The character of seismo-turbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece

Sedimentology (1991) 38,717-733

The character of seismo-turbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece

G E O R G E C. A N A S T A S A K I S * and D A V I D J . W. P I P E R ?

*National Centre for Marine Research, 16604 Hellenikon, Athens, Greece f Atlantic Geoscience Centre, Geological Survey of Canada, Bedjord Institute of Oceanography, PO Box 1006,

Dartmouth, Nova Scotia, Canada B2Y4A2

ABSTRACT

The early Holocene S-l sapropelic sequence in the northwest Hellenic Trench has been studied in six piston cores from the Zakinthos and Strofadhes basins. The S-l sequence, 0.7-3.5 m thick, consists principally of silt to mud turbidites, with rare, thick, disorganized, sandy turbidites. These lithofacies are described and compared with fine-grained turbidites from the literature. Petrographical data, including the abundance of organic carbon and planktonic microfossils, indicate that the principal source of sediment to the turbidites was from the continental slope. On the basis of composition and texture, five turbidite units can be correlated between the two basins. These basins are fed by separate but adjacent drainage systems. The apparently synchronous occurrence of turbidites in the two drainage systems suggests that the turbidity currents were seismically triggered. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of travel. Turbidites were deposited more frequently in the S-l sapropelic interval than in the over- and underlying sediments. Application of slope stability analysis shows that on the 8" slopes above the basins, a 10-cm-thick sapropel would have a factor of safety of about 2, and would fail with earthquake accelerations in excess of 0.08 g . The frequency of earthquakes likely to produce such accelerations is similar to the observed frequency of turbidites. The low strength of the sapropelic sediment makes it particularly susceptible to such failure. Similar thin-skinned slumping may be an important process for the initiation of turbidity currents in other environments where there are steep slopes or high sedimentation rates.

INTRODUCTION

Most turbidity currents are thought to be initiated by slumps, in many cases triggered by earthquakes (Heezen & Hollister, 1971). Details of this process have not been well documented, especially with regard to the thickness and extent of the initial slump masses. Furthermore, except for very large 'seismo-turbidites' (Mutti et al . , 1984), there are few established criteria to distinguish turbidites of slump origin from those in which the turbidity currents are initiated by other mechanisms (Kastens, 1984; Adams, 1990).

We have examined piston cores from the Zakinthos and Strofadhes basins on the continental margin off western Greece-an area of active seismicity-in order to determine if seismically triggered turbidites

can be distinguished. The early Holocene S-1 sapropelic sequence (Ryan et al., 1970) of these basins is used as a stratigraphic marker to correlate turbidites. Bioturbation is reduced in the sapropel and the presence of resedimented sapropelic sediment provides information on sediment source, since, the sapropelic sequence was not deposited on the shelf (Anastasakis, 1986). Sediment types are documented and correlation between cores is supported by petrographical and textural data; petrography is also used to identify the sediment source. These data are used to test the hypothesis that the turbidites are seismically triggered and to look for general features of seismically triggered turbidites.

718 G. C. Anastasakis and D. J . W. Piper

GEOLOGICAL SETTING

Physiography and sediment sources

The study area is located on the continental margin 20-60 km southwest of the island of Zakinthos, the southernmost of the larger Ionian Islands off the west coast of Greece. The Zakinthos and Strofadhes basins represent the northwest extension of the Hellenic Trench. The shelf west of Zakinthos is only a few hundred metres wide, with a shelf break at 30-50 m. The continental slope is steep (5-8’) and is dissected by narrow incised submarine valleys. These valleys lead to two basins, both about 4200 m deep. The larger Zakinthos Basin is 18 km long, 8 km wide and orientated NW-SE. The smaller Strofadhes Basin is elliptical, 10 km long and 3 km wide.

Extensive 3.5-kHz surveys (Got et a[., 1977) have defined the morphology of the slope and basins west of Zakinthos Island. Much of the sea bed appears hummocky (‘cobblestone’ morphology) on 3.5-kHz profiles, reflecting an irregular topography controlled by fault blocks. Relative motion on bounding faults is of the order of 1 mm yr-’ (Stanley, 1978a). Sediment is ponded between some uplifted fault blocks. Uncon- solidated sediment is less than 100 m thick over much of the margin. The greatest thicknesses of unconsoli- dated sediment are 800 m in the Zakinthos Basin and 300m in the Strofadhes Basin (Got el al., 1977). Stanley (198 1) has proposed that observed basinward fining and progressive downslope textural and com- positional homogenization of surficial sediments on this margin is due to the continued failure and short residence time of the sediment on relatively steep and locally tectonically mobile slopes.

There is no direct fluvial input to this shelf-slope- basin system, except for a few ephemeral streams on Zakinthos Island. Fluvial input from the western Greek mainland is trapped by the submarine valley of the Zakinthos Channel (Ferentinos et al., 1985; Cramp et al., 1987), which leads to a perched basin separated by the Zakinthos-Strofadhes Rise from the basins studied here.

Study material

Cores were collected on the continental margin off Zakinthos Island by D. J . Stanley in 1976 from the R/VTrident(Blanpied&Stanley, 1981 ;Stanley, 1983). The centres of the Strofadhes and Zakinthos basins were sampled in cores 36 and 35 respectively (Fig. 1).

Core 34 is from the margin of the Zakinthos Basin, 80 m above core 35, where the ponded sediments of the basin pinch out (Stanley, 1978a, Fig. 1 ; Blanpied & Stanley, 1981, Fig. 2). Core 33 is located on rough, hummocky topography about 300 m above the basin floor. Core 32 is located on a valley floor 25 km upslope from the Zakinthos Basin. Core 37 is further upslope at 1200 m water depth and consists of a hemipelagic slope sequence. All other cores contain turbidites (mostly muddy) interbedded with hemipelagic sediments. The sapropelic interval present in the upper part of all the cores is the regional S-1 sequence (Blanpied & Stanley, 1981).

Blanpied & Stanley (1981) and Stanley (1983) distinguished four main sediment types (principally below S-1) on the basis of sedimentary structures and petrography. Hemipelagic muds are generally biotur- bated, with > 5%sand fraction, principally planktonic foraminifera. Mud turbidites show distinct grading and sequence of structures. Both ‘unifites’ (uniform or faintly laminated muds) and ‘parallel-laminated muds’ (lacking prominent grading or sequencesof structures) show petrological evidence for relatively rapid deposition from nepheloid layers or the tails of turbidity currents.

