The influence of physico-chemical material properties on erosion

Geomorphology 81 (2006) 235 – 251
www.elsevier.com/locate/geomorph
The influence of physico-chemical material properties on erosion
processes in the badlands of Basilicata, Southern Italy
Marco Piccarreta a , Hazel Faulkner b,⁎, Mario Bentivenga c , Domenico Capolongo a
a
c
Dipartimento di Geologia e Geofisica, Università degli Studi di Bari, Via F. Orabona, 4, 70125 Bari, Italy
b
Flood Hazard Research Centre, Middlesex University, Enfield EN34SA, Middlesex, UK
Dipartimento di Scienze Geologiche, Università della Basilicata, Contrada Macchia Romana, Potenza, Italy
Received 16 June 2005; received in revised form 3 February 2006; accepted 14 April 2006
Available online 12 July 2006
Abstract
Piping (tunnelling) appears to be one of the more significant methods of erosion in the badlands in the Plio-Pleistocene marinesourced clays of the Basilicata area in Southern Italy. A detailed field investigation of the pedological, textural, mineralogical and
geochemical properties of the clay-rich terrains widely outcropping in two selected badland field sites (the Fossa Bradanica area
and the Bacino di Sant'Arcangelo) was undertaken to further elucidate process variability across the complex site. Paralleling
findings from other piped badlands, certain physico-chemical properties of the clays were found to influence the different erosional
processes in fundamental ways. The very dispersive nature of the materials enhances pipe enlargement so that subsurface flow
rapidly becomes the dominant process, causing pipe enlargement by a process of positive feedback. The paper speculates on the
progressive role of surface and subsurface processes in the genesis of all evolutional badland forms (calanchi, calanchi
mammellonari and biancane). In an early stage, the calanchi mammellonari are produced, as slope mounds become increasingly
separated by a dense network of small inclined pipes in the intermediate part of the slopes, a site which is presumed to favour
elevated subsurface flow. When the overburden collapses, many cones are isolated in these portions of the slope. A second stage
follows in which overland flow dominates, with the water canalizing into small gullies generated by the collapsed crust. At the base
of many slopes, this same mechanism produces biancane as residual cones. Because of the structural weakness of piped materials,
mass movements cannot be ruled out. In a few locations, landsliding may be involved, especially in the formation of calanchi
mammellonari. In this case the intersection of the vertical pipes with the impermeable substratum focuses subsurface flow, and
collapse of the surface along this failure plane. Subsequent remodelling occurs by surface processes, since the ‘catchment volume’
for large pipes no longer exists. This interpretation differs from that of others who have argued that in other locations the biancane
formation is linked to the development of large vertical pipes along tectonic joints.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Biancane; Calanchi; Piping; Tunnelling; Dispersivity; Gully; Badland
1. Introduction
⁎ Corresponding author. Tel.: +44 2084115531; fax: +44
2084115403.
E-mail address: [email protected] (H. Faulkner).
0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2006.04.010
Badland landscapes are very widespread in the PlioPleistocene marine clays of the Basilicata region of
Southern Italy. The badland morphology is dominated
by the classic calanchi, calanchi mammellonari and
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
biancane1 forms that have been described extensively
elsewhere in Italy by Alexander (1982) and Farifteh and
Soeters (1999). Calanchi are large degraded landscapes
that are present in the form of single morphological units
with steep, bare slopes and channels which rapidly
incise and extend headwards (Alexander, 1982). However, biancane are dome-shaped forms dominated by
rills and micropipes developed in bedrock and surrounded by a basal micropediment (Torri and Bryan,
1997) and have been interpreted by some authors as the
end-product of calanchi erosion (Alexander, 1982; Del
Prete et al., 1994). Calanchi mammellonari are arguably
transitional between calanchi and biancane, due to their
intermediate morphological and physico-chemical characteristics (Castelvecchi and Vittorini, 1967; Vittorini,
1977; Alexander, 1980; Sdao et al., 1984; Pinna and
Vittorini, 1989). Despite some recent investigations
which focus on surface erosion rates (Clarke and
Rendell, 2005) we argue here, as have many other
geomorphologists, that these landforms cannot be fully
interpreted without considering their highly dispersive
nature and the tendency for extensive pipe development.
In the current investigation, the extensive literature
on these badland slope forms is further developed by
assessing the success of new material diagnosis tools to
further elucidate the manner in which material properties, geomorphological connectivity and developmental
stage interplay to explain the differences between the
various slope forms. The results are used to develop an
explanatory geomorphological model that includes both
the geomorphological and geochemical autostabilisation
potential of the site as part of the story.
