Aegean Sea ridge barrier-and-basin sedimentation patterns

Marine Geology, 24 (1977) 97--107 ©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

AEGEAN SEA RIDGE BARRIER-AND-BASIN SEDIMENTATION PATTERNS

DANIEL JEAN STANLEY and CONSTANTINE PERISSORATIS Smithsonian Institution, Washington, D.C. (U.S.A.) Institute of Geological and Mining Research, Athens (Greece)

(Received September 8, 1976; accepted December 9, 1976)

ABSTRACT Stanley, D.J. and Perissoratis, C., 1977. Aegean Sea ridge barrier-and-basin sedimentation patterns. Mar. Geol., 24: 97--107. An isopach map of the Aegean Sea, based on a regionally comprehensive seismic-profile network, reveals the highly irregular distribution of unconsolidated sediments of postMiocene age. Geologically recent structural activity has considerably modified the seafloor configuration of the north and south Aegean, and depositional patterns are to varying degree related to the complex Aegean physiography. From north to south, a series of ridges, islands, and plateaus have acted as effective barriers behind which sediments are trapped, primarily in depressions. Sources of sediment in the north Aegean troughs and basins include rivers and suspensate-rich water masses; material was also provided by the erosion of plateaus and ridges during phases of Pliocene uplift and regressions and Quaternary eustatic low sea-level stands when the Aegean became virtually land-locked and isolated from the Black Sea, Volcanic as well as terrigenous material has accumulated in the central and south Aegean. However, the rapid depositional rates in the south Aegean are more closely related to the flow exchange with the Levantine Basin and the consequent ponding of material behind the Peloponnesus--Crete and Crete--Rhodes ridges. INTRODUCTION T h e Aegean Sea in the eastern M e d i t e r r a n e a n is an e p i c o n t i n e n t a l sea o c c u p y i n g a s u b s i d e d p o r t i o n o f the Alpine chain which, until r e c e n t geologic time, u n i t e d the Hellenides and T a u r u s orogenic belts. R e c e n t b a t h y m e t r i c and p h y s i o g r a p h i c charts s h o w the e x t r e m e l y c o m p l e x n a t u r e o f the Aegean Sea (Maley and J o h n s o n , 1971; C a r t e r e t al., 1972; A n o n y m o u s , 1 9 7 5 ; Morelli et al., 1975). Earlier g e o p h y s i c a l studies suggest t h a t p h y s i o g r a p h y and t e c t o n i c s in the Aegean are closely i n t e r r e l a t e d . E v i d e n c e o f geologically r e c e n t and intense s t r u c t u r a l d i s p l a c e m e n t is p r o v i d e d b y p a t t e r n s o f seismicity ( P a p a z a c h o s , 1974), gravity a n d m a g n e t i c s (Allan a n d Morelli, 1971; Makris, 1973), a n d v o l c a n i s m (Pe a n d Piper, 1972). H o w e v e r , little is k n o w n o f the r e l a t i o n b e t w e e n p h y s i o g r a p h y and s e d i m e n t a t i o n . With the e x c e p t i o n o f the area n o r t h o f C r e t e (Wong e t al., 1971; J o n g s m a et al., 1976) t h e r e h a v e been n o

98 published seismic profiles or s u b b o t t o m sediment distribution charts of the Aegean. This note considers the distribution and thickness of unconsolidated sediments in the Aegean based on recent data, and evaluates the degree of correlation between post-Miocene depositional patterns and physiography. METHODS A series of deep-penetration continuous seismic profiles (csp) covering much of the Aegean (airgun and sleeve exploder energy source, M/V "Seismic Explorer", 1972, 1974) was made available to us by the Institute of Geological and Mining Research of Greece. Additional deep-penetration profiles (Flexotir, M/S " L a d y Diana", 1969) covering the Thermaikon Plateau, as well as higher-resolution profiles in the south Aegean (airgun, F.F.S. "Meteor", 1974, and R.R.S. "Shackleton", 1974, cf. Jongsma et al., 1976) and west of the island of Karpathos (R/V "Chain" profile 5, Wong et al., 1971) also were used for this study. The position of these seismic lines is shown on Fig. 1. The unconsolidated sediment cover is defined as a stratified band of reflectors overlying acoustically less-well defined series; the contact between unconsolidated and consolidated sediment in the csp is frequently unconformable, particularly on ridges and plateaus. This contact has been designated event A on airgun profiles in the south Aegean by Jongsma et al. (1976) and, in places, may correspond to the top of the M reflectors (Messinian-Lower Pliocene boundary as defined by Ryan et al., 1971, and others).. In the north and central Aegean, we correlate the base of the unconsolidated series identified on deep-penetration "Seismic Explorer" profiles with this event A marker. However, the age of the unconformity separating the unconsolidated and consolidated series probably varies from region to region. Stratigraphic correlation is also established with several drill sites, i.e., JOIDES leg 42A site 378 in the Cretan Trough (Hsi] et al., 1975), and petroleum drill sites (Limnos 1) in the north Aegean (Fig. 2). The upper unconsolidated sequence, which in many sectors includes both Pliocene and Quaternary terrigenous series, is assigned an average velocity of 2000 m/sec.; it probably is in large part equivalent to the s u b b o t t o m series termed Q - P by Morelli (1975) and others elsewhere in the eastern Mediterranean. A geometric scale (< 0.25, 0.25--0.50, 0.50--1.0 and > 1 . 0 km), used to depict the unconsolidated sediment thickness, emphasizes the regional depositional trends in Fig.2; generalized data are listed in Table I. The Aegean Sea is divided into north, central and south sectors based on both physiographic and depositional criteria. Sedimentation thicknesses are evaluated in the three major environments characteristic of these three zones: plateau (flat-topped sea floor of considerable extent), ridge (long, narrow elevation with steep sides and irregular topography) and basin (depression of more or less equidimensional form) or trough (long depression). Some of the geographic names in Fig.2 are used by MaJey and Johnson (1971) and Morelli et al. (1975); some new names are applied here (see Table I and Fig.2).

99 TABLE I Approximate thickness of the unconsolidated sediment cover on the major submarine physiographic features in the Aegean Sea (see Fig.2). Physiographic regions

Approximate area (km 2)

Thickness of unconsolidated sediment section

North Aegean Thermaikon Plateau Thasos--Samothraki Plateau North Aegean Trough Sporadhes--Limnos Ridge Limnos--Imroz Plateau North Skiros Basin Skiros--Lesvos Ridge South Skiros Basin Evvoia--Lesvos Ridge

4300 8400 15,200 1300 8300 3100 1200 1000 1700

< 300 < 350 100--1100 < 350 < 250 < 350 350 100--700 < 400

Central Aegean Lesvos Basin Kalogeroi Basin Andros--Khios Ridge North Ikarian Basin South Ikarian Basin Samos--Kos Plateau Cyclades Plateau Amorgos--Leros Ridge

600 2000 1100 3900 1000 4500 20,500 700

350--600 100--800 < 400 100--700 100--700 50--350 100--550 < 200

South Aegean Myrtoon Basin Argolikon Basin South Aegean Volcanic Arc Cretan Trough Peloponnesus--Crete Ridge Crete--Rhodes Ridge

3400 3900 -13,500 1500 1800

100--1000 100--1000 -100--1200 100--300 100--500

OBSERVATIONS

North Aegean In this sector, c o m p r i s e d b e t w e e n T h r a c e - - M a c e d o n i a and the Evvoia-Lesvos Ridge, the t h i c k e s t s e d i m e n t a c c u m u l a t i o n s ( > 1 0 0 0 m) o c c u r in the N o r t h Aegean T r o u g h (cf. N e e d h a m et al., 1 9 7 3 ) and the N o r t h Skiros Basin. The S o u t h Skiros Basin also includes a t h i c k s e d i m e n t p o n d . The csp r e c o r d s reveal acoustically well-stratified sections; these are a l m o s t always t r u n c a t e d at basin margins. Thus, isopach c o n t o u r s in depressions c o n f o r m r a t h e r closely with the s u b m a r i n e t o p o g r a p h y . In c o n t r a s t , the three plateaus ( T h e r m a i k o n , T h a s o s - - S a m o t h r a k i , and L i m n o s - - I m r o z ) are c o v e r e d b y t h i n n e r sequences o f u n c o n s o l i d a t e d s e d i m e n t (for the m o s t p a r t < 250 m); these tend to thin landward. S e d i m e n t on the ridges ( S p o r a d h e s - - L i m n o s ,

100 Skiros--Lesvos, Evvoia--Lesvos) are intermediate in thickness (to 350 m) and vary laterally.

Central Aegean This sector, lying between the Evvoia--Lesvos Ridge and the volcanic arc in the south Aegean {position of volcanic centers shown on Figs.1 and 2), includes the broad Cyclades Plateau (Aseismic Central Plateau of Maley and Johnson, 1971). The central Aegean is generally shallower and morphologically less complex than either the north or south Aegean. The sediment thickness is regionally variable and depositional trends are n o t as closely conformable with topography as in the north Aegean, although the thickest section (to 800 m) occur in basins (Lesvos, Kalogeroi, North Ikarian, and South Ikarian). The two plateaus (Cyclades and Samos--Kos) are covered, for the most part, by a thin sedimentary cover {about 250 m); unconsolidated deposits on ridges (Andros--Khios and Amorgos--Leros) are more variable in thickness (100--400 m).

