Catena 61 (2005) 1–29

www.elsevier.com/locate/catena

Weathering and pedogenesis in the Sila

Grande Massif (Calabria, South Italy):

From field scale to micromorphology

Fabio Scarciglia*, Emilia Le Pera, Salvatore Critelli

Dipartimento di Scienze della Terra, Universita della Calabria, Via P. Bucci,

87036 Arcavacata di Rende (CS), Italy

Received 18 November 2003; received in revised form 31 December 2004; accepted 1 February 2005

Abstract

Weathering and soil features in the upland landscapes developed on granitic terrains of the Sila

Grande Massif (Calabria, South Italy) have been studied in this work. The attempt to link field scale

and micromorphological observations in order to better understand the main weathering processes

and their controlling factors is revealed to be very powerful. Six soil profiles in the area around the

Cecita Lake, representative of different geomorphological units, were characterised by field work,

optical microscopy and SEM-EDS analysis. A strong influence on weathering and pedogenesis by

micro-scale features of primary minerals and parent rock fabrics is shown. Key roles are played also

by relief, in the development and preservation or stripping of regolith + soil mantles in relation to

natural and man-induced phenomena; by time, controlling rates and degree of development of the

main weathering features; by climate, in terms of type and intensity of dominant processes.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Granite weathering; Micromorphology; Quartz textures; Grus; Calabria

0341-8162/$ -

doi:10.1016/j.

T Correspon

E-mail add

(S. Critelli).

see front matter D 2005 Elsevier B.V. All rights reserved.

catena.2005.02.001

ding author. Tel.: +39 984 493549; fax: +39 984 493554.

resses: scarciglia@unical.it (F. Scarciglia)8 emilia.lepera@unical.it (E. Le Pera)8 critelli@unical.it

F. Scarciglia et al. / Catena 61 (2005) 1–292

1. Introduction

Weathering of crystalline rocks and soil features of the Sila Grande Massif, in the

Calabria region (South Italy), have been widely investigated with special emphasis on

granular, in situ disintegration of granitoid/gneissic rocks (grus production) and soil

management, respectively. In more detail, previous papers dealt with the description of

typical weathering landforms, the interpretation of their formational factors and

processes, and their distribution (Matano and Di Nocera, 1999; Le Pera and Sorriso-

Valvo, 2000a). The evaluation of micropetrographic indices has permitted (i)

quantification and comparison of different degrees or stages of weathering (Le Pera

et al., 2001b), (ii) assessment of their effects on rock stability and landslide

phenomena (Critelli et al., 1991; Cascini et al., 1992), and (iii) application to

provenance studies linking siliciclastic sediments in fluvial catchments to source rocks

(Le Pera et al., 2001a). Petrographic and mineralogical analyses performed by

Mirabella et al. (1996) on parent rock materials and secondary clay minerals,

respectively, indicated connections between weathering and pedogenetic processes.

Other works report data concerning detailed soil survey, taxonomy and mapping,

mainly to estimate soil sensitivity to degradation under different land management, for

agriculture purposes (seed-potato crop suitability), pasture and forestry, or to test soil

conservation measures (Lulli et al., 1992; Costantini, 1993; Lulli and Vecchio, 1996,

2000).

Nevertheless, published studies lack an integrated approach to weathering and

pedogenesis, as well as any climatic interpretation or soil age assessment. This paper

attempts to address these questions and some wider issues in weathering studies. The

major factors responsible for weathering of crystalline rocks are widely reviewed and

discussed by Whalley et al. (1982), Twidale (1986), Migon and Lidmar-Bergstrfm(2001), Migon and Thomas (2002), where also the importance of linking different scales

of observations (Viles, 2001) to interpret weathering processes and landforms is

highlighted. The present work approaches this issue, scaling from field investigation to

micromorphological analysis, and combines both weathering and pedogenetic features,

in order to have a more complete understanding of their driving processes and to

interpret them in terms of environmental conditions and landscape evolution. In addition,

the controversial issue about relative roles of physical and chemical factors in grus

production is discussed.

Six selected soil profiles, representative of the main soil types (Umbrisols,

Cambisols and Luvisols, e.g. Lulli and Vecchio, 2000) associated with the major

landforms surrounding the Cecita Lake (central sector of the Sila uplands) are

considered in this paper, with a special focus on Late Quaternary soil evolution, the

most recurrent patterns of weathering, their degree of development and the main

alteration products. Both in situ granite weathering and soil development on granitic

fluvial deposits are taken into account, in order to include a variety of features and

consider weathering in its widest meaning. Physical, chemical and biological processes

operate changes on parent materials exposed to atmospheric agents at or near the

Earth’s surface, whether they consist of basement rocks, rock fragments, sediments,

colluvia or single minerals (cf. Butzer, 1976; Govett et al., 1992; SSSA, 2001).

F. Scarciglia et al. / Catena 61 (2005) 1–29 3

2. Geological, geomorphological and climatic setting

The study area (Fig. 1) is situated between 39810V and 39815VN latitude and between

16825V and 16835VE longitude, in the Sila Massif. This area represents a section of the

Hercynian orogenic belt of western Europe, where allochthonous crystalline basement

rocks are exposed to form the highest tectonic units (Calabrian Arc) of the southern Italy

Fig. 1. Geological sketch map of the study area (after Messina et al., 1991, modified) and location of the studied

soil profiles: (1) clastics (Recent to Tortonian); (2) sedimentary and metamorphic rocks (Oligocene to Devonian);

(3) phyllites+schists and ophiolitic rocks (Cretaceous to Palaeozoic); (4) gneissic rocks (Palaeozoic); (5) Sila

batholith (Palaeozoic); (6) stratigraphic contact; (7) thrust fault; (8) soil profile.

F. Scarciglia et al. / Catena 61 (2005) 1–294

fold–thrust belt (Amodio Morelli et al., 1976). The Sila Massif consists of Palaeozoic

intrusive and metamorphic rocks, locally with an unmetamorphosed, Mesozoic sedimen-

tary cover. A sedimentary succession of Miocene to Pleistocene age, about 2000 m thick,

crops out along the eastern border of the Sila Massif (Roda, 1964; Critelli, 1999). The

Palaeozoic rocks consist of gneiss, amphibolite, schist and phyllite, affected by various

Alpine metamorphic events and intruded by the Late Hercynian Sila batholith (Messina et

al., 1991). Gneisses consist of decimetre- to metre-thick biotite–sillimanite–garnet-rich

layers, varying in structure from massive to migmatitic. Contacts with plutons are marked

by a network of pegmatite dikes and an irregular thermal aureole with amphibolite facies

(Caggianelli et al., 1994). The Sila batholith consists of multiple intersecting intrusions,

heterogeneous in texture and fabric, ranging in composition from granodiorite to gabbro

and leucomonzogranite (Messina et al., 1991). Fission track analyses of apatite and zircon

from the basement rocks of the Sila Massif indicate a major period of exhumation ranging

from ~35 to ~15 Myr (Thomson, 1994).

