Geo.Cro.2-3-61-KB.pdf 379 � Borna Lužar-Oberiter1, Snježana Mikulčić Pavlaković2, Marta Crnjaković2 and Ljubomir Babić1 AB STRA CT The composition of beach sands from the Islands of Rab and Susak (northern Adriatic) has been studied in order to determine how individual beaches are supplied with detritus. The beaches on both islands are composed of quartz dominated siliciclastic sand, with subordinate carbonate content. Three end-member heavy mineral assemblages have been identifi ed among the studied beach sands, each one associated with a specifi c source rock: (1) a garnet dominated assemblage and (2) a zircon, rutile and tourmaline dominated assemblage on Rab Island, as well as (3) an assemblage dominated by unstable minerals on Susak Island. Sands from individual beaches contain one of these specifi c assemblages or display a mixing of two varieties. The end-member assemblages are very comparable with those of Eocene and Pleistocene sediments which crop out on the two islands, identifying them as the principal sourc- es of detritus. Cretaceous and Eocene carbonate rocks, although present to a considerable extent in the study area, have shown to be a negligible source of sandy material. Thus, the supply of detritus to the beaches is primarily con- trolled by erosion of siliciclastic rocks in the immediate or nearby hinterland. Keywor ds: beach sands, sediment supply, framework composition, heavy minerals, Rab, Susak, Croatia 1 Department of Geology, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia; (bluzar@geol.pmf.hr) 2 Croatian Natural History Museum, Demetrova 1, 10000 Zagreb, Croatia Variable sources of beach sands of north Adriatic islands: examples from Rab and Susak Geologia Croatica 61/2–3 379–384 4 Figs. 2 Tabs. Zagreb 2008 Geologia CroaticaGeologia Croatica � 1. INTRODUCTION Analysis of framework composition and heavy mineral as- semblages are commonly used tools for delineating prove- nance of both ancient and recent sediments (WELTJE & VON EYNATTEN, 2004; GARZANTI & ANDO, 2007). In par- ticular, many studies have employed such methods to deter- mine sources of detritus and reconstruct supply pathways by which coastal areas are supplied with sand (e.g. GILES & PILKEY, 1965; GANDOLFI et al., 1982; CLEMENS & KO- MAR, 1988; POTTER, 1994; MARCHESINI et al., 2000; SABEEN et al., 2002; GARZANTI et al., 2002, 2003; CAS- CALHO & FRADIQUE, 2007). Beaches represent a delicate coastal environment, one that is easily subject to destructive processes if an imbalance is created between the supply of detritus and erosion (KOMAR, 1998). In the European Union, comprehensive coastal investigations have shown that its coastal beach environments are experiencing increased ero- sion due to lack of sediment, attributed primarily to human activity (SALMAN et al., 2004). Given the rising pressure from development, the coastal environments of Croatia can be expected to become more and more vulnerable to such pro- cesses in the future. A better understanding of how beaches are supplied with sandy material is crucial when considering the protection and management of such environments, which is essential if their aesthetic, recreational and economic values are to be preserved. With the above notions in mind, we have conducted a comparative beach sediment study on the Islands of Rab and Susak, located in the Kvarner region of the NE Adriatic (Fig. 1). To date our research on beach sands from Rab Island has shown that considerable variability exists in their heavy min- eral compositions (BABIĆ et al., 1997; CRNJAKOVIĆ et al., 1998; LUŽAR-OBERITER et al., 2005; LUŽAR-OBERIT- ER, 2006). In the current study, data has been obtained from Geologia Croatica Geologia Croatica 61/2–3 380 both modern beach sands and ancient rocks from both islands, which we use to document the presence of three end-member heavy mineral assemblages and more precisely defi ne the re- lationships between individual beaches and their source(s) of sand (MIKULČIĆ PAVLAKOVIĆ et al., 2006). 