Geo-Marine Letters DOI 10.1007/s003670100070

Original

Lithology and mineralogy of surface sediments in the vicinity of the Kafireas Strait (Aegean Sea) K. Pehlivanoglou(

)

K. Pehlivanoglou Department of Oceanography, Hydrographic Service, Hellenic Navy, Strat. Papagou, Holargos, TGN 1040 Athens, Greece

E-mail: [email protected]

Received: 2 May 2000 / Revision accepted: 15 May 2001 / Published online: Abstract. The surface sediments of the Kafireas Strait and Petalion Gulf, the area between the islands of Evvoia, Andros, Gyaros and Kea in the Aegean Sea, were examined for grain size, mineralogical composition and geotechnical properties. Contents of sand, silt, clay and carbonates as well as porosity, absolute and relative water contents, grain density and bulk mineralogy were determined in samples collected at 35 localities during two oceanographic cruises in April 1986 and July 1991. In addition, current measurements were carried out at three localities in the periods May-June 1988 and July 1991. Muddy sand and sand (52-96% sand) cover the continental shelf, whereas sandy mud (3-49% sand) predominates in deeper areas. Absolute water content varied between 22 and 42%, whereas the values for relative water content were 28-73%. Porosity showed a moderate range of values (36-62%), whereas grain density varied between 2.52 and 2.77 g/cm 3 . Carbonate content showed a wide range of values (30-72%). Mica and kaolinite are the dominant minerals (average contents 32 and 16.6%, respectively), and quartz, plagioclase, calcite and aragonite showed lower contents of 12.9, 9.8, 13.5 and 9.6%, respectively. Higher mica, kaolinite, chlorite and plagioclase contents closer to Evvoia Island may be related to local mica-amphibolitic-chloritic schists and cipollin outcrops, whereas higher percentages of mica, calcite and aragonite probably relate to the mica schists and marbles of Andros Island. Strong currents were recorded near the seabed in the vicinity of the Kafireas Strait (24 and 38 cm/s maximum velocity on the continental shelf off Evvoia and Andros islands, respectively). The regional geomorphology and bathymetry, the texture and mineralogical composition of terrigenous material, and the particular meteorological and hydrodynamic conditions in the area are the main factors which control the texture, mineralogy and distribution of seabed sediments in the vicinity of the Kafireas Strait.

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Introduction Kafireas Strait, which connects the Aegean Sea with the Petalion Gulf and Myrtoon Sea (Fig. 1), is of considerable interest to both scientists and navigators because of its geographical position and abrupt relief as well as the extreme and often hazardous climatic and oceanographic conditions prevailing in the region. High waves and strong currents characterize the strait, resulting from the extended fetch and very strong N and NE winds blowing in the Aegean Sea during the winter and summer.

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Fig. 1. Locations of sediment samples (circles), current-meter deployments (triangles at sites A, B and C), and sub-bottom profiles (SSS/SBP routes A and B) Previous investigations of the area documented the influence of geological and anthropogenic hazards on pipelines and cables (Papatheodorou et al. 1995), as well as the bulk mineralogy of the surficial seabed sediments (Pehlivanoglou 1991; Pehlivanoglou and Souri-Kouroumbali 1997). In addition, Karagiorgis (1992), studying the Holocene sediments of the area Attica-Evvoia Island-North Cyclades, examined the mineralogy, geochemistry and stratigraphy of the sediments in the westernmost sector. The present study focuses on the grain-size distributions, bulk mineralogy, carbonate contents and geotechnical properties of surface bottom sediments in the Kafireas Strait and the approaches from the Central Aegean Sea, the Petalion Gulf, and the Cyclades Plateau. In addition, current velocities were recorded in the region in summer. By assessing the interrelationships between these parameters with respect to the regional hydrodynamic conditions and bathymetry, the ultimate aim was to investigate sediment transport pathways and deposition in the area.

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Physical setting The study area is bounded by the islands of Evvoia and Andros in the north, separated by the 11-km wide Kafireas Strait. The islands of Kea and Gyaros lie at the southern limit of the study area, and are separated by the Cyclades Plateau (Fig. 1).