It has long been recognised that the local Mediterranean climate, tectonics and human impact interact to
determine the gross morphology and surface conditions
of this landscape. Revisiting early interpretations by
Alexander (1982), attention has recently been given to
the explanatory role of lithology, in particular sediment
size and clay mineralogy, in explaining the badland
forms. For instance, on biancane sites, Battaglia et al.
(2002) found clay fractions to be over 65%, significantly
higher than on the calanchi sites; whereas the latter were
coarser with sand fractions between 6% and 18%. Both
sites have been reported to possess clay minerals in the
smectitic group, double-layer clays that are dispersive.
This study would suggest that material properties can be
morphologically diagnostic. Early work in badland
areas has always affirmed that physico-chemical
properties of the materials play an important role in
1
Torri (personal communication) suggests these terms were devised
originally by Gausparri (1978).
both rill initiation (Hodges and Bryan, 1982; Imeson et
al., 1982; Gerits et al., 1987; Imeson and Verstraten,
1988) and surface crust development (Bryan et al.,
1978; Torri and Bryan, 1997). Smectitic (2:1:1 clays)
will swell on wetting, sometimes sealing the surface,
encouraging overland flow and rill erosion. Additionally, for these clay minerals, high exchangeable sodium
on the exchange complex promotes dispersion (deflocculation) of the clays. The exchangeable sodium
percentage (ESP), sodium adsorption ratio (SAR),
sodium percentage (PS) and total dissolved salts
(TDS) are commonly used to measure the dispersive
state. Sediment containing deflocculated clay can be
entrained at much lower stream powers than normal
(Sherard et al., 1976; Torri et al., 1994), so these
geochemical properties strongly affect physical erosion
rates and patterns (Faulkner, 2006). However, if suitable
macropores are available within the material for
enlargement, dispersion can encourage the rapid
enlargement of subsurface pipes (Benito et al., 1993;
Gutierrez et al., 1997), a process sometimes referred to
as piping or tunnelling.
It has been argued that the threshold values for
deflocculation are ESP = 12% and SAR > 15. However,
although Battaglia et al. (2002) found the SAR to be
significantly higher in biancane than in calanchi
forms, the difference alone did not seem to be able
to distinguish the two features. To predict the tendency
of materials to pipe, Faulkner et al. (2000, 2003,
2004) explored the effectiveness of the relationship
between electrical conductivity (EC) and SAR,
originally used by Rengasamy et al. (1984). Whilst
this improves diagnosis over the use of ESP or SAR
values alone, it seems that this analysis is also
insufficient in itself to distinguish between badland
surfaces in terms of their morphology. It was found
that the percentage presence of the swelling clays in
the overall material mass is very important. Where
clay percentages are high, the material mass is
rendered impermeable on swelling, encouraging surface wash erosion and reducing infiltration. Where
clay percentages are low, the deflocculation of the clay
fraction merely destructures a material already lacking
in other sources of cohesion, encouraging subsurface
erosion. Given the presence of a suitable hydraulic
gradient through a site, this distinction will separate
materials that are dispersive but which do not develop
large pipes from those that do. Therefore, the
relationship between sediment size and SAR was
demonstrated to play an additional role in diagnosing
the development of a non-dispersive crust at a single
badland site.
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Robinson and Phillips (2001) found that high
organic matter (OM), iron and aluminum oxides can
promote aggregate stabilization in the top horizons.
The effective organic buffering content of surfaces in
their study areas was found to be present when OM
contents > 20 g·kg− 1, this value being sufficient to
stabilize aggregates. Data from another badland site
near Vera in SE Spain (Faulkner et al., 2003)
demonstrated that through time, buffering of site
geochemistry of several sorts can be involved in local
surface stabilization. In some cases, crusts may lose
sodium over time, probably because of the buffering
by calcium during leaching or by hydrogen on the
cation exchange sites under a vegetation cover or in
237
the presence of algae (for example, see Alexander et
al., 1994). The relationship between pH and SAR can
be used to indicate the extent of material buffering as
dispersivity changes. Paralleling similar findings by
Robinson and Phillips (2001), Faulkner et al. (2004)
found in SE Spain that the latter condition may
present morphologically as a stable (buffered) crust
and a dispersive subsurface horizon into which
shallow subsurface pipes preferentially locate. When
the non-dispersive crusts collapse into the pipe, rills
and gullies form (“pipe-origin” rills and gullies).
Provided no change to basal connectivity occurs
through time, the reduction of SAR in surface
materials may progress to the point where a
Fig. 1. Location of the two study areas in SE Italy.