Methods

Core sites were defined by 3.5-kHz profiling. Naviga- tion was by transit satellite and radar fixes on land (Stanley, 1978a). The trigger weight cores show that insignificant amounts of sediment are missing from the top of the piston cores. Cores were split and photographed in colour. The S-1 sapropel was recognized visually from its dark colour and was confirmed by determination of the organic carbon (C0J content of samples. An X-ray was taken of the S-1 interval of the cores. A total of 116 samples (listed in the Appendix) were selected from the cores on this basis and analysed for Corg and total carbonate (by Leco analyser), grain size (by wet sieving and Coulter counter: model TAII with 140- and 3 0 - ~ m apertures),, and bulk composition by X-ray diffraction (see details in footnote to Table 1). In addition, sand fraction petrography was determined by counting under the polarizing microscope over 300 grains in 42 samples : selected samples are listed in Table 2. Some samples are also illustrated in Fig. 3. Systematic down-core description of the cores is difficult because all the sediments appear muddy and the intensity of X-ray

Seisrno-turbidites, Zakinthos and Strofadhes basins, Greece

20" 21"E

719

38' N

37' 37"

20' 21 "E 22"

Fig. 1. Map showing bathymetry of the study area (based in part on Got er al., 1977). Core locations are shown by solid dots,

images is a function of both grain size and composition.

Grain-size analysis of sapropels presents problems because aggregates form with the organic matter. Counter analyses were made on low-concentration suspensions that were ultrasonically disaggregated just before analysis. Although this procedure may not completely disaggregate sediments, it does yield reproducible size analyses that show no systematic variation with C,,,.

S-1 S A P R O P E L I N T E R V A L

Basin-wide studies in the eastern Mediterranean have established that the uppermost organic-rich sapropel sequence (S-1) is developed in water depths > 180 m in the Aegean Sea and >230 m in the Ionian and Levantine Seas (Anastasakis, 1986; Anastasakis & Rohling, 1989). The Corg content of S-1 is from 2.0 to 3.6% in water depths > 1600 m, decreasing to as little as 0.5% in shallower water (Anastasakis & Stanley, 1986).

The age of the S-1 sapropel has been generally cited as 7000-9000 yr BP (Stanley, 1978b). Radiocarbon

dating of bulk samples requires correction for dead carbon (Stanley et al., 1978; Anastasakis & Stanley, 1986). Recent AMS dating of individual shells from sapropelic sequences in the northern Aegean indicates an age for the sapropel of 6600-9200 yr (C. Perissor- tatis & D. Piper, unpublished observation). We use this age range because the AMS dates on individual shells are more likely to be accurate than corrected bulk sediment dates.

The S-1 layer in cores 32-36 is of the episapropelic type (Anastasakis & Stanley, 1984) in which hemipelagic sapropelic sediments are interspersed with gravity-flow units. Core 37, recovered on the slope west of Zakinthos Island, shows a 25-cm-thick sapropel sequence which is similar to that found in other areas of the outer Hellenic Arc, i.e. it is dominated by hemipelagic sedimentation (Anasta- sakis & Rohling, 1990).

L I T H O F A C I E S P R E S E N T I N T H E S-1 I N T E R V A L

The sediments in the S-1 interval consist of silty sands, silts and muds, with colours ranging from olive grey

720 G. C. Anastasakis and D . J . W. Piper

(SYS/2), greyish olive (10Y4/2) to greyish-olive green (SGY3/2). Several bed types can be distinguished with the aid of X-ray radiographs (Fig. 3) (see 1-4 below). All are sharp-based, and are more or less graded. They vary in thickness from a few millimetres to a few decimetres, with the coarser-grained units being thicker. By analogy with structures described by Piper & Stow (1991), they are interpreted as fine-grained turbidites. Laminae may be more distinct in these sapropelic turbidites than in many fine-grained turbidites because of the potential for segregation of Corg- and carbonate-rich detritus.

(1) Sharp-based silty sand beds, the top few millimetres of which are graded. Some show indistinct internal grading, whereas in others fluctuations in grain size are irregular (Fig. 3a). Some beds have a massive interval (Fig. 3a, c) that may be overlain by a laminated interval (Fig. 3c, i).

(2) Graded sandy silt to mud beds, with massive and/ or laminated sandy silt at the base, in some cases with clusters of laminae. This grades upward to a silty mud, only rarely laminated, and then to mud (Fig. 3b, d, h, i).

(3) Silty mud to mud couplets. The base of the couplet is sharp. The basal sediment is commonly massive silty mud, passing up into laminated silty mud. Laminae spacing and prominence decrease upwards as the sediment becomes muddier. Clusters of laminae may occur within the muddy part of the couplet. Recognition of depositional units is based principally on the presence of a thick muddy top to the bed and the massive base, where present (Fig. 3c, f, h). A few such beds resemble the ‘parallel laminated muds’ of Stanley (1983) and lack well-developed overall grading.

(4) At the base of the sapropelic unit in several cores there is a distinctive couplet consisting of a sharp- based mud (with a high terrigenous component in the sand fraction) about 1.5 cm thick overlain by about 4 mm of laminated organic detritus, some of which is of macroscopic size (Fig. 3e; see also Stanley, 1983, Fig. 7).

Intervals at the top of turbidite beds that are of distinct pelagic origin are not recognized with confidence. A massive biogenic ooze (bo in Fig. 2) occurs at the top of S-1 (illustrated in Fig. 3g). Distinct laminae, which appear dark in X-ray radiographs, immediately below the bases of silty mud turbidites may represent hemipelagic sapropelic sedimentation, for example at 204 cm in Fig. 3(c) and at 482 cm in Fig. 3(e).

PETROGRAPHY

Petrographical data provide information on the provenance of the turbidite beds and are used for correlation. Bulk analysis shows a mean composition for the S-1 sapropelic unit of 27% quartz, 25% mica, 10% feldspar, 20% carbonate, with the rest mostly clay minerals and pyrite. A significant proportion of X-ray amorphous material occurs in the sapropel (as discussed in the footnote to Table 1). The sand fraction consists of terrigenous grains and biogenic material (principally foraminifera). Very sandy samples have an unusually high bulk carbonate content (see Appen- dix) and a high proportion of bioclastic debris (e.g. 393, 446, Table 2), reflecting a source on the continental shelf. A few suchsands(e.g. 123, Appendix 1) have very high Corg content, suggesting transport and concentration of coarse organic detritus. Most sapropelic turbidites have a carbonate content of about 18% (mostly nannoplankton and planktonic foraminifera) and 1.0-2.S% Corg, with <5% sand (Table 1). Such sediments could have been resedimented from the continental slope and lack a significant continental shelf component. Comparison of the sand fraction from the silt-mud couplets (Table 2) shows mica and heavy minerals to be more abundant in the upper part of the couplet but that the relative proportions of various terrigenous and biogenous components do not vary greatly within a single bed. Biogenic ooze overlying the S-1 interval (BO in Table 2) is composed predominantly of planktonic foraminifera with a very low percentage of terrigenous grains in the sand fraction.

