sand-granule-deposits and very coarse sands with disseminated pebbles lenses. (ii) This is overlain by continuous, at the outcrop level, unit with apparently massive to slightly laminated sandy to granule deposit some with sparse pebbles, enclosing some pebbly lenses and small cut-and-fill structures. This second unit is cut in the up-dip part by a small channel filled with pebble to cobble openwork conglomerate. (iii) Everything is uncomformably overlain by massive, disorganized pebbly sand- to granule- deposits with sparse pebbles and rare boulders, capped by organic rich, dark colored soil of similar lithology (Fig. 5b).
The entire deposit at G2 records the infilling of a deep gully by powerful events. The lower unit is dominated by granular not-cohesive debris flow (the coarser lenses) (gf) and sandy/muddy matrix rich watery debris flows (df). The overlaying unit is dominated by sheetfloods (sf) possibly reworking also debris flows, and local channel flow (cf). The capping unit resembles the underlying one, being however partially reworked by man (hm). A thin, dark-brown, sandy pebbly soil cap the unit (Fig. 5b). No ancient soil horizons has been encountered in these sections
The adjacent (20 m apart) thinner exposure at G1 location a thin (~ 3 m) near top-fan surface exposure shows a regular alternation of clast-supported, fine pebble to granule conglomerate lenses resting with sharp bases on predominantly sandy layers with disseminated fine pebbles, and very fine pebble to granule laminas (Fig. 5c). This unit is best interpreted as an alternate of grain flow (gf) and sheetflood deposit (sf) with some very small cut-and-fill structures (cf). A thin sandy pebble to cobble medium-thick conglomerate lens occurs at the base of this section possibly associated with a debris flow (df) event (Fig. 5c).
The transversal, middle, roadside exposures of both the Stadium and Gustui alluvial fans (G4, G5, G6, Fig. 2b) shows sandy pebble conglomerates occurring in sequences predominantly of cuts-and-fills channel flow and quasi regular alternation of thin lenses of sandy, pebble conglomerates, and granular very coarse-grained sandstones with sparse fine pebbles (Fig. 5d). These deposits here and along the road outcrops are interpreted as formed by debris flows reworked in part by sheetflood (sf) and shallow channel flows (cf), possibly in a braided system (Fig. 5d, e). Occurrence of unsorted sandy pebbles to pebbly sands local deposits suggest the presence of mid to distal parts of debris flow (Fig. 5e).
Rock-falls of carbonate blocks occur over the northeast lateral terminal part of the Gustui fan (Fig. 2b). This indicates that tail ends of debris flows, muddy sandy gravely floods (and local and minor rockfalls) did reach the edge of the present village and possibly could also reach the entire area down to the coast.
Coastal alluvial fan (P). The Plio-Pleistocene small volcanic vents poured a basaltic cover (10–20 m thick) on the carbonate terrace of the south-eastern lower part of the Cala Gonone reentrance (Fig. 2a). Part of it forms the substrate of the village almost up to the brink of the conglomerate scarp at Palmasera (P) (Fig. 2). The morphology of the sedimentary deposits exposed along the coastal scarp, arcuate away from a centrally located feeding stream (Rio Sos Dollores Codula = sd, Figs. 2b, and T of Fig. 6a), and their sedimentological characteristics, such as sedimentary structures, indicate these outcrops to be the inner remnant exposures of a relatively large, ancient coastal alluvial fan (Palmasera fan) (Fig. 6a, b). The transversal section has width of 450 m (terminating to the northeast against basalt and to the southeast against carbonate bedrock, Figs. 2a, 6b), maximum thickness of 24–28 m, and a maximum remnant ~ 20 m thick-slice on the scarp-face (Fig. 6c). The longitudinal sedimentary sections along the flanks of the Rio Sos Dollores Codula are not as well exposed but have similar measurements with a length of 250 m (Fig. 2b).
The coastal alluvial-fan deposits (F) generally rest over a basaltic key-bed (B) composed primarily of rounded pebbles and large to very large, sub-angular to sub-rounded basaltic boulders and few carbonate ones (Figs. 6b, c, 7). This basaltic boulder bed overlies thinner layers (T) composed primarily of carbonate and minor basaltic openwork rounded and flattened pebbles, frequently imbricated. This in turn overlies a Late Pliocene-early Pleistocene fine to medium grained well rounded, openwork, fine stratified and well-imbricated clasts refereed to beach deposit (L) (Massari and Dieni ,1973). The T carbonate pebble layer and the basaltic basal B boulder key deposits are interpreted as beach deposits and referred to last interglacial highstand Marine Isotopic Stage (MIS) 5e (Sarria et al., 2016). These last occur now at about 6 + m above the present sea level.
