Ž .Catena 42 2001 1–15www.elsevier.comrlocatercatena

Weathering of gneiss in Calabria, Southern Italy

Emilia Le Pera), Salvatore Critelli 1, Marino Sorriso-ValvoConsiglio Nazionale delle Ricerche-Istituto di Ricerca per la Protezione Idrogeologica nell’Italia

( )Meridionale ed Insulare, Via CaÕour, 87030 Roges di Rende CS , Italy

Received 26 October 1999; received in revised form 4 April 2000; accepted 5 April 2000

Abstract

The effects of weathering in a Mediterranean climate on the mineralogy and microfabric ofPaleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observationsshow highly weathered rock forms a residual soil. Micromorphological and mineralogical proper-ties of bedrock and saprolite show that the weathering process is characterized by at least twomajor stages, having two distinct rock microfabrics. In the first stage, the morphological featuresof the original rock are preserved and weathering is manifested mainly by microfracturing, andlarge portions of the rock remain unaltered. The second stage of weathering involves furtherdevelopment of microcracks and progressive chemical attack on the minerals. This latter stageoccurs along both compositional and microstructural discontinuities, with etch pitting of feldspar,and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, andiron-bearing garnet. The determination of quantitative petrographic indices provides a measure ofthe various stages of weathering. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Gneiss; Saprolite; Weathering; Micromorphology; Mineral transformation

1. Introduction

Ž .Calabria southern Italy experiences a Mediterranean climate today. As a result ofŽ . Ž . Ž .relief 0–2000 m , mean annual precipitation 1151 mm and temperatures 10–168C

Ž . ŽCaloiero et al., 1990 , and of a leaching factor among the highest in Europe Le Pera.and Sorriso-Valvo, 2000b , weathering has led to alteration of all primary minerals

) Corresponding author. Tel.: q39-984-835298; fax: q39-984-835319.Ž . Ž .E-mail addresses: lepera@irpi.cs.cnr.it E. Le Pera , critelli@irpi.cs.cnr.it S. Critelli , sorriso@irpi.cs.cnr.it

Ž .M. Sorriso-Valvo .1 Present address: Dipartimento di Scienze della Terra, Universita della Calabria, 87036 Arcavacata di`

Ž .Rende CS , Italy.

0341-8162r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0341-8162 00 00117-X

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Ž . ŽMoresi, 1987; Mongelli, 1993 and to widespread deep sandy regoliths Critelli et al.,. Ž1991 . The weathering products of gneiss are kaolinite, halloysite and Fe-oxides Critelli

. Žet al., 1991 , and kaolinite, illite, an expandable 2:1 clay phase and goethite Mongelli

.et al., 1998 , for the Sila and Aspromonte massifs, respectively.Many of the weathering profiles reported from other European sites appear to be

Ž .pre-Pleistocene features Millot, 1970 ; weathering on the Sila massif may have startedŽ . Žin the Late Cenozoic–Quaternary Nossin, 1973 . It is argued that an AoldB early

.Pleistocene or older regolith surface is partially preserved in the gentler slope areas ofŽ .the Sila massif landscape Le Pera and Sorriso-Valvo, 2000b .

Paleoclimatic information on the Pleistocene is inferred from well developed strati-graphic sequences that filled the Crotone basin, at the eastern border of the Sila MassifŽ .e.g. Gliozzi, 1987; Rio et al., 1996; Massari et al., 1999 . These Pleistocene strati-graphic sequences can be correlated to Pleistocene high-amplitude sea-level oscillations.The lower Pleistocene stratigraphic sequence is correlated to the major climatic transi-

Ž .tion which occurred from marine oxygen-isotope stage 25 to 24–22 Rio et al., 1996 ,and is known to correspond to a major transition of the Earth’s climatic system, marking

Ž .the gradual onset of the glacial–interglacial 100-ka cycles Ruddiman et al., 1989 . TheMiddle–Upper Pleistocene stratigraphic sequences developed during the sea-level fall

Ž .that ended with stage 18 Rio et al., 1996 . On the hilltops of the Sila Massif, remnantsŽ . Ž .of older Early to Middle Pleistocene morphological surfaces paleosurfaces probably

Žcorrelated with unconformities within the Crotone basin Sorriso-Valvo, 1993; Le Pera.and Sorriso-Valvo, 2000b are preserved. The morphological surface is truncated by

Ž .several erosional episodes, such as the 21–18 ka glacial traces Palmentola et al., 1990 .In this study, micromorphological and mineralogical transformations of a biotiteq

sillimaniteqgarnet-bearing gneiss are assessed during weathering to saprolite in aŽMediterranean climate. The study area is the Sila Massif, Calabria, southern Italy Fig.

