Quaternary sedimentary history and neotectonic evolution of the eastern part of Central Aegean Sea, Greece

ELSEVIER

Marine

Quaternary

Geology

128 (1995) 59-71

sedimentary history and neotectonic evolution of the eastern part of Central Aegean Sea, Greece

V. Lykousis,

C. Anagnostou,

P. Pavlakis,

G. Rousakis,

M. Alexandri

National Centre for Marine Research, Ag. Kosmas, 16604 Athens, Greece

Received 5 April 1994; revision accepted 10 May 1995

Abstract

Four distinct unconformities can be traced below the E. Cyclades margins (S. Ikarian Basin), that separate four oblique progradational delta sequences. These have been deposited during sea-level lowstands within Middle-Late Quaternary glacial stages (oxygen isotopic stages 2, 6 and possibly 12-8, 22-16). The topset-foreset transition of these sequences, located at 110, 152 and 212 m below present sea level, reflect continuous subsidence of the S. Ikarian margins, with a calculated rate of 0.33-0.57 m kyr-‘. The mean sedimentation rates were estimated as 20-90 cm kyr-’ for the Late Pleistocene turbiditic deposits (250 kyr B.P.-today). For the post-isotopic 6 (128 kyr B.P.-today) sediments the accumulation rates were lo-30 cm kyr-’ on the slopes and 45-60 cm kyr-’ within the basins, while for the post-late glacial sediments (18 kyr B.P.-today) the calculated maximum accumulation rates were 30 cm kyr-’ within the basins and 6-14cm kyr-’ within the basin margins of North and South Ikarian Basins. A pronounced compressional phase during the Middle-Early Quaternary within the North and South Ikarian Basin can be recognised. Within the N. Ikarian Basin indications of Quaternary strike-slip deformations appeared to be related with en echelon of transpressional reverse faults. From the Middle Quaternary onward continuous subsidence and the reduction in size of subaerial regions are attributed to extension tectonics.

1. Introduction The area under investigation comprises mainly the eastern termination of the Cyclades Plateau and the greater area of North and South Ikarian Basin (Stanley and Perissoratis, 1977) (Fig. 1). The geodynamic evolution of the area (as the Central Aegean domain) implies intense compressional deformation (Late Jurassic-Miocene) with vertical movements and block tilting (Angelier, 1976, 1979; Papanikolaou, 1977, 1987). The extensional tectonic regime was established during Late Miocene or Early Pliocene times (Mercier et al., 1979) associated with normal and listric faulting, and resulting in the formation of the central Aegean Sea grabens (basins) (N. and S. Ikarian 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDZ 0025-3227(95)00088-7

Basins, Lesbos Basin etc.). An episode of compressional tectonics in the Early Quaternary is also described by Mercier et al. (1976). According to Mascle and Martin (1990) the Central Aegean display a “puzzle-like” morphological and structural pattern with variously oriented normal faults. The eastern part of the Central Aegean appears to be dominated by clearer trends (NE-SW to ENE-WSW), while the western part (S. Skiros Basin, Kalogeri Basin, Andros-Chios ridge etc.) chiefly displays NW-SE oriented extensional features. These trends clearly result from a NW-SE extension, and therefore may progressively have been activated as incipient translational fault zones as suggested by the local strike-slip deformation. Such strike-slip deformation has also been sug-

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60

2

2d36

Fig. 1. Baythymetric (dashed lines).

map with seismic

profiling

tracks

(solid lines), core locations

gested for the S. Aegean Sea (Pavlakis, 1992, Anagnostou et al., 1993) and the western part of Central Aegean (Argolikos Gulf) (Van Andel et al., 1993). Subsequently Plio-Quaternary sediments were deposited within the slowly subsiding basins, as a result of river discharge and eustatically controlled delta progradation during the Quaternary. Although there have been several studies of Late Quaternary subsidence and sedimentary history in the N. Aegean, the Central Aegean is very poorly studied. The Quaternary sedimentary evolution of the Aegean sediments have been revealed mainly in the N. and E. Aegean (Perissoratis and Mitropoulos, 1989; Perissoratis and Van Andel, 1988; Van Andel et al., 1990, 1993; Piper and

(circles)

and selected

profiles

used in the text

Perissoratis, 1991; Lykousis, 1991) and along the Western coasts of Turkey from delta progradational patterns (Aksu and Piper, 1983; Aksu et al., 1987a,b). The purpose of this work is the investigation of the Middle-Late Quaternary sediment sequences and neotectonics with the possible geodynamic implications during this period.