The study area consists of flat to gently-inclined dissected landforms (summit

Paleosurface), cut across the Palaeozoic plutonic rocks and the Miocene–Pliocene

deposits and therefore developed during Pliocene(?)–Lower Pleistocene (Dramis et al.,

1990; Sorriso-Valvo, 1993; Matano and Di Nocera, 1999; Caruso et al., 2000), and of

younger paleosurface limbs, approximately spanning the whole Pleistocene, often sharply

separated by steep slopes (sometimes corresponding to fault planes), uplifted to different

elevations between 1650 and 1150 m a.s.l. (Ferrini and Moretti, 1997; Caruso et al.,

2000). Quaternary (Sorriso-Valvo, 1993; Matano and Di Nocera, 1999) terraced lacustrine

and fluvial deposits (of the paleo-Cecita Lake), morphologically entrenched within the

higher, uplifted paleosurface limbs and overlying the lower, downfaulted counterparts,

range from about 1270 to 1135 m a.s.l. (Fig. 2). Three main orders can be identified: the

lowest and intermediate terraces (III and II order) cut the lacustrine basin infilling of Late

Pleistocene age (Sorriso-Valvo, 1993), whereas the highest order (I) predates them and

postdates the paleosurfaces, tentatively being ascribed to Middle Pleistocene–Upper

Pleistocene pro parte(?).

A weakly reworked, weathered, yellowish tephra layer can be found towards the top

of Late Quaternary fluvial terraces both at the western and the eastern ends of the

Cecita Lake.

The study area has a typical upland Mediterranean climate (Csb, sensu Koppen, 1936),

i.e. a temperate humid climate with mean monthly temperatures ranging from �1 8C/1 8C(in January) to 16–18 8C (in August) and a not particularly prolonged dry summer

(Tmaxb22 8C), with July (and sometimes August) having less than 30 mm rainfall. Mean

annual precipitation reaches 1400–1600 mm according to elevation and aspect (Versace et

al., 1989; Lulli et al., 1992; Colacino et al., 1997). During winter the snow cover can be

preserved up to 6 months (Lulli et al., 2000). In more details, temperature data recorded at

Camigliatello Silano (1250 m a.s.l.) and Monte Scuro (1720 m a.s.l.), the thermopluvio-

metric stations closest to the study area, are available for the periods 1959–1970 and

1952–1980, respectively. Mean monthly values during the four coldest months (December

to March) range from �0.7 (recorded in January) to 3.8 8C (in March), with mean monthly

minima between �2.6 and 0.0 8C. Daily temperatures below zero commonly occur from

November to April and, although no detailed information about amount and frequency of

P2 P5 P1

P4

P3

Pliocene(?)-Lower Pleistocene

III

III

CecitaLake

Paleosurface limbs

Lower-MiddlePleistocene

Middle-UpperPleistocene(?)

Upper Pleistocene-Holocene

Tephra layer

? ?

II

P6

Clay coatings

Pumice fragments(Holocene)

Slope deposits(Late Holocene)?

Granite bedrock(Palaeozoic)

Lacustrine clays(Upper Pleistocene)

SpheroidalbouldersFaults

Fluvial sands andgravels (Middle-Upper Pleistocene )?

Fluvial sands andgravels (UpperPleistocene-Holocene )?

P1 P6 Soil profiles-

I-III Orders oflacustrine andfluvial terraces

A

R

Ap

Bw

C

P2

P5P6 P3 P4P1

R

AOi

A1

2Bw1

2BC

A2

A3

2Bw2

OiA1

A2

Bw

C

A

Bt

C

CBt

A

2Ab

2BC

2Bwb

Bw1

Bw2

Bw3

Bw4

50 cm

B

Fig. 2. Schematic geomorphological cross-section of the study area with the location (A) and logs of soil profiles

(B). See the text for details (Sections 2 and 5).

F. Scarciglia et al. / Catena 61 (2005) 1–29 5

diurnal cycles is available, oscillations a few degrees around zero can be supposed to occur

rather frequently. Mean monthly temperature of the warmest month (August) is 16–18 8C,with a mean monthly maximum of 24 8C.

The pedoclimate is characterised by a mesic soil temperature regime and a udic soil

moisture regime (sensu USDA, 1998).

As a consequence of these climatic conditions, an important role seems to be played

during the coldest months by a variety of cryonival phenomena, influencing rock

degradation and fragmentation, slow mass movements and soil processes. In particular,

seasonal snow-melt produces prolonged seepage and through-flow (inducing solifluction

and shallow soil slips or leaching phenomena and illuviation of silt particles in some sub-

F. Scarciglia et al. / Catena 61 (2005) 1–296

surface horizons) and overland-flow (inducing surface erosion, often concentrated into rills

or gullies).

Moreover, the leaching factor proposed by Crowther (1930), which depends on mean

annual precipitation and temperature, calculated for the Sila Massif, appears to be among

the highest in Europe (Le Pera and Sorriso-Valvo, 2000a).

3. Methods

A detailed geomorphological survey was carried out to describe the main weathering

features and landforms in the field, and to assess the associated geomorphic factors and

processes controlling the landscape evolution of the study area. Six representative

weathering/soil profiles corresponding to the main, different soil types (Section 1)

associated with the major landforms of this area (fluvial terraces, paleosurface limbs and

slopes with different steepness) were also described and sampled for bulk and

mineralogical analyses and for micromorphological and geochemical investigations.

Weathering features were first described using optical microscopy (OM) on thin

sections (10�5�0.003 cm), prepared after impregnation with a polyester crystic resin and

hardening of undisturbed samples for each soil horizon (FitzPatrick, 1984).

SEM-EDS analyses were performed on bulk samples (sand-textured mineral grains and

soil matrix clays) and thin sections, in order to (i) identify primary mineral species, (ii)

describe their pattern and degree of weathering (morphoscopic study), and (iii) recognise

neogenesis products (clay minerals due to pedogenesis). Samples were mounted on

aluminium stubs, coated with gold and examined with a Stereoscan 360 scanning electron

microscope (Cambridge Instruments), equipped with an energy dispersive X-ray analyser

with a Si/Li-SUTW detector (EDAX, Philips Electronics). X-ray intensities were

converted into weight percentages of oxides applying the ZAF standard matrix correction

procedure (Wilson, 1987).

4. Results

4.1. Weathering processes and landforms: field observations

The granitic bedrock constitutes the substratum both for lacustrine or fluvial deposits

and soils. Due to mechanical stresses building-up during history of tectonic activity and

unloading phenomena related to the emplacement and uplift of the Sila batholith, granites

exhibit very frequent shear zones, fault planes differently striking and dipping, and other

irregularly-arranged joint systems, sometimes intersecting (Fig. 3A and B). These and

other discontinuities (such as subordinate lithologic differences and contacts with

pegmatite dikes) isolate rock blocks and wedges of variable shape and size, which are

extremely prone to failure and occasionally fall down when slope stability thresholds are

overcome, especially from steep slopes and sub-vertical cliffs (Fig. 3B; Critelli et al.,

1991; Cascini et al., 1992). Joint widening is also observed to result from tree root

penetration or windthrow, ice intrusion related to freeze/thaw cycles and chemical

Fig. 3. Two typical outcrops of granites in the study area: (A) the bedrock exhibits intersecting fault planes (white

arrow) and other irregular joint systems, which (B) isolate rock blocks occasionally undergoing falls from cliffs

and steep slopes and accumulating at the foot-zone together with gritty and loosen, grussified debris. Note the

abrupt boundary between the thin soil and the underlying saprolite.

F. Scarciglia et al. / Catena 61 (2005) 1–29 7

weathering. Very often an intense grussification can be observed, leading to a further rock

flawing and crumbling to small polymineral aggregates and even single mineral particles

(grain-by-grain disintegration, sensu Butzer, 1976), as typical of crystalline rocks.

Rockfall debris from the cliffs and associated hillslope deposits appear often underlain

by 1–101 m thick, gritty and loosen grus (Fig. 3B), unsteadily stabilised at high scarp

angles (approximately 408–458) and repeatedly undergoing dry grain flow processes.

Despite the great depth of weathering profiles, ranging from one to some tens of metres,

up to about 100 m (Matano and Di Nocera, 1999; Cascini et al., 1992), the loosen and fine-

textured topsoils are often incomplete and lack of well-differentiated diagnostic horizons.

