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Development of quaternary calcrete in the Tensift Al Haouz area, Central Morocco: Characterization and environmental significance
Catena 149 (2017) 331–340
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Development of quaternary calcrete in the Tensift Al Haouz area, Central Morocco: Characterization and environmental significance Elidrissi Soukaina a,⁎, Daoudi Lahcen a, Arabi Badr a, Fagel Nathalie b a b
Laboratory of Geosciences and Environment (LGSE), Department of Geology, Faculty of Sciences and Technologies, Cadi Ayyad University, BP 549 Marrakech, Morocco UR, Clay, Geochemistry and Sedimentary Environment, Department of Geology, Liege University, B18, Sart-Tilman, Liège B 4000, Belgium
a r t i c l e
i n f o
Article history: Received 8 March 2016 Received in revised form 22 September 2016 Accepted 10 October 2016 Available online xxxx Keywords: Calcrete Arid Morphology Mineralogy Geochemistry Genesis
a b s t r a c t This study utilizes features of calcrete to elucidate the main environmental factors that affect its formation and distribution in the Tensift Al Haouz area, Central Morocco, one of semi arid regions where calcrete is ubiquitous and occurs in a variety of forms. More than hundred calcretes profiles were examined and described by field observations. Selected samples were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), Xray Diffraction (XRD), and chemical analysis (XRF). Two types of evolution and genesis are evidenced: 1) calcrete of pedogenic origin in the central and eastern part of the studied area, developed in the vadose zone with an upward gradational maturity pattern, ranging from incipient, nodular, to concretionary and to hardpan. 2) Combination of groundwater and pedogenic origin in the western part; the paired calcrete profiles consist from bottom to top of massive calcrete suggested to have a groundwater origin; it might be formed under wet to semi-arid climatic conditions and the gravel hardpan situated at the top suggested to have developed in the vadose zone by pedogenic processes under arid climate. In this study, we conclude that the vertical and lateral distribution of calcrete is related to differences in the topography, the nature of the bedrock, the texture of parent material and the climate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The carbonate accumulations in the soil, named calcrete or caliche, are widely distributed throughout arid and semi arid regions (Gile et al., 1966; Hay and Reeder, 1978; Estrela and Vogt, 1989). It occurs in a variety of forms and range in consistence from soft to extremely hard (Goudie, 1973; Watts, 1980; Eren and Hatipoglu-bagci, 2010; Kaplan et al., 2013). This phenomenon has received much attention from a variety of scientists including geomorphologists, pedologists and sedimentologists. Many aspects of their physical and chemical evolution maintained wide debates for more than one century. Exhaustive reviews on calcrete have been provided by Esteban and Klappa (1983), Wright and Tucker (1991), Watson and Nash (1997), Alonso-Zarza (2003), Wright (2007) and Pfeiffer et al. (2012). According to these previous studies, most calcrete profiles are polygenetic, where different processes may act during their evolution leading to fabric transformation and facies superimposition (Durand et al., 2007; Khalaf and Gaber, 2008). Calcrete is commonly developed within the vadose zone; however, its formation within the phreatic zone is also observed (Wright, 1994; Candy et al., 2004). It is also well known that the development of calcrete profiles requires long periods (Alonso-Zarza et al.,
⁎ Corresponding author. E-mail address: [email protected] (E. Soukaina).
http://dx.doi.org/10.1016/j.catena.2016.10.009 0341-8162/© 2016 Elsevier B.V. All rights reserved.
2010). Candy et al. (2004) shows that the hardpan took between 73 and 31 ka to form and the mature profile took between 121 and 69 ka. Nevertheless, we still know little about the genesis of calcrete and the clues they may hold for understanding paleogeomorphic and paleoclimatic processes (Brock and Buck, 2009). It is worth noting that soils with carbonate accumulation are ubiquitous features in Maghreb (Ballais and Ben Ouezdou, 1991; Gallala et al., 2010). As for the Moroccan context, calcretes cover large areas of the territory; this is well reported in studies especially on the low Moulouya and the Anti Atlas (Ruellan, 1967; Ducloux and Laouina, 1986; Kaemmerer et al., 1991). In the lowland of Tensift Al Haouz region located in the central part of Morocco (Fig. 1), calcretes are widespread. They occur in a number of forms of various thicknesses; however, except very limited studies (Ruellan, 1967; Moreau, 1981), the morphology, the distribution and evolution of calcretes in this vast region have not been characterized and dealt with in detail. The present study was performed on more than one hundred profiles distributed throughout the Tensift Al Haouz region. Morphological, micromorphological, mineralogical and chemical data on different types of calcretes were examined. The aim is to construct the sequence of events that lead to the development of calcrete profiles, to interpret their origin and their distribution in relation to the prevailing paleoclimatic conditions and geomorphic context of formation and to determine the main factors controlling their formation. This study emphasizes the paleoclimatic history and leads to represent features
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Fig. 1. Geological map and location of the studied area and calcrete profiles.