2. Materials and methodology The area was surveyed (PAR BOLT) and a 3.5 resolution profilers. The carried out from the R/V

using l-10 in3 Air-Gun kHz (ORE LTD) high entire field work was Aegaio. Position fixing

V. Lykousis

et aL/Marine

was obtained by an integrated positioning system (Global Position System-G.P.S. TRIMBLE 4000 SURVAYOR) with an accuracy of f50 m. A narrow beam (8’) echo sounder (FURUNO) was also used during the whole survey. Ten sediment gravity cores were recovered with a 3 m BENTHOS INSTR. gravity corer for chronostratigraphic control and relative dating of the Late Quaternary seismic reflectors. AMS radiocarbon dating analysis of six core samples was carried out at BETA ANALYTIC (U.S.A.) laboratory.

3. Results and discussion 3.1. Sequence stratigraphy

The continuous reflection seismic profiles (AirGun and 3.5 kHz) have been studied according to the method of Mitchum et al. (1977). This technique distinguishes different sediment sequences and depositional conditions based on the analysis of correlatable reflections, successive reflectors and associated unconformities. The characteristic seismic profile across the eastern margin of the Cyclades to S. Ikarian Basin (Fig. 2) reveal the existence of characteristic major unconformities, which define four successive progradational sequences (Psi-P&). These sequences display an oblique progradational configuration indicating sediment prodelta progradation during sea-level stillstand. These are interpreted as delta sequences that prograded basinward during sealevel lowstands (Pleistocene glacial stages), while the unconformities that separate the depositional sequences represent major transgressions during interglacial stages. Similar sequences have been detected in the NW Aegean sea (Lykousis, 1991) and along the western coasts of Turkey (Aksu and Piper, 1983; Aksu et al., 1987a,b) and have been interpreted as Middle-Late Pleistocene delta prograding sequences. The topset-foreset transition (FT,) of the shallowest prograding sequence (PS,) (Fig. 2), located on the shelf edge in a water depth of 110 m (146 ms), indicates the lowest stage of delta evolution during the late glacial maximum (ca. 18 kyr B.P.)

Geology 128 (1995) 59-71

61

and corresponds to the maximum fall of sea level (ca. 110-120 m) in oxygen isotopic stage 2 (2.2) (Chappell and Shackleton, 1986; Imbrie et al., 1984). The maximum fall of sea level was established also for the Aegean sea by Van Andel and Lianos ( 1984) ( 118-120 m). The chronostratigraphically relatively older (deeper) topset-foreset transition (FT,), located in a water depth of 152 m (203 ms), indicates delta progradation (PS,) during the oxygen isotopic stage 6.2 (ca. 146 kyr B.P.). The oldest lower edge of the topset-foreset transition (FT,) of the prodelta sequence PS3 (2 12 m below the present sea level) is interpreted to represent the latest stage of delta progradation during isotopic stage 8.2 (ca. 250 kyr B.P.). The possible fourth (and probably fifth) delta sequence (PS,-PS,?) (Fig. 2) is partly deformed and eroded and it is assumed to be of Early-Middle Quaternary age. A sequence of deformed reflectors (B) appeared below the fourth (and possibly fifth) prodelta sequence that probably represents Pliocene (and/or Earliest Pleistocene) deposits, while the older hard substrate (basement rock-C) is emerged below the sea-bed of the eastern margin of the Cyclades and S. Ikarian Basin. From the reflection character (well stratified, continuous, intense, parallel-subparallel without onlap or offlap terminations) of the seismic sequence (A) clinoforms (Fig. 2), it is inferred that the basin deposited sediments are mainly (deeper) prodelta facies. These have been deposited at sealevel lowstands as prodelta muds interbedded by sandy and/or silty horizons, deposited directly during high river plumes or from resuspention on the edge of palaeoprodelta platforms during stormy conditions. During interglacial stages (sealevel highstand) appreciably thinner (a few meters) and finer grained (muddy) hemipelagic sediments were deposited. The uppermost part of seismic sequence (A) is characterised by decreased stratification and continuity of the seismic reflectors (A,) (Fig. 2) indicating differentiation of sedimentary conditions and probably of sediment texture. High resolution 3.5 kHz profiles from the western margin of the S. Ikarian Basin display a sequence of alternatingly stratified and transparent or semi-transparent intervals (Fig. 3). This stratigraphically independent seismic unit is identified

