late quaternary evolution of the northern aegean shelf

QUATERNARY RESEARCH 32, 36-50 (1989)

Late Quaternary Evolution of the Northern Aegean Shelf

C.PERISSORATIS AND D. MITROPOULOS Institute of Geology and Mineral Exploration, 70 Mesoghion Str.. 11527 Athens, Greece

Received February 9, 1988

A dense network of 3.5 kHz and Uniboom seismic profiles revealed the geological evolution of the Ierissos-Alexandroupolis Shelf area of the northern Aegean during the latest sea-level rise. Near the end of the Pleistocene, ca. 14,000 yr B.P., the sea was at about - 120 m, and almost 5300 km2 of shelf was exposed to subaerial erosion. Two permanent lakes existed in the Ierissos and Kavalla Gulfs, as well as a number of ephemeral lakes scattered throughout the rest of the area. Kavalla Gulf was drained by the Nestos River which joined the Strymon River at the outer Strymonikos Plateau. On the flat Samothraki Plateau were dune fields, marshes, and a number of minor seasonal rivers, while the Evros River flowed east of the plateau. By ca. 13,000 yr B.P. the sea had risen to - 70 m and covered 30% of the previously exposed shelf. The lakes at Ierissos and Strymonikos Gulfs were shallower and the sea approached to within about 5 km of them. Kavalla Gulf and the adjacent eastern Strymonikos Plateau lay 10 m above sea level, while the sea intruded along river mouths at the Samothraki Plateau. During this transgression of the sea river-bed gravels and sands were covered by silts and clays. At the Pleistocene-Holocene transition, ca. 10,500 yr B.P., the sea lay at about -50 m and in areas of steep slope the coast was close to its present position. At Kavalla Gulf, the sea advanced along the paleochannel of the Nestos River, and the island of Samothraki was separated from the mainland. By ca. 7500 yr B.P. the sea was only 15 m below its present level and the northeastern Aegean shelf assumed nearly its present morphology. The Nestos River changed course to the east and Thassos Island was separated from the mainland. Coarse sediment formed wedges nearshore, whereas tine-grained sediments were distributed widely by current action. In many areas, relict sediments are present. 0 1989 University of Washington.

INTRODUCTION

During the Pleistocene, with its altemat- ing growth and melting of continental ice sheets, large sea-level changes were fre- quent. A major drop occurred during the latest Pleistocene when the sea fell to a low of about 120 m (e.g., Milliman and Emery, 1968; Bloom, 1977; van Andel and Lianos, 1984). This paper deals with the geological evolution of the north Aegean shelf, between Ierissos and Alexandroupolis (Fig. I), during the most recent rise of the sea, which began between 15,000 and 13,000 yr ago (Ruddiman and Duplessey, 1985) and ended sometime during the last several thousand years as the sea reached its present level.

Although the Aegean domain is characterized by high seismic and tectonic activity, the studied area has experienced little recent tectonism, because it is part of the Servomacedonian and Rhodope massifs.

As a result, at the sector from Ierissos to Alexandroupolis the shelf is better devel- oped compared to the rest of the Aegean. This area has been studied for several years with regard to sediment texture and com- position and to subsurface stratigraphy in a search for placer deposits. Therefore, the general geologic setting and the density of the data present a rare opportunity to examine the relationship between land, sea, and depositional processes during the late Quaternary. The two major factors consid- ered here, sea-level changes and sediment deposition, are induced mainly by the rate of ice melting and the input of river sediment, respectively.

Prior marine-geological work in this region was carried out by Lalechos and Sa- voyat ( 1977)) Konispoliatis (1984)) Vele- grakis (1985), Pechlivanoglou (1986), and Perissoratis (1986, 1987a); studies covering a larger area were reported by Perissoratis et al. (1984, 1987b). Paleo-coastlines for ca.

36 0033-5894189 $3.00 Copynght 0 1989 by the University of Washington. All rights of reproduction in any form reserved.