Sometimes, especially along steep slopes, they are extremely thin, reaching only few

centimetres in thickness, may appear truncated and/or exhibit abrupt boundaries – rather

than gradual transitions – towards the underlying weathered bedrock (Fig. 3B; Le Pera and

Sorriso-Valvo, 2000a; Le Pera et al., 2001b). It is also not uncommon to observe a

complete lack of soil cover, with exposed grus or bare rock, where tree roots directly

penetrate at depth within fractures. Frequently, where the saprolite has been removed,

F. Scarciglia et al. / Catena 61 (2005) 1–298

isolated rounded core-stones and core-boulders (Butzer, 1976; Ollier, 1988), derived from

deep spheroidal weathering (e.g. Ollier, 1967, 1971; Twidale, 1986; Thomas, 1994; Migon

and Lidmar-Bergstrom, 2001; Migon and Thomas, 2002), have been exhumed and outcrop

at the ground surface (Le Pera and Sorriso-Valvo, 2000a; Fig. 4A and B). Especially on the

highest and oldest paleosurface limbs they form wide boulder fields (Fig. 4B). The

exposed boulders are built of medium-grained and equigranular-textured granite, may

reach up to 2–3 m in diameter and appear occasionally split into large blocks along planar

or curved cracks. They have an essentially unweathered inner core surrounded by a weakly

weathered, outer shell, exhibiting encrusting lichens and thin exfoliation (flaking, sensu

Ollier, 1967, 1971) with millimetric to centimetric sheets – often not well defined,

incomplete and/or fragmented – at the rock/air surface (Fig. 5); also oxidation and cleaving

of biotite minerals can be observed, and local reddish iron staining frequently spreading

out from these crystals. More rarely, massive exfoliation (large-scale spalling or sheeting,

sensu Ollier, 1967, 1971) characterise the outline of boulders, showing curved, edge-

tapering sheets about 10–30 cm thick. On the other hand, thicker (up to 1.5 m), well-

developed soil profiles are preserved in protected flat landforms or depressions and along

gentle footslope belts, where also intense reworking and accumulation of colluvial

A

B

Fig. 4. Lots of spheroidal boulders, often splitted into large blocks (A), are exposed at the ground surface of the

highest paleosurface limbs (B).

Fig. 5. A spheroidal boulder showing various thin, curved sheets due to exfoliation (flaking), with incrusting

lichens on the outer shell.

F. Scarciglia et al. / Catena 61 (2005) 1–29 9

material – often including core-stones on the paleosurfaces – is observed. They often

exhibit humbric epipedon and cambic horizon/s, sand to sandy loam texture, very frequent

skeletal grains, sometimes lithic lower boundary towards underlying grus or bedrock, sub-

acid reaction.

4.2. Macromorphological soil features

The main pedological features of the six soil profiles sampled are reported in Table 1.

On the basis of the data produced they have been classified according to the WRB

taxonomy (ISRIC-FAO-ISSS, 1998), as follows: profiles P1, P3 and P6 are Hapli-Humic

Umbrisols, profile P2 is a Hapli-Dystric Cambisol, P4 a Hapli-Leptic Umbrisol and P5 a

Dystri-Cutanic Luvisol. They are soils representative of fluvial terraces, slope landforms

and paleosurfaces surrounding the Cecita Lake.

Soil profile P1 is located on a fluvial terrace along the southeast sector of the lake (1138

m a.s.l.). It is developed on sandy matrix-supported to clast-supported gravels of granitic

(and subordinately metamorphic) rocks, spherical to elongate in shape (approximately 2–

10 cm), with subrounded to subangular outlines.

Profile P2 was sampled on a depositional glacis (1150 m a.s.l.), gently sloping (with an

inclination not higher than 68–78) towards the valley floor of a stream, at the footslope of a

granitic hill along the northwestern margins of the lake. The slope of this glacis, grassy to

shrubby-vegetated, extends a hundred metres in length, after a headward break of slope

from a steeper portion (~208) about 150 m long. It consists of a weakly pedogenised,

stratified slope deposit, with very fine gravel to fine sand texture, overlying a well-

structured buried soil, developed on fluvial sands.

Profile P3 represents the oldest fluvial terrace elevated above the more recent (Late

Quaternary) lacustrine infilling of the basin, at an approximate height of 1270 m in the

Cecita Valley, at the eastern end of the lake. A man-restored pine forest represents its

vegetation cover. Beneath a thick litter and three distinct brownish A horizons – possibly

at least in part representing colluvial layers, as supported by the slight inclination (~58)

Table 1

Some selected features of the soil profiles studied (after Scarciglia et al., in press)

Soil

Profile

Horizon Depth

(cm)

Colour

(moist)

Sand

(%)

Clay

(%)

pH

(H2O)

Org. Mat.

(%)

CEC

(meq/100 g)

Mg

(meq/100 g)

Ca

(meq/100 g)

K

(meq/100 g)

Na

(meq/100 g)

P1 A1 0–5/10 10YR 2/2 56.4 8.0 5.4 9.3 12.4 0.1 1.6 0.7 0.1

A2 5/10–55 10YR 2/2 44.6 10.0 5.8 3.1 10.5 0.7 0.5 0.5 0.1

Bw 55–75 10YR 4/4 59.4 10.0 6 1.0 7.0 0.7 0.5 0.5 0.1

C 75–110+

P2 A 0–10 10YR 4/2

Bw1+Bw2+Bw3 10–125 10YR 5/4–4/4 75.4 8.0 6.1 0.6 5.6 0.7 0.7 0.2 0.1

Bw4 125–158 10YR 4/2 68.4 12.0 6.1 1.1 6.5 0.6 0.5 0.1 0.0

2Ab 158–190 10YR 2/2 55.8 12.0 6.2 3.2 9.0 0.6 0.5 0.1 0.1

2Bwb 190–230 10YR 6/6 75.4 6.0 6.5 0.5 5.3 0.5 0.5 0.4 0.1

2BC 230–260+ 10YR 6/6

P3 Oi 0–5

A1 5–20/30 10YR 2/2 54.6 8.0 5.7 5.7 12.0 0.5 1.0 0.4 0.1

A2 20/30–45 10YR 2/2 46 9.0 6.1 5.4 13.2 0.4 0.7 0.4 0.1

A3 45–60/65 10YR 3/2 46.8 8.0 5.9 3.7 11.5 0.7 0.5 0.4 0.0

2Bw1 60/65–80 7.5YR 4/3 57 8.0 6.4 0.9 7.0 0.8 0.8 0.4 0.1

2Bw2 80–100 7.5YR 4/4 61.8 10.0 6.2 0.6 4.8 0.7 0.6 0.3 0.1

2BC 100–120+ 7.5YR 5/4

P4 Oi 0–2

A 2–25 10YR 3/3 80.6 2.0 5.8 3.6 6.4 0.6 0.9 0.4 0.1

R 25–60+

P5 A 0–30/38 10YR 4/4 42.6 13.6 5.0 3.1 12.4 0.3 1.3 0.4 0.7

Bt 30/38–60 7.5YR 4/6 37.8 25.6 5.5 0.9 10.5 0.6 1.2 0.5 0.8

CBt 60–75 7.5YR 4/3 74.4 15.6 5.6 0.4 7.0 0.5 0.9 0.4 0.6

C 75–100+

P6a Ap 0–35 7.5YR3/2 50.7 10.9 5.6 5.6 23.2 1.5 1.1 0.2 0.4

Bw 35–60 10YR4/4 60.3 11.0 6.0 1.9 31.8 0.9 1.0 1.5 1.5

C 60–65 10YR5/6 73.2 4.9 5.9 0.8

R 65–100+

Org. Mat.: organic matter; CEC: cation exchange capacity; Mg, Ca, K, Na: exchangeable bases.a Data from Mirabella et al. (1996).