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K Lykousis

et al.lMarine

Geology 128 (1995) 59-71

63

A1

Fig. 3. 3.5 kHz (upper) and Air-Gun (lower) profiles from the S. Ikarian Basin showing the characteristic Locatlon of the core Td5, used for chronostratigraphic correlation of the reflector V. is also indicated.

by a basic unconformity and a series of parallel and stratified reflectors (Z) that can be traced along the basin and the slopes of S. and N. Ikarian Basins. The regional package of these reflectors (Z) at the base of the seismic unit (A,) displays a reflection configuration similar to the widespread reflectors (Cu) in the basins of N. and E. Aegean Sea (Piper and Perissoratis, 1991; Van Andel et al., 1993). The top of this reflector sequence (Cu) is correlated with the termination of the glacial period (end of isotopic stage 6.1 ca. 128 kyr B.P.). The reflectors (Z) is assumed to be of the same age, representing distal prodelta facies deposited during a sea-level lowstand, chronostratigraphitally equivalent to the (PS,) prodeltaic prograding sequence. The upper surface of the stratified reflectors (Z) can be traced within the Late Quaternary sediments of the eastern margin of S. Ikarian Basin as the upper termination of the prograding sequence (PS,) (stage 6.1) and marks the initiation of the 6 to 5 stage transgression (Fig. 2).

Late Pleistocene

reflectors.

A section of a seismically transparent or semitransparent sequence appears above the stratified reflectors (Z) (Fig. 3) that terminates upward to another package of stratified reflectors (X). This transparent sequence (Y) principally represents muddy hemipelagic sediments deposited during sea-level highstands (isotopic stages 5.5 to 3.3) for a period of about 70.000 years (ca. 120-50 kyr B.P). The well stratified reflectors (X) correspond to a relatively sea-level lowstand in isotopic stages 3.2-3.1 (about 50-30 kyr B.P.). Their lower and uppermost boundary reflectors are correlated with the b, and b, reflectors from the N. Aegean Sea sediments (Piper and Perissoratis, 199 1). During the following short period of relatively higher sea level (stage 3) sediments of semi-transparent to transparent acoustic character were deposited (W). This sequence is upward bounded by a thin but strong stratified reflector (V) that reflects sandy-silty mud horizons deposited during the late glacial (stage 2.2) about 25-18 kyr B.P. During

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V. Lykousis et al/Marine Geology 128 (1995) 59-71

this period sea level was about 110-120 m below present level and the sediment deposits are mainly basinward silty sandy muds, derived from the deltaic prograding deposits on the shelf break (PS,). The reflector (V) also marks the initiation of the late-glacial transgression (stages 2.0-l .O) and is buried below the near bottom post-glacial muddy sediments (transparent section U ) . 3.2. Chronology of acoustic reflectors The chronology of the acoustic reflectors V, W and X was estimated by three radiocarbon dates obtained for core T,, (Fig. 4). This sediment core was collected on the western margin of S. Ikarian Basin where the reflectors V, W and X almost pinch-out at a subbottom depth of 0.2-2 m. Radiocarbon date analysis was undertaken on three different sediment lithofacies: (1) The muddy biogenic sand layer, toward the bottom of the core ( 1555 180 cm), (2) the sapropelic layer (core depth 118-140 cm), and (3) the carbonate fraction of the turbiditic silt horizon in the upper part of the core (35-45 cm). These sediment horizons was yielded radiocarbon ages of 35,090+710 yrs B.P. (Beta 74238), 29,400*250 yrs B.P. (Beta 74239) and 21,000 + 130 yrs B.P. (Beta 74240), respectively. Consequently the muddy biogenic sand layer should be correlated with the top of stratified