.1 . The Sila Massif is a section of the Hercynian orogenic belt of western Europe, whereŽallochthonous crystalline rocks are uplifted to form the highest tectonic units Calabrian

. Ž .Arc of the southern Italy fold–thrust belt Amodio-Morelli et al., 1976 . The SilaMassif consists of Paleozoic intrusive and metamorphic terranes, locally with an

Ž .unmetamorphosed, transgressive Mesozoic sedimentary cover Fig. 1 . The Paleozoicterranes consist of gneiss, amphibolite, schist and phyllites, affected by an Alpinemetamorphic overprint, and intruded by Late Hercynian plutonic rocks of the Sila

Ž .Batholith Messina et al., 1991 . The gneisses are more than 800 m in thickness, varyingin structure from massive to migmatitic. Fission track data for the basement rocks of theSila Massif indicate a major period of exhumation ranging from c.35 to c.15 Ma B.P.Ž .Thomson, 1994 .

Ž .The sampled area extends over a zone underlain by the gneiss Fig. 1 . Weatheringand erosion vary markedly across the Sila Massif according to the topographic setting.The central portion of the massif is dominated by a broad upland plateau where

Žweathering exceeds the rate at which slope processes can remove detritus Le Pera,.1998 . In this portion, on flat depressions, gentler slopes and flat hill tops, a thick soil

Ž .cover is present Le Pera and Sorriso-Valvo, 2000b . The highland plateau abruptlypasses to the valley sides with a sharp increase of slope gradient. These steeper slopes

Ž .are subject to frequent landslides Cascini et al., 1992 , and mechanical erosion is more

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Ž .Fig. 1. Geologic sketch map of the Sila Massif modified from Messina et al., 1991 with location of thestudied weathering profiles.

Ž .pronounced than chemical weathering Le Pera and Sorriso-Valvo, 2000a . In theseinstances, the dynamics of weathering is strongly controlled by relief characteristicsŽ .e.g., Thomas, 1994; Teeuw et al., 1994 .

2. Methods

Samples were collected in five weathering profiles developed in coarse-grained,Ž .garnet–biotite-bearing gneiss Fig. 1 . Sampling sites of the gneissic weathering profilesŽ . Žare on both flat depressions sites 1, 2, 3, 4; Fig. 1 , and on plateau remnant site 5;

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.Fig. 1 . Based on visual inspection in the field, different weathering stages, from freshrock to saprolite, were distinguished on the basis of hardness, and the loss of texture andcolor of biotite and garnet. Weathering stages have been defined according to the

Ž . Ž .scheme of Irfan and Dearman 1978 , which recognizes six stages from fresh rock I toŽ .residual soil VI . For each site, the zones of the weathering profile were reconstructed,

and samples of weathered material were collected as changes in color and texture wereŽobserved. Samples described in this study fall dominantly into stage IV highly

. Ž . Žweathered and V completely weathered ; only one sample falls into stage II slightly

.weathered .

Table 1Ž .Characterization of Sila Massif gneiss in terms of decomposition index X ; Lumb, 1962 and microfabricd

Ž .Baynes and Dearman, 1978b

Weathering Sample N N Decomposition Microfabricq q0Ž .profile index Xd

1 ELP5 0.65 0.44 0.37 granular-framework1 ELP51 0.64 0.44 0.36 granular-framework1 SC5 0.7 0.44 0.46 clay-matrix

a1 SC18 0.81 0.44 0.66 clay-matrixa1 SC16 0.86 0.44 0.75 clay-matrix

2 ELP52 0.6 0.44 0.29 granular-framework2 ELP53 0.48 0.44 0.07 granular-framework2 ELP54 0.61 0.44 0.3 granular-framework

a2 SC17 0.84 0.44 0.71 clay-matrix2 SC5 0.7 0.44 0.46 clay-matrix

a2 SC2 0.72 0.44 0.5 clay-matrix3 1 0.6 0.3 0.43 granular-framework3 2 0.61 0.3 0.44 granular-framework3 3 0.61 0.3 0.44 granular-framework

a3 4 0.7 0.3 0.57 clay-matrixa3 5 0.7 0.3 0.57 clay-matrix

3 6 0.67 0.3 0.53 clay-matrix3 SC4 0.63 0.3 0.47 clay-matrix

a4 ELP41 0.58 0.3 0.4 granular-framework4 ELP3 0.43 0.3 0.18 granular-framework4 ELP31 0.37 0.3 0.1 granular-framework