F.Scarcig

liaet

al./Caten

a61(2005)1–29

10

F. Scarciglia et al. / Catena 61 (2005) 1–29 11

of the flat surface for about 250 m towards the valley bottom – a buried soil (2Bw1–

2Bw2–2BC) is present. It exhibits reddish colours and includes, in a loam-sandy

matrix, very rare, rounded and elongated rock fragments (F max=15 cm), themselves

reddish to yellowish in colour as intensely weathered. The dearth of good outcrops and

the inability to drill deeper soil profiles made it impossible to determine whether they

represent core-stones typical of spheroidal weathering or coarse fluvial deposits;

however, the presence of nearby fluvial sediments, thinly stratified and sandy in texture

(eastward and upstream along the same valley terraces) better supports the latter

hypothesis. Scarce silt coatings on ped surfaces and skeletal grains occur in the same

buried horizons.

Profile P4 exhibits the granite saprolite, along an E-facing slope (incompletely

covered by a degraded grassland used for grazing) along the southeast border of the

lake (1160 m a.s.l.); slope inclination is about 158 while its length reaches 150 m. The

profile is extremely simple, consisting of a thin A horizon with a clear, linear lower

boundary (lithic contact) to the fissured and weathered bedrock. The organic-mineral

horizon includes frequent, very small skeletal grains (V1 cm), consisting both of single

minerals and irregular, polycrystalline rock fragments. The saprolite appears intensely

fragmented with very thin inter- and intra-granular cracks separating even small

(millimetre to centimetre) mineral aggregates loosely retained together; some crystals of

feldspar are quite weathered, have lost their original translucent aspect and acquired an

almost powdery consistency, with dirty white to pale yellow colours, whereas micas,

often oxidized, may show splitting along cleavage planes or distinct edge exfoliation.

Occasional Fe-oxide segregations and scarce clay coatings occur in inter-granular

spaces.

Soil profile P5 is cut at the top of an alluvial terrace in the southern sector of the

Cecita Lake (about 1150 m a.s.l.). It is developed on fluvial sands, and includes

frequent, rounded, granitic and metamorphic gravels. It is characterised by a yellowish

brown A horizon overlying a Bt and a CBt reddish horizons with abundant and thick

clay coatings.

Profile P6 is located on a grass-vegetated paleosurface limb along the southwestern

border of the Cecita Lake. It is about 1 m deep, with a dark brown Ap horizon

overlaying a yellowish brown Bw horizon and the poorly cohesive gritty grus (C), in

turn developed on the fractured granite rock (R). Parent rock fragments and single

mineral grains (F=1–15 mm) are frequent within the whole soil, but decrease from

bottom to top; C and R show occasional, reddish iron staining and rare illuvial clays

in inter-granular cracks, and slightly to moderately weathered feldspar (often with

whitish, non-translucent appearance) and biotite crystals (exfoliated and/or cleaved

apart).

Particle size distribution analysis showed the dominance of the sand fraction in

almost all soil horizons, with an extremely low amount of clay, except in the argillic

horizon of profile P5. Soil structure, moderately to strongly developed, is usually

angular or subangular blocky (medium to coarse), and crumby to granular in some A

horizons.

Organic matter appears coherently high in diagnostic A horizons. CEC values are low

as a whole, but increase with the organic matter content with the exception of profile P6;

F. Scarciglia et al. / Catena 61 (2005) 1–2912

exchangeable bases are always very low; pH (H2O) is weakly to moderately acid, and

strongly acid in the uppermost A horizon of profiles P1 and P5.

4.3. Micromorphology

All soil horizons have shown the dominance of quartz, feldspars and mica minerals, as

expected from the nature of the bedrock. The main weathering features observed in thin

sections are the following:

Soil profile P1 shows abundant quartz and feldspar grains (rarely microcline and albite

plagioclases) and biotite: they have unweathered surfaces and mechanical cracking

fissures only. Occasional moderate pitting or clay neogenesis (giving a speckled

appearance) and reddish-brown iron oxide staining occur along cleavage lines and

fractures.

In profile P2 the surface cambic horizons (Bw1 to Bw4) consist of very abundant coarse

sand-sized rock fragments (granite and rare phyllites) and monomineralic grains (quartz,

feldspars, biotite). The latter have euhedral, elongated shapes, and mainly show

mechanical fractures with low-intensity chemical weathering. Other grains (anhedral)

show severe etching pits, clay neogenesis and oxidation. Some vesicular pumice fragments

are present and appear extremely weathered. In the buried organic-mineral horizon (2Ab)

quartz dominates among detrital grains; feldspars (some consisting of microcline; Fig. 6A)

are quite weathered, with surface etching, clay formation and iron segregation (Fig. 6B and

B

C

A

D

Fig. 6. Micrographs from Profile P2 (horizon 2Ab): (A) microcline feldspar with typical microperthites (XPL);

(B, C) weathered feldspar grain showing surface etching, Fe-segregation and clay formation (PPL, XPL); (D)

weathered biotite crystal with etching holes, iron staining and exfoliation occurring along cleavage lines (XPL).

F. Scarciglia et al. / Catena 61 (2005) 1–29 13

C); biotite appears from strongly weathered to rather fresh with some edge exfoliation

(Fig. 6D). Very rare pumice fragments occur, spherical, with vesicular structure and

intensely weathered. Also phyllite fragments have been rarely identified. Similar

micromorphological features characterise the 2Bwb horizon, except the presence of

volcanoclastic products.

Profile P3 is characterised by topsoil horizons (A1–A3) which include mineral grains

consisting of quartz, feldspars (among which microcline is frequent, and rarely zoned and

albite-twinned plagioclase can be identified), and micas (biotite dominating, muscovite

uncommon). Rare grains of phyllite occur. In the buried horizons (2Bw1 and 2Bw2), the

degree of crystal weathering is much higher than in the surface soil, sometimes leading to

unrecognisable pseudomorphous grains. Weathering features appear concentrated along

preferential weakness zones (cleavage lines, fractures and Na-microperthites within K-

feldspar grains) or most likely widespread over great part of grain surfaces. Very rare

metamorphic rock fragments and pumices, the latter with elongated, subrounded shape and

intense clay neogenesis associated with iron staining, have been observed.

Mineral grains of profile P4, in horizon A as well as in the saprolite (R), comprise

dominant quartz, abundant feldspars and subordinately biotite. They show essentially

physical weathering, with linear (cleavage-controlled) to irregular cracks and fissures

(Fig. 7A), and reddish-brown iron segregation (Fig. 7B) or scarce clay infillings along

preferential lines. Some fractures appear widened and deepened by chemical solution,

occasionally showing neoformed clay inside; similar features characterise some outer

edges of mineral grains (mostly K-feldspars and plagioclases), thinned, worn and made

irregular by deep etching. Sometimes also moderate pitting occurs on mineral surfaces

(Fig. 7C). Biotite crystals are commonly exfoliated and at least partly split into two or

more fragments along cleavage lines; small fractures are often developed in feldspar

and quartz grains surrounding fragmented biotite minerals in the granite rock

originating from these crystals; in addition, inter-granular cracks may also radiate

from cleaved biotite, separating irregular polycrystalline rock fragments and single

mineral grains.