reflectors X (stage 3.1), the Sapropel (S,) with the beginning of the transparent semi-transparent unit W (stage 3) and the turbiditic silt horizon with the stratified reflector V (stage 2.2). Short sediment cores (2-3 m) recovered from the N. and S. Ikarian Basins (Fig. 5) have confirmed the age of the reflector (V) as well. Within a subbottom sediment depth of 1.2-2.5 m (approximate subbottom depth of reflector V) the Y-5 ash layer was recognised and was estimated to have been deposited in the Aegean Sea 20-25 kyr B.P. (Cramp et al., 1989). A few tens of centimetres shallower than Y-5 ash layer the sapropelic layer S, was recognised and yielded AMS radiocarbon dates (three samples) from 7 to 9 kyr B.P. (Fig. 5). 3.3. Basin subsidence and accumulation

rates

The foreset-topset transitions observed in the eastern margin of the Cyclades and S. Ikarian Basin appear shallower of a few meters from W. Turkey and a few tens of meters from the N. Aegean Sea (Table 1). This implies relative differential subsidence rates between the associated margins of these areas. By comparing the vertical displacement from the topset-foreset transition of two successive prodelta sequences, we can estimate the relative subsidence rates, assuming similar sealevel lowstands, with an uncertainty of no more

T46 “/“‘“R--

silty

IU

mud c21000

5130

yrs

BP 1 v

Fig. 4. Pinch out of the Late Pleistocene reflectors in 3.5 kHz profile across the western margin of the S. Ikarian Basin with their chronostratigraphic correlation, using radiocarbon dates from core T,,.

V. Lykousis et al./Marine

N. IKARIAN I

-0s

(cm) 0

B\SIN 0,

T17

65

Geology 128 (199.5) 59-71

S. IKARIAN

BASIN MARGINS

.

TIS

T20

‘T21

c

T19’

BASIN

\ T44 T45

50 g650+

100

60 yrs

BP

,739O

-+60yrs

BP

,a940

+60yrs

BP

150 200 I LEGENT

u

Hemipelagic

- turbiditic

n

Sapropalic

layer

0

Y-5

ash

layer

mud

(7-Q

Ka

(ZO-25Ka

BP)

BP)

Fig. 5. Simplified Late Pleistocene lithofacies logs of the N. and S. Ikarian Basins used in this study. Numbers in the log margins are AMS radiocarbon age determinations of sapropel S,.

Table 1 Comparative observations by various authors in the North and Central Aegean Sea Foreset to topset transitions

Isotopic events

NW Aegean (Lykousis 1991)

N Aegean (Piper and Perissoratis, 1991)

Kusadasi Gulf (Aksu et a1.,1987)

Izmir-Bay (Aksu et al., 1987)

E Aegean (this Study)

FTi (18 kyr B.P.) FTZ (145 kyr B.P.) FT3 (250 kyr B.P.)

2.2 6.1 8.0

120 m 240 m 430 m

-125 m

112.5 m -173 m

112.5 m 169 m 236 m

109.5 m 152 m 212 m

0.95 m kyr-’ 1.8 m kyr-’

0.3-1.5 m kyr-’

0.48 m kyr-’

0.45 m kyr-’

0.33 m kyr-’ 0.57 m kyr-’