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Alexander, 1982). Pipe enlargement, surface geomorphology and evolution style depend on all these
issues, not just the material's dispersive signature.
2. Aims
This paper uses these previous findings in relation to
badland material diagnosis (Faulkner et al., 2000;
Battaglia et al., 2002) and material autostabilisation
(Torri and Bryan, 1997; Robinson and Phillips, 2001;
Faulkner et al., 2003, 2004) as tools with which to
reinterpret two new slope sites in the Basilicata badland
areas. Specific objectives are to
➢ assess the efficacy of the ‘three signature’
diagnostic plots as tools to interpret the sites;
➢ explore whether downslope patterns of autostabilisation found in other similar badland areas are
present;
➢ use findings to explore the link between physicochemical properties of the clays and pipe
frequency and slope morphology;
➢ use further local geomorphological factors to
speculate as to how the erosional processes then
operate through time to explain the evolution of
the various badland forms (calanchi, calanchi
mammellonari and biancane).
3. Regional setting
Fig. 2. Block diagram suggesting the manner in which lithology and
fracture patterns have affected the development of the monoclinal
landscape and Aliano.
geochemical autostabilisation occurs at some depth,
as in the Tabernas badlands (Faulkner et al., 2003).
Even these three ‘site signatures’ (SAR/EC;
particle size/SAR; pH/SAR) are in themselves not
sufficient to explain the morphological variety found
in badland areas because the topographic setting of
the exposed materials, and changes to that topographic setting through time, are also crucial. At any point
within a dispersive, infiltrating landscape where there
is a sufficiently large ‘up-net’ catchment volume of
water (which depends on the availability of a
considerable infiltrating surface ‘up-net’, which is
topographically determined at any one point in time),
then significant pipe flow volumes can be generated
‘down-net’. In the presence of good hydraulic
gradients through the site, pipes can reach colossal
sizes when all these conditions are met (Heede, 1971;
The investigated area in the Fossa Bradanica is
located at Serra Pizzuta, near Pisticci, while the other
study area is at Masseria Mastrosimone, Aliano, within
Bacino di Sant'Arcangelo, a part of the middle Agri
basin that has been mapped as having some of the most
environmentally sensitive2 landscapes in southern Italy
(Basso et al., 2000) (Fig. 1). The Masseria Mastrosimone area is dominated by silty and marly PlioPleistocene marine-sourced clays and rare beds of
sands, with a total thickness estimated at 900 m. The
Serra Pizzuta site, south-west slope of the Pisticci hill,
is dominated by silty clays interspersed with rare
sandier units and capped discordantly by polygenetic
conglomerates. In both study areas, the topography has
a gentle dip and morphology is expressed as a typical
monoclinal landscape. However, the causes for the
Basso et al. (2000) define an environmentally sensitive area as “…
a specific and delimited entity in which environmental and socioeconomic factors are not balanced, or are not sustainable for that
particular environment”.
2
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
239
Fig. 3. Block diagram suggesting the manner in which landslide features have produced a topography at Pisticci which has a superficial similarity to
that at Aliano.
monoclinal topography differ in the two regions. The
Aliano (Fig. 2) site has been interpreted as a simple
monoclinal system, whereas at the Pisticci site, landslides are particularly widespread on the SE-facing
hillslopes (Guerricchio and Melidoro, 1982). Whereas
the gentle dip slopes around Masseria Mastrosimone
near Aliano, when not resculpted by land-levelling,
retain a fairly good vegetation cover and in places are
used for agriculture, the scarp slopes are devoid of
cover and are back-wearing by a mixture of surface
wash, piping and mass movements. Near Pisticci the
surficial pattern is repeated, although the direction of
(and reasons for) the failure planes that separate the
monoclinals is interpreted differently (Fig. 3), and the
slopes are generally somewhat steeper. In both settings,
the rapidity of the geomorphic processes on the
relatively steeper scarp slopes generally prohibits
vegetation from securing a stable function. On vertical
imagery, this monoclinal topography displays a stripy
appearance, the paler linear elements (the bare surfaces
of the badlands) are formed in every case on the scarp
in the monoclinal field.
Although the monoclinal morphology has differing
origins in the two areas, in both settings, the existing
primary and secondary network of fractures and
joints appears to influence the genesis and development of surface drainage (Del Prete et al., 1994).
Farifteh and Soeters (1999) argue that at the large
scale, the initiation, development, spatial distribution
and intensity of the pipe systems in the area are
influenced by the geostructural lineated characteristics
of the area. At a more detailed scale, it is the
secondary failure and joint features in both settings
that appear to influence the nature and direction of
the smaller scale evolution of scarp slope forms
(Boenzi et al., 1976). At Aliano, down-valley
structural features have been focussed around the
weaker parts of the structural sequence of marine
Fig. 4. Aspects of the slope morphology suggestive of pipe activity in the Plio-Pleistocene sediments.