Also soil profile P5 is dominated by quartz and to a minor extent by feldspars (among

which microcline is present), Na-plagioclases, micas (dominantly biotite and subordinately

muscovite) and rare opaques. Occasional granite and phyllite fragments as well as scarce

metamorphic rocks are present. All of them show evidence of weak to moderate

weathering (mainly exfoliation and solution features), rarely with extremely abundant clay

neogenesis and iron oxide segregation. Fe staining involves also the matrix (up to about

50%), in the form of irregular segregations and subspherical concretions. Some rock

fragments show preferential fragmentation along inter-granular contacts between minerals

weathered at various extents.

In profile P6, mineral grains are dominated by quartz, with abundant K-feldspars (some

consisting of microcline) and twinned or zoned plagioclases, and minor micas (frequent

biotite and rare muscovite), both in soil horizons and in the saprolite/parent rock. Physical

weathering prevails, with linear or irregular fracture patterns, producing a loose, open

fabric in horizon C, coupled with reddish-brown iron segregations and scarce clay

coatings/infillings in cracks. In horizons Ap and Bw chemical features are more common

and a higher degree of weathering is observed on primary minerals. Feldspar grains may

A

B

C

Fig. 7. Micrographs from profile P4 (horizon A and R): (A) physical weathering characterised by linear to

irregular inter- and intra-granular cracks (XPL); moderate chemical weathering consisting of (B) iron segregation

along preferential lines (PPL) ad (C) pitting on mineral surfaces (XPL).

F. Scarciglia et al. / Catena 61 (2005) 1–2914

show outer weathering rims and moderate surface etching, especially along cleavage and

twinning lines (where also neoformed clays may occur); biotites have exfoliated, and show

frequently etched, outer boundaries (sometimes with clay neogenesis or Fe-oxide staining)

and may separate along cleavage lines.

F. Scarciglia et al. / Catena 61 (2005) 1–29 15

4.4. SEM analysis

Scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy

(EDS), permitted to observe, at major magnification, the same weathering patterns

identified on sand grains in optical microscopy, and to assess further features recognisable

at a few microns scale, only. In addition, it permitted identification of some secondary

(pedogenetic) mineral components forming the clay fraction.

SEM morphoscopy of sand grains in profile P1 confirmed the optical microscopy

observations: mineral grain surfaces appear rather fresh, sometimes locally covered by Fe

coatings (Fig. 8A). Biotite crystals have exfoliated outer edges (presumably due to

hydration processes; Fig. 8B), sometimes showing early clay neogenesis (chloritization),

evidenced by a change in chemical composition with an increase in Al and Mg.

Plagioclases are generally more weathered, exhibiting typical etch pitted surfaces.

The analysis of the clay fraction showed that clay platelets are irregularly shaped

and arranged, as typical of pedogenetic origin. They have an extremely low content in

K, Na and Mg, whereas the Si/Al ratio is close to 1, suggesting that clays mainly

consist of 1:1 phyllosilicates, such as the kaolinite or halloysite group. In contrast,

where a weak increase in the Si/Al ratio and in the same bases were detected, possible

illite, chlorite or vermiculite and other interstratified (mixed layers) clay minerals may

be present.

Also in profile P2 the different primary mineral species show clear, polished

surfaces, as a whole. Some K-feldspars contain Na-rich perthites. The morphoscopic

analysis of sand particles in profile P2 showed rather fresh surfaces (sometimes with

solution pits in feldspars and plagioclases) in the top soil as well as in the buried one,

as a result of a weak weathering degree, as a whole. Iron segregations and coatings

preferentially occur on external grain surfaces, along cleavage planes and exfoliated

outer edges (e.g. in biotite minerals; Fig. 8C). In the buried soil of P2, occasional quartz

crystals show extremely superficial etch pits with subspherical, rounded shapes (Fig.

9A). Very rare amphiboles show a typical surface pitting pattern. SEM analysis of the

same horizon in thin sections showed very small glass fragments, consisting of rounded

to subrounded pumices with a vacuolar structure, moderately to strongly weathered,

with evidence of clay neogenesis and disruption of primary vacuoles towards very

irregular holes.

Scanning electron microscopy also allowed identification of the major components of

the clay fraction: halloysite in form of small spherules (e.g. Tazaki, 1979, 1982; Malucelli

et al., 1999; Adamo et al., 2001; Fig. 9A). EDS spectra exhibit Si/Al ratios approximately

comprised between 1 and 2, very high oxygen and iron content (the former suggesting it

occurs in a hydrated state), and extremely low phosphorous amounts, presumably deriving

from soil organic matter and the high phosphate retention of andic soils. The dominant

clay platelets are represented by illite and clay minerals with SiNAl and low amounts of

Mg and Fe, possibly vermiculite or chlorite.

Profile P3 revealed the most peculiar features in the buried soil: K-feldspar grains

locally show Na-microperthites, some possible zeolites (related to an early stage of

diagenesis), and the typical weathering pattern consisting of intense surface pitting and

deep solution lines (along preferred cleavage planes; Fig. 8D). Even most of quartz grains

B

C

A

D

Fig. 8. SEM images of mineral grains: fresh unweathered surface locally covered by Fe coatings (A); biotite crystal with exfoliated outer edges (B, C); K-feldspar with

zeolites (white arrows) and the typical weathering pattern consisting of intense surface pitting and cleavage-controlled solution lines (D).

F.Scarcig

liaet

al./Caten

a61(2005)1–29

16

B

C

A

D

Fig. 9. SEM images of clay minerals: halloysite spherules on grain surfaces, concentrated close to small cavities

(A, B); in Fig. 8A the quartz grain shows extremely superficial etch pits with subspherical, rounded shapes;

chaotically arranged clay platelets with irregular shapes (C); pseudo-hexagonal kaolinite crystals (D).

F. Scarciglia et al. / Catena 61 (2005) 1–29 17

exhibit very widespread weathering microtextures (Fig. 10). The main features are irregular

solution pits, angular to well-rounded holes and deep grooves, as a result of progressive

deepening and coalescence of originally regular etch pits (Fig. 10A and B). Sometimes they

produce an extremely vacuolar, cavernous structure in quartz grains (Fig. 10C). Never-

theless, triangular, hexagonal or rhomboidal outlines – sometimes concentric – of some

etched surfaces, regularly oriented along parallel or crossed preferential lines, are still

preserved on most samples.

Some grains show the presence of at least one flat, relatively sharp and fresh (or

weakly weathered) face, often with stepped fracture lines and angular edges, whereas

the other surfaces appear more irregular and pitted at a different extent (Fig. 10C). The

former can be interpreted as more recent breakage (mechanical) fracture (Mazzullo et

al., 1986). Other minerals exhibit smooth and flattened, sheet-like regular areas of

former grain contacts (Newsome and Ladd, 1999), suggesting an originally polycrystal-

line nature of the grains, detached from crystalline-rock sources (Higgs, 1979), namely

granites or gneisses.

Also in this buried soil spheroidal particles of halloysite were recognised (Fig. 9B). The

fine fraction shows clay platelets with irregular shapes and chaotically arranged (Fig. 9C),

A

B

C

Fig. 10. SEM images of quartz grains: irregular solution pits and angular to well-rounded holes (A); coalescing

deep grooves with a pseudo-dendritic pattern (A, B); vacuolar, cavernous appearance of a quartz grain (C), with

one flat, relatively sharp and fresh face.

F. Scarciglia et al. / Catena 61 (2005) 1–2918

F. Scarciglia et al. / Catena 61 (2005) 1–29 19

among which pseudo-hexagonal kaolinite crystals with Si/Alc1 can be clearly identified

(Fig. 9D); also possible illite and chlorite or vermiculite phyllosilicates occur.