Subsidence

FT-FT, FT3-FT,

rates

than 20 m. This is the approximate water depth where the recent transitions are formed in the Aegean Sea. Accordingly the relative subsidence from isotopic stage 6.2 (146 kyr B.P.) to isotopic stage 2.2 (18 kyr B.P.) is 0.34 m kyr-’ (42.5 m subsidence 130 kyr-‘). This value is lower than the corresponding values from the NW Aegean, N. Aegean and W. Turkey margins (0.95, 0.3-1.5 and 0.45-0.48 m kyr-‘, respectively). The relative subsidence from isotopic stage 8 (ca. 250 kyr B.P.) to isotopic stage 6.2 (ca. 146 kyr B.P.) is 0.57 m kyr - ’ (60 m subsidence 104 kyr - ‘), a value appre-

ciably lower than that of the NW Aegean Sea margin (1.8 m kyr-‘) (Table 1). The estimated subsidence rates during the Late Quaternary are relatively similar with the values calculated by Van Andel et al. (1990, 1993) for the Argolikos Gulf (western part of Central Aegean). The thickness of the delta prograding sequences decreases from 150 m (PS,) to about 30 m (PS,) indicating a significant decline of the E. Cyclades drainage systems (reduction in size of subaerial land) during the subsidence, particularly from isotopic stages 8 to 6.2. This is associated with

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K Lykousis et al.JMarine Geology 128 ( 1995) 59-71

lower depositional rates and restriction of delta progradation. The mean Late Quaternary sedimentation rates, calculated from the thickness of post-isotopic stage 8.2 undifferentiated sediments (50-230 m seismic sequence A) (Fig. 2) range between 20 and 90 cm kyr-’ in the E. Cyclades margin to about 80 cm kyr-’ within the S. and N. Ikarian Basins. The maximum thickness of the seismic unit (A,) is 12.5-40 m (Fig. 3) on the basin margins of the S. and N. Ikarian Basins, implying mean sedimentation rates from 10cm kyr-i (12.5 m 128 kyr-l) in the upper slope to 30 cm kyr - ’ (40 m 128 kyr-‘) in the lower slope. Within the S. and N. Ikarian Basins the thickness of the seismic unit (A,) is up to 60-80 m and the corresponding mean sedimentation rates range from 4.5 cm kyr-’ to 60 cm kyr-‘. The thickness of these sediments is

fairly comparable with sediments of similar age in the N. Aegean sea (20-50 m) (Piper and Perissoratis, 1991). The post-late glacial deposits ( 18 kyr-today) are represented by the transparentsemi-transparent uppermost section (U) of seismic unit (A,) and by the younger (upper) sediment sequences than Y-5 ash layer along seven cores taken from the N. and S. Ikarian Basins (Fig. 5). The thickness of these sediments ranges from 1.2 to 2.5 m (locally 7 m within the basins) and the corresponding values of mean sedimentation rates are estimated from 6 to 14 cm kyr-’ on the basin margins up to 30 cm kyr-’ (locally) within the basins. It is clear that the sedimentation rates display a gradual but significant decrease during the Late Quaternary and are in accordance with the decreasing thickness of the chronostratigraphitally equivalent prograding sequences (P’S_PS,).

Fig. 6. High resolution seismic profiles from the southwestern part of the N. Ikarian Basin. (A) 3.5 kHz profile with (locally) normal faulted Upper Quaternary reflectors. (B) Air-Gun seismic profile illustrating characteristic tilted blocks. These blocks are associated with reverse faulting of the pre-Middle Quaternary reflectors.

J-TLykousis et al.jMarine Geology 128 (1995) 59-71

3.4. Neotectonic deformationalprocesses Intense sediment deformation are observed in seismic profiles from the N. Ikarian Basin (Figs. 6 and 7) an area located within the possible NE-SW strike-slip zone (Mascle and Martin, 1990). In the S. Ikarian Basin, the deformations are relatively weaker and concern exclusively the older (and deeper) (Early Pleistocene-Pliocene) sediment strata (Fig. 2). A series of tilted blocks, is seen in the NNE-SSW striking seismic profile between the N. and S. Ikarian Basin (Fig. 6). These blocks are associated with a sequence of reverse faults which display an increasing fault offset towards the northeast. The en echelon arrangement of these faults implies a transcurrent component during the strike-slip deformations as suggested, also, by Mascle and Martin (1990) from the morphotectonics of the slopes of Ikaria and Samos islands (Fig. 8). These deformational processes could be interpreted as the surficial result