EVOLUTION OF AEGEAN SHELF 37

18,000 and 9000 yr B.P. were reconstructed by van Andel and Shackleton (1982).

Data used in this study consist of seismic reflection profiles obtained with Uniboom and 3.5kHz equipment and bathymetric profiles obtained with a Furuno echo- sounder (Fig. 1). Ship’s positions were determined by Motorolla Miniranger, Loran C, and radar.

GEOLOGY OF THE ADJACENT LAND AND MORPHOLOGY OF

THE SEAFLOOR

The area of study extends from Ierissos to Alexandroupolis and from the present coast to a depth of approximately 150 m, just beyond the deepest late Pleistocene shoreline. The surrounding land consists of metamorphic and matic igneous rocks be- longing to the Servomacedonian and Rhodope massifs, as well as Neogene and Quaternary sediments (Bornovas and Ron- doyannis, 1983) which are found both in nearshore areas and in interior Neogene basins.

Three of the largest rivers of Greece en-

ter the sea in the northern Aegean region, the Evros, Nestos, and Strymon Rivers, as well as a number of minor streams.

The study area is divided into a western and an eastern part by Thassos Island. West of this island the coast is complex and indented by the large Ierissos, Strymoni- kos, and Kavalla Gulfs. East of Thassos the coastline is relatively straight with only three minor embayments at Porto Lago, Xilagani, and Alexandroupolis. The bathymetric map, based on sounding traverses, permits distinction, from west to east, of the following physiographic units (Figs. 2 and 3).

(1) Zerissos Gulf has an elliptical shape and a NW-SE orientation. The bottom is steep near the coast, the 50-m isobath lying very close to the shore. Seaward, the slope diminishes and the central part of the Gulf is marked by a depression with a greatest depth of about 80 m and is closed at the mouth by a southeast-sloping sill with a depth of ca. 65-70 m.

(2) Strymonikos Gulf has a rectangular shape and steep nearshore slopes, except

FIG. 1. Map of the study area, with sample locations and survey lines. Bold lines represent locations of the profiles displayed in Figures 8-l 1.

38 PERISSORATIS AND MITROPOULOS

near the mouth of the Strymon River delta. Beyond the 40-m isobath the bottom slopes gently southeastward. The central part of the gulf is slightly elevated, thus creating a small basin in the north which is connected to the open shelf by a 2-km-wide and 8- m-deep channel that continues as far as Loutra Eleutheron.

(3) The Gulf of Kavalla is circular and, unlike the two others, slopes smoothly from the shore to its center. In two places it opens to the shelf: between Thassos and Keramoti in the southeast and between Thassos and Nea Peramos in the southwest. In the latter entrance the seafloor, rising to about 40 m, or 10 m above the maximum depth at the center of the Gulf, forms

a sill traversed in the middle by a channel 2 km wide and 10 m deep.

(4) The Strymonikos Plateau occupies the sector between the three aforemen- tioned gulfs. It extends to the shelf break and consists of an eastern part between Nea Peramos and Thassos and a western part between Thassos and the Strymonikos and Ierissos Gulfs. These two parts are separated by a fault-generated southwest- facing scarp extending from Loutra Eleutheron eastward and thence southeastward, to west of Thassos. This steep slope between the 70- and 100-m isobaths con- trolled the pretransgressional flow of the ancestral Strymon River, the channel of which remains as a submerged shelf valley.

FIG. 2. Bathymetric map of the western part of the study area. 1, lerissos Gulf; 2, Strymonikos Gulf; 3, Gulf of Kavalla; 4, Strymonikos Plateau. Depths are given in meters.


FIG. 3. Bathymetric map of the eastern part of the study area (Samothraki Plateau). Depths are given in meters.

The scarp continues then along the south- western shore of Thassos, forming a steep marine slope covered by sandy deposits. The western part of the Strymonikos Pla- teau slopes gently southeast while the eastern part is higher and slopes gently southwestward.

