On the origin of both a recent and submerged tombolo on Prvić Island in the Kvarner area (Adriatic Sea, Croatia)

This paper analyzes the origins of two tombolos (one recent and another fossil/submerged) on the southwestern coast of Prvić Island, which is located in the Kvarner area in the northeastern part of the Adriatic Sea. A recent tombolo on Cape Pipa was formed by the erosion of Quaternary deposits and Palaeogene siliciclastic rocks. The submerged tombolo is much bigger, clearly visible on the sea bed toward the southwest. The conditions for forming a submerged and recent tombolo have occurred during a slow rise and then stagnation of sea level of the Adriatic Sea in the Holocene. The sea flooded the fossil tombolo probably in the final part of a period of rapid sea-level rise at the beginning of the Holocene when a large proportion of the Quaternary sediments were eroded. Waves from the northwest (tramuntana) and southeast (jugo) refract and diffract around the tombolo. The nourishment of the beach body happens permanently on both sides of the cape. Accumulated sediments are protected by resistant rocky blocks of breccia on the peak of Cape Pipa, acting as a natural tombolo. Due to the fact that wave directions are perpendicular to the beach coastline, they do not generate longshore currents that would erode sediment in beach bodies around Cape Pipa. Therefore, the recent tombolo is probably stable in the present climatic and oceanographic conditions in the Kvarner area. the most common but there are also tombolo pairs and triplets (GOSSEAUME, 1973; MARRINER et al., 2008; CEYLAN, 2012). The preconditions for tombolo formation are: (i) high se­ diment supply, (ii) a physical barrier against the swell and (iii) coastal processes conducive to the development of a sand bank, i.e., bidirectional currents converging towards this physical ba­ rrier (DAVIES, 1980, according to MARRINER et al., 2008). Geomorphological forms like tombolos are rare on the Croatian coast of the Adriatic Sea and have not been studied in the scien­ tific literature so far. This is the first known scientific research of the morphogenesis of recent and submerged tombolos at nearby locations.


INTRODUCTION
Sea level is a global boundary with weathering and erosional pro cesses prevailing above it, whereas the accumulation of sediments occurs below it. In accordance with sea level changes during geo logical history, the intensity and location of erosion and accumu lation of sediments also changed (COWELL & THOM, 1997). Remnants of the effects of sea level changes have been found on the coastal and submarine relief in the Kvarner area in the north eastern part of the Adriatic Sea (BENAC & JURAČIĆ, 1998).
This paper analyses the origins of two tombolos (one recent and one submerged) and their conditional dependence on the Ho locene sea level rise. Both tombolos are located on the southwest ern coast of Prvić Island in the Kvarner area, in the northeastern part of the Adriatic Sea (Fig. 1A).
A tombolo is a depositional geomorphological landform (sandbar, barrier or spit) that joins an island or a barrier with ei ther the mainland or another island, resulting from longshore drift or the migration of an offshore bar toward the coast (WARD, 2004).
Tombolos length varies from few tens of metres behind small obstacles, up to more than ten kilometers. Single tombolos are the most common but there are also tombolo pairs and triplets (GOSSEAUME, 1973;MARRINER et al., 2008;CEYLAN, 2012). The preconditions for tombolo formation are: (i) high se diment supply, (ii) a physical barrier against the swell and (iii) coastal processes conducive to the development of a sand bank, i.e., bidirectional currents converging towards this physical ba rrier (DAVIES, 1980, according to MARRINER et al., 2008. Geomorphological forms like tombolos are rare on the Croatian coast of the Adriatic Sea and have not been studied in the scien tific literature so far. This is the first known scientific research of the morphogenesis of recent and submerged tombolos at nearby locations.

