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, Aegean Sea, Greece K. Pehlivanogloua, A. Tsirambidesb and G. Trontsiosb a
Department of Oceanography, Hydrographic Service, Hellenic Navy, T.G.N. 1040, Holargos, Athens, Greece; e-mail:
[email protected]. b Department 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 Evros 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 The general scarcity of clay sized sediments in shallow water, due to winnowing through the action of waves, tides and currents, contrasts with the huge accumulations of argillaceous deposits in the rest of the oceans. Where they do occur, source mixing during transportation, flocculation and differential settling processes appear to be the main mechanisms for their distribution (Griffin et al., 1968; Chamley, 1989). Gibbs (1977) reports that the most probable explanation for the clay mineral variations off the mouth of the Amazon River is a physical size segregation. Additionally, Chamley (1989) suggests that the aggregation of clay particles by marine organic matter appears to be a widespread phenomenon, mainly responsible for the rapid sinking of land-derived materials. Clay is mainly incorporated in fecal pellets and other mucous matter within the surface water masses where high planktonic productivity develops seasonally. Gorbunova (1962) observed a decrease in kaolinite abundance with increasing distance from the Indian coast because of changes in conditions of marine transportation. The occurrence of grain sorting was suggested later by many researchers. Brewer et al. 0272–7714/00/000000+00 $35.00/0
(1976) and Biscaye and Eittreim (1977) showed that suspended particulate material in the Atlantic ocean ranges from 5 to 300 g kg 1 mainly depending on regional characteristics. Chamley et al. (1977) observed an increase of relative abundance of smectite and palygorskite from 1000 to 3000 m water depth in sediments off NW Africa, which is attributed to a late deposition of these fine and low-flocculable minerals. Preferential clay settling was also proposed for the Ganges sediments, where increased amounts of smectite in hemipelagites are reported offshore from the coastline (Bouquillon & Chamley, 1986). Preferential settling of smectite and expandable mixed minerals represents a common phenomenon in the western Mediterranean Sea. Chamley (1971), Monaco (1971) and Roux and Vernier (1977) report a frequent increase of expandable minerals at increased distance from the shore of the Gulf of Lion. In the Adriatic Sea, mechanical sorting and flocculation account for the distribution of clay suites derived from the Po and other rivers (Veniale et al., 1972; Tomadin & Borghini, 1987). In this study the mechanisms responsible for the distribution of the clay minerals in the Alexandroupolis Gulf are investigated. 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'
r
ve
i sR
ro
Ev
40° 45'
20 30
50 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 Hydrography The Gulf of Alexandroupolis covers an area of 350 km2 and constitutes the NE part of the Thrace Sea (Figure 1). The Evros River flows into the eastern part of the Gulf through a lobate type delta, which has an area of 188 km2. The total drainage basin of the Evros is 52 500 km2, extending from Bulgaria through Turkey to Greece. The length of the main branches of the Evros is 410 km (Psilovikos & Hahamidou, 1987). The wide range of estimates on sediment discharge of the Evros River in the Greek literature is very confusing. Psilovikos et al. (1993) estimated the sediment discharge of the Strymon River (about 200 km west of Evros), which has a drainage basin of 11 000 km2, at 1 000 000 m3 annually. They used geomorphological data of the whole drainage basin, examined about 1000 samples taken close to the main bed during floods for a period of 2 years and compared old and new maps of the morphology of the area. Thus, taking these data into account, the annual sediment discharge of the Evros River should well exceed 1 000 000 m3. Aksu et al. (1995), using a mathematical formula, suggest an average annual sediment yield of approximately 107 tonnes. In addition, a number of small creeks flow into the N and NW part of the Gulf.
