Alternating marine and lacustrine sedimentation during late Quaternary in the Gulf of Corinth rift basin, central Greece

Marine Geology 167 (2000) 391–411 www.elsevier.nl/locate/margeo

Alternating marine and lacustrine sedimentation during late Quaternary in the Gulf of Corinth rift basin, central Greece C. Perissoratis a,*, D.J.W. Piper b, V. Lykousis c b

a Marine Geology Department, Institute of Geology and Mineral Exploration, 70 Messoghion Street, 11527 Athens, Greece Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, N.S., Canada B2Y 4A2 c Geological Oceanography Department, National Center of Marine Research, Greece

Received 8 December 1998; accepted 24 March 2000

Abstract The Gulf of Corinth in central Greece has a maximum depth of about 900 m and is separated from the open sea by the Rion Strait, with a sill depth of 62 m marked by an extensive submarine terrace. During eustatic sea-level lowstands, the Gulf of Corinth was a lake. Under lacustrine conditions, stratified sediments accumulated in the deep-water basins and turbid underflows from rivers eroded the basin slopes. As the sea level rose, marine waters flooded the Gulf and deltas prograded across the shelves. In shallow-water areas, two key reflectors termed Z and X are terrace surfaces, commonly erosional, which mark the base of the overlying deltaic sequences. In deep-water basins, Z and X mark the top of the acoustically stratified sediments interpreted as lacustrine turbidites; reflectors Y and W mark the base of these stratified intervals and overlie acoustically transparent sections similar to the Holocene section. The last lacustrine conditions in the Gulf (Z to Y) were during isotopic stage 2 and terminated about 12 000 yr ago. Age estimates based on sedimentation rates suggest that the X to W interval corresponds to the stage 4 lowstand of sea level. In the western Gulf of Corinth, shoreline transgressive surfaces corresponding to minor transgressions in stage 5 and to major transgression at the end of stage 6 are recognised. This seismic stratigraphy permits a detailed interpretation of the history of the Gulf of Corinth in the past hundred thousand years. It also provides a general model for sedimentation in rift basins in which marine and lacustrine sediments alternate. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Sea-level changes; Stratigraphy; Sedimentation; Upper Quaternary; Central Greece

1. Introduction 1.1. Purpose and materials Quaternary sea-level changes have left their imprint on the sediment record on continental shelves, where unconformities and hiatuses alternate with periods of * Corresponding author. Tel.: ⫹ 30-1-7795-093; fax: ⫹ 30-17752-211. E-mail address: [email protected] (C. Perissoratis).

sedimentation. A complete record, however, can be obscured in areas of insufficient sediment input because of the presence of hiatuses, but can be imaged better in areas where a barrier separates a bay or a semi-enclosed marine area from the open sea. In these cases, low sea-level stands result in lake formation, whereas during high sea-level stands open marine conditions are established. Thus marine and lacustrine conditions alternate and this is usually depicted clearly in seismic records (e.g. Perissoratis and Van Andel, 1988, 1991; Ryan et al., 1997).

0025-3227/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0025-322 7(00)00038-4

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Fig. 1. Location map showing the geological setting of the Gulf of Corinth and the location of detailed study areas (boxes).

Chronostratigraphic markers distinguished in these records can commonly be correlated with the standard sea-level curves (Piper and Perissoratis, 1991). Such an area is the Gulf of Corinth (Fig. 1), a rapidly subsiding half graben in central Greece. The Gulf is 105-km long, 30-km wide and has a maximum depth of 900 m. Its connection with the Ionian sea is via the Rion Strait between Rion and Antirion, which is 2-km wide and about 60-m deep. The data used in the following study are bathymetric and shallow penetration 3.5-kHz and Uniboom seismic profiles collected from the eastern and western parts of the Gulf. In most cases the quality of the records was very good. The positioning system was the ship’s radar or Motorola miniranger with an accuracy of about 100 m to a few meters, respectively. 1.2. Previous studies The Gulf of Corinth attracted early the interest of the marine geologists due to its peculiar morphological characteristics (the presence of a shelf, slope, and deep basin) which resemble those of a small-scale ocean basin. Heezen et al. (1966) provided the first modern understanding of the marine geology of the Gulf, reporting the results of a RV Vema cruise and identifying alternating marine and lacustrine sediments in a core. Detailed research throughout the entire Gulf was carried out by RRS Shackleton in

