Kaolinitic Paleosols in the South West of the Iberian Peninsula (Sierra Morena Region, Spain)

Catena 70 (2007) 388–395 www.elsevier.com/locate/catena

Kaolinitic paleosols in the south west of the Iberian Peninsula (Sierra Morena region, Spain). Paleoenvironmental implications ⁎ M.A. Núñez, J.M. Recio

Ecology Department (Physical environment and Geomorphology), Rabanales Campus, University of Cordoba. 14071-Cordoba, Spain Received 5 October 2005; received in revised form 27 September 2006; accepted 13 November 2006

Abstract

This paper studies the physical and chemical characteristics of eight different soil profiles with paleohorizons rich in kaolinites and developed on different lithologies along the southern border of the Hercinian Iberian peninsula (Sierra Morena region). The paleohorizons of four of these soils are defined by low pH levels, desaturation of the exchange complex and proportions of kaolinite of over 75%; these characteristics are associated with subtropical pedogenic conditions during the Pliocene period. A further two soil profiles are contemporary and defined by the following features: they occupy depression areas, the clay fraction is dominated by kaolinites and smectites, the exchange complex is saturated and the pH is above neutral values. The last two soil profiles are developed on Pliocene continental deposits and have more moderate pedogenic features, their kaolinite levels are around 50% and their evolution is associated with shorter periods of exposure to these subtropical alteration conditions. © 2006 Elsevier B.V. All rights reserved.

Keywords: Paleoweathering; Pliocene; Western Mediterranean; Tropical pedogenesis

1. Introduction around the Pliocene or Late Neogene period (Espejo, 1987; Martín Serrano, 1989; Rodríguez Vidal, 1989). The neoformation of clays such as kaolinite and smectite is These chronologies might also be supported by several a pedogenic process predominant in subtropical areas (Millot, paleoenvironmental studies such as those of Suc et al. 1964; Tardy et al., 1973; Duchaufour, 1984; Pedro, 1984; Van (1995a,b), Fauquette et al. (1998, 1999) and Capozzi and Wambeke, 1992). Thus, the fact that a significantly high Picotti (2003). They propose subtropical climatic conditions proportion of kaolinite is found in the clay fraction of soils for the north west sector of the Mediterranean basin during exposed to non-tropical alteration conditions is important the Early Pliocene (3.6–5.32 Ma). These conditions gradu- from a climatic and paleoenvironmental point of view. ally evolved towards the marked thermal and rainfall oscil- In a clearly Mediterranean inland region of the Iberian lation characteristics of the region during the Quaternary peninsula, Chaput (1971) and Vaudour (1979) consider all (Thunell et al., 1990; Verguand Grazzini et al., 1990; Agustí surface formations that contain significant amounts of et al., 2001). kaolinite in their alteration complex to be paleosols. Sub- This study presents a regional pedogenic interpretation of sequent studies of certain surface formations of the Iberian the different variants of kaolinitic paleosols located in the massif have characterized the pedogenic aspects (Espejo, region of Sierra Morena (southern sector of the Iberian massif). 1985; Santisteban et al., 1991; Cantano, 1996; Cano and Recio, 1996; Núñez and Recio, 1999) and stratigraphic 2. Study area characteristics of these alterites, and most authors date them Sierra Morena is a mountain range that extends around ⁎ Corresponding author. Tel.: +35 957 21 85 97; fax: +35 957 21 85 97. 400 km from east to west, in the south of the Iberian E-mail address: [email protected] (J.M. Recio). Peninsula. Its lithology is diverse and its structure complex.

Fig. 1. Situation and UTM position of soil profiles.