67

of en echelon of transpressional reverse faults with a relatively similar mechanism with that described by Reading (1980) for the San Andreas fault. High resolution profiling (3.5 kHz) from the same area (Fig. 6A) exhibits continuous Late Quaternary stratigraphic reflectors with local deformation resulting from normal rather than reverse faulting. This configuration related to the intensively faulted and tilted Early QuaternaryPliocene continuous and parallel reflectors (Fig. 6B), is an indication of an intense middleEarly Quaternary compressional tectonic phase (associated with strike-slip deformation), that was followed by extensional tectonics in the Middle-Late Quaternary. A further characteristic indication for strike-slip deformation is displayed in a typical seismic profile from the deeper part of N. Ikarian Basin (Fig. 7). A sequence of reverse faults (RF) associated with horsts (H), folded reflectors (F) and deeper chaotic reflectors (CH), implies local.depression and horizontal slip move-

A

W Fig. 7. Air-Gun seismic profile from the N. Ikarian Basin displaying seismic reflectors. RF= reverse faults; F= folds; H= horst; CH= chaotic

intense strike-slip deformations of the Upper reflectors; sub-vertical thin lines = faults.

Quaternary

68

Y Lykousis

25.30’

et al. JMarine Geology 128 (1995) 59-71

26.

2&ci

3

1

Fig. 8. Geotectonic sketch map of the investigated area with the strike-slip region and local compression or extension regime indicated by small thick arrows. Middle-Late Miocene volcanic activity in the surrounding islands is shown by the symbols + or v. The Late-Miocene freshwater lake deposits in Samos I. and marine Pliocene deposits on SE Ikaria I. are indicated as well.

ment. The syn-sedimentary character, up to the sea-bed, of these deformations, indicates constant continuous and persistent deformational activity throughout the (Late) Quaternary. In the S. Ikarian Basin a compressional tectonic regime is documented during the Early-Middle Quaternary by the deformed reflectors of the Pliocene-Early Quaternary seismic sequences (B) and the PS-PS,? (Fig. 2), and by the relatively unaffected prograding sequences Psi-P&. Nevertheless, this compressional phase is not as pronounced, as its stratigraphic equivalent in the N. Ikarian Basin (Fig. 6), as deduced from the limited faulting activity with insignificant vertical displacement. From the Middle Quaternary, con-

tinuous slow subsidence (extension) took place in the S. Ikarian Basin. A pronounced NE-SW horstlike feature, due to the uplift of basement rock (C), was detected below the upper slope of the eastern margin of the Cyclades between the north cape of Naxos I. and the small Chtapodia I. (Figs. 2 and 8). The character of the seismic reflectors (strongly hyperbolic without internal reflectors), the external mounded configuration and their NE-SW direction possibly suggests indurated rock, although it could well correlate with Alpine basement metamorphic rocks and granites exposed on Naxos and Mykonos islands. Granodioritic intrusions of Middle-Late Miocene age are widely spread on the surrounding islands (Patmos I.,

V. Lykousis et aL/Marine

Naxos I., Mykonos I., Chtapodia I., W. Ikaria I.) and are associated with NE-SW structures (faults and thrusts) (Durr et al., 1978; Innocenti et al., 198 1). Late Miocene-Early Pliocene basaltic volcanics are also extended to Patmos I., (Fytikas et al., 1976). The fault trends are organised in two main directions NE-SW and ESE-WSW (Fig. 8). The NE-SW trend is mainly associated with Miocene tectonics and the ESE-WSW principally with Early-Middle Quaternary strike-slip deformations.

