Estuarine, Coastal and Shelf Science (2000) 00, 000–000 doi:10.1006/ecss.1999.0620, available online at http://www.idealibrary.com on

Origin and Distribution of Clay Mineralsin the Alexandroupolis Gulf, ,

K. Pehlivanogloua, A. Tsirambidesb and G. Trontsiosb aDepartment of Oceanography, Hydrographic Service, Hellenic Navy, T.G.N. 1040, Holargos, Athens, Greece; e-mail: [email protected]. bDepartment of Geology, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece

Received 29 March 1999 and accepted in revised form 18 December 1999

The mechanisms of clay mineral distribution in Alexandroupolis Gulf are studied. The annual solid supply of the River, flowing into the Gulf, amounts to at least 1 000 000 m3. The surficial bottom sediments are commonly fine-grained and are distributed along zones almost parallel to the coastline. In the central part of the Gulf clay-silt size sediments predominate. The main clay minerals in the size fractions (2–1, 1–0·25 and <0·25 m) are illite, smectite, kaolinite and in small amounts interstratified illite/smectite. Quartz, feldspars, amphiboles and chlorite occur in traces in the coarser fraction (2–1 m) of some samples. All the above minerals are the weathering products of the Evros River drainage basin, as well as of the Neogene and Quaternary unconsolidated sediments of the coast. The hydrodynamic regime and physical grain size are the main mechanisms, which control the distribution of the clay minerals in the Gulf. The low content of kaolinite in all samples and the presence in traces of chlorite and amphiboles in some coarse clay fractions may be due to the unfavourable climatic and physicochemical conditions, as well as to the rapid transport and deposition of freshly weathered material.  2000 Academic Press

Keywords: clay mineral distribution; river solid supply; Alexandroupolis Gulf; Aegean Sea

Introduction (1976) and Biscaye and Eittreim (1977) showed that suspended particulate material in the Atlantic ocean  The general scarcity of clay sized sediments in shallow ranges from 5 to 300 gkg 1 mainly depending on water, due to winnowing through the action of waves, regional characteristics. Chamley et al. (1977) ob- tides and currents, contrasts with the huge accumula- served an increase of relative abundance of smectite tions of argillaceous deposits in the rest of the oceans. and palygorskite from 1000 to 3000 m water depth in Where they do occur, source mixing during transpor- sediments off NW Africa, which is attributed to a late tation, flocculation and differential settling processes deposition of these fine and low-flocculable minerals. appear to be the main mechanisms for their distribu- Preferential clay settling was also proposed for the tion (Griffin et al., 1968; Chamley, 1989). Gibbs Ganges sediments, where increased amounts of smec- (1977) reports that the most probable explanation for tite in hemipelagites are reported offshore from the the clay mineral variations off the mouth of the coastline (Bouquillon & Chamley, 1986). Amazon River is a physical size segregation. Addition- Preferential settling of smectite and expandable ally, Chamley (1989) suggests that the aggregation of mixed minerals represents a common phenomenon in clay particles by marine organic matter appears to be a the western Mediterranean Sea. Chamley (1971), widespread phenomenon, mainly responsible for the Monaco (1971) and Roux and Vernier (1977) report rapid sinking of land-derived materials. Clay is mainly a frequent increase of expandable minerals at in- incorporated in fecal pellets and other mucous matter creased distance from the shore of the Gulf of Lion. In within the surface water masses where high planktonic the Adriatic Sea, mechanical sorting and flocculation productivity develops seasonally. account for the distribution of clay suites derived from Gorbunova (1962) observed a decrease in kaolinite the Po and other rivers (Veniale et al., 1972; Tomadin abundance with increasing distance from the Indian & Borghini, 1987). coast because of changes in conditions of marine In this study the mechanisms responsible for the transportation. The occurrence of grain sorting was distribution of the clay minerals in the Alexandroupo- suggested later by many researchers. Brewer et al. lis Gulf are investigated.

0272–7714/00/000000+00 $35.00/0  2000 Academic Press 2 K. Pehlivanoglou et al.

40° 55' N 0 n.m. 3 n.m. 6 n.m. 9 n.m. 12 n.m. 15 n.m.