CORRELATION OF TURBIDITES BETWEEN CORES

Recognition and correlation of units

The upper and lower limits of the S-1 interval provide two marker horizons for correlation between cores, and show that sediment thickness is three times greater in the Strofadhes Basin (core 36) than in the, other areas studied (Fig. 2). There are three distinct units that can be recognized in the cores (units A, C and F in Fig. 2). All cores except 36 display a massive or faintly laminated sandy silt at the top of S-1 (unit A in Fig. 2): a thick laminated silty mud in core 36 may be equivalent. A lighter coloured, relatively thick bed (unit C) occurs near the middle of S-1 in all cores. This bed appears darker and almost structureless in X-ray

Seisrno-turbidites, Zakinthos and Strofadhes basins, Greece 72 1

KEY

32 34 35

15

20

2 5

30

Z A K I N T H O S B A S I N STROFADHES

SLOPE VALLEY EDGE CENTRE BAS" 37 32 33 34 35 36

1200m 3345m 3820m 4060m 4140m 4147m

\ \ \ \ \ \ \ \

\ \ \ \25:

\

\ \

36

15

,O

25

30

3 5

40

4 5

50

Fig. 2. Detailed lithological logs of the S-1 interval in cores 32, 34, 35 and 36, plus generalized logs of all the cores, to show setting of sapropel and facies of overlying sediments. Facies above S-1 interval: bo= biogenic ooze; Im=laminated mud; hm = homogenous mud ; sm = sand-mud couplets.

722 G . C. Anastasakis and D. J . W. Piper

Fig. 3. Examples of X-ray radiographs showing type of structures visible in the turbidites. Sequential numbers are depths (cm); numbers in boxes are sample numbers (cf. Tables 1, 2 &Appendix). (a) Graded sand (141-131 cm) passing up into laminated silty mud (131-123 cm) immediately overlying sapropel in core 36. Base of sand shows inverse grading, with finer grain size and low C,,, content at base; coarser grain size and higher C,,, content in middle of bed. Top of bed is sorted silt with high Corg content, overlain by laminated mud. Core 36, 119-146 cm. (h) Three depositional units of laminated sandy mud, with bases at 191, 182 and 176 cm, overlain by laminated mud in which recognition of depositional units is more difficult. Unit B, core 36, 165-192 cm. (c) Massive sandy mud (base at 204 cm) overlain by laminated silty mud, with laminae dying out upwards as the sediment becomes muddier. Below this is a unit of silty mud to mud with the silty base at 219 cm. This also shows an upward decrease in the spacing and number of laminae, with laminae becoming indistinct from 207 to 21 1 cm. Laminated sediments from 204 to 207 cm may represent another depositional unit. Unit B, core 36, 197-220 cm. (d) Poorly laminated unit of sandy silt (base at 347 cm), capped by a thin laminated mud (33&327 cm), overlain by sand at base of unit C (sample 123). Core 36, 320-349 cm. (e) Base of sapropel (at 498 cm) in core 36 showing three complex laminae of concentrated organic fragments at 498-502 cm. Each lamina overlies a massive mud bed. At 486 is the base of a laminated silt-mud couplet, overlain by a more massive silty mud at 482 cm. Core 36,475-506 cm. (f) multiple laminated silty mud units. Light-coloured layers are slightly sandy with enhanced Corg contents (samples 101, 103; compared with 100, 102). Most are sharp based and grade up into relatively unlaminated mud. Core 35, 239-272 cm. (g) Top of sapropel in core 32, showing silty sand unit A with faint laminae (26-32 cm), overlain by thin biogenic ooze (sample 51), and non-sapropelic turbidite muds and sands (18-25 cm).

Seismo-turbidites, Zakinthos and Strofadhes basins, Greece 123

240

m

a m

76(

[103/

27

Fig. 3. (continued).

1 O(

1621 [631

11[

m 121

Note that this is stratigraphically equivalent to the section in Fig. 3(a). (h) Couplets of light-coloured laminated silty mud (with 1.1% C,,, and a few per cent sand) overlain by homogenous mud, in places with faint laminae. Unit B, core 32, 44-65 cm. (i) Units D and E, core 32. Lighter tone of lower part of section results from increasing sand content. Light-coloured beds of massive to laminated silty or sandy mud are overlain by laminated or massive mud. Core 32,94-120 cm.


Table 1. Summary of the mean bulk mineralogical composition (%) by unit and core.

Core Unit No. Sand Carbonate Corg Quartz Mica Orthoclase Plagioclase

32 A 4 17.7 22.5 1.2 28.4 28.2 3.3 4.8 33 A 2 10.2 ~

34 A 2 - 19.6 1.8 26.8 23.1 5.1 5.6 35 A 3 2.6 19.5 1.3 25.6 26.4 4.8 5.2 36 A 2 0.9 19.1 1.6 26.3 22.0 5.0 6.4 32 B 3 3.1 19.9 1.2 24.9 22.0 7.3 3.4 34 B 3 - 17.8 1.9 28.2 26.1 5.1 5.2 35 B 2 3.2 18.4 1.6 29.9 26.6 5.0 6.4 36 B 8 I .9 19.3 1.5 29.2 27.2 5.3 6.1 32 C 2 10.9 22.9 2.3 26.2 23.9 4.1 5.6 34 c 4 - 20.6 2.2 26.8 25.2 4.9 5.1 35 c 1 3.3 19.0 2.1 26.1 23.5 4.1 5.6 36 C 5 22.6 21.1 1.2 21.6 27.1 5.2 5.1 33 D 4 4. I ~

34 D 3 - 22.6 1.7 24.8 23.6 4.9 5.8 35 D 3 3.0 18.6 2.2 26.4 23.6 4.9 6.5 36 D 10 3.8 19.2 1.8 29.8 26.9 4.7 5.4 32 E 4 14.1 18.0 2.2 29.9 23.5 4.4 6.1 33 E 3 12.4 34 E 3 - 13.2 2.1 24.9 24.0 5.1 5.1 35 E 2 2.2 19.2 1.8 29.1 26.4 5.3 5.8 36 E 4 2.4 18.6 1.9 21.6 26.4 5.3 5.4

- - - - -

- - - - -

- - - - - -

X-ray diffraction was made with unorientated powder samples, using a Philips diffractomer with Cu K, Ni filtered radiation at a scan speed of 1" 2% min- '. Relative abundance of minerals such as quartz and feldspar were determined from peak height above background; for minerals of variable crystallinity (e.g. carbonates) weighted peak area above background was used. The proportions of carbonates were calculated using the inorganic carbon calculated by the LECO method ; the non-carbonate minerals were estimated from the ratios of the peak heights assuming that they represent all the non-carbonate component of the bulk samples. It appears as though considerable (?2&40%) amorphous minerals are present, but the error in calculating this component is greater than the amount of amorphous minerals present (Mtlieres et al., 1978).

radiographs, with faint laminae near the top and a concentration of sand near the base. At the extreme base of S-I, cores 34-36 contain several layers of megascopic organic detritus (unit F). All cores except 33 contain a bed of biogenic ooze (typically 5 cm thick) immediately above the sapropel (BO in Fig. 2).