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 5. Images of the two study slopes: (a) the Masseria Mastrosimone site at Aliano; (b) the Serra Pizzuta slope at Pisticci.
clays, although in the case of Pisticci, since these are
failure planes not lithological features, these lineaments are more discontinuous in their down-valley
pattern as might be imagined. Certainly these
fractures and joints would appear to give a rectilinear
look to some of the resulting morphology around
Aliano. Farifteh and Soeters (1999) conclude that the
joints in the clay bedrock provide macropore foci for
pipe enlargement.
Several authors (Alexander, 1982; Phillips, 1998;
Clarke and Rendell, 2000) have demonstrated that
abundant montmorillonite and mean ESP values
between 36% and 45% characterize the extensively
piped clays of this area, so pipe development is very
commonly found along these features (Fig. 4). Given
that larger pipes will collapse to produce a sort of
surface drainage feature (or gully), the overall progression of the morphology by predominantly piping and
then subsequent pipe roof collapse has underlying
structural control. At a smaller scale still, dissection
cracks in the clay will further focus the development of
micropipes; however, these may follow the hydraulic
gradients produced by the development of secondary
morphology, rather than the original structural patterns,
a view supported by the field research of Torri et al.
(1994).
4. Data and methods
After a detailed field study of the widely
outcropping Plio-Pleistocene clay terrain at both
sites, pedological, textural, mineralogical and geochemical investigation was undertaken within each of
the two study sites. Sets of soil samples were collected
at top, middle and base of two characteristic badland
slopes, one in each area (Fig. 5). Two sampling
procedures were followed. The first sampling, from
the crust to the bedrock (found to a maximum depth
of about 50 cm at Masseria Mastrosimone and 90 cm
at Serra Pizzuta), was aimed to reconstruct the
pedologic profile and to define OM, pH, EC3 in
order to evaluate the soil aggregate stability. To
reassure ourselves that the sampling on the basis of
visual identification of soil profile features was not
biased, a second sampling was made at a regular
centimetre scale, moving from crust to bedrock. The
analysis of mineralogy was determined in greater detail
from this sampling. X-ray diffraction was used for total
and clay fraction samples using a Siemens D5000
diffractometer. Chemical elements of bulk rock were
determined by X-ray fluorescence using a Philips PW
1480/10 spectrometer with Cr radiation. The chemical
characterisation also includes pH, SAR = (Na+/[(Ca2++
Mg 2+ )/2] 1/2 ), PS = ([Na + /(Na + + K + + Ca 2+ + Mg 2+ )] ⁎
100); and TDS (total dissolved salts, used here as a
substitute for electrical conductivity in the signature
plots adopted by Faulkner et al., 2000). Whilst the
SAR ,TDS and pH would allow an interpretation of
the first and third signatures used for material
diagnosis by Faulkner et al. (2001), to obtain particle
size for signature 2, the samples were prepared for
granulometric, mineralogical and chemical characterisation at IMAA (Istituto di Metodologie di Analisi
Ambientale, Italian CNR, Potenza). A detailed grain
size analysis (Shepard, 1954) was carried out for the
samples adopting a laser diffraction method; the
separation of the granulometric fractions was made
using the fractionated sedimentation technique. Since
mean sediment sizes were all so convergent, a decision was made to use the clay % in the sample
3
In the event, TDS was used as a surrogate parameter for EC due to
analytic difficulties with the sample. This does not affect the sense of
this signature analysis because of the strong correlation between TDS
and EC.
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
241
Table 1
Data related to the preliminary set of material analyses
Location
Horizon
Depth (cm)
Masseria Mastrosimone (Aliano)
Top
C1
0–24
R
24–54+
Middle
C1
0–24
R
24–54+
Base
C1a
0–6
C1b
6–12
C1c
12–18
R
18–42+
Pisticci
Top
A
0–15
BC
15–30
CB
30–50
CR
50–90+
R
90+
Middle
C1
0–12
R
12–34+
Base
C1
0–12
R
12–34+
pH
Organic Matter (g/kg)a
Total CaCO3 (g/kg)
Electrical conductivity (μ S/cm)
SAR
18.21
8.48
9.75
8.48
9.75
8.55
9.38
9.97
9.23
6.13
5.16
6.13
5.16
8.43
8.79
7.80
6.80
143.08
138.28
143.08
138.28
127.44
150.73
138.42
145.89
2850
1879
2850
1879
5230
5180
1038
1422
8.55
8.21
8.38
8.56
10.02
4.01
3.67
4.02
6.80
5.11
6.78
5.11
6.78
155.60
171.57
163.59
171.57
145.89
91.42
158.03
91.42
158.03
140.6
2060
2400
1889
1422
4830
4360
4830
4360
8.44
9.06
8.44
9.06
rather than the mean particle size for use when
plotting signature 2.
