Physical-chemical and mineralogical properties of parent materials and their relationship with the morphology of badlands

Geomorphology 354 (2020) 107047

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Physical-chemical and mineralogical properties of parent materials and their relationship with the morphology of badlands Asunción Romero-Díaz a,⁎, José Damian Ruíz-Sinoga b, Francisco Belmonte-Serrato a a b

Department of Geography, University of Murcia, Spain Department of Geography, University of Málaga, Spain

a r t i c l e

i n f o

Article history: Received 28 October 2019 Received in revised form 16 January 2020 Accepted 17 January 2020 Available online 20 January 2020 Keywords: Badlands Landscape morphology Parent material South-east Spain

a b s t r a c t Badlands are present throughout the world, but not everywhere do they have the same morphology. The main objective of this study is to try to understand why these different morphologies occur by analysing the physical-chemical and mineralogical characteristics of the parental material in which badlands have developed. The study was carried out in three areas of the Region of Murcia (south-east Spain): Abanilla, Mula and Gebas. Several analyses were carried out: (i) field analysis (orientation, slope, length and shape of the slope, presence and characteristics of the rills, vegetation, vegetation pedestals, exposed roots, stoniness, physical and biological crusts, saline efflorescence, cracks, popcorn, piping processes, sedimentation areas and mass movements); (ii) physical-chemical analysis of the soils (colour, texture, aggregate stability, bulk density, organic matter, pH, electrical conductivity, cations and cation exchange capacity, total or equivalent calcium carbonate, sodium absorption ratio and exchangeable sodium percentage); and (iii) mineralogical analysis. The results of the analyses reveal how the rounded shapes in the badlands that appear in Abanilla have significant differences with respect to the angled shapes of Mula and Gebas, which could explain the different morphologies. In general, Abanilla has very small amount of sand, higher bulk density, small amount of organic matter, lower cation exchange capacity, and very high salinity, as shown in the values of electrical conductivity, sodio, SAR and ESP. On the other hand, similarities were found between the badlands of the Region of Murcia with the calanchi (angled shapes) and biancane (rounded shapes) of Italy. © 2020 Published by Elsevier B.V.

1. Introduction The definition of badland is not unanimous, and has been defined by different criteria as indicated by Martínez-Murillo and Nadal-Romero (2018), such as lithological conditions, weathering processes, landform features, agriculture potentiality, and even by their difficulty of being crossed by humans. An old definition is due to Bryan and Yair (1982) who defined badlands “as intensely dissected natural landscapes, in which vegetation is sparse or absent, that are unsuitable for agriculture”. Another definition that we consider appropriate is that which gives the Encyclopedia of Geomorphology describes badlands as “deeply dissected erosional landscapes, formed in soft rock terrain, commonly but not exclusively in semi-arid regions” that have a high drainage density of rill and gully systems, and are dominated by overland flow and with sparse vegetation (Harvey, 2004). In summary, the term “badlands” refers to regions that have soft and poorly consolidated material outcrops, limited vegetation, reduced or no human activity, and a wide range of

⁎ Corresponding author. E-mail address: [email protected] (A. Romero-Díaz).

https://doi.org/10.1016/j.geomorph.2020.107047 0169-555X/© 2020 Published by Elsevier B.V.

geomorphic processes, such as weathering, erosion, landslides, and piping (Martínez-Murillo and Nadal-Romero, 2018). Regarding the origin of the badlands, Moreno-de las Heras and Gallart (2018), after an extensive review study of badland mechanisms and predisposing factors, indicates that badland development is controlled by the incidence of four general terrain instability factors: relief vigour in the form of topographic gradient or active base-level conditions, weatherable and erosion-susceptible soft lithology, an erosive climate and, finally, a disturbance or environmental condition limiting the development of protective vegetation. Among the lithologies most prone to the action of external modelling agents and especially to the action of water erosion, are marls. In general, in the Mediterranean area, marls constitute the sediments of the neogenic basins (Picarreta et al., 2006; Arana-Castillo, 2007), although with very varied characteristics, linked to the period of formation and subsequent evolution. Marls are extremely fragile because of water erosion, both mechanical (splash and runoff), and chemical (dissolution). And this, coupled to the low plant cover they usually have (especially in semi-arid areas), due to the poor shallow soils that develop on them (mainly calcareous regosols), and to the meagre but often torrential Mediterranean rainfall and human action, promote the