1982 and RRS Discovery in 1983 (Brooks and Ferentinos, 1984; Higgs, 1988; Cramp et al., 1989). Anderson and Carmack (1973) and Poulos et al. (1996) summarised the physical and chemical oceanographic aspects of the Gulf. Bariagin (1972) provided tidal data and Myrianthis (1984) some industrial multichannel seismic data. Detailed studies of parts of the Gulf include work on sedimentation processes and the impact of the 1981 earthquake in the eastern Gulf (Perissoratis et al., 1984, 1986a; Papatheodorou and Ferentinos, 1993), the impact of the 1995 earthquake in the central Gulf (Papatheodorou and Ferentinos, 1997; Perissoratis et al., 1997), sediment instabilility on steep slopes of the Gulf (Ferentinos et al., 1988; Ferentinos, 1992), sediment distribution in Antikyra Bay (Varnavas et al., 1986; Lykousis, 1990; Papatheodorou, 1990), sediment distribution in the western part of the Gulf (Piper et al., 1980) and surveys of the Rion Strait for possible bridge construction (Perissoratis et al., 1986b; Papanikolaou et al., 1987). The above studies have emphasised the structural framework and the Holocene sediment distribution of the Gulf, however only limited work has been done on its Quaternary stratigraphic evolution due to the lack of chronostratigraphy. The purpose of this paper is to evaluate the existing data in order to develop such a chronostratigraphic tool to reveal the sedimentation conditions during Late Quaternary sea-level changes.

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1.3. General setting The Gulf of Corinth is the greater part of a discontinuous east–west trending graben that runs across the Greek mainland. It is seismically very active and shows a high rate of tectonic subsidence (Brooks and Ferentinos, 1984). Multichannel seismic data show 1.5–2 s of Neogene–Quaternary strata above the alpine basement (Myrianthis, 1984). Neotectonic studies on land (Doutsos et al., 1988; Doutsos and Poulimenos, 1992; Armijo et al., 1996) and earthquake focal mechanism studies (McKenzie, 1978; Jackson et al., 1982) suggest an approximate north– south extension. The subsidence rates of the northern margin of the Gulf of Corinth during the last 250 ka was calculated as 0.9–1.3 m ka ⫺1 and the differential displacement between the north and the south margin of Corinth graben is estimated to be about 2.8 m ka ⫺1 (Lykousis et al., 1998). The land on the northern side of the Gulf is occupied by Mesozoic limestones, whereas the southern side is covered by Pliocene marine and lacustrine strata overlain by Pleistocene fluvial and lacustrine sediments (Fig. 1) (Doutsos and Piper, 1990; Seger and Alexander, 1993; Gawthorpe et al., 1994; Armijo et al., 1996).

393

recent fault offsets. Thus the absolute depth of the sill is likely to have increased only slightly in the late Pleistocene. Based on published data on sea-level rise since the latest sea-level lowering (Bard et al., 1989, 1996), a 60-m deep sill would have been flooded by the sea at about 12 000–13 000 C-14 years before present, so that during the last glacial maximum (of isotopic stage 2) the Gulf of Corinth would have been a lake. In earlier low sea-level stands, during isotopic stages 4 and 6, the Gulf may also have been transformed into a lake. The sea level during isotopic stage 3 is not well defined (Chappell and Shackleton, 1986; Fairbanks, 1989; Skene et al., 1998), but during much of this time the water level in the Gulf was higher than that of a lake but lower than today, perhaps at times leading to brackish conditions. 2.2. River derived sediment input The largest rivers draining into the Gulf are the Mornos in the northwest and a series of short steep rivers that enter the southern Gulf draining the mountains of the northern Paloponnese. From these, the discharge data for the Selinous, Vouraikos, Krathis and Asopos rivers are reported by Therianos (1974). Rivers draining to the north-eastern part of the Gulf

2. Sedimentation and sub-bottom stratigraphy 21˚46 'E

2.1. Deltaic sedimentation and sea level changes in the Gulf

20

Corinth

40

30 50

70

60 50

70

60 70

80

60

60

3b

38˚ 19'N

3a

60 70

meters

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60

Patras 0

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50 30

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20

The effect of sea-level change on the Gulf of Corinth is influenced by the sill at the Rion Strait (Fig. 2). The Strait has a maximum depth of 62 m, and a prominent terrace is formed at a 60–62 m water depth (Fig. 3a and b). This terrace is interpreted as a coastal erosional terrace in a large lake with a long fetch. A sinuous incised channel is present to the west of the Strait (Figs. 2 and 3b), that can be interpreted as a fluvial outlet channel during low sea-level stands. On the sides of the Strait, Holocene sedimentary wedges thin seawards and unconformably overlie the terrace (reflector Z in Fig. 3a). Data from the northeastern Gulf of Patras indicate a subsidence rate of less than 1 mm a ⫺1 (Chronis et al., 1991) and the terraces at the Rion Strait sill show no evidence of

Gulf of

Antirion

Rion

Fig. 2. Bathymetric map of Rion Strait (from IGME suveys with ⬍300 m line spacing). The area shallower than 64 m is shaded.

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Fig. 3. Boomer profiles from Rion Strait: (a) east of the Strait, showing prominent 60–64 m terrace forming reflector Z, overlain by Holocene coastal and deltaic deposits; (b) central part of the Strait, showing the terrace and the outlet channel. Location in Fig. 2.