a4 ELP32 0.51 0.3 0.3 granular-framework4 ELP21 0.72 0.3 0.6 clay-matrix4 ELP42 0.75 0.3 0.64 clay-matrix4 ELP23 0.72 0.3 0.6 clay-matrix4 ELP24 0.71 0.3 0.58 clay-matrix5 SC31 0.4 0.3 0.14 granular-framework

a5 SC33 0.6 0.3 0.42 granular-framework5 SC25 0.68 0.3 0.54 clay-matrix5 SC26 0.64 0.3 0.48 clay-matrix5 SC27 0.75 0.3 0.64 clay-matrix5 SC4 0.66 0.3 0.51 clay-matrix5 SC6 0.78 0.3 0.68 clay-matrix5 SC32 0.8 0.3 0.71 clay-matrix

a Ž .Samples also used for calculating micropetrographic index I in Table 2.p

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Gneiss presents a complex weathering profile, with a thin soil horizon topping a thicksaprolite, overlying the fresh rock. Both the presence of pegmatitic dykes and quartzveins traversing the rock, and the preservation of joint planes, indicate that the profilesare developed in situ. The organic soil thickness does not exceed 1.0 m, it is dark brownin color, and has mostly a coarse-to-medium sandy texture. The soil grades down to a

Ž . Ž . Ž .thick 10–20 m or to a very thick 40–60 m or more Critelli et al., 1991 saprolitehorizon.

Samples, from fresh rock to saprolite, were thin sectioned for microfabric andmineralogical analysis. Besides petrological microscope analysis, some saprolite grains

Ž .were also examined in a scanning electron microscope SEM . The progress of weather-Žing has been characterized through calculation of the decomposition index X ; Lumb,d

.1962 , indicating the extent to which the rock microfabric and composition are affectedŽ .by weathering, leading to soil formation Table 1 . This index, according to Lumb

Ž .1962 , has been defined as X sN yN r1yN , where N is the weight ratio ofd q q0 q0 q

quartz and feldspar in the soil sample, N is the weight ratio of quartz and feldspar inq0

the original rock. N has been determined by separating the grains by hand pickingq

under a microscope using the soil fraction retained on a 500-mm aperture sieve. N wasq0

obtained by artificial crushing of the original bedrock. This artificially crushed rock wasŽ .successively dry sieved to obtain the same sand fraction 500 mm as the associated soil

samples.Ž .A selection 10 samples of the original uncrushed bedrock samples, thin sectioned

Žand point counted, was also used for calculating the micropetrographic index I ; Irfanp. Žand Dearman, 1978; Table 2 , to assess different stages of weathering e.g. Irfan and.Dearman, 1978; Tsidzi, 1986 . This index is defined as I s% sound constituentsr%p

unsound constituents, where sound constituents are primary and accessory unalteredminerals, and unsound constituents are secondary minerals such as clay minerals,chlorite, sericite, iron-oxides, together with microcracks and voids. To distinguishbetween the artificially crushed sample and the entire sample, we used two different

Žsample numbers adding AaB to the latter for example, SC18 sample used for calculating

Table 2ŽCharacterization of Sila Massif gneiss in terms of micropetrographic index, I , and weathering stages Irfanp

.and Dearman, 1978

Weathering Sample Unaltered Altered Microfracture Micropetrographic WeatheringŽ . Ž . Ž . Ž .profile minerals % minerals % qvoid % index I stagep

1 SC18a 58 43 0 1.35 V1 SC16a 56 37.4 6.9 1.26 V2 SC17a 45 54 1 0.82 V2 SC2a 53.2 44.8 2 1.14 V3 4a 54.1 42.3 3.6 1.17 V3 5a 43.1 54.8 2.1 0.75 V4 ELP41a 79 21 0 3.76 IV4 ELP32a 77.2 22.3 0.5 3.38 IV5 SC33a 73.3 26 0.7 2.74 IV5 Sm12b 88.6 11.4 0 8.09 II

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.N , and SC18a sample used for calculating the I ; Tables 1 and 2 . Only one bedrockq0 pŽ .sample Sm12b; Table 2 , collected at site 5, represents the freshest rock we studied.

3. Results

3.1. Parent rock petrography

The fresh rock is a medium-coarse grained and garnet-rich gneiss with a porphyrob-Ž .lastic texture. Its main constituents are plagioclase, quartz, k-feldspar microcline ,

Ž . Ž .biotite, iron-bearing garnet almandine , and sillimanite fibrolite ; accessories includeŽ .opaques ilmenite, pyrite , zircon, sphene, apatite, and cordierite. The plagioclase is

oligoclase, constituting the largest crystals of the rock. Fine-grained sericite occursŽ . Žwithin plagioclase, preferentially along twin planes Fig. 2a , or more pervasively Fig.

