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
The neoformation of clays such as kaolinite and smectite is
a pedogenic process predominant in subtropical areas (Millot,
1964; Tardy et al., 1973; Duchaufour, 1984; Pedro, 1984; Van
Wambeke, 1992). Thus, the fact that a significantly high
proportion of kaolinite is found in the clay fraction of soils
exposed to non-tropical alteration conditions is important
from a climatic and paleoenvironmental point of view.
In a clearly Mediterranean inland region of the Iberian
peninsula, Chaput (1971) and Vaudour (1979) consider all
surface formations that contain significant amounts of
kaolinite in their alteration complex to be paleosols. Subsequent studies of certain surface formations of the Iberian
massif have characterized the pedogenic aspects (Espejo,
1985; Santisteban et al., 1991; Cantano, 1996; Cano and
Recio, 1996; Núñez and Recio, 1999) and stratigraphic
characteristics of these alterites, and most authors date them
⁎ Corresponding author. Tel.: +35 957 21 85 97; fax: +35 957 21 85 97.
E-mail address: [email protected] (J.M. Recio).
0341-8162/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.catena.2006.11.004
around the Pliocene or Late Neogene period (Espejo, 1987;
Martín Serrano, 1989; Rodríguez Vidal, 1989).
These chronologies might also be supported by several
paleoenvironmental studies such as those of Suc et al.
(1995a,b), Fauquette et al. (1998, 1999) and Capozzi and
Picotti (2003). They propose subtropical climatic conditions
for the north west sector of the Mediterranean basin during
the Early Pliocene (3.6–5.32 Ma). These conditions gradually evolved towards the marked thermal and rainfall oscillation characteristics of the region during the Quaternary
(Thunell et al., 1990; Verguand Grazzini et al., 1990; Agustí
et al., 2001).
This study presents a regional pedogenic interpretation of
the different variants of kaolinitic paleosols located in the
region of Sierra Morena (southern sector of the Iberian massif).
2. Study area
Sierra Morena is a mountain range that extends around
400 km from east to west, in the south of the Iberian
Peninsula. Its lithology is diverse and its structure complex.
M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395
389
Fig. 1. Situation and UTM position of soil profiles.
It constitutes the southern border of the peninsular Hercynian
massif and is essentially formed by Palaeozoic and
Precambrian materials. The Alpine tectonic fracture has
divided the region into distinct peneplain surfaces, ranging
from 200 to 1000 m.a.s.l. in the direction of the Guadalquivir
river basin located in the south (Díaz del Olmo and
Rodríguez Vidal, 1989). In these areas, vast peneplains,
Appalachian reliefs, paleokarst forms and hollows from the
pre-Quaternary period can be found (Baena et al., 1993;
Núñez and Recio, 1998; Recio et al., 2002). However, in the
study area these features are predominantly combined with
steep slopes associated with the Quaternary evolution of the
river network, with strong retrogressive erosion.
On the diverse Hercynian materials (slate, schist, limestones, dolomites, gneiss, granites, etc.) and in a clearly
Mediterranean climate (average annual temperatures of
between 14 and 17 °C and an annual rainfall between 600
and 1000 mm), poorly evolved soils developed (Leptosols
and Regosols according to F.A.O., 1989), owing to the
dominance of erosive processes, intensified over centuries
by the deforestation of this territory by human activities. In
areas with more stable geomorphological conditions, better
evolved soils still exist, such as Chromic luvisols (Mediterranean red soils) as well as other kaolinitic and relict
paleosols that are discussed in this paper.
3. Materials and methods
Eight soil profiles have been studied, distributed along the
longitudinal axis of the Sierra Morena (Fig. 1), from a range of
altitudes between 280 and 745 m.a.s.l. These are developed on
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
(soils C.1 and C.2) and gneisses (soil profile B.2). The
pedogenic features of each soil are strongly related to the
geomorphological position and this enables us to group the soil
profiles according to their position: upper position of relief
(A.1, A.2, A.3 and A.4), depression zone (B.1 and B.2) and
soils developed on pre-Quaternary alluvial deposits, currently
in summit positions (soil profiles C.1 and C.2) (Fig. 2).
