Holocene climate variability and soil erosion hazards on the margins of the lower Ebro Depression, SW of Zaragoza, Spain

Abstract

The Huerva Valley (NE Spain, 30 km south of Zaragoza) is the contact area between the Iberian mountain range and the lower Ebro Depression. Properties of both geographic regions exist: one can identify Jurassic hard limestone, typical for the Iberian Mountains in the ‘Massive of Mezalocha’ as well as soft horizontal layered Tertiary limestones on the ‘Plana’, typical for the Ebro Depression. Quaternary deposits are present in valley bottoms and hill slope sections. Numerous geomorphological features, mostly of Quaternary age are present in the area: piping, glacis, debris cones, terraces, incisions, etc. Triangular slope facets appear on the southeast slope of the hill ‘Las Coronas’. The facets indicate alternating periods of sedimentation and erosion associated with climate variability during the Holocene. The evidence lies in their shape and relative position. They can only be formed when, after a period of accumulation of colluvium, a facet individualizes due to erosion and incision processes. Their actual position, one facet behind the other, proves climate cycli stimulating now sedimentation, then erosion. Severe climatic conditions (wind, drought, heat and cold) and agriculture make the current landscape extremely vulnerable to erosion. A soil erosion hazard classification for the study area was done using the Van Zuidam (1979) methodology, resulting in an erosion risk map. Gully areas and places with a high degree of incised slopes are most vulnerable to erosion, followed by fluvial ravines situated in the Jura limestone and steep slopes linked to the flat Tertiary platforms. Investigations are currently done to sequence and laterally correlate the observed phenomena