The fan deposits (F) have similar characteristics throughout, such as poorly cemented sandy conglomerates dominated by carbonate sub-angular pebbles with local cobbles and isolated boulders (Fig. 7). Isolated basalt pebbles, cobbles and few boulders occur sparsely at different horizons. Most layers do not present preferred depositional fabric. Predominant sedimentary structures include definable bedding generally discontinuous and with erosional boundaries (Fig. 6b, c). Along the transversal section subparallel to the coast, slight accumulations of poorly sorted coarser material suggests either channeled or, if arcuate, overland deposits.
Vertical and lateral differences mainly related to changes in grain size and sedimentary structures, and the local presence of poorly developed/preserved, discontinuous palaeosols, contributes to a gross subdivision of the F deposits into four subunits: F1, F2, F3 (Figs. 6b, c, 7). Along the transversal section, they are thicker near the codula and thin out to the northeast (Fig. 6).
Sub-unit F1 is exposed in the lower SW part of the Palmasera section. It has a pale yellowish brown color and is characterized by carbonate with scattered basaltic clasts deposits containing numerous cobbles and few disseminated boulders (Fig. 7a, b). It shows irregular, lenticular, thin- to medium-thickness beds of alternating cobble to coarse pebble conglomerate and of sandy pebble conglomerate, and few gravelly (fine pebbles) very coarse-grained sandstone. Some clasts layers are openwork or with very little matrix, (Fig. 7c). Carbonate clasts are prevalently sub-prismoidal to sub-discoidal, angular to sub-angular. Conversely, most of the basaltic clasts, except some of the largest boulders, are sub-rounded to well rounded (Fig. 7b, c). No definite grain size grading occurs within the beds, some have sub-discoidal clasts showing preferred imbrication (Fig. 7c). The beds are generally sub-horizontal to-slightly inclined (less than 10°), with sharp basal contact, flat in most layers, slightly concave upward in others indicating shallow cuts-and-fills (Fig. 7a, b).
Sub-unit F2 dominates the central part of the Palmasera section (Figs. 6b, c,). It has a pale yellowish brown coloration similar to F1 and contains a predominance of sandy carbonate fine pebble to granule conglomerates, and pebbly very coarse-grained sandstone beds (Fig. 7a, d, e). The clasts are mostly of carbonate with minor basalt. It has a few isolated large clasts (up to boulders) (Fig. 7f). The beds have thin- to medium-thickness, thinning and lensing out toward the northern termination of the fan body (Fig. 6b, c). They generally have flat, sub-horizontal to slightly concave-up lower boundaries (Fig. 7a, d, e, f). Some shallow, wide cut-and-fill structures occur involving both gravelly layers and, at a smaller scale, sand/granule beds (Fig. 7g). A large slab of basaltic lava was emplaced in the central lowermost part of the F2 sequence, indicating rockfall and sliding from an adjacent, still- or re-exposed basaltic scarp (Fig. 7d.
Sub-unit F3 is a carbonate sandy and gravelly deposit with no (or rare) basaltic clasts. It is separated from F2 locally by few remants reddish palaeosol and it has a marked difference in structures (Figs. 6c, 7a, d). It has a predominance of thinner lensing beds with more numerous sandy interlayers, fewer cobbles and boulders of carbonate (Fig. 7a, d). The principal structures are shallow cannels, cut-and-fills, cross-bedding and laminar structures. Locally has also openwork pebbly gravels alternating with lenticular sandy laminas (Fig. 7g, h).
F1 clasts mainly deposited ad consequence of mega-floods and debris flow from the local coastal scarp filling large channels (Figs. 6, 7). Local channel flows (cf) and sheetfloods (sf) may develop as well as (Fig. 7b, c). Most likely, the basalts and some of the carbonates boulders have been added to the inland-derived sediments by rockfall (rf).
F2 was not greatly affected by debris flows but by sparse channels and sheetwash (overland flow) flood events (Fig. 7e, f).
F1 and F2 sub-units record the coarsest channelized flood deposits in both subunits. Fairly good palaeosols are clearly identifiable (Figs. 6c, 7a, d) suggesting time of inactivity. Furthermore the northeastern side of the Rio Sos Dollores Codula mouth (T of Fig. 6a) shows a sequential stepwise erosion of the original sequence and large carbonate boulders are found toward the center of the codula (Fig. 6a, b).