2. MATERIAL AND METHODS Six separate beaches have been chosen from various bays on Rab Island, while for the smaller Island of Susak, the beach in Bok bay has been selected as a representative location (Fig. 1). Samples of beach sands were taken from the foreshore of each beach (Samples R1 to R6 and S1 to S3). To identify the sources from which the individual beaches are supplied with detritus, potential source rocks which crop out on the islands were identifi ed and sampled. These included Eocene sand- stones (Samples ER1 and ER2) which are ubiquitous on Rab Island (Fig. 1), as well as Pleistocene sediments from both is- lands (Samples PR1, PR2, PS1 to PS3). Thin sections were produced from both hard rock and dis- aggregated sand samples. For disaggregated samples, which included beach and Pleistocene sands, the material was fi rst artifi cially cemented with epoxy. Thin sections were inspect- ed under a polarizing light microscope to determine frame- work composition. All samples were treated with 3% hydrochloric acid to remove carbonates. Rock samples were crushed prior to acid treatment, for which only fragments larger than 1mm were used. Carbonate content was calculated from the weight dif- ference before and after acid treatment. Obtaining heavy min- eral concentrates was performed by gravity separation with bromoform (MANGE & MAURER, 1992), using the 0.05– 0.125 mm size fraction after sieving. Heavy mineral propor- tions were determined by ribbon counting 300–400 grains in each sample. We have chosen to display these results as pro- portions of three groups of heavy mineral species, which have been determined to best express the differences between our studied samples: 1) garnet, 2) zircon, tourmaline and rutile (ZTR), and 3) other transparent grains. 3. RESULTS 3.1. Ancient Rocks In the two Eocene sandstone samples from Rab Island angu- lar to subangular quartz predominates among the framework grains. The remainder of the siliciclastic component is made up mostly of rounded chert grains. Feldspars, mica and other lithic fragments (pelitic, quartzite type, quartz-feldspar aggre- gates) also occur. The measured carbonate content in these two samples is 35% and 48%, (Table 1) most of which is pres- ent in the form of matrix. The Pleistocene sands of Rab Island are largely dominat- ed by monocrystalline quartz grains, along with rarer chert, commonly with a limonitic coating. Other lithic fragments are brown limonitized fi negrained clasts. Feldspars are rare. Un- like in the Eocene sandstones carbonate is almost completely absent (1–4%), in these sands. Fi gu re 1: Maps of the study area with marked locations of the analyzed samples. Basic lithologies are simplifi ed after MAMUŽIĆ (1962, 1968) and MAMUŽIĆ et al. (1969, 1973). Geologia CroaticaBorna Lužar-Oberiter et al.: Variable sources of beach sands of north Adriatic islands: examples from Rab and Susak 381 The samples of Pleistocene sand from Susak Island con- tain 16–34% carbonate (Table 2) which is present as grains of micritic or microsparitic limestone. Rare mosaic grains with large crystals or individual larger carbonate crystals were iden- tifi ed as probable dolostones. The siliciclastic component is mostly quartz, along with lithic fragments, feldspars and mica. The lithic fragments are aggregates of mica and quartz, epi- dote or zoisite, or may be entirely composed of sericite. Mi- cro-quartzite and chert fragments are much rarer. Heavy mineral assemblages from the analyzed rock types which crop out on the two islands demonstrate obvious dissim- ilarities (Tables 1 and 2). Eocene sandstones from Rab Island are characterized by the dominance of garnet and a low propor- tion of ZTR (66% and 11% in average, respectively). The ana- lyzed Pleistocene sands from Rab Island have a much lower garnet content together with a high proportion of ZTR (5% and 63% in average, respectively). The Pleistocene sands of Susak Island demonstrate yet a third compositional type with low gar- net and low ZTR content (17% and 4% in average, respective- ly), while the bulk of the heavy mineral fraction is made up of unstable minerals, mostly amphiboles and epidote/zoisite. 3.2. Beach Sands All the beach sands analyzed from Rab Island are predomi- nantly composed of siliciclastic material, and contain only a small amount of carbonate. Those from the Kalifront penin- sula (samples R1, R2, R3; Fig.1) are clearly dominated by quartz, along with rarer chert grains. Lithic fragments of weath- ered quartz sandstone occur occasionally. Feldspar is almost completely absent. Carbonate grains do occur (6–9% carbon- ate), but these are primarily shell fragments of recent organ- isms. Only rarely are they detrital carbonate grains of a some- what larger grain size than the siliciclastic material. On the Lopar peninsula (samples R5 and R6), quartz also dominates the beach sands. Lithic fragments are fairly common and are mostly chert grains, along with occasional fragments of Eo- cene sandstone. Other rarer fragments are quartzite types, phy- litic or quartz-feldspar aggregates. Carbonate grains are rare (4–9% carbonate), and consist of recent shell fragments and redeposited foraminifera (Nummulites) from the Eocene sand- stones. Sample R4 (Fig. 1) is comparable in framework com- position to samples R5 and R6 described above. The siliciclastic component in the beach sands from Bok bay on Susak Island is almost identical to that of the Pleis- tocene sand present on the island. Only the amount of mica varies signifi cantly due to hydraulic sorting. The carbonate content in two of the beach sand samples is similar (26% and 28%) to that of the Pleistocene sands. However, the third sam- ple contains a considerably higher proportion of carbonate (66%). This excess carbonate comes from shell fragments of recent organisms, mostly foraminifera. Heavy mineral assemblages of various beach sands from the two studied islands show major differences. In the beach sands from Rab Island, the variations in the proportion of gar- net relative to ZTR are most conspicuous. Among the indi- vidual beaches of Rab Island, the garnet content varies be- tween 12% and 72%, while the ZTR vary from 11% to 67% (Table 1). A higher proportion of garnet relates to a lower pro- portion of ultrastable minerals, and vice versa. Among the other heavy minerals encountered are epidote/zoisite, Cr-spi- nel, pyroxenes, staurolite and kyanite, the proportions of which do not differ signifi cantly among the studied samples. Unlike those of Rab Island, the beach sands from Susak Island dem- onstrate relatively low amounts of garnet and ZTR (12–26% and 4–5%, respectively), and instead contain a very high pro- portion of unstable minerals, mainly amphiboles and epidote (Table 2). 4. DISCUSSION Analysis of thin sections reveals a strongly dominating silici- clastic component in the composition of the studied beaches on Rab Island. Although Cretaceous and Eocene carbonate rocks cover a large area of the island, they obviously play a negligible role in supplying sandy material to the studied co- astal areas. Indeed, the carbonate material that is present in the sands is more commonly composed of recent shell frag- ments or redeposited foraminifera than detrital carbonate gra- ins. Even most of the carbonate which makes up a consider- Table 1: Carbonate content and proportions of specifi c heavy mineral groups in samples of beach sands (R1–R6), Eocene sandstones (ER1, ER2) and Pleistocene sands (PR1, PR2) from Rab Island. ZTR = zircon+tourmaline +rutile; OT = other transparent heavy mineral grains. Sample Locality % CaCO3 % ZTR % Garnet % OT R1 Planka 9 67 13 20 R2 Vardarika 6 62 12 26 R3 Valafta 9 52 18 30 R4 Gonar 4 34 41 25 R5 Crnika 4 32 42 26 R6 Saramić 9 11 72 17 ER1 Lopar 35 13 72 15 ER2 Palit 48 9 60 31 PR1 Kalifront 4 63 6 31 PR2 Fruga 1 62 4 33 Table 2: Carbonate content and proportions of specifi c heavy mineral groups in samples of beach sands (S1–S3) and Pleistocene sediments (PS1–PS3) from Susak Island. ZTR = zircon+tourmaline+rutile; OT = other transparent heavy mineral grains. Sample Locality % CaCO3 % ZTR % Garnet % OT S1 Bok 28 3 15 83 S2 Bok 66 5 25 70 S3 Bok 26 2 10 88 PS1 Bok 22 4 12 84 PS2 Bok 16 5 26 69 PS3 Bok 34 4 13 83 Geologia Croatica Geologia