and factors that may be used to interpret other developed calcrete profiles worldwide. 2. Study area The Tensift Al Haouz is both a political region and a large watershed including several small sub-watersheds. It is located at the central part of Morocco and occupies an area of about 20,500 km2 extending between 31° and 32°30′ North and between 7° and 10° West (Fig. 1). From a geomorphological point of view, the region is composed of three distinct areas: the piedmont of the High Atlas at the south, the coastal plain of Essaouira at the west and the plain of Marrakech Al Haouz and Chichaoua which constitutes the major part of the region (Chehbouni et al., 2012). The study area lies within a semi-arid climatic region with two tendencies; in the western part of the region where the oceanic influences interfere, the climate is characterized by a sub humid tendency, whereas the eastern part is characterized by clear continental influences. Water is irregularly and unevenly distributed. The Tensift Wadi, which is the main natural water resource, originates from the High Atlas Mountains and flows in to the Marrakech-Al Haouz plain before reaching the Atlantic Ocean in the West (Fig. 1). Two different aquifer systems characterize the Tensift Al Haouz region. In the East, the plain of Marrakech- Al Haouz contains an unconfined groundwater flow system composed of Pliocene-Quaternary alluvium and Neogene formations. While in the west, a multi-aquifer was identified constituted by detrital deposits of the Plio-quaternary and dolomitic limestone of the Turonian. The Plio-quaternary aquifer is unconfined. The Turonian aquifer is confined by the Senonian marls and in direct contact with the Plioquaternary. On the geological side, the studied area offers different types of lithology (Fig. 1). The upstream part of the Atlas Mountain consists of a Precambrian and Paleozoic basement composed of igneous and metamorphic rocks wish form the Atlas chain platform. On the northern side of the chain, these facies are topped with sedimentary formations of varied composition and age. In the western part, the overlying
sedimentary sequences comprises epicontinental and marine sediments of Cretaceous and Eocene dominated largely by limestone, calcareous sandstones and marls (Sinan, 2000). In the central and eastern part of the region, the plain of Marrakech-Al Haouz and Chichaoua is composed by alluvium resulted from the dismantling of the mountain range of Atlas and accumulated in the Neogene and in the Quaternary (Soltanian); these facies are formed by conglomerates, sandstones, silts and clays. The geomorphological environments of these formations are marked by two major groups of forms of deposits: the dejection cones and fluvial terraces. These two units are intimately linked in time and space. Finally, in the coastal zone, the Plio-Quaternary outcrops are composed of calcareous sandstones of coastal and eolian dunes (Weisrock, 1980). The soil cover of the Tensift Al Haouz region is composed of three main types of soils; while the eastern and central parts consist of heterogeneous alluvial soil, the western part had sandy calcareous humus-bearing soil and the coastal zone is composed of sandy soil. 3. Materials and methods Hundred ten soil profiles (Fig. 2) were examined; the following observations and criteria were considered in situ for each profile: the thickness of the profile, the nature of the bedrock, the nature and texture of the soil, the size, the shape and situation of calcretes. The most representative profiles were surveyed and sampled; 4 to 5 samples were collected for each profile (one sample from the bedrock and the others from different types of horizons), collected samples underwent petrographic, mineralogical and chemical studies. The petrographic study was performed on thin sections of 30 μm in thickness using binocular Optical Microscope (Leitz). The scanning electron microscope (SEM) observations were performed using a Zeiss DSM-950 (equipped with a LINK Systems energy-dispersive Xraymicroanalysis system). Samples were prepared for SEM observation by dispersing powdered material onto double sided conductive tape fixed on SEM stub mounts. These were then coated with a film of silver using an Edwards S150B sputter coating unit.
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Fig. 2. Distribution of the studied calcrete profiles in the studied area.
The mineralogical analysis was performed by X ray diffraction (XRD), with a Bruker D8 Advance diffractometer, using Cu Ka radiation on powdered bulk sediment and on the b 2 μm fraction. From the same profiles, samples were chosen to undergo a mineralogical study of their oriented clay powders after air-drying at 25 °C, heating at 500 °C for 2 h, and adding glycol solvated. The chemical composition of representative calcrete samples were determined by X-ray fluorescence spectroscopy in Philips PW 2400 equipment. The total carbonate amount was determinate by Bernad Calcimetry method and confirmed by XRD analysis. 4. Results 4.1. Field description In order to bring out the vertical and lateral evolution of the calcretes within Tensift Al Haouz region, a series of lithostratigraphic profiles were surveyed on North–South and East-West section (Fig. 1). The calcrete forms were grouped into five types based on their size and morphological characteristics. 4.1.1. Incipient calcrete This type of accumulation particularly described in the east and central part of the studied area is represented by two forms (Fig. 2). The first one is characterized by fine particles of carbonate sporadically scattered in the fine soil. They are invisible to the naked eye, but they react differently to the acid (HCl) depending on their nature. These forms are principally encountered on the slopes and river banks. The second one is represented by small and chalky calcareous patches, scattered within the matrix of the host sediments. The color of these patches is white to cream, their shape is irregular and their size range from 1 to 10 cm (Fig. 3.A). These forms are more abundant in lowland profiles with a higher concentration at the base of the profiles. 4.1.2. Pebble and nodular calcrete Carbonate has accumulated as fine and indurate pebbles randomly scattered within the matrix of the host sediments (Fig. 3.A). They are