Surface horizons of profile P3 contain very small vacuolar pumice fragments with a

rhyolitic composition, similar to those described in P2 buried soil, and occurring also in

the topsoil of P5.

The most interesting microtextures worthy to note in profiles P4 and P6 are the physical

breakage patterns along preferred lines, structurally controlled by cleavage planes

exhibited by sand grains, often associated with some iron staining and coatings, and

occasional solution features.

The most peculiar characters detected by electron microscopy in soil profile P5 have

been found in the argillic horizon. Mineral grains show both physical and chemical

weathering features. More specifically, a regular, geometric weathering pattern can be

observed, in places made irregular by severer etching: dominantly linear, sub-parallel or

rhombic to square-angled edges of cleavage planes, as well as subrectangular etch pits,

occur on surfaces of quartz, mica and feldspar grains. As well as in the P3 buried soil, flat

and relatively fresh grain surfaces with stepped, angular fracture lines, in contrast with the

other (irregular and weathered) ones occur, representing recent mechanical fractures or

former grain contacts of polymineral aggregates. The weathering degree of sand grains

appears moderate to strong, as a whole, although quite fresh micas and feldspars occur.

Even quartz shows chemical microtextures in forms of surface etching (solution) with rare

linear and parallel-oriented grooves, and silica precipitation (silica plastering and/or quartz

overgrowths) (e.g. Krinsley and Doornkamp, 1973; Eswaran and Stoops, 1979; Pell and

Chivas, 1995).

Clay minerals from the soil matrix have Si/Al ratios approximately close to 1 with low

content in K and extremely low Mg, Na and Fe (suggesting illite- or kaolinite-like

phyllosilicates) or slightly higher amounts of the latter elements, probably signifying

mixed layers clay minerals with dominant 1:1 species.

5. Discussion

By dealing with typical granite landforms, grus and/or soil profiles, this work

contributes to understanding the interplay between weathering and pedogenetic features,

forcing factors and related processes. Although some soils described formed on parent

material consisting of fluvial and slope deposits – i.e. on granite-derived sediments (rather

than to in situ granite grussification) and are, therefore, conceivably polycyclic or

polygenetic – a variety of moderately to intensely developed pedogenetic features has been

described, otherwise not recorded in the Sila Grande uplands as a consequence of erosion,

and the interpretation of driving processes are discussed.

On the basis of the data obtained, petrographic and mineralogical controls appear to

play key roles in the development of weathering products, cumulatively bpropagatingQtheir effects to the macro-scale. All discontinuities observed on the bedrock/grus and

spheroidal boulders in the field, can be interpreted as related to physical fracturing

(tectonics, pressure release, thermal stresses, freezing and thawing, impacts upon falling or

during transport-sedimentation, etc.) or chemical weathering, possibly often acting

F. Scarciglia et al. / Catena 61 (2005) 1–2920

concurrently, and thus producing a number of weathering patterns. The large variety of

microtextures described in the primary mineral component of studied saprolite and soil

horizons (twinning or cleavage planes; physical breakage patterns produced by inter-

granular or trans-granular micro-cracks; exfoliation and expansion of crystals such as

biotite; etch pitting of plagioclases, feldspars and even quartz; clay neogenesis or iron

release and oxide staining along cracks), could be among the effective preconditioning

factors which cause the weakening of rock fabric (e.g. Newsome and Ladd, 1999; Migon

and Thomas, 2002) and progressively lead to core-stone separation, grussification and

pedogenesis. Factors of physical and chemical weathering can be considered, at least in

part, as linked by mutual cause/effect relationships (Whalley et al., 1982) both at the

micro- and the macro-scale. In particular, inherent, mechanically or (bio)chemically

produced micro-flaws, as well as stress-induced large-scale joints, tend to increase access

to rock surfaces by groundwater, accelerating weathering processes (solution, hydration,

hydrolysis and oxidation); by aiding water penetration, they enhance rock splitting and

disintegration by wetting/drying or freeze/thaw cycles, progressively widening and

deepening (up-scaling effect) pre-existing joints (Ehlen, 2002; Migon and Thomas, 2002).

Cryoclastic phenomena due to freeze/thaw cycles are very likely to be active in the study

area – and presumably even more effective for rock shattering during past Quaternary

colder stages of glacial periods – according to its high altitude and temperature values/

oscillations during winter months (see Section 2). The long-lasting snow cover and the

occurrence of a variety of cryonival processes, support this hypothesis. Also thermal

stresses in response to diurnal insolation cycles can be assumed to be effective for rock

fatigue and physical breakdown in the study area, where changes in temperature can be

relevant (see Section 2), and differential responses to such variations are expected from

outer and inner rock layers, respectively, as well as from different-coloured mineral grains

(e.g. biotite and feldspar) which expand/contract at different extent because of a different

power of reflection/adsorption of solar radiation (Ehlen, 2002). This may be relevant for

weathering processes, especially in the case of small-scale boulder flaking (Ollier, 1967).

The spheroidal boulders which are found at the ground surface are very likely to have

previously formed at considerable depths (real spheroidal weathering, sensu Ollier, 1967),

where hydrolysis of silicate minerals dominate (Ollier, 1988) under constant volume

conditions (Ollier, 1967, 1971), and exposed by subsequent erosion. It is not to be

excluded that subaerial weathering processes could partly inherit previously developed

features due to stress release and/or subsurface chemical weathering (polygenetic origin).

The role of biotite in the weathering of granitoid rocks seems to be of prominent

importance (e.g. Blum and Erel, 1997; Le Pera and Sorriso-Valvo, 2000a). Hydration,

oxidation, chloritization or vermiculitization may lead to expansion and/or cleaving of this

mineral species with consequent weakening of the total rock mass strength (Taboada and

Garcıa, 1999a,b). More specifically, the occurrence of micro-fractures radiating from

exfoliating/splitting biotite minerals into adjacent mineral grains and inducing rock

fragmentation (Section 4.3), strongly confirms this hypothesis (Isherwood and Street,

1976). This is also supported by the macro-scale observations of weathering features both

on spheroidal core-stones/boulders and bedrock/detrital grains of profiles P4 and P6,

reported in Sections 4.1 and 4.2, respectively. The increase in the amount of biotite crystals

in granite soil sands than the related unweathered parent rock, is likely to be produced by

F. Scarciglia et al. / Catena 61 (2005) 1–29 21

the breaking of biotite along cleavage plains (Cullers, 1988; Le Pera et al., 2001a), as also

observed in thin section and on bulk mineral grains by scanning electron microscopy

(Sections 4.3 and 4.4). Also feldspars represent particularly unstable constituents leading

to rock disintegration while undergoing weathering. Plagioclases in particular are revealed

to be generally more weathered than K-feldspars, presumably related to a differential

dissolution for slight variations in chemical composition (FitzPatrick, 1986). Moreover,

recent dissolution experiments and SEM observations of reacted surfaces on different

types of alkali feldspars (treated with pH 2 HCl at 25 8C) have shown correlation between

dissolution rate and dislocation density and some visible signs of dissolution at particularly

reactive sites (Lee et al., 1998). Both intra-granular microtextures and secondary

microstructures induced by straining are highly significant in determining the weathering

behaviour of these minerals. In particular, strained structure dissolves at rates which are

orders of magnitude greater than unstrained feldspar, progressively leading to growth and

coalescence of etch forms with progressive pseudomorphic transformation (Sections 4.3

and 4.4), and presumably responsible for the dirty, non-translucent appearance in the field

(Section 4.2). At a late stage, mechanical flaking occurs, thus increasing available surface

area and exposing further reactive sites, until grain surfaces start to disintegrate to a

microgranular material (arenization process, sensu Power and Smith, 1994; Teeuw et al.,