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development of abundant rills and gullies, giving rise to land scapes of badlands (Alexander, 1982; Calvo-Cases et al., 1991; López-Bermúdez et al., 1998). Badlands can develop in almost all climates, however, badlands are more common in semi-arid areas and less common in sub-humid and humid regions. From a climate perspective, Gallart et al. (2002) classified the badlands into three main types: (i) arid badlands (annual precipitation b200 mm and very little vegetation); (ii) semi-arid badlands (annual precipitation between 200 and 700 mm, and discontinuous herbaceous cover); and (iii) humid badlands (annual precipitation N700 mm, and full vegetation cover, but it is limited by climatological and geomorphological factors). Nadal-Romero and García-Ruíz (2018) also spatially located the places in the world where these are abundant, highlighting their presence in the Mediterranean basin, South Africa, western USA, Canada, Mexico, India, China, Taiwan and Australia. Research on the badlands has increased progressively throughout the 20th century (Torri et al., 2000; Nadal-Romero and Regüés, 2010; Gallart et al., 2013) and recently Martínez-Murillo and Nadal-Romero (2018) have classified the main research topics in badlands during recent decades as: origins, lithology, human activities and land uses, vegetation, hydrology, piping, erosion processes and erosion rates, new emerging methodologies, reclamation and restoration, geoheritage and geotourism, and modelling. However, much more research is still needed on badlands to help understand the many questions that still exist, such as when and how badlands are generated, what the erosion rates are, or the evolution of badlands in the context of global change… Much of the studies on badland landscapes have focused on erosive processes, and it has been found that both the behaviour of the processes and erosion rates or hillside evolution, can be very different for the same lithology. That is why we share the opinion of Kasanin-Grubin et al. (2018), who indicate that it is necessary to address aspects such as grain size and the physical-chemical and mineralogical characteristics of the parental material. Several authors have analysed the effect of different erosion processes and the physical-chemical and mineralogical properties on the development of badlands in order to explain the different forms of hillside modelling. Already, several decades ago, Schumm (1956) found that creep is the main process that forms round slopes on the clay-rich Chadron formation, while wash processes produce straight slopes on the Brule formation in the South Dakota badlands, USA. Harksen and Macdonald (1969) also analysed the relationship between processes and forms of modelling in the Chadrom (round slopes) and Brule (straight slopes) formations of the South Dakota badlands in the USA. Vittorini (1977), Alexander (1982) and Sdao et al. (1984) analysed, the differences between Calanchi (straight slopes) and Biancane (round slopes) in Italy. Grain size has been considered as one of the factors that can control the shape of the badlands. Thus, according to Battaglia et al. (2002) and Pulice et al. (2012) Calanchi develop in coarser sediments with sand fraction present and dominant silt fraction over clay fraction; and Vergari et al. (2013) found a considerable sand fraction in calanchi samples. Couper (2003) considers that sediments composed of silt-clay particles are most susceptible to weathering processes and mass-movements. On the other hand, Grabowski et al. (2011), analysed the stabilizing role of organic content on cohesive sediment. Regüés et al. (1995) showed that bulk density is a useful indicator in studying the development, weathering and dynamics of the regolith. Faulkner et al. (2000) have tried to explain the morphological variety found in areas of badlands, considering the size of the grain and the geochemical and mineralogical properties of the parent material. And other authors such as Picarreta et al. (2006), Farifteh and Soeters, 2006, Kasanin-Grubin and Bryan (2007), Romero-Díaz et al. (2007, 2009), Vergari et al. (2013), Summa and Giannossi (2013), Moreno-de las Heras and Gallart (2016), or Kasanin-Grubin et al. (2018), among other have found that the lithological differences may be important factors to justify morphological diversity.

The main objective of this work was to analyze the influence that the physical-chemical and mineralogical characteristics of the parental material can have on the resulting morphologies in three badlands areas of the Region of Murcia (south-east Spain), in which different forms of modelling also appear.

2. Study areas 2.1. General framing of the Region of Murcia The Region of Murcia is located in the southeast of the Iberian Peninsula (Fig. 1). From a geological point of view, it is located at the eastern end of the Baetic Ranges. In addition to limestone and metamorphic rocks of the Baetic Ranges that are linked to the main tectonics, there are other post-orogenic materials, like marls, well developed in the inner depressions, such as those analysed in this study. Because of the latitudinal position (37–39° north latitude), the region is located in a transition zone between typically Mediterranean climates and semi-arid climates that introduce the characteristics of the North African desert (Alonso-Sarria, 2007). This, together with the foehn effect caused by its position leeward of the Baetic ranges, which hinder the passage of the fronts coming from the Atlantic, make it one of the most arid regions in Europe (AEMET, 2011). Average annual temperatures range from 12 °C in highland areas to 20 °C in the Vega of the Segura river, Mula and Abanilla-Fortuna basins, Campo de Cartagena and the south coast. Maximum temperatures can far exceed 40 °C and minimum temperatures can go as low as −10 and −15 °C in the N and NW of the region. The days of the year with temperatures below 0 °C (frosts) are between 20 and 40, between 60 and 80 in the NW and between 0 and 5 in the rest of the southern half of the Region (AEMET, 2011). Rainfall is scarce, between 200 and 400 mm, but often of torrential character, associated with the passage of one “Isolated Depression at High Levels” (IDHL). These situations are responsible for most of the annual rains and, above all, for the activation of erosion processes and runoff generation that often cause significant flooding. The areas with erosion in gullies and ravines (badlands) occupy a large area in the Region of Murcia (Fig. 1) and constitute one of its most representative landscapes (Belmonte-Serrato et al., 2019). In general, these are territories widely devoid of plant cover, formed by marlyclay substrates, and equipped with a high-density drainage network and very active morphological dynamics. The high waterproofness and little cohesion of the substrate, the presence of a very open xerophytic plant cover, more or less steep slopes and, above all, the few but intense rains characteristic of the region, are the factors responsible for the great erosive action that gives rise to the appearance of numerous ravines and gullies, leading to landscapes of badlands. The gullies progress through the action of various processes, especially fluvial-related ones; however, the role played by mass movements, gravitational slips and subsurface erosion or piping (Romero-Díaz and Belmonte-Serrato, 2002) is also important.

2.2. Areas of study In this study we have analysed three badland areas (Fig. 1), henceforth referred to as Abanilla, Mula and Gebas, which are located in the municipalities of Abanilla and Mula, and in the area of Gebas (corresponding to the municipalities of Alhama de Murcia and Librilla). The three badland areas analysed are located in three sedimentary basins, which were covered by sedimentation pediments and today, after the pediments have been eroded, the badland landscapes have been developed. In some sectors it is possible to observe remains of pediments with general slopes of around 5% in Abanilla and 10% in the other two areas (Fig. 2).

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Fig. 1. Areas of badlands and location of study areas in the Region of Murcia.

2.2.1. Abanilla Abanilla is located in the central part of the Abanilla-Fortuna Basin. This Miocene basin is one of the most representative badland areas of the Region of Murcia, and its gully landscape is one of its characteristic (Belmonte-Serrato and Romero-Díaz, 2007). Marine sedimentation of the basin consists, mainly, of loamy deposits from the Tortonian and Messinian period, sometimes interspersed with sandstone, or covered by quaternary sediments, giving rise to important glacis accumulations (Belmonte-Serrato et al., 2019). In the area chosen for this work, called “Los Barrancos”, the marly deposits have lost their quaternary covert almost entirely, suffering an intense erosion that has given rise to a landscape of badlands, with a succession of gullies and ravines with rounded shapes (Fig. 1) very different from the other badland landscapes of the Region.

