Temporal variations in sedimentation patterns: NW Aegean Sea

Marine Geology, 43 (1981) M39--M48 M39 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

L e t t e r S e c t i o n

T E M P O R A L V A R I A T I O N S I N S E D I M E N T A T I O N P A T T E R N S : N W A E G E A N S E A

M.B. COLLINS 1 , V. LYKOUSIS 1 and G. FERENTINOS 2

~Department of Oceanography, University College, Wales (United Kingdom) :Department of Geology, University of Patras, Patras (Greece)

(Received July 17, 1980; revised and accepted June 24, 1981)

ABSTRACT

Collins, M.B., Lykousis, V. and Ferentinos, G., 1981. Temporal variations in sedimentation patterns: NW Aegean Sea. Mar. Geol., 43: M39--M48.

Cores are examined from the northern continental slope environment of the Sporadhes Basin, the western part of the North Aegean Sea Trough. Three distinct lithofacies have been identified, which correspond with similar environments identified elsewhere in the western and southeastern Mediterranean. These layers, turbiditic mud --hemipelagic mud -~ calcareous ooze, represent late Quaternary climatic and eustatic sea level changes and tectonic activity.

Based on benthic foraminiferal evidence from the eastern part of the North Aegean Sea Trough, it is suggested that contemporary sediment supply and physical oceanographic condit ions in the area under investigation might be conducive to sapropel formation; also, that the depositional sequence in the cores represents a gradual recovery in the Mediter- ranean Sea, since its last stagnation and sapropel formation some 8000 years ago.

INTRODUCTION AND METHODS

T h e g r a v i t y c o r e l o c a t i o n s a re p r e s e n t e d in F ig . 1, in a s s o c a t i o n w i t h t h e r e c e n t l y d e r i v e d b a t h y m e t r y ( L y k o u s i s e t al . , 1 9 8 1 , f ig. 3) . S a m p l i n g was c a r r i e d o u t f r o m R R S Shack le ton , Cru i se 7 / 7 8 , w i t h p o s i t i o n f i x i n g b y sa te l - l i t e a n d r a d a r . C o r e l e n g t h s r a n g e d f r o m 1 .93 t o 2 . 4 0 m , c o r r e s p o n d i n g w i t h w a t e r d e p t h s o f 4 5 0 t o 6 4 0 m .

C o r e s w e r e s p l i t a n d t h e n X - r a y e d , u s i n g a H e w i t t P a c k a r d 4 3 8 0 x s y s t e m . B a s e d o n t h e r a d i o g r a p h s , a t o t a l o f 59 s a m p l e s w e r e s e l e c t e d f o r p a r t i c l e s ize a n a l y s i s a n d d e t e r m i n a t i o n o f c a r b o n a t e c o n t e n t (F ig . 2) a n d t h e b i n o c u l a r a n d S E M m i c r o s c o p i c e x a m i n a t i o n o f t h e s a n d a n d c l a y ( S E M ) f r a c t i o n s , r e s p e c t i v e l y .

C o m p o s i t i o n a l a n d t e x t u r a l c h a r a c t e r i s t i c s o f l i t h o f a c i e s w i t h i n t h e c o r e s a re c o m p a r e d w i t h t h e s e d i m e n t a r y c h a r a c t e r i s t i c s o f e x t e n s i v e c o r e s a m p l i n g p r o g r a m m e s f r o m t h e w e s t e r n a n d s o u t h e a s t e r n M e d i t e r r a n e a n .

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R E S U L T S

Visually all the cores appeared muddy and homogenous. Colour ranged from yellowish-brown or brown at the top, through greenish-grey, to olive green at the base. In general, passing downwards throughout each of the cores, the relative amount of coccoliths within the sand fraction and the clay and calcium carbonate c o n ~ t s decreased; conversely, the silt content in- creased (Fig. 2).

Based on the interpretation of the results of compositional, textural and X-ray analyses, three distinct layers have been identified as follows (with

M41

mean percentage values presented): (a) L a y e r (1): upper surface down to 45--50 cm. Characterised by low

sand (2.4% mean) and silt (36%), and high clay (62%) and CaCO3 (approximately 30%) contents. The layer is generally lacking in any sedimentary structure.

The sand fraction is entirely biogenous, consisting of planktonic and benthonic foraminifera, p teropod tests, shell fragments and echinoid spicules.

The silt/clay ratio is low within this sediment type, at 0.58. The clay fraction consists mainly of clay aggregates and a relatively high proportion of coccoliths, dominated by Ernil iania h u x l e y i .