South Aegean This arcuate region, enclosed between the volcanic arc (Fig.l) and the Peloponnesus--Crete and Crete--Rhodes ridges, includes the deepest depressions in the Aegean ( > 2 5 0 0 m west of Karpathos). As elsewhere in this sea, the thickest unconsolidated sediment accumulation occurs in depressions. In the deepest of these, the Cretan Trough, the depositional trends do n o t everywhere conform closely with topography. Recent faults have offset both unconsolidated and consolidated series within, as well as at the basin margins. The thickest section, a b o u t 1200 m, occurs north of eastern Crete and west of Karpathos Island; another {about 1000 m) is also present north of central Crete. Deposits in the Myrtoon and Argolikon basins exceed 1000 m. Sediment is thicker (to 500 m) on the Crete--Rhodes Ridge than on the Peloponnesus--Crete Ridge (to 300 m) to the west. Sediment thickness increases markedly northward of these two ridges. The csp show that thickening north of the Hellenic island arc is in part related to vertical displacement by post-Miocene faults (Jongsma et al., 1976). DISCUSSION The distribution of Plio-Quaternary deposits actually tends to conform in a general way with topography, i.e., thick in depressions and thin on highs such as ridges and plateaus which have acted as partial barriers to sediment dispersal. Thus, alternations of thick elongate and more irregular thin sediment accumulations occur from north to south. However, some anomalies are observed particularly in the central and south Aegean. Seismic profiles clearly show the role of structural displacement on the configuration of unconsolidated section and underlying series. Sediments generally thin

Fig.1. Chart of the Aegean Sea showing isopach map. Some of the major basins Greek I.G.M.R. chart (1975).

position of seismic lines used to compile the are highlighted. Depth in meters. Modified from

220

ad 230

Id

240 30’

25’

30‘

LO.-+

-.-

26O

30’

270

EUROPEAN TURKEY

MI

30’

26*

__ ... .: T.:.,::;

NOR

(ANADOLU)

LEGEND

Fig.2.

Isopach

of unconsolidated

deposits

in the Aegean

Sea. Basin contours

are emphasized.

\

1 i i i

105

abruptly between basins and topographic highs (ridges and plateaus) as a result of truncation by faulting, or pinch-out, or both. It is apparent that sedimentation per se in the Pliocene and Quaternary has played a lesser role than tectonics in modifying and smoothing the physiography of the Aegean (Wong et al., 1971; Stanley, 1974, 1976; Morelli et al., 1975). Fluvial apport, however, has been important insofar as the north Aegean is concerned where large rivers of Thrace (Evros), Macedonia (Nestos, Strimon, Axios and Aliakmon) and Thessaly (Pinios) have significantly contributed to the fill of the North Aegean Trough. This supply was partially dammed behind the Sporadhes--Limnos Ridge. Some sediment also has been transferred southward across topographic lows of the Sporadhes-Limnos Ridge; this by-passing accounts for thick sections in the North Skiros Basin. Another important sediment source is the erosion of large areas around the islands of Sporadhes, Ag. Eustratios and Limnos, particularly during phases of uplift and subaerial exposure in the Pliocene and Quaternary. Suspensates from the Black Sea entering via the Dardanelles, and to a lesser extent from the Menderes River in northwest Turkey, also may account for some of the post-Miocene sedimentary cover of the Limnos--Imroz Plateau and adjacent north Aegean basins. The thick section in the South Skiros Basin may record the erosion of the adjacent Evvoia and Skiros islands. Large volumes, fed by the Sperkhios River into the northwest Aegean, have been trapped behind E w o i a Island, although some of this material presumably also was distributed further seaward by water masses flowing in the North and South Skiros basins. The fill of the central Aegean basins (Lesvos, Kalogeroi, North Ikarian, South Ikarian), somewhat reduced compared to those in depressions to the north, has been derived from the erosion of large areas (Sterea Hellas, Cyclades Plateau, islands of eastern Aegean, borderland off Turkey) exposed during periods of marked uplift and regression in the Pliocene (Keraudren, 1975) and Quaternary low sea-level stands. Large volumes carried by the Sperkhios River were trapped behind Evvoia Island although sediments from this source probably were distributed further seaward into the central Aegean. Terrigenous materials supplied by rivers flowing from Turkey (Gediz Nehri, Bi]ji]kmenderes Nehri) were deposited in embayments off Turkey and also contributed to the fill of the Lesvos and South Ikarian Basin. Volcanics were supplied by the arcuate belt of south Aegean volcanoes (Fig.2). The greater uniformity of sediment thickness in this region is explained by the more gentle configuration of the central Aegean and less effective damming by ridges. In contrast, sediment thickness is more variable in the south Aegean and more closely follows structural trends. The particularly thick sediment section in the Myrtoon Basin was derived from the Peloponnesus and Athenian--Cyclades Plateau which became subaerially exposed during periods of Pliocene regressions and more recent glacial eustatic low stands. Erosion of the Cyclades Plateau and volcanic supply from the adjacent volcanic islands of Santorini and Milos contributed particularly large volumes to the shallower

