Some aspects of paleoweathering in the Iberian Hercynian Massif

I CATENA

vol. 17, p. 333-346

Cremlingen 1990

SOME ASPECTS OF PALEOWEATHERING IN T H E I B E R I A N H E R C Y N I A N M A S S I F ~ E. Molina, M. Cantano, M.A. Vicente, Salamanca E Garcia Rodriguez, Madrid Summary The Iberian Hercynian Massif occupies most of the western half of the Iberian Perninsula and forms the basement over which Mesozoic, Cenozoic and Quaternary materials rest unconformably. Over it can be found remains of weathering profiles that are fossilized by marine and/or continental sediments of different ages. On the southwestern border of the Tertiary basin of the Duero and below the series attributed to the lower Tertiary (Palaeocene), lies a weathering mantle that has been displaced by the effects of alpine tectonics. Accordingly, its remains may appear truncated and/or covered by other more recent deposits, The present work studies this ancient weathering mantle in the zones in which

an expandible smectite-like mineral. This first phase reflects the disappearance of albite, and chemical analysis points to a pronounced loss of Na. In' the most advanced stage the chlorites and smectites have disappeared, giving way to the presence of kaolinite, goethite and hematites. This weathering phase is associated with a strong loss of Mg. The main processes occurring in these profiles are monosiallitisation and ferruginisation.

Resumen El Macizo Hercinico Ibbrico ocupa la mayor parte de la mitad oeste de la Peninsula Ibrrica y forma el basamento sobre el que se apoyan, discordante-

mente, los materiales depositados durante el Mesozoico, Cenozoico y el Cuit appears developed over the slaty se- aternario. Sobre este zrcalo se pueden ries of the basement in the province encontrar restos de antiguos perfiles of Salamanca, central west Spain. The de alteracirn fosilizados por materiales profiles chosen reveal the existence of a marinos y/o continentales de diferentes strong chemical weathering that involves edades. En el borde suroccidental de the destruction of some primary materials (Fe-bearing chlorites and feldspars) la cuenca terciaria del Duero, y por deand the formation of kaolinites and Fe- bajo de las series atribuidas al Terciario mas inferior (Paleoceno), existe un anoxihydroxides. The start of the degrada- tiguo manto de alteracirn desnivelado tion of chlorite occurred during the first por los efectos de la tectrnica alpina. weathering phases; this evolved towards Los restos de este manto de alteracirn ISSN 0341-8162 @1990 by CATENAVERLAG, D-3302 Cremlingen-Destedt,W. Germany 0341-8162/90/5011851/US$ 2.00 4- 0.25

pueden aparecer truncados y/o cubiertos * This work was supported by the CAICYT (Spain) under Projects 84/1113 & 84/0140-006-00.

CATENA--An Interdisciplinary Journal of SOIL SCIENCE--HYDROLOGY--OEOMORPHOLOGY

Molina, Garcia Rodrigucz, Cantano & Viccnte

334 +

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series

profiles studied

1

Faults

Pre-

Otdavician

Groniles il

and

series

(schi$1- greywacWe complex

Gronitolde$

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Fig. 1: Geographical and geological situation of the profiles studied. par materiales mas recientes, En el presente trabajo se estudia este antigua manta de alteraci6n cuando aparece desarrollado sabre las series pizarrosas del basamento hercinico en la provincia de Salamanca, centro oeste espafiol. Los perfiles elegidos revelan la existencia de una alteraci6n quimica que lleva consigo la destrueci6n de algunos primarios (feldespatos y cloritas ferriferas) y la formaci6n de kaolinitas y oxihidroxidos de Fe. En las primeras fases de la alteraci6n, las cloritas se transforman en un mineral hinchable de naturaleza e smectitica y desaparece la

C?AIENA--An

albita. En los anhlisis quimicos se aprecia una fuerte p6rdida de Na. En las partes mhs superiores las cloritas y smectitas han desaparecido y aparecen par el contrario kaolinitas, goetita y hematites. Esta fase de la alteration va asociada con una fuerte p6rdida de Mg. Los procesos fundamentales que suceden en estos perfiles son la monosialitizacion y ferruginizacion.