Oceanography Seasonal measurements of oceanographic parameters (salinity, density etc.) during maximum discharge periods have shown that water of low salinity, 26–34 (using the Practical Salinity Scale) and low density (1·0019 g cm3) diffuses at shallow depth (up to 5 m) into the sea, in a SE direction (Hydrographic Service, 1984). This water, with slightly changed parameters, can be traced up to 30 km from the river mouth. Towards the bottom, the salinity and density gradually increases, thus confirming that the initial surface layer gradually sinks and is mixed with the ambient sea water with increasing distance from the river mouth. From current measurements during the minimum discharge periods (July), at distances of about 10 km from the river mouth, a distinct retrogressive motion of the seawater is noticed with NW to SE direction. It shows changeable range of 10–30 km, average velocity of 17 cm s 1 and maximum velocity of 40 cm s 1, with a WNW to ESE direction. In contrast, current measurements during March and under the action of south winds, showed motion of the seawater towards west, parallel to the northern coastline (Hydrographic Service, 1984). According to the data of the National Meteorological Service of Greece (Alexandroupolis Airport Station) the broader area presents average annual
Origin and distribution of clay minerals
3
Legend Fluvial deposits (sands, pebbles etc) Conglomerates, sandstones, schists, mudstones, marbles, andesites Volcanic rocks interstratified with sedimentary rocks Schists, limestones, dolomites, marbles
Greece
Pliocene-pleistocene deposits Crystalline rocks and migmatites (leptites, gneis, schists) Granites and diorites
Turkey
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 have mainly NE (25%), N (13%) and SW (10%) directions. Calm period is 32%. Bathymetry The bathymetry of the Gulf was studied from the bathymetric diagrams of the Hydrographic Service of the years 1966, 1967 and 1978 with scale 1:5000, as well as from the bathymetric charts of the area with scale 1:50 000 and 1:75 000. The relief of the Gulf bottom as well as of the surrounding area is smooth with very low gradient. Thus the Gulf bottom seems almost flat (Figure 1). The sea depth, even at large distances from the coast, is low and does not exceed 35 m. The bottom gradient is less than 1% except in the SE and NW edges of the Gulf where the relief is more intense (1–2% dip). After the isobath of 30 m, an elongate undersea platform exists at an average depth of 35 m with SE–NW direction. Geology The drainage basin of the Evros River is part of the Rhodope geotectonic zone which consists mainly of
metamorphic rocks (Figure 2). Plutonic intrusions, as well as volcanic rocks of rhyo-dacitic composition, 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 unconformably on the rocks of the greater region (Mountrakis, 1985). Calc-alkaline volcanism of Eocene-Oligocene age produced extensive deposits of volcaniclastic tuffs interbedded with a variety of other sediments (Arikas, 1979). Finally, a transgressive sequence of Miocene coastal sediments was deposited on the Oligocene sediments (Papadopoulos, 1980; Solakius and Tsapralis, 1987). The meta-alpine sediments (conglomerates, sandstones, marls, marly limestones etc.) were rapidly deposited during periods of high seasonal rainfall. As a result, physical and chemical weathering processes and reworking of these sediments was of limited influence (Trontsios, 1991). The large discharge of sediments from the land, into the coastal basin, while initially was periodically submerged and later crossed by the Evros River, resulted in the formation of 3000 m thick sediment beds in front of the river mouth and 1500 m thick in the western part of the Gulf (Lalechos, 1986). The Holocene sediments reach a thickness of 10 m in front
4
K. Pehlivanoglou et al. 40° 54' N
Makri
Alexandroupolis A1 A2
A8
A3
A26
A7
A4 A9
A6
A30
A10
A11
A13 A14
40° 48'
A15
C/M
A12
er
iv
A16
r
Ev
A28
A20 A21 A29
A19
A18 A23
A22
R os