(b) L a y e r (2): from approximately 45--50 cm down to 130--135 cm. Characterised by lower sand (1.7%), higher silt (39%), lower clay (59%) and carbonate content (27%) than the overlying Layer (1). The layer is without any primary structure, but with relatively well-developed signs of bioturbation, together with pyritised filaments and tubes. In some of the cores, at aroud 65--90 cm, there is a faint lamination; this is high in carbonate (31--40%) and clay (60--66%) contents, but low in silt (32--37%).

The sand fraction consists mainly of planktonic and benthonic foraminifera, p teropod tests, shell fragments and echinoid spicules.

The silt/clay ratio is slightly higher (0.66) within this layer than in the Layer (1), with a reduction in the amount of coccolithis within the clay fraction.

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The layer is fine-grained (7.25 ~ mean), highly sorted and greenish-grey in colour.

(c) Layer (3): from 130--135 cm to the bot tom of the cores. Characterised by low sand (1.7%), high silt (47%), relatively low clay (50%) and very low carbonate (17%) content. The layer is characterised by thin (< 1 cm thick) and delicate laminations, by the absence of bioturbation and by the parallel nature of the laminations of pyrite filaments and pyrite congregates.

The sand fraction consists of biogenic material, mainly foraminifera, inorganic terrigenous material and sand-sized pyrite aggregates. The terrigenous material is more pronounced towards the bot tom of the cores.

The silt/clay ratio within the layer is exceptionally high at 0.94. In one of the cores (NA/78/39), two silt turbidite layers occur, wherein

the silt fraction is high (62% and 70%) and the sand fraction is relatively high (3% and 5%). Other textural and compositional characteristics of these silt layers are similar to those of the rest of Layer (3).

Relative to the overlying Layers (1) and (2), the mud fraction is coarser {6.85 ¢ mean) and the material is poorly sorted or very poorly sorted: it is olive grey in colour.

INTERPRETATION AND DISCUSSION

The overall sedimentary characteristics of the cores appear to represent deep-water marine conditions in their upper part, changing to periods of increasing influence of terrigenous sediment supply in their lower section. This cycle of events can be considered within the context of regional patterns of sedimentation within the Mediterranean, based on comparison of the identified lithofacies with core descriptions presented elsewhere.

Layer (1), with biogenous sand and high clay content represents a near- surface expression of the "outer margin muds (Province 3)" identified in the distribution of modern sediments over the area (Lykousis et al., 1981). Such sediments reflect contemporary sediment transport processes over the region and have been explained in terms of biogenic productivity, a dominant supply of fine-grained terrigenous material and the presence of an anticlockwise surface and sub-surface water circulation pattern (Lykousis et al., 1981).

Due to the high coccolith composition of the muds and their entirely biogenous sand fraction, this sedimentary province could be termed "calcareous (coccolith) ooze". Such a classification compares with the "coccolithic-foraminiferal calcareous" sediment of this area (Emelyanov, 1972), described on the basis of the analysis of a single sample just outside the area under investigation, and the "coccolith ooze" of the Black Sea (Ross and Degens, 1974). Calcareous ooze layers have been identified in cyclothem patterns of sedi- mentat ion (Maldonado and Stanley, 1.976) in cores from the Nile Cone -- Levantine Sea, on the basis of detailed analysis of the lithostratigraphic record in the southeastern Mediterranean (Stanley and Maldonado, 1979). The uppermost of these layers (C1), less than 10 cm in thickness (table III, Stanley and

M44

Maldonado, 1979), represents the last phase in a cycle of sedimentation. This cycle, Cycle 1 (Late Wisconsin to Holocene, about 17,000 yr B.P. to Present~ coincides with a general warming in the palaeoclimatic trend and an associated rise in sea level. Although the carbonate content of the calcareous ooze of the NW Aegean Sea (approximately 30%) is less than that of the southwestern Mediterranean (up to 70%), it is quite possible that sediments of the latter area contain terrigenous carbonate. Consequently, the sediment types might be directly comparable, in terms of depositional processes, between the two areas. For the southeastern Mediterranean, such sediments have been interpreted as being representative of suspensite deposits of the region related to "fine-grained sediment input, biogenic product ion and the flow patterns and degree of stratification of water masses" (Stanley and Maldonado, 1979). Such conditions are analogous with the contemporary patterns observed over the area under investigation, with outer margin muds (Province 3, Lykousis et al., 1981} being deposited within a lowered salinity surface water circulation pattern.