5. Results and interpretation
The results relating to the first set of samples show
that all the aggregates have OM contents below the
critical threshold of 20 g/kg and SAR values
exceeding the threshold of 15 (Table 1). As a
consequence they suffer physico-chemical breakdown
on contact with water. The pH values range from 8.29
18.02
8.12
17.94
15.84
13.69
15.30
4.54
27.44
30.41
50.56
26.65
37.97
to 8.85 at Masseria Mastrosimone, and from 8.22 to
9.19 at Serra Pizzuta. In both cases, pH values increase
towards the base of the examined profiles. The EC
values show the same pattern with depth as pH.
Signature 1, the relationship between EC and SAR
plotted in relation to the Rengasamy et al. (1984)
domains shows that all the samples have a dispersive
character (Fig. 6). The sampling on the basis of
horizons did not appear to be sufficient to differentiate
the samples so that they can be inspected for process
changes with depth.
Fig. 6. The first sample survey data plotted for the Aliano and Pisticci slopes in relation to the Rengasamy domains of dispersivity (Rengasamy et al.,
1984).
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
90%; (b) more than 50% of particles are smaller than
8 μm; (c) the clay fraction <2 μm represents about the
20% of the total. Further inspection of Table 2
demonstrates that in relation to the top and immediate
subcrust data, a coarse crust is often underlain by inwashing of finer particles in the layer beneath, whereas
the subsoil increases in coarseness once again. On Fig.
8a and b, the parameters influencing dispersivity at both
the Masseria Mastrosimone (Aliano) and Serra Pizzuta
(Pisticci) sites, calculated from the first survey data, are
plotted as profiles (data listed in Table 1) for top, middle
and slope base positions. The values of pH show the
same general increase with depth down the profiles of
both investigated slopes, whereas SAR demonstrates
very different behaviour between the two sites. At the top
and at the base sites of Serra Pizzuta slope (Fig. 8b), SAR
increases up to a depth of 20–25 cm, and then slightly
The more detailed second set of samples was
analysed for textural, mineralogical and chemical
characteristics in the same way as before. Mineralogical
analyses demonstrated that phyllosilicates dominate
both the sites, followed in order of abundance by quartz,
calcite, feldspars and dolomite. Traces of gypsum and
hematite are present only in some samples. Clay
mineralogy is dominated by illite and kaolinite, followed
by chlorite and expandable clays. These data do not
agree with previous literature, which has suggested that
montmorillonite is the main clay mineral. In both the
investigated slopes, there are no significant mineralogical differences from the top to the base site of the slopes.
The textural data in Table 2 and Fig. 7 demonstrate
that the samples of both investigated sites are clayey silts
with a sandy fraction <3%. The analysis shows that (a)
the percentage of the fraction smaller than 32 μm is about
Table 2
Data related to the second set of material analyses
Sample
Na+
K+
Masseria Mastrosimone (Aliano)
TA1
9.23
0.58
TA2
21.99
0.80
TA3
13.75
0.56
TA4
10.56
0.01
MA1
8.79
0.67
MA2
15.39
0.60
MA3
11.21
1.03
MA4
6.82
1.23
BA1
11.87
0.91
BA2
8.14
0.65
BA3
5.50
0.91
BA4
6.38
0.65
BA5
7.04
0.65
Pisticci
TP1
TP2
TP3
TP4
TP5
MP1
MP2
MP3
MP4
MP5
MP6
BP1
BP2
BP3
BP4
BP5
BP6
BP7
0.63
6.52
13.27
17.83
19.57
21.97
33.06
23.05
18.49
16.53
19.36
15.01
26.10
15.66
23.27
16.09
13.05
16.09
0.55
0.45
0.58
0.36
0.46
0.56
0.69
0.55
0.58
0.43
0.54
0.69
0.66
0.52
0.69
0.51
0.49
0.69