STUDY AREA
Prvić Island is elongated in the NWSE direction. The surface of the island is 12.76 km 2 , maximum length is 7.84 km, maximum width is 2.6 km and the total length of the coast is 23.12 km (DUPLANČIĆLEDER et al., 2004). The highest peak of the is land is 356 m above mean sea level (m.s.l), (Fig. 1C).
This island is mostly formed of carbonate rocks consisting of Upper Cretaceous limestone and dolomitic limestone, Palaeogene foraminiferal limestone, and partially of Palaeogene siliciclastic rocks i.e. marls and flysch (MAMUŽIĆ et al., 1969;BENAC et al., 2013). The bare karstic plateau in the central area of the island has an elevation of > 300 m above m.s.l. (Fig. 1B, C).
The northeastern slopes between cape Stražica (NW) and Cape Šilo (SE) are mostly very steep, with an average inclination > 70°, and many vertical scarps are located above the shore. The carbonate rock mass is partially covered by active talus. Subma rine slopes are also very steep, resulting in the 50 m isobath being  mostly <100 m from the coastline. The slopes are more gently inclined in the western part of the island. A dry karstic valley, the Smokova draga is a dominant exogenous geomorphological form (Fig. 1C).
Cape Pipa is located in the central part of the southwestern coast of Prvić Island where the coastal relief and geologic fabric are very different in relation to other coastal parts of this island. Steep or vertical upper parts of the slopes are formed of carbo nate rocks, and the tectonic fabric is complex and not well known. The contact between these carbonate and siliciclastic rocks in the lower part of the slope is a NWSE trending reverse fault (MAMUŽIĆ et al., 1969), (Fig. 1B).
Large blocks of limestone breccia form Cape Pipa itself and sandy and gravely beaches have formed along both sides of the cape (Fig. 2). The sea bed is at a shallow depth around this cape.
Approximately 340 metres SSE from Cape Pipa lies the Nji vi ce Rock (Fig. 2). It is a small rock with a length of about 50 m striking WE. Its largest width is about 12 m, its height is 4 m and its surface is approximately 500 m 2 . The Njivice Rock is also built of carbonate breccia, probably of Palaeogene age. The investi gated part of Prvić Island is exposed to waves from both the southeast and the northwest which are relatively low compared to waves in the open sea due to their small fetch (see below).

METHODS
Three different methods were used in this study. In the prepara tory phase, various maps were used to provide information on the geomorphological phenomena of the area: topographic map 1: 25,000, Croatian base map (1:5000) and Digital orthophoto map 1 . According to information from the visual interpretations of these maps, we found this specific location and recognized active and submerged tombolos at nearby locations. Further, we used orthophoto map as a base for the field investigation.
The second method was field investigation. The main pur pose of the field work was to collect data on landforms and their relationship with the geological structure of the terrain in the wider area of Cape Pipa. Data were obtained by aerial photogra phy from an aircraft, geological and geomorphological mapping, and reconnaissance of the underwater part of the exploration area using scuba diving equipment. For geomorphological mapping, a customized methodology was used according to GAMS et al. (1985) and PAVLOPOULOS et al. (2009). This methodology is based on the morphogenetic determination of observed geomor phological processes, forms and conditions which are related to the morphogenesis of tombolos. The geomorphological mapping involved recognizing, locating and drawing on the map the main forms and processes of the coastal and slope morphogenetic type of relief with the main purpose of determining their spatial rela tionships.
In order to determine the influence of waves on the forma tion of the tombolos, numerical modeling was used. Numerical simulation was performed using the Simulating Waves Nearshore (SWAN) numeric model, which is the third generation model for application in coastal areas, based on Euler's equilibrium equa tion of spectral waveform (BOOIJ et al., 1999). A JONSWAP wave spectrum with parameter γ = 3.3 was used. Numerical simu lation of waves was carried out in the wider area of the Grgur Channel for waves of northwestern (generated by tramuntana wind) and southeastern directions (generated by jugo). The nu merical simulation was done for the 50year return period and for mean sea level. Relevant wind strengths for numerical simulation (southeastern direction: 27.4 m/s, northwestern direction: 22.2 m/s) were taken from the wind climate study of the nearby Vinodol Channel 2 .
Based on the data collected from the maps and field work, geomorphological analysis and synthesis were carried out, focu sing on the geological and oceanographic conditions, forms and processes that created the recent features of the relief on Cape Pipa.

RESULTS
Cape Pipa is formed by sea erosion of the Palaeogene siliciclastic rocks (marls and flysch) which are partially covered with younger cohesive sediments, probably of Quaternary age (Fig. 3). This  Quaternary sediment body has a form of an irregular triangle with a base length of around 50 m. The height of this Quaternary deposit is 10 to 12 m, and outcrops of flysch bedrock are partially visible on its southeastern side. The stratification of the Quater nary sediment body is clearly visible and bedding planes have a gentle inclination opposite to the coastal slope. According to pre liminary field observations, silty sand prevails in this sediment body. Some layers contain angular fragments and blocks that originate from the carbonate rock mass. The coastal cliff is formed on both sides of this sediment body.
Cape vertex itself is formed on collapsed blocks of talus breccia and provides an obstacle to wave motion (Fig. 3). Behind this obstacle is the accumulation of beach sediments that connect Cape Vertex with the coast. Cape Pipa, according to geomorpho logical classification (WARD, 2004), can be characterized as a tombolo. Grains of gravel with an average diameter between 2 and 6 cm prevail in the beach body. Coarse sand and subangular pebble grains are partially visible. A much bigger triangular tom bolo form is clearly visible on the sea bed southwest of Cape Pipa. The edges of this form are built of submerged beach sediments. So, we can consider it as a palaeocoastline, approximately 300 m long. Concave traces of ancient coast are clearly visible on the western side and less noticeable on the southeastern side (Fig. 4). This submerged tombolo form was formed behind another ob stacle. The Njivice Rock is built of carbonate breccia, probably   Fig. 4).
of Palaeogene age. This submerged tombolo lies at an approxi mate depth of 10 metres below the recent sea level.
Unusual dark parallel lines are clearly visible on the ortho photo image where these submerged outcrops look like artifi cially built walls (Fig. 4). Exploration using scubadiving equip ment has revealed that they are outcrops of vertical layers of rocks (Fig. 5). At first glance, they seem to be carbonate members of a flysch rock mass (calcarenite and calcrudite).  Geomorphological and geological conditions in the sur rounding area are shown in Figures 6. and 7. The recent coastline in the area around the tombolos was formed in Palaeogene silici clastic rocks and limestone. A little further away, the coast was formed in Cretaceous carbonate rocks. Palaeogene (siliciclastic and carbonate) rocks are present only at lower elevations whereas Cretaceous carbonate rocks are more distant from the beach, on higher elevations with steep slopes.
The inland of the Prvić Island is predominantly formed of Cretaceous carbonate rocks with a few deep gullies. These car bonate rocks are karstified, but on the steep slopes intensive line ar erosion is also present. According to the shape of the contours and isobaths on topographic maps, we can hypothesize that sedi ment eroded from the gullies formed some prolluvium fans, which are now submerged (Fig. 6).
Geological crosssections indicate several favourable factors that enabled the development of the investigated tombolos (Fig.  7). In addition to the above mention obstacles of different geo logical origins, the source of beach accumulation material is im portant. These are most likely the Quaternary sediment body from Cape Pipa and probably Palaeogene flysch (Fig. 6). It is pos sible that a source of material for the submerged tombolo was the supposed submerged prolluvium fans, but probably in small pro portions. The vertical position of the more resistant layers in fly sch rock mass (Figs. 5, 7) was also likely to have helped protec tion against marine erosion of these sediments.
The southeastern and northwestern wind directions generate the highest waves in the Grgur Channel. Figure 8 shows signifi cant wave height and wave direction simulations using the SWAN model (BOOIJ et al., 1999), which assumes a uniform wind dis tribution. According to model results, wind waves coming from the southeast (H S 50 =2.40 m) are higher than ones coming from the northwest (H S 50 =2.00 m) in the Grgur Channel. The recent tombolo and the adjacent coast of Prvić Island are protected from waves from the southeast direction. Waves from southeastern di rections break on the Rock of Njivice, and significant shoaling  occurs at the submerged tombolo (Figs. 6,9,10). Wind waves from northwestern directions have higher significant wave heights of the same return period than waves from southeastern directions on Cape Pipa (Fig. 10).
The effects of wave diffraction and shoaling are clearly mani fested around Cape Pipa. A majority of the incoming wave's energy dissipates on Cape Pipa and the surrounding seabed. Be hind this cape, there is a shadow zone with low wave energy which provides tombolo stability.