It constitutes the southern border of the peninsular Hercynian between 14 and 17 °C and an annual rainfall between 600 massif and is essentially formed by Palaeozoic and and 1000 mm), poorly evolved soils developed (Leptosols Precambrian materials. The Alpine tectonic fracture has and Regosols according to F.A.O., 1989), owing to the divided the region into distinct peneplain surfaces, ranging dominance of erosive processes, intensified over centuries from 200 to 1000 m.a.s.l. in the direction of the Guadalquivir by the deforestation of this territory by human activities. In river basin located in the south (Díaz del Olmo and areas with more stable geomorphological conditions, better Rodríguez Vidal, 1989). In these areas, vast peneplains, evolved soils still exist, such as Chromic luvisols (Mediter- Appalachian reliefs, paleokarst forms and hollows from the ranean red soils) as well as other kaolinitic and relict pre-Quaternary period can be found (Baena et al., 1993; paleosols that are discussed in this paper. Núñez and Recio, 1998; Recio et al., 2002). However, in the study area these features are predominantly combined with 3. Materials and methods steep slopes associated with the Quaternary evolution of the river network, with strong retrogressive erosion. Eight soil profiles have been studied, distributed along the On the diverse Hercynian materials (slate, schist, lime- longitudinal axis of the Sierra Morena (Fig. 1), from a range of stones, dolomites, gneiss, granites, etc.) and in a clearly altitudes between 280 and 745 m.a.s.l. These are developed on Mediterranean climate (average annual temperatures of different lithologies such as plutonic rocks (profiles A.1, A.2

Fig. 2. Hypothetical geomorphological situations and pedogenetic aspects of soil profiles studied. 390 M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395 and B.1), limestone (A.4), greywacke (A.3), alluvial deposits was determined using a Bernard calcimeter, in accordance (soils C.1 and C.2) and gneisses (soil profile B.2). The with the Duchaufour (1975) method, and the organic matter pedogenic features of each soil are strongly related to the was determined by Sims and Haby (1971). Analysis of the geomorphological position and this enables us to group the soil exchange complex (interchangeable cations and total profiles according to their position: upper position of relief cationic exchange capacity) was carried out according to (A.1, A.2, A.3 and A.4), depression zone (B.1 and B.2) and the methods proposed by Pinta (1971) and Guitián and soils developed on pre-Quaternary alluvial deposits, currently Carballas (1976). The clay minerals were quantified by X- in summit positions (soil profiles C.1 and C.2) (Fig. 2). ray diffraction following treatment of the samples with The field description and classification of the soils were magnesium chloride and ethylene glycol according to the carried out according to the F.A.O. (1977, 1989). In order to method described by Brindley and Brown (1980), using a estimate the particle size distribution (b2 mm) the samples Siemens D5000 instrument with CuKα radiation. were sieved and treated with H2O2 in order to eliminate organic material. Subsequently, the pipette method was used 4. Results (Soil Survey of England and Wales, 1982). The colour was defined using the Munsell scale (Munsell Colour, 1990), the The macromorphological analysis of the soil profiles pH (1:1) was measured potentiometrically in a 1:1 soil– studied (Table 1) indicates that these soils have been water suspension, in a Schott CG818 instrument using the truncated and buried by aggradative processes under more Guitián and Carballas method (1976), the carbonate content recent organic horizons, as indicated by the clear lithologic