2.2.2. Mula The badlands of the Mula Basin are also very representative, much visited at an educational level and perhaps the most studied at a scientific level of all the areas (Belmonte-Serrato et al., 2019). In this Miocene basin, the neogenous sedimentary filling consists mainly of a marly-sandy series with calcareous and conglomeratic collations of marine character attributable to Tortonian and Messinian periods, just as in the Abanilla-Fortuna basin (with which it shares its origin). Quaternary sedimentation is linked to the installation and fitting of the Mula river and also to the development of alluvial fans and glacis systems, formed from the new reliefs generated during the emersion and immersion of the Neogenic Basin during the PlioQuaternary (Del Ramo-Jiménez and Guillén-Mondejar, 2009), at the foot of which systems of alluvial fans and glacis were developed, but currently highly dissected by erosion (Romero-Díaz and LópezBermúdez, 2009).

2.2.3. Gebas “Los Barrancos de Gebas”, were declared Protected Landscape of the Region of Murcia in 1992 (Law 4/92 of Planning and Protection of the Territory of the Region of Murcia) and included in the Catalogue of places of geological interest of the Region (Arana-Castillo et al., 1992). They are located in the central sector of the Rambla de Algeciras and today constitute a badland landscape, whose origin is similar to that of the Mula Basin (Romero-Díaz and López-Bermúdez, 2009). It is a natural space located between three important reliefs: Sierra Espuña to the west; La Muela and El Castellar to the south; El Cura to the east; and the plateau of Fuente Librilla to the north. Its main drainage systems are the Rambla de Algeciras and Librilla that flow into the Guadalentín river. The rocks that make up the Rambla de Algeciras are mainly yellowish, grey and bluish marls of the upper Tortonian period, which usually have frequent intercalations of gypsum and other more soluble salts. At the top of Tortonian part, marls are replaced by sandy limestones, more or less organogenic, sometimes arrecifals (AranaCastillo et al., 1992). The geomorphogenesis of these ravines during the Holocene, seem to be linked to the confluence of several factors: the neotectonic of the Alhama de Murcia fault, the endorheic exorheic nature of the Guadalentín River, climate changes and human activity (Calmel-Ávila, 2000). 3. Methods 3.1. Field methods We analysed the different physiographic characteristics of the three areas of study in different orientations (south-facing and north-facing slopes): slope, length and shape of the hillside, presence and characteristics of rills, vegetation, vegetation pedestals, exposed roots, stoniness, physical and biological crusts, salt efflorescence, cracks, popcorn, piping processes, sedimentation areas and mass movements.

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Fig. 3. Soil sampling: Top, Middle and Bottom profile.

The international Munsell colour code (2000) was used for the determination of floor colour. The textural analysis was carried out using the England and Wales Soil Survey method (Avery and Bascomb, 1982; Avery, 2006). Sands were separated by dry sieving and silts and clays by decanting. The Kemper and Rosenau (1986) methodology was used to determine the AS. This method obtains the stability of the soil aggregates for different sizes, the curve and the aggregate stability index. Above 0.25 mm are considered macro-aggregates and, from 0.25 to 0.053, micro-aggregates (Haynes, 2000). The BD (ρap) was measured with the “cylinder method”, first described by Coile (1936) which consists of extracting a soil sample using a metal cylinder of known volume (in our case of 98 cm 3) and drying it at 105 °C for 24 h to obtain its constant weight (Blake and Hartge, 1986). This is a reliable method in cohesive clay and silt soils (as is the case with our marly substrate) (Campbel and Henshall, 1991). 3.3. Chemical determinations

Fig. 2. Overview of the three areas of badland studied. Abanilla (above), Mula (centre) and Gebas (below).

In each study area, three gullies (south-facing and north-facing slopes) were sampled in three different parts of the hillside (top, middle and bottom) (Fig. 3), obtaining a total of 54 samples. In each gully, 6 samples were obtained (3 in south-facing and 3 in north-facing) and in 3 slope positions. The total number of samples in each area was 18. Subsequently a physical-chemical analysis of these soils was made in the laboratory.

3.2. Physical determinations The physical determinations of the sediment made were of colour, texture, aggregate stability (AS) and bulk density (BD).

The following chemical determinations were made: Organic Matter (OM), pH, Electrical Conductivity (EC), cations and Cation Exchange Capacity (CEC), total or equivalent calcium carbonate (CaCo3), Sodium Absorption Ratio (SAR) and Exchangeable Sodium Percentage (ESP). The determination of Organic Carbon (OC) was made by oxidation with potassium dichromate in the presence of sulphuric acid, using a photometer for detection. The amount of oxidized organic carbon is calculated from the amount of dichromate reduced. OM is calculated by multiplying the OC by 1.72 (Sims and Haby, 1971). The pH of the water was obtained by a pH-meter, following the official soil and water analysis method (BOE, 1976). EC was determined by measuring the electrical resistance between two parallel electrodes submerged in the aqueous solution (BOE, 1976). After obtaining the cations, the CEC was calculated according to the official soil and water analysis method of the BOE (1976). For determination of the calcium carbonate content (CaCO3) a Bernard calcimeter was used, following Duchaufour's (1975) method. The SAR was measured following Richards (1954) in milliequivalents/ l (meq/l), according to the following formula: Na SAR ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ca þ Mg 2

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Finally, the exchangeable sodium percentage (ESP) was obtained from the formula: ESP ¼ 100  Na=CEC ðcmol=kg or meq=100 gÞ

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frequent in Gebas and Mula. The density of cracks is very high in Abanilla, as is the presence of popcorn. Piping processes in Abanilla are mostly of medium grade, while in Mula mass movements and sedimentation areas predominate. 4.2. Physical analysis