Using units A, C and F as guides, three further units can be correlated. Unit B, between A and C, consists of graded laminated silt-mud couplets, with 4-9 couplets recognizable in different cores. Unit D, below C, consists of similar graded laminated silt- mud couplets. Four couplets are recognizable in cores 36 and 32, with the upper couplets being thicker; two or more couplets are visible in core 35. The underlying unit E also consists of silt-mud couplets, but the base of the couplet consists of massive or laminated sandy silt. Four such couplets are visible in cores 32, 34, 35 and 36. The boundary between units E and F is indistinct in all cores: unit F consists of a structureless massive sandy silt topped by organic-rich laminae.

Unit A in all cores is represented by a massive or faintly laminated thick sandy silt bed. Cores 32, 34

and 35 show consistent and comparable grain-size fluctuations : fining-upward at the base of the bed, coarsening in the middle, then fining-upward to near the top and coarsening again at the very top. In contrast, this bed in core 36 shows a steady fining- upward sequence. Unit A in core 32 contains < 27% sand and in core 34 <20% sand. The median grain size decreases progressively from core 32 through 34 to 35. This unit has a relatively high carbonatecontent, particularly in the coarser parts of cores 32 and 36. The sand fraction consists mainly of foraminifera1 tests, except in proximal cores 36 (45% terrigenous minerals) and 32 (1 5% terrigenous minerals). Propor- tions of feldspar are highest in core 36 (Tables 1 & 2).

Unit B consists of graded laminated silt-mud or fine sand-mud couplets. Silt-mud couplets are consistently coarsest in cores 32 and 36; the finest beds occur in core 35. The CDrg and carbonate contents fluctuate; the carbonate content is highest in coarse sediments. Some beds (e.g. sample 441 in core 36; Table 2) include significant proportions of shelf-derived bioclastic material. Such beds typically have a coarse

Tabl

e 2. S

elec

ted s

and

frac

tion

petr

ogra

phy

data

(%)

437

361

417

438

418

363

440t

44

1t

393

419$

42

11

442$

44

3$

445

446

423

450

452

453

36

T 32

B

O

35

BO

36

BO

35

A

32

B 36

B

36

B

34

c

35

c 35

c

36

C

36

C

36

D

36

D

35

E 36

E

36

F 36

U

65.3

1 .

O 0.

8 11

.6

0.1

0.1

0.9

13.1

51

.6

0.9

0.3

4.1

5.6

1 .o

40.4

3.

8 1.

8 1.

5 1.

2

17.5

0.

8 2.

8 15

.9

-

1 .o

4.0

-

1.2

37.1

1.

6 4.

7 13

.4

-

2.8

12.7

-

5.0

44.1

1.

6 6.

9 44

.3

-

14.5

15

.1

0.6

4.2

4.9

1.2

1.2

6.3

0.6

13.5

48

.2

0.9

6.8

40.7

0.

6 7.

0 50

.5

0.7

9.2

43.4

0.

5 10

.6

2.2

-

0.8

19.4

-

9.4

23.1

0.

6 7.

9 42

.9

-

15.3

1.2

1 .o

2.3 -

-

3.5

5.3

0.9

5.0

1.2

0.6

3.2

2.0

5.6

5.4

4.1

0.3

3.3

~

22.3

18

.0

5.2

45.7

16

.2

21.8

56

.9

59.7

24

.9

8.5

21.0

59

.1

50.3

66

.0

59.9

3.

8 32

.9

31.9

61

.5

19.4

1.

6 80

.1

87.0

25

.5

0.5

54.5

-

66.5

0.

3 26

.3

1.6

18.9

1.

6 53

.6

0.3

79.4

0.

3 70

.1

23.8

0.

8 31

.2

20.3

0.

7 14

.5

0.8

73.0

47

.8

-

30.4

0.

8 13

.8

0.8

-

-

-

-

-

10.7

-

0.6

0.8

2.4

1.6

0.8

0.6

0.3

-

-

-

-

0.3

0.5 -

2.9 1.5

0.5

7.5

0.3

1 .o

-

-

-

1.3

2.1

0.8 -

-

1.2

1.1

0.6 -

~ 3.8 1.8

1.1

36.2

75

.4

80.1

6.

6 94

.5

21.5

48

.5

24*2

79

.5

7.1

76.3

8.

2 43

.1

13.5

40

.3

18.8

74

.3

10.2

90

.5

9.5

79.6

12

.7

38.8

14

.5

46.8

10

.5

32.4

21

.5

37.3

13

.7

86.7

18

.3

72.8

19

.3

53.8

-

8.0

24.2

~~

~ ~

~

Sam

ple

Cor

e U

nit

Sand

Q

uart

z M

ica

Ligh

t H

eavy

To

tal

Plan

k-

Ben

thic

Pt

erop

od

Gas

trop

od

Bro

ken

Tota

l fr

actio

n fr

actio

n te

rrig

enou

s to

nic

fora

ms

frag

men

ts

frag

men

ts

shel

ls*

biog

enou

s m

ater

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fora

ms

mat

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l 3 i; z

__

*May

incl

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som

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for

am fr

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. ?S

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. $,

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from

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t C re

spec

tivel

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om n

ear t

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nd n

ear b

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of b

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g. 2

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=un

it b

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th u

nit F

at t

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ase

of S

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= un

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the

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(BO

) at t

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rp

726 G. C. Anastasakis and D. J . W. Piper

silt or sand at the base of the couplet, higher carbonate content (bioclastic and foraminifera]), lower Corg content and a more complex terrigenous mineral assemblage. The terrigenous mineral content of the sand fraction overall is higher than in unit A and the carbonate content is the lowest of all units. The C,, content is about half that of similar lithologies in units D and E.

Unit Cis represented by a relatively thick bed which consists of fine sand or coarse silt at the base quickly passing up into a structureless mud. The upward decrease in grain size is a little irregular, similar to trends described from ‘unifites’ below the S-1 sapropel in the same cores by Blanpied & Stanley (1981). The sediments differ from those of unit A in having a much higher terrigenous component; a relatively high mica component is distinctive (Table 2), The carbonate content is relatively high, particularly in the coarser sediment of cores 32 and 36, although the proportions of shell fragments and foraminifera are quite different in the two cores (Table 2).

Units D and E comprise principally laminated graded silt-mud couplets. In unit D, the sand fraction in the Zakinthos Basin cores has consistently < 15% terrigenous material, whereas core 36 in the Strofadhes Basin has 30-66% terrigenous material and the coarser beds lower in the unit have a high proportion of shell fragments at the base of the beds. Unit E has a higher silt proportion than unit D and has the highest Corg contents of any unit.

