Morfología Fluvial.-Controls on Late Quaternary Landscape development in the Upper Guadalentin Basin, Murcia, SE Spain

Abstract

ABSTRACT Landscapes in South-eastern Spain have developed in response to tectonics, climatic fluctuations and, more recently, to human action. In the valleys, fluvial and colluvial sediments are found in the form of river terraces, pediments and slope deposits. We studied these sediments to evaluate landscape dynamics and the processes of sedimentation and erosion in the semi-arid region of the Guadalentín Basin, Murcia Province, SE Spain. The objective of the study is to deduce controls on Late Quaternary and Holocene landscape development. Fieldwork was carried out on the reach of the Upper Guadalentin, upstream of the city of Lorca, and two of its tributaries (Rio Velez and Rambla de Torrealvilla). River terrace levels were mapped using GPS and presence of gravel layers in outcrops. For the Rambla de Torrealvilla, more detailed sediment descriptions show their build-up. Charcoal was found and dated (14C) on ten locations, while 15 samples for Optically Stimulated Luminescence (OSL) dating were taken to have an age-control on processes of sedimentation and erosion. While the OSL dating measurements and analysis are still in process at this time, some preliminary results are given. Several terrace levels are identified along the Rio Guadalentin and the Rio Velez. These have formed in response to regional and/or global changes in climate. However, local mechanisms have played an important role in the area, overruling regional dynamics. From finely layered sediments, it is deduced that a lake existed during some time in the area, caused by a blockage of the valley. This lake was filled in with sediments and a delta was build at its end, interfering with terrace levels. When the blockage was broken through, lake sediments have been removed and after incision, younger terraces have developed. The Torrealvilla tributary is draining the Lorca Basin, and sediments seem to be younger than those along the Guadalentin and Velez rivers. Infilling of the basin and incomplete removal has shaped this valley. Outcrops in the gully sidewall show stacked layers of large to fine gravels and fine sediments with smaller gravel layers in between. We will present a conceptual model of landscape development since the Late Quaternary based on the age control (14C and OSL) and field observations. This reconstruction is correlated to climatic fluctuations and rates of sedimentation and erosion are approximated on a millennial timescale. Following this approach of longer-term geomorphological investigation of landscape development, we can ultimately put the relative contribution of human actions in the context of natural erosion and sedimentation processes.