.2b , as bright yellowish birefringent laths a few microns in size. A pre-weathering originŽfor sericite has been considered Dixon and Young, 1981; James et al., 1981; Velbel,

. Ž1983 , but a formation during deep weathering conditions cannot be ruled out Irfan and.Dearman, 1978; Ollier, 1983 .

3.2. Saprolite petrography and grain micromorphology

The saprolite preserves the texture of the parent rock, even if most of the rock has aŽ .deep-brown iron-staining Fig. 3a . The transition from rock to saprolite is marked by

both granular disintegration and chemical alteration. Incipient alteration shows that themost unstable constituents of the saprolite environment are feldspars, biotite and garnetŽ .Fig. 3a,b .

Feldspars have an opaque, brownish appearance, due to a microcrystalline secondaryŽproduct, that is probably a clay mineral replacement during weathering e.g. Velbel,

.1983; Tazaki and Fyfe, 1987 . A combination of kaolinite and halloysite has beenpreviously identified as a possible feldspar derivative occurring on Sila Massif weath-

Ž .ered gneiss Critelli et al., 1991 .Feldspar grains from saprolite exhibit both chemical alteration, indicated by uniform

clay coatings under the SEM, and an incipient disintegration, indicated by transgranularŽ .and intragranular microcracks Fig. 4a,b that have started to open. Biotite, in various

degrees of chloritization, or replaced by iron oxides, along cleavage planes, has alsoŽ .been observed Fig. 3a . Garnet alters to ferruginous products, grown as linings along

Ž .grain cleavage, and around grain surfaces Fig. 3b , that are very likely to develop withŽ .increased weathering in the oxidized, vadose, zone of saprolite Velbel, 1984 .

3.3. QuantitatiÕe indexes and microfabric

Petrographic analysis of the Sila massif gneiss revealed that variable X , and relateddŽ .microfabrics, characterize the rock Table 1 . X values range from 0.07 to 0.75, withd

increased decomposition. X )0.5 samples exceed those with X -0.5. According tod dŽ .Baynes and Dearman 1978b , these two microfabric types can be described as

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Ž . Ž .Fig. 2. Incipient stage of plagioclase alteration in fresh rock crossed polarized light . A Early stage alongŽ . Ž . Ž .twin planes 10= Ps relict Plagioclase; arrow indicates preferential and oriented alteration ; B more

Žadvanced stage of plagioclase alteration with pervasive sericite 10=; arrows indicate small plagioclase.relicts .

Ž . Ž .granular-framework microfabric X -0.5 , and as clay-matrix microfabric X )0.5 .d d

The granular-framework microfabric consists of an interlocking granular aggregateŽ .enclosing isolated, decomposed minerals Fig. 5a , and is clay-poor, whereas, the

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Ž . Ž .Fig. 3. Saprolite micromorphology plane polarized light, 40= . A Original biotite replaced by iron oxides,Ž . Ž .and plagioclase crystals replaced by clay minerals Psplagioclase; Bsbiotite ; B highly altered iron-bearing

Ž .soil garnet almandine , with ferruginous weathering products. Note the presence of subconchoidal cleavage-controlled growth of the weathering coatings.

clay-matrix microfabric is typical of a rock where decomposition products, mainlyŽ .clayey phases, dominate and enclose the remnant original minerals Fig. 5b .

Ž .Results of the micropetrographic index I , Irfan and Dearman, 1978 are shown inpŽ .Table 2. Based upon this index, two main stages IV and V of weathering have been

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Ž . Ž . Ž .Fig. 4. SEM of feldspar from saprolite. A Feldspar grain F in a matrix of platy kaolinite Ka particles.Ž .Note prismatic etch pits penetrating feldspar surface small arrows , and intergranular microcracks in a

Ž . Ž . Ž .sub-parallel arrangement big arrows . Localized opened microcracks creating voids v . B Close up ofclay-coated feldspar grain; arrow indicates intragranular microfracturing.

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Ž . Ž .Fig. 5. Two extreme saprolite microfabric types crossed polarized light . A Granular-framework microfabricŽ . Ž .10= for X -0.5 gneiss samples, constituted by completely weathered feldspar and biotite grains F , andd

Ž .fresh quartz Q , embedded in an oxidized argillaceous matrix. Small arrow indicates intragranular feldsparŽ . Ž .microfracture. B Clay-matrix microfabric 10= for X )0.5 samples, with dominating clay product, andd

enclosing very few original quartzo-feldspathic grains.