3.4. Mineralogy X-ray diffraction was performed on some of the samples from each of the study areas with the aim of identifying and quantifying their crystalline components. These analyses were carried out using a PANalytical diffractiometer, model X'Pert PRO MPD. Diffractograms were obtained and phase identification was performed, for which High Score Plus software was used, comparing the diffractogram with those in the existing database. Once the crystalline phases were identified, they were quantified using the Rietveld method (Young, 1993) using PANalytical High Score Plus software. It should be noted that these quantifications are a semi quantification, since the presence of clays and/or feldspars makes analysis much more difficult and there is no standardized method established in the literature for use with the Rietveld settings. Quantitative analysis refers to 100% crystalline matter using the Rietveld methodology. 4. Results 4.1. Parameters observed in the field Observations and field measurements (Table 1) showed the similarity of the slopes in Abanilla and Mula, but they were more steep in Gebas. The hillside lengths are shorter in Abanilla and, unlike the rest, their shapes are convex. The presence of rills is greater in Gebas but they are less deep, and in all areas, there are parallel grooves which form small drainage networks. The vegetation is almost non-existent in Abanilla, whether on south-facing or north-facing slopes. By contrast, Gebas has a very noticeable south-facing and north-facing dissymmetry. In Mula, in both orientations exposed roots are common and, in all areas, pedestals of vegetation are visible. Stoniness is very low in all areas. Physical crusts occur very often everywhere, while biological ones are only present in Mula on south-facing slopes and, especially, in Gebas. Saline efflorescences are very visible in Abanilla and less

4.2.1. Texture In general, the parent material of the three areas has a loamy-silty texture. However, it should be noted that, in some of the samples, since it had not dispersed well, a part of the silt would be clay. Abanilla has a low proportion of sands (1%), which increases in Gebas (5.0%) and especially in Mula (9.0%). The silts behave in the opposite way, Abanilla has the largest amount (78.2%) and Mula the least (73.4%). Gebas has intermediate textural characteristics between the other two areas. Regarding clays, the lowest proportion is in Mula (15.2%) with similar, slightly higher values in Abanilla and Gebas (19.1 and 19.3% respectively) (Fig. 4). As for the variations between south-facing and north-facing, in the north-facing slopes of all areas, there is a higher percentage of clay than in south-facing slopes; by contrast, silts show lower values in the north-facing slopes. This could be related to a higher soil moisture in the shadows at the onset of erosive rains, which results in an earlier start to runoffs and, consequently, a greater mobilization of the silts relative to the clays (Fig. 5). Variations in the different stretches of hillside in Abanilla were almost imperceptible. However, in Gebas the clays increased at the bottom of the slope and in Mula the percentage of sand was higher in the high parts, unlike the silts, which were more abundant at the bottom of the slope. 4.2.2. Bulk density (BD) Average BD values range from 1.15 in Abanilla to 1.26 in Mula. These values correspond to fine-textured surface parent material, as demonstrated in the textural analyses of these substrate. Slight differences in BD were observed between south-facing and north-facing slopes. With smaller BD values on the north-facing slopes of Gebas and Abanilla and, higher values on the north-facing slopes of Mula (Fig. 5). Regarding the variations of BD along the hillside, in

Table 1 Parameters analysed in the field in the different areas of study. Parameters

Abanilla

Gebas

Mula

S-facing N-facing S-facing N-facing S-facing N-facing Slope Hillside length (m) Hillside morphology Rills (spacing in cm) Rills (depth in cm) Presence of vegetation Exposed roots Vegetation pedestals Stony Physical crusts Biological crusts Crack density Salt outcrops Popcorn Presence of piping Sedimentation areas Mass movements

30–45° 10 Convex

30–45° 10 Convex

N45° 24 Straight

N45° 24 Straight

30–45° 35 Straight

30–45° 35 Straight

5–10 10–20

0–5 10–20

0–5 5–10

0–5 5–10 **

5–10 10–15

0–5 5–10 *

*

* *

*

*

* *

* *

* ****

* ****

* ****

* ****

**** **** **** **

**** **** **** **

**** * ** * *

* **** *** ** *

* **** ** *** *

*

*

**

Frequency: * low, ** medium; *** high; **** very high.

*** * *

* ***

*

***

*

Fig. 4. Texture of the parent material of the different areas. Larger symbols correspond to the average values for each area.

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TEXTURE

TEXTURE

Sand

Silt

Mula South

Clay

75.8

79.1

Mula North

73.4

6

Gebas North

40% Silt

60% Clay

80% 100% ABANILLA

GEBAS

BD

1.2

BOTTOM

Abanilla

Gebas

1.3

Mula

47.5 30.5 39.7

37.5

9.1

%

35.3

64

76.4

69.7

Mula

43.2

36.5

NORTH FACING

AS

Gebas

48.9

58.6

%

1.2

1.2

SOUTH FACING

AS Abanilla

Mula

1.1

1.1

1.3

1.2

MIDDLE

Gebas

1.2

Abanilla

Mula 1.3

Gebas

1.2

1.4 1.2

1.1 TOP

MULA

1.1

BD Abanilla

43.4

20% Sand

25.1

0%

14.6

5.0

1.0

Abanilla South

9.0

19.1

Abanilla North

19.3

%

Gebas South

SOUTH FACING

NORTH FACING

TOP

MIDDLE

BOTTOM

Fig. 5. Texture, bulk density and aggregate stability: on south-facing and north-facing slopes and in the different parts of the slopes in the three study areas.

Abanilla and Mula the BD was greater in the middle section, while in Mula it was higher in the highest section (Fig. 5).

vary much, with the exception of Mula where it increases in its lower section (Fig. 6).

4.2.3. Aggregate stability (AS) Aggregate stability values are very similar in Gebas (39.9%) and in Mula (39.5%), and much higher in Abanilla (53.8%); also, Abanilla has greater AS on south-facing slopes, contrary to what happens in Gebas and, especially, in Mula (Fig. 4). In Abanilla and Gebas the highest AS occurs on the middle stretch of the hillside, and in Mula in the lower and middle sections (Fig. 5). 4.3. Chemical analysis

4.3.2. pH Abanilla has the highest pH with a mean value of 8.2, while Gebas and Mula have the same pH (7.9). However, there are slight variations between south-facing and north-facing values in Abanilla and Mula (Fig. 6). In Abanilla the pH on the south face is higher, while in Mula it is higher on north-facing slopes. Regarding the pH in the hillside position, in Abanilla pH is higher in the upper and middle parts of the slopes, while in Mula it higher is in the middle stretch and in Gebas in the lower section (Fig. 6).

