The Origin of the Sierra De Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain
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Geomorphology 45 (2002) 197–209 www.elsevier.com/locate/geomorph The origin of the Sierra de Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain J.M. Recio Espejo a,*, D. Faust b, M.A. Nun˜ez Granados a aEcology (Physical Environment-Geomorphology), Campus de Rabanales, University of Co´rdoba, 14071-Co´rdoba, Spain bLehrstuhl Physische Geographie, Katholische Universita¨t Eichsta¨tt, Ostenstra e, 26, D-85072, Eichsta¨tt, Germany Received 1 March 2001; received in revised form 10 September 2001; accepted 28 September 2001 Abstract Hollows in the Sierra de Aracena, part of western sector of Sierra Morena region (Huelva, Spain), are geoecologically unusual macroforms. They are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation. Paleosols with very dark colours, a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows. An evolutionary model is proposed for the hollows, involving differential weathering during the Mesozoic on plutonic and amphibolitic rocks, alpine tectonic activity followed by Quaternary erosion and exhumation leading to formation of erosional terraces. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hollows macroforms; Deep weathering; Hercynian massif; Sierra Morena; Spain 1. Introduction nary erosional river terraces developed on both the planation surfaces and occurring mainly in narrow The western sector of the Sierra Morena, the Sierra valleys. of Aracena, is formed mainly of Precambrian and A series of enclosed hollows up to 3 km2 in area Palaeozoic rocks typical of the Iberian Hercynian and 150 m deep stand out in the landscape because of massif (Fig. 1). This sector is characterised by large their unusual geoecological characteristics. They morphological features such as planation surfaces and occur all over the western Sierra Morena (Fig. 2) Appalachian morphologies. The planation surfaces are and are delimited by a different vegetation from the cut across plutonic rocks and schists, forming two surrounding areas, by their great depth and by the main levels at about 600–700 and 400–500 m above eutrophic nature of the soils on the floors of the sea level (Nu´n˜ez and Recio, 1998); these are termed hollows. We studied the morphology and genesis of surfaces I and II, respectively. The Appalachian mor- the hollows of the Sierra de Aracena, paying special phologies occur mainly on carbonate and metasedi- attention to palaeo-weathering features in them. mentary lithologies. These is also a series of Quater- The western Sierra Morena lies between 300 and 900 m above sea level, has a relatively high precip- itation of 800–1000 mm/year and average annual * Corresponding author. temperatures of 14–17 °C. These climatic factors E-mail address: [email protected] (J.M. Recio Espejo). explain the establishment of umbraphile communities, 0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0169-555X(01)00154-4 198 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 Fig. 1. Main lithological zones of the western Sierra Morena. such as gall oak and chestnut groves (with Quercus Together with tectonic stability this allowed weath- faginea and Castanea sativa as the basic species) and ering processes to dominate Mesozoic morphogenesis oak and cork oak groves (Q. suber and Q. rotundifo- (Molina, 1991; Martı´n Serrano, 1988). Some traces of lia) as the most frequent communities all over the the resulting soils in the north-western sector of the Sierra Morena. The communities are part of one of the Iberian basement (River Duero basin) have been typical cultural landscapes of grazing land with sparse described by Molina et al. (1990). forest in the Andalucian region. Hollows similar to these of the western Sierra Under conditions of maximal rainfall and north- Morena have been described by Godard (1977), Twi- ward exposure, this climatic regime would produce dale (1982) and Ollier (1984) on plutonic rocks of the acidic umbric soils rich in organic matter. However, French Central Massif, USA (Davil’s Marble) and the all the soils of the western Sierra Morena are poorly Murrmungee Basin in Australia, respectively. For these developed Regosols, Leptosols and Cambisols authors increased weathering compared with surround- because of erosive processes accelerated by human ing areas and fluvial removal of weathering products activities over the last two millennia. More strongly were the main factors responsible for formation of the developed soils, such as Luvisols and Acrisols are enclosed hollows. Godard (1977) pointed out the relict from an earlier period (Cano and Recio, 1996). importance of different rock properties, and Twidale Tropical conditions dominated the environment of (1982) related the genesis of hollows to differential the Iberian basement during the Mesozoic (Rat, 1982). weathering in