rounded in shape and have generally a diameter of 1 cm. They are usually white to cream in color. As for the nodules (Fig. 3.A, B), they range in size from 1 to 4 cm, they are spheroidal to elliptical in shape, and their colours vary generally from light brown to white. The core of these nodules can be hard and red in color. 4.1.3. Concretionary calcrete This form is characterized by hard and coalescing nodules with a diameter up to 20 cm (Fig. 3.B). These flattened and irregular calcrete concretions are surrounded by laminated calcrete (Fig. 3.C). They form a layer of about 0.5 to 1.5 m thick and usually occur above the nodular calcrete. 4.1.4. Massive calcrete Massive calcrete, which thickness reaches approximately 2.5 m, are soft to moderately indurated (Fig. 3.D). The color varies from white, brown, beige to gray; this variation of color seems to be in relation with the bed rock color. Very commonly, they form a transitional form between the bedrock and the hardpan. 4.1.5. Hardpan This form consists of hard and relatively impervious layers of indurated carbonate with a thickness ranging from 0.5 to 2.0 m. This form represents the most indurated of the different calcrete morphologies. The hardpan calcrete is exposed in the plains and in the low lying areas. The morphology is complex; we can distinguish pisolithic hardpan (Fig. 3.E) when it covers nodular calcrete and coated gravels hardpan (Fig. 3.F) when it covers massive calcrete. The pisoliths (Fig. 3.E) are more or less rounded grains up to 10 cm across and consist of a nucleus and cortex. The nucleus is mostly reworked fragment sourced from a previous calcrete horizon. The cortex, redder than the nucleus, is usually asymmetric and shows prominent underside development. They are commonly coated with irregular laminae formed of dark and lighter layers. Gravel hardpan (Fig. 3.F) consists mostly of quartz, carbonates and fragments of bedrocks. They are etched by a thin and dark carbonated layer and commonly cemented by calcite.
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Fig. 3. Photo of calcrete profile within Tensift Al Haouz region: A: Incipient and nodular calcrete profile: (Inc) Incipient calcrete, (Pd) Pebble, (Nd) Nodules. B: Nodular and concretionary calcrete: (Nd) Nodules, (Co) Concretionary calcrete. C: Concretionary calcrete with coalescent nodules. D: Massive calcrete profile: (Ma) Massive calcrete, (Hp) Hardpan. E: Pisolithic hardpan. F: Coated gravel hardpan. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.2. Microscopic features The observations of thin sections by the (OM) and by (SEM) reveal that the hardpan shows various microscopic features. 4.2.1. Cement Calcretes are generally cemented by a micritic or microsparitic calcite. In the pisolithic calcrete, the cement is composed of quartz grains randomly-distributed in micritic calcite groundmass and surrounded by microsparite (Fig. 4.A). In the gravel hardpan, coated grains are an important component which can be very variable in size (Fig. 4.B). The nucleus of the grains can include relics of the host rock, micrite clasts of quartz, feldspar, mica and heavy minerals. Wright and Alonso-Zarza (1990); Alonso-Zarza et al. (1992); Alonso-Zarza (2003) proposed that the formation of these grains requires the generation of the nuclei, either by desiccation or by root activity, and the formation of the coating, which is controlled by roots and associated microorganisms, especially fungal filaments and cyanobacteria. 4.2.2. Microsparitic veins Veins of calcite cut across randomly in thin sections of pisolithic hardpan (Fig. 4.C). These veins are filled with microsparite, and directly linked with the width of the veins. Khadkikar et al. (2000); Moussavi-Harami et
al. (2009) believed that shrinkage cracks are formed through successive wet-dry cycles, which ultimately caused spar/microspar filled veins. 4.2.3. Alveolar septal structures In the studied profiles, networks of alveolar septal structures are shown in pisolithic hardpan (Fig. 4.D). Under the petrographic microscope, they are mostly seen in irregular pores within the calcrete groundmass. The pores also show an irregular network of micritic septa and in some cases, the pores are filled with microspar calcite cement. Alveolarseptal structures are basically interpreted as products of fungal activity formed in fungal mycelia commonly, but not wholly, associated with roots (Wright, 1986; Wright and Tucker, 1991; Meléndez et al., 2011). 4.2.4. Needle-fibre calcite crystals Needle-fibre calcite crystals are common in gravel hardpan (Fig. 4.E). SEM images indicate that micro-rods of calcite are an important component of these calcretes. They are 1–4 μm long and 0.4 μm wide, but can be very variable in size. These needle-fibers are seen around rounded pores where they form an interwoven mass of crystals. The origin of needle-fibre calcite has been the subject of detailed studies, their formation is due to either high levels of supersaturation or microbial activity especially that of fungi or cyanobacteria (Callot et al., 1985; Phillips and Self, 1987). Most other authors attribute these crystals to microbial activity, but not necessarily to that of fungi.
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Fig. 4. A: photomicrograph of pisolithic hardpan showing quartz grains floating in micritic calcite groundmass. B: photomicrograph of gravel hardpan showing coated grains. C: Microsparitic veins. D: Alveolar septal structures. E and F: SEM image showing Needle-fibre calcite crystals. G and H: Palygorskite fibers developed on preexisting minerals: (Ca) Calcite, (Pl) Palygorskite, (Sm) Smectite.
The coated grains as well as the alveolar septal structures and the needle-fibre calcite crystals characterized by biogenic features are considered as beta micro-fabrics, while alpha micro-fabrics are characterized by non-biogenic features (Wright, 1990).