Layer (2), with an increasing silt/clay ratio (0.66) in relation to the overlying calcareous ooze {0.58), is also lower in its carbonate content; it is vir- tually uniform and homogeneous in texture. Such uniformity, with only few laminations, is characteristic of hemipelagic muds (see footnote on p. 3! , Rupke and Stanley, 1974) as have been identified elsewhere in cores from the Alg6ro--Balearic Basin, western Mediterranean (Type B Muds: Rupke and Stanley, 1974), from the southeastern Mediterranean (Maldonado and Stanley, 1976; Stanley and Maldonado, 1979) and the Strait of Sicily (Mal- donado and Stanley, 1977). These mud deposits, with high clay content and very poor sorting "exclude any normal bo t tom current emplacement" and are considered to be "typical of autochthonous deepwater accumulations". The almost perfect correlation between the age and thickness of the hemipelagic mud layers in the western Mediterranean is considered to be in accor- dance with a cont inuous pelagic settling process, with a sedimentation rate (10 cm/1000 years) which was constant throughout all cores recovered from the Balaeric Abyssal Plain. Further, this correlation indicates that no signifi- cant erosion of these layers by bo t tom or turbidity currents has occured.

As with the calcareous ooze, hemipelagic mud forms integral parts of the cyclothems identified for the southeastern Mediterranean (Maldonado and Stanley, 1976}. Within the generalised lithostratigraphic sections for this region (fig. 5, Stanley and Maldonado, 1979), the deposit identified in the NW Aegean lies within the upper port ion of Cycle 1, underlying the calcareous ooze. The suspensite core assemblage trends record enhanced deposition during this phase of accumulation, consistent with a continuous settling process.

Finally, the sedimentary characteristics of Layer (3), with its delicate laminations in the X-ray photographs, is considered to represent a turbiditic

M45

m u d deposit. Such deposits have been identified in cores from the western Mediterranean (Type A Muds: Rupke and Stanley, 1974), southeastern Mediterranean (Stanley and Maldonado, 1979) and the Strait of Sicily (Mal- donado and Stanley, 1977). The overall thickness of the turbiditic mud layer in the western Mediterranean has been found to be greater in cores at greater water depth and in areas with access to fluvial sediment sources. The inverse relationship between thickness of deposit and water depth is thought to reflect deposition from turbidity currents; this "is to be expected where turbidity currents seek the lowest parts of the basin plain". Conversely, for the same area, an inverse relationship between thickness and water depth was found for the hemipelagic muds described previously. The turbiditic muds here are "likely to be a distal portion of a turbidite body, deposited by the slowly moving, or ponded, less dense tail of a normal, high-density turbidity current" (Rupke and Stanley, 1974). It is also suggested that the triggering mechanism for such deposits in the western Mediterranean could be agitation of fine material on the shelf break during storms. In the NW Aegean, tectonic ac tivity could be an additional mechanism (see Brooks and Ferentinos, in press). PDR and continuous seismic profiling data from the region suggested already that gravity-induced deformation of the postorogenic sedimentary sequence has taken place. The uppermost sedimentary layers are deformed by several gravity-induced processes including slumping, sliding and creep contempora- neous with sediment deposition. Earthquakes associated with the regional tectonism may act as a triggering mechanism for the slides and slumps (Ferentinos et al., in press).

CONCLUDING REMARKS

Depositional processes over the Thermaicos Gulf/Sporades Basin, NW Aegean Sea, can be identified in terms of a vertical sequence of lithofacies. These lithofacies compare directly with those identified in other areas of the Mediterranean Sea and have been identified as turbiditic mud -- hemipelagic mud -- calcareous ooze. Such a sequence of deposits reflects a general de- crease in gravitite and increase in suspensite mechanisms of deposition, coin- ciding with a eustatic rise in sea level. Within the context of cyclothems identified in the southern Mediterranean, the lithofacies here coincide with the upper part of Cycle I (17,000 yr. B.P. to Present: Late Wisconsin to Holocene).

Modern sedimentation patterns in this area of the NW Aegean Sea have been shown to be strongly influenced by terrigenous sediment inputs, from rivers draining into Thermaicos Bay, to the north, and from the River Pinois to the west. Such a terrigenous influence on offshore deposits may be compared with that associated with the River Nile. Hence, based on comparison with an idealised Nile Cone - -Levant ine Sea lithostratigraphic section (fig. 2; Stanley and Maldonado, 1979) and in the absence of radiocarbon dating, the calcareous ooze would appear to have been deposited between 2700 yr.B.P.