DISCUSSION
Geomorphologic forms like a tombolo are rare along the Croatian coast of the Adriatic Sea, due to the prevalence of relative resis tant carbonate rocks in which processes of mechanical marine erosion are less well expressed . The same situation occurs in the Kvarner area, where a very slow pro cess of bioerosion prevails, while marine erosion is more strongly expressed in the less resistant siliciclastic rocks and Quaternary deposits . The origin of the described de posits at Cape Pipa had not been investigated in detail. According to MARJANAC (2012) these deposits (Fig. 3) can be considered as the remains of the glacial terrace.
The question arises as to why recent and submerged tombo los were formed in nearby locations? The cause may be three geomorphological factors: (1) a specific geological fabric on the southwestern side of Prvić Island, (2) favourable oceanographic conditions in the Grgur Channel and (3) the sea level change dur ing the Holocene.
Exogenous slope geomorphological processes are very in tense on the southwestern coast of Prvić Island, especially where active talus formation and rock sliding are visible (Fig. 2). The erosion of the Quaternary sediment body on Cape Pipa is perma nent. Sheet wash and rilling types of erosion caused by surface water are visible and consequently, additional feeding of the beach sediment body is active on both sides of the cape (Figs. 3, 6).
Behind barriers, a complicated mechanism of waves, cur rents, sediment transport, and morphology occurs. At Cape Pipa, wave energy will be reflected dissipated and transmitted by over topping, waves also diffract around the tombolo (Fig. 10). As a result, they generate longshore currents of opposite directions and support erosion or accumulation of sediment in beach bodies around Cape Pipa.
Accumulated sediments are then protected by a resistant block of breccia on the peak of Cape Pipa, creating a natural tom bolo (Fig. 11), as wave directions on the windward side are per pendicular to the beach coastline, and sediment longshore trans port on the tombolo is minimal (Fig. 10). Wave setup in the lee is a combination of diffraction, transmission and partly refraction. The wave direction and energy behind Cape Pipa seem favoura ble for sediment accumulation and retention due to reduced ero sion caused by longshore currents.
The recent tombolo is in a state of equilibrium in the present climatic and oceanographic conditions in the Kvarner area (PEN ZAR et al., 2001). Stagnation of sea level occurred in the last 7000 years, after the rapid Late Pleistocene-Holocene rise in the Adri atic Sea (LAMBECK et al., 2004;SURIĆ, 2009;BENJAMIN et al., 2017). Erosion of an ancient large sediment body has been intensive during this period. Namely, the described Quaternary sediment body has features of a cohesive soil. Therefore, me chanical weathering and erosion are intensive and permanent. This sediment body has lower resistance to destructive wave at tack in relation to the carbonate rocks. The coarsegrained sedi ments found at the bottom of the southeastern part of the Grgur Chanel near Šilo Cape  probably originated from these deposits.
The conditions for forming a submerged tombolo occurred during the global sea level rise in the Holocene (LAMBECK et al., 2004;BENJAMIN et al., 2017). The relatively resistant car bonate breccia played an important role in the formation of the tombolo. These large rocky blocks provide an obstacle to waves in the shallow and flattened area. This has created conditions for forming beach sediment bodies between the Njivice Rock and Prvić Island. The top of the rock is today visible above the sea surface (Fig. 2). The sea flooded the tombolo probably during the last phase of rapid sea level rise at the beginning of the Holocene and a great proportion of the Quaternary sediments was eroded and the coastal cliff was formed. However, the traces of ancient beaches are clearly visible: better on the western side, and less so on the southeastern side (Fig. 4).
Based on the depth of the submerged tombolo of approxi mately 10 m, and according to Holocene sealevel rise, it can be assumed that the fossil tombolo was formed approximately 7,000 years ago (Fig. 12). Local oceanographic conditions may have been responsible for formation of the tombolo, because the winds from southeastern directions (jugo) are more frequent and make higher waves than the winds from northwestern directions (tramuntana), driving the sediment behind the outcrop (Figs. 8,9,10). Sea level rise in the range of 2.0 ± 0.9 3.4 ± 1.1 mm/yr has been recorded from the beginning of instrumental measurements in the Adriatic Sea (TSIMPLIS et al., 2009). According to new analyses, predicted sea level rise could be 62 ± 14 cm by the end of the 21 st century (ORLIĆ & PASARIĆ, 2013). Possible indications of accelerated sea level rise include extremely high tides recorded in the mareo graph in Bakar city: + 117 cm above MSL from 1 st December 2008, +122 cm above MSL from 1 st November 2012 and + 127 cm above MSL from 29 th October 2018 3 . This indicates that the recent tom bolo is (under these given conditions) an unstable form and it could therefore be eroded and submerged in the future. These recorded extremely high tides are probably the start of a new sustained trend, which will have a negative impact on tombolo stability. Ac  cording to the expected sealevel rise and more frequent events of extremely high water levels (Acqua Alta) in the Kvarner area, coastal processes might become more intensive. This could con tribute to the acceleration of marine erosion, cliff retreat and ex pressive changes of beach bodies including tombolos.

CONCLUSIONS
Within this research, the existence of two (recent and fossil/sub merged) tombolos in neighbouring locations on Prvić Island in the northern Adriatic Sea has been reported.
There are three geomorphological factors important for the emergence of these tombolos: (1) specific geological fabric, (2) oceanographic conditions and (3) sea level change during the Holocene.
The atypical appearance of Palaeogene flysch, Quaternary clastites, and submerged subrecent potential prolluvial fans, all in an area dominated by karstified carbonates, enabled the exis tence of obstacles as a prerequisite for the formation of tombolos. In addition, geological specificities have enabled the source of material to feed both (recent and submerged) tombolos. Windy waves from the northwest (tramuntana) and southeast (jugo) to gether with generated longshore currents are responsible for coastal erosion and accumulation of sediment in the beach bo dies at Cape Pipa.
Sea level rise during the Holocene allowed the emergence of two tombolos at different heights at different times. Regarding the depth of approx. 10 m it is supposed that the fossil tombolo could have been submerged approximately 7,000 years ago.
Two generations of tombolos coexisting at close proximity is a unique phenomenon in the Adriatic Sea, and possibly in the Mediterranean Sea.