4.3.1. Electrical conductivity (EC) Electrical conductivity is one of the parameters that most differentiates Abanilla from the other two study areas. The average value for Abanilla is 6.6dS/m, while in Mula it is 2.1 and in Gebas 1.6. It goes without saying that EC is higher on south-facing than on north-facing slopes in all areas (Fig. 6). In the different parts of the slope the EC does not

4.3.3. Organic matter (OM) The mean values of OM are very low, as corresponds to a semi-arid area: 1% in Abanilla, 1.9% in Gebas and 2.2% in Mula. In Abanilla there are no differences between south-facing and north-facing values, but there are differences in the other areas where, as would seem logical, the values are higher on the north-facing slopes (Fig. 6). Regarding the

A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

EC

EC Mula

South facing

North facing

7.3

Gebas

MIDDLE

0.8 ABANILLA

BOTTON

GEBAS

pH

pH

MIDDLE

BOTTON

ABANILLA

GEBAS

OM

MULA

OM

Gebas

Mula

South facing

North facing

1.1

BOTTON

ABANILLA

2.4 2 74.9 67.6 68.1

69.1 70.3 70.0

Mula

MIDDLE

BOTTON

%

%

67.9

68.2

69.6

69.7

Gebas

66.7

Abanilla

North facing

78 67.6

MULA

CaCO3

CaCO3 South facing

GEBAS

74.6 71.2

MIDDLE

1

1

% 0.7

TOP

2.1

1.7

1.8

2.1

2 1.8 1.2

%

2.2

2.6

Abanilla

7.9

7.9 7.8

7.7

7.9

7.8

TOP

7.8

8.2

North facing

8.1

South facing 8.1

8.1

Mula 8.1

Gebas 8.2

8.2

Abanilla

MULA

7.9

TOP

1.2

1.8

3.1

dS/m

2.4 1

1.9 2

1.9 2

dS/m

5.9

6.4

6.4

7.1

Abanilla

7

ABANILLA

GEBAS

MULA

TOP

Fig. 6. EC, pH, OM and CaCO3 on south-facing and north-facing slopes and in the different parts of the slopes in the three study areas.

OM in the different stretches of hillside, Gebas and Mula show similar behaviour, the OM increasing in the middle and lower sections, which does not occur in Abanilla (Fig. 6). 4.3.4. Total or equivalent calcium carbonate (CaCo3) On average, total calcium carbonate represents 73% in the soils of Abanilla, 70% in Gebas and 68% in Mula. Again, Abanilla has

different values compared to the other areas and, likewise, it also shows different behaviour on south-facing and north-facing slopes and in the different stretches of the hillside (Fig. 6). On northfacing slopes, Abanilla has a much higher value than the corresponding south-facing slopes, and practically the total calcium carbonate content is similar along the hillside, which is not the case in Gebas and Mula.

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4.3.5. Cation exchange capacity (CEC) The mean CEC in Abanilla (44.5 meq/100 g) is lower than in other areas (Gebas 49.7 and Mula 50.2 meq/100 g). The variations between south-facing slopes and north-facing slopes are very few, but they behave differently in Abanilla, as do the values found along the hillside (Fig. 8). 4.3.6. Assimilable calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg) With regards Ca cations present in the samples analysed, Abanilla has a lower concentration (16 meq/100 g) than Mula (17.8 meq/100 g) and Gebas (18.4 meq/100 g), and the same applies to Mg (mean value of 0.9 meq/100 g in Abanilla, 1.3 in Mula and 1.8 in Gebas). K, on the other hand, is slightly higher in Abanilla (0.4 meq/100 g), compared to Mula and Gebas (0.3 meq/100 g). However, the differences are very great in terms of the Na content: an average of 9.9 meq/100 g in Abanilla, 1.2 meq/100 g in Mula and only 0.2 meq/100 g in Gebas. The variations between south-facing and north-facing slopes follow the same trend as Ca, with higher amounts of this cation on the southfacing slopes of all areas. Mg is also higher on south-facing slopes in Gebas and Mula, but not in Abanilla. K is similar on both hillsides in Abanilla and Gebas and higher on south-facing slopes of Mula. Sodium (Na) is much higher in Abanilla on south-facing slopes (Fig. 7). When we analyses the stretches of hillside, the most marked differences are in sodium (Na), which is much higher in the highest parts of the hillsides in all areas. In the case of magnesium (Mg) the highest values are located in the middle stretches, while K and Ca have no significant trend (Fig. 7). 4.3.7. Sodium Adsorption Ratio (SAR) SAR denotes the relative proportion of sodium relative to calcium and magnesium, divalent cations that compete with sodium for soil exchange sites. Mean values matching assimilable cation values are much higher in Abanilla (3.43%), compared to Mula (0.36%) and Gebas (0.06%). In Abanilla the SAR is higher on south-facing slopes and slightly higher in the middle stretch of the slopes (Fig. 8). 4.3.8. Exchangeable Sodium Percentage (ESP) ESP, which represents the percentage of sodium compared to other adsorbed cations, relative to previous mentioned data, is much higher in Abanilla (22.64%), followed at a distance by Mula (2.20%) and Gebas (0.35%). According to Massoud (1971) who classified soils according to its ESP, Abanilla would be strongly sodium, while Mula and Gebas would be non-sodium. Just like SAR, ESP in Abanilla is higher on south-facing slopes and slightly higher in the middle stretch of the slopes (Fig. 8). 4.4. Mineralogical analysis Of the minerals present in all areas, calcite is of particular note, followed, to a lesser extent, by silicates of Al and K, Quartz and Dolomite. In a smaller amount there are also Ankerite and illite. Thenardite only occurs in low amounts in Gebas. Comparing the values of the three study areas, Abanilla has less dolomite and ankerite and more aluminium and potassium silicates, as well as illite and kaolinite (Fig. 9). 5. Discussion 5.1. Comparative analysis of the variables analysed in the three study areas From a physiographic point of view, Abanilla differs from Mula and Gebas mainly in the morphology of its convex slopes, shorter hillside length, lack of vegetation, a greater presence of gullies, the density of cracks, popcorn, piping processes and saline outcrops (Fig. 10). We should also mention the higher density of cracks and the presence of popcorn, which Schumm (1956) mentions in the badlands of the