4.2.5. Palygorskite Under Scanning Electron Microscopy (SEM), the palygorskite was clearly noticed within the hardpan. Individual and flexuous palygorskite fibers are 0.5 μm to 2 μm long. Fibre bundles appear generally as overgrowths on pre-existing minerals (Fig. 4.G, H).
4.3. Mineralogical composition and geographic distribution The arrangement of different forms of calcrete allows identifying four types of profiles (Fig. 2) according to their morphology, their carbonate content, their mineralogy and their clay assemblages.
4.3.1. Profile type 1 It is characterized by incipient calcrete and occurs on slopes and river banks (Fig. 2). The mineralogical study shows that in these types of profiles, the percentages of calcite, clays and quartz decrease
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upwards, counterbalanced by increase of dolomite and plagioclase (Fig. 5). The percentage of calcite doesn't exceed 20%. The clay fraction (b 2 μm) consists mainly of illite, palygorskite, chlorite and kaolinite. Illite is the major component of the clay fraction (N 50%). The palygorskite, identified in all the samples, does not exceed 20% of the clay fraction; it decreases progressively upward. 4.3.2. Profile type 2 This type of profile has a vertically gradual transition from incipient (calcareous patches) to nodular calcrete passing through pebbles (Fig. 3.A). This type of profile is developed mainly in the east-central part of studied region in areas with low slopes (Fig. 2). The thickness varies from approximately 1 m to 1.5 m. The percentage of CaCO3 increases towards the top; calcareous patch and pebbles consist of approximately 30% of CaCO3, while the nodules consist of 60%. 4.3.3. Profile type 3 The profile has a vertically gradual transition from Pebbles, nodular, concretionary, to pisolithic calcrete (hardpan) (Fig. 3.B). This type of profile is usually encountered in the central part of the studied area from Mzouda to Taftechet (Fig. 2). Mineralogical study of the profile (Fig. 6) shows that calcrete samples consist mainly of calcite, clay and quartz associated with minor amounts of plagioclase and K feldspars. The calcite amounts range between 60% in the nodular calcrete and 70% in the hardpan. Palygorskite is the dominant clay mineral within the clay fraction; its amount increases from the surface horizon of soils (40%) towards the nodular calcrete (80%). Palygorskite is associated with illite, kaolinite and chlorite in variable proportions (Fig. 8). A cross section of the large nodule shows that the mineralogical composition is characterized by calcite as the dominant mineral. It increases from the outside (65%) towards the core (75%) (Fig. 7). Quartz and clays show similar content between the interior and the exterior of the nodule (~20%). Palygorskite is the more abundant and ubiquitous clay mineral phase, which increases from 65% in the peripheral part of the nodule to 85% in the core. It is associated with illite and kaolinite (Fig. 7). 4.3.4. Profile type 4 It corresponds to the complete calcrete profile; it is characterized by vertical gradational maturity pattern which is manifested by the occurrence of well differentiated forms that range from massive calcrete to gravel hardpan (Fig. 3.D). This type of profile is usually encountered in
the western part of the studied area, between Taftechet and Essaouira (Fig. 2). The mineralogical composition of calcrete samples is mainly calcite associated with clays, quartz and accessory K-feldspar and plagioclases (Fig. 7). The calcite content increase upward (65 to 85%). Smectite and palygorskite are the dominant clay mineral phases associated with variable amounts of illite, chlorite and kaolinite. Smectite constitutes the major component of the clay fraction of the bed rock (55%) and massive calcrete (65%) (Fig. 8), it decreases progressively towards the top of the profile (30%). Palygorskite increases from 10% in the bed rock to 25% in the massive calcrete (Fig. 7). 4.4. Geochemical analysis Calcrete samples (Table 1) are characterized by high amount of Ca related to calcite identified both by XRD analysis and calcimetry. Clear differences in Ca among the calcrete samples were likely due to variable amount of calcite. Si (2.16 to 45%) is related to quartz and feldspar, the lower quantity of Al (0.60 to 12.50%) and K (0.26 to 4.22%) are ascribed to clay minerals and alkali feldspar, both identified in XRD analysis. Mg might be assigned to dolomite and palygorskite occurrence; nodular calcrete samples have relatively high concentrations of Mg (1.28 to 4.77%, mean: 2.18%), whereas massive calcrete samples have relatively low concentrations (1.12 to 1.15%, mean: 1.14%). 5. Discussion The analysis of calcrete in a rich and varied observation field as the region of Tensift Al Haouz enabled us to specify the important types of accumulations: incipient, nodular, concretionary, massive calcrete and hardpan. The shape of these forms as well as their consistency and hardness is clearly related to the contents of CaCO3. The vertical and lateral distribution indicates a complex evolution of different calcareous forms resulting from a succession of agents and processes. 5.1. Mechanisms of calcrete profiles development The occurrence of calcrete profiles within the studied area indicates two types of vertical evolution each materializing a genetic process. 5.1.1. Pedogenic calcrete In the eastern and central part of the studied area, the calcrete sequence is characterized by alluvial sedimentation followed by calcrete
Fig. 5. Mineralogy and clay composition of profile type 1.