Unit F is well developed only in the basinal cores 34-36. It consists of poorly organized beds (Fig. 3e) that display a thin strongly laminated layer of megascopic disseminated organic matter (e.g. at 500 cm in Fig. 3e) and/or a poorly sorted coarse layer with ‘floating’ aggregates of organic matter (some pyritized) (e.g. at 505-506 cm in Fig. 3e). The petrography is similar to unit E except that Corg is < 1%.

Style of deposition in the section above S-1

In cores from outside the basins, post-sapropel sediment is < 35 cm thick and consists principally of brown muds with rare silty laminae. In cores from the basin centres (35 and 36), post-sapropel sediment is about 1.5 m thick, but at the edge of the Zakinthos Basin is only 0.5 m thick. In the Strofadhes Basin (core 36), four prominent graded sand to mud beds immediately overlie the organic ooze at the top of the S-1. Above the sands is 70cm of brown mud with common silt laminae. In the Zakinthos Basin, three

or four graded sand to mud beds occur in both cores 35 and 34, but in core 35 are overlain by 0.5m of uniform grey-brown mud, which may be a basinal ‘unifite’ (Stanley, 1981) not represented in basin- margin core 34. The sand has a higher proportion of terrigenous grains, bioclastic material and gastropods than in S-1 (Table 2).

SEDIMENTATION HISTORY

Turbidite origin of the basin sediments

The occurrence of graded silt and mud beds with sequences of sedimentary structures, such as graded laminated muds, similar to those described by Piper (1978) and Piper & Stow (1991) indicates that many beds are turbidites. This is supported by the presence of resedimented shelf carbonates in some beds. Structureless muds at the top of turbidite beds are probably of turbidite origin ; thin laminated intervals immediately below the bases of turbidite beds may represent hemipelagic sedimentation. It is, however, in many cases difficult unequivocally to distinguish individual small sedimentation events in the cores, particularly where laminated muds are present. Some laminated units might represent deposition from nepheloid layers (Stanley, 1983). In addition to the larger sedimentation events correlated from core to core, there may be thin turbidites of only local distribution. Only sedimentary events identified with confidence are shown in Fig. 2: possible thin local events are illustrated in Fig. 3(b, c).

Many of the coarser beds have a relatively disorganized character. The sharp-based silty sand beds show only indistinct internal grading. In some cases beds with internal inverse grading or cross-lamination are overlain by more massive sediment (Fig. 3a). The base of many of the graded sandy silt to mud beds consists of massive sediment, and the silty mud is only rarely laminated (Fig. 3b, d). The silty mud-mud couplets do show regular organized structures similar to typical mud turbidites (Piper & Stow, 1991), except that some show clusters of laminae within the muddy part of the couplet (Fig. 3c, f). In the basin cores, the irregular vertical sequences of structures (Fig. 3f) might result, in part, from reflection of turbidity currents from basin walls (Pickering & Hiscott, 1985) or from sequential arrival of turbidity currents through different slope drainage systems (Hughes Clarke, 1988). The turbidites sampled in the cores represent transport distances of 30-60 km, which is less than

Seisrno-turbidites, Zakinthos and Strofadhes basins, Greece 727

from most described fine-grained turbidites (cf. Piper, 1978, Fig. 12-7): the disorganized character may reflect insufficient time for the current to develop a steady internal distribution of suspended sediment. The better organized and sorted sediments of units B and D incorporate some shelf-derived material (Table 2), suggesting that an increase in transport distance may produce significantly better bed organization.

Interpretation of dispersal pathways

The sequential characteristics of the units within S-1 show that much of the sediment results from synchronous events in two physiographically distinct basins. Cores 32, 34 and 35 belong to the Zakinthos Basin system, with the west Zakinthos Island margin as the dominant sediment source. Core 36, in the Strofadhes Basin, is supplied by sediment derived from the southeast Zakinthos Island margin, including the Zakinthos-Strofadhes rise. This source for the Stro- fadhes Basin is richer in terrigenous minerals with a higher proportion of feldspar, mica and shell fragments in the sand fraction (Table 2). In bulk mineralogy (Table I), only the mica content is distinctly higher.

Sediment dispersion can be interpreted from the correlation of depositional units. The presence of correlatable beds in cores 32 and 36, which are within different valley systems (Fig. I), suggests that turbidity currents are initiated over an extensive area of the slope. In units B and C, sediment is coarser in core 36 than in the shallower-water core 32. Although it might be argued that turbidity currents with a point of initiation upslope from core 32 spilled over the intervening topography into the Strofadhes Basin, this is difficult to reconcile with the rapid down-system changes in sediment thickness and grain size between core 32 and the Zakinthos Basin. Furthermore, for units B and C, including both a thick turbidite and thin laminated silty mud-mud couplets, the deposits in the Strofadhes Basin are coarser than those in the shallower slope valley (Table 1).

There are fewer depositional events (Table 3) recorded within units B and D in the more distal cores 34 and 35 than in core 32, which lies further upslope. The larger depositional events in the other units appear equally represented in all three cores. Core 36 probably contains the same number of depositional events as core 32, except that the correlation of unit A and the precise number of events in unit B are unclear. This is evidence that smaller turbidity currents may not reach the distal part of the Zakinthos Basin.

Table 3. Summary of information on the frequency of turbidites during S-1.

Number of clearly identified beds within S-1

Core 32 33 34 35 36

Unit A 1 1 1 1 Unit B 9 4 4 9 Unit C 1 1 1 1 Unit D 4 3 3 4 Unit E 5 5 5 5

Total 20 6-10* 14 14 20

Mean recurrence interval (yr) 125 25Cb450 179 179 125

*Part of core removed for I4C dating.

Core 33, on an elevated area of sea bed 300 m above the floor of the Zakinthos Basin, contains at least six depositional events, but probably no more than 10. Unit A in this core contains 19% sand and the base of one couplet in unit E contains 24% sand. Thus only a few of the turbidity currents entering the basin were thick enough to deposit material on the high where core 33 is located; they included currents in which fine sand was in suspension high in the current.

Stratigraphic and spatial variation in petrography and sedimentary structures

In unit A, cores in the Zakinthos drainage system display similar down-core grain-size fluctuations but a progressive downslope fining, indicating deposition of coarser material during flow. The poor sorting and irregular grading of the deposit may be due to its short distance of transport (< 30 km). The bed of unit A in core 36 is finer and shows a more organized sequence of structures: it is unclear whether this is a result of the type of source material, a long distance of flow, or trapping of coarse sediment in depressions by flow stripping (Piper & Normark, 1983) or relief bypassing (Blanpied & Stanley, 1981). The unit shows a distal decrease in terrigenous material, ascribed to petrographical sorting in the flow by Blanpied & Stanley (1981).