(5) The Samothraki Plateau (Interna- tional Bathymetric Map, 1981) extends eastward from Thassos to Samothraki Is- land and Alexandroupolis (Fig. 3). It slopes gently southward and is generally smooth except for a slight rise between Alexan- droupolis and Samothraki Island and a few NE-SW oriented channels in its eastern part. Between the 50- and 70-m isobaths its rippled bottom is almost flat. West of Thas- SOS, two subhorizontal physiographic features exist in the same depth zone, the eastern Strymonikos Plateau and the sill off Ier- issos.

Beyond the shelf break, the seafloor falls steeply to more than 1000 m into the North Aegean Trough.

SEDIMENTATION, STRATIGRAPHY, AND STRUCTURE

At a depth of up to 25 m below the seafloor seismic profiles show an erosional unconformity associated with the various exposed or buried channels and valleys. This unconformity, a distinct feature on many seismic reflection records, has been inter- preted as a land surface of the last Pleisto- cene low stand of the sea (Perissoratis and van Andel, 1988). However, it is not seen within the Ierissos Gulf and in the inner part of the Strymonikos Gulf where it appears to be replaced by a transparent horizon con- formable with the beds above and below (Fig. 10, profiles 4 and 5; Fig. 11, profiles 6 and 7). Apparently in those two areas sedimentation continued without interruption even when the shelf was exposed. The two depressions probably were lakes during the latest Pleistocene.

The unconformity generally lies buried under fine-grained sandy silts, clayey silts,

40 PERISSORATIS AND MITROiOULOS

FIG. 4. Map of sediment distribution in terms of sand-silt-clay ratios. Wavy lines separate Ho- locene transgressional sands from relict sands.

and silty clays (Fig. 4), but in some places it crops out or is covered by a thin veneer of coarse sediments, usually well-sorted sands and silty sands. These relict deposits (Per- issoratis et al., 1987a) are found on the elevated or shallower portions of the Ierissos and KavaIla Gulfs, on the hill in the eastern part of the Strymonikos Gulf, in sandy areas of the Samothraki Plateau, and near the shelf break (Fig. 4).

A detailed examination of the reflection records indicates that two types of deposits make up the postunconformity sequence. The lower part usually consists of distinct or indistinct, discontinuous, opaque or transparent beds, occasionally with “chaotic” structure. These probably are coarse deposits that tilled channels and valleys during the shallow, littoral stage of the transgression. In contrast, the upper part consists of alternating thin opaque and transparent, or entirely transparent, beds. This unit represents the fine-grained silts

and clays deposited during normal marine post-transgressive conditions (Fig. 5, pro- tile 13; Fig. 6, profile 21).

Holocene sediments overlying the late Pleistocene and early Holocene unconformity are thickest in the Ierissos and Stry- monikos Gulfs (~24 m, Fig. 7). Assuming that the postglacial transgression began about 14,000 yr ago (Ruddiman and Du- plessy, 1985), the maximum rate of sedimentation in the area is about 1.8 m/1000 yr.

A few faults can be seen on the seismic profiles, most in the western part of the area. The faults appear to be responsible for the presence of the Ierissos, Strymoni- kos, and Kavalla Gulfs; they also define the steep slope between the eastern and western parts of the Strymonikos Plateau. Evi- dence for minor local tectonic activity can also be seen east of Thassos Island. The generally low level of tectonic activity, compared to the rest of Greece, is in accord with the relative stability of the Serbomace-


NW SE

IAS AS 50 m

l- 1 / 13 0 Km

NE

FIG. 5. Seismic profiles of the Gulf of Kavalla. The numerous channels of the eastern (profile 12) and central (profile 13) areas join in a single one in profiles 14, 15, and 16. The channel shoals seaward. AS indicates the late Pleistocene and early Holocene unconformity; I and II refer to two groups of reflectors discussed in the text. For location of profiles, see Figure 1.

donian and Rhodope massifs to which the LATE QUATERNARY GEOLOGIC region belongs. The tectonic displacements EVOLUTION OF THE NORTHEASTERN of at most a few meters during the latest AEGEAN SHELF Pleistocene and Holocene have had little ef- fect compared to a sea-level rise of about The time of onset of ice melting and of 120 m and sediment deposition of several to the postglacial rise of the sea is in dispute, several tens of meters. with claims ranging from 16,000 to 14,000


W E

/

.,. .A.*, , .