Croatica 61/2–3 382 able proportion of the Eocene sandstones, (up to 48%), does not arrive on the beaches which contain a maximum of 9% carbonate. This is probably because most of it is present as matrix in the sandstones, and is lost during weathering. Fur- thermore, this also excludes coarse grained Quaternary sedi- ments, (Holocene after MAMUŽIĆ et al., 1973), common on the SE part of the island as a source of material for the beaches included in this study, as they contain predominantly detrital carbonate fragments. Instead, Eocene sandstones and Pleisto- cene quartz sands are the obvious dominant sources of detri- tus, which is refl ected in the general similarity between their framework components and the grain composition of the beach sands. Lithic fragments identifi ed in some of the beach sam- ples, are an additional indicator of these source rocks. The general similarity in the dominant framework com- ponents between the Eocene and Pleistocene source rocks on Rab Island does not allow for a more precise evaluation of the contribution each of these sources makes to a specifi c beach. In this regard, a comparison of heavy mineral assemblages showed to be a much better indicator. The consistent variabil- ity in the garnet versus ZTR proportions in heavy mineral as- semblages of the studied beaches indicates the derivation of these sands. While the garnet-rich and ZTR-poor beach sands (Lopar peninsula), closely resemble those of Eocene sand- stones, the beach sands rich in ZTR and poor in garnet (Kali- front peninsula) are comparable to those of Pleistocene sands (Fig. 2). These relationships suggest that some beaches are supplied exclusively from Eocene rocks (e.g. Saramić – R6), some from Pleistocene sands (e.g. Planka – R1), while other beaches receive detritus from both sources (e.g. Gonar – R4 and Crnika – R5). This is consistent with the previously pro- posed view on the provenance of the Rab beach sands which was based on the study of only three beach samples (CRNJA- KOVIĆ et al., 1998), and a similar conclusion has been reach- ed for the northern Dalmatian coast (PAVIČIĆ et al., 2000), where analogous rocks were identifi ed as important sources of detritus for beach sands. The beach sands from Susak Island show almost identi- cal compositions to those of the analyzed Pleistocene sands on this island, both in framework components and heavy min- erals (Fig. 3). Such a conspicuous similarity results from the large coverage of Pleistocene sand (Fig. 1) which is charac- terized by a homogenous composition throughout the island (MUTIĆ, 1967; BOGNAR et al., 1983; CREMASCHI, 1990; MIKULČIĆ PAVLAKOVIĆ, 2006). Furthermore, consider- ing that the carbonate content in the beach sand has been at- tributed to shells of recent organisms, the carbonate rocks on Susak Island represent a negligible source of sandy material to Bok bay. The heavy mineral assemblages of the Susak beach sands are considerably different from those of Rab Island (Tables 1 and 2). Consequently, we have identifi ed three end-member heavy mineral assemblages in the beach sands of the two is- lands: 1) garnet dominated assemblage from Rab Island, 2) ZTR dominated assemblage from Rab Island and 3) assem- blage dominated by unstable minerals from Susak Island. These correspond to the heavy mineral assemblages of the three studied ancient rock types from the two islands: 1) Eo- cene sandstones, 2) Pleistocene sands of Rab Island and 3) Pleistocene sands of Susak Island, respectively (Fig. 4). Con- sequently, we have shown that studying the heavy mineral as- semblage of beach sand allows the assessment of the contri- bution each source rock makes to a specifi c beach. The substantial variability of beach sand compositions detected across a relatively short distance of shoreline, as wit- nessed on Rab Island, allows us to conclude that the supply of detritus to the beaches must be primarily controlled by ero- sion of siliciclastic sediments in the immediate or nearby hin- terland. Other possible supply paths such as longshore trans- port or a derivation from a more distant