Geology The area forms part of the Attic-Cycladic geotectonic zone which comprises three units. 1. The Attiki unit in the north includes the southern part of Attiki as well as part of Evvoia Island. It comprises a lower carbonate series with marbles and dolomites, an intermediate series of marbles, ultrabasic rocks, micaceous schists, and an upper series of carbonate metamorphic rocks. 2. The North Cyclades unit more to the south includes part of Evvoia Island and the islands of Andros, Siros, Tinos, Gyaros and Kea. It comprises schists (mica, quartz and chlorite schists), marbles, quartzites, metavolcanic rocks, and clastic sediments of Upper Triadic-Lower Jurassic age. 3. The South Cyclades unit in the south includes the islands of Paros, Naxos, Ios, Sifnos, etc. It comprises a crystalline basement with gneiss, amphibolites and schists of Paleozoic age, a schist series of Permo-Triadic age, a carbonate series of Upper Triadic-Upper Cretaceous age, and a metaflysch series of Tertiary age (Mountrakis 1985). Schists cover the southern part of Evvoia Island, whereas marbles, cipolines and amphibolites have more limited coverage in the central and northern sectors of the study area (Fig. 2). Schists on Evvoia Island contain muscovite, quartz veins or microlenses, sericite, chlorite, and calcite, whereas the amphibolites contain amphiboles, ferrous minerals and minor amounts of feldspars and quartz. Lenticular marble outcrops are included in the schists. Metalliferous deposits containing Pb, Zn, Cu, Mn, and Hg have been detected along the north shoreline of the strait. (Andronopoulos 1962; Heliotis 1987).

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Fig. 2. Bathymetric, petrographic and geotectonic sketch of the Kafireas Strait (depths in m; data extracted from IGME 1978; Papanikolaou 1979; HNHS 1984; IGME 1989; Papatheodorou et al. 1995) On Andros Island, schists in the north and northwest contain muscovite, albite, amphiboles, sericite, calcite, weathered quartz and chlorite. Local marble and cipolline outcrops of considerable size are present among the schists (Papanikolaou 1979). Serpentinic outcrops of limited size are also situated in the northern part of Andros Island (Marinos 1953). Debris slides and modern alluvial deposits are present locally in the southern coastal zone of Evvoia Island as well as around Andros Island. (IGME 1967, 1978). Mica schists, micaceous gneiss and limestones cover Gyaros and Kea islands. A NNE-SSW fault is located in the western half of the Kafireas Strait between the islands of Evvoia and Andros, extending southwest to the Gyaros-Kea Strait (Fig. 2; IGME 1989). Two shorter faults have developed WSW and ESE of the major fault.

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Bathymetry Bathymetric charts (HNHS 1984; Papatheodorou et al. 1995) indicate very steep slopes along the coasts of the islands (Fig. 2). The gradient is between 0.3 and 11% along the Evvoia Island continental shelf, whereas the values lie in the range 0.3-16% for Andros Island. The continental shelf break occurs at 120-150 m water depths along the NNE-SSW fault. At 250-m water depth in the southern sector of the Kafireas Strait, a submarine ridge oriented E-W divides the area into two basins. The northern basin has a maximum water depth of 540 m whereas the southern basin, which extends to the Gyaros-Kea Strait, has a maximum water depth of 350 m. The seafloor topography is smoother to the south of Evvoia Island in the western sector of the study area, where an extensive plateau is located at a depth of about 100 m.

Physiography and climate The islands have an abrupt relief and a steep, rocky intertidal zone. The northern part of Andros Island exhibits a well-developed drainage network, whereas to the south drainage is poor. A relatively high rainfall and very strong winds, especially during the summer, characterize the climate of this island (Papanikolaou 1979). The climate influences the weathering of the rocks, and cavities are commonly observed in soft rocks intercalated between harder ones. The drainage network of the southern part of Evvoia Island is also well developed, but the climate is drier than on Andros Island. The annual mean monthly rainfall is 63 mm, the annual minimum and maximum values being 1.5 and 146 mm, respectively (NMS 1997). The mean annual frequency of N, NE, and NW winds is 53.5%, the value being 16.7% for S winds (NMS 1997). Thus, stormy north and northeastern winds are common in the area. These strong winds, the relatively high rainfall and the abrupt relief result in the erosion of the islands and the supply of terrigenous material to the coastal waters.

Materials and methods Thirty-five samples were collected from the seabed in the Kafireas Strait, Petalion Gulf and on the Cyclades Plateau (Fig. 1), during two cruises of the R/V Nautilus of the Hydrographic Service, Hellenic Navy. Samples were taken in the upper 15-20 cm layer by means of a Dietz La Fond bottom sampler and a 1-m-long Tsurumi-Seki gravity corer (sites B, K4, K8, K11 and K15). A Trisponder positioning system (accuracy ±1 m) was used to determine the vessel’s position. Grain-size composition was determined by means of standard sieve analysis for the fraction >63 µm, and by pipette analysis for the fraction 63-1 µm. The textural classification was based on Folk (1968), and the textural parameters were determined by moment statistics (McBride 1971). Carbonate contents were determined in 16 samples by means of a carbonate bomb according to Müller and Gastner (1971). Relative and absolute water contents (weight percent) were also determined by oven drying aliquots of these samples (cf. Flemming and Delafontaine 2000). Grain density was determined in six undisturbed samples using the pycnometer (volumetric bottle) method (Lambe 1961), and these data were then used to calculate porosity for these samples.