1

Introduction

According to H E R N A N D E Z PACHECO (1932)the "Hesperian Massif" is the

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Paleoweathering in the Iberian Hercynian Massif

335

ensemble of geological materials which forms the fundamental structure of the Central Plateau of the Iberian peninsula, In this Massif SOLE & LLOPIS (1952) distinguish what they call the "Hercynian Massif", which is formed by the materials affected by the Hercynian Orogeny (fig.l). Its geological structure is defined by a set of blocks whose changes in level during the Alpine Orogeny created the mountain and basin systems in the Peninsula. One of the features of the relief of this Hercynian Massif are the remnants of

(ALIA 1972) is located on the southern side of the Central System. It is a set of fractures directed East to West and because of them the River Tajo flows westwards within a tectonic graben. Accordingly, its drainage pattern is more deeply entrenched than other rivers of the Central Plateau. The Central System and the Montes de Toledo range are the elevated blocks that border this graben.

extensive planation surfaces which can be found even at the summits of the elevated blocks in the Central System, in the Montes de Toledo, or in the Sierra Morena range. These levels are the remains of ancient erosion surfaces whose ages, until now, have not been well established. Most authors agree that at the end of the Mesozoic period this Hercynian Massif displayed an almost flat relief (the "fundamental polygenic peneplain" according to PEDRAZA 1978) which was later unevenly displaced during the Alpine Orogeny. Its fundamental deformation began more or less during the Mesozoic-Tertiary transition with the development of the "CastilianExtremaduran Dome" as defined by ALIA (1976). The later evolution of this dome gave rise to the present Central System (PORTERO & AZNAR 1984). On its northern side appear remnants of the peneplain fossilized by continental series of siderolithic nature and attributed to the Palaeocene. The age of the top of this series is about 58 m y. (BLANCO et al. 1982) which confirms the pre-Palaeocene age of the ancient peneplain, The "Structural Band of Toledo"

In the contact zones between the Hercynian basement and the Mesozoic or Tertiary covering, there are remnants of weathering profiles of several metres thick. These alterites can be found beneath different covering: Triassic (VIRGIL et al. 1974); Cenomanian (MOLINA & BLANCO 1980); Tertiary deposits (CORROCHANO 1977, ALONSO 1982, MARTIN SERRANO 1985); the Plio-Pleistocene "Rafias" (ESPEJO 1978, VAUDOUR 1979, MOLINA et al. 1987); or even appearing at the surface on the summits of some mountains in Spain (RIEDEL & SCHMIDT LORENZ 1978, GUTIERREZ & RODRIGUEZ 1978) and Portugal (DAVEAU 1969). According to mineralogical and morphostructural studies, different authors have concluded that the Hercynian Massif has been affected by several periods of weathering (SAAVEDRA & MARTIN PATINO 1983, MOLINA et al. 1987). GARCIA ABBAD & MARTIN SERANO (1980) report that at the end of the Mesozoic period this basement had a polygenic weathering mantle. The lower part of this mantle, which is the weathering front, had a differential topography depending on the resistance of the geological

1.1

Weathering on the Iberian Hercynian Massif

CATENA--An Interdisciplinary J o u m a / o f SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y

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materials to being weathered. The first tectonic Alpine phases gave rise to the following: 1. the beginning of the CastilianExtremaduran dome with the regression of the Cretaceous sea to the East: fini Cretaceous-Palaeocene regression, 2. a continental deposition of siderolithic sediments on the cretaceous marine limestones, the contact being a single unconformity (FERN A N D E Z G A R C I A 1987, CENTENO CARRILLO 1987), 3. the beginning of the exhumation of the etchplain beneath the weathering front,

other younger Tertiary deposits (CANTANO & M O L I N A 1987). The relief currently shown by the Hercynian basement is a result of the following factors: 1. the tectonics of the blocks, that are Alpine in age, 2. the erosion of the ancient weathering mantle, thus giving rise to the etchplain, 3. a repetition of the fossilization and exhumation processes of the etchplain during the whole of the Tertiary.