Ca2+
Mg2+
TDS (meq/l)
PS
SAR (meq/l)
pH
Clay %
1.56
2.78
0.51
0.68
1.49
0.63
0.81
0.95
2.59
1.14
0.88
0.95
0.76
0.94
2.50
0.58
1.14
0.67
0.58
1.14
1.87
0.71
1.77
1.35
1.46
1.25
12.32
28.07
15.39
13.39
11.62
17.20
14.20
10.86
16.07
11.68
8.64
9.43
9.69
74.98
78.36
89.32
78.83
75.69
89.49
78.98
62.75
73.88
69.63
63.64
67.66
72.63
8.26
13.55
18.64
11.05
8.48
19.76
11.35
5.74
9.25
6.75
5.20
5.82
7.03
8.29
8.43
8.44
8.64
8.40
8.68
8.85
8.67
8.60
8.70
8.68
8.70
8.79
28.70
33.70
32.30
32.00
31.00
31.00
32.30
34.30
33.30
31.70
32.00
33.70
31.30
0.40
5.55
11.35
0.50
0.94
4.12
6.74
0.75
0.30
0.15
0.17
3.12
3.06
0.17
0.44
0.22
0.20
0.50
0.66
1.13
1.75
0.21
0.51
0.41
0.62
0.25
0.33
0.25
0.29
1.54
0.82
0.45
0.82
0.29
0.33
0.82
2.24
13.66
26.94
18.90
21.48
27.06
41.10
24.60
19.69
17.36
20.36
20.36
30.64
16.81
25.22
17.12
14.06
18.11
28.18
47.78
49.24
94.38
91.11
81.18
80.43
93.72
93.89
95.21
95.09
73.71
85.17
93.15
92.27
94.02
92.79
88.89
0.87
3.57
5.18
30.04
22.99
14.60
17.24
32.68
32.98
37.12
40.24
9.83
18.74
27.96
29.32
31.79
25.38
19.80
8.22
8.40
8.63
9.01
8.97
8.59
8.98
9.19
9.00
8.85
8.93
8.41
8.74
8.78
8.91
8.79
8.75
8.82
41.70
40.00
42.30
40.00
36.70
35.30
36.70
34.00
38.70
37.70
36.00
37.00
35.00
35.00
36.00
35.00
34.00
36.50
The sample code’s first symbol refers to the position on the slope , i.e.T, M, and B refer to top, middle and base slope samples. The second code
symbol refers to locations at Pisticci and Aliano, respectively. Final code numbers suggest depth of sample.
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
243
Fig. 7. Grain size plots in which materials for the two sites are distinguished on the basis of sediment class: (a) Aliano; (b) Pisticci.
decreases towards the bedrock, whereas it continuously
increases in the middle-slope profile. At Masseria
Mastrosimone (Fig. 8a), SAR shows the same behaviour
as at Serra Pizzuta at the top of the slope, whereas
SAR decreases or remains constant at the middle and
base slope positions. The data in both Fig. 8a and b
indicate an increase in dispersivity favouring piping at
depths and geochemical autostabilisation at the
surface. The effect is more pronounced in the case
of the Pisticci data.
The saturated paste composition of the second set of
samples is given in Table 2, together with the calculated
SAR, PS and TDS parameters. The monovalent cations
appear mainly represented by sodium, and potassium is
only present in small concentrations. Calcium and
magnesium are always dissolved in pore water in
significant amounts, with Ca2+ often dominating over
Mg2+ at the surface, whereas Mg2+ prevails at depth.
From Fig. 6, all the samples of Masseria Mastrosimone
and those of the middle and base sites of slope of Serra
Pizzuta are dispersive, according to the Rengasamy et al.
(1984) criteria. However, Sherard et al. (1976) explored
SAR, TDS and PS as a function of clay dispersivity, and
this plot was attempted as Fig. 9. On this diagram, clays
plotting in zone A have a high tendency for spontaneous
dispersion and can be distinguished from the sediments
of zone C (potentially dispersive or nondispersive) and
those of zone B (ordinarily erosion-resistant). In Fig. 9,
the samples from the top site of Serra Pizzuta slope are
differently interpreted: the surficial (crust) sample PT1
shows a non-dispersive tendency; the dispersive status
of the middle samples PT2 and PT3 is ambiguous, and
the deepest samples PT4 and PT5 are dispersive. These
data again reflect the tendency, clearer for Serra Pizzuta
site, towards the geochemical autostabilisation, starting
at the top of the slope.