source are obviously less signifi cant in this area. In the bottom sediments surrounding Rab Island, ŠKRI- VANIĆ & MAGDALENIĆ (1979) determined sand domi- nated by quartz along with heavy mineral assemblages with Fi gu re 2: A comparison in the proportions of ZTR, garnet and other transparent grains among diff erent beach sands from Rab Island (R1–R6). Results from a Pleistocene sand sample (PR1) and an Eocene sandstone sample (ER1) are added for additional comparison. ZTR=zircon+tourmaline+rutile; OT=other transparent grains. Geologia CroaticaBorna Lužar-Oberiter et al.: Variable sources of beach sands of north Adriatic islands: examples from Rab and Susak 383 increased amounts of zircon and garnet, which is similar to the framework composition and dominant heavy minerals we detected in the studied beaches on the island. This suggests that the supply of sand material from Eocene sandstones and Pleistocene sands probably extends to the bottom sediments of the surrounding seaways. Similarly, heavy mineral assem- blages analogous to those of the Pleistocene sands on Susak Island can be found in the bottom sediments of the northern Adriatic (PIGORINI, 1968), including the western part of the Kvarner region (ŠKRIVANIĆ & MAGDALENIĆ, 1979). The source areas and the natural transport directions of beach sands from the Islands of Rab and Susak have been suc- cessfully assessed using framework composition and heavy mineral assemblages. Since haphazard construction can po- tentially cut off vital sediment supply paths and disrupt the beach sediment budget, which eventually leads to sediment defi cit and coastal erosion, we propose that the information derived from this and other similar coastal studies should be taken into account when planning future development along the relevant coastline. In our continuing research we are ex- tending our study area to embrace a greater coverage of the Croatian coastline, including other Islands as well as the main- land. 5. CONCLUSIONS This study has shown that the studied beach sands of the Is- lands of Rab and Susak are made up primarily of siliciclastic, quartz dominated detritus. Carbonate particles are mostly a minor component. Three end-member heavy mineral assem- blag es, each related to a different source rock, have been identi- fi ed in the studied beaches: 1) a garnet dominated assemblage and 2) a zircon, rutile and tourmaline dominated assemblage on Rab Island, as well as 3) an assemblage dominated by un- stable minerals on Susak Island. Intermediate compositional types of beach sands result from the mixing of detritus sup- plied from two sources. Consequently, erosion of siliciclastic sediments in the immediate or nearby hinterland, specifi cally Eocene sandstones and two varieties of Pleistocene sands, has a primary role in providing the studied beaches with sandy material, while longshore transport or derivation from a dis- tant source are much less signifi cant. Meanwhile, the carbon- ate, where present, is more commonly derived from shells of recent organism’s than from eroding Cretaceous and Eocene carbonate rocks. Furthermore, for the studied area, the heavy mineral as- semblage of sand from a specifi c beach allows the successful identifi cation of its specifi c source rock(s). We propose that the conclusions derived from this study should be taken into account when planning future develop- ment of the relevant coastline. ACKNOWLEDGMENT This work was funded the Croatian Ministry of Science, Education and Sports project 119-1191155-1159 “Evolutionary Changes of the Dinarides from Subduction to Modern Adriatic Beaches”. We would like to thank Dr. Giovanni Vezzoli and Dr. Dunja Aljinović for their critical reviews of the manuscript and their constructive suggestions which signifi cantly improved the fi nal version. REFERENCES BABIĆ, LJ., CRNJAKOVIĆ, M. & ZUPANIČ, J. (1997): Complex nour- ishment of sand beaches of the Island of Rab (eastern Adriatic, Croatia). – Meeting on Environmental Sedimentology, IAS-SEPM, Venice, Abstracts, W3S–26. 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