Erosion and stabilisation sequences in relation to base level changes in the El Cautivo badlands, SE Spain
Introduction
Process-form relationships are at the heart of much geomorphological enquiry. Geomorphology has made significant and successful efforts in relating landscape forms to the operating processes, as well as in the identification of the processes that, at different times, are responsible for these forms. Another key aim of geomorphology is to discover the speed at which landscapes change. The usual slow rate of operation of many processes in relation to the human lifespan, and the complexity of process interactions in space and time as a consequence of environmental changes or the occurrence of intrinsic thresholds, have created the need for landform comparison at different points in their evolution in order to assess the rate of change. In most places the only way to study this is by the ergodic approach or the analysis of present-day landforms that in different places have different ages; for example, Savigear (1952) and Calvo-Cases (1987), both studying paleosea-cliff retreat, give good examples of this assumption and an idea about the magnitude of the changes over time. In other cases, mathematical models and laboratory and computer simulations have been used in order to develop and test theories concerning landform development. Badlands have been seen as an environment where processes operate with sufficient speed to study their development by direct measurement, and thus they have often been used as natural laboratories (Campbell, 1997). Despite the apparent rapidity of their development dominated by overland flow processes (Bryan and Yair, 1982), many badlands sites show considerable complexity of process interactions (Harvey and Calvo-Cases, 1991, Harvey, 2002) changing over time in response to both intrinsic and extrinsic factors (Faulkner, 2008-this issue).
Badlands may be unconnected to a drainage system at their base and thus develop independently of the base level, or they may be connected to a drainage system (Howard, 1997, Howard, 1999, Harvey, 2002) and thus respond to fluctuations in base levels. In the latter case development may be driven from below with erosion being (re-)activated in response to a lowering of base level and aggradation occurring when base level rises. Whilst badlands do have a role to play as microcosms or analogues for larger, more slowly evolving landscapes, a thorough understanding of badland development and form requires the study of their spatial context. This study can benefit considerably from the use of digital data to simulate and model landscape development over longer time periods.
The high sensitivity of badlands, their response with relative rapidity to changes in both intrinsic and, particularly, extrinsic controlling factors, and the consequential changes in process suites, leave evidence of the old forms in the landscape from which reconstruction can be attempted. The El Cautivo badlands near Tabernas in Almería, SE Spain (Figs. 1 and 2) represent an example of such a situation. They contain a series of remnant surfaces that represent earlier phases of stability during the episodic erosion of the area. Fluctuating base levels in particular, have caused a switch between phases of incision and stabilisation with the incision phases frequently being too short to erode the whole area. Work at the site in the early 1990s (Alexander et al., 1994), identified six stages of episodic erosion and subsequent stabilisation with ages, assumed by regional correlation, ranging from the late Pleistocene to the present day. A developmental sequence, involving changing process interactions, was proposed, suggesting three early stages (A, B and C) characterised by extensive pediment surfaces, and three younger stages (D, E and F) characterised by basally incising streams with more limited pediment development (Fig. 3). Since this work was published, the site has been instrumented and surveyed in detail from both the ground and the air, and a considerable volume of hydrological (Solé-Benet et al., 1997, Cantón et al., 2002), geomorphological (Spivey, 1997, Nogueras et al., 2000, Cantón et al., 2001a, Cantón et al., 2003, Cantón et al., 2004a), pedological (Cantón et al., 2001b, Cantón et al., 2004b) and ecological (Lázaro et al., 2000) research has been carried out. New insights have also been gained into the broader context of the site through the recognition and study of lacustrine sediments on its lower fringes (Harvey and Mather, 1996, Harvey et al., 1999, Harvey et al., 2003).
Section snippets
Study site
The El Cautivo badlands (Fig. 1) lie approximately 20 km north of the city of Almería and some 5 km south-west of the village of Tabernas in an area known as the Tabernas Desert. The area is thermo-Mediterranean semi-arid with a mean annual temperature of 17.8 °C and mean annual precipitation of 218 mm (Lázaro and Rey, 1990).
The study area lies between 245 and 385 m above sea level. The badlands, cut into Upper Miocene (Tortonian) marls and mudstones, are dominated by deeply incised gullies
Aims and objectives
We use knowledge gained from previous work at the site together with new field survey data, digital elevation models, and orthorectified aerial photographs, to re-interpret the development sequence of the site. We examine the ways in which changing drainage directions and phases of incision and stabilisation are related to the filling and subsequent incision of the Rambla de Tabernas, which during the late Quaternary, has been affected by localised uplift and fault movement and converted into a
Methods
To achieve the research objectives, a combination of methods was used, including field and photo interpretation-based mapping, photogrammetry, and GIS analysis.
The basic cartographic information was obtained from stereo air photographs acquired in 1996 by the UK Natural Environment Research Council (NERC, mission 95/10). The photographs were scanned to the highest possible resolution (0.2 m per pixel) and orthocorrected using Erdas Imagine 8.7 Orthobase. From the same images a digital elevation
Results and interpretation
The reconstruction of each developmental stage and its 3D visualisation (represented by contours in Fig. 5) provides insights into the evolution of the site. From an analysis of these images, we propose that the erosional phases differed considerably in both time-span, derived from the rate of landform evolution, and depth of incision.
Conclusion
This work substantiates the general interpretation of the site's development given by Alexander et al. (1994) by providing evidence for the key controls and drivers of change and an initial quantification of process rates. The major phase of incision at the site is shown to have occurred within the last 14 ka and to have been driven by channel capture induced by a rapid lowering of base level. Production of simulated DEMs for each developmental stage has allowed volumes of sediment removal to
Acknowledgements
We are grateful to Adrian Harvey for his comments on an earlier draft of this paper, the Natural Environment Research Council (Mission 95/10) for provision of the aerial photographs upon which much of the work was based, and to Phil Toms of the University of Gloucester for the OSL date (2002). The work reported here forms a part of project REN2003-04570/GLO del Plan Nacional de I+D+I. We are also grateful to the Viciana brothers, proprietors of the site, for allowing access and to Dr. Elias
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