Makri Alexandroupolis

40° 50'

Evros River

° 40 45' 20

50 30 30

40° 40'

40° 36' 25° 30' 25° 35' 25° 40' 25° 45' 25° 50' 25° 55' 26° 00' 25° 05' E F 1. Bathymetry of the Gulf of Alexandroupolis (depths in m).

Geographic setting of the study area Oceanography Seasonal measurements of oceanographic parameters Hydrography (salinity, density etc.) during maximum discharge The Gulf of Alexandroupolis covers an area of periods have shown that water of low salinity, 26–34 350 km2 and constitutes the NE part of the Thrace (using the Practical Salinity Scale) and low density Sea (Figure 1). The Evros River flows into the eastern (1·0019 g cm3)diffuses at shallow depth (up to 5 m) part of the Gulf through a lobate type delta, which has into the sea, in a SE direction (Hydrographic Service, an area of 188 km2. The total drainage basin of the 1984). This water, with slightly changed parameters, Evros is 52 500 km2, extending from through can be traced up to 30 km from the river mouth. Turkey to Greece. The length of the main branches of Towards the bottom, the salinity and density gradu- the Evros is 410 km (Psilovikos & Hahamidou, 1987). ally increases, thus confirming that the initial surface The wide range of estimates on sediment discharge of layer gradually sinks and is mixed with the ambient the Evros River in the Greek literature is very confus- sea water with increasing distance from the river ing. Psilovikos et al. (1993) estimated the sediment mouth. discharge of the Strymon River (about 200 km west of From current measurements during the minimum Evros), which has a drainage basin of 11 000 km2,at discharge periods (July), at distances of about 10 km 1 000 000 m3 annually. They used geomorphological from the river mouth, a distinct retrogressive motion data of the whole drainage basin, examined about of the seawater is noticed with NW to SE direction. It 1000 samples taken close to the main bed during shows changeable range of 10–30 km, average velocity floods for a period of 2 years and compared old of 17 cm s1 and maximum velocity of 40 cm s1, and new maps of the morphology of the area. Thus, with a WNW to ESE direction. In contrast, current taking these data into account, the annual sediment measurements during March and under the action of discharge of the Evros River should well exceed south winds, showed motion of the seawater towards 1 000 000 m3. Aksu et al. (1995), using a mathemati- west, parallel to the northern coastline (Hydrographic cal formula, suggest an average annual sediment Service, 1984). yield of approximately 107 tonnes. In addition, a According to the data of the National Meteoro- number of small creeks flow into the N and NW part logical Service of Greece (Alexandroupolis Airport of the Gulf. Station) the broader area presents average annual Origin and distribution of clay minerals 3

Legend Fluvial deposits (sands, pebbles etc) Greece Conglomerates, sandstones, schists, mudstones, marbles, andesites Volcanic rocks interstratified with sedimentary rocks Schists, limestones, dolomites, marbles Pliocene-pleistocene deposits Turkey Crystalline rocks and migmatites (leptites, gneis, schists) Granites and diorites Flysch Ultrabasic rocks