Unit B in all cores consists of graded silt-mud couplets. There are more such beds in core 32 than in the more distal parts of the Zakinthos Basin, probably because small flows may die out within the channel system before reaching the basin floor. Piper & Normark (1983, Fig. 9) have shown similar turbidite behaviour on several fans. Unit B of core 36, in the


Strofadhes Basin, also includes a large number of beds; the basal laminae of silt-mud couplets in both cores 32 and 36 tend to be coarser than those in cores 34 and 35. Some couplets in these two latter basinal cores are topped by thick, fine, unlaminated muds. These might represent either the type of high- concentration deposition postulated by McCave & Jones (1988) or the distal parts of several of the events recorded in core 32. Petrography demonstrates that some resedimented material was derived from the continental shelf; the variability in Corg content may also indicate variable source areas.

Unit C contains the thickest turbidite bed in S-1. It has a relatively high (shelf-derived) carbonate content, low Corg content, and a high proportion of terrigenous sand grains, suggesting a large resedimentation event derived from the upper slope or outer shelf.

Unit D consists of graded couplets with significant variations in Corg and carbonate contents, especially in the coarser beds. These coarser beds are found in the Strofadhes Basin and in the most proximal core 32 in the Zakinthos Basin system. The sand fraction of the carbonate-rich beds contains shelf-derived bioclastic material and enhanced proportions of terrigenous detritus.

In unit E the number of silt-mud couplets is the same in all cores examined. The beds are not well graded and display insignificant carbonate and Corg content fluctuations. This unit displays the highest mean Corg contents and the lowest terrigenous mineral contents. The sand fraction of the upper beds of the unit contains a larger proportion of bioclastic fragments.

Holocene variations in sediment type and sedimentation rates

Assuming that the age of the S-1 sapropel is approximately 6600-9200 yr BP, sedimentation rates within cores in ponded basins and valleys (32,34,35,36) are between two and 11 times greater in the sapropel than in the overlying section (Table 4). In slope core 37 and in core 33 located on a bathymetric high, sedimentation rates remained approximately constant.

The biogenic ooze immediately above the sapropel represents a short-lived episode of reduced terrigenous sedimentation, that is present on the upper slope (core 37) as well as in the turbidite basins. Most cores include only 2-5 turbidites above the sapropel, so that the post-sapropel section has a lower sedimentation rate and fewer depositional events.

Table 4. Sedimentation rates (mm kyr ~ ').

Core Within S-1 Above S- 1

37 34 35 32 412 39 33 I28 54 34 536 61 35 480 242 36 1416 220

Rates based on the assumption that S-1 lasted from 9200 to 6600 yr BP.

Causes of variation in Holocene sedimentation patterns

Several factors may be responsible for the systematic changes in thickness, texture and petrography of turbidite units through the Holocene and for the presence of biogenic ooze. Climatic change in the Holocene may have influenced supply of sediment from rivers. Changes in the rate of sedimentation and sediment type may have influenced susceptibility to slumping. Rising sea-level would have increased the distance of the continental slope from river deltas, and fluvial sediment from the Alfios and Acheloos rivers may have been largely trapped in estuaries in bedrock valleys. Mid-Holocene shifting of the Pinios river (Raphael, 1969) may also have been important.

The coarse sediment in units A and C of the S-1 sapropelic interval is interpreted to result from particularly large turbidity currents derived from sandier upper slope or even shelf-break sediments. The fluctuations in the abundance of Corg and pelagic carbonate in units B and D-F probably result from variability in oceanographic conditions during the accumulation of the sapropelic unit. The higher Corg content of the lower units in S-1 corresponds to the greater Corg content of the lower part of autochthonous S-1 sapropels elsewhere (Anastasakis & Rohling, 1989). The abundant foraminifera in unit A and the biogenic ooze overlying the sapropelic interval corresponds to the time of maximum flooding of river valleys following the late Pleistocene-Holocene transgression. It would, therefore, have been a time of minimum delivery of suspended sediment to the sea, as sediment would have been trapped within estuaries. As rivers such as the Acheloos, Pinios and Alfios prograded to the regional coastline, delivery of fine terrigenous sediment increased again, although most sediment would have been trapped in the Zakinthos Channel.


S E I S M I C TRIGGERING OF TURBIDITY CURRENTS

Modern seismicity

The area southwest of Zakinthos Island is one of the most seismically active in Greece. There is a wealth of data on earthquakes both from historical accounts (Papazachos & Comninakis, 1982) and seismographs (Makropoulos & Burton, 1981, 1984). The recurrence interval of earthquakes of different magnitudes per unit length of continental margin (Table 5 ) has been calculated using the data of Makropoulos & Burton (1981) and Papazachos & Comninakis (1982) within 150 km of the study area. Although this recurrence interval may have changed since the deposition of the S-1 sapropel, the correlation between the 2000-yr historical record and the modern instrumental record suggests that regional seismicity has not changed significantly over this time span.

Evidence for seismic triggering of turbidity currents

The sediments cored consist principally of multiple turbidite beds, recognized from distinctive sedimentary structures, such as graded laminated silty muds. Structureless muds at the top of turbidite beds may be of turbidite, mixed nepheloid layer or possibly hemipelagic origin (Stanley, 1983). Comparison with thicknesses of the S-1 sapropel in non-turbidite environments (Anastasakis & Rohling, 1989) suggests

Table 5. Estimates of earthquake frequency.

s- 1 Lutite interval above S-1

Acceleration for failure* @) 0.08 0.3 Limiting distance? (D) for M6.5 40 8 Limiting distance? (D) for M7.0 55 10

Probability of M>6.5 < D in total 40 20 Total accumulation time (ka) 2.6 6.6

accumulation time

accumulation time Probabilityof M>7-0<Din total 0.75 0.4

Probability of earthquakes per year per km of margin$ : M > 7 = 3 ~ 10-6 ;M>6.5=200~ M>6.0=500~ 10-6; M > 5 . 5 = l l 00x lo-?

See text for details of simplifying assumptions in this table. *Based on nomogram of Booth et al. (1985, Fig. 4). ?Based on nomogram of Idriss (1985, Fig. 1-B). $Based on data of Makropoulos & Burton (1984) and

Papazachos&Comninakis (1982) within 150 kmofthestudy area.

that sediment deposited hemipelagically is unlikely to have a total thickness of more than 10-20cm. Sapropels on the continental slope in core 37 contain 1.1-1.5% Corg and in other areas the Corg content of hemipelagic sediment increases downslope (Anasta- sakis & Stanley, 1986). The relatively high Corg content of the turbidites indicates that their source must consist principally of sapropelic muds; a deep-water source is confirmed by the high proportion of planktonic foraminifera (Table 2). Some thicker beds have higher carbonate content (with a greater proportion of bioclastic material) and lower Corg, suggesting admixture of shallow-water sediment.