1994). The existence of greater surface energy in fine-grained rocks enhances a greater

instability mainly along inter-mineral contacts and therefore an easier grain-by-grain

disintegration of the material (Taboada and Garcıa, 1999a). As soon as this process

progresses, further chemical reactions occur, increasing the production of a pedogenetic

matrix (neogenesis of clay minerals and iron oxides), which in turn favours chemical

attack as a prolonged interaction between mineral surfaces and the circulating soil solution

is promoted. Similarly, the organic-rich matrix of A horizons seems to induce some

etching of primary crystals, due to the acidic behaviour of humic substances. This is

consistent with an overall higher degree of weathering of mineral grains assessed in proper

soil horizons rather than in grus layers and bedrock (Section 4.3). In particular, the

rubification of the matrix observed in the buried soil of profile P3 and in the argillic

horizon of P5 correlates with the highly weathered primary minerals, which allowed

abundant amounts of iron to be released and segregated as oxides. Extremely etched and

thinned, irregular outer edges, described on feldspar and biotite grains (Section 4.3),

frequently lined by clay neogenesis/illuviation and/or Fe-segregations, suggest they could

represent previous cleavage planes, intra- or inter-granular fractures (often appearing

particularly deepened and widened by weathering processes), also observed in the field

(Sections 4.1 and 4.2). In fact, they correspond to weakness zones which could have

become preferential sites for further chemical attack, progressively leading to the isolation

and separation of intervening, less weathered portions of the crystals, until complete

fragmentation of the former integral crystals themselves. The presence of neoformed clay

along some of these fissures well could be another direct cause of fragmentation through

its expansion under wet conditions.

A very interesting point of discussion emerges from the morphoscopic analysis of

quartz sand grains. Many of them represent mere physical fracture surfaces related to

crystal ruptures (Krinsley and Doornkamp, 1973; Al-Saleh and Khalaf, 1982). Apart from

mechanical features, all the other microtextures can be considered as derived from

F. Scarciglia et al. / Catena 61 (2005) 1–2922

chemical dissolution (e.g. Krinsley and Doornkamp, 1973; Al-Saleh and Khalaf, 1982;

Newsome and Ladd, 1999). Hexagonal, rhombic and regularly oriented triangular etch

pits, as well as parallel crevasses suggest a selective, crystallographic control over

dissolution (Al-Saleh and Khalaf, 1982; Howard et al., 1995) presumably related to micro-

structural zones of weakness (differential composition, cleavage or twinning planes,

mechanical cracks).

Particularly worthy to note are the dissolution features of quartz grains observed both in

the buried soil of profile P3 and in the Bt horizon of P5: whether they derive from

diagenetic (pre-weathering) or pedogenetic processes cannot be precisely evaluated.

Although hydrothermal dissolution of quartz at different, elevated ranges of temperature

(102 8C) has been widely assessed (Dove and Nix, 1997; Dove, 1999), intensely etched

quartz has been observed in tropical weathering environments, in ancient (long time

forming, maybe polycyclic) or organic-rich soils (Eswaran and Stoops, 1979; Mazzullo

and Magenheimer, 1987; Stoops, 1989; Howard et al., 1995; Schulz and White, 1999).

Quartz weathering essentially depends on mineral mass, morphology and surface area, as

well as on environmental conditions (Krinsley and Doornkamp, 1973; Pell and Chivas,

1995; Schulz and White, 1999). More baggressiveQ climates, characterised by abundant

rainfall and humidity and relatively high temperatures, can induce a severe leaching effect

and acidic pH conditions under which silica solution is strongly enhanced (Krinsley and

Doornkamp, 1973; Summerfield, 1991; Malengreau and Sposito, 1997). Moreover, studies

show that quartz can suffer dissolution at a temperature of 25 8C also in neutral or more

alkaline pH conditions, as a consequence of alkali and alkaline-earth cations interactions at

the soil solution/mineral surfaces (Dove and Nix, 1997; Karlsson et al., 2001). This

availability of bases is possibly favoured by the hydrolysis of other primary minerals (e.g.

feldspars and micas), which characterises the soil weathering environment itself. Past

climatic conditions warmer and moister than the present are therefore suggested to explain

the solution quartz microtextures, in full agreement with the intense rubification and

kaolinite clay mineralogy identified in the buried P3 soil and in the argillic horizon of P5

(see Sections 4.2 and 4.4).

The studied soil profiles – and their epipedons – denote a relatively young pedogenesis,

as a whole, with weak horizon differentiation, variably loamy and sandy textures, still

witnessing a strong influence from the coarse-grained parent material. Conversely, they

have a very low amount of pedogenetic clay (except in the argillic horizon of profile P5).

The overall absent to moderate weathering of primary minerals, apart from the quartz

etching patterns present in the old soil of P3 and partly in P5, points out well the early

stages of weathering and soil development (cf. Le Pera et al., 2001b), under the favourable

temperate humid climate and udic soil moisture regime of the study area (Section 2).

The identification of pumice fragments in some humbric epipedons of these soils (the

2Ab horizon of profile P2 being included; Section 4.4) gives a further contribution to

estimate their age. The specific rhyolitic composition of glass fragments indicates the

Lipari Volcano eruptive activity (Aeolian Islands, more than 200 km SW, in the Tyrrhenian

Sea) during the last 10 kyr (Crisci et al., 1991; De Rosa et al., 2003) as the only likely

source of these volcanic products: thus, an essentially Holocene soil development for

topsoil horizons – clearly overlying older ones in profiles P3 and P5, at least (see below) –

is pointed out, in accordance with the chronological constraint represented by the youngest

F. Scarciglia et al. / Catena 61 (2005) 1–29 23

(Late Pleistocene) age of the basin infilling. The extremely small size and amount of

pumice suggest it should represent windblown ash (distal deposits of high-energy

eruption/s) progressively assimilated by pedogenic alteration within soil horizons.

The assemblage of clay mineral types suggested by SEM-EDS analyses, such as illite,

halloysite, chlorite and/or vermiculite, with possible mixed layers of 1:1 and/or 2:1 clays

(and kaolinite mainly in the buried soil of profile P3), accords well with the moderate

weathering environment assessed, as well as with the age supposed. This clay mineralogy

is also consistent with the CEC values determined in the studied soils (Table 1) and with

pre-existing mineralogical data related to analogous soils of the same area (Mirabella et al.,

1996).

Illite, chlorite and vermiculite minerals are the likely alteration products of primary

micas, formed by hydration and/or isomorphous substitution of cations in the structural

units, and are frequently found in weathered granitic saprolites under temperate climates

(e.g. Sequeira Braga et al., 2002), such as the Holocene – and present-day – temperate

humid, upland Mediterranean climate. In such conditions a common trend of biotite

weathering leads first to hydrobiotite (interstratified biotite-vermiculite; Blum and Erel,

1997; Taboada and Garcıa, 1999b), frequently associated with iron oxyhydroxides, and

then to vermiculite. The vermiculitization reaction is perhaps the earliest and most rapid

weathering reaction which takes place on fresh granitic materials exposed to weathering

(Blum and Erel, 1997). Both illite and chlorite often represent intermediate stages of

weathering towards vermiculite (or HIV, i.e. hydroxy-interlayered vermiculite) and

smectite (Barnhisel and Bertsch, 1989; Selby, 1993); chlorite is specially favoured in

aluminium-rich soil environments (acidic pH conditions), which are common in silicate

rocks (and specifically in the study area), where leaching of SiO2 creates an excess of Al3+

ions (Duchafour, 1982; Thomas et al., 1999). As for kaolinite, it could be the alteration

product of biotite (Sequeira Braga et al., 2002), possibly throughout an intermediate

transformation step from vermiculite, observed in highly leached soils and saprolites,

maybe in tropical regions (Blum and Erel, 1997). Kaolinite, which characterises the buried

soil in profile P3, presumably records a higher degree of pedogenesis (Bronger and Bruhn,

