Fluvial terrace formation in a mountainous area (2): influence of eustatism, tectonics and altitudinal distribution of watersheds based on an allostratigraphic study (Albania)

Terraces are highly developed along the Albanian rivers and eleven levels of terraces are recognized in the area, nine occurring during the last glacial cycle (MIS 5d to end of MIS 2). An allostratigraphy study of the fluvial terraces shows a large variety of the geometry of the sedimentary units beneath this set of terraces. This variety is controlled by the respective amount of the three parameters: lateral erosion, aggradation and difference between incision and aggradation. Active faults offset the paleoriver profiles at throw rates locally greater than 1 mm·yr -1 and the tectonic uplift influences the long term incision: Nested strath terraces or nested filled terraces with substratum risers occur in zones of high uplift rate (greater than 0.4 mm·yr -1 ), superposed units and nested fill terraces with hidden substratum risers occur in the weakly uplifted zones of the intra-mountainous graben-like structures. Most of the Albanian terraces are located above straths (nearly flat erosional surfaces) linked to phases of lateral beveling. The thickness of the sedimentary units above the strath surfaces is mostly influenced by the altitudinal distribution of the river catchments: thin strath terraces are found in the low elevation catchments, while thick fill terraces are found in large catchments and we suggest here that the deposition of fill terrace sediments occurred very rapidly at the cold-warm transitions when the high elevation areas of the large catchments were not protected by vegetation and heavily affected by hillslope processes that delivered a large volume of sediments. The thick Holocene valley fill, locally affected by fill-cut terraces, extends several tens kilometer within the mountain valleys and is probably linked to the mid-Holocene sea-level highstand.