The relief of the northern border of the graben is flat, due to the exhumation of the etchplain, which was first fossilized 2 Materials by the Palaeogene series, and still being exhumed. The relief of the southern borThe area studies is located in the Duero der is also a result of the faulting, lifting basin, at the western side of the Central and uncovering of the ancient etchplain, Plateau, close to the Portuguese border, which is partially fossilized by the NeoThe regional geological structure of the gene alluvial fans. Today the erosive prozone can be defined as a tectonic graben cesses are removing this cover, exposing (the "Ciudad Rodrigo" graben). This is the more or less weathered basement. Alpine in origin and is filled with Tertiary Mineralogical studies of these alterites and Quaternary sediments (fig. 1). On the have mainly been carried out when they northern border of the graben the Her- are developed on granitic rocks, but are cynian basement gradually dips towards much scarcer in non-granitic materials. the Tertiary covering while the south- Recently, M A R T I N SERRANO (1985) ern border is an elevated block which and VICENTE et al. (1987) have reis the origin of the "Sierra de Francia" ferred to this problem on the south-west range. Beyond this southern border, im- border of the Duero basin (Central west portant alluvial fans are inclined towards Spain). the graben. For the development of the present In different parts of this graben the paper three weathering profiles in the sediments attributed to Palaeocene fos- province of Salamanca have been chosen silize a weathering mantle developed on (fig. 1, profiles 1, 2 and 3). Morphologthe Hercynian basement of more than ical study of this area (CANTANO & 30 m deep. The Alpine tectonic phases M O L I N A 1987) confirms that these alhave removed this mantle and its rein- terites belong to the remnants of the prenants are often eroded and/or covered by Paleocene mantle which has been fracCA'lENA

An Interdisciplinary Journal of SOIL SCIENCE

H Y D R O L O G Y I)Ef,)MORPHOLOt)Y

Paleoweathenng in the Iberian Hercynian Massif

1O00mlJ

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W.Q. West Quarry

PalaeogeneSandstones

E.Q. East Quarry

Ordovician Slates Leucogranites

w /

Weatheringprofiles Faults

Situation of the samples-studied

Fig. 2: Morphostructural situation of profile 1. Geological scheme of the two quarries sampled, and situation of the samples.

C A T E N A - - A n hlterdiseiplinary Journal o f S O I L S C I E N C E - - H Y D R O L O G Y - - - G E O M O R P H O L O G Y

338

Molina, Garcia Rodriguez, Cantano & Vicente

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Fig. 3: Morphological situation of profile 2. Geological scheme and situation of the

samples taken. tured and deformed in this zone. A general description of these profiles is as follows: Profile 1 (fig. 2)

Morphologically it appears on a peneplain at about 820 m above s.l. and geologically it exhibits a basement of folded Ordovician slates affected by a contact metamorphism (DEE OF PETROL-

The profile studied is located on the northern border of the "Cuidad Rodrigo" graben, at the contact between the Hercynian basement and the Tertiary covering, in fact one is working in two very proximal tin quarries that,

OGY, UNIV. OF SALAMANCA 1983) and is very strongly weathered. Resting unconformably over this basement, are Tertiary sandstones of Palaeogene age (CANTANO & M O L I N A 1987): In the east quarry there is a fault within a direction of N 130° whose east side has overall, have permitted the construction been elevated leading to the exhumation of a general profile, of the alterites. Eastwards from the fault This profile is about 22 km west of the Ordovician slates have been intruded Salamanca (41°1'30" N, 5°55'10" W). (ATENA

An Imerdisciplinary Journal of SOIL SCIENCE

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PMeoweathenng in the Iberian Hercynian Massif

339

%i¢1,1 l/L-).