Pyroclastic rocks Igneous rocks

F 2. Petrographic sketch map of the Evros river drainage basin (Pehlivanoglou, 1995). rainfall of 576 mm and the predominating winds metamorphic rocks (Figure 2). Plutonic intrusions, as have mainly NE (25%), N (13%) and SW (10%) well as volcanic rocks of rhyo-dacitic composition, directions. Calm period is 32%. outcrop in small areas of the basin. During early Tertiary time the meta-alpine basin of Thrace was formed and filled with molassic sediments that lie Bathymetry unconformably on the rocks of the greater region The bathymetry of the Gulf was studied from the (Mountrakis, 1985). Calc-alkaline volcanism of bathymetric diagrams of the Hydrographic Service of Eocene-Oligocene age produced extensive deposits of the years 1966, 1967 and 1978 with scale 1:5000, as volcaniclastic tuffs interbedded with a variety of other well as from the bathymetric charts of the area with sediments (Arikas, 1979). Finally, a transgressive se- scale 1:50 000 and 1:75 000. The relief of the Gulf quence of Miocene coastal sediments was deposited bottom as well as of the surrounding area is smooth on the Oligocene sediments (Papadopoulos, 1980; with very low gradient. Thus the Gulf bottom seems Solakius and Tsapralis, 1987). The meta-alpine sedi- almost flat (Figure 1). The sea depth, even at large ments (conglomerates, sandstones, marls, marly lime- distances from the coast, is low and does not exceed stones etc.) were rapidly deposited during periods of 35 m. The bottom gradient is less than 1% except in high seasonal rainfall. As a result, physical and chemi- the SE and NW edges of the Gulf where the relief is cal weathering processes and reworking of these sedi- more intense (1–2% dip). After the isobath of 30 m, ments was of limited influence (Trontsios, 1991). The an elongate undersea platform exists at an average large discharge of sediments from the land, into the depth of 35 m with SE–NW direction. coastal basin, while initially was periodically sub- merged and later crossed by the Evros River, resulted in the formation of 3000 m thick sediment beds in Geology front of the river mouth and 1500 m thick in the The drainage basin of the Evros River is part of the western part of the Gulf (Lalechos, 1986). The Rhodope geotectonic zone which consists mainly of Holocene sediments reach a thickness of 10 m in front 4 K. Pehlivanoglou et al.

40° 54' N

Makri Alexandroupolis

A1 A2 A26 A3 A8 A7 A4 A30 A6 A9 A10 A11 A13 A12 A14 A15 A16 40° 48' C/M Evros River A20 A19 A18 A17 A21 A28 A29 A22 A23 A24

A25

A27

40° 42' 25° 42' 25° 48' 25° 54' 26° 00' 26° 06' E F 3. Locations of the sediment samples and current meters moorings (C/M). of the delta. At the western and southern parts of the sample stations were determined by a ‘ Trispomder ’ Gulf this thickness is greatly reduced (Perissoratis and positioning system. Mitropoulos, 1989). Grain size determination of the material and tex- The Gulf area with the adjacent coast belongs to the tural classification was performed on each sample Perirhodope geotectonic zone which is represented by following the Folk (1968) method. A 20 g split of each two distinct lithologic and stratigraphic units (Kouris, sample was subjected to the following chemical treat- 1980; Papadopoulos, 1980, 1982). The Makri unit ments (Jackson, 1974): 1N sodium acetate-acetic acid overlies the Rhodope massif. It includes a lower buffer solution with pH=5·0 for carbonate removal; carbonate series, 300 m thick, which consists of mar- 30% H2O2 for organic matter and Mn-oxides bles, dolomites, limestone, calcitic schists and sericitic removal; 0·3M sodium citrate and 1M sodium bicar- phyllites and an upper greenschist series, 200–300 m bonate buffer solution with pH=7·3 to which 1 g thick, consisting of albitic, chloritic, talc and mica- increments (up to 3 g) of sodium dithionite were ceous schists. The Makri formations have Jurassic to periodically added to remove free Fe-oxides and Fe-/ Lower Cretaceous age (Kouris, 1980). The Drymos- Al-hydroxides. Melia unit has a thickness of 800–900 m and consists The <2 m fractions of the samples were separated of shales, marls, sandstones, conglomerates and into three fractions (2–1, 1–0·25 and <0·25 m) by an volcaniclastics. The age of these formations is Upper IEC centrifuge. The sample fractions were then oven- Cretaceous (Kouris, 1980) or Jurassic-Lower dried at 100 C. Subsequently, random and oriented Cretaceous (Papadopoulos, 1980, 1982). mounts were prepared for XRD analysis. All the oriented mounts were reanalysed after treatment with an ethylene-glycol solution to distinguish the expand- Materials and methods able mineral phases. Some were heated for 2 h at Twenty-five surficial samples were collected from the 550 C for chlorite detection. XRD analysis was car- Gulf bottom (Figure 3) using a Dietz La Fond bottom ried out using a Philips X-ray diffractometer with sampler, from the RV Nautilus of the Hydrographic Ni-filtered Cu-Ka radiation. Semi-quantitative esti- Service, Hellenic Navy. The position of the mates of the mineral abundance based on the peak Origin and distribution of clay minerals 5