3.5. Geotectonic evolution Most of the workers agreed that during the Miocene the Central Aegean was a very shallow domain with extensive emerged regions and small elongated basins, as a consequence of prolonged intense compression (Angelier, 1976, 1979; Mercier et al., 1976, 1979). During the Pliocene the S. Ikarian Basin should be a rather shallow epicontinental sea. Shallow marine Pliocene deposits were found along the southeastern coasts of Ikaria I. (Ktenas, 1969) (Fig. 8). These deposits are intensely faulted, tilted, and overlain unconformably by Middle Quaternary unfaulted deposits with parallel bedding and low-angle dips (Georgalas, 1953). This is indicative of an Early-Middle Quaternary intense compressional phase (related to strike-slip tectonics), an assumption that seems to be compatible with the tilted blocks and the analysis of the high resolution seismic profiles (Fig. 6). This compressional tectonic regime was particularly pronounced within the N. Ikarian relatively to the S. Ikarian Basin. Basin extension and subsidence were established throughout the Middle-Late Quaternary, except probably within the deeper part of the N. Ikarian Basin where strike-slip deformation is still active. Even during the Early Quaternary the S. Ikarian Basin was still a shallow and probably a N-S elongated basin, as evidenced by the occurrence of toplap reflector terminations (TL) of the MiddleEarly Quaternary prograding sequences (PS, and PS,?). This indicates delta progradation in a shallow marine environment without sediment depos-

Geology 128 (1995) 59-71

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ition (by passing) and minor erosion due to current activity (Mitchum et al., 1977). Graual subsidence continued during the lateLate Quaternary with basin extension and size reduction of subaerial surrounding regions.

4. Conclusions Sea-level changes and geotectonic activity have contributed to the Quaternary sedimentary evolution in the eastern part of the Central Aegean Sea. Sea-level fluctuations and tectonic subsidence are the predominant factors controlling the development of the offshore sediment sequences in the S. Ikarian Basin and their associated margins, while geotectonics related to strike-slip deformations are the main factors of the evolution of the N. Ikarian Basin. During the Middle-Late Quaternary, four major prograded delta sequences were constructed on the eastern Cyclades margin (western margin of the S. Ikarian Basin) reflecting four major sea-level lowstands possibly during the oxygen isotopic stages 2, 6 and possibly 12-8, 22-16 maxima. Topset-foreset transitions in the first three prograding sequences, located at ca. 110, 152 and 212 m, respectively, below present sea level, suggest a continuous subsidence, which has been estimated at 0.33-0.57 m kyr-l for the Late Quaternary. Basin sedimentation rates were calculated from characteristic seismic sequences and reflectors and was correlated with sediment cores. The mean sedimentation rates during the Middle-Late Quaternary were estimated to 20-90 cm kyr-’ (250 kyr B.P.-today), lo-60cm kyr-’ (128 kyr B.P.-today) and 6-30 cm kyr-’ for the post-glacial sediments (18 kyr B.P.-today). A compressional tectonic phase during the Middle-Early Quaternary can be recognised within the S. Ikarian Basin and the southern part of N. Ikarian Basin where it was especially pronounced. Within the N. Ikarian Basin indications of Quaternary strike-slip deformations appear that are related with en echelon of transpressional reverse faults. These deformations were probably active throughout the Late Quaternary in the deeper southeastern part of N. Ikarian Basin.

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V. Lykousis et al.IMarine Geology 128 (1995) 59-71

The main geodynamic events may be summarised as follows: (1) Pliocene. The S. Ikarian was a rather shallow epicontinental sea. Marine conditions probably were established following basin extension and subsidence. (2) Early-Middle Quaternary. This was a relatively short but intense compressional episode (due to strike-slip tectonics), particularly pronounced in the N. Ikarian Basin. The Plio-Lower Quaternary strata were tilted and faulted. The S. Ikarian Basin was still a shallow elongated basin. (3) Middle-Late Quaternary. A period with gradual basin extension and subsidence, where the conditions were similar to present. Delta progradation on the margins, and prodelta muddy-silty and/or fine sandy facies within the basins are the predominant sediment infill processes.

Acknowledgements

Professors T. van Andel and H. Got and Dr. D.J.W. Piper are gratefully acknowledged for their constructive and upgrading reviews. Dr. A. Aksu is acknowledged for his critical reviewing, as well. Dr. S. Stavrakakis and K. Chronis are gratefully acknowledged for their significant contribution to computer applications and seismic profiling, respectively, and T. Kostopoulou for typing the manuscript. Finally we thank the officers and crew of the R/V Aeguio for their significant assistance at sea.

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