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Bulk sample XRD analysis was performed on thirty of the samples. Measurements were done on a Siemens diffractometer (Ni filter, Cu-Ka radiation, 40 kV, 20 mA). The slides were scanned from 2 to 60° 2 . Semiquantitative estimates of mineral abundance were based on the peak area of the diffractograms, using the method described in Rex and Murphy (1970). Sediment thickness was estimated by conducting bottom profiling by means of a 100/3.5-kHz Klein Assoc. sub-bottom profiler combined with side-scan sonar. Profiles were run in the western and southeastern sectors of the Kafireas Strait (profiles A and B, respectively; Fig. 1). Current measurements were performed close to the seabed by means of Aanderaa current meters mounted on moorings. Measurements were carried out during July (2-29 July 1991) on the continental shelf off the Evvoia and Andros islands (location sites B and C, Fig. 1). Current measurements were also carried out in the northern part of the Cyclades Plateau from 2 May 1988 to 17 June 1988 (location site A, Fig. 1). The data for the grain-size composition, the mineralogical composition, and the textural and mass physical parameters were investigated by linear correlation analyses (Pearson correlation) by means of Microsoft Excel 97. T-test analysis was performed by means of Analyse-It Version 1.61 for the determination of p values at 5, 1 and 0.1% significance levels.

Results Composition and textural parameters The seabed sediments in the Kafireas Strait and the surrounding area are mostly coarse grained. Average sand content was 64.2%, the average values being 18.2 and 17.8% for silt and clay, respectively (Table 1). High sand contents (24-56%) were recorded even in some samples from the deeper parts (>300-m water depth) of the strait, shallower areas of the continental shelf nevertheless showing the highest values (52-96% sand). Many sand fractions, especially those of the continental shelf off Andros and Evvoia islands, contained substantial amounts of shell fragments, foraminifers and gastropods. Shell fragments were also observed in the fine-grained fractions of samples from the deeper areas. The highest silt and clay contents were documented towards the deeper areas, the values varying in the range 28-41 and 35-41%, respectively. Table 1. Grain-size composition and textural parameters of the bulk samples. SD Standard deviation; n.d. not determined

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Mean grain Sand Silt Clay Lithology size (Mz)

Absolute Sorting H O 2 ( ) cont.

Relative H2 O cont.

Porosity

Grain density

(%)

(%) (%)

(phi)

(phi)

(%)

(%)

(%)

(g/cm 3 )

E143

3

72

25

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

E20

12

37

51

8.4

3.2

42

73

n.d.

n.d.

E209

49

30

21

5.3

3.3

n.d.

n.d.

n.d.

n.d.

K4

22

44

34

6.9

3.8

39

63

62

2.72

E16

24

41

35

7.0

3.9

32

46

n.d.

n.d.

K22

29

30

41

6.0

3.7

n.d.

n.d.

n.d.

n.d.

K29

34

28

38

6.5

4.2

35

55

n.d.

n.d.

K3

37

35

28

6.1

3.8

n.d.

n.d.

n.d.

n.d.

K28

41

24

35

6.1

4.1

n.d.

n.d.

n.d.

n.d.

K30

22

38

40

7.1

3.9

36

56

60

2.77

Sample location

Sandy mud

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Mean grain Sand Silt Clay Lithology size (Mz)

Absolute Sorting H O 2 ( ) cont.

Relative H2 O cont.

Porosity

Grain density

(%)

(%) (%)

(phi)

(phi)

(%)

(%)

(%)

(g/cm 3 )

E17

52

20

28

5.5

4.2

n.d.

n.d.

n.d.

n.d.

A

58

27

23

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

B

53

26

21

4.4

4.2

28

40

59

2.64

K8

54

17

29

4.4

4.8

38

62

57

2.52

K36

56

21

23

n.d.

n.d.

29

41

n.d.

n.d.

K12

63

19

18

3.3

4.2

n.d.

n.d.

n.d.

n.d.

K18

64

19

17

3.0

4.1

38

60

n.d.

n.d.

K5

64

20

16

4.4

3.2

n.d.

n.d.

n.d.

n.d.

K24

73

16

11

2.5

3.3

34

52

n.d.

n.d.

K7

73

17

10

2.9

2.6

n.d.

n.d.

n.d.

n.d.

K21

75

15

10

2.2

3.5

n.d.

n.d.

n.d.

n.d.

K10

78

13

9

2.6

2.3

n.d.

n.d.

n.d.

n.d.