Ž .recognized e.g., Irfan and Dearman, 1978; Tsidzi, 1986 . Stage IV is a weakened andiron-stained rock in which the main petrological changes have led to a nearly completealteration of plagioclase and biotite and to a slight decomposition of K-feldspar. Stage V

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Ž . Ž .comprises weakly cemented I )1 and friable I -1 rock that can be intenselyp p

Ž .microfractured by transgranular microcracks Fig. 4a . In this stage feldspar grains areŽ .clay-coated, and most of grain surface is etched Fig. 4b and has intragranular

Ž .microfracturing. Newly formed clay phases Fig. 4a,b gradually extend into the entirerock microfabric, leaving few remnants of the original mineral. Voids and microcracks

Žwithin the rock microfabric are more numerous during later stages of weathering Table. Ž .2 , contributing to the increase of rock porosity Fig. 4a . The stages IV and V are

characterized by petrological changes, and extreme microfracturing, which are typical ofŽ .soil material behavior e.g., Irfan and Dearman, 1978 .

4. Discussion

ŽThe selective weathering process has long been established e.g. Goldich, 1938;.Harriss and Adams, 1966; Tardy et al., 1973; Nesbitt et al., 1980 . Petrographic

observations of rocks that have experienced varying degrees of weathering haveŽdetermined that biotite and plagioclase alter before K-feldspar and quartz e.g.

Isherwood and Street, 1976; Irfan and Dearman, 1978; Pye, 1986; Taylor and Howard,.1999 .

The compositional and textural study of the Sila massif gneiss shows that the initialstage of weathering of biotite produces mainly precipitates of iron-oxides along biotitecleavage planes. Newly formed chlorite, replacing biotite along rims and lamellae, has

Ž .also been observed in a later stage of biotite weathering e.g. Stoch and Sikora, 1976 .The advanced stage of weathering is also marked by growth of feldspar etch pits that areregarded as evidence of mineral solution, and their development as the most importantstage in the chemical breakdown and weathering of feldspar, both in the field and in the

Ž .laboratory e.g. Berner and Holdren, 1979 .The influence of altered biotite and feldspars on the microfabric of the rock can be

Žexerted in different ways e.g. Berner and Holdren, 1979; Velbel, 1983; Taboada and.Garcıa, 1999 , and for the Sila gneiss a greater influence is exerted upon microfabric by´

Ždisintegrating the rock, leading to the process of arenization e.g. Power and Smith,.1994; Teeuw et al., 1994; Taylor and Howard, 1999 . Both the products of oxidation and

the boundaries between biotite and chlorite constitute high-probability sites for rockŽ .disaggregation e.g. Eggleton, 1986; Heins, 1995 . Furthermore, newly formed phases

along cleavage planes expand the mineral, thereby producing a wedge effect whichŽ .fractures adjacent grains e.g. Eggler et al.,1969; Baynes and Dearman, 1978a . Etch pits

Žtend to coalesce as weathering proceeds e.g. Baynes and Dearman, 1978a; Berner and.Holdren, 1979 , increasing crystal microfracturing. As a consequence, etched feldspar

grains are weakened, and can more easily break up to form fragments finer than theoriginal. Indeed, the arenization process, as mainly induced, or enhanced, by chemicaland physical alteration of biotite and feldspar during the weathering process of granitoid

Žrocks, has been found elsewhere in Calabria Moresi, 1987; Mongelli and Moresi, 1990;.Critelli et al., 1991; Calcaterra et al., 1996; Le Pera and Sorriso-Valvo, 2000a,b . The

degree of decomposition index X , having values-0.5, is in accord with the observedd

arenization process, which produces a granular framework microfabric of the gneiss,

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consisting of dominant sand and silt fraction, and a lower percentage of clay. Thismicrofabric is indicative of considerable leaching and high porosity with 30% to 50%

Ž .weathered minerals in the framework e.g. Irfan, 1986; Baynes and Dearman, 1978b .Ž .Consistently, this microfabric is correlated to low I values Table 2 . The progressivep

Žweathering stage is outlined by the occurrence of a clay-matrix microfabric e.g. Baynes.and Dearman, 1978b; Irfan, 1986 . This seems to apply for the Sila Massif gneiss, where

more weathered samples are characterized by fresh quartz grains set in a clay matrix,released from highly decomposed feldspars and biotite with the clay matrix acting as acementing agent. This microfabric, with an X )0.5, and a highest percentage of alteredd

Ž .minerals indicates no leaching Baynes and Dearman, 1978b . The contemporaneouspresence of both granular- and clay-matrix microfabric reflects a complex weathering

Žprofile development, combined with a varying amount of eluviation e.g. Baynes and.Dearman, 1978a; Teeuw et al., 1994 .

