Calcrete 'fossilisation' of alluvial fans in SE Spain: the roles of groundwater, pedogenic processes and fan dynamics in calcrete development

Stokes, M., Nash, D.J. and Harvey, A.M. (2007) Calcrete 'fossilisation' of alluvial fans in SE Spain: the roles of groundwater, pedogenic processes and fan dynamics in calcrete development Geomorphology, 85 (1-2). pp. 63-84. ISSN 0169-555X

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Abstract

Dryland alluvial fans developed along the northern flanks of the Sierra Lisbona in the north-western Vera basin (Almeria region, southeast Spain) displaying negligible amounts of incision despite distal base-level lowering of >30m by the modern drainage network. The fans are of early middle Quaternary age and are graded to a coeval river terrace, now isolated from the modern drainage network. The negligible incision is due to the encasement and 'fossilisation' of the alluvial fans by calcrete. This paper explores the reasons for such fossilisation and assesses the relative importance of pedogenic and groundwater mechanisms for calcrete formation within an alluvial fan setting. A single fan was selected for detailed examination. The geomorphological and sedimentological features of the fan, its catchment area and their relationship to the distal river terrace were documented. Qualitative and semi-qualitative petrographic and scanning electron microscope analyses of calcrete samples collected from transects across the fan surface, and within its distal top river terrace surface, enabled the style, pattern and relative timing of calcrete development to be assessed. Calcrete fabrics comprised initial micritic grain-coating cements, pellets and glaebular carbonate nodules, with interstitial spaces infilled by equal sparite and microsparite mosaics. It is proposed that the early phases of calcrete development were dominated by pedogenic processes with increasing groundwater calcretisation over time. Point count data indicated increased quantities of interstitial sparite and microsparite cement within near-surface proximal fan calcretes and at depth across the fan, suggesting that groundwater processes played a more important role in calcrete formation in these locations. The contribution of groundwater to calcrete development can be best explained by the intrinsic funnelling of groundwater from the catchment through the proximal fan head area, a zone where the fan gravels are thinnest. Calcrete 'fossilisation' appears to have followed a reduction in the fan catchment area as a result of rockfalls and watershed stream capture, which reduced and sediment supply to the fan and enabled surface stabilization and calcrete development to take place. The reduced sediment/water supply, combined with calcrete fossilisation, appears to have protected the alluvial fans from regional base-level lowering. The implications of these results for existing pedostratigraphic models of calcrete development in alluvial fans are subsequently explored.

Item Type: Journal article
Additional Information:
Uncontrolled Keywords: Alluvial fans, calcrete, carbonate soils, groundwater
Subjects: F000 Physical Sciences > F800 Physical Geography and Environmental Sciences > F840 Physical Geography
F000 Physical Sciences > F800 Physical Geography and Environmental Sciences > F820 Geomorphology
F000 Physical Sciences > F600 Geology
DOI (a stable link to the resource): 10.1016/j.geomorph.2006.03.020
Faculties: Faculty of Science and Engineering > School of Environment and Technology > Applied Geosciences
Faculty of Science and Engineering > School of Environment and Technology
Depositing User: editor environment
Date Deposited: 02 Mar 2007
Last Modified: 25 Jun 2013 12:04
URI: http://eprints.brighton.ac.uk/id/eprint/1772

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