2.5

3.7

32

46

55

2.69

Sample location

Muddy sand

K15

78

8

14

E19

82

12

6

1.5

2.6

33

49

n.d.

n.d.

K20

82

9

9

1.3

3.0

n.d.

n.d.

n.d.

n.d.

E21

84

8

8

2.0

2.8

n.d.

n.d.

n.d.

n.d.

E18

85

6

9

1.3

3.8

32

47

n.d.

n.d.

K13

85

9

6

2.9

2.0

n.d.

n.d.

n.d.

n.d.

E13

87

7

6

1.0

2.3

22

28

n.d.

n.d.

K14

87

3

10

1.1

1.4

33

48

n.d.

n.d.

K11

88

5

7

1.5

2.3

25

34

36

2.66

K17

87

11

2

2.5

1.6

n.d.

n.d.

n.d.

n.d.

K35

88

11

1

1.2

1.7

n.d.

n.d.

n.d.

n.d.

E10

90

4

6

2.7

2.2

n.d.

n.d.

n.d.

n.d.

K9

96

2

2

0.7

1.0

n.d.

n.d.

n.d.

n.d.

Average 64.2 18.2 17.8

3.5

3.1

33

50

55

2.66

SD

22.9 11.2 12.7

2.1

1.0

5.2

11.4

9.5

0.09

Range

3-96 2-72 1-51

0.7-8.4

1.0-4.8 22-42

28-73

36-62

2.52-2.77

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A ternary diagram (Fig. 3) shows two major lithological types in the study area. 1. Sandy mud in the deeper areas where the northern basin is located, as well as on the submarine ridge. Here, sand contents varied between 3 and 49%, silt contents between 24 and 72%, and clay contents between 20 and 41%. The mean grain sizes were 5.4-8.4 phi and these sandy muds were poorly sorted ( =3.2-4.2 phi, Table 1). 2. Sand and muddy sand in the shallower areas and the southern basin. Here sand contents varied from 52 to 96%, silt contents from 2 to 27%, and clay contents from 1 to 29%. The mean grain sizes showed values between 0.8 and 5.1 phi and these muddy sands were poorly sorted ( =1.4-4.8 phi).

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Fig. 3. Sediment composition (textural classification after Folk 1968) High carbonate contents (>30%) were recorded in the bulk samples. Average porosity was low (55%), the values varying between 36 and 62% (Table 1). Lower values were found in the muddy sands. Average grain density was 2.66 g/cm 3 , and the values decreased from 2.77 to 2.52 g/cm 3 as sediment type changed from sandy mud to muddy sand or sand. The average relative water content was 50%, and the values ranged between 28 and 73%. Both porosity and relative water content showed significant positive correlations (p<0.05 or p<0.01) with sand, silt, and clay contents as well as mean grain size (Table 2). Table 2. Correlation coefficients for various lithological and mineralogical parameters. * p<0.05, ** p<0.01, *** p<0.001. Mz Mean grain size; GD grain density

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Sand

Silt

Sand

1

Silt

***-0.96 1

Clay

Mz

Sorting

Chlorite Mica

Kaolinite Quartz Plagioclase Calcite Aragonite Carbonates Porosity

Clay

***-0.97 ***0.86 1

Mz

***-0.98 ***0.93 ***0.95 1

Sorting

***-0.67 ***0.60 ***0.68 ***0.62 1

Chlorite

***-0.58 ***0.60 **0.52

Mica

***-0.61 ***0.60 ***0.57 **0.48

***0.62 **0.55

Kaolinite

***-0.61 ***0.60 ***0.58 **0.55

***0.70 ***0.79 ***0.66 1

Quartz

*0.44

**0.52

H2 O GD (relative)

***0.64 1 1

*-0.40

*-0.44

-0.36

*-0.45

*-0.38

**-0.55 **-0.50

1

Plagioclase *0.39

-0.29

*-0.44

-0.25

*-0.41

-0.25

*-0.42

-0.30

**0.56 1

Calcite

0.01

-0.06

0.03

0.05

-0.15

-0.20

-0.35

-0.26

-0.26

Aragonite

**0.54

**-0.53 **-0.51 **-0.55 ***-0.67 ***-0.59 **-0.50 ***-0.71 0.35

-0.06

0.15

1

Carbonates 0.43

-0.41

-0.41

-0.45

*-0.54

-0.38

0.12

*-0.55

-0.14

0.05

0.01

0.42

1

Porosity

-0.79

*0.75

*0.79

*0.80

*0.82

0.61

0.45

0.56

-0.15

-0.45

-0.82

-0.44

-1.00

1

H2 O (relative)

