TAFONI IN THE EL-CHORRO AREA, ANDALUSIA, SOUTHERN SPAIN

Using a combination of field,laboratory and micromorphological evidenc e, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the pro cesses responsible for their formation. Twenty-five tafoni were random ly selected for field examination. The morphology of these cavernous r ock domes is characterized by a helmet-shaped outer roof and an arched -shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, ove rhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, o btained using a Schmidt hammer, are relatively low but significantly h igher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and f loors indicates predominantly sandy textures, alkaline pH values and s ome base cation enrichment. Micromorphological analysis of thin sectio ns, prepared from undisturbed blocks, reveals large quantities of pore -filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodule s with a metallic appearance. In terms of their formation, different p rocesses appear to act on different parts of the landform. On the oute r roof surfaces, case hardening, resulting from near-surface cementati on and surface varnish development, is dominant. On the inner cavern s urfaces, however, core softening, resulting from granular disintegrati on and flaking, dominates. Exfoliation weathering, running water and w ind deflation also appear to play an important role in tafone formatio n. A phased model of tafone evolution is proposed whereby the features pass through four phases of development-initiation, enlargement, amal gamation and degradation; in the study area there are examples of tafo ni in each of these phases. Much of the evidence suggests that the taf oni are actively developing under current environmental conditions. (C ) 1997 by John Wiley & Sons, Ltd.