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Miocene conglomerates Palaeogenedeposits Ordovician-Silurian series

0 Situation of the samples studied ~

Yellowish zones within the fissures

Pre-Ordovician series

Fig. 4: Geological scheme and morphological situation of profile 3. The locations of

the samples taken are shown in the scheme of the profile (upper part of figure). by some leucogranitic ring dykes which are also strongly weathered. Owing to their small surface area these leucogranites are not mapped in fig. 1. Profile 2 (fig. 3) The profile is located close to km 15'400 on the road from Salamanca to Tamames (40°52'10 " N; 5045'30' ' W) at about 870 m above s.1. Morphologically, this zone forms part of a pediment gently inclined towards the N and NW over which there is a complex system of Miocene red alluvial

fans (MAZO & J I M E N E Z 1982) arising from the S. The current fluvial network has eroded the Tertiary covering, and remains of an ancient weathering mantle affecting the pre-Ordovician schistgreywacke complex, can be seen under the red alluvial fans in the valleys. It was not possible to obtain a vertical series of samples since there is not a quarry. However, taking into account the fact that the degree of weathering of the slates always decreases progressively with the depth under the red alluvial fans, we consider that all of the

CATENA--AnInterdisciplinaryJournalof SOILSC1ENCE--HYDROLOOY--GEOMORPHOLOGy

Molina, Garcia Rodriguez, Cantano & Vicente

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samples belong to a single truncated and fossilized weathering profile, Profile 3 (fig. 4) This is located close to the village of "El Maillo" on the road towards Ciudad Rodrigo (40030'30 '' N; 6°11'30" W) and between 1000-1040 m above s.l. In view of its morphological position it belongs to the head area of the piedmont of the Sierra de Francia formed by Ordovician quartzites resting unconformably over the '~schistgreywacke complex" (MACAYA 198 l, DIEZ BALDA 1986). The profile studied is developed from the pre-Ordovician slates which have been fossilized towards the north by the quartzite conglomerates of the Neogene alluvial fan systems (MOLINA & JORDA 1984). In it, below a thin covering of detritus of the Neogene conglomerates, there are slates with a strong degree of weathering that varies depending on their depth, According to field observations throughout the profile three "horizons" or weathering gradients can be distinguished. From bottom to top, are as follows: a. the lower level (L.L., fig. 4). The slates appear with no appreciable degree of weathering, conserving the geological schistosity and lamination. The fracture surfaces of these slates are 10Y 6/1 in tone. b. The middle level (M.L., fig. 4). At a depth of some 30 m the schistosity is still conserved but the stratigraphic lamination becomes progressively less obvious towards the top, with the appearance of joint planes defining elongated splinters with a major axis of 10-15 cm. The ( A I ENA

size of these splinters decreases towards the top as the lithological and the tone become more brownish: 10YR 5/1. c. Upper level (U.L., fig. 4). At some 8-10 m depth the schistosity persists but the colour of the weathered slate is strong red (10R 4/4). Towards the topmost part the discontinuity planes have a yellowish tone (10YR 6/8) that become more important and frequent the higher they are in the profile. There are also white discoloured zones (10YR 6/1). This is the most recent process and is usually associated with organic remains. Owing to its good development and easy observation this profile has been considered as a model and, apart from being studied from the mineralogical point of view, a geochemical analysis has also been carried out on it. 3

Methods

A detailed sampling procedure was performed in the different weathering levels of the three profiles chosen. All the samples were studied by X-ray diffraction (XRD). Furthermore, in profile nr.3 ("El Maillo') a geochemical study was conducted with a total chemical analysis of all the levels distinguished together with differential thermal analysis (DTA). In the case of the XRD, the nonoriented powder and the oriented aggregate techniques were used. To differentiate between the different species the method proposed by ROBERT (1975) was employed. For analysis of total elements (Fe, A1, Ca, Mg, Mn, K and Na) the method

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Paleoweathedng in the Iberian Hercynian Massif