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from predominantly limestone areas are relatively small. Total mean discharge to the Gulf is estimated at 40 m 3 s ⫺1. There are no published sediment discharge load data for these rivers, but if the relation of water discharge to sediment load for the Acheloos River to the west of Rion Strait (Piper and Panagos, 1981) can be applied to the Peloponnese, then the total suspended sediment supply to the Gulf is about 0.85 × 10 6 ton/year. This corresponds to an average sedimentation rate for suspended sediment of about 0.25 mm a ⫺1; coarser sand deposition would increase the overall sedimentation rate. Changing sea level and the creation of a lake in the Gulf of Corinth would have predictable effects on the sedimentology of the Gulf. The Gulf of Corinth receives large amounts of sediments through a number of rivers that form either low gradient (suspended load) deltas, such as the deltas of a number of small rivers entering Corinth Bay, or deltas with steep gradients (bed load), as is the case with most of the rivers entering the Gulf on its southern side. In low gradient deltas, the sediments are dispersed through plumes but the coarse sediment load is trapped in the coastal (shelf) zone (Piper and Panagos, 1981; Lykousis, 1990). In bed load deltas, channels extend down the delta front from the river mouth (Piper et al., 1990). Information from cable breaks (Heezen et al., 1966; Ferentinos et al., 1988) shows that some coarse sediment passes through the channels into deep water where it is deposited as turbidites. Thus under conditions of high sea level, only the steep bed load deltas deposit turbidites on the deep basin floor. When a lake formed in the Gulf, all the rivers entering the lake would form underflows and deposit turbidites on the deep basin floor. These underflows would cause erosion of the pro-delta slopes. When the sea level rose again, underflow turbidite deposition would cease. These differences are usually clearly depicted in the examination of seismic profiles collected in the various parts of the Gulf and are described in the following sections on the stratigraphy in the shallow and the deep areas of the Gulf (from east to west, Fig. 1). 2.3. Corinth and Alcyonides bays In Corinth Bay (Fig. 4), little sediment is supplied from the north whereas modern suspended-load deltas

395

prograde from the south. The Holocene deltas consist of wedge-like bodies that thin seaward (Fig. 5). The upper part of the section consists of weakly reflective clinoforms, locally obscured by shallow gas and in places broken up into rotational slump blocks. Such features are common in seismic reflection profiles of suspended sediment deltas in the northeastern Mediterranean Sea (Chronis et al., 1991; Lykousis, 1990). The lower part of the Holocene section consists of one or more high amplitude reflections parallel to a planar erosional surface, reflector Z, that cuts off the top of older prograded clinoforms. Z is generally found at a depth of about 80 msec. The seaward end of the erosional surface Z appears offset by normal faults and occurs at ⫺90 msec (68 m). Reflector Z forms an abrupt inflection at about 80 msec depth and can be traced seawards as a strong but irregular and clearly erosional reflector. The underlying reflectors appear continuous into the clinoforms beneath Z and the Holocene delta. Some of these reflections are subparallel and of moderate amplitude but other intervals have incoherent and point source reflections or are transparent and appear to be debris flow deposits. The section below Z is locally offset by faults but these cannot be discerned in the Holocene delta. At the eastern end of the bay, the sequence below Z also passes into a deltaic section. A second regional reflector, Y, occurs generally 5– 10 msec below Z, but appears to pass beneath thick clinoforms at the eastern end of the bay. It is a high amplitude reflector overlying a thick, generally acoustically transparent unit that includes a few subparallel moderate reflections. The depositional style is much more continuous than between Y and Z and generally resembles that of the section above Z. In Alcyonides Bay, on the shelf, an erosional transgression surface correlative to Z can be recognised at about ⫺80 msec (e.g. profile B of Perissoratis et al., 1986a). In places normal faulting has lowered this reflector to as deep as ⫺120 msec (Fig. 6). The sediment thickness above this reflector is 7–15 msec. Another more deeply buried erosional transgression surface, termed X, occurs at ⫺220 msec in Fig. 6. In the deep areas of Alcyonides Bay, near-surface sediment is acoustically transparent, forming a seismic unit 3–10 msec thick, the base of which is correlated with the Z surface (Figs. 6 and 7). The unit is

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22050'

23000'E

23010'

200m

Germeno Bay 380 10' Fig.6

Fig.15 600

380 10'

m

Fig.7

KO-7

300m

80 0m

Central Basin

- KO3 40

Alcyonides Bay

Psatha Bay

30

0m

0m

1 30000m m

20 10 0m 0m

380 00' N

Fig.5

3.5 kHz profile

Corinth Bay 0

km

6

l

na

Ca

CORINTH

core

0

22 50'

23000'

Fig. 4. Bathymetric map of eastern Gulf of Corinth, showing 3.5-kHz track lines and cores with radiocarbon dates.

Fig. 5. 3.5-kHz profile from Corinth Bay showing seismo-stratigraphic interpretation.

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Fig. 6. 3.5-kHz profile from northeastern Alcyonides Bay showing seismo-stratigraphic interpretation.

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W

TWTT (msec.)