The full set of site signatures is displayed for the
second, fuller survey in Fig. 10. Firstly, we consider the
Serra Pizzuta (Pisticci) signatures. The SAR/TDS
signature suggests that the slope is stabilising from the
top–down, and since pH was found to be always lower in
the crust than in the substratum, the idea that a more
acidic (buffered) surface develops through time can be
inferred. This is confirmed by the strong positive
correlation between pH and SAR at the top of the
Pisticci slope, reinforcing the view that autostabilisation
of the crust at Pisticci is occurring by a buffering
mechanism. These findings match our Spanish results
from Vera (Faulkner et al., 2003). At Masseria
Mastrosimone (Aliano), however, the least dispersive
sites are found at the slope base. Since the slope top is
host to a considerable cover of Pistacia lentiscus and
Lygeum spartum, these results were surprising. Apart
from the sporadic presence of L. spartum, the erosional
slopes are normally bare at Serra Pizzuta. It appears that
stabilisation of the materials at Aliano by organic
buffering has not proceeded very far, if at all.
However, Table 1 shows that despite the absence of
current vegetation, the top of the Pisticci slope has the
highest organic content of all samples analysed (10 g/
kg). Although lower than the threshold for aggregate
formation noted by Robinson and Phillips (2001) in
similar materials, this difference may hold a key to the
contrasting pattern of signatures described above.
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 8. (a) Surface-to-depth plots of cations, SAR and pH for the top, middle and slope base sites, Aliano. (b) Surface-to-depth plots of cations, SAR
and pH for the top, middle and slope base sites, Pisticci.
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 8 (continued).
245
246
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 9. Sherard's suggested diagnosis shows that from our first survey, site PT1 is not dispersive, PT2 and PT3 are intermediate, and all others are
dispersive. Sites labelled PT = Pisticci top.
Piccarreta et al. (2006) argue that semi-natural vegetation may have been present at the Pisticci site at the
beginning of the 20th century, and that cultivation may
have been intense from this period onwards. We argue
here that this explains this disparity, and that the
unexpected differences found can be possibly attributed
to the buffering exchange between hydrogen and
sodium ions at this site due to the availability of organic
acids during at an earlier period when the cultivation of
the site started.
None of the relationships between particle size and
SAR (not shown) were significant, probably owing to the
generally similar mean sediment sizes, which failed to
effectively spread the SAR values. As a result, the
second site signature (SAR/particle size) was replaced by
SAR/clay % of the sample (Fig. 10) for both sites. In this
figure, the value r is the Pearson's product moment
correlation for the best fit lines, which are plotted where
the relationship is significant (p > 0.01). The expected
inverse nature of the second signature, as found at
Mocatan, SE Spain (Faulkner et al., 2000) and which
proved to vary downslope as geochemical autostabilisation progressed in Vera, SE Spain (Faulkner et al., 2003),
was picked up again in the case of the Serra Pizzuta
(Pisticci) sites, although this effect was not noticeable in
the Masseria Mastrosimone (Aliano) data set. All the
slope top sites have finer material than the slope bases.
6. Geomorphological implications
Our results suggest that Alexander's (1982) view of
the Basilicata badland landforms as an evolutionary
sequence is correct, and that Battaglia's differences are
explicable in terms of geochemical site stabilisation, i.e.,
geochemical changes of the same material as calanchi
geostabilised into biancane features. The results
obtained for these samples from relatively short
rectilinear slopes on the two badland sites show clearly
that Basilicata clays are highly dispersive and prone to
piping and tunnelling in the middle and basal part of the
slopes, and that, especially at Pisticci, they show a
tendency to autostabilisation at the surface, paralleling
findings elsewhere. The results provide tentative
support for the use of the site signature approach in
characterising these effects. Also, as observed by several
authors (Alexander, 1982; Sdao et al., 1984; Farifteh
and Soeters, 1999), subsurface flow is frequently
focussed along macropores associated with cracks,
bedding planes or failure planes at both sites. It can be
inferred that the geochemical changes cause a deflection
of the subsurface flows at the interface with unweathered subsurfaces, so that on progressive degradation and
pipe collapse a hummocky degraded state progressively
evolves (calanchi mammellonari).
It is also not surprising that the forms evolve in midslope and are degraded finally into biancane, which
dominate at the base of the evolving escarpment because
the upslope pipes will be relatively larger, collapse and
degrade earlier. Therefore, the pattern is progressive
with distance downslope, because up-slope catchment
‘volume’ for pipe flow during any one event is distancedependent. Both tectonic and/or failure features dictate
the gross form of the mounds. It is also possible to
hypothesize that calanchi mammellonari are produced in
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 10. The three site signatures for Aliano (above) and Pisticci (below).