17

E NW CF

30 m

L---L

AS

1 0 Km

19

I' ~ jl. "2, :,

N S 4 A I ,

/ I 1 .- _ __- ..- --

50 m

0 q-

!

Km 2

.; i

I

FIG. 6. Seismic profdes of the Samothraki Plateau. No channels or valleys occur in the areas south of the present Nestos delta (profile 17) or between Alexandroupolis and Samothraki (profile 22). Elsewhere channels can be seen in profiles 18, 19,20, and 21. The late Pleistocene and early Holocene unconformity (AS) and the two groups of reflectors above it appear in the same profiles. For location of profdes see Figure 1.


FIG. 7. Isopach map (m) of deposits above the late Pleistocene and early Holocene unconformity.

yr B.P. However, it is clear that the rise was discontinuous, with a major stillstand or even reversal between about 13,000 and 10,500 yr ago (Ruddiman and Duplessy, 1985). The rapid sea-level rise that resulted from ice melt came to an end ca. 7500 yr ago (Fig. 8a); since then the sea has slowly risen to its present position, whether with episodic small reversals or stillstands is still being debated. Thus, it is convenient to dis- cuss the late-glacial and postglacial evolution of the shelf in four distinct phases: Tl ca. 14,000 (or perhaps 16,000) yr B.P.; T2 at

about 13,000 yr B.P.; T3 at ca. 10,500 yr B.P.; and T4 at ca. 7500 yr B.P. According to van Andel and Lianos’ (1984) sea-level curve for the southeastern Peloponnese, the beginning of the four stages correspond to depths below present sea level of approximately 120, 70, 50, and 15 m.

Although in Figure 8b the beginning of the stage Tl sea-level rise corresponds to about 16,000 yr ago, in the description below we attribute both dates to this stage (14,000 or 16,000 yr ago). We do this because the matter is not yet clear, and wish

\ ICE \

VOLUME \

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50. \ '--

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\ 100.

0 4 a 12 16 20

YRX 103

50 -

loo- I.

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a b

FIG. 8. (a) Curve of ice-sheet melting (after Ruddiman and Duplessy, 1985); (b) curve of sea-level rise for the eastern Peloponnese (after van Andel and Lianos, 1984).


Ia 2j3 3&l 4m 5m aI""\ 7191 f3m Ql’rl 1opJ

FIG. 9. The northern Aegean shelf at about 14,000 (or perhaps 16,000) yr B.P. (Tl): 1, Present line; 2, ancient coast; 3, ephemeral lakes; 4, permanent lakes; 5, sandy hills; 6, sand dunes; 7, faults; 8, steep coast; 9, swamps; 10, 5 m isobath of stage I.

only to examine and present the geological picture of the area at the onset of the latest sea-level rise.

Tl: Late Pleistocene (ca. 14,000 (or perhaps 16,000) yr B.P.; Fig. 9)

At this time the sea was about 120 m below its present level, exposing some 5300 km* of shelf to subaerial weathering and erosion. Close to the present shoreline the slope was quite steep, as shown by the 5-m isobath, but, except for the terrace edge between Thassos and Nea Peramos, the shelf plain was flat out to the shoreline and between 20 and 40 km south of its present position. The region was drained by two major permanent rivers, extensions of the present Strymon and Nestos, and by a number of minor and probably ephemeral ones. For the most part, the plain was covered by sandy dunes and a few larger sandy hills, and overlooked by the Thassos and Samothraki mountains, which were 1225 and 1731 m high, respectively.