On the whole, F3 structures are typical of a braided stream, perhaps with frequent channel migrations, erosions and overland deposits. In better, smaller exposures along the top accessible right codula section, debris flow, cross-bedding and other small scale structures and various types of vertical and lateral grain size sorting indicate predominance of relatively shallow channels and sheetfloods (cf) complex attributable to a braided stream (Fig. 7h).
The proposed depositional model where sheetwash (sf) alternate with channelized (cf) deposits in places resembling Ostler lenses (Osl) (Martini, 1973, Fig. 7g) is in agreement with erosion and sedimentation processes related to recurring strong floods.
On the whole the Rio Sos Dorroles (could have been very active in the past and, under appropriate climatic conditions, it may have acted as the filling-and-eroding system of a ‘deep valley’ and fed the frontal coastal alluvial fan.
Sediment transport and timing
Similarly to other world areas, various transport and sedimentation processes can be invoked for the colluvial and alluvial fans of Cala Gonone. Some of them are azonal, such as slump, rock fall, debris falls, debris avalanches, debris flows (cohesive and non-cohesive), hyperconcentrated to high-density flows, and fluid flows (water and wind) (Bertran et al., 1992; Blikra and Nemec, 1998; Nemec Kazanci, 1999; García-Ruiz et al., 2001; van Steijn et al., 2002; van Steijn, 2011). Most of them require water in some form to perform. When in quantity, water actively increases the weight of the sediment and passively reduces the friction and cohesion between the particles or layers favoring mass movement on slopes (Lorenzini and Mazza, 2004). In central-east Sardinia occasional extreme rainfall events have ben experienced pouring in a short time, order of few hours or day, large quantity of water, order of 500 mm in in few hours (Cossu et al. 2007), leading to debris flows, channel floods and overland sheetfloods (Bodini and Cossu, 2008, 2010a, 2010b; Betro et al. 2008; Hewson et al., 2021). The composite Stadium and particularly Gustui fan systems are dominated by remnants of debris flows heavily reworked by sheetfloods close to the feeding valley and upper part of the fan. The sediments were, however, mostly redistributed by sheetfloods and channelled flow over the mid-lower portions of the fans, and contributed to the feeding of the coastal alluvial Palmasera fan. Other sediments transported by stream to the south of the Stadium fan contributed to those funneled through the Rio Sos Dollores Codula into the coastal fan. Additional coarse carbonate and basaltic clasts were added to the coastal fan as rockfall from the open coastal scarp. The codula acted as a deep narrow valley filling up and feeding repeatedly the coastal fan that was totally or in part eroded and reformed during upper Pleistocene recurring sea level oscillations occurred.
The relatively low costal carbonate scarp partially capped by basaltic lava could not have supplied sufficient amount of material to form the deposits of the coastal alluvial fan (P). Such deposit can, thus, be considered an extension of the inland upper valley and of the alluvial-fan aprons including the Stadium and Gustui ones. A complex waterway system may have conveyed floodwaters and sediments from the carbonate highlands directly to the inland talus and alluvial cones and reworked material funneled through various passages to the most important channel, being the Codula Sos Dollores (Fig. 2b). Such codula area may have been a lowland partially filled by lava in early Pliocene, later eroded into a narrow deep coastal valley. At the present, the codula starts about 250 meters inland and its sequential excess infilling led to the development of the Palmasera coastal alluvial fan (Fig. 8a). The development of such fan would have occurred primarily during period of major climatic changes that led to variation in sea-level and to periods of intense precipitation and floods. Quiescent times or temporarily inactive areas of the fan led to the formation of palaeosols.
This coastal system seems to have several times repeated deposition mainly during sea regression, and erosion of the fan during sea transgression. Evidence of this has been preserved in the remnant deposits along the coastal scarp at the present sea level. It has not been yet possible to obtain reliable numerical dates from the F carbonate deposit. Dates available in the area are limited to a pIRIR290 age of 87 ± 4 ka of putative basal paleosol at the base of the Palmasera fan (Fig. 6c). The paleosol separates Eemian MIS5e (about 136 − 116 ka) beach B from fan F deposits. The fan deposits in their central part completely erode those of the beach. It has to be noted that same paleosol, however, has also been dubiously dated at 19—17 ka using 14C (Coltorti et al., 2010; Thiel et al., 2010). This age is probably affected from contamination having been sampled in a site reached by modern storm wave. Comparable ages have been derived in the close by Bure Marino Cave where a colluvium filling a tidal notch scour, referred to MIS 5e (Carobene and Pasini, 1982) has been pIRIR290 dated to 86 ± 4 ka (Andreucci et al., 2017).
This age uncertainty had led to two interpretations of formation of the Palmasera coastal fan outcrop.