40° 54' N Sand Clayey sand Clay-silt

Sandy clay-silt Sandy clay Sandy silt

Makri Alexandroupolis

40° 48' Evros River

40° 42' 25° 42' 25° 48' 25° 54' 26° 00' 26° 06' E F 4. Sediment composition (after Folk, 1968).

area of the oriented mounts were made from the XRD The clay minerals that occur in all fractions are illite, data using the method described in Biscaye (1965). smectite and kaolinite (Figure 5). In the 2–1 m fraction of some samples chlorite is detected. It is poorly crystallized and has a high Fe content (Pehliva- Results noglou, 1995). In the <0·25 m fraction of some The bottom sediments of the Gulf are mostly fine- samples the interstratified phase illite/smectite is dis- grained and distributed in zones with a SE to NW tinguished. In addition, quartz, feldspars and amphi- orientation i.e. running almost parallel to the coastline boles are present in traces in some 2–1 m fractions. (Figure 4). Lithologically the sediments on the central The abundance of smectite is greater in the finer Gulf are composed of clay-silts. Towards the north fraction (<0·25 m). The reverse is true for illite and they become coarser, comprising of sandy silt, sandy kaolinite. The distribution of clay minerals in the Gulf clay-silt and sand close to the coast. Towards the is shown in Figures 6, 7 and 8. south, there are zones of sandy clay and clayey sand. Illite is the most abundant clay mineral, being Finally, relict sand dominates the largest part of the present in almost all the coarser fractions (2–1 m and remaining continental shelf (Pehlivanoglou, 1989; 1–0·25 m), but with a smaller range of values (Figure 1995). Sakellariadou (1987) found high contents of 6). In contrast to the distinct geographic grain size Fe and Al in the Gulf sediments, which form a lobe differentiation of smectite, illite has a more ubiquitous withaWtoNWorientation in front of the Evros distribution. An exception is the <0·25 m fraction mouth. The organic content of modern and Paleogene which shows a higher content in front of the Evros River sediments discharged into the gulf is usually less mouth. The distribution of illite content in the 2–1 than 2% (Trontsios, 1991; Pehlivanoglou, 1995). and 1–0·25 m fractions is almost uniform through The results of XRD analyses of the different size out the Gulf. fractions are listed in Table 1, following the method of In the <0·25 m fraction a significant increase in Biscaye (1965). Different minerals in variable propor- smectite content is observed. The reverse is true for tions are concentrated in different grain size fractions. illite and to a lesser extent also for kaolinite (Table 1). 6 K. Pehlivanoglou et al.

T 1. Mineralogical composition (wt. %) of separated size fractions (m) of the analysed sediments

Samplea Size S I K I/S Sample Size S I K I/S

A1 2–1 3 75 22 A17 2–1 354916 1–0·25 4 78 18 tr 1–0·25 50 36 14 tr <0·25 74 18 8 tr <0·25 25 70 5 tr A4 2–1 17 64 19 A18 2–1 4 79 17 1–0·25 30 56 14 tr 1–0·25 17 66 17 tr <0·25 61 22 17 tr <0·25 81 12 7 tr A5 2–1 13 64 23 A19 2–1 5 77 18 1–0·25 21 66 13 tr 1–0·25 20 59 21 tr <0·25 9 72 19 tr <0·25 81 10 9 tr A6 2–1 23 57 20 A20 2–1 5 82 13 1–0·25 18 65 17 tr 1–0·25 28 59 13 tr <0·25 78 16 6 tr <0·25 86 3 11 tr A7 2–1 5 80 15 A21 2–1 3 83 14 1–0·25 32 51 17 tr 1–0·25 23 60 17 tr <0·25 77 18 5 tr <0·25 73 7 20 tr A8 2–1 10 76 14 A22 2–1 8 66 26 1–0·25 61 27 12 tr 1–0·25 9 77 14 tr <0·25 79 16 5 tr <0·25 83 11 6 tr A10 2–1 10 72 18 A23 2–1 8 77 15 1–0·25 24 60 16 tr 1–0·25 14 68 18 tr <0·25 76 19 5 tr <0·25 81 12 7 tr A11 2–1 4 73 23 A24 2–1 5 79 16 1–0·25 2 85 13 tr 1–0·25 8 75 17 tr <0·25 86 11 3 tr <0·25 11 73 16 tr A12 2–1 12 67 21 A25 2–1 592813 1–0·25 32 52 16 tr 1–0·25 59 28 13 tr <0·25 65 24 11 tr <0·25 89 5 6 tr A13 2–1 9 74 17 A26 2–1 8 70 22 1–0·25 10 81 9 tr 1–0·25 17 65 18 tr <0·25 91 3 6 tr <0·25 47 26 27 tr A14 2–1 5 70 25 A27 2–1215821 1–0·25 29 52 19 tr 1–0·25 38 46 16 tr <0·25 60 23 17 tr <0·25 57 41 2 tr A15 2–1 5 71 24 A28 2–1106426 1–0·25 10 69 21 tr 1–0·25 43 33 24 tr <0·25 79 11 10 tr <0·25 33 59 8 tr A16 2–1 6 72 22 AVERAGE 2–1126919 1–0·25 7 78 15 tr 1–0·25 24 60 16 <0·25 22 67 11 tr <0·25 64 26 10

