Pre-Holocene sediments in the Barranco de las Lenas, Central Ebro Basin, Spain, as indicators for climate-induced fluvial activities

In the Central Inner Ebro Basin, valleys infilled with young alluvial sediments are very common. They are usually incised by deep gullies. Previously, these sediments were assumed to be of Holocene age, and their infilling was seen in connection with different periods of climatic deterioration and increasing human impact. In the Barranco de las Lenas, an eastern tributary of the Huerva River south of Zaragoza, the sediments are up to 15 m thick and consist of 20-60 cm thick interbedded layers, of argillaceous silt, pure silt, sandy-loamy silt, sandy silt with fine grit which are interrupted by 5-25 cm thick discontinuous cobble and gravel layers with silty matrix. The sediments contain horizontal organic layers and charcoal pieces of remarkable size. A 2.2 m high outcrop in the upper part near the recent end of the incision was radiocarbon dated, with the ages ranging from 17765-15440 cal BP to 13140-12650 cal BP. A 1.2 m sediment sequence between two organic mud layers with an age of 14040-12975 cal BP at the base and 13140-12650 cal BP at the top indicates a climatically induced highly active phase of morphodynamic processes within a period of 1400 years at the longest. Older sediments from the Pleniglacial have been reworked. The analyses of the sediments and data suggest that these few single events are indicators for strong climatic changes in the Late Glacial (Allerod Period). The results show that similar highly dynamic processes can occur without human impact. These results strongly contrast with previously published investigations that dated such fillings as Younger Holocene. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved.