Quaternary evolution of the fluviokarst Rosandra Valley (Trieste, NE Italy).

The stratigraphy of terraced sediments as well as morphological features of the middle reach of the Rosandra Valley (Trieste, NE Italy) were studied in order to reconstruct its Quaternary evolution. This sector forms a fluviokarst valley, which is characterized by a deep incised gorge with abrasional features. Downstream, the gradient is reduced, the valley widens and terraced deposits occur. Alluvial/colluvial terraces crop out for about 1 to 1.5 km along the creek. They show the coalescence of the alluvial deposits with debrisfalls from the steep limestone slopes and colluvial fans related to some minor tributaries. The highest terrace is roughly 15 m high. 14C datings on a frustule plant collected in the lower part of the deposit (> 45.000 yrs. BP), together with geological and geomorphological considerations, suggested that the terrace scarp could be Middle Pleistocene in age. The geomorphic regime of the creek changed from aggradation to erosion, as recorded by fanhead trenching and incision of fluvial terrace scarps, and it still persists. The tectonic uplift, which is partly responsible for the downcutting of the terrace and is still active, could be related to the SE-NW tilting of the Karst plateau.