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Fig. 6. Mineralogy and clay composition of profile type 3.
formation. The plain area of Al Haouz is located within a structurally low area, bounded by the High Atlas and Jbilet highlands respectively from the north and south (Fig. 1). These highlands are mostly formed of Precambrian and Paleozoic basement composed of igneous and metamorphic rocks. It has been suggested that the alluvial sedimentation in the Eastern and Central part of the studied area derives from the highlands by drainage systems during wet period, when humid climate prevailed (Weisrock, 1980). This phase was followed by calcrete development, under more arid conditions. The calcrete profile displays upward increasing carbonate content from an incipient calcrete to a nodular and concretionary calcrete and finally to hardpan. Field occurrence and microscopic descriptions suggest that the incipient calcrete represents the earliest stage of calcrete formation, in which calcite has precipitated within the intergranular pores of the matrix of the host rock, forming a calcareous patches. These patches grow by first displacing and then replacing the siliciclastic muddy matrix of the host sediment. The pebbles and nodules are probably formed by replacement of the varied siliciclastic framework grains by authigenic calcite. The calcrete nodules grow laterally until they coalesce and form calcrete concretions. This stage represents the beginning of hardpan formation and by a complete replacement of the parent material, displacement and partial replacement of framework grains. The pisolithic hardpan can be the result of the intense reworking of the underlying calcrete. This can be deduces from the composition of the pisoliths nuclei which are commonly
composed of hardpan fragments. Furthermore, the pisoliths and beta textures suggest a pedogenic origin of calcrete, formed by precipitation of calcium carbonate in the vadose zone (Alonso-Zarza, 2003; Candy et al., 2003; Khalaf and Gaber, 2008). Their occurrence within the calcrete of the eastern and central part of the studied area suggests that they are best interpreted as pedogenic calcrete. 5.1.2. Combination of groundwater and pedogenic calcrete In the western part of the region (between Taftechet and Essaouira) the vertical evolution and thus, the formation process of calcrete seems to be different. Unlike pedogenic calcretes profiles of the eastern and central part, profiles sampled in this zone are thicker and mostly massive lacking any horizonation and differentiation (Fig. 3.D). In this respect, the profiles are similar to many groundwater calcrete profiles described in many regions around the word (e.g. in Australia (Mann and Horwitz, 1979; Arakel and McConchie, 1982) and Spain (Nash and Smith, 1998). These studies suggest that calcretes formed in a phreatic zone are mostly massive in character. Wright and Tucker (1991) also suggest that, in general, thicker horizons are likely to be present in groundwater calcretes when compared to pedogenic varieties. However, coated grains structures (Fig. 3.F), which appear similar to those of pedogenic calcretes (Fig. 3.E), are frequently present in the uppermost section of these profiles. Otherwise, the occurrence of beta fabrics structures (Needles fibre) in these upper levels suggests that they are best interpreted as pedogenic calcrete formed in the vadose zone.
Fig. 7. Mineralogy and clay composition of profile type 4.
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Fig. 8. X-ray diffraction patterns of oriented clay fractions of samples collected from nodular and massive calcrete.
Consequently, on the basis of these morphological and micromorphological observations, it would appears most likely that the calcrete profile in the western part of Tensift Al Haouz have formed by a combination of groundwater and pedogenic calcretes. 5.2. Environmental control on the development of calcretes The occurrence of calcrete profiles in the studied area shows a lateral evolution from south to north and from east to west (Fig. 2). This suggests that these forms of calcrete do not appear randomly but seems to be related to well define environmental conditions. In the literature, a range of factors has been suggested to affect calcrete development (Goudie, 1973; Wright and Tucker, 1991; Alonso-Zarza, 2003; Candy et al., 2006; Wright, 2007; Reith et al., 2009). In the case of Tensift Al Haouz region, four main factors seem to have controlled the variation and evolution of calcrete: topography, bedrock, parent materiel and climate. 5.2.1. Topography The topography of the studied region has not experienced any significant change since the late Pliocene; the major uplift of the western High Atlas chain has occurred since the Oligocene (Giese and Jacobshagen, 1992; Frizon de Lamotte et al., 2000; El Harfi et al., 2006). Thus, in order to visualize the topography in which the calcrete profile lie, actual slope map was produced with ArcGis spatial analyst function using 30 m resolution DEM (digital elevation model). The projection of
Table 1 Chemical composition of calcrete samples from Tensift Al Haouz region.