M46

and the present day, the hempelagic mud between approximately 5700 yr. B.P. and 2700 yr. B.P., and the turbiditic muds at around 5700 yr. B.P. Be- fore 5700 yr. B.P., when sea level was approximately 10 m below the present level, precipitation over the Mediterranean land masses was appreciably higher than at present (Fairbridge, 1972), so that sediment supply from river waters would have been greater.

A further influence of terrigenous sediments supply is seen in core NA/78/39 from the western part of the slope. Here, the calcareous ooze is absent and there are two distinct silt layers within the turbiditic mud. In- creased terrigenous sediment supply, together with bot tom current activity at this station, as observed in underwater photographs (Lykousis, 1979), could explain this variation.

Within the NW Aegean Sea cores, the upper sapropelic layer ($1) observed elsewhere, in the Ionian Sea (Stanley, 1978) and the Southern Aegean (Pas- touret, 1970), was not present; however, the turbiditic mud layer at the bottom of the cores is considered to have commenced its deposition after that of the S~ layer, at around 7000--9000 yr B.P. In terms of absolute core length, the upper sapropel layers have been observed at between 20 cm and 120 cm from the top of the cores in the Southern Aegean (Pastouret, 1970) and at approx. 2 m in a generalised gravitite assemblage core from the Nile Core region of the eastern Mediterranean (Cycle 1 in fig. 5, Stanley and Mal- donado, 1979). The latter can be compared with a depth of 40 cm in a deep basin of the eastern Mediterranean away from a terrigenous supply of silt and clay (Mullineaux and Lohman, 1981). These depths confirm that, in the area under investigation which is one of high fluvial sediment supply, the upper sapropel layer might be expected to occur below the present length of core recovery.

The overall rate of sedimentation in the cores, assuming that the bases are dated at 7000 yr. B.P., is 20 to 34 cm/1000 yr. This can be compared with a sedimentation rate in the western Mediterranean, for total sediment thickness, of 33 cm/1000 yr.

In general, information relating to sediment deposition in the northwest- ern Aegean Sea has been restricted to the description of contemporary processes (Lykousis et al., 1981} and the recovery of some deep-sea cores from the Sporadhes (Trough) Basin during the 1972 cruise of the Jean Charcot (Needham et al., 1973). The preliminary results described for the latter re- vealed that, below 2 m to 4 m within cores from the basin, there was a strong terrigenous influence, represented by several layers containing up to 40% of coarse inorganic detritus. The importance of the supply of terrigenous material is emphasised by the results of the present study. Further, some evidence is provided to support the idealised "Aegean Sea" core sequence presented recently by Stanley and Blanpied (1980).

Interestingly, the surface water circulation over the area under investigation consists of an anticlockwise water gyre, incorporating freshwater and less-saline waters from the Rivers Axios and Aliakmon in Thermaicos Bay

M47

and freshwater discharge from the River Pinois (see Fig. 2, Lykousis et al., 1981). This circulation pat tern extends to a depth of 50--100 m. Water masses between 100 and 350 m appear to follow the surface circulation, with a level of "no mo t ion" at depths > 350--500 m (Sultan, 1981). Simi- lar, although a great deal more extreme, oceanographic condit ions exist at the eastern end of the North Aegean Sea Trough, where density stratifica- t ion is caused by freshwater input f rom the Black Sea. The significance of this assocation is that the latter condit ions conform with those considered to be required for the format ion of sapropels (Stanley, 1978). Indeed, the benthic foraminiferal assemblages present in modern sediments of the deep basin at the eastern end of the North Aegean Sea Trough are considered those to be most closely associated with a sapropel layer in a core from the Levantine Basin, eastern Mediterranean. Faunal changes are also thought to be associated with stagnations lasting more than 10,000 years (Mullineaux and Lohmann, 1981). This correlation infers that contemporary conditions here are conductive to sapropel formation and, further, that the cores from the NW Aegean, if underlain by a sapropel layer, reflect sediment deposits representing the recovery of the eastern Mediterranean " f rom its last stagnation about 8000 years ago" (Mullineaux and Lohmann, 1981).

ACKNOWLEDGEMENTS

This s tudy was supported by the Natural Environment Research Council. The cooperat ion and assistance provided by the officers and crew of the R.R.S. "Shack le ton" (Cruise 7/78) are gratefully acknowledged, together with the technical assistance provided by Messrs. I. Chivers, S. Jones and P. Taylor.