106

northwestern sector of the Cretan Trough. The thick fill of the Argolikon Basin ( > 1 0 0 0 m) and Cretan Trough ( > 1 2 0 0 m) which are bound by the Cyclades Plateau to the north and the Peloponnesus--Crete and Crete--Rhodes ridges to the south, probably received much sediment from both surface and deep water flow between the Aegean and the Levantine Basin (Moskalenko and Ovchinnikov, 1965). An average sedimentation rate of 20 c m / 1 0 0 0 years is calculated if it is assumed that the unconsolidated sections in basins are as old as Lower Pliocene (dated at about 5.0 m.y.; cf. Berggren, 1973). This corresponds well with a rate measured in the Cretan Trough (Opdyke et al., 1972). In the south Aegean we see the particularly effective role of the ridges forming the Hellenic Arc which have acted as effective barriers and dammed material north of the arc. There are some thick sediment pods trending normal to the Hellenic Arc (see for example belt extending NNE of Antikithira Island) which do not follow the general tectonic trend but reflect major dispersal patterns across the Arc. The thicker sediment cover on the Crete--Rhodes Ridge is attributed to the greater depth of this feature and more pronounced deposition from suspensate-rich flows in this sector. ACKNOWLE DGEMENTS

We thank Dr. G. Machairas, Institute of Geological and Mining Research of Greece, for providing unpublished Aegean seismic data. This study, part of the Mediterranean Basin (MEDIBA) Project, was partially funded by Smithsonian Research Foundation grant 460132. REFERENCES Allan, T.D. and Morelli, C., 1971. A geophysical study of the Mediterranean Sea. Boll. Geofis. Teor. Appl., 13: 99--142. Anonymous, 1975. Bathymetric map of Aegean Sea. Institute of Geological and Mining Research, Athens Greece (scale 1:1,000,000). Berggren, W.A., 1973. Biostratigraphy and biochronology of the Late Miocene (Tortonian and Messinian) of the Mediterranean. In: C.W. Drooger (Editor), Messinian Events in the Mediterranean. North-Holland, Amsterdam, pp. 10--20. Carter, G.T., Flanagan P.J. et al., 1972. A new bathymetric chart and physiography of the Mediterranean Sea. In: D.J. Stanley (Editor), The Mediterranean Sea -- A Natural Sedimentation Laboratory. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp.1--23. Hsfi, T.K., Montadert, L. et al., 1975. Glomar Challenger returns to the Mediterranean Sea. Geotimes, 20: 16--18. Jongsma, D., Wissmann, G., Hinz., K. and Garde, S., 1976. The southern Aegean Sea: an extentional marginal basin without spreading? Meteor-Forschungsergeb., Reihe C., in press. Keraudren B., 1975. Essai de stratigraphie et de pal~og~ographie du Plio-P16istoc~ne ~g~en. Bull. Soc. G~ol. Fr., 17: 1110--1120. Makris, J., 1973. Some geophysical aspects of the evolution of the Hellenides. Bull. Geol. Soc. Greece, 10: 206--212. Maley, T.S. and Johnson, G.L., 1971. Morphology and structure of the Aegean Sea. DeepSea Res., 18: 109--122.

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