Several lines of evidence suggest that most or all of the turbidites sampled are seismically triggered. The source sediment is derived principally from the continental slope and is contemporaneous with S-1 deposition. Rates of sedimentation on the slope are relatively low. Thus an origin related to oceanographic processes at the shelf break, or to slumping due to sediment loading, is unlikely. A similar sequence of turbidite events is recognized in core 32 (lower slope valley leading to the Zakinthos Basin) and core 36 (Strofadhes Basin), even though they are fed sediment from different parts of the slope. Adams (1989, 1990) has argued that such correlation of events in different drainage basins is evidence of a seismic origin.

The concentration of organic matter in many turbidite units in the sapropel interval in the basins is similar to or greater than that in the slope core 37 (1.1-1.5% Corg). This fact indicates that the source material for the turbidites must have been principally sapropelic, undiluted by older non-sapropelic sediments. Slumping thus involved only a thin surface layer of sediment. A similar inference can be drawn from the lack of sapropelic material in the section above the S-1 sapropel.

The mean Corg content in basinal turbidites is 1.6- 1.9%, greater than the 1.1-1.5% Corg on the upper slope. This difference is not the result of differential sorting within the turbidites, since Corg contents of <1.5% are rare (Table 1, Appendix 1). Rather, it implies that the principal source area for the turbidites was on the lower slope (where sediments have a higher Corg content) below the 1200-m depth represented by core 37. The presence of shelf carbonates in some turbidites indicates that the source was not restricted to the lower slope. Either seismicity resulted in slumping over a very wide area, or turbidity currents initiated on the steeper upper slope eroded surficial material from the lower slope.

The abundance of turbidite events within the

730 G . C. Anastasakis and D . J . W. Piper

sapropel sequence compared with the over- and underlying sediments may result from the different geotechnical properties of sapropels. Sapropels have low shear strengths and bulk densities and high water contents and porosities compared with non-sapropelic sediments (Keller & Lambert, 1972). Sapropels from the Mediterranean Ridge at burial depths of 1.5-2 m have shear strengths of 1-2.5 kPa, saturated bulk density of about 1.4 g ml- and porosity of up to 83% (Horn e ta / . , 1967).

We use infinite slope analysis (Graham, 1984) to evaluate the stability of a thin layer (10 cm thick) of sapropelic mud on the continental slope off Zakinthos Island, with mean gradient of 8". Since the sapropels here have a greater proportion of terrigenous sediment than those from the Mediterranean Ridge, for which physical properties were reported, we assume a saturated bulk density of 1.5 g ml- ' . The shear strength determination from the Mediterranean Ridge of 1.25 kPa at 2 m depth gives a ratio S,/a', of 0.3, so that the estimated shear strength at 10 cm depth is about 0.12 kPa. This gives a safety factor of about 2 for an 8" slope. The nomogram of Booth et a / . (1985, Fig. 4) suggests that earthquake accelerations of about 0.08 g are required to reduce the factor of safety to 1, neglecting effects such as cyclical loading leading to generation of excess pore pressures. The graph of Idriss (1985, Fig. B-1) indicates that such accelerations are experienced within 55 km of the epicentre of an M = 7.0 earthquake (Table 5). Using generalized values of lutite bulk density and strength from Keller & Lambert (1972) we derive estimates of the safety factor of 5-30 for non-sapropelic muds, with corre- sponding earthquake accelerations > 0.25 g to give failure. An acceleration of 0.3 g is assumed in Table 5 for the purposes of illustration.

Table 5 shows the critical distances from epicentres within which earthquakes of different magnitudes must occur to produce the ground accelerations necessary to cause failure in both the S-1 sequence and the overlying Holocene lutites. It also provides estimates of the probability of earthquakes of different magnitudes occurring within the critical distances during the accumulation time of the two sequences. The evidence of similar turbidite sequences in the Strofadhes and Zakinthos basins indicates that triggering earthquakesmust result in failureover distances of tens of kilometres in order to supply sediment to both sedimentation systems. Thus earthquakes of M<6.5 are unlikely to result in large correlatable turbidites in S-1 (Table 5 ; see also Keefer, 1984; Idriss, 1985): some of the thin mud beds that are not

clearly distinct correlatable events (e.g. at 204 cm in Fig. 3c) may result from small local earthquakes. The predicted number of correlatable resedimentation events for M>6.5 earthquakes in S-1 is about 40, compared with the 14-20 beds recognized in cores (Table 3). In the overlying lutite sequence, a greater minimum magnitude of earthquake is required to produce a large correlatable turbidite : the estimates are consistent with the small number of turbidites above S-1. Given the simplifying assumptions involved in making the calculations in Table 5, the correspondence between predicted and observed events is striking. The greatest uncertainty is probably in determining the minimum magnitude of earthquake required to generate a failure large enough to produce a turbidity current that flows as far as the basin.

The role of thin-skinned failure

It is a widely held concept that seismically triggered turbidity currents are catastrophic (Mutti et a / . , 1984) and that a single instantaneous trigger will generate a surge current that will begin to deposit a short distance from the source (Kirwan et a / . , 1986). The petrographical composition of turbidites in S-1, derived from an autochthonous layer no more than a few decimetres thick, indicates that turbidity currents transport sediment derived from only the top few centimetres to decimetres of the sea bed. Since the gradient of the upper slope is several times that of the lower slope, entrainment solely by sliding would be expected to give a current transporting predominantly upper-slope sediment: this is not consistent with the Corg contents of the turbidites. Rather, entrainment of poorly consolidated lower-slope sediment by a current initiated on the upper slope is more likely.

Such thin-skinned failure is probably a common geological feature, not a unique consequence of the sapropelic layer. Adams (1990) inferred shallow, seismically triggered failure on the Oregon-Washing- ton margin. From submersible observations, there is anecdotal support for failure on steep eroded slopes that receive a thin veneer of modern sediment (K. Moran, pers. comm., 1990). Failure affecting a few decimetres of sediment is seen in ancient sequences (e.g. Fig. 9 of Kleverlaan, 1987). Furthermore, not all slump failures result in surge-type flows: for example, in the 1929 Grand Banks earthquake, failure of the upper few metres of gas-charged, muddy upper-slope sediments led to steady flow of ignitive muddy turbidity currents for several hours after the earthquake (Hughes Clarke, 1988; Piper et a/ . , 1988).