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of GARCIA & SAAVEDRA (1983) was Fe-oxihydroxides. In the reddish zones of used, consisting in attack of the sam- this level the smectite content decreases. pies by a disaggregating agent (Acua re- An interesting aspect is that the smectites gia and concentrated HF), later adding are trioctohedral in nature. a boric-lithium solution (Li2CO3, HCL Profile 3 (fig. 4) is very well-developed and H3BO3). in the vertical sense and conserves rocks ranging from practically unweathered 4 Results and discussion material (LL) to the highly weathered level (UL). Tab. 3 and 4 show the reTab. 1 shows the data corresponding to suits of the mineralogical and geochemthe mineralogy of the clays of the sam- ical analyses of the profile, respectively. pies taken from the two quarries in proThe hard grey slate (M 1) is comfile 1 (fig. 2). Comparative study of the posed of iron-bearing chlorite (bavalite data reveals that in the deepest part sam- or chamosite, according to the autors), pied (G 4) the main components are dioctahedral mica, quartz and feldspars. smectite, mica and a small amount of A small part of the chlorite has dekaolinite. As one moves towards the veloped into an expandible material top of the profile the smectite disappears (smectite-like). The M 2 sample, soft while the kaolinite content increases and slate, has a similar mineralogical compoFe-oxihydroxides begin to appear. In sition to the one below, although the the Tertiary covering (G 3 and G 7) the effect at 6.41 A corresponding to alsmectite appears again, bite disappears (fig. 5) and the content Although some of the smectites ap- in expandible chorite increases. In the pearing in the Tertiary may have been highest sample (M 3) the mica is conneoformed, many of them have been in- served, the chlorite-smectite disappears herited from the weathered basement. It altogether and kaolinite and the Feis well known that at a medium level oxihydroxides, goethite and hematite, apin the weathering mantle there are abun- pear (fig. 5). The DTA curves (fig. 6) condant smectites that originated in the evo- firm the disappearance of chlorite and lution of the primary chlorites, as will be the presence of kaolinite and goethite in detailed in the study of profile 3. When M 3. the Tertiary erodes the weathering manThe results of the geochemical data tie at this level the smectites become part (tab. 4) are in agreement with the aboveof the sediment, findings. No significant variations can In profile 2 (fig. 3) sampling was per- be seen in the contents of the different formed only in the slates of the schist- elements, except for a strong loss in the greywacke complex, the mineralogical Na + content in the M 1 and M 2 trandata are shown in tab. 2. In the lowest sition associated with the destruction of level (V 1), apparently not very weath- albite and a loss of Mg 2+ in the M 2 ered, smectite, mica and a small amount to M 3 transition associated with the of chlorite can be found. As one moves destruction of chlorite-smectite. The reup the profile, the chlorite disappears maining elements are conserved in situ, while the mica and smectite remain. In forming kaolinite and Fe.oxihydroxides. the highest level (V 4) kaolinite apIn the three profiles chosen, one over pears together with small amounts of the Ordovician series (profile 1) and two CATENA An Interdisciplinary Journal of SOIL SCIENCE--HYDROLOGY 43EOMORPHOLOGY

Molina, Garcia Rodriguez, Cantano & Vicente

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Samples

Material

Depth (m)

G 7

Sandstone (Tertiary) Slate Slate Slate

G 6 G 5 G 4 G 3

Sandstone (Tertiaryj Slate Slate

G 2 G 1

Semctite

Mica

I

.!

+

2 4 8

T +++

T +++ +++

3

+++

3,5 4

MicaVermiculite

Kaolinite

Goethite and/or Hematite

+++ ++÷+ + +

+ T

~

++

T

+++ +++

++ ++

++ +

+ T

+, ++, +++, + + + + means respectively: small, moderate, high and very high contents. T means traces.

Tab. 1: Clay minerals from profile I at different levels.

Samples

Material

V4

Red slate Ochre slate Slate Slate Slate

V3 V2 V 1

Depth (m) ]3 13 13 15 20

Chlorite

Smectite

Mica

Kaolinite

+++ ++ ++ ++ ++

+ + T T

+

++ ++ ÷ +++ +++ +++

Goethite and/or hematites T T

+, ++, +++, + + + + mean respectively: small, moderate, high and very high contents. T means traces.

Tab. 2: Clay minerals from profile 2 at different levels.

Samples

Material

M 3 M 2 M 1

Slate Slate Slate

Depth (m) 8 25 40

Chlorite

Smectite

Mica

Kaolinite

Goethite and Hematite +

T

++ + ~ ++

++

++-~ +++

Feldspars

Quartz

T T +

++ ++ ++

Tab. 3: Mineralogical composition of slates fi'om the different levels in profile 3.