Massive calcrete
Nodular calcrete
Samples
Ca
Si
Al
K
Mg
Mn
Fe
SK_2 SK_3 SK_4 SK_5 AO_2 AO_3 VON_Z Car 2 Car 3 Car 4 SIM2 SIM3 MZ_2 MZ_3 OMNII D OMNII E TMSLT D PMTG D AV CHI D AVCHI E TIGMI D
37.26 38.17 34.06 40.23 31.07 18.87 42.52 39.29 38.49 38.45 66.18 60.60 31.99 32.42 82.53 70.30 68.39 89.14 81.58 26.86 71.79
5.49 4.71 7.31 2.72 7.90 16.21 2.16 3.90 4.32 4.74 19.55 23.43 8.66 8.75 10.68 18.12 15.81 4.23 9.91 45.01 15.57
0.64 0.70 1.29 0.57 1.22 2.49 0.64 0.85 1.04 0.91 6.08 6.97 1.99 1.79 2.77 5.10 6.37 2.21 2.32 12.53 5.02
0.27 0.26 0.47 0.31 0.39 0.77 0.39 0.32 0.33 0.41 2.85 3.17 0.61 0.52 0.90 1.58 2.53 0.40 0.52 4.22 1.84
– – – – – – – – – – 1.15 1.12 4.77 – 1.28 1.57 1.71 2.25 1.72 2.65 1.45
0.03 0.01 0.02 – – 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 – 0.01 0.02 0.01 – 0.06 0.02
0.43 0.62 1.10 0.41 0.79 1.39 0.42 0.45 0.59 0.47 2.81 3.16 0.87 0.91 1.73 3.08 4.58 1.69 1.63 7.03 4.12
calcrete profiles on the slope map (Fig. 2) reveals that the calcretes developed on slopes (piedmont of High Atlas) lack the hard forms and occur as powdery patches and friable nodules, whereas down slope, the calcretes are thicker and show well developed features (concretionary calcrete and hardpan). In high slope areas, the rate of erosion is higher than the rate of soil formation (and CaCO3 accumulation) which explain the presence of more developed soils on the flat areas. In addition, in the low topography, the solutions infiltration can reach deeper horizons which enable the formation of thick and mature calcrete profiles. 5.2.2. Bedrock and source of carbonate In the central and eastern part of the studied area, the bedrock is mainly composed of alluvial deposits (conglomerates, sandstones, siltites and clays) from the High Atlas. The formation of mature nodular calcrete profiles in the absence of calcareous bedrock is related to the fact that the origin of calcium seems predominantly allochthonous. In this context, calcrete may have been formed through detrital input or by lateral transfer of Ca at landscape scale from High Atlas Ca rich source rocks from the south. However, as suggested by several authors (Goudie, 1973; Reeves, 1976; Monger and Gallegos, 2000 and Eren et al., 2004), CaCO3 dust and the water-soluble Ca in that dust may be considered as another source of calcium carbonate. The types of clays and mineral assemblages found in the studied profiles, with a significant presence of quartz, feldspars, illite and kaolinite show that detrital supply by either wind or water transport seems to be a fundamental mechanism during the accumulation of these materials. However, in the western part, massive calcrete formation is due to interstitial cementation, displacement and replacement of sediment bodies by carbonates within shallow aquifer systems. Calcrete in this case have developed in situations where CaCO3 was sufficiently available due to the presence of cretaceous calcareous aquifer. 5.2.3. Parent material The morphology of calcretes seems to depend on the texture of parent material. Calcrete developed over gravelly sediments in the central and eastern part of the study area display a different morphological sequence; the authigenic calcite replaced the siliciclastic framework grains and therefore allows the formation of pebbles, nodules and concretionary calcrete. However, in the west, on sandy calcareous humusbearing soil, cementation, displacement and replacement of sediment bodies by carbonates are interstitial allowing the development of mostly massive calcrete lacking any horizonation and differentiation. 5.2.4. Climate Precipitation regime is one of the main factors that control calcrete formation (Candy and Black, 2009). Data on average rainfall may be obtained from calcretes by studying the mineralogy of the clays they contain (Alonso-Zarza, 2003). Khadkikar et al. (2000) show that calcrete,
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when associated with palygorskite, suggest an arid climate (mean annual rainfall = 50–100 mm) and when associated with smectite it indicates a semi-arid climate (mean annual rainfall = 100–500 mm). In the Tensift al Haouz region, mineralogical analyses show that most of the analyzed samples are characterized by the presence of variable amount of palygorskite. The content of this fibrous clay varies between 20% of the clay fraction of the incipient calcrete, 40% in the pebbles to 85% in the nodules. However, in the massive calcrete profiles (western part of the studied area); palygorskite doesn't exceed 25%, whereas smectite represents the abundant clay mineral phase (up to 65% of the clay fraction) (Figs. 7 and 8). Accordingly, the abundance of palygorskite in the nodular calcrete in central part of the studied area suggests their formation under arid climatic conditions. On the other hand, the occurrence of smectite in the massive calcrete in western part may indicate that they formed under semi-arid climate. Furthermore, geochemical study can clarify the position of carbonate accumulation with respect to the water table. Khadkikar et al. (2000) and Khalifa (2005) show that during dry periods that are related to high evaporation rates, Mg concentration in meteoric waters increases, however, when the water table rises and under low evaporation rate conditions, fresh water is capable of leaching Mg ions from calcite crystals. Mg values in samples collected from massive calcrete profiles of the studied area are lower than those of nodular group samples; this confirms that massive calcrete reflects the rising groundwater phase. 