aNumbers in Alexandroupolis (A) Gulf samples denote collection sites. S=smectite, I=illite, K=kaolinite, I/S=illite/Smectite, tr=trace.

The geographic distribution of smectite in the increasing grain size. In the <0·25 m fraction the <0·25 m fraction (Figure 7) shows higher values in highest contents are found close to the river mouth, the NE of the inner part of the Gulf, as well as in the whereas in the 2–1 and 1–0·25 m fractions a small central and western parts. The 1–0·25 m fraction has increase is noted at the western and south-western high smectite contents closer to the Evros mouth, as stations. well as in the western part of the Gulf, whereas the lowest contents occur in the central part. Finally, the Discussion 2–1 m fraction shows the highest contents only close to the river mouth. Illite, smectite and kaolinite are the minerals, which Kaolinite occurs in very low contents in all fractions predominate, in the fine-grained fraction while chlo- and also shows little geographic differentiation (Figure rite, amphiboles and interstratified illite/smectite also 8). In general the kaolinite content increases with occur in small quantities or traces. Origin and distribution of clay minerals 7

il il 2–1 µm

il

a

k k

b

q

c

1–0.25 µm

d sm

k

e

sm f

<0.25 µm sm

g

h 264 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Angle 22 F 5. Representative XRD patterns of clay fractions of sample A14. (a, f) Heated at 550 C;(b, d, g) ethylene-glycolated; (c, e, h) parallelly oriented. sm=smectite, il=illite, k=kaolinite. 8 K. Pehlivanoglou et al.

40.9° N Fraction 2–1 µm

Makri Alexandroupolis

80

<80% <70%

70 70 40.8° 70 70–80% 60

Evros River 60 70 50 <40% 70 60–70%

40 50

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction 1–0.25 µm

Makri Alexandroupolis

<60% 60 40.8° 60 60 40 60 60–80% Evros River

60 <40% 60 40–60% 40

40

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction <0.25 µm

Makri Alexandroupolis

40 20

° 20 40.8 40

<20% >60% Evros River 20 50

40 <20% 40 20 20–40%

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E F 6. Distribution of illite (weight %). Origin and distribution of clay minerals 9

40.9° N Fraction 2–1 µm

Makri Alexandroupolis

10

10

<10% 40.8° 20 <10% 20–30% Evros River 10

10 >40% 10–20%

40

10 20 30

40.7° 25.6° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction 1–0.25 µm

Makri Alexandroupolis

40 20

20–40% >40% <20% <20%

40.8° 20

Evros River

40 20 20 >40%

40

40.7° 25.6° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction <0.25 µm

Makri Alexandroupolis

60 80 60 40.8° 40 40 >80% <40%

Evros River <40% 80 80 60

40–60% 60–80% 60 40 40.7° 25.6° 25.7° 25.8° 25.9° 26.0° 26.1° E F 7. Distribution of smectite (weight %). 10 K. Pehlivanoglou et al.

40.9° N Fraction 2–1 µm

Makri Alexandroupolis

30

25 20 >30% 20 >20% 30 20 20 40.8° 25–30% 25 20 15–20% 20 15 20 Evros R.