Dr. H.F. Shaw (Geology Department , Imperial College, London) and Mr. S. Phethean (University College, London) are warmly thanked for their guid- ance in the use of the X-ray radiography and Dr. D.K. Harris for bringing to our a t tent ion the research on benthic foraminifera.

Miss Tracey Brewer, Mr. K. Naylor and Mrs. A. Carr are thanked for their typing, drawing and reprographic contributions, respectively. Finally, one of the authors (Lykousis) was in receipt of a Schilidzi Foundat ion Studentship. S.J. Wakefield kindly read through the manuscript.

REFERENCES

Brooks, M. and Ferentinos, G., in press. Structure and evolution of the Sporadhes Basin of the North Aegean Trough, Northern Aegean Sea. Tectonophysics.

Emelyanov, E.M., 1972. Principal types of recent bottom sediments in the Mediterranean Sea: their mineralogy and geochemistry. In: D.J. Stanley (Editor), The Mediterranean Sea: A Natural Sedimentation Laboratory. Dowden, Hutchinson and Ross, Strouds- burg, Pa.

Fairbridge, R.W., 1972. Quaternary sedimentation in the Mediterranean Region controlled by tectonics, palaeoclimates and sea level. In: D.J. Stanley The Mediterranean Sea: A Natural Sedimentation Laboratory. Dowden, Hutchinson and Ross, Stroudsbourg, Pa.

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Ferentinos, G., Brooks, M. and Collins, M.B., in press. Gravity-induced deformation on the north flank and floor of the Sporadhes Basin of the north Aegean Sea Trough. Mar. Geol.

Lykousis, V., 1979. Aspects of the Modern and Holocene Sedimentation of the N.W. Aegean Sea. M.Sc. Thesis, University of Wales, 108 pp. (unpubl.).

Lykousis, V., Collins, M.B. and Ferentinos, G., 1981. Modern sedimentation in the N.W. Aegean Sea. Mar. Geol., 43::111~--130.

Maldonado, A. and Stanley, D.J., 1976. The Nile Cone: Submarine Fan Development by Cyclic Sedimentation. Mar. Geol., 20 : 27--40.

Maldonado, A. and Stanley, D.J., 1977. Lithofacies as function of depth in the Strait of Sicily. Geology, 5: 111--117.

Mullineaux, L.S. and Lohmann, G.P., 1981. Late Quaternary stagnations and recirculation of the Eastern Mediterranean: Changes in the deep water recorded by fossil benthic foraminifera. J. Foraminiferal Res., 11(1): 20--39,

Needham, H.D., Le Pichon, X., Melguen, M., Pautot, G., Renardy, V., Avedik, F. and Carre, D., 1973. North Aegean Sea Trough: 1972 Jean Charcot Cruise. Bull. Geol. Soc. Greece, 10: 152--153.

Pastouret, L., 1970. Etude sedimentologique et Pal6oclimatique de carottes Pr61ev6es en Mediterann6e Orientale. Tethys, 2 ( 1): 227--266.

Ross, D.A. and Degens, E.T., 1974. In: The Black Sea -- Geology, Chemistry and Biology. Am. Assoc. Pet. Geol. Mere., 20: 183--199.

Rupke, N.A. and Stanley, D.J., 1974. Distinctive Properties of Turbiditic and Hemipelagic Mud Layers in the Alg~ro Balearic Basin, Western Mediterranean Sea. Smithsonian Contribution to the Earth Sciences, 13, Smithsonian Institution Press, 40 pp.

Stanley, D.J., 1978. Ionian Sea Sapropel distribution and late Quaternary Palaeoocean- ography in the eastern Mediterranean. Nature, 274: 149--152.

Stanley, D.J. and Blanpied, C., 1980. Late Quaternary water exchange between the eastern Mediterranean and the Black Sea. Nature, 285: 537--541.

Stanley, D.J. and Maldonado, A., 1979. Levantine Sea--Nile Cone lithostratigraphic evolution: Quantitative analysis and correlation with palaeoclimatic and eustatic oscilla- tions in the Late Quaternary. Sediment. Geol., 23: 37 -65 .

Sultan, S.A.R., 1981. Water Masses and Circulation in the N.W. Aegean Sea. Ph.D. Thesis, University of Wales {in prep.).

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Temporal variations in sedimentation patterns: NW Aegean Sea