A17 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 Gulf this thickness is greatly reduced (Perissoratis and Mitropoulos, 1989). The Gulf area with the adjacent coast belongs to the Perirhodope geotectonic zone which is represented by two distinct lithologic and stratigraphic units (Kouris, 1980; Papadopoulos, 1980, 1982). The Makri unit overlies the Rhodope massif. It includes a lower carbonate series, 300 m thick, which consists of marbles, dolomites, limestone, calcitic schists and sericitic phyllites and an upper greenschist series, 200–300 m thick, consisting of albitic, chloritic, talc and micaceous schists. The Makri formations have Jurassic to Lower Cretaceous age (Kouris, 1980). The DrymosMelia unit has a thickness of 800–900 m and consists of shales, marls, sandstones, conglomerates and volcaniclastics. The age of these formations is Upper Cretaceous (Kouris, 1980) or Jurassic-Lower Cretaceous (Papadopoulos, 1980, 1982). Materials and methods Twenty-five surficial samples were collected from the Gulf bottom (Figure 3) using a Dietz La Fond bottom sampler, from the RV Nautilus of the Hydrographic Service, Hellenic Navy. The position of the
sample stations were determined by a ‘ Trispomder ’ positioning system. Grain size determination of the material and textural classification was performed on each sample following the Folk (1968) method. A 20 g split of each sample was subjected to the following chemical treatments (Jackson, 1974): 1N sodium acetate-acetic acid buffer solution with pH=5·0 for carbonate removal; 30% H2O2 for organic matter and Mn-oxides removal; 0·3M sodium citrate and 1M sodium bicarbonate buffer solution with pH=7·3 to which 1 g increments (up to 3 g) of sodium dithionite were periodically added to remove free Fe-oxides and Fe-/ Al-hydroxides. The <2 m fractions of the samples were separated into three fractions (2–1, 1–0·25 and <0·25 m) by an IEC centrifuge. The sample fractions were then ovendried at 100 C. Subsequently, random and oriented mounts were prepared for XRD analysis. All the oriented mounts were reanalysed after treatment with an ethylene-glycol solution to distinguish the expandable mineral phases. Some were heated for 2 h at 550 C for chlorite detection. XRD analysis was carried out using a Philips X-ray diffractometer with Ni-filtered Cu-Ka radiation. Semi-quantitative estimates 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
er
iv
r
40° 48'
40° 42' 25° 42'
R os
Ev
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 data using the method described in Biscaye (1965). Results The bottom sediments of the Gulf are mostly finegrained and distributed in zones with a SE to NW orientation i.e. running almost parallel to the coastline (Figure 4). Lithologically the sediments on the central Gulf are composed of clay-silts. Towards the north they become coarser, comprising of sandy silt, sandy clay-silt and sand close to the coast. Towards the south, there are zones of sandy clay and clayey sand. Finally, relict sand dominates the largest part of the remaining continental shelf (Pehlivanoglou, 1989; 1995). Sakellariadou (1987) found high contents of Fe and Al in the Gulf sediments, which form a lobe with a W to NW orientation in front of the Evros mouth. The organic content of modern and Paleogene River sediments discharged into the gulf is usually less than 2% (Trontsios, 1991; Pehlivanoglou, 1995). The results of XRD analyses of the different size fractions are listed in Table 1, following the method of Biscaye (1965). Different minerals in variable proportions are concentrated in different grain size fractions.
The clay minerals that occur in all fractions are illite, 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 (Pehlivanoglou, 1995). In the <0·25 m fraction of some samples the interstratified phase illite/smectite is distinguished. In addition, quartz, feldspars and amphiboles are present in traces in some 2–1 m fractions. The abundance of smectite is greater in the finer fraction (<0·25 m). The reverse is true for illite and kaolinite. The distribution of clay minerals in the Gulf is shown in Figures 6, 7 and 8. Illite is the most abundant clay mineral, being present in almost all the coarser fractions (2–1 m and 1–0·25 m), but with a smaller range of values (Figure 6). In contrast to the distinct geographic grain size differentiation of smectite, illite has a more ubiquitous distribution. An exception is the <0·25 m fraction which shows a higher content in front of the Evros mouth. The distribution of illite content in the 2–1 and 1–0·25 m fractions is almost uniform through out the Gulf. In the <0·25 m fraction a significant increase in smectite content is observed. The reverse is true for 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