The field description and classification of the soils were
carried out according to the F.A.O. (1977, 1989). In order to
estimate the particle size distribution (b 2 mm) the samples
were sieved and treated with H2O2 in order to eliminate
organic material. Subsequently, the pipette method was used
(Soil Survey of England and Wales, 1982). The colour was
defined using the Munsell scale (Munsell Colour, 1990), the
pH (1:1) was measured potentiometrically in a 1:1 soil–
water suspension, in a Schott CG818 instrument using the
Guitián and Carballas method (1976), the carbonate content
was determined using a Bernard calcimeter, in accordance
with the Duchaufour (1975) method, and the organic matter
was determined by Sims and Haby (1971). Analysis of the
exchange complex (interchangeable cations and total
cationic exchange capacity) was carried out according to
the methods proposed by Pinta (1971) and Guitián and
Carballas (1976). The clay minerals were quantified by Xray diffraction following treatment of the samples with
magnesium chloride and ethylene glycol according to the
method described by Brindley and Brown (1980), using a
Siemens D5000 instrument with CuKα radiation.
4. Results
The macromorphological analysis of the soil profiles
studied (Table 1) indicates that these soils have been
truncated and buried by aggradative processes under more
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
discontinuities that appear at a depth of around 50 cm. The
paleohorizons studied have mainly well-developed polyhedral and prismatic structures, except the A.1 profile developed on quartz diorites, where no types of aggregates
were found (single-grain structure). The depth of these
paleohorizons is between 55 cm (A.3) and 200 cm (A.4);
beneath this layer, deeper horizons, associated with the
boundary of current illitic alteration (Table 2), gradually give
way to the parent material.
The kaolinitic palaeosols (A.1, A.2, A.3 and A.4 profiles)
are associated with upper positions in the relief and therefore
with good drainage; they have reddish yellow and yellowish
red colours (5 YR (d)), pH values between 4.6 and 5.4; their
exchange complexes are desaturated (Table 3) and their clay
fraction is dominated by kaolinite with values around 75%
(Table 2) (Fig. 3). Textural analysis of these paleohorizons
(Table 4) confirms the dominance of clay, with values
between 31 and 47%, with a particularly high content of 2Bt1
in A.4, 64.6%, corresponding to a Terra Rossa formation.
The Fed/Fet ratio for the palaeohorizons where this value has
been determined (Table 5) is around 50% and the average is
between 43 and 60%.
The paleohorizons corresponding to the soils B.1 and B.2
present a set of alteration features that are associated with lower
relief positions (B.1) or palaeo-bottom positions (B.2). They
Table 2
Physico chemical characteristics of profiles studied
391
display 10 YR hues (yellowish brown and very dark brown),
saturated exchange complexes and slightly acid pH values
between 6.1 and 7.0; clay contents are always over 34% (57.7%
2Bw in B.2). The clay mineralogy appears to be dominated by
smectite (64–70%). Kaolinite is only an accompanying mineral
(24–36%), and illite was not detected in these paleohorizons
(Table 2). The Fed/Fet ratio gives values close to 25%.
The soil profiles C.1 and C.2 are located respectively on
fluvial Pliocene deposits and alluvial-fan deposits (Rañas)
that are disconnected from the current Quaternary river
network. The palaeohorizons studied in soil C.1 correspond
to the chroma 7.5 YR6/8(s) (reddish yellow), whereas the
colours of the C.2 profile range from 5 YR5/8(s) hues
(yellowish red) in the horizon 2B2 to the colourless ones
found in horizon 2B3 caused by hydromorphy. The exchange
complex is desaturated and has acid pH levels of 4.5–4.7;
kaolinite and illite in the clay fraction are close to 50%.