granitic landscapes. Ollier (1984) des- J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 199 Fig. 2. Main hollows of the Aracena Massif and location of the studied soil profiles. cribed hollows 100 m deep below a planation surface, (1976), carbonate by the method of Duchaufour their floors covered with alluvial sediments. (1975), organic matter by the Sims and Haby (1971) method, granulometry according to Soil Survey of England and Wales (1982), and exchangeable ions by 2. Materials and methods For a morphological study of the hollows we used topographic maps at scales of 1:50,000 (National Topographic Map) and 1:10,000 (Andalusian Carto- graphic Service). The up-dated goelogical maps at a scale 1:50,000 issued by the Spanish Geological and Mining Institute (IGME, 1982, 1983, 1984; ITGE, 1990) were used to identify the bedrock around and beneath the hollows, and air photos at a scale 1:30,000 for their detailed geomorphological characteristics. Soil profiles were described in the field and classi- fied using FAO (1977, 1989). Colours were defined according to Munsell Colour (1990). pH in water was Fig. 3. Geomorphology and bedrock geology of the hollow of determined by the method of Guitia´n and Carballas Dehesa de Valle Torres. 200 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 Fig. 4. Topographic plans and sections of a plutonic hollow (Santa Eulalia) and an amphibolitic hollow (Calabazares). J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 201 Fig. 5. Topographical relationship of hollows to planation surfaces levels NI and NII. the methods of Pinta (1971) and Guitia´n and Carballas The forms of iron were determined according to Mehra (1976). Clay minerals were quantified according to and Jackson (1960), Barro´n and Torrent (1986) and Montealegre (1976) and Brindley and Brown (1980). Torrente and Cabedo (1986). The mineralogy of sand Fig. 6. Current generalised relationships between lithology, soils and vegetation in the hollows. 202 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 fractions was determined by the methods of Partenoff slopes are usually of acid metasedimentary rocks et al. (1970). (phyllites and schists) (Figs. 3 and 4). This suggests that the main factor controlling the presence of hol- lows is differential weathering of the various bedrock 3. Results types. Weathering would have affected the amphib- olitic and plutonic lithologies to a greater extent as 3.1. Morphological features they are richer in weatherable minerals and more permeable than the acid metasedimentary rocks, A total of 16 hollows was described within the which are composed mainly of quartz. In some 3250 km2 of our study area (Fig. 2). Most are near hollows, there is a clear relationship between the fault Sierra de Aracena, which is in the central sector of the pattern (determining changes in bedrock) and the western Sierra Morena. The hollows are 0.2–3 km2 in margin of the hollows. In other situations, the role area, with a circular or subcircular outline and steep of tectonics in the genesis of these forms is less clear. ( f 15°) side slopes and depths of 100–150 m. All the hollows have been captured and excavated Amphibolitic and plutonic rocks (quartz–diorites by the present fluvial systems. Fluvial action seems to and diorites) occur on their floors, and their marginal account for the appearance of two different morpho- Table 1 Macromorphological properties of profiles I–VI Profile Horizon Depth (cm) Colour (dry) Colour (moist) Structure Reaction HCl Boundary Umbric Leptosol (H: 680 m, slope: 32–46%, Par. mat.: slates, veg.: rockroses) I A/C1 0–15 7.5YR5/4 7.5YR3/3 Granular Nil Abrupt C 15– > – – – Nil – Eutric Regosol (H: 600 m, slope: 4–8%, Par. mat.: colluvium, veg.: grazing land) II Ap 0–30 10YR5/6 10YR3/4 Massive Nil Sharp C1 30–> 10YR4/4 10YR3/3 Massive Nil – Eutric Cambisol (H: 540 m, slope: 4–8%, Par. mat.: quartz diorites, veg.: pasture) III Ap 0–100 10YR5/3 10YR3/4 Granular Nil Abrupt 2Bw 100–115 10YR6/8 10YR5/8 Prismatic Nil Diffuse 2BwC1 115–> 10YR6/6 10YR4/6 Prismatic Nil Diffuse Eutric Cambisol (H: 520 m, slope: 32–46%, Par. mat.: gneiss, veg.: Genista sp.) IV A1 0–40 10YR5/4 10YR3/6 Massive Nil Abrupt 2Bw 40–60 10YR2/2 10YR2/1 Prismatic Nil Diffuse 2BC1 60–100 10YR2/2 10YR2/1 Prismatic Nil Diffuse R 100–> – – – Nil – Eutric Cambisol (H: 280 m, Par. mat.: quartz diorites, veg.: pasture) VAp 0–40 10YR6/8 7.5YR4/4 Granular Nil Abrupt 2Bw1 40–80 10YR5/4 10YR3/4 Prismatic Nil Diffuse 2Bw2 80–100 10YR5/6 10YR4/6 Prismatic Nil Diffuse C1 100–> 10YR5/6 10YR4/6 Single-grain Nil – Eutric Regosol (over Palaeoacrisol) (H: 700 m, Par. mat.: quartz diorites, veg.: chestnut woodland) VI A1 0–05 7.5YR6/4 7.5YR4/4 Granular Nil Sharp A1C1 05–35 7.5YR5/4 7.5YR3/4 Granular Nil Sharp 2C1 35–100 5YR6/6 5YR5/8 Single-grain Nil Diffuse 2C2 100–> 5YR7/6 5YR5/6 Single-grain Nil Diffuse 2C3 100–300 10YR7/8 10YR6/8 Single-grain Nil Diffuse J.M.