300

transparent interval between Y and X

stratified interval between X and W stratified interval between Z and Y

local erosion at Y

transparent interval above Z

E 200

250 Z

350 Y X 400

300

W 0

km

water depth (mbsl)

398

1

450 350 Fig. 7. 3.5-kHz profile from Alcyonides Bay showing acoustic stratigraphy. W, X, Y and Z are correlated reflectors described in text.

thickened locally by debris flows deposits (Perissoratis et al., 1986a, Fig. 6). The stratified sequence below Z passes laterally into lowstand delta clinoforms. The base of the stratified sequence, reflector Y, shows local erosion and channelling, probably indicating sudden development of underflows. Below reflector Y there is a second transparent section, similar to but thicker than that between reflector Z and the surface, followed by a deeper stratified section, floored by reflector W (Fig. 7), which locally shows erosion at its base. 2.4. Antikyra and Itea bays These two bays are located on the northern side of the Gulf (Fig. 1). 3.5-kHz profiles (G. Ferentinos, unpublished data, pers. commun.) clearly show a shallow transgression surface corresponding to reflector Z, that terminates seaward at about ⫺85 msec (63 m). Long cores from these bays suggest a low sedimentation rate, consistent with the small size of the streams entering them. Core IS-82-37 from Antikyra Bay (Cramp et al., 1989) contains the Y-5 tephra horizon (considered to be ca. 35 ka in age: St. Seymour and Christanis 1995) at 1.13–1.17 m

depth. Core V-10-30 from Itea Bay (Heezen et al., 1966) contains brackish faunas at 0.5–2.5, 5.0–6.5 and 8.5–9.0 m, interbedded with marine faunas. 2.5. Rion Strait In the western part of the study area, Rion Strait is subject to intense erosion by very strong currents of up to 1 m s ⫺1, which result in the absence of loose surface sediments in the central part of the Strait: only hard Pleistocene and Pliocene conglomerates are present. At the sides of the Strait a Holocene deltaic sequence is up to 20-msec thick (Fig. 3a). The base of this prograding deltaic sequence lies at about 80 msec (60 m) (Z in Fig. 3a) and is apparently the same reflector as observed in the eastern part of the Gulf described above. 2.6. Mornos Delta Eastward of Rion Strait (Fig. 8), the Gulf of Corinth consists of a central fault-bounded basin that slopes gradually eastward to water depths of up to 500 m. Sedimentation on the northern side is dominated by the Mornos delta (Piper et al., 1980, 1990; Lykousis,

C. Perissoratis et al. / Marine Geology 167 (2000) 391–411

399

0

38 25'

Alluvium 3.5 kHz profile core

50m 100m

NAVPAKTOS

Mornos delta

Akra Marathias

Akra Mornos

200m

Nisos Trizonia

Fig.11 200

50 m

m

Fig.9

Akra Drepanon 380 20'

0m

on Ri

m

Gulf of Corinth

Arahovitika fan

10

50 m

ait Str Fig.3

300

Fig.10

50m

PSATHOPIRGOS

0

km

Erineos delta

6

30

0m

20 0m

21050'

21055'

22000'

Fig. 8. Bathymetric map of western Gulf of Corinth, showing 3.5-kHz track lines. Box at Rion Strait is area of detailed boomer survey by IGME with ⬍300 m line spacing; the two lines within the box are illustrated in Fig. 3.

km

0.5

N

S

80

60 Z

erosion by tidal currents 100 TWTT (msec.)

80

120

T

100

water depth (mbsl)

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140

160

120

Fig. 9. 3.5-kHz profile north of Arahovitika fan, showing reflectors Z and T.

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Fig. 10. 3.5-kHz profile from southeast of Mornos delta showing stratified Holocene sediments overlying slump and channel fill deposits below reflector Z.

1990), from which a subaqueous channel system connects to the axis of the Gulf. Some fan deltas have prograded from the south side that is marked by a major fault. On both sides of the Gulf, off the Mornos delta and in the Psathopirgos area, a strong reflector (Z) occurs at 75–95 msec (Fig. 9), below which a deltaic sequence is recognisable. Above the reflector the sediment sequence is little more than 20 msec (15 m) thick on the upper Mornos delta, thinning to about 10 msec in more distal environments (Fig. 9). This horizon, corresponding to the lowstand terrace, continues eastward into deeper water, where it marks the change from stratified sediments to a sequence that includes slump and channel fill deposits (Fig. 10) analogous to the sequence in Corinth Bay. In Navpaktos Bay (Fig. 8) reflector Z can be traced from the Mornos delta to the bay in water depths of 40–60 m (Fig. 11). The Holocene sequence above Z is about 3-msec thick. Below Z there is a 6-msec well stratified unit underlain by a 20-msec transparent unit with a central more stratified interval that is tentatively correlated with the X to W section observed in the Alcyonides Bay.

Below the unit a series of erosive transgression surfaces were identified (V, U, T, Fig. 11), the lowest of which (horizon T) can be widely correlated throughout the Mornos delta as a high amplitude single or double reflector. This reflector appears to

Fig. 11. 3.5-kHz profile from Navpaktos Bay showing acoustic stratigraphy. V, U and T are erosional transgression surfaces discussed in text.