Seismo-turbidites, Zakinthos and Strofadhes basins, Greece 73 1

CONCLUSIONS

In the sapropelic S-1 sequence of the northwest Hellenic Trench, compositionally and texturally correlatable units can be distinguished in two separate but adjacent drainage systems : the Zakinthos and Strofadhes basins. Petrographical data, including the abundance of planktonic foraminifera and Corg content, indicate that the principal source of sediment was from the continental slope and the extremely similar sequences of turbidites in two different drainage systems suggests that the turbidity currents were seismically triggered. Analysis of slope stability is consistent with the observed frequency of larger earthquakes in the area. The high frequency of turbidites in the S-1 sapropelic interval was due to the low strength of the sapropelic sediment. Seismic triggering resulted in failure of an upper layer of sediments centimetres to tens of centimetres in thickness. Such thin-skinned slumping may be an important process for the initiation of turbidity currents. The high organic carbon contents indicate erosional entrainment of sediments from the lower slope, suggesting that surge-type models for slump- generated turbidity currents may be inappropriate. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of transport. Many turbidite silt- mud couplets show well-organized alternations of silt and clay laminae; more irregular vertical sequences may be due to different travel times for turbidity currents down valley systems that lead to the basin plain. This study shows that there are no unequivocal sedimentological characteristics of seismically triggered turbidites.

ACKNOWLEDGMENTS

Our thanks go to Dan Stanley who collected the cores and organized much of the data acquisition on which this work was based. Kate Moran provided the slope stability calculations and helped with the geotechnical assessment. John Adams advised on earthquake frequency and effects. Some of the analytical work was carried out by John Knight. Thorough reviews by K. Moran, J . Adams, D. Stanley, Kick Kleverlaan and Dave Mosher improved the manuscript. Geolog- ical Survey of Canada contribution 20490.

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(Manuscript received 29 June 1990; revision received 6 February 1991)


Appendix 1. Summary of grain-size and petrography variation within the S-1 sapropel

Sample Unit Depth Sd Si C1 Carbonates C,,, ( 4

Sample Unit Depth Sd Si C1 Carbonates Corg (m)

Core 32 50 51 53 54 55 56 57 58 59 60 61 62 63 64 65 66

Core 33 68 69 70 71 72 73 74 75 75A 75B 75C 75D

T 23 17.1 74.7 8.1 22.0 0.2 BO 25 1.2 86.1 12.8 21.9 0.2 A 31 26.4 65.1 8.4 20.9 1.3 A 32 19.4 73.6 7.0 23.4 1.4 A 43 9.0 77.3 13.7 20.5 1.0 B 45 4.6 81.5 13.9 25.3 1.2 B 47 0.7 85.7 13.6 19.8 1.3 B 49 5.8 87.8 6.4 19.8 1.3 B 61 3.7 85.5 10.8 20.1 1.1 C 80 0.9 91.2 7.9 21.6 1.2 C 82 20.9 72.3 6.8 24.2 1.4 E 102 17.6 72.8 9.6 18.3 2.1 E 103 14.5 77.3 8.2 18.3 2.2 E 119 4.3 88.4 7.3 15.0 2.7 E 121 20.1 73.5 6.4 20.5 1.8 U 128 7.5 87.6 5.0 20.3 0.5

A 37 2.1 ~ ~ 35.1 0.9 A 39 18.3 61.8 19.9 18.7 1.9 D 51 5.0 72.4 22.6 17.6 1.7 D 54 4.4 75.5 20.0 17.9 1.7 D 57 2.9 78.8 18.3 19.4 1.0 E 60 12.5 68.3 19.2 18.7 2.1 E 62 0.4 85.4 14.2 20.5 1.0 E 65 24.5 62.1 13.4 18.6 2.1 F 67 0.9 82.2 16.9 ~ -

F 69 6.0 75.7 18.2 ~

F 72 1.5 80.7 17.7 ~

U 76 2.7 83.9 13.4 - ~

- -

Core 35 92 A 158 1.4 79.9 15.6 19.1 0.6 93 A 164 2.7 87.8 9.4 18.6 1.4 94 A 169 3.9 85.2 10.9 20.2 1.6 95 A 171 2.5 88.6 8.9 20.2 1.5 96 B 189 4.7 77.6 17.6 17.2 2.2 97 B 209 1.8 84.1 14.2 19.4 1.1 98 C 237 3.3 79.2 17.5 19.1 2.1 99 D 241 2.1 83.3 9.5 18.3 2.6

100 101 102 103 104 105

Core 36 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 I26 127 128 129

D 249 1.0 89.4 9.5 20.9 1.4 D 250 5.8 76.9 17.2 17.6 2.7 E 260 0.6 55.3 44.1 21.6 1.2 E 266 3.8 87.1 9.1 15.8 2.4 F 272 4.3 83.4 12.3 18.8 0.9 U 281 0.2 83.7 16.1 21.2 0.3

T 134 12.5 78.6 8.9 20.2 0.5 T 140 60.8 36.4 2.8 11 .1 0.2

BO 142 3.6 78.3 18.1 21.2 0.4 A 148 0.4 44.7 55.0 18.0 1.6 A 160 1.4 89.7 8.9 20.3 1.7 B 174 2.2 90.6 7.3 18.5 1.8 B 183 1.3 86.8 12.0 16.5 1.5 B 188 4.5 75.3 20.2 18.7 2.1 B 192 1.4 91.6 6.9 18.3 1.2 B 204 38.2 59.8 2.0 32.1 0.4 B 206 1.3 83.0 15.8 18.3 1.4 B 217 2.4 84.2 13.3 16.1 1.9 B 223 0.4 87.3 12.3 15.9 1.5 C 233 0.8 81.3 17.9 15.2 0.9 C 268 5.3 81.9 12.8 26.9 0.9 C 288 4.2 83.3 12.4 16.7 0.8 C 317 47.1 45.3 7.6 27.9 0.6 C 324 56.0 39.7 4.3 18.7 3.0 D 332 4.3 75.4 20.3 19.8 1.8 D 334 3.6 83.1 13.4 17.8 1.7 D 346 3.1 83.5 13.3 23.5 0.6 D 357 3.6 80.1 16.3 17.2 1.8 D 367 4.5 80.0 15.4 17.1 1.3 D 382 3.2 78.8 18.0 18.4 2.1

130 D 397 3.5 82.4 14.0 18.9 1.6 131 D 410 3.6 89.0 7.4 19.8 2.1 132 D 424 4.5 83.7 11.8 19.4 1.6 133 D 440 3.6 79.4 17.0 20.2 2.4 134 E 446 1.0 83.1 15.9 19.4 1.5 135 E 475 2.2 81.0 16.8 18.1 2.0 136 E 484 1.8 81.6 16.6 18.2 1.7 137 E 494 4.6 78.1 17.3 18.3 2.4 138 F 497 2.0 83.9 14.1 19.0 0.8 139 U 504 0.7 81.5 17.8 18.8 0.3

The character of seismo-turbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece

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