***-0.64 **0.55

**0.68

**0.64

0.43

0.40

0.38

0.47

-0.19

-0.22

-0.10

-0.47

-0.40

0.71

1

GD

-0.40

0.27

0.40

-0.39

-0.48

-0.50

-0.46

0.14

-0.18

0.32

0.53

-1.00

0.15

-0.03

0.48

-0.29

1

1

Mineralogy The minerals identified were quartz, plagioclase, chlorite, mica, kaolinite, calcite and aragonite whereas K-feldspar was observed in only a few samples (Table 3). Table 3. Mineralogical composition of the bulk samples. SD Standard deviation; n.d. not determined Sample location Chlorite Mica Kaolinite Quartz Plagioclase Calcite Aragonite Carbonates

E20

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

5

44

22

10

3

15

2

30

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Sample location Chlorite Mica Kaolinite Quartz Plagioclase Calcite Aragonite Carbonates (%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

K4

3

44

20

10

3

7

14

n.d.

E16

6

40

19

10

7

17

2

50

K22

3

45

26

5

5

16

0

n.d.

K29

6

34

29

10

4

17

0

41

K3

5

44

21

8

4

18

2

n.d.

K28

4

35

29

11

4

16

1

n.d.

E17

4

33

21

11

6

20

3

34

K8

5

50

24

6

5

7

3

n.d.

K36

4

44

13

9

3

2

17

68

K12

6

38

24

10

3

20

0

n.d.

K18

3

50

20

9

8

7

3

65

K5

2

37

13

10

14

23

2

n.d.

K24

6

35

26

10

13

6

6

55

K7

0

28

11

11

23

18

10

n.d.

K21

4

38

24

15

13

4

3

n.d.

K10

0

20

10

30

8

15

16

n.d.

K15

0

10

6

2

0

27

4

n.d.

E19

4

45

27

9

12

1

2

55

K20

3

31

26

11

3

2

14

30

E21

2

11

7

15

6

23

38

57

E18

3

28

7

33

18

4

7

43

K13

1

13

10

26

39

1

6

n.d.

E13

0

44

5

16

3

4

28

64

K14

0

27

0

9

8

19

30

72

K11

0

23

10

10

12

32

14

n.d.

K17

2

10

5

27

40

7

10

n.d.

K35

5

28

15

5

4

29

14

n.d.

K9

0

0

0

31

9

22

37

n.d.

B

7

34

28

10

13

7

1

44

E10

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

51

C

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

67

Average

3.1

32.1 16.6

12.9

9.8

13.5

9.6

50

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Sample location Chlorite Mica Kaolinite Quartz Plagioclase Calcite Aragonite Carbonates (%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

SD

2.2

12.9 8.9

7.9

9.4

8.8

10.7

14.5

Range

0-7

0-50 0-29

2-33

0-40

1-32

0-38

30-72

Mica was the most abundant mineral in the study area (average content 32.1±12.9%; Table 3). Except for one sample in which no mica was detected, the mica contents fluctuated between 10 and 50%. Highest values (>40%) were observed mostly in the sandy muds of the deeper part of the Kafireas Strait as well as at shallower sites (approximately 100-m water depth) to the south of Evvoia Island (Fig. 4). A significant negative relationship (p<0.001) was recorded between mica and sand contents, whereas there were highly significant positive relationships (p<0.001) between mica content and sorting as well as silt and clay contents (Table 2).

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Fig. 4. Distribution of the mica content (wt%) A less abundant mineral was kaolinite, with an average content of 16.6±8.9% (range 0-29%). Highly significant positive relationships (p<0.001) were recorded between kaolinite content and sorting as well as silt, clay and mica contents, and there was also a significant positive relationship (p<0.01) between kaolinite and mean grain size (Table 2). By contrast, kaolinite content was negatively correlated to sand content. The highest kaolinite contents (>20%) were commonly recorded in the deep areas of the Kafireas Strait, in proximity to the amphibolitic sericitic outcrops of southern Evvoia Island and northwestern Andros Island (Fig. 5).

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Fig. 5. Distribution of the kaolinite content (wt%) Quartz showed a low average content (12.9±7.9%, range 2-33%; Table 3). Some areas with higher quartz contents (>30%) had higher sand contents (>75%) as well, (e.g., sample E18 on the continental shelf of Evvoia Island, and sample K10 in the proximity of Andros Island; Tables 1, 3 and Fig. 6). Indeed, there was a positive (albeit not significant) relationship between the quartz and sand contents, and a significant positive relationship (p<0.01) was observed between the quartz and plagioclase contents (Table 2). Significant negative relationships (p<0.05) were recorded between quartz and sorting as well as silt, clay and chlorite contents, and there were highly significant negative relationships (p<0.01) between quartz and mica as well as kaolinite contents.

