Quarternary evolution of “Ancient Lake Ohrid”, FYROM/Albania

The cross-boundary Lake Ohrid (40°54´ – 41°10‘N, 20°38‘ – 20°48‘E) located at the border of FYR of Macedonia and Albania is situated within a karstic environment in an active tectonic region in the Balkanides and stretches over a length of c. 30 km and a width of c. 15 km. The regional basin and range setting in an extensional back-arc system, that is controlled by the roll-back of the subducted plate of the Northern Hellenic Trench, produces an elongated N-S trending basin with relatively straight shorelines along the lake. These in particular are linked mostly to N-S trending active faults that are formed by extension and point to active subsidence. The evolution of the shorelines during the Holocene was investigated by studying extensive parts of the present-day coastline, including locations in the northern and southern plains as well as deltas of inflowing rivers and mass movement bodies on the eastern shore where a steep relief is exposed. Ground Penetrating Radar and electric resistivity have been applied as non-invasive shallow subsurface mapping methods to image sedimentary and tectonic structures and unconsolidated sediment cores were taken to support the geophysical data. The southern plain is dominated by alluvial plain deposits and deltaic foresets, generated by the meandering Cerava River. This could be validated by the geophysical data. The northern plain shows fluvial-dominated sequences including channel structures which are underlain by deltaic sediments and foresets. The Velestovo site to the east of the basin provides evidence for a shallow lagoon or marsh environment by peat deposits with periodical clastic input of the Velestovo creek. A change in the drilled sediments from peat to clayey marls at a depth of 8 m suggests a change in the depositional environment, which can be related to a sudden lake-level drop or to tectonic activities. No evidence for a higher lake-level during the Holocene was found in the plains north and south of the lake, except rare temporal floodings, which are also documented historically and lacustrine faunal elements (ostracods, Chara) encountered within the sediments. The abrupt change in sediment composition in the core of the east coast can be related to a sudden lakelevel drop, enhanced discharge of the karstic springs or to tectonic activities. Considering the tectonic activity of the region and the landscape architecture a tectonic event is likely the cause of this effect. In conclusion, the plains north and south of the lake are dominated by clastic input related to climate variations and uplift/erosion, whereas the steep western and eastern margins are controlled tectonically by normal faulting. Mapping of the limestone cliffs around Lake Ohrid yielded no evidence for abrasional platforms or notches as indicators for past higher lake-levels. Hydroacoustic survey exhibited several drowned platform like terraces at depth ca. of 30 and 60 m below present-day lake level.