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22˚05'E

401

22˚10'E 0

km

4

Egion Bay

Fi 1 g. 4

EGION

38˚15'N borehole Fig. 13

Selinous 100m 50m

20m

ite Ke rin

ikos

a Vour

s

38˚12'N

Fig. 12. Bathymetric map of southern shelf of western Gulf of Corinth between Selinous and Vouraikos deltas, showing boomer seismic track lines.

have very little relief on its surface, except at fault offsets. 2.7. Shelf area near Egion The southern side of the Gulf of Corinth is very steep and crossed by numerous channels that originate

in river mouths and carry sediments from the bed load deltas to the basinal areas (Ferentinos et al., 1988; Papatheodorou and Ferentinos, 1997). The shelf east of Egion, between the Selinous and Vouraikos river deltas (Fig. 12), is 1–2-km wide. Seismic profiles show a prominent reflector terminating seaward at about ⫺80 msec and shallowing landward (Fig. 13),

Fig. 13. Boomer profile from shelf edge west of Selinous delta showing late Pleistocene erosional terrace (Z) at about ⫺60 m overlain by Holocene muds.

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again correlated with reflector Z. On the outer shelf, the reflector appears to overlay a prograded set of clinoforms, whereas on the inner shelf it has higher amplitude and appears to be an erosion surface. The overlying sediments are predominantly acoustically transparent and have a thickness from 5 msec at the outer shelf to over 35 msec east of the modern Selinous mouth. The age of reflector Z is confirmed by two radiocarbon ages from silts immediately above the reflector of 9450 ^ 160 and 10170 ^ 470 C-14 years BP, obtained from an offshore borehole (Schwartz and Tsiavos, 1979) (Fig. 14). Older channels of the Selinous are found immediately west of the present river course, with a Roman channel identified from a bridge (Katsonopoulou, 1998). This outlet deposited a buried sandy interval near the seaward continuation of the Egion fault (Fig. 14), which is marked by lines of pockmarks (Soter, 1999). More reflective sandy sediments up to 25 msec thick occur off an apparently older (mid-Holocene?) western channel of the Selinous (Fig. 14). Similar sediments off the Vouraikos mouth are up to 12msec thick and a channel extends across the shelf seaward of the mouth of Vouraikos. The transparent character of the sediments off the modern Selinous river mouth indicates that today fine-grained sediments are deposited there, suggesting that this channel diversion is of recent age. Below reflector Z, lenticular sediment bodies up to 20-msec thick, resting above another erosion surface, are developed off the eastern channel of the Selinous (Fig. 14) and off the Vouraikos. These we interpret as Late Pleistocene alluvial fans. They are similar in thickness to Holocene alluvial fans at the southern side of the Gulf (Kontopoulos and Stamatopoulos, 1991) and are acoustically rather incoherent, as would be expected from alluvial fan facies. A similar stratigraphy is recognised in seismic profiles in Egion Bay, where an upper weakly stratified unit overlies a lower well stratified unit, separated by a transgressive horizon at a depth of about 80-msec (60 m), which corresponds to the Z horizon observed in the other areas. Boreholes show that the upper unit consists of clayey mud, the transgressive surface of silty sand and gravel, and the lower unit of silty sand (Perissoratis et al., 1997). The area between Selinous and Vouraikos deltas is also of particular archaeological interest because it was the site of the ancient city of Eliki, which was

reported to have been submerged by the sea during an earthquake in 373 BC (Marinatos, 1960), although recently reported sidescan and borehole data suggest that the city is buried beneath the present coastal plain (Soter and Katsonopoulou, 1998). Our data indicate no evidence of either major slumping or active faults on the shelf between these two rivers. The 1995 earthquake activity that caused large scale damage in the city of Egion, produced abundant sediment liquefaction phenomena that were observed in the coastal zone near the city, having a distinct NW–SE direction, parallel to the main earthquake fracture zone. In the shelf area west of the Selinous river delta, extensive sediment creep was recognised in seismic profiles (Perissoratis et al., 1997). If the city was flooded by the sea following the earthquake, the suggestion of Georgalas (1962) that it disappeared as a consequence of tectonic subsidence and perhaps sediment liquefaction, seems more plausible. 2.8. Central basin of the Gulf The large normal faults that exist all along at the north and south side of the Gulf of Corinth graben have formed the central basin and separate it from the shelves and the perched basins at the eastern and western ends of the Gulf. The central basin has a flat floor with a slight gradient toward the east. 3.5-kHz profiles collected in the eastern part of the central basin indicate an accumulation of mass- and debris flows at the basin slopes (our unpublished data; Perissoratis et al., 1986a; Ferentinos, 1992; Poulos et al., 1996). In the central part of the basin the tails of these sediment masses, were deposited together with turbidites and hemipelagic sediment (Cramp et al., 1989; Varnavas et al., 1986). High amplitude reflections in the basin pass laterally into debris flow deposits. Reflectors Z and Y are tentatively recognised as the top and bottom of a packet of more reflective sediment layers (Fig. 15), with a deeper erosion surface perhaps corresponding to W. 3. Interpretation 3.1. Age of reflector Z Interpretation of the age of reflector Z is based on

TWTT (msec.)

borehole (projected)

0

late Pleistocene alluvial fan

Holocene muds

Z Z

km

1

30 water depth (mbsl)

Egion fault

50

100

Sands ot Roman mouth of Selinous

60 F

Fig. 14. Boomer profiles from shelf off Selinous delta showing Holocene prodelta deposits, late Pleistocene erosional terrace, and late Pleistocene Selinous fan delta. Site of borehole dated by Schwartz and Tsiavos (1979) is projected into the profile.

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SE

NW sandy deposits off mid Holocene mouth of Selinous

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N

S

passes laterally on to debris flow deposit

0

TWTT (msec.)

1.10

4

800

Z Z

1.15

km

X ?W

850

Y Y

1.20

X F

F 900

water depth (mbsl)

404

1.25 950 Fig. 15. 3.5-kHz profile from the central Basin of the Gulf of Corinth showing offset of correlatable reflectors on faults.

both radiocarbon dating and consideration of eustatic sea level at the sill at Rion Strait. Since the data from the Rion area suggest that the Rion sill was relatively stable at about ⫺60 m during the late Pleistocene, the last lacustrine condition in the Gulf of Corinth occurred before about 12 000 years ago. This prominent transgressive surface of reflector Z is presumably diachronous, starting at about 12 000 years ago and continuing through the rather slow rise in sea level between 12 and 10 ka. The age of reflector Z is confirmed by radiocarbon dating from cores. In the deep part of Alcyonides Bay, two new radiocarbon dates were obtained from cores collected during a survey in 1981 (Perissoratis et al., 1986a). Core KO-3 (170-cm long) was taken in Psatha Bay (Fig. 4) at a depth of about 320 m and contains a upper mud section, 110-cm thick, that overlies a faintly stratified sequence below. The seismic data indicate that at this location the depth of reflector Z is at about 3.5 m. Foraminiferal tests taken at the base of the mud section (103–105 cm) give an age of about Table 1 Radiocarbon dates from cores in Alcyonides Bay Core

Depth

Material

Lab number

Age a

KO-3 KO-7

(cm) 102–105 138

Foraminifera Mollusc shell

TO-3590 TO-3591

(yr BP) 3230 ⫹ 170 11 380 ⫹ 80

a Ages in radiocarbon years are corrected for isotopic fractionation, but are not corrected for reservoir effect.

3000 yr (Table 1) which, assuming a constant Holocene sedimentation rate under the present open marine sediment conditions, gives an age of about 12 000 yr for reflector Z. The other date is from core KO-7 (230cm long) taken in a water depth of about 220 m on the northern slope of the same basin (Fig. 4). The lower 100-cm core section is an intensively disturbed sandy mud, probably a debris flow deposit, overlain by a non-stratified bioturbated mud. In the seismic records, the thickness of the Holocene section is about 1 m. Dating of a mollusc shell taken from the top of the sandy mud layer gave an age of about 11 000 yr, for reflector Z in the area. The age of Z is also confirmed by the dates in boreholes on the shelf near Egion, discussed above (Schwartz and Tsiavos, 1979). Piston cores (up to 7 m) from the deep central basin, described by Heezen et al. (1966), Cramp et al. (1989) and Poulos et al. (1996), did not penetrate brackish or lacustrine sediments, implying that the top 7 m of sediments accumulated in less than 12 000 yr. These cores, however, were obtained at the slopes of the central basin where Holocene sediment thickness is greater due to sediment deposition by mass movement processes. Varnavas et al. (1986) estimated sedimentation rates of 1–2 mm a ⫺1 by studying bauxite tailings discharge on the northern slope of the central basin. We speculate that the penetration of piston cores was stopped by sands corresponding to reflector Z, in which case sedimentation rates in the central basin average about 0.5 mm a ⫺1. This is a little lower than the rates estimated by Varnavas et al.

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405

(1986) which may be artificially high due to recent tailings discharge.

from slumps and other mass movement processes triggered in part by slope erosion.