5.3. Correlation with regional paleoclimate data Calcrete features described in the Tensift Al Haouz region reflect the sequence of events enabling the reconstitution of paleoclimate history. In the eastern and central parts of the studied area, the deposition of Soltanian silts and alluvium ended with the formation of soil around 17,300 ± 240 and 22,400 ± 400 yr B.P. (Rognon et al., 1984). These silts and alluvium were covered by incipient and calcrete nodules (Weisrock and Rognon, 1977). This could explain the formation of incipient and nodular calcrete in the wetter climate during upper Pleistocene. Weisrock and Rognon (1977) describe a gradual trend to a more arid climate. This would have caused the formation of large and flattened nodules at the top. With sustained aridity, these large nodules became coalescent. This stage represented the beginning of hardpan formation. The thick hardpan requires a significant time to form and reflects the absence of any erosion events. This may indicate the formation during the Lower Holocene under a drier climate than that of the present. The abundance of rotated and re-incorporated petrocalcic fragments in the hardpan (Fig. 3E) suggests probably an increase in aridity. Brock and Buck (2009) explained that the climate was not wet enough for the dissolution of the fragments that were exposed at the surface. As regards to calcretes developed in the western part, after a major pluvial period of the Soltanian that lasted throughout the Upper Pleistocene, the rainfall regime seems to have been more irregular and climatic fluctuations more frequent Rognon (1987). The warm climate prevailed and the formation of massive groundwater calcrete may be promoted by intense evaporation from a water table close to the surface supplied by cretaceous and plio-quaternary aquifers. The increasing aridity in the lower Holocene would have caused the formation of hardpan. The gravel hardpan could be formed in the same conditions as pisolithic hardpan by erosion, deposition and recementation of fragment under arid climate during the Lower Holocene. 6. Conclusion This study is a result of an extensive survey of soil profiles with calcium carbonate accumulations. Field and microscopic observations as well as mineralogical and geochemical studies, were used to reveal the calcrete formation mechanism. The calcrete features offer valuable
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data to determine the main environmental factors that operate in the calcrete formation and distribution and to interpret the geomorphic and climatic major events. Pedogenic calcrete profiles (eastern and central part of the studied area) were developed as a result of two main sequential processes, namely; distal alluvial deposition and calcrete formation. This sequence started by the deposition of alluvial sediments during a humid period in the topographically low area, was followed by pedogenic calcrete formation. It is further suggested that incipient calcretes followed upward by nodular calcretes and hardpan may indicate an episode of low sedimentation rate followed by a period of stagnation in a semi-arid climate. The early stage of calcrete development (incipient and nodular calcretes) was probably developed in the vadose zone within floodplain during periods of low terrigenous supply under semi-arid climatic conditions. Cementation of the relatively thick hardpan may correspond to the relatively more arid climate. Whereas, combined groundwater and pedogenic profiles (western part of the studied area) shows the occurrence of groundwater calcrete at the base, it reflects wet to semi-arid climatic conditions, rising of groundwater, and the pedogenic calcrete at the top of the sequence which reflects arid climate. The differentiation and evolution of calcrete throughout the study area is under the influence of three major factors: 1) the climate which is responsible for the evaporation and crystallization of carbonate; 2) the availability of carbonate rich solution which is controlled by the topography, the nature of underlying bedrock and the detrital processes; 3) the texture of the parent materiel which controlled also the morphologies of calcrete. Acknowledgements The financial support is provided by the PPR-CNRST program (grant: PPR1/2015/63) and by the “Projet de Coopération Bilatérale Wallonie Bruxelles-Maroc” (grant 2.7) that are all gratefully acknowledged. The first author acknowledges the financial support provided by the CNRST as part of scholarship program (009UCA2014). References Alonso-Zarza, A.M., 2003. Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record. Earth Sci. Rev. 60, 261–298. Alonso-Zarza, A.M., Calvo, J.P., Garcia del Cura, M.A., 1992. Palustrine sedimentation and associated features —grainification and pseudo-microkarst—in the middle Miocene (Intermediate Unit) of the Madrid Basin, Spain. Sediment. Geol. 76, 43–61. Alonso-Zarza, A.M., Wright, V.P., Alonso-Zarza, A.M., Tanner, L.H., 2010. Calcretes. Carbonates in Continental Settings: Facies, Environment, and Processes: Developments in Sedimentology. 61, pp. 225–267. Arakel, A.V., McConchie, D., 1982. Classification and genesis of calcrete and gypsum lithofacies in palaeodrainage systems of inland Australia and their relationship to carnotite mineralization. J. Sediment. Petrol. 52, 1149–1170. Ballais, J.L., Ben Ouezdou, H., 1991. Forms and deposits of the continental quaternary of the Saharan margin of eastern Maghreb (tentative synthesis). J. Afr. Earth Sci. 12 (1/2), 209–216. Brock, A.L., Buck, B.J., 2009. Polygenetic development of the Mormon Mesa, NV petrocalcic horizons: geomorphic and paleoenvironmental interpretations. Catena 77, 65–75. Callot, G., Guyon, A., Mousain, D., 1985. Interrelations entre aiguilles de calcite et hyphes mycéliens. Agronomie 5, 209–216. Candy, I., Black, S., 2009. The timing of quaternary calcrete development in semi-arid southeast Spain: investigating the role of climate on calcrete genesis. Sediment. Geol. 218, 6–15. Candy, I., Black, S., Sellwood, B.W., 2004. Quantifying time scales of pedogenic calcrete formation using U-series disequilibria. Sediment. Geol. 170, 177–187. Candy, I., Black, S., Sellwood, B.W., Rowan, J.S., 2003. Calcrete profile development in Quaternary alluvial sequences, southeast Spain: implications for using calcretes as a basis for landform chronologies. Earth Surf. Process. Landf. 28, 169–185. Candy, I., Rose, J., Lee, J.R., 2006. A seasonally ‘dry’ interglacial climate in Britain during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK. Boreas 35, 255–265. Chehbouni, A., Escadafal, R., Duchemin, B., Boulet, G., et al., 2012. An integrated modelling and remote sensing approach for hydrological study in arid and semi-arid regions: the SUDMED programme. Int. J. Remote Sens. 29 (17–18), 5161–5181 Taylor & Francis: STM, Behavioural Science and Public Health Titles, 2008. Ducloux, J., Laouina, A., 1986. L'encroûtement calcaire des formations caillouteuses; faciès et types d'évolution. Observations sur formations alluviales et colluviales dans le Maroc nord-oriental. Rev. Géogr. Maroc 1-2, 97–114.