A1 A4 A5 A6 A7 A8 A10 A11
1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25
A12 A13 A14 A15 A16
1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25 2–1 1–0·25 <0·25
S
I
K
I/S
Sample
Size
S
I
K
I/S
2–1 4 74 17 30 61 13 21 9 23 18 78 5 32 77 10 61 79 10 24 76 4 2 86 2–1 32 65 9 10 91 5 29 60 5 10 79 6 7 22
3 78 18 64 56 22 64 66 72 57 65 16 80 51 18 76 27 16 72 60 19 73 85 11 12 52 24 74 81 3 70 52 23 71 69 11 72 78 67
75 18 8 19 14 17 23 13 19 20 17 6 15 17 5 14 12 5 18 16 5 23 13 3 67 16 11 17 9 6 25 19 17 24 21 10 22 15 11
22 tr tr A18 tr tr A19 tr tr A20 tr tr A21 tr tr A22 tr tr A23 tr tr A24 tr tr 21 tr tr A26 tr tr A27 tr tr A28 tr tr AVERAGE tr tr
A17
2–1 1–0·25 <0·25 4 1–0·25 <0·25 5 1–0·25 <0·25 5 1–0·25 <0·25 3 1–0·25 <0·25 8 1–0·25 <0·25 8 1–0·25 <0·25 5 1–0·25 <0·25 2–1 1–0·25 <0·25 8 1–0·25 <0·25 21 1–0·25 <0·25 10 1–0·25 <0·25 12 1–0·25 <0·25
35 50 25 79 17 81 77 20 81 82 28 86 83 23 73 66 9 83 77 14 81 79 8 11 59 59 89 70 17 47 58 38 57 64 43 33 69 24 64
49 36 70 17 66 12 18 59 10 13 59 3 14 60 7 26 77 11 15 68 12 16 75 73 28 28 5 22 65 26 21 46 41 26 33 59 19 60 26
16 14 5
tr tr
17 7
tr tr
21 9
tr tr
13 11
tr tr
17 20
tr tr
14 6
tr tr
18 7
tr tr
17 16 13 13 6
tr tr
18 27
tr tr
16 2
tr tr
24 8
tr tr
2–1 2–1 2–1 2–1 2–1 2–1 2–1 A25 2–1 2–1 2–1 2–1
tr tr
16 10
a Numbers 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 <0·25 m fraction (Figure 7) shows higher values in the NE of the inner part of the Gulf, as well as in the central and western parts. The 1–0·25 m fraction has high smectite contents closer to the Evros mouth, as well as in the western part of the Gulf, whereas the lowest contents occur in the central part. Finally, the 2–1 m fraction shows the highest contents only close to the river mouth. Kaolinite occurs in very low contents in all fractions and also shows little geographic differentiation (Figure 8). In general the kaolinite content increases with
increasing grain size. In the <0·25 m fraction the highest contents are found close to the river mouth, whereas in the 2–1 and 1–0·25 m fractions a small increase is noted at the western and south-western stations. Discussion Illite, smectite and kaolinite are the minerals, which predominate, in the fine-grained fraction while chlorite, amphiboles and interstratified illite/smectite also 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 2
4
6
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.
K. Pehlivanoglou et al. 40.9° N
Fraction 2–1 µm Makri
Alexandroupolis 80
<70%
<80%
70
70
70
40.8°
60
70–80%
r
ve
s ro
Ri
Ev
60
70 70
50
<40%
40.7° 25.7°
25.8°
40
50
60–70%
25.9°
26.0°
40.9° N
26.1° E
Fraction 1–0.25 µm Alexandroupolis
<60%
60
60
40
40.8°
60
Makri
er
iv
60
60–80%
R os
r
Ev
60 <40%
60
40–60%
25.8°
25.9°
40
40
40.7° 25.7°
26.0°
26.1° E
40.9° N Fraction <0.25 µm Makri
Alexandroupolis
20
40
40
20
40.8°
er
iv
<20%
o vr
E
20
40
40.7° 25.7°
sR
>60% 50 <20%
20
8
25.8°
40
20–40%
25.9°
26.0°
F 6. Distribution of illite (weight %).
26.1° E
Origin and distribution of clay minerals 40.9° N
Fraction 2–1 µm Makri
Alexandroupolis
10 10
<10%
10 10
E vr os
20 20–30%
<10%
R iv er
40.8°
>40%
20
10
25.7°
25.8°
25.9°
40
40.7° 25.6°
30
10–20%
26.0°
40.9° N
26.1° E
Fraction 1–0.25 µm Makri
Alexandroupolis 40 20
<20%
20–40%
<20%
20
40
E vr os
R iv er
>40%
20
40.8°
>40% 40
20
40.7° 25.6°
25.7°
25.8°
25.9°
26.0°
26.1° E
40.9° N Fraction <0.25 µm Alexandroupolis
40
>80% <40%
80
60 40 <40%
R iv er
80
60
40.8°
E vr os
Makri
80 60
60–80%
40–60% 40
60
40.7° 25.6°
25.7°
25.8°
25.9°
26.0°
F 7. Distribution of smectite (weight %).
26.1° E
9
10 K. Pehlivanoglou et al. 40.9° N
Fraction 2–1 µm Alexandroupolis 20
>30%
>20%
30
20
20
25
25–30%
20
40.8°
.
sR
ro
20
Ev
15
<20%
20
20
25
20
15–20%
15
25
20–25%
40.7° 25.7°
20
30
25
Makri
25.8°
25.9°
26.0°
40.9° N
26.1° E
Fraction 1–0.25 µm Makri
Alexandroupolis 15
15
<15%
20
20
15
40.8°
15
15–20%
20
<20% 20
R.
r Ev
15
20
os
15
15
<20% 20
15
40.7° 25.7°
25.8°
25.9°
26.0°
26.1° E
40.9° N Fraction <0.25 µm
10 15
10
10 10
15
40.8°
15 10
Alexandroupolis 20
Makri
>15%
os
R.
r Ev
5
15
10
10 >10%
5–10%
10–15%
10
5
<5%
10
40.7° 25.7°
25.8°
25.9°
26.0°
F 8. Distribution of kaolinite (weight %).