5. Discussion
Paleohorizons of soil profiles A.1, A.2, A.3 and A.4 have
enabled us to determine a relict ferruginous (Duchaufour,
1984), acric (F.A.O., 1989) or ultisol pedogenesis (Soil
Taxonomy, 1975) in the southernmost sector of the Iberian
massif, developed from diverse lithologies such as limestone,
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M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395
Table 3
Exchange complexes characteristics
quartz diorite, granite or greywacke. The paleoenvironmental
significance of the kaolinite contents indicates subtropical
conditions for the genesis of these paleoalterites. Furthermore, the red hues of the paleohorizons might indicate drastic
climatic contrasts, including pronounced dry seasons.
The clay mineralogy of deep horizons from B.1 and B.2
profiles seems to be dominated by smectites, with kaolinite
being present only as an accompanying mineral; the absence of
illite suggests an imbalance of these alterites with current
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
and kaolinites could be neoformation clays associated with
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
Table 5
Different forms of iron of selected profiles. (Fed: dithionite iron; Fet: total iron.)
394
M.A. Núñez, J.M. Recio / Catena 70 (2007) 388–395
from 3.1 Ma onwards (Suc et al., 1999a; Capozzi and
Picotti, 2003).
Although significant quantities of kaolinite were not
registered in soils and deposits from the Late Pliocene in the
northern regions of the Iberian Peninsula (Anadón et al.,
2002) and Italy (Leone et al., 2000), our soil profiles C.1 and
C.2, developed on Pliocene alluvial deposits displayed
moderate proportions of kaolinite (50%), and an absence of
smectite. A shorter exposure time to these Pliocene alteration
conditions (during the Piacenzian period?) could well
explain the presence of these moderate levels of kaolinite.
The absence of smectite in the paleohorizons of C.1 and C.2
soils could be an indication that their pedogenic conditions
favoured good drainage, associated with the movement of the
alluvial deposits on which they developed towards higher
positions. This inversion of the relief formation, associated
with the rising dynamic of the Plio-Quaternary river network
of Sierra Morena, developed from the continentalisation of the
Guadalquivir basin, would have occurred before the definitive
disappearance of these subtropical conditions in the Hercynian
zone, at around 3.1 Ma.
Finally, the paleopalynological studies of Suc et al.
(1995b) and Fauquette et al. (1999) highlight a clear
biogeographical contrast during the Early Pliocene (Zanclean), between the north western Mediterranean regions
with subtropical flora and the south western regions with
flora similar to that which exists nowadays in the area. These
authors have managed to mark out a boundary running
approximately from the centre of Portugal along the bed of
the river Maior, close to Lisbon, to Cataluña (Garraf-1, close
to Barcelona). The absence of evidence pointing to
subtropical conditions to the south of our study area (Agustí
et al., 2001; Günster and Skowronek, 2001) leads us to
propose the region of Sierra Morena as the southernmost
boundary of this subtropical sector of the NW Mediterranean region.
6. Conclusions
The existence of paleosols that are rich in kaolinite in the
southern sector of the Iberian massif allows us to determine a
subtropical ferruginous pedogenic phase, occurring in the
early (Zanclean) and middle Pliocene period (Early
Piacenzian).
Other paleosols, associated with more low-lying areas
in depressions, which have smectite-kaolinitic clay mineralogy and no illite presence, could be considered pedogenic
variants of this same Pliocene subtropical phase.
The moderate proportions of kaolinites (around 50%)
found in the cross-sections developed on alluvial deposits,
could be linked to shorter exposure times to subtropical
conditions (3.6–3.1 Ma ??), after they had become disconnected from the Quaternary river network and moved
into higher positions.
All the paleoenvironmental evidence we have so far about
the south of the Iberian Peninsula suggests that during the
Pliocene subtropical conditions did not occur any farther south
than the southernmost boundary of the Hercynian massif.
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