Chadron formation (South Dakota, USA) as one of the causes of its rounded shapes. Comparing together the physical-chemical analyses performed of the three areas (Table 2), we observe that Abanilla (with the badlands of rounded shapes) differs significantly with respect to Gebas and Mula (with badlands of angled shapes). From a textural point of view, Abanilla has a more silty and not very sandy texture compared to Gebas and Mula. The BD is similar to that of Gebas, but lower than in Mula, and its higher AS of the three areas analysed should be mentioned. Abanilla also has a very high EC (which is not the case in the other areas), a higher pH and also higher CaCO3 content. Conversely, it has the lowest OM content (1%), and the lowest CEC. Regarding cations, K and Na are the most common (especially the latter) and Ca and Mg have lower values. This higher salt concentration relative to the other two areas may be related to two important factors: (i) the AbanillaFortuna basin was the last basin to dry and, consequently, its waters would have concentrated many more salts in its final stages; and (ii), today this is the most arid area in the entire region of Murcia, with potential evapotranspiration values above 1200 mm and a water deficit of N900 mm/y, which undoubtedly favours the concentration of salts. In summary, the most significant differences between Abanilla and the other areas, which could explain, among other factors, the different morphologies, are: (i) the very small amount of sand, (ii) the higher BD, (iii) the small amount of OM, and consequently the lower CEC, and (iv) the very high salinity of its soils, as shown in the value of EC, Na, SAR and ESP. (i) With regard to differences in texture, it should be noted that the risk of runoff and erosion is affected by small differences in this parameter. This is because texture influences the degree of water percolation through the soil. Soils containing large proportions of sand have relatively large pores through which water can drain freely and therefore these soils are less at risk of causing runoff. When the clay ratio increases, the size of pore space decreases, restricting the movement of water through the soil and increasing the risk of runoff. According to Cerda's (1997) rain simulation experiments, in sandy soils the infiltration rate is higher and waterlogging and surface runoff is delayed, so that in areas with lower amounts of sands runoff happens more quickly. This could be what happens in Abanilla: less sand and a higher proportion of silts and clays. According to Battaglia et al. (2002), size has a strong influence on the shapes of the badlands. (ii) Aggregate stability (AS) generally reduces soil susceptibility to runoff and erosion. AS determines the ability of the soil to maintain cohesion and a stable structure facing physical-chemical disintegration processes (Nadal-Romero et al., 2009). Cerdà (1996) noted the positive influence of vegetation on AS and Boix-Fayos et al. (1998) showed that aggregates are more stable in vegetation-covered spaces than in bare soils, even if the aggregates on bare soils are larger. Lithology is another conditioning factor of aggregate stability. The results obtained by NadalRomero et al. (2007) indicated that the marly lithologies on which the gullies are developed are very susceptible to processes of meteorization and erosion. Phillips and Robinson (1998) demonstrated that there is a positive correlation between OM and AS. However, our results do not match those expressed in the literature that relates the aggregation and stability of structure to a higher content of OM and vegetation. In the case of Abanilla this does not happen, since, although it has a higher AS than the other areas, the OM is much lower and the vegetation is almost non-existent. (iii) The content of OM in the soil or sediments is an important factor in soil erosion (Grabowski et al., 2011), considering eroded soils those that have b2% OM (Morgan, 1986). Moreover, OM is

A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

contributions of organic matter (in addition to causing an increase in CEC) improve the physical properties of the soil (Bot and Benites, 2005), increase water infiltration, improve soil structure and reduce erosion losses. In the case of Abanilla,

responsible for a high percentage of CEC, and the reduction in the content of OM in the soil usually causes a decrease in its CEC (Peinemann et al., 2000). We have seen this with our analyses there is a relationship between OM and CEC (Fig. 11). The

CA

CA

Mula

24.6 17.4

15.4

meq/100g

15

20.5

TOP

MULA

NA

MIDDLE

BOTTON

NA Abanilla

North facing

Gebas

Mula

ABANILLA

MULA

TOP

1.8

0.2 1

MIDDLE

K

BOTTON

K North facing

Abanilla

Gebas

Mula

TOP

0.3

0.4

0.4

MIDDLE

BOTTON

MG

MG Abanilla

North facing

Gebas

Mula

ABANILLA

GEBAS

MULA

TOP

MIDDLE

1.5 1

0.8

1.1

1.3

1.4 0.9

meq/100g

1.5 1

1

0.8

meq/100g

1.6

2

1.9

2.2

South facing

0.3

0.3

MULA

0.2

GEBAS

meq/100g

0.4 0.4

0.4 0.3

0.3 ABANILLA

0.3

0.4

meq/100g

0.4

0.5

South facing

0.3

0.1 0.8

1.6

GEBAS

0.8

0.1

0.3

meq/100g

meq/100g

8.4

8.9

9.7

11

11.3

South facing

Gebas

16.7 14.5 17.8

GEBAS

Abanilla

16 16.1 18.2

ABANILLA

16.8

19.9

16.5

North facing

15.6

meq/100g

South facing

9

BOTTON

Fig. 7. Assimilable calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg) on south-facing and north-facing slopes, and in the different parts of the slopes in the three study areas.