26.1° E
Origin and distribution of clay minerals 11
The illite, smectite and kaolinite contents of the Gulf sediments are due to the very large river input of suspended load rich in micas, derived from the weathering of parent rocks in the drainage basin of the Evros River. In addition, some of the clay minerals are derived from the weathering of the unconsolidated Neogene and Quaternary coastal sediments. Acid and mafic igneous as well as metamorphic rocks cover the western and central parts of the Evros river drainage basin, Neogene sediments cover the eastern part and the coastal zone. The weathering of these rocks supplies at least 1 000 000 m3 year 1 of suspended material to the gulf of Alexandroupolis. Perissoratis et al. (1987) accept that the Evros is the largest supplier of fine grained Fe-Al-rich continental detritus to the offshore area which then is dispersed westwards along the coast by local currents. Tsirambides et al. (1989) studied the clinoptilolite contained in volcaniclastic sediments (connected with rhyo-dacitic volcanism of the Upper Eocene-Lower Oligocene) from the Metaxades area, which is drained by Erythropotamos (tributary of Evros River). They found that the only clay minerals present, (usually in traces), are illite, smectite and mixed-layer illite/ smectite. Kirov et al. (1990) who studied the zeolite-bearing Tertiary sediments (conglomerates, breccias and sandstone) from Petrota area (about 150 km north of Alexandroupolis) which belongs to the drainage basin of the Evros River, found only traces of celadonite, kaolinite and chlorite among the clay minerals. Studying the weathering products of trachyrhyolites from the eastern Phodope (part of the Evros drainage basin), Popov and Michalev (1990) concluded that the prevailing conditions (altitude, climate, vegetation, etc.) prevent intensive weathering and thus the formation of kaolinite. The dispersal of the Evros River suspended load in the area of the gulf of Alexandroupolis takes place under the prevailing meteorological and oceanographic conditions: (a) dispersal to the west and north-west takes place under the action of south, south-east and south-west winds. (b) dispersal in a north-west/south-east direction occurs under the action of north and north-east winds. The modern sediments of the Gulf have adopted their characteristic zonal distribution in response to the coastal topography, the water motion and the quality of the supplied material. The hydrodynamic conditions and to a lesser extent also the grain size of the clay minerals, are the main factors controlling their distribution in the Gulf. The average clay mineral composition of the 25 samples analysed is (Table 1): illite 52%, smectite
33% and kaolinite 15%. Mixed-layer illite/smectite were also detected in traces in some samples. Comparatively the average clay mineral content in Thermaikos Gulf, NW Aegean Sea, 250 km west of Alexandroupolis Gulf, was reported by Lykousis et al. (1981) as illite 50%, smectite 34% and kaolinite+chlorite 16%. The terrigenous input, the water mass circulation and to a lesser extent the wave activity, control the sedimentation within the NW Aegean Sea. In the Strymonikos Gulf, (an area geomorphological similar to the Alexandroupolis Gulf 200 km westward), the illite content in the clay fraction varies between 45 and 73%, with higher values close to the Strymon delta, while smectite varies between 11 and 38% and kaolinite between 6 and 15% (Conispoliatis, 1984). The distribution of clay minerals is interpreted as the result of differential settling and is controlled by the water circulation. According to Conispoliatis and Perissoratis (1987) the clay mineral distribution in the Ierissos Bay (an enclosed gulf, N Aegean Sea) is mainly related to the rock composition of the drained land and to the dispersion by currents. The average clay mineral content of the 37 samples analysed was: illite 63%, smectite 20%, kaolinite 9% and chlorite 8%. Also, low content of mixed-layer illite/smectite was detected in some samples. Volcanic rocks occur extensively in the drainage basin of the Evros River. Micas exist as primary mineral in all samples examined, while chlorite in some of them. The abundance of illite and smectite in the Gulf sediments is due to presence of these minerals in the volcanic rocks of the drainage basin of the Evros River. Especially, the abundance of smectite enhanced by the high Fe and organic content, which results in a rapid flocculation and settling out of smectite grains. The distribution pattern of smectite in the Gulf shows that the dispersion of this mineral has not been strongly influenced by the hydrodynamic conditions and that it should thus not be used as a hydrodynamic index. Chronis (1986) confirmed the predominance of smectite over the rest of the clay minerals in front of the pro-delta platform and close to the coast at depths not exceeding 30 m in the Thermaikos Gulf, (250 km west of Alexandroupolis gulf). The main reason of the high smectite content is the high Fe and organic content of the discharged sediments, which results in a rapid flocculation and settling out of smectite grains. This flocculation process is enhanced by the physico-chemical conditions of the seawater (i.e. salinity, temperature, pH and Eh). Kaolinite content expresses the strong climatic dependence controlled by the intensity of hydrolysis of
