Quaternary International 93–94 (2002) 65–72 Pre-Holocene sediments in the Barranco de las Lenas, Central Ebro Basin, Spain, as indicators for climate-induced fluvial activities Wolfgang Andres a,{ , Johannes Ries a, *, Manuel Seeger b a Institut f . ur Physische Geographie, Johann Wolfgang Goethe-Universit . at Frankfurt am Main, Senckenberganlage 36, D-60054 Frankfurt am Main, Germany b Instituto Pirenaico de Ecolog ! ıa (C.S.I.C.), Avenida de Monta * nana 177, Aptdo. 202, E-50080 Zaragoza, Spain Abstract 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 (Aller . od 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. r 2002 Elsevier Science Ltd and INQUA. All rights reserved. 1. Introduction The Val de las Lenas is situated in the Inner Central Ebro Basin, an eastern tributary of the Huerva River, which flows into the Ebro at Zaragoza, about 15 km north. Here, flat-lying Miocene gypsum, marl and clay series with interbedded limestone and sandstone form the bedrock of the highly dissected slopes of the mesa Plana de Zaragoza (ITME, 1998). The series between 350 and 600 m a.s.l., dominated by gypsum in the middle part, forms an impressive erosion landscape with out- liers and island mounds with mostly straight to slightly convex upper and mid slopes. These are flanked sharply by scree-covered glacis, which grades downslope smoothly into the planar valley bottom. The slopes are covered by low scattered matorral (Tomaselli, 1981) of Rosmarinus officinalis and Thymus vulgaris, and the glacis support scattered Lygeum spartum. Both slopes and glacis show indicators of heavy land degradation due to deforestation of the natural vegetation (Juniper- eto-phoeniceo-thuriferaesigmetum) and grazing activities by the transhumant sheep flocks from the Pyrenees during recent centuries (Braun-Blanquet and Bol " os, 1987; Rivas-Mart ! ınez, 1987). The soil surface is covered by stone pavements, the results of intensive sheet wash: on these surfaces, runoff coefficients (as measured by artificial rainfall simulation) can be as high as 57%. Rill erosion and gully erosion are obvious on the slopes. The flat valley bottoms (Span. val) are infilled with Quaternary alluvial sediments and are characteristic elements of the landscape in the Central Ebro Basin. They are usually incised by deep gullies (Span. barrancos), which provide the opportunity to study the composition and thickness of the deposits. In the Barranco de las Lenas, the flat surface declines from 580 m a.s.l. at the top end of the valley to 380 m a.s.l. at the junction with the Huerva Valley. The sediments are up to 15 m thick and consist of 20–60 cm thick interbedded layers, of argillaceous silt, pure silt, *Corresponding author. E-mail addresses: j.b.ries@em.uni-frankfurt.de (J. Ries), mseeger @ipe.csic.es (M. Seeger). { Deceased. 1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII:S1040-6182(02)00042-3