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A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

ABANILLA

51.4

48.4

GEBAS

MULA

SAR

SA R Mula

South facing

3.9

Gebas

3.0

North facing

TOP

MIDDLE

ABANILLA

BOTTON

ES P

0.4

0.3

GEBAS

MULA

ES P Mula 23.1

North facing

TOP

MIDDLE

GEBAS

2.4

2.0

0.5 ABANILLA

BOTTON

0.2

2.3

0.4

2.9

0.5

1.4

0.2

%

%

18.5

20.2

South facing

26.7

Gebas 24.6

Abanilla

0.0

0.1

0.4

0.1

0.5

0.1

0.0

0.2

%

%

3.2

3.3

3.9

Abanilla

North facing

50.9

46.1

42.8

BOTTON

meq/100g

MIDDLE

43.2 48.9 46.6

meq/100g

TOP

South facing

Mula 44.7 49.3 49.3

Gebas 45.6 50.9 54.7

Abanilla

C EC

49

C EC

MULA

Fig. 8. CEC, SAR and ESP on south-facing and north-facing slopes, and in the different parts of the slopes in the three study areas.

70 60 50

%

40 30 20 10 0 Calcite

Quartz

Dolomite Abanilla

Thenardite Silicate Al y K Mula

Gebas

Fig. 9. Mineralogy of the three study areas.

Ankerite

Illite

A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

11

Table 2 Comparison of the physical-chemical properties analysed in the three study areas. Properties

Abanilla

Gebas

Mula

Sand (%) Silt (%)a Clay (%) BD AS (%) EC (dS/m) pH OM (%) CaCo3 (%) CEC (meq/100 g) Ca (meq/100 g) Na (meq/100 g) K (meq/100 g) Mg (meq/100 g) SAR (%) ESP (%)

1 78.2 19.1 1.2 53.8 6.6 8.2 1 72.8 44.5 16.0 9.9 0.4 0.9 3.43 22.64

5.0 75.8 19.3 1.2 39.9 1.6 7.9 1.9 69.7 49.7 18.4 0.2 0.3 1.8 0.06 0.35

9.0 73.4 15.2 1.3 39.5 2.1 7.9 2.2 68.3 50.2 17.8 1.2 0.3 1.3 0.36 2.20

a

Possibly part of the % of silts corresponds to clays.

the high percentages of ESP and SAR, which measure the presence and behaviour of sodium in the soil, indicate the high sodicity of the soil solution in the study area and spontaneous dispersion in contact with water (Richards, 1954).

5.2. The role of the orientation and position of the hillside

Fig. 10. Popcorn (above), small pipes (centre) and salt outcrops (below) in the badland of Abanilla.

with less OM and lower CEC, we can assume that erosion processes would be more intense than in the rest of the areas. (iv) Large quantities of salts present in Abanilla, they may also be responsible for the different morphologies. The high presence of salts in areas with arid and semi-arid climates can be explained by the imbalance in the water balance of the soil profile, since evapotranspiration far exceeds precipitation. The soluble salts contained in the water do not evaporate and are concentrated on the surface, forming a whitish film on the outer layer of the soil formed by crusts of different salts, generating salty soils naturally (Paniza-Cabrera, 2002), as is the case in Abanilla. This results in the decrease in the production capacity of the land (when it is for agricultural use), the emergence of halophilous vegetation or the definitive disappearance of vegetation (FAO, 1984). In the case of Abanilla, the increased presence of salts and lower amounts of OM would explain the near total absence of vegetation (Table 1). It is worth mentioning that in Abanilla,

5.2.1. Hillside orientation The orientation of the hillsides, especially in arid and semi-arid regions, is an important factor to consider. The north face is usually damper and therefore there is the possibility of higher vegetation density, and that some soil characteristics may be modified. In the case of Abanilla, orientation does not intervene at all in the presence of vegetation, since this is almost non-existent on both slopes (Table 1); on the contrary, in Gebas it is possible to observe a marked difference of vegetation between south-facing and north-facing slopes, as is the case with biological crusts on the soils. Several authors have studied the surface meteorisation profiles of soil, both in the laboratory and in the field (Regüés et al., 1995; Solé et al., 1997; Cantón et al., 2001; KasaninGrubin, 2013) and have observed significant differences in soil moisture, which, in theory, could be the result of hillside orientation. Table 3 summarizes the weight of the different physical-chemical variables in relation to the orientation of the slopes; as can be seen, there seems to be a relationship between some parameters and orientation. As regards texture, not considering the sands, which hardly exist in Abanilla, silts predominate on the south face and clays on the north face. Parameters such as AD, EC and the content of Ca and Na are higher on the south-facing slopes of the hillsides in the three areas, doubtless a consequence of the fact that in this orientation the sun in semi-arid areas is stronger. SAR and ESP are higher on south-faces slopes in Abanilla and Gebas. On the other hand, Aggregate Stability and pH are higher in Abanilla on the south-facing slopes, while in the other two study sites they are higher on the north-facing slopes. The scarce organic matter in all areas has slightly higher values on the north-facing slopes, where there would be a little more moisture.

5.2.2. The position on the hillside In this study, we also wanted to see whether, in the physicalchemical parameters analysed, there are differences depending on the position of the hillside in which they are located (Table 4). Regarding texture, the sands appear to a greater extent in the high sections of the three studied areas, while in Abanilla and Mula, the largest amount of silts are in the lower sections and clays in the middle sections, implying that clays wash off the upper parts and are deposited in the lower parts.

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A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

Fig. 11. Relationship between Organic Matter and Cationic Exchange Capacity in the three study areas.