A number of lakes existed; most were probably ephemeral, as indicated by thin sediment and the scarcity of influent and

exfluent river channels around them, but two were permanent. One occupied the greater part of the Ierissos Gulf and had a depth of about 35 m. Its margins were sandy and in its center were deposited silts and clays. This lake was the product of Quaternary deformation that produced steep slopes on its northern, southern, and eastern sides. East of the lake existed a 5- m-high hill. A channel south of the hill, and probably another one north of it, connected the lake to the sea (Fig. 10: profile 3).

In Strymonikos Gulf, a similar steep- sided basin formed during the late Quater- nary; sediments supplied by the Nestos River accumulated in its central part in a small lake with a depth of about 30 m. To the southeast of this basin, a channel runs parallel to the fault scarp (Fig. 11: profiles 8 and 9), continuing southeastward parallel to the present coast. This channel was occupied by the Strymon River which flowed southeastward to Loutra Eleutheron and there turned southward between this place and Nea Peramos, following along the foot of the fault scarp that terminates the eastern part of the Strymonikos Plateau (Fig.

EVOLUTION OF AEGEAN SHELF

i 100 m ‘-- .’

1

.- --

50 C

-

FIG. IO. Seismic profiles of the Ierissos Gulf. Continuous sedimentation can be observed in most of the central part (profiles 4 and 5). A channel is present at the outer part of the gulf (profile 3, C). Figure 1 shows location of profiles.

11, profiles 10 and 11). Farther south the valley of the Strymon River became shallow and widened to about l-2 km before it joined the Nestos River a short distance from the -120-m shore. At the shore, the two rivers constructed a delta of coarse and fine deposits.

The Gulf of Kavalla was drained by the Nestos River. Today this river runs eastward to the northeast of Thassos (Fig. l), but seismic data south of the present Nes- tos delta show no channels or valleys on the seafloor and very little Holocene deposition (Fig. 6: profile 17; Fig. 7). On the other hand, the seismic profiles of the Gulf of Kavalla indicate the presence of many erosional channels and valleys. These features

can be traced southwestward until they join in a single channel that connects with the Strymon valley (Fig. 5: profiles 12-16). Geologic studies of the present Nestos delta show that the older lagoons, bars, and channels are concentrated in its western part (Psilovikos et al., 1986; Stournaras, 1984). Evidently, the present easterly course of the Nestos is a recent phenome- non.

The Samothraki Plateau (submerged shelf) was almost flat and drained only by minor seasonal rivers, as is the case today for most of the Thracian land to the north. No surficial or buried channels were seen on seismic reflection profiles between Samothraki and Alexandroupolis (Fig. 6:


NW

100 m

50

0 L Km 2

I

i

S / N

S

01 .!Km 10 I

I I

7 : ii .i, 7 I

/ 'i

FIG. 11. Seismic profiles of Strymonikos Gulf and Plateau. Continuous sedimentation is evident in the central part (profiles 6 and 7). The paleochannel of the Strymon River can be seen in the northern gulf in profiles 8 and 9 and in the eastern part in profiles 10 and 11. Figure 1 shows location of profiles.

profile 22); the seafloor is slightly elevated flowed east of Samothraki, as indeed is in- here and covered with relict deposits. This dicated by the presence of a channel south suggests that the Evros River, the only of the current Evros delta (International larger, permanent stream in this region, Bathymetric Map, 1981). Erosional chan-

CVOLU’I ION Ol- - - - - - - - - - - - - -7 AEGEAN SHELF 47

nels and valleys are also seen on seismic profiles taken northeast of Samothraki. Thus, a late Pleistocene Evros delta at the shelf edge well east of Samothraki seems likely.

physiographic features and earlier deposits were not covered by transgressive sediment and today remain on the surface as relicts from the late Pleistocene.

Other small rivers today enter the bays of Pot-to Lago, Xilagani, and Alexandroupo- lis. These continued their courses southward, forming lakes here and there and bringing minor amounts of sediments. Be- cause their discharge and sediment load were too small (cf. the Filiouris River), their precise courses cannot be determined. Sometimes the entire sediment load was trapped in small depositional basins, as off Alexandroupolis where a thick accumula- tion of Holocene deposits covers the distinct late Pleistocene unconformity (Fig. 6: profile 21), presumably filling a tectonic depression.