Samples

Material

M 3 M 2 M 1

Slate Slate Slate

Depth (m)

Fe203 %

AI203 %

CaO %

MgO %

MnO %

K20 %

Na20 %

SiO 2 %

8 25 40

6.4 6.0 5.6

18.9 17.0 17.1

0.04 0.05 0.06

0.3 3.6 3.2

0.05 0.10 0.06

2.6 2.5 2.5

0.1 0.2 1.1

71.6 70.4 70.3

Tab. 4: Chemical analysis of slates from the different levels in profile 3.

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Paleoweathelqng in the Iberian Hercynian Massif

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r<

.;

~a

j

70 67 6'3 59 5`5 5'I 47 '-~ 3'9 3'5 3'1 z7 2'3 19 1'5 ,

7

~.

Fig. 5 : RXD diagrams of samples from

profile 3.

2~

102

~0

. 220

.

. 400

539.6 . 580 .

.

760

i 940

Fig. 6: DTA diagrams of samples from profile 3.

TEMPERATURE (C)

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over pre-Ordovician series (profiles 2 and 3), the results of the different types of analysis point to the existence of a kaolinizing-type weathering. In profiles 2 and 3, where it has been possible to sample deeper weathering levels, chlorite appears in the least weathered rocks. As one moves up each profile, the degree of weathering increases and the chlorite begins to develop into a smectitic material and this is paralleled by destruction of the less stable mineral (albite). At the end of the process, chlorite and smectite are destroyed; the mobile elements are washed away (Na + and Mg 2+) and the more stable ones (Fe 3+ and A13 ~) persist, forming kaolinite and Fe-oxihydroxides. The remains of this weathering profile appear truncated at different depths owing to later erosion processes,

5

Conclusions

In the central western zone of the Iberian Peninsula and under the series attributed to the Palaeocene, the Iberian Hercynian Massif displays a thick weathering mantle that is currently displaced and fractured by alpine movements. A detailed study was carried out on this weathering manthe where it appears developed over Ordovician slate series and the preOrdovician series of the schist-greywacke complex. The main results of the study are as follows: 1. From the point of view of the evolution of the eastern border of the Iberian Hercynian Massif the existence is assumed of a marine regression that occurred during the Cretaceous-Palaeoeene transition, at least for this border of the Duero basin,

2. The slaty series that form part of the schist-greywacke complex are cornposed of Fe-bearing chlorite, dioctohedral mica, quartz and feldspars. 3. In the first weathering phases on this Hercynian basement, the evolution of chlorite to smectite-like minerals and the destruction of sodium feldspar took place. This process was accompanied by a loss of Na + 4. As one moves up the profiles, the degree of weathering of the minerals increases. In the upper level ohiorites and smectites have diappeared, kaolinite and Fe-oxihydroxides appearing. The dioctohedral mica persists throughout the profile. In this weathering phase the geochemical data point to a strong toss of Mg 2+. 5. It may be concluded that one is dealing with a process in which chemical weathering is dominant. The phenomena of moaosiallitisation and ferrugini~ation can be seen; P E D R O (1966, 1968) defined these as being typical of zones with minimum rainfalls of 500 mm/year and mean annual temperatures above 15-20°C.

Acknowledgements We wish to extend our gratitude to Dr. E. Pellitero, to Mr. V. Nieves Paz and Mrs. E Rodriguez Palacios for technical collaboration.

References ALIA MEDINA, M. (1972): Evolution posth~rcyni~ane darts les rrgions ccntrales de la Mesetaespagnole. 24 th Intern. Geol. Congres, sect. 3, 265-272, Montreal.

CATENA An Interdisciplinary Journal of SOIl, SCIENCE HYDROLOGY GEOMORPHOLOGY

Paleoweathedng in the Iberian Hercynian Massif

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CATENA--An Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y

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Molina, Garcia Rodriguez, Cantano & Vicente

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Addresses of authors: E. Moliaa M. Cantano

Departamento de Geologla, Fac. Ciencias Univ. Salamanca 37071 Salamanca Spain P. Garcia Rodril~aez Inst. Edafologla (C.S.I.C.) Serrano 115 28006 Madrid Spain M.A. Vicente C.E.B.A.S, (C.S.I.C.) Apdo. 257 37071 Salamanca Spain

CATENA An Interdisciplinary Journal of SOIL SCIENCE--HYDROLOGY- GEOMORPHOLOGY