Depending on the parameter analysed, the three sites show different behaviour. Thus, BD presents the highest values in the middle section for Abanilla and Gebas, while AS also has high values in the middle section in Abanilla and Mula. The parameters that indicate salinity in the soil, such as EC, sodium content (Na) and SAR, are highest in the middle sections of the hillsides in Gebas, while in Abanilla the highest amount of Na is at the top, and the highest EC in Mula is in the lowest section. Despite these anomalies, in general, it can be said that there is a mobility of salts from the top to the middle stretch of the hillsides. 5.3. Comparison of the badlands of the Region of Murcia with the Italian Calanchi and Biancane In various places on our planet there are badlands with different morphologies, some of which have been studied, such as in the badlands of South Dakota in the USA (Schumm, 1956), but the reason for these differences have rarely been analysed. However, in various parts of Italy, the differences between “Biancane and Calanchi” have been more comprehensively studied for decades (Vittorini, 1977; Alexander, 1982; Sdao et al., 1984; Pinna and Vittorini, 1990; Battaglia et al., 2002; Farifteh and Soeters, 2006; Vergari et al., 2013). The Italian badland landscapes are quite similar to the morphologies that exist in the Region of Murcia and analysed in this work. Thus, the Biancane (rounded shape), would be similar to the badlands of Abanilla; while

Table 3 Physical-chemical properties of soils and relationship with the orientation of the hillside. Properties

Abanilla S-facing

Sand Silt Clay BD AS EC pH OM CaCo3 Ca Na K Mg CEC SAR ESP

Gebas N-facing

S-facing

+ +

= +

= + +

Mula N-facing

S-facing

+ +

+

+ + + + + = + + =

+ +

+ higher value; = equal value.

+ +

+ + = + =

=

N-facing

+ +

+ = = + + = + + + +

+ + + =

+ +

= + + + + +

the Calanchi (angled shape) looks more like the badlands of Gebas and Mula. According to Alexander (1982) “the Calanchiare composed of an extremely dissected, rapidly developing land scape, characterized by rill and gully landforms and a very dense dendritic drainage network, and the Calanchi landscapes are dominated by processes such as land sliding, mudflows, slope collapse and pipe development, which produce a drainage network characterized by a greater internal disorder than badland. On the contrary, the Biancane are terrains characterized by bare cones and hummocks separated by flatter areas of surface wash deposits. Their drainage network is highly disordered, presenting two indistinct forms, one on the surface and one subsurface in the form of solutional cracks and piping”. In their study in Tuscany (north-eastern Italy), Battaglia et al. (2002) concluded that Biancane tend to develop on very fine sediments, such as silt clays, while Calanchi appear in places with sandy sediments, which is consistent with the studies of Vergari et al. (2013), who found more sands in the Calanchi. These results would be in line with our study, as there is a small proportion of sands in Abanilla and they are more abundant in Gebas and Mula. However, another Italian study carried out in La Basilicata (southern Italy) by Farifteh and Soeters (2006) states that, after laboratory analyses, the Biancane and Calanchi showed no significant differences, a finding that, according to these authors, questions the previous studies (Vittorini, 1977; Alexander, 1982; Sdao et al., 1984; Pinna and Vittorini, 1990; Battaglia et al., 2002) that attributed the differences between the two forms of land to the different materials. In the cases analysed by us, and according to our results, we suggest, among other causes, the presence of different grain size. The study carried out in Tuscany by Battaglia et al. (2002) pointed to the greater amount of sodium present in Biancane compared to Calanchi, which also occurs in Abanilla (where the quantities are very high) compared to Gebas and Mula. On the other hand, the existence of piping has been described as another cause of the formation of the Biancanes (Torri and Bryan, 1997). We also found this in the Region of Murcia, where we observed a greater number of small pipes in the badlands of Abanilla (Table 1) than in the other two study areas. Picarreta et al. (2006) also mentioned the importance of the piping processes in the formation of the badlands, especially in the middle and lower part of the hillside, but in this case, for both the Calanchi and Biancane formations. 6. Conclusions Our studies, both in field observations and by physical-chemical and mineralogical analyses of the soils, identified differences that may

A. Romero-Díaz et al. / Geomorphology 354 (2020) 107047

13

Table 4 Physical-chemical properties of soils and relationship to the position of the hillsides. Properties

Abanilla Top

Sand Silt Clay BD AS EC pH OM CaCo3 Ca Na K Mg CEC SAR ESP

Gebas Middle

Bottom

+

Top

Bottom

+ +

= =

Mula Middle

+ + + + = = =

Top

+ + + =

+ + +

=

+ +

+ +

+ + + +

= + + + = =

+ + +

+

+ +

Bottom +

+

= +

Middle

+

=

+ + + +

= =

+ = =

= =

+ higher value; = equal value.

explain the different morphologies of the badlands existing in the region of Murcia. Physiological analyses conclude that Abanilla (with rounded shapes) has a greater presence of salt efflorescences, popcorn, gullies, cracks and piping processes, while vegetation and biological crusts are almost non-existent. From a physical-chemical point of view, the soils of Abanilla are more silty and have a very low proportion of sands compared with those of Mula and Gebas (angled shapes); they have high salinity, which is reflected in a high Na content and a higher EC, SAR and ESP; they also have higher AS and higher CaCO3 content. The most abundant mineral in all three areas is calcite, but in Abanilla aluminium and potassium silicates, along with illite, are also frequent. With regard to the role that the orientation of the slopes may play, in semi-arid environments it is usually related to the higher humidity existing on the north-facing slopes. The three areas coincide in having the highest BD and EC values and highest Ca and Na contents on south-facing slopes. Of note is the absence of vegetation on both hillside orientations in Abanilla. Considering the position of the hillside, the middle section stands out as the place where most of the variables analysed reach the highest values, while there is movement of salts from the upper sections to the middle ones. Finally, from the comparison made with the Calanchi and Biancane, it is concluded that the badlands of Abanilla have morphological and physical-chemical characteristics similar to those of Biancane, whereas the badlands of Gebas and Mula are more like the Calanchi. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References AEMET, 2011. In: Agencia Estatal de Meteorología e Instituto de Meteorología de Portugal (Ed.), Atlas climático ibérico (Madrid). https://www.aemet.es/documentos/es/ conocermas/publicaciones/Atlas-climatologico/Atlas.pdf. Alexander, D., 1982. Difference between “Calanchi” and “biancane” badland in Italy. In: Bryan, R., Yair, A. (Eds.), Badland Geomorphology and Piping. Geo Books, Norwich, UK, pp. 71–88. Alonso-Sarria, F., 2007. El clima. Romero Díaz, A. (Coord.). Atlas Global de La Región de Murcia. La Verdad CMM S.A., Murcia, pp. 166–175.

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