1. The basal palaeosol is indicator of the change from interglacial to glacial conditions and the other palaeosols of the successions of climate oscillations occurred during the glacial time.
2. The coastal fan sequence formed during climate deterioration occurred post Eemian; that is post 116 ka.
The first hypothesis (our preferred) is in agreement with reconstructions made in other places of northwest Sardinia in Alghero (Andreucci et al., 2010) and Argentiera (Andreucci et al., 2014; Pascucci et al., 20014) (Fig. 8a), whereas the second was the preferred interpretation of Massari and Dieni (1973) based on the regional geological analysis of the area.
If Cala Gonone alluvial fan developed during the cold post MIS5 glacial stages, it is possible to hypothesize that most of the material available for the fan was formed during repetitive periods of cold and dry and transported during the more wet and humid. In this case, the Cala Gonone alluvial fans may have started to form during MIS4 and reached their maximum activity during MIS3 when repetitive high frequency warm and cold cycles (Dansgaard–Oeschger and Henrich events, Rahmstorf, 2003) occurred. Thus, fans could be related to Unit 4 and 5 of northwest Sardinia. In the Argentiera area (NW Sardinia), water dominated MIS3 deposits (alluvial fan) are associated to Dansgaard–Oeschger interstadial events when warm and humid conditions occurred, while wind dominated deposits (aeolian) to cold and very dry stadial conditions (Heirich events) (Pascucci et al., 2014) (Fig. 8b).
The processes involved in the generation of loose sediments and then transport and deposit them in fans have been long studies under several climatic zones. The basic processes are generally similar although the result may vary for flood dominated, debris flow dominated or mixed alluvial fans (Harvey et al., 2016). Great effort is made globally to recognize the hazards and the risks of alluvial fans and possible remediation, if necessary. Much information on the occurrence of disasters is available and good guidelines have been written for detailed surveys of the landscape, taking also account of climatic and geological/geomorphological conditions (such as structure, stratigraphy, sedimentology, numerical dating procedure and geotechnical characteristic of the bedrock and sedimentary deposits). Geotechnical and other scientific analyses can help in establishing any probable recurrences and intensity of events. Guidelines and specifications for flood hazard mapping in papers, books and reports (Fema, 2000, 2021; Lancaster et al., 2015; Da Silva Nascimento and Alencar, 2016). Lancaster et al. (2015, p.1) stressed the geological approach to “identify the general distribution of alluvial fans, the relative age of alluvial deposits, and the relative likelihood of alluvial fan flooding”. Extending this approach to the sedimentological and numerical dating of the deposit also allow the analysis of long dormant fans that could be revived under changed climatic condition such as the high frequency Dansgaard–Oeschger or Henrich similar events. In addition, the short intense meteorological events nowadays increasing the frequency of extreme short-lived rainstorm.
How does all this apply to the Cala Gonone area? The Cala Gonone village is expanding on an inland mixed dormant alluvial fans down to the brink of the coastal cliff coated with residual conglomerate remnants of the coastal fan (P), subject to persistent sea wave erosion (Figs. 2b, 9a, b).
Cala Gonone historically became a small harbour for Italian fisherman at the beginning of 20th century but remained isolated from the rest of Sardinia because of the impervious high costal hill barrier. In 1860 a small walking tunnel for people and animals was perforated near of the saddle between the Mt Tuli and Mt Irveri. As the village prospered, particularly for the recent touristic explosive increase, modern motor-vehicle tunnels were open along the road to Dorgali on the northwest flank of the coastal hill barrier (Fig. 2a). Cala Gonone is now a preferred touristic area for the magnificent landscape. Beach and hill places and various permissible activities such as unusual canyoning, climbing, paragliding, and all other activities offered by pleasant touristic resorts.
All this happened despite the hazards facing the area. The municipality of Dorgali (to which Cala Gonone belongs) has provided a detailed survey of the landscape of the area determining the best use of the land. Recognition of the various existing natural hazards, risks, and in recommending operas that are continuously attended to. Extensive reports and maps on the methodology used and the observations and recommendations made have been written and available at https://www.comune.dorgali.nu.it/area-tecnica/attivita-e-servizi/piano-di-protezione-civile.html, including the hydrogeological risk map (Fig. 9c). These reports, however, seem to be more addressed to rockfalls (Hg3 and Hg4, Fig. 9c) from the nearby high cliffs than to potential flood of the village (Hg1, Fig. 9c). In Cala Gonone the conditions of potential instability are, instead, numerous. In particular, the areas close to Rio Ischirtiore, the adjacent coast, the beaches of Palmasera and Sos Dorroles Codula mostly classified as minor Hg1 risk (Fig. 9c).