Our results on gneiss generally confirm the conclusion reached by Baynes andŽ . Ž .Dearman 1978a,b and Irfan and Dearman 1978 that in an early stage the weathering

of granitoids involves primarily biotite and feldspar decomposition, leading to theopening of grain boundaries, microfracturing, and to the development of an intragranularand transgranular porosity. A later stage should be characterized by mineral dissolution,represented by etch pits on feldspar grains, the formation of authigenic clays, andferruginous products occurring as pseudomorphous replacements of feldspar, biotite, andgarnet, and partially filling interstices between grains. The microfabric of the rockchanges as weathering progresses. Thus, the initial stage of rock decay, namely increasein porosity and microfracture, is reflected in a granular-framework microfabric, whereasprolonged weathering, detected by higher amounts of decomposition products, isidentified by a clay-matrix microfabric.

Clay minerals formation, such as halloysite and kaolinite, in weathered graniteŽ . Ž .Mongelli, 1993 and gneiss Critelli et al., 1991; Mongelli et al., 1998 in Calabriasuggests that weathering of these rocks has occurred for a protracted period of time.

Ž .Halloysite is found in cases of a tropical climatic regime Gerrard, 1994 rather than in aMediterranean environment, so it could be a pre-Holocene relict product of paleoweath-ering, probably forming during warmer and more humid climatic conditions. Kaolinite

Žcan be generated under a variety of climatic conditions Harriss and Adams, 1966;.Gerrard, 1994 , therefore it may have been produced during colderrdrier periods of the

Pleistocene or earlier. The development of the weathering of landsurface could be aresult of intense tectonic uplift and climatic oscillations since, at least, the Pleistocene

Ž .time Nossin, 1973; Le Pera and Sorriso-Valvo, 2000b .

5. Conclusions

The nature of gneiss alteration in Calabria is consistent with the results of mostexperimental and empirical studies. Fundamental changes in the Sila Massif gneisscharacteristics can be related to an initial stage of weathering, mainly involving granulardisintegration, and to a later stage during which chemical decomposition prevails. Thesechanges, and the intensity of weathering, are reflected by the coexistence of two rock

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microfabrics, clay-poor and clay-rich, respectively, which substitute the original rockfabric. These microfabrics are a useful tool to discriminate between weathering stages;nonetheless the saprolite microfabrics are useful to gain a better understanding of theprocesses that control landscape evolution in highly weathered rocks.

Acknowledgements

We are grateful to H. Ibbeken, G. Mongelli, V. Perrone and R. Valloni fordiscussions in the field and suggestions. We are also very grateful to Bob Cullers, CliffOllier and Michael Thomas for suggestions and review of the manuscript. This worksupported by funds of the CNR-Istituto di Ricerca per la Protezione Idrogeologica

Žnell’Italia Meridionale ed Insulare, Italy Project: The effects of climate, morphology andgeology on generation and transport of sediment to the coastal and marine enÕiron-ments of the Mediterranean Sea; Resp. S. Critelli, and Project: Topographic and

( )geotechnical monitoring of the Serra di Buda Landslide Acri, Calabria ; Resp. M..Sorriso-Valvo .

References

Amodio-Morelli, L., Bonardi, G., Colonna, V., Dietrich, D., Giunta, G., Ippolito, F., Liguori, V., Lorenzoni,S., Paglionico, A., Perrone, V., Piccarreta, G., Russo, M., Scandone, P., Zanettin-Lorenzoni, E., Zuppetta,A., 1976. L’Arco Calabro-Peloritano nell’Orogene Appenninico-Maghrebide. Mem. Soc. Geol. Ital. 17,1–60.

Baynes, F.J., Dearman, W.R., 1978a. The microfabric of a chemically weathered granite. Bull. Int. Assoc.Eng. Geol. 18, 91–100.

Baynes, F.J., Dearman, W.R., 1978b. The relationship between the microfabric and the engineering propertiesof weathered granite. Bull. Int. Assoc. Eng. Geol. 18, 191–197.

Berner, R.A., Holdren, G.R., 1979. Mechanism of feldspar weathering: II. Observations of feldspar form soils.Geochim. Cosmochim. Acta 43, 1173–1186.

Calcaterra, D., Parise, M., Dattola, L., 1996. Caratteristiche dell’alterazione e franosita di rocce granitoidi nel`Ž .bacino del Torrente Alaco massiccio delle Serre, Calabria . Boll. Soc. Geol. Ital. 115, 3–28.

Ž .Caloiero, D., Niccoli, R., Reali, C., 1990. Le precipitazioni in Calabria 1921–1980 . CNR-IRPI Geodata 36,53 pp.