Table 1 Macromorphological properties of profiles studied (shaded rows are the kaolinitic paleohorizons) M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395 391 discontinuities that appear at a depth of around 50 cm. The display 10 YR hues (yellowish brown and very dark brown), paleohorizons studied have mainly well-developed polyhe- saturated exchange complexes and slightly acid pH values dral and prismatic structures, except the A.1 profile de- between 6.1 and 7.0; clay contents are always over 34% (57.7% veloped on quartz diorites, where no types of aggregates 2Bw in B.2). The clay mineralogy appears to be dominated by were found (single-grain structure). The depth of these smectite (64–70%). Kaolinite is only an accompanying mineral paleohorizons is between 55 cm (A.3) and 200 cm (A.4); (24–36%), and illite was not detected in these paleohorizons beneath this layer, deeper horizons, associated with the (Table 2). The Fed/Fet ratio gives values close to 25%. boundary of current illitic alteration (Table 2), gradually give The soil profiles C.1 and C.2 are located respectively on way to the parent material. fluvial Pliocene deposits and alluvial-fan deposits (Rañas) The kaolinitic palaeosols (A.1, A.2, A.3 and A.4 profiles) that are disconnected from the current Quaternary river are associated with upper positions in the relief and therefore network. The palaeohorizons studied in soil C.1 correspond with good drainage; they have reddish yellow and yellowish to the chroma 7.5 YR6/8(s) (reddish yellow), whereas the red colours (5 YR (d)), pH values between 4.6 and 5.4; their colours of the C.2 profile range from 5 YR5/8(s) hues exchange complexes are desaturated (Table 3) and their clay (yellowish red) in the horizon 2B2 to the colourless ones fraction is dominated by kaolinite with values around 75% found in horizon 2B3 caused by hydromorphy. The exchange (Table 2)(Fig. 3). Textural analysis of these paleohorizons complex is desaturated and has acid pH levels of 4.5–4.7; (Table 4) confirms the dominance of clay, with values kaolinite and illite in the clay fraction are close to 50%. between 31 and 47%, with a particularly high content of 2Bt1 in A.4, 64.6%, corresponding to a Terra Rossa formation. 5. Discussion The Fed/Fet ratio for the palaeohorizons where this value has been determined (Table 5) is around 50% and the average is Paleohorizons of soil profiles A.1, A.2, A.3 and A.4 have between 43 and 60%. enabled us to determine a relict ferruginous (Duchaufour, The paleohorizons corresponding to the soils B.1 and B.2 1984), acric (F.A.O., 1989) or ultisol pedogenesis (Soil present a set of alteration features that are associated with lower Taxonomy, 1975) in the southernmost sector of the Iberian relief positions (B.1) or palaeo-bottom positions (B.2). They massif, developed from diverse lithologies such as limestone,

Table 2 Physico chemical characteristics of profiles studied 392 M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395

Table 3 Exchange complexes characteristics

quartz diorite, granite or greywacke. The paleoenvironmental The clay mineralogy of deep horizons from B.1 and B.2 significance of the kaolinite contents indicates subtropical profiles seems to be dominated by smectites, with kaolinite conditions for the genesis of these paleoalterites. Further- being present only as an accompanying mineral; the absence of more, the red hues of the paleohorizons might indicate drastic illite suggests an imbalance of these alterites with current climatic contrasts, including pronounced dry seasons. Mediterranean Quaternary conditions. Following Duchaufour

Fig. 3. X ray diffractograms of clay minerals of some paleohorizons studied. CuKα radiation (I = illite, K = kaolinite and Sm = smectite). M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395 393

Table 4 Semiquantitative mineralogical analysis of clay fractions (b2 mm), in selected profiles

n.d.=not detected.

(1984), Pedro (1984) and Tardy et al. (1973), these smectite Table 5 and kaolinites could be neoformation clays associated with Different forms of iron of selected profiles. (Fed: dithionite iron; Fet: total iron.) similar subtropical climatic conditions, although they are usually characteristic of depression areas. The scarcity of soils located on post-palaeozoic deposits makes it difficult to date these paleosols; however, their geomorphological position on pre-Quaternary relief formations seems clear (Núñez and Recio, 1998). Authors such as Espejo (1987) and Rodríguez Vidal (1989) have suggested the Pliocene period for similar kaolinitic paleosols, developed on post-Hercynian deposits in the Hercynian massif. Other studies such as those of Suc et al. (1995a), Fauquette et al. (1998) and Muñiz et al. (1999) in the NW Mediterranean region (Iberian Peninsula and southern of France) and Capozzi and Picotti (2003) in the northern Apennines, provide paleobotanical and stratigraphic evidence that suggests the prevalence of subtropical conditions (wetter and warmer) during the early Pliocene period (3.6–5.32 Ma) this would reinforce our hypothesis that these paleosols are from the Pliocene period. From 3.6 Ma during the Piacenzian period (Late Pliocene), these subtropical conditions slowly gave way to other typical Mediterranean conditions; this tendency is more evident 394 M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395 from 3.1 Ma onwards (Suc et al., 1999a; Capozzi and Pliocene subtropical conditions did not occur any farther south Picotti, 2003). than the southernmost boundary of the Hercynian massif. 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