Consideration of modern sendimentary processes of the Evros river delta front and prodelta, NE Aegean sea

Abstract

Gravity cores data from the Evros River delta front and prodelta confirm the aspect that the Evros River deltaic deposits tend to west-northwest, along the main direction of the local hydrodynamic regime, preventing the construction of a symmetrical Holocene prism. In the delta front, the terrigenous sediment consists mostly of fine-grained material. Its vertical succession is monotonous with slight differences in grain size or colour, and lack of internal structure or current-produced laminae, indicating almost immutable sedimentary processes as well as long-term discharge fluctuations. The river-borne sand is limited because of its entrapment in the river mouth but sometimes, during extreme conditions (periods of high river discharge, short-lived catastrophic events etc.), can be transported seawards covering an extensive area of the Alexandroupolis Gulf. The negligible biogenic content and the absence of bioturbation effects are attributed to the high rates of deposition. Some bioturbated horizons are produced when sedimentation rates decrease temporarily. The prodelta sediment distribution patterns occur a distinctive zonation along an east-southeast to westnorthwest trend. In the central part, mud dominates, while on both sides of this area, the content of sand gradually increases and becomes the prevalent facies near the coast and in the outer plateau of the gulf. The vertical facies sequences of the upper sedimentary cover are complicated, with many variations in grain size, colour and biogenic content reflecting a complicated manner of deposition. The major feeder of this area is the Evros River providing great amounts of suspended load. Another remarkable source of sand is derived from the coastline, which during storm conditions provides coarse-grained material in the shoreface area. Finally, a sediment supplier of local importance, Loutros River, affects the eastern area of Alexandroupolis building up a small subaqueous fan. In the open sea, the consequences of the modern sedimentation are negligible. The sandy character of the surface and sub-surface sediments, the analysis of biogenic fragments, the long distance from the present-day terrigenous sources are some evidences which lead to the view that the upper sediment body of the southwestern part of the study area has a presumable relict origin. 210Pb profiles are, more of less, consistent showing a sedimentation rate more than 2 cm/yr in the delta front, which decreases to 0.2 cm/yr at about 8 km seaward.