Cascini, L., Critelli, S., Di Nocera, S., Gulla, G., Matano, F., 1992. Grado di alterazione e franosita negli` `Ž .gneiss del massiccio silano: l’area di San Pietro in Guarano CS . Geol. Appl. Idrogeol. 27, 49–76.

Critelli, S., Di Nocera, S., Le Pera, E., 1991. Approccio metodologico per la valutazione petrografica del gradoŽ .di alterazione degli gneiss del massiccio silano Calabria settentrionale . Geol. Appl. Idrogeol. 26, 41–70.

Dixon, J.C., Young, R.W., 1981. Character and origin of deep arenaceous weathering mantles on the BegaBatholith, southeastern Australia. Catena 8, 97–109.

Eggler, D.H., Larson, E.E., Bradley, W.C., 1969. Granites, grusses, and the Sherman erosion surface, southernLaramie Range, Colorado–Wyoming. Am. J. Sci. 267, 510–522.

Eggleton, R.A., 1986. The relationship between crystal structure and silicate weathering rates. In: Coleman,Ž .S.M., Dethier, D.P. Eds. , Rates of Chemical Weathering of Rock and Minerals. Academic Press,

Orlando, FL, pp. 21–40.Gerrard, J., 1994. Weathering of granitic rocks: environment and clay mineral formation. In: Robinson, D.A.,

Ž .Williams, R.B.G. Eds. , Rock Weathering and Landform Evolution. Wiley, New York, pp. 3–20.Ž .Gliozzi, E., 1987. I terrazzi del Pleistocene Superiore della penisola di Crotone Calabria . Geol. Rom. 26,

17–79.

( )E. Le Pera et al.rCatena 42 2001 1–1514

Goldich, S.S., 1938. A study of rock weathering. J. Geol. 46, 17–58.Harriss, R.C., Adams, J.A.S., 1966. Geochemical and mineralogical studies on the weathering of granitic

rocks. Am. J. Sci. 264, 146–173.Heins, W.A., 1995. The use of mineral interfaces in sand-sized rock fragments to infer ancient climate. Geol.

Soc. Am. Bull. 107, 113–125.Irfan, T.Y., 1986. Description and classification in granitic soils from Shouson Hill. Hong Kong, Geotechnical

Control Office, Spec. Project Rep., April 1986, 25–35.Irfan, T.Y., Dearman, W.R., 1978. The engineering petrography of a weathered granite in Cornwall, England.

Q. J. Eng. Geol. 11, 223–244.Isherwood, D., Street, F.A., 1976. Biotite-induced grussification of the Boulder Creek Granodiorite, Boulder

County, Colorado. Geol. Soc. Am. Bull. 87, 366–370.James, W.C., Mack, G.H., Suttner, L.J., 1981. Relative alteration of microcline and sodic plagioclase in

semi-arid and humid climates. J. Sediment. Petrol. 51, 151–164.Le Pera, E., 1998. Relazioni composizionali tra aree fonte e sabbie fluviali, costiere e marine attuali e recenti

in Calabria settentrionale. PhD Thesis, University of Bologna, Italy, 278 pp.Le Pera, E., Sorriso-Valvo, M., 2000a. Weathering, erosion and sediment composition in a high-gradient river,

Calabria, Italy. Earth Surf. Processes Landforms 25, 277–292.Le Pera, E., Sorriso-Valvo, M., 2000b. Weathering and morphogenesis in a Mediterranean climate, Calabria,

Italy. Geomorphology, in press.Lumb, P., 1962. The properties of decomposed granite. Geotecnique 12, 226–243.Massari, F., Sgavetti, M., Rio, D., D’Alessandro, A., Prosser, G., 1999. Composite sedimentary record of

Ž .falling stages of Pleistocene glacio-eustatic cycles in a shelf setting Crotone Basin, south Italy . Sediment.Geol. 127, 85–110.

Messina, A., Compagnoni, R., De Vivo, B., Perrone, V., Russo, S., Barbieri, M., Scott, B., 1991. GeologicalŽ .and petrochemical study of the Sila Massif plutonic rocks northern Calabria, Italy . Boll. Soc. Geol. Ital.

110, 165–206.Millot, G., 1970. Geology of Clays. Chapman & Hall, London.Mongelli, G., 1993. REE and other trace elements in a granitic weathering profile from ASerreB, southern Italy.