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Fig. 6. Distribution of the quartz content (wt%) The carbonate minerals calcite and aragonite had moderate average contents of 13.5±8.8 and 9.6±10.7%, respectively, with a wide range of values in each case (1-32 and 0-38%, respectively; Table 3). Calcite showed no significant relationship with any of the other variables. Higher values (>20%) were observed to the southwest of Evvoia and Andros islands (Fig. 7). Aragonite content was positively correlated (p<0.01) with sand content. By contrast, aragonite was negatively correlated (p<0.01) with mean grain size and silt, clay, and mica contents (Table 2). Highly significant negative correlations (p<0.001) were documented between aragonite and sorting as well as chlorite and kaolinite contents. Higher values (>25%) were documented west of Andros Island (Fig. 8). Carbonate content was positively correlated (though not significantly) with aragonite content (Table 2). There was no relationship between carbonate and calcite contents.

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Fig. 7. Distribution of the calcite content (wt%)

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Fig. 8. Distribution of the aragonite content (wt%) Plagioclase had a low average content (9.8±9.4%), and a wide range of values (0-40%; Table 3) in the area. In addition to a highly significant positive relationship (p<0.01) with quartz, plagioclase showed a significant positive relationship (p<0.05) with sand. Significant negative relationships (p<0.05) were observed with sorting as well as with clay and mica contents. There were no relationships with any other lithological or mineralogical parameters (Table 2). Chlorite had a low average content (3.1±2.2%), and a limited range of values (0-7%; Table 3). Highly significant positive correlations (p<0.001) were observed between chlorite and sorting as well as silt and kaolinite contents (Table 2). In addition, chlorite was positively correlated (p<0.01) with mean grain size and clay content. By contrast, highly significant negative correlations (p<0.001) were recorded between chlorite and sand as well as aragonite contents (Table 2).

Current velocity and sediment thickness Strong tidal currents were recorded near the seabed in the vicinity of the Kafireas Strait (sites B and C in Fig. 1). On the continental shelf off Evvoia Island (site B), the maximum velocity was 24 cm/s, increasing to 38 cm/s off Andros Island (site C). At sites B and C mean current velocities were 7 and 14 cm/s, respectively. By contrast, the maximum velocity close to the seabed at site A near the Cyclades Plateau was only 12 cm/s, the mean velocity being 5.8 cm/s. The main current direction was SSW at this site.

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The results of the bottom profiling show that essentially no sediment cover exists in the western part of the Kafireas Strait. A low rock outcrop indicated by rough topography is visible on the left-hand side of profile A in Fig. 9A. By contrast, the southeastern part of the strait is smoother and is covered by a thin layer of unconsolidated sediments (1-2 m thick). Low outcrops occur also in this area, as illustrated on seismic profile B in Fig. 9B.

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Fig. 9A-B. Examples of bottom profiles in the A western and B southeastern sectors of the Kafireas Strait (see Fig. 1 for locations of profiles)

Discussion Earlier studies have shown that the regional geomorphology and meteorological conditions as well as the mineralogical composition of terrigenous material are important in controlling the texture and composition of sediments supplied to the Kafireas Strait region. According to Papanikolaou (1979), high humidity and strong winds promote the weathering of rocks on Andros Island, forming cavities along soft rocks, loose breccias with some huge boulders, and deposits of red clays thicker than 3 m. Marinos (1953), studying the geology and mineral deposits of Andros Island, found mica schists and phyllites containing muscovite, albite, sericite chlorite and eroded quartz as well as carbonate minerals and muscovite in the marbles and cipollin outcrops. Andronopoulos (1962) documented the geology of south Evvoia Island, and Heliotis (1987) the geochemistry of sediments derived from streams on this island. These authors found that muscovite, calcite, feldspars, sericite, chlorite, and quartz are common components of the mica-schists, chloritic schists and sericitic schists, with amphiboles, ferrous minerals and minor amounts of quartz and feldspars occurring in the amphibolites of the area. The erosion of these schists, amphibolites, carbonate rocks, etc., and the rapid transport of the weathered material seawards due to the abrupt relief (see above) supply the area with coarse-grained material and rock fragments consisting of terrigenous mica, carbonate minerals, clay minerals, quartz, etc. In the present study the high current velocities which were measured close to the seabed in the Kafireas Strait would inhibit the deposition of fine-grained suspended material produced by the reworking of terrigenous material and the erosion of seabed sediments. By implication, this fraction is transported to, and deposited in the deeper parts of the Kafireas Strait, and even in the more distant

