Development of a fore-mountain alluvial fan of the Olza River southern Poland) during the Pleistocene

A series of Pleistocene deposits with different lithology is present where the Olza River flows out from the Carpathian foothills to the O wi cim Basin. The deposits are mostly composed of gravels forming several series of different ages, which are intercalated with much finer silts and sands as well as organic silts. A complex of glaciogenic deposits is also found as a thin layer of till, glaciolacustrine and glaciofluvial sediments. Loess-like deposits occur in the top part of the section under study.Gravels were deposited in the zone of a fore-mountain fan. The co-occurrence of lithologically different deposits reflects a great variability of sedimentation conditions, which depended mostly on climate changes. However, the formation of fan was also controlled by other factors. Neotectonic movements probably played an important role in its evolution. In this paper, we describe the successive stages of fan development and the factors determining this process. The interpretation is based on the analysis of deposits exposed in the eastern part of the fan, at the Kończyce site.The fan of the Olza River was built up with alluvia mostly during successive glaciations. It was dissected towards the end of each glaciation. During interglacials the fan was only slightly transformed. A special period of fan development occurred during glaciation when the ice sheet advanced on the fan surface. The aggradation of the fan was probably stopped due to uplift of the area. Then, aeolian loess-like deposits started to accumulate on a considerable part of the fan surface. Former opinions about the stratigraphy of the fan deposits are strongly diversified. Precise age of the successive series is still difficult to establish. In the light of contemporary studies, it can not be excluded that age of the Olza fan might be younger than previously suggested.