Chem. Geol. 103, 17–25.ŽMongelli, G., Moresi, M., 1990. Biotite–kaolinite transformation in a granitic saprolite of the Serre Calabria,

.southern Italy . Mineral. Petrogr. Acta 33, 273–281.Mongelli, G., Cullers, R.L., Dinelli, E., Rottura, A., 1998. Elemental mobility during weathering of exposed

lower crust: the kinzigitic paragneiss from the Serre, Calabria, southern Italy. Terra Nova 10, 190–195.Ž .Moresi, M., 1987. L’alterazione dei graniti delle Serre orientali Calabria . Rend. Soc. Ital. Mineral. Petrol. 42,

237–248.Nesbitt, H.W., Markovics, G., Price, R.C., 1980. Chemical processes affecting alkalies and alkaline earths

during continental weathering. Geochim. Cosmochim. Acta 44, 1659–1666.Ž .Nossin, J.J., 1973. Use of air photos in studies of slope stability in the Crati basin Calabria, Italy . Geol. Appl.

Idrogeol. 8, 261–287.Ollier, C.D., 1983. Weathering or hydrothermal alteration? Catena 10, 57–59.Palmentola, G., Acquafredda, P., Fiore, S., 1990. A new correlation of the glacial moraines in the southern

Apennines, Italy. Geomorphology 3, 1–8.Power, E.T., Smith, B.J., 1994. A comparative study of deep weathering and weathering products: case studies

Ž .from Ireland, Corsica and Southeast Brazil. In: Robinson, D.A., Williams, R.B.G. Eds. , Rock Weatheringand Landform Evolution. Wiley, New York, pp. 21–40.

Pye, K., 1986. Mineralogical and textural controls on the weathering of granitoid rocks. Catena 13, 47–57.Rio, D., Channell, J.E.T., Massari, F., Poli, M.S., Sgavetti, M., D’Alessandro, A., Prosser, G., 1996. Reading

Ž .Pleistocene eustasy in a tectonically active siliciclastic shelf setting Crotone peninsula, southern Italy .Geology 24, 743–746.

Ruddiman, W.F., Raymo, M.E., Martinson, D.G., Clement, D.M., Backman, J., 1989. Pleistocene evolution:Northern Hemisphere ice sheets and North Atlantic Ocean. Paleoceanography 4, 353–412.

Sorriso-Valvo, M., 1993. The geomorphology of Calabria. A sketch. Geogr. Fis. Din. Quat. 16, 75–80.Stoch, L., Sikora, W., 1976. Transformation of micas in the process of kaolinization of granites and gneiss.

Clays Clay Miner. 24, 156–162.

( )E. Le Pera et al.rCatena 42 2001 1–15 15

Taboada, T., Garcıa, C., 1999. Pseudomorphic transformation of plagioclases during the weathering of granitic´Ž .rocks in Galicia NW Spain . Catena 35, 291–302.

Tardy, Y., Bocquier, G., Paquet, H., Millot, G., 1973. Formation of clays from granite and its distribution inrelation to climate and topography. Geoderma 10, 271–284.

Taylor, R.G., Howard, K.W.F., 1999. Lithological evidence for the evolution of weathered mantles in Ugandaby tectonically controlled cycles of weathering and stripping. Catena 35, 65–94.

Tazaki, K., Fyfe, W.S., 1987. Primitive clay precursors formed on feldspar. Can. J. Earth Sci. 24, 506–527.Teeuw, R.M., Thomas, M.F., Thorp, M.B., 1994. Regolith and landscape development in the Koidu basin of

Ž .Sierra Leone. In: Robinson, D.A., Williams, R.B.G. Eds. , Rock Weathering and Landform Evolution.Wiley, New York, pp. 303–320.

Thomas, M.F., 1994. Ages and geomorphic relationships of saprolite mantles. In: Robinson, D.A., Williams,Ž .R.B.G. Eds. , Rock Weathering and Landform Evolution. Wiley, New York, pp. 287–301.

Thomson, S.N., 1994. Fission-track analysis of the crystalline basement rocks of the Calabrian Arc, southernItaly: evidence of Oligo-Miocene late-orogenic extension and erosion. Tectonophysics 238, 331–352.

Tsidzi, K.E.N., 1986. A quantitative petrofabric characterization of metamorphic rocks. Bull. Int. Assoc. Eng.Geol. 33, 3–12.

Velbel, M.A., 1983. A dissolution–reprecipitation mechanism for the pseudomorphous replacement ofplagioclase feldspar by clay minerals during weathering. Petrologie des Alterations et des Sols, vol. I. In:

Ž . Ž .Nahon, D., Noack, Y. Eds. , Sc. Geol. Mem. Strasbourg 71, pp. 139–147.Velbel, M.A., 1984. Natural weathering mechanism of almandine garnet. Geology 12, 631–634.