3.2. Interpretation of sediment facies

3.3. Age of deeper reflectors and correlation with eustatic sea level variations

The modern deltas on the inner shelf and the parallel continuous reflectors on the outer shelf above reflector Z observed in Corinth, Egion and Alcyonides bays and elsewhere apparently correspond to deposits of the present highstand of sea level. The progradational Holocene sequence is probably younger than 6500 yr, when sea-level rise markedly decelerated, enhancing progradation of marine deltas globally (Stanley and Warne, 1994). The flat lying basal sequence is probably older and formed during the early part of the latest marine transgression. The erosional surface Z on the shelf was formed by marine transgression after 12 000 years ago as sea level flooded the Rion Strait. The deltaic section between Z and Y on the shelf has clinoforms that are pinched off upwards at the ⫺60 m Late Pleistocene lake level (Figs. 5 and 6). The abundance of erosional features and debris flows in this stratigraphic interval on the slope (Fig. 5) is a consequence of erosion by underflows that in turn produced over-steepening of channels and sediment failure. The relatively thin stratigraphic section between Z and Y on uneroded slopes (Fig. 6) suggests that much of the sediment by-passed into the deeper basins. In the deeper basinal areas, Z is interpreted to represent the change from marine conditions (transparent section between Z and seabed, see also Piper and Perissoratis, 1991; Perissoratis and Van Andel, 1991) to lacustrine conditions (stratified sequence between Z and Y). The beginning of the lacustrine sedimentation is probably marked by the local erosional truncation of reflectors observed at the base of this well structured sequence (Fig. 7). With the same reasoning as above, the transparent section between Y and X may also represent open marine conditions while the X to W stratified section with local erosion at its base represents an earlier lacustrine interval. During the open marine conditions, deltaic sediments were deposited on the shelf areas while finegrained sediments accumulated offshore in the basins, intercalated with slumps and rare turbidites. During lower water levels, the shelf areas underwent erosion while the basinal areas received coarser sediments originating from lacustrine turbidity currents, and

Two approximate methods can be used to independently estimate the age of reflectors below Z, which are beyond the limit of sediment sampling and thus radiocarbon dating. One is to assume that on timescales of tens of thousands of years, subsidence rates along faults are relatively uniform. The other is to base ages on sedimentation rates. We apply both of these techniques to estimate the range of possible ages for deeply buried reflectors. Then we compare these ages with eustatic sea-level variations, as recently summarised by Skene et al. (1998). At the shelf break, in some cases more than one deltaic sequence was noted below Z. In Germeno Bay (Fig. 6), the Z reflector is at ⫺120 msec and another truncation surface (correlated with X in the deep basin) occurs at ⫺220 msec. The normal depth of the Z reflector is 80–85 msec (see Figs. 3a, 5, 9 and 14), so that there has been 35–40 msec of subsidence of Z in 12 000 yr. If the deeper truncation surface was also graded to the level of the Rion Strait, it must have subsided 140 msec and by assuming a constant subsidence rate, the age of reflector X is estimated to about 42 000 yr. In the deep areas of the Gulf of Corinth, synsedimentary faulting produces increasingly large offsets with depth (Figs. 7 and 15). If fault movement has been approximately constant through time and assuming that reflector Y is about 32 000 yr, then reflectors X and W would date around 45 000 and 75 000 yr, respectively. Order of magnitude estimates of the age of reflectors can be made from sediment thicknesses in any one location. Several assumptions are involved here, notably that sedimentation rates were reasonably constant and not strongly influenced by Late Quaternary climatic change, in which glacial stages were times of greater aridity (Van Zeist and Bottema, 1982). Thus in Corinth Bay, the thickness of the Z to Y deltaic sequence is about 1.5 times that of the Holocene delta: it therefore probably took 10 000– 15 000 yr to accumulate, suggesting that reflector Y dates from 22 000 to 27 000 years ago. In Alcyonides

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Eustatic sea-level 0 lacustrine

marine

Estimated age of reflectors

Age (thousand years)

2.2

50

RION SILL DEPTH

Z Y

Basin stratigraphy Z Y

Shelf stratigraphy deltaic

stratified

erosion deltaic

X W

4.2

X

stratified

erosion

W 5.1 5.2

100

5.3 5.4

deltaic

V U

5.5

T

6.1

150

stratified

erosion

?

-100 -50 0 Relative sea-level (m)

Fig. 16. Correlation of Gulf of Corinth seismo-stratigraphy with global eustatic sea-level curve (from Chappell and Shackleton, 1986; Skene et al., 1998), showing the effect of the Rion Strait sill. Age of reflector Z based on correlation with the sea level curve of Bard et al., 1989), confirmed by radiocarbon dating. Age of Y, X and W based on rates of sedimentation discussed in text. Age of V, U and T based on correlation with the eustatic sea level curve.

and Navpaktos bays, the transparent Y to X interval is 2–3 times thicker than the Z to surface interval, suggesting that marine deposition lasted some 25 000–35 000 yr. This line of reasoning gives an age of 47 000–62 000 years ago for reflector X. The X to W lacustrine interval is thinner than the Z to Y interval by a factor of 2, indicating a duration of perhaps 5000 years. By comparing these order of magnitude estimates of the age of horizons Z, Y, X, and W with estimates of global changes in eustatic sea level (Chappell and Shackleton, 1986; Skene et al., 1998) it is clear that reflector Z is correlated with isotopic stage 2–1 transgressive phase, and reflector Y probably correlates with the isotopic stage 2–3 regressive phase (Fig. 16). The older estimates of the age of the lacustrine X to W interval suggest that this interval corresponds to isotopic stage 4 (65 000–75 000 years). Reflectors V, U and T in Navpaktos Bay, which show transgressive erosional features, are interpreted as marine transgressive shoreline deposits that have tectonically subsided. Their sub-bottom depth in relation to the overlying dated reflectors suggests that they may