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Aegean and Myrtoon seas where lower current velocities were observed. At first glance, the high mica contents in the sediments of the continental shelf would appear to contradict this hydrodynamic control mechanism. Indeed, according to Doyle et al. (1968), the presence of micaceous minerals can be used as an index of low kinetic energy favorable to deposition. It is proposed that high mica contents in coarse-grained sediments in the Kafireas Strait can be explained by (1) the erosion of the rocky outcrops on the bottom of the strait, suggested to be continuous with the land outcrops, schists, cipolins, marbles, etc., of the Evvoia and Andros islands, and (2) the reworking of terrigenous coarse-grained material and rock fragments, resulting from the strong currents. Thus, the data of the present study demonstrate that the sediments closer to the schists and cipolins of these islands have higher mica contents. Moreover, the presence of kaolinite, a terrigenous mineral par excellence, deriving from the erosion of feldspars, indicates the intense erosion of the island landmasses and the supply of terrigenous material to the area. Due to the large diameter and rapid flocculation, kaolinite can settle close to land (Whitehouse et al. 1960). The high average content and the geographical distribution of kaolinite in the area, demonstrating higher enrichment closer to the Evvoia schists, enforces the suggestion of a partly terrigenous origin of the bottom surface sediments. The significant positive relationships between kaolinite and mica as well as between these minerals and the silt and clay contents demonstrate the common origin and similar distributions of the minerals, especially in the finer sediment fractions. Quartz, which is the most resistant and abundant mineral, has average contents of 21 and 5% in terrigenous/marine and modern pelagic sediments, respectively (Blatt et al. 1972). Due to its hardness and resistance to weathering, this mineral can be transported over very long distances. Weathered quartz infiltrated by calcite, as well as quartz veins or microlenses are common in the schists of the Evvoia and Andros islands. On Evvoia Island amphibolites also contain small amounts of quartz (Andronopoulos 1962; Heliotis 1987). The advanced erosion of the island outcrops indicates a local terrigenous source of this mineral. By contrast, the content of feldspars is lower than that of other minerals. These are mainly sodic feldspars. Sparse occurrence in the rocks of the area (Marinos 1953), as well as poor resistance and rapid alteration to clay minerals result in a limited geographic distribution closer to the Evvoia and Andros island schists. Among the carbonate minerals, calcite is evidently related to the marbles and cipolins of Andros Island, whereas aragonite is related to biogenic material. Marinos (1953) and Papanikolaou (1979) consider that marbles and cipolines are the next most abundant rocks on Andros Island whereas they are more rare on Evvoia Island (Andronopoulos 1962). This is consistent with the distribution of calcite in the surface sediments, enrichment values close to Andros being higher than those recorded near Evvoia Island. Aragonite, which shows higher contents westward of the two islands where high carbonate contents were also observed, seems to be related with the biogenic component of the sand fraction. The very significant positive correlation between the aragonite and sand contents, as well as the very significant negative correlations between aragonite and several other grain-size and mineralogic parameters enforce the suggestion of a biogenic origin. Due to the rapid transport of the reworked terrigenous suspended material and the erosion of the seabed sediments caused by the strong bottom currents which prevail on the continental shelf, only coarse-grained sand and muddy sand settle on the continental shelf, covering the major part up to the Kea-Gyaros Strait. By contrast, the mean and the maximum current velocities on the continental shelf towards the Cyclades Plateau were substantially lower than those recorded closer to the Kafireas

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Strait. Low current velocities are suggested for the deep northern basin of the strait, too. The sediments of the deep area along the axis of the Kafireas Strait, and at some limited shallower sites towards the Petalion Gulf and Cyclades Plateau are mostly fine-grained sandy mud, suggesting less energetic hydrodynamic conditions. Grain density and porosity also demonstrate distinct differences between the sediments of the two hydrodynamic regimes. Thus, the coarse-grained sediments had lower values of grain density and porosity than those documented for the fine-grained sediments. According to the Hjulstrom curve (Easterbrook 1993), current velocities which exceed 15 cm/s are sufficient to erode fine-grained material, whereas current velocities greater than 25 cm/s can erode the fine-sand fraction of seabed sediments. The maximum current velocities which were measured on the continental shelf near the islands of Evvoia and Andros were 24 and 38 cm/s, respectively. These currents would be able to transport the fine- and medium-grained terrigenous fractions, as well as to resuspend and transport towards deeper areas the fine-grained fraction produced by the erosion of seabed sediments. Moreover, any fluctuations in current direction and velocity would affect the sorting of the sediments, which were found to be poorly sorted even in the deeper areas. The limited thickness of the surface sediments, the prevalence of coarse-grained fractions, the paucity of finer fractions even in deeper areas, and the strong currents which were recorded on the continental shelf together support the suggestion that the sediments are relict in character. These sediments were partly derived from the last sea transgression, and were mixed with terrigenous material supplied from the islands. This hypothesis agrees with Karagiorgis (1992) who proposed that the sediments of the south Evvoikos Gulf are relicts of the Flandrian transgression. It is concluded that the main factors which control the texture, mineralogy and distribution of the surficial sediments in the vicinity of the Kafireas Strait are (1) the regional geomorphology and bathymetry, (2) the texture and mineralogical composition of terrigenous material supplied by the adjoining islands, and (3) the particular meteorological and hydrodynamic conditions reigning in the area. Acknowledgements. The author wishes to acknowledge the Hellenic Navy Hydrographic Service for making available the summary data on the current measurements as well as the technical personnel of the Oceanography Department for assistance in the laboratory analyses and field work.