247
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M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
Fig. 11. Schematic diagrams and plates illustrating the manner in which cones or mounds become isolated by surface processes through time (b and
c); and small pipe development suggested for the Aliano site (d and e).
the Serra Pizzuta (Pisticci) case by the remodelling of
the small landslides bodies (Fig. 3), although how the
interplay between pipe enlargement and landslide
generation works requires further investigation. For
instance, it is possible that local infilling of joints and
failure planes provides a site for vegetation to settle
which can have a locally important role in increasing or
decreasing erosion and the pH of subsurface flow.
At Aliano, field surveys showed that in the intermediate part of the slopes, the calanchi mammellonari are
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
separated by a dense network of small inclined pipes,
which favour elevated subsurface flow (Fig. 11). When
the crusts collapse, many cones are isolated in these
portions of the slope. However, once some erosion has
occurred, the new slopes develop minor pipe forms (of
much smaller size and relatively shallow because of a
very restricted catchment at this stage), these will be
inclined along new, local hydraulic gradients. Together
with desiccation cracks which can develop at the contact
between cracked surface and impermeable substratum,
they produce a highly ramified or reticulated network of
subsurface horizontal pipes and micropipes. At the base
of many slopes, this same mechanism produces biancane
as residuals cones, the surface of which appear to
become relatively fossilised with a relative preponderance of overland flow, with the water canalizing into
small rills generated by the crust's collapse. Thus,
alongside the important physico-chemical predispositions of the sites, the geomorphology of each site, and the
progressive hydraulic conditions as erosion proceeds
(and their distance-dependency) gives rise to the
calanchi/calanchi mammellonari/biancane sequence
down the scarp slopes.
7. Conclusions
Paralleling findings from other piped badlands, this
research has demonstrated that certain physico-chemical
properties of the local sodic Plio-Pleistocene clays
249
influence the different erosional processes in the two
study slopes at Basilicata in fundamental ways. The
physico-chemical properties of the two monitored
badland sites show an important tendency (clearer
around Pisticci) to geochemical autostabilisation on
the top of the slope. Clay materials in the middle and
base of slopes retain a dispersive character. Piping plays
a main role in badland morphology, especially in the
intermediate and basal part of the slope, favouring the
formation and development of calanchi mammellonari
and biancane. The very dispersive nature of the
materials enhances pipe enlargement along planes of
weakness, so that subsurface flow rapidly becomes the
dominant process, causing pipe enlargement by a
process of positive feedback. The pH/SAR relationship
shows the tendency of the crust to autostabilise,
confirming it is really an effective site signature.
This paper has also speculated on the progressive role
of surface and subsurface in the evolution of badland
forms (calanchi, calanchi mammellonari and biancane)
at both sites. Our interpretation is as follows: at Aliano,
an early stage following the exploitation of local planes
of weakness (joints, bedding plans at Aliano), calanchi
mammellonari are produced as slope mounds are
increasingly separated by a dense network of little
inclined pipes in the intermediate part of the slopes, a
site which is presumed to favour elevated subsurface
flow (Fig. 11). When the overburden collapses, many
cones are isolated in these portions of the slope. A
Fig. 12. Schematic diagram and plate illustrating the development of calanchi mammellonari and biancane forms on the slipped surfaces at Pisticci.
250
M. Piccarreta et al. / Geomorphology 81 (2006) 235–251
second stage follows in which overland flow dominates,
with the water canalizing into small gullies generated by
the collapsed crust. At the base of many slopes, this
same mechanism produces biancane as residual cones.
Because of failure planes at Pisticci, by contrast, the
macropore development is controlled by failure planes.
In a few locations in both areas, a secondary (post-pipe
development) role for mass movements cannot be ruled
out, especially in the formation of calanchi mammellonari at Pisticci (Fig. 12). In this case the intersection of
the vertical pipes with the impermeable substratum
focuses subsurface flow and collapse of the surface
along this failure plane. Subsequent remodelling occurs
by surface processes, since the ‘catchment volume’ for
large pipes no longer exists. This interpretation differs
from that of others who have argued that in most
locations biancane formation is linked to the development of large vertical pipes along tectonic joints.
Geochemical stabilisation signals at both locations
support these interpretations and suggest that ‘top–
down’ geochemical slope stabilisation is occurring as
calanchi evolve into biancane.
Acknowledgments
This research was supported by the European Union
funding for PhD Courses in Geomorphology and
Environmental Dynamics grant to Marco Piccarreta
and the PRIN-COFIN 2005 grant to Mario Bentivenga
and Domenico Capolongo. We are indebted to Dr. Vito
Summa and Dr. Luca Medici of the Istituto di
Metodologie per l'Analisi Ambientale (IMAA) (Tito
Scalo, Potenza) for providing unpublished data in
relation to the clay samples and their chemical analyses.
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