correlate with the isotopic stage 5.2 to 5.1, 5.4 to 5.3, and 6.1 to 5.5 transgressions, respectively (Fig. 16). Reflector T represents a widespread erosional surface in the area over which sediments of various ages were deposited. A similar erosional surface was noted in the adjacent Patraikos Gulf (profile F in Chronis et al., 1991) and on the N. Aegean shelf where it was associated with the large scale Eutyrrenian (isotopic stage 6) lowstand of sea level (Piper and Perissoratis, 1991) which ended about 128 000 years ago. This chronology can be tentatively applied to the seismic reflection profiles from the deep basin of the Gulf of Corinth published by Brooks and Ferentinos (1984) and Higgs (1988). These show intervals with numerous strong reflections alternating with more transparent intervals (Fig. 17). The more reflective intervals show evidence of channel-like erosion near the southern margin of the basin. Using a mean sedimentation rate of 0.5 mm a ⫺1, reflectors, Z, Y, X, W and T are identified, implying that reflector I3 of Higgs (1988) dates from the last interglacial (isotopic stage 5e).

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Fig. 17. Part of seismic profile 15 of Higgs (1988) on the southern margin of the central basin of the Gulf of Corinth, showing interpretation of key reflections, acoustic facies and chronology, e ˆ erosion.

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a)MARINE HIGHSTAND

low gradient delta

high gradient delta

SURFACE PLUME

earthquake -triggered slope failure

RARE SANDY HYPERPYCNAL FLOW

suspension fallout of mud in deep water,rare turbidites

b)LACUSTRINE LOWSTAND

high gradient delta

low gradient delta

SURFACE PLUME

FREQUENT HYPERPYCNAL FLOW

FREQUENT HYPERPYCNAL FLOW

sediment failure promoted by slope erosion

deep-water turbidite deposition Fig. 18. Model showing contrasts in sedimentation processes between: (a) highstand fully marine conditions and (b) lowstand lacustrine or brackish conditions.

4. Discussion and conclusions Late Quaternary sedimentation in the Gulf of Corinth has been controlled by two factors: the position of sea level relative to the Rion Strait sill, and the sediment input from rivers. When the sea level fell

below the level of the Rion Strait sill (which is today at ⫺60 m), the Gulf was transformed into a lake or, if the sea level was a few meters above the Rion Strait sill, into a marine area with a high fresh water content. When sea level was higher, normal open marine conditions were restored. Low gradient (suspended

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load) deltas, such as those entering Corinth Bay, deposited most of their sediment on the shelf under marine conditions, but supplied lacustrine turbidites by underflow when the Gulf was a lake (Fig. 18). Steep gradient (bed load) deltas like the Selinous and Vouraikos supply turbidites to the basin floor under both marine and lacustrine conditions, but the importance of mass movement appears greater under lacustrine conditions. A distinct seismic stratigraphy was distinguished in the shelf areas of the Gulf that was extended into the deep-water areas. In the shallow areas, Z and X are terrace surfaces, commonly erosional, which mark the base of overlying deltaic sequences. In deep-water basins, Z and X mark the top of acoustically stratified sediments interpreted as lacustrine turbidites; reflectors Y and W mark the base of these stratified intervals and overlie acoustically transparent sections similar to the Holocene section. According to standard eustatic sea-level curves (Fig. 16), the last lacustrine conditions in the Gulf were during stage 2. The lacustrine conditions terminated about 12 000 years ago when sea-level rise overtopped the Rion Strait sill. Therefore reflector Z is about 12 000 years old and corresponds to the isotopic stage 2/1 transgression. During the initial part of the transgression over the suspended load deltas, a thin veneer of sediments was deposited at the outer shelf due to the fast advance of the sea. The maximum extent of the sea was reached about 6500 years ago and then Holocene deltaic sequences began to prograde across the inner shelf (e.g. Corinth and Egion bays). On the bed load deltas, only minor changes were noted due to their steep gradient. Age estimates based on sedimentation rates suggest that the Z to Y lacustrine interval corresponds to the stage 2 lowstand of sea level and the X to W interval to the stage 4 lowstand of sea level. This correlation is also consistent with the relative duration of the two lacustrine phases. In the western Gulf of Corinth, shoreline transgressive surfaces corresponding to minor transgressions in stage 5 are present, and the major transgression at the end of stage 6 is recognisable as a reflector (T) over a large area around the Mornos delta. T also marks the top of a thick packet of acoustically wellstratified sediment about 120 m subbottom on the basin floor of the Gulf of Corinth.

409

Acknowledgements This paper was dedicated to the late Dr A.G. Panagos, recently Professor of Geology at the Polytechnic University of Athens and former Professor and Chancellor of Patras University. We thank the Institute of Geology and Mineral Exploration and the National Center of Marine Research of Greece for permitting publication of the data. Geological Survey of Canada contribution number 1999039.

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