1989; Scarciglia et al., 2003b), in agreement with the older age (presumably Middle or

early Upper Pleistocene) of the terrace on which the soil developed, and indicates humid

but well drained environmental conditions (Righi et al., 1999). Also halloysite, detected in

the clay fraction of profiles P2 and P3 and possibly occurring also in the other profiles,

may represent a precursor to kaolinite (Selby, 1993). Halloysite is often a typical

weathering product of volcanic deposits (Tazaki, 1979, 1982; Driessen and Dudal, 1989;

Mizota and van Reeuwijk, 1989; Jongmans et al., 1999; Nieuwenhuyse et al., 2000). It

denotes a pedogenetic environment with particularly high silica availability (e.g. Buol et

al., 1989; Cronin et al., 1996; Malucelli et al., 1999), which is consistent with the Si-rich

soil environment, deriving from both the silica-rich parent bedrock (granite) and the silica-

rich pumice fragments of volcanic supply assimilated into the studied soil profiles. In

previous papers (Tazaki, 1979, 1982), halloysite morphology has been hypothetically

related to chemical composition and the spheroidal type considered as an early-

intermediate stage deriving from and evolving to other forms, as weathering products of

primary minerals after progressive iron removal. The dearth of other morphologies (but the

rounded ones) and the extremely great amount of Fe detected in our samples seem rather to

F. Scarciglia et al. / Catena 61 (2005) 1–2924

be in contrast. The occurrence of hydrated but confined conditions (possibly with silica

supersaturation), which could have promoted only restricted growth (Adamo et al., 2001),

could be more reliable, since spheroidal halloysite was found, in profiles P2 and P3, just

close to small cavities of mineral grains. This somewhat accords with the limited leaching

conditions and restricted drainage suggested by Nieuwenhuyse et al. (2000) for

pedogenetic halloysite formation.

The whole set of features described for the six profiles can be interpreted also in terms

of relief influence on weathering and pedogenetic processes. All soil profiles but the soil of

P4 record a substantial geomorphological stability (consistent with the flat, terraced, and

persistently exposed, landforms on which they developed), which could permit enough

horizon differentiation and deepening of the pedogenetic front. A key role appears to be

played also by time and climate: in particular, the deep horizons of profiles P3 and P5

show clearly more developed weathering features, as well as some specific diagnostic

properties (etched quartz grains, reddish colours, illuvial clay) presumably related to older

periods, possibly even characterised by humid but warmer conditions than the present-day

climate and pedoenvironment (Scarciglia et al., 2003a,b), such as Middle to Late

Pleistocene interglacial/s. In particular, the deep horizons of profile P5, developed on the

top of the Late Quaternary basin infilling, could be tentatively ascribed to the Last

Interglacial; the lack of analogous features in P1, occurring in a similar stratigraphic

position, could be related to intense erosion, typical of the pulsing erosive/sedimentary

processes of the fluvial environment, as also suggested by the extremely limited and

scattered outcrops of Luvisols at the study sites. This would indicate a complex feature, in

places recording a more bcompleteQ soil development (P5) and in other places lacking a

bcontinuousQ pedogenetic record (P1). In addition, the multicycle origin of their parent

material (fluvial deposits), very likely inheriting some degree of weathering of primary

minerals, clearly should have enhanced more advanced in situ alteration and pedogenesis.

Profile P4, in contrast, appears extremely weakly developed and shows evidence of severe

surface erosion, continuously rejuvenating pedogenetic processes. Similarly, the slope

deposit in the uppermost portion of profile P2 records intense morphodynamics,

tentatively ascribed to historical times (since being very weakly pedogenised and sealing

the buried soil including Holocene volcanic supply). These features are consistent with the

absence of a protective vegetation cover, as well as with the topographic context of steep

slopes, where erosive phenomena are more pronounced than chemical weathering (e.g. Le

Pera and Sorriso-Valvo, 2000b), and especially in those areas that experienced severe

phases of forest clearance from Greek to recent historical times (Sorriso-Valvo, 1993),

inducing a continuous soil truncation with a rejuvenation of the weathering front. The

widespread (i) rare preservation – even on the oldest/stable landscapes – of virtually

bcompleteQ profiles grading from unaltered granite to in situ grus (still including not

exposed core-stones/boulders) up to thick, mature and well-differentiated soils, (ii) the

small thickness (such as in P6) or absolute lack of soil cover, (iii) the frequent presence of

abrupt, lithic contacts between soil horizons and the underlying bedrock, (iv) the

recurrence of clay-poor saprolites, clearly indicate important disturbance and rejuvenation

of pedogenesis by erosion. Also the exhumed boulder fields dominating the landscape of

the highest and oldest paleosurface limbs in the area can be easily explained in this way.

On one hand, if a long time of exposure of paleosurfaces could account for the

F. Scarciglia et al. / Catena 61 (2005) 1–29 25

development of deep spheroidal weathering, on the other hand, their old age, along with

the high local relief and deep dissection produced by Quaternary tectonic uplift, represent

reliable causes of the complete exhumation of these landforms. Therefore, the present

thickness of the weathering mantle in the study area cannot be considered a reliable

indicator of the effective grus and soil formation (cf. Migon and Thomas, 2002) as well as

of their age, as a full record of the time and of the various processes leading to soil

development is lost.

6. Conclusions

This paper has shown how different scales of observation, by means of appropriate

specific methods (namely field work, micromorphology in optical and scanning

electron microscopy both on bulk samples and thin sections), give some contributions

to better understand weathering and pedogenesis during landscape evolution of granitic

terrains.

A possible combination of the effects of deep (inherited) and surface weathering acting

with different modes can be assumed. The importance of variable microtexture fabrics,

such as twinning and cleavage planes, physical micro-flaws, etch pitting, clay neogenesis

or iron release and oxide staining along cracks, seems to be prominent, inducing great part

of the macro-scale effects which lead to the progressive disintegration of the crystalline

rock mass in the Sila Grande uplands. Therefore, both physical and chemical factors

appear to contribute to grus production and subsequent enhancement of chemical attack by

increasing mineral surface area, in turn accelerating soil development.

Moreover, topographic factors revealed to be of primary importance in the development

and preservation or removal of grus/soil mantles, which may be regarded as a response of

weathering systems to relief differentiation; the implications of weathered materials in

relation with topography in determining the deepening of the weathering/soil profile and

the outcome of erosional processes results emphasised in an evolutionary approach to the

development of landscape (Thomas, 1994). On the other hand, time and climate seem

rather to influence modes, intensity and rates of rock degradation and pedogenesis, since

weathering/soil mantles are widespread across climatic zones (from the humid tropics to

cool temperate areas), but often show very different degree of development. Furthermore,

climatic conditions exert a strong control also on weathering features and soil types,

leading to the development of specific textures and diagnostic pedofeatures typical of

different environments, possibly in response to Quaternary climatic changes.

Acknowledgments

We thank Dr M. Davoli (University of Calabria) for his technical assistance and helpful

discussion in SEM-EDS analyses, and Mr F. Colonnese (University of Calabria) and Dr B.

Di Matteo (CNR-ISAFoM, Ercolano, Napoli) for thin section preparation. We are grateful

to the anonymous reviewers and to Prof. M. Thomas for their critical comments and

suggestions which improved our manuscript.

F. Scarciglia et al. / Catena 61 (2005) 1–2926

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