The lake in the Ierissos area, although shallower than before because of gradual infilling with sediments, continued to exist. The seashore, however, was now a mere 5 km away. In the Strymonikos Gulf the seashore was similarly ca. 5 km away from the lake, but there it formed a narrow, tongue- like embayment. The remaining plain was smooth, except south and east of Loutra Eleutheron where the relief was steeper. Little change took place in the Kavalla area because the high eastern part of the Stry- monikos Plateau remained about 10 m above sea level of the time, thus preventing intrusion of the sea toward the northeast.

T2: End of the Late Pleistocene (ca. 13,000 yr B.P.; Fig. 12)

During the early melting of the ice sheets, the sea rose some 50 m and by 13,000 yr ago it was about 70 m lower than today. The shoreline advanced considerably, and at time T2 about 30% of the previous land area had become submerged. Because the land- ward shift of the shore was so rapid, many

South of Thassos and Samothraki islands the coast had receded to within 3 km of the present shores, and the new sea bottom was steep, with many bedrock outcrops, and covered only with a little sand. The Samothraki Plateau land area itself remained wide, with incursions of the sea mainly at river mouths. The Evros delta withdrew slightly to the north. During this retreat the gravels and sands in the river beds and valleys became blanketed with fine marine sands and silts.

FIG. 12. The northern Aegean shelf at 13,000 yr B.P. (T2). Legend as in Figure 9.


FIG. 13. The northern Aegean shelf at 10,500 yr B.P. (T3). Legend as in Figure 9.

T3: Beginning of the Holocene (ca. 10,500 yr B.P.; Fig. 13)

After the first substantial ice melting and sea-level rise, a cold interstade occurred and the sea level stopped rising or rose no more than 20 m at most. This slowdown permitted waves and currents to sort and differentiate the coarse deposits on the submerged shelf hills and plateaus between the present 50- and 70-m isobaths on the shelf.

Although the rise of the sea was slow and relatively minor during this cold period, it took place across a very flat part of the shelf and eventually 70% of the former shelf land was lost to the sea. In many places the coast was close to its present position. The Ierissos and Strymonikos Gulfs took on their modern outline, with minor differ- ences such as the hill on the central- western side of the Strymonikos which remained as a peninsula in front of the shore. In the Gulf of Kavalla, the sea advanced up the Nestos channel, forming a tongue-like embayment that protruded into the tectonic basin of Kavalla Bay. Dune fields still situ- ated a little above sea level bordered this marine channel.

Thassos remained connected with the mainland, but many of its shores were close to their present position. A large advance of

the sea took place across the Samothraki Plateau, especially east of Xilagani, and Samothraki became separated from the mainland by a shallow sea channel. Fine- grained sediments were deposited in the sea to the east of the island by the Evros River. In general, the slow rise of the sea during the cold period caused a significant redis- tribution of tine-grained sediments across a wide area, except on the eastern and outer parts of the Strymonikos Plateau and the greater part of Samothraki Plateau where relict nearshore transgressive deposits remain at the surface even today.

T4: Mid-Holocene (ca. 7500 yr B.P.)

After about 10,500 yr B.P. the rapid rise of the sea resumed, and at 7500 yr B.P. sea level was only 15 m below its present position. The shores at this time were near and similar to those of today (Figs. 2 and 3). Sands and gravels brought by the rivers formed a thick wedge of Holocene sediments near the shore (Figs. 4 and 7). How- ever, the fine sediment load was distributed by marine currents over a wide area and partly buried the coarser transgressive deposits of the now-submerged river channels and valleys. Gradually, sediments deposited by the Nestos River tilled the estuaries produced in the west by the transgression


and so shifted the river to its present more easterly course. Thassos became completely separated from the mainland during this stage.