12 K. Pehlivanoglou et al.
continental rocks which occur in the drainage basin. The low content of kaolinite however, may be due to unfavourable climatic and physicochemical conditions, as well as to the detrital origin, rapid transport and deposition of the weathered material in the Gulf. Furthermore, the low content of amphiboles observed even in the clay fractions of the discharged material confirms the limited reworking and weathering of the primary ferromagnesian minerals because of the high river discharge over short time periods and rapid deposition in the Gulf. The traces of the interstratified illite/smectite confirm the limited reworking and weathering of the primary minerals occurring in the sediments of the broader area. Trontsios (1991) studying Paleogene sediments from drilling cores of the Evros delta, found that the clay fraction consisted exclusively of illite, chlorite, vermiculite and the interstratified phases of illite/ smectite and chlorite/vermiculite. The climate that prevailed in the broader area of that time was hot and semi-arid. In a detailed study of the interstratified minerals occurring in the above Paleogene sediments, Tsirambides and Trontsios (1993) found that mixedlayer illite/smectite predominates at depths <1650 m and vermiculite, expanded chlorite and interstratification of the two occur in great abundance at 1650–2500 m . At depths >2500 m the interstratified minerals are completely missing, the more discrete illite and chlorite prevailing instead. Variations within the samples and their different distribution in the deeper layers of the sediments are the result of diagenetic processes. They concluded that the absence of discrete smectite and kaolinite is partly due to the unfavourable physicochemical conditions but most probably due to the rapid transport and deposition of the weathered materials. References Aksu, A. E., Yasar, D. & Mudie, P.J. 1995. Origin of late glacialHolocene hemipelagic sediments in the Aegean Sea: clay mineralogy and carbonate cementation. Marine Geology 123, 33–59. Arikas, K. 1979. Geologische und Petrographische Untersuchungen in der Umgebung von Kirki (Thrazien, Nordgriechenland). Mitt. Geol. Paleont. Inst. Univ. Hamburg 49, 1–26. Biscaye, P. E. 1965. Mineralogy and sedimentation of recent deep sea clay in the Atlantic Ocean and adjacent seas and oceans. Geological Society American Bulletin 76, 803–831. Biscaye, P. E. & Eittreim, S. L., 1977. Suspended particulate loads and transports in the nepheloid layer of the abyssal Atlantic ocean. Marine Geology 23, 155–172. Bouquillon, A. & Chamley, H. 1986. Se´ dimentation et diagene`se re´ centes dans l’ e´ ventail marin profond du Gange (Oce´ an Indien). C. R. Academic Science, Paris 303, 1461–1466. Brewer, P. G., Spencer, D. W., Biscaye, P. E., Hanley, A., Sachs, P. L., Smith, C. L., Kadar, S. & Fredericks, J. 1976. The distribution of particulate matter in the Atlantic ocean. Earth Planetary Science Letters 32, 393–402.
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Trontsios, L. G. 1991. Granulometric, Mineralogical and Chemical Study of the Paleogene Sediments from Drilling Cores of Evros Delta. Ph.D. thesis, Aristotle University of Thessaloniki, 254 pp. (in Greek with English abstract). Tsirambides, A. & Trontsios, G. 1993. Study of the interstratified clay phases of Paleogene sediments of Evros delta. Bulletin Geological Society Greece 28, 55–67. (in Greek with English abstract). Tsirambides, A., Kassoli-Fournaraki, A., Filippidis, A. & Soldatos, K. 1989. Preliminary results on clinoptilolite-containing volcaniclastic sediments from Metaxades, NE Greece. Bulletin Geological Society Greece 23, 451–460. Veniale, F., Soggetti, F., Pigorini, B., Dal Negro, A. & Adami, A. 1972. Clay mineralogy of bottom sediments in the Adriatic sea. In Proceedings of the 4th International Clay Conference, Madrid, pp. 301–312.
