Evros delta: Evolution of continental shelf sediments

Evros delta: Evolution of continental shelf sediments

Marine Geology, 87 (1989) 27 29 27 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands EVROS DELTA: EVOLUTION OF CONTINENTAL...

171KB Sizes 1 Downloads 18 Views

Marine Geology, 87 (1989) 27 29

27

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

EVROS DELTA: EVOLUTION OF CONTINENTAL SHELF SEDIMENTS K. PEHLIVANOGLOU Department of Oceanography, Hydrographic Service of the Hellenic Navy Cholargos, Athens (Greece) (Accepted February 22, 1988)

Abstract Pehlivanoglou, K., 1989. Evros delta: Evolution of continental shelf sediments. In: J. Makris (Editor), Geological Aspects and Tectonic Evolution of Mediterranean Seas. Mar. Geol., 87:27-29 Investigations of the continental shelf sediments off the Evros river delta involved a study of 40 bottom samples and three short piston cores which were examined concurrently with current measurements, aerial photographs and physical oceanographic parameters. Sedimentological processes are more related to the suspended load discharged from the Evros river and near-bottom currents rather than to the bathymetry of the area.

The study of the sediment distribution, shelf morphology, origin of sedimentation and physical properties of sediments was conducted using 40 surface samples and three short cores from the widest continental shelf area of the Evros river delta. The area lies on the Rhila-Rhodope crystalline massif. The onshore area surrounding the Evros delta consists mainly of Quaternary terrigenous and deltaic, Neogene and Paleogene sediments. The Evros river, one of the largest rivers entering the North Aegean Sea, discharges about 3250 x 106 m 3 water/yr through a 15-km wide delta on the eastern side of the study area (Fig.l). In comparison with other great rivers of Northern Greece, this discharge corresponds to about 170,000 t sediment/yr carried in suspension and bottom load. The sediments are distributed mainly along SE NW-oriented zones parallel to the shore (Fig.2). From both sides of a central muddy zone, which begins from the river estuary and trends to the northwest, sediments are gradu0025-3227/89/$03.50

ally sorted into a coarse-grained fraction up to a sand fraction in the exterior zones. The carbonate (CaCO3) fraction of the surface sediments increases from 3% in the central muddy zone to 50% seaward, and to 15% in the landward sandy zone. The quartz content increases from 10°/0 in the landward finegrained zones to 84% in the seaward coarsegrained zones, while the mica content increases to 78% in the muddy zone. Feldspars decrease from 40% in the exterior coarsegrained zones to 10 15~o in the interior zones. The fine-grained central zones (mud, sandy mud and sandy silt) appear to be related to higher proportions of suspended material and mica in the sand fraction (>50%), higher porosity, water content and shear strength (Lykousis and Pehlivanoglou, 1985), and to lower proportions of carbonates (< 10%) and lower bulk densities. The coarse-grained exterior zones (sand, clayey sand and muddy sand) appear to be associated with lower proportions of suspended material and mica in the sand fraction, a lower water content, and a lower

© 1989 Elsevier Science Publishers B.V.

28

45'

50'

2~

55'

]

05"

)



.~_*

".~.~.~.~.~.~.~.~.~".~....'~.~ :~

~J

b

/I

\

[4S

2

Fig.1. Map of the Evros river delta showing the study area, bathymetry and locations of bottom samples. Depth contours are in metres.

9

'

• . . ~ •

. "~----....__~_ ,

:

• ::d~-~.

~

.

. , , ~

. . . . .

'~

.

--_----.--__~--_--....~._--

..

~

,

.

.

. . . .

.......

:-:...~'~.

,

..

.

:

-..-.'..

,

"....

. - . . . . . . ~ ~ - _ - : - - - - - - - - . . . ~ - -

.

.

.

.

.

:f!::

,..

...,....

/

.

-..

:

L

'..'.

. ~ I

.

.

.: ~

" ~

~

//

"

'

-

:--\I:I:'~H-4-~

' '

I -'

"

)

~;

~ .

r:

~

~u~ ,~i,:

"'

• •

.

10k-

..

.

~y~-)-..~/

:: : : ~ ~ _ : _ - _ : - _ •

..

.

,

: .

.'

"

~-o

J

MUBDY

E L s,y E ¥

~

~

1

Fig.2. Distribution of surface sediments.

vane shear strength, and with higher bulk densities, water content, carbonate content (15-50%) and percentage of quartz in the sand fraction. The mean size (7-9 ~b in the central zone and

2 5 ¢ in the exterior zones), the inclusive graphic standard deviation (almost all the studied samples are "very poorly sorted") the inclusive graphic skewness (symmetrical or nearly symmetrical values along the central

29

zone and extreme positive values at the exterior zones), the distribution of the sediment texture, and the conditions of sedimentation appear to be more related to the quantity and the composition of the material discharged by the Evros river in relation to the predominant conditions of energy and layer mixing near the river estuary than to the bat hym e t r y and topography of the shelf. Using the distribution of statistical parameters (mean size, sorting, etc.) as indicators of the energy conditions in the area (Folk, 1974), the distribution of mica in the sand fraction, the low textural m a t ur i t y of the sediments, the wave action, and in-situ cur r ent measurements, it was ascertained t ha t the proportion of energy (current energy and wave action) seems to be relatively high while the sedimentation in the area is extremely high because of the large sediment discharge from the Evros river and the neighbouring coasts. It is believed that most of the suspended material, as observed from the study of the above-mentioned factors and aerial photographs, moves in the surface layer to the

northwest while there is a near-bottom current which retrogrades either northeast to southeast or west-southwest to east-northeast. Under these conditions of movement, mixing of layers with the sediment load is created and the fine-grained sediment is trapped, flocculated and settles (Drake, 1975). At the same time, material originated from the neighbouring coasts, is transported, mixed with the discharged material from the river and settles seaward creating the landward fine-grained zones between the central muddy zone and the coast. Finally, the seaward sand zone seems to be related to low quantities of fine-grained sediment and high-energy conditions.

References Drake, E.D., 1975. Suspended sediment transport and mud deposition on continental shelves. In: Marine Sediment Transport and Environmental Management. Wiley, New York, pp.127 157. Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphill, Austin. Lykousis, V. and Pehlivanoglou, K., 1985. Geotechnical Properties of Shelf Sediments from the Aegean Sea. Rapp. P.-V. Reun. Cons. Int. Explor. Mer, 29:171 pp.