20

25 <20% 15 20 20–25% 25

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction 1–0.25 µm

Makri Alexandroupolis

15

15 15–20% 15 <15% 20 20 40.8° 15 <20% 20 15 20 20 Evros R.

<20% 15

15 15

20

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E

40.9° N Fraction <0.25 µm

Makri Alexandroupolis

10 20 15 15 10 10 10 40.8° 15

10 >15% 10 5–10% 10 Evros R. 15 >10% 5 10–15% 5 10 <5% 10

40.7° 25.7° 25.8° 25.9° 26.0° 26.1° E F 8. Distribution of kaolinite (weight %). Origin and distribution of clay minerals 11

The illite, smectite and kaolinite contents of the 33% and kaolinite 15%. Mixed-layer illite/smectite Gulf sediments are due to the very large river input of were also detected in traces in some samples. suspended load rich in micas, derived from the weath- Comparatively the average clay mineral content in ering of parent rocks in the drainage basin of the Evros Thermaikos Gulf, NW Aegean Sea, 250 km west of River. In addition, some of the clay minerals are Alexandroupolis Gulf, was reported by Lykousis derived from the weathering of the unconsolidated et al. (1981) as illite 50%, smectite 34% and Neogene and Quaternary coastal sediments. kaolinite+chlorite 16%. The terrigenous input, the Acid and mafic igneous as well as metamorphic water mass circulation and to a lesser extent the wave rocks cover the western and central parts of the Evros activity, control the sedimentation within the NW river drainage basin, Neogene sediments cover the Aegean Sea. eastern part and the coastal zone. The weathering of In the Strymonikos Gulf, (an area geomorphologi- these rocks supplies at least 1 000 000 m3 year1 of cal similar to the Alexandroupolis Gulf 200 km west- suspended material to the gulf of Alexandroupolis. ward), the illite content in the clay fraction varies Perissoratis et al. (1987) accept that the Evros is the between 45 and 73%, with higher values close to the largest supplier of fine grained Fe-Al-rich continental Strymon delta, while smectite varies between 11 and detritus to the offshore area which then is dispersed 38% and kaolinite between 6 and 15% (Conispoliatis, westwards along the coast by local currents. 1984). The distribution of clay minerals is interpreted Tsirambides et al. (1989) studied the clinoptilolite as the result of differential settling and is controlled by contained in volcaniclastic sediments (connected with the water circulation. rhyo-dacitic volcanism of the Upper Eocene-Lower According to Conispoliatis and Perissoratis (1987) Oligocene) from the area, which is drained the clay mineral distribution in the Ierissos Bay (an by Erythropotamos (tributary of Evros River). They enclosed gulf, N Aegean Sea) is mainly related to the found that the only clay minerals present, (usually in rock composition of the drained land and to the traces), are illite, smectite and mixed-layer illite/ dispersion by currents. The average clay mineral con- smectite. tent of the 37 samples analysed was: illite 63%, Kirov et al. (1990) who studied the zeolite-bearing smectite 20%, kaolinite 9% and chlorite 8%. Also, low Tertiary sediments (conglomerates, breccias and content of mixed-layer illite/smectite was detected in sandstone) from Petrota area (about 150 km north of some samples. Alexandroupolis) which belongs to the drainage basin Volcanic rocks occur extensively in the drainage of the Evros River, found only traces of celadonite, basin of the Evros River. Micas exist as primary kaolinite and chlorite among the clay minerals. mineral in all samples examined, while chlorite in Studying the weathering products of trachy- some of them. The abundance of illite and smectite in rhyolites from the eastern Phodope (part of the Evros the Gulf sediments is due to presence of these miner- drainage basin), Popov and Michalev (1990) con- als in the volcanic rocks of the drainage basin of the cluded that the prevailing conditions (altitude, cli- Evros River. Especially, the abundance of smectite mate, vegetation, etc.) prevent intensive weathering enhanced by the high Fe and organic content, which and thus the formation of kaolinite. results in a rapid flocculation and settling out of The dispersal of the Evros River suspended load in smectite grains. The distribution pattern of smectite in the area of the gulf of Alexandroupolis takes place