The fast rise between 10,500 and 7500 yr ago prevented many physiographic features from being completely buried, such as the elevated parts of the Ierissos and Kavalla Gulfs, the hill on the west side of the Stry- monikos Gulf, and most of the midshelf part of the Samothraki Plateau. These areas remain covered by relict nearshore deposits.

DISCUSSION AND CONCLUSIONS

During the low-sea-level stage (Tl, about 14,000 yr ago) the sea was about 120 m below its present position and an emergent area of about 5300 km2 was exposed to subaerial erosion. Two permanent lakes existed, at the gulfs of Ierissos and Strymon- ikos, and a number of ephemeral ones were distributed over the rest of the area. The shelf plain was flat, with scattered dunes and hills, covered by sandy sediments and drained by two major rivers, the extensions of Strymon and Nestos, and a number of minor ones. The high mountains of Thassos (1225 m) and Samothraki (1731 m) domi- nated the area. During this stage much of the sediment brought by the rivers was trapped at the lakes and the rest was deposited along river beds and in deltas and estuaries. At the shoreline, elongate sand bodies were formed by the reworking of previously deposited sediments.

During subsequent transgressive stages (Tl to T4, from 14,000 yr ago until today) the sea advanced in the various parts of the shelf at different rates. In the areas of flat topography, such as the plateaus, the rising sea level resulted in a rapid migration of the shoreline, even at periods of slower rate of submergence (such as between stages T2 and T3). This resulted in the reworking of sands over the major part of the plateaus, while over the rest a thin veneer of muds transported by current action began to be deposited on the sands. In the drowned

Pleistocene river valleys and channels, sands were deposited over pretransgressive alluvial sequences. On areas of high slope, however, such as south of Thassos and Samothraki, the shoreline migration was much slower. The minor sediment input resulted in little or no deposition and, at places, even in erosion, as indicated by the many rock outcrops present. By 7500 yr ago the sea was only 15 m below its present level and the shore configuration was similar to present. In the river valleys and the channels, the transgressive sands were buried under marine muds. First Samothraki, and later Thassos, were separated from the mainland while the Nestos estuaries were gradually filled by sediments and shifted the river’s course to its present position. The coarser sediments were deposited near the shorelines forming thick wedges, while the fine load was distributed by the currents over part of the plateaus, mixed with sands, and formed palimsest sediments. Even today a considerable part of the area remains free of recent sediments.

Thus, the Holocene transgression in the northern Aegean shelf can be characterized as depositional at the gulfs and along river valleys and channels; as discontinuous depositional on the plateaus; and as erosional at the shelf sectors south of Thassos and Samothraki islands (terminology after Cur- ray, 1964). This process explains the fact that almost 75% of the examined shelf is covered by relict and palimsest sediments, a common characteristic of most continental shelves today.

As indicated in the bathymetric maps (Figs. 2 and 3), the basins and the Pleisto- cene channels and valleys have not yet been filled by sediments. The sediment load of rivers has been diminishing during the last few decades due to the construction of dams and reservoirs. If this continues, the basins at Ierissos and Strymonikos gulfs and the small embayments of the Samoth- raki Plateau will derive sediments from the adjacent coastal areas, which will eventually be eroded and destroyed.


Based on our study, the islands of Thas- SOS and Samothraki were connected with the mainland until a few thousand years ago, when the rising sea separated them (Figs. 9, 12, and 13). These data are partic- ularly useful to archeologists who study prehistoric settlements on both of the islands.

ACKNOWLEDGMENTS

We thank Dr. C. Papavasiliou, Director General of IGME, for permitting the publication of the seismic data; Dr. T. van Andel, Professor of Stanford Univer- sity, for his valuable corrections and suggestions; and Dr. J. Kraft, Professor of Delaware University, for reviewing the paper. Mrs. D. Tsotakos patiently typed the manuscript. The research was partly funded by the Research and Development program of the European Economic Communities.

This paper received the C. Ctenas Award from the Athens Academy of Science for the best geological paper of 1987.

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