References Andronopoulos B (1962) Geological construction of the South Evvoia Island (in Greek with English abstract). PhD Thesis, University of Athens Blatt H, Middleton G, Murray R (1972) Origin of sedimentary rocks. Prentice-Hall, Englewood Cliffs Doyle JL, Cleary JW, Pikley HO (1968) Mica and its use in the determination of the shelf depositional regimes. Mar Geol 6:381-389 Easterbrook D (1993) Surface processes and land forms. Macmillan, New York Flemming BW, Delafontaine MT (2000) Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications. Cont Shelf Res 20:1179-1197 Folk RL (1968) Petrology of sedimentary rocks. Hemphill’s, Austin, TX

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Heliotis G (1987) The strategic geochemistry of the sediments and the streams of the map sheets "Karystos" and "Platanistos"(in Greek). Tech Rep Institute of Geological and Mineral Exploration, Athens HNHS (1984) Nautical chart Gulf Petalion to Naxos Island, sheet 421, scale 1:150000. Hellenic Navy Hydrographic Service, Athens IGME (1967) Geologic map of Greece, sheet Evvoia Island, scale 1:200000. Institute of Geological and Mineral Exploration, Athens IGME (1978) Geologic map of Greece, sheet Evvoia Island, scale 1:50.000. Institute of Geological and Mineral Exploration, Athens IGME (1989) Seismotectonic map of Greece, scale 1:500000. Institute of Geological and Mineral Exploration, Athens Karagiorgis A (1992) Mineralogy geochemistry and stratigraphy of the sea area Attica-Evvoia-N. Cyclades Holocene sediments (in Greek with English abstract). PhD Thesis, University of Thessaloniki Lambe TW (1961) Soil testing for engineers. Wiley, London Marinos G (1953) Geology and mineral deposits of Andros Island (in Greek). In: The mineral wealth of Greece. Institute of Geological and Mineral Exploration, Athens, pp 201-225 McBride EF (1971) Mathematical treatment of distribution data. In: Carver RE (ed) Procedures in sedimentary petrology. Wiley, New York, pp 109-127 Mountrakis D (1985) Geology of Greece (in Greek). University Studio Press, Thessaloniki Müller G, Gastner M (1971) The "Karbonat-Bombe" - a simple device for the determination of the carbonate content in sediments soils and other materials. Neues Jahrb Mineral 10:446-469 NMS (1997) Wind data of the coastal stations of the central Aegean Sea and Evvoikos gulf, Vol C. National Meteorological Service, Athens Papanikolaou D (1979) Contribution to the geology of the Aegean Sea: the island of Andros. Ann Geol Pay Hell 29/2:477-553 Papatheodorou G, Hasiotis Th, Ferntinos G, Vogiatzakis I (1995) Marine geological and anthropogenic hazards and their influence on the submarine pipelines and cables (in Greek). Laboratory of Marine Geology and Physical Oceanography, Patra Pehlivanoglou K (1991) Mineralogical study of the seabed surface sediments of the area S. Evvoikos-Kafireas-Cyclades (in Greek with English abstract). Abstr 1st Congr Geosciences and Environment, 15-18 April 1991, University of Patras, pp 104-105 Pehlivanoglou K, Souri-Kouroumbali F (1997) Mineralogical study of Kafireas Strait surface sediments (in Greek with English abstract). Proc 5th Hellenic Symp Oceanography and Fisheries, 15-18 April 1997, National Center of Marine Research, Kavala, vol 1, pp 387-390

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Rex RW, Murphy B (1970) X-ray mineralogy studies Leg 4 Deep Sea Drilling Project. Init Rep Vol 4, University of California Whitehouse NG, Jeffrey ML, Debrecht DJ (1960) Differential settling tendencies of clay minerals in saline waters. In: Swinelord A (ed) Clay and clay minerals. Pergamon, Oxford, pp 1-79

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