the Gulf shows that the dispersion of this mineral has under the prevailing meteorological and oceano- not been strongly influenced by the hydrodynamic graphic conditions: (a) dispersal to the west and conditions and that it should thus not be used as a north-west takes place under the action of south, hydrodynamic index. Chronis (1986) confirmed the south-east and south-west winds. (b) dispersal in a predominance of smectite over the rest of the clay north-west/south-east direction occurs under the minerals in front of the pro-delta platform and close to action of north and north-east winds. the coast at depths not exceeding 30 m in the Ther- The modern sediments of the Gulf have adopted maikos Gulf, (250 km west of Alexandroupolis gulf). their characteristic zonal distribution in response to The main reason of the high smectite content is the the coastal topography, the water motion and the high Fe and organic content of the discharged sedi- quality of the supplied material. The hydrodynamic ments, which results in a rapid flocculation and set- conditions and to a lesser extent also the grain size of tling out of smectite grains. This flocculation process the clay minerals, are the main factors controlling is enhanced by the physico-chemical conditions of the their distribution in the Gulf. seawater (i.e. salinity, temperature, pH and Eh). The average clay mineral composition of the 25 Kaolinite content expresses the strong climatic samples analysed is (Table 1): illite 52%, smectite dependence controlled by the intensity of hydrolysis of 12 K. Pehlivanoglou et al. continental rocks which occur in the drainage basin. Chamley, H. 1971 Recherches sur la se´dimentation argileuse en Me´di- The low content of kaolinite however, may be due to terranee. Sci. Ge´ol., Strasbourg, me´m. 35, 225 pp. Chamley, H. 1989 Clay Sedimentology. Springer-Verlag, Berlin, unfavourable climatic and physicochemical condi- 623 pp. tions, as well as to the detrital origin, rapid transport Chamley, H., Diester-Haass, L. & Lange, H. 1977. Terrigenous and deposition of the weathered material in the Gulf. material in East Atlantic sediment cores as an indicator of NW African climates. Meteor Forsch. Ergebn. 28, 44–59. Furthermore, the low content of amphiboles observed Chronis, G. 1986. The Modern Dynamics and Holocene Sedimentation even in the clay fractions of the discharged material in the Inner Plateau of Thermaikos Gulf. Ph.D. thesis, University of confirms the limited reworking and weathering of the Athens, 228 pp. (in Greek with English abstract). Conispoliatis, N. 1984. Study of the Recent Sediments of the Stry- primary ferromagnesian minerals because of the high monikos Gulf. Ph.D. thesis, National Technical University, Ath- river discharge over short time periods and rapid ens, 109 pp. (in Greek with English abstract). deposition in the Gulf. Conispoliatis, N. & Perissoratis, C. 1987. Distribution and origin of clay minerals in the bottom sediments of the Ierissos bay. In The traces of the interstratified illite/smectite con- Proceedings of the 2nd Hellenic Symposium Oceanography and Fish- firm the limited reworking and weathering of the ery, Athens, pp. 485–492. (in Greek). primary minerals occurring in the sediments of the Gibbs, R.J. 1977. Clay mineral segregation in the marine environ- ment. Journal Sedimentary Petrology 47, 237–243. broader area. Gorbunova, Z. N. 1962. Clay and associated minerals of the Indian Trontsios (1991) studying Paleogene sediments ocean sediments. Trans. Inst. Oceanol. U.R.S.S. 61, 93–103. ffi from drilling cores of the Evros delta, found that the Gri n, J. J., Windom, H. & Goldberg, E. D. 1968. The distribution of clay minerals in the World Ocean. Deep-Sea Research 15, clay fraction consisted exclusively of illite, chlorite, 433–459. vermiculite and the interstratified phases of illite/ Folk, R. L. 1968. Petrology of Sedimentary Rocks. Hemphill’s, smectite and chlorite/vermiculite. The climate that Austin, Texas, 170 pp. Hydrographic Service, 1984. Oceanographic Data of Alexandroupolis prevailed in the broader area of that time was hot and Gulf. Unpublished report, Athens, v. 1–3. (in Greek). semi-arid. In a detailed study of the interstratified Jackson, M. L. 1974. Soil Chemical Analysis. 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