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Development of quaternary calcrete in the Tensift Al Haouz area, Central Morocco: Characterization and environmental significance Elidrissi Soukaina a,⁎, Daoudi Lahcen a, Arabi Badr a, Fagel Nathalie b a b
Laboratory of Geosciences and Environment (LGSE), Department of Geology, Faculty of Sciences and Technologies, Cadi Ayyad University, BP 549 Marrakech, Morocco UR, Clay, Geochemistry and Sedimentary Environment, Department of Geology, Liege University, B18, Sart-Tilman, Liège B 4000, Belgium
a r t i c l e
i n f o
Article history: Received 8 March 2016 Received in revised form 22 September 2016 Accepted 10 October 2016 Available online xxxx Keywords: Calcrete Arid Morphology Mineralogy Geochemistry Genesis
a b s t r a c t This study utilizes features of calcrete to elucidate the main environmental factors that affect its formation and distribution in the Tensift Al Haouz area, Central Morocco, one of semi arid regions where calcrete is ubiquitous and occurs in a variety of forms. More than hundred calcretes profiles were examined and described by field observations. Selected samples were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), Xray Diffraction (XRD), and chemical analysis (XRF). Two types of evolution and genesis are evidenced: 1) calcrete of pedogenic origin in the central and eastern part of the studied area, developed in the vadose zone with an upward gradational maturity pattern, ranging from incipient, nodular, to concretionary and to hardpan. 2) Combination of groundwater and pedogenic origin in the western part; the paired calcrete profiles consist from bottom to top of massive calcrete suggested to have a groundwater origin; it might be formed under wet to semi-arid climatic conditions and the gravel hardpan situated at the top suggested to have developed in the vadose zone by pedogenic processes under arid climate. In this study, we conclude that the vertical and lateral distribution of calcrete is related to differences in the topography, the nature of the bedrock, the texture of parent material and the climate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The carbonate accumulations in the soil, named calcrete or caliche, are widely distributed throughout arid and semi arid regions (Gile et al., 1966; Hay and Reeder, 1978; Estrela and Vogt, 1989). It occurs in a variety of forms and range in consistence from soft to extremely hard (Goudie, 1973; Watts, 1980; Eren and Hatipoglu-bagci, 2010; Kaplan et al., 2013). This phenomenon has received much attention from a variety of scientists including geomorphologists, pedologists and sedimentologists. Many aspects of their physical and chemical evolution maintained wide debates for more than one century. Exhaustive reviews on calcrete have been provided by Esteban and Klappa (1983), Wright and Tucker (1991), Watson and Nash (1997), Alonso-Zarza (2003), Wright (2007) and Pfeiffer et al. (2012). According to these previous studies, most calcrete profiles are polygenetic, where different processes may act during their evolution leading to fabric transformation and facies superimposition (Durand et al., 2007; Khalaf and Gaber, 2008). Calcrete is commonly developed within the vadose zone; however, its formation within the phreatic zone is also observed (Wright, 1994; Candy et al., 2004). It is also well known that the development of calcrete profiles requires long periods (Alonso-Zarza et al.,
⁎ Corresponding author. E-mail address: [email protected] (E. Soukaina).
http://dx.doi.org/10.1016/j.catena.2016.10.009 0341-8162/© 2016 Elsevier B.V. All rights reserved.
2010). Candy et al. (2004) shows that the hardpan took between 73 and 31 ka to form and the mature profile took between 121 and 69 ka. Nevertheless, we still know little about the genesis of calcrete and the clues they may hold for understanding paleogeomorphic and paleoclimatic processes (Brock and Buck, 2009). It is worth noting that soils with carbonate accumulation are ubiquitous features in Maghreb (Ballais and Ben Ouezdou, 1991; Gallala et al., 2010). As for the Moroccan context, calcretes cover large areas of the territory; this is well reported in studies especially on the low Moulouya and the Anti Atlas (Ruellan, 1967; Ducloux and Laouina, 1986; Kaemmerer et al., 1991). In the lowland of Tensift Al Haouz region located in the central part of Morocco (Fig. 1), calcretes are widespread. They occur in a number of forms of various thicknesses; however, except very limited studies (Ruellan, 1967; Moreau, 1981), the morphology, the distribution and evolution of calcretes in this vast region have not been characterized and dealt with in detail. The present study was performed on more than one hundred profiles distributed throughout the Tensift Al Haouz region. Morphological, micromorphological, mineralogical and chemical data on different types of calcretes were examined. The aim is to construct the sequence of events that lead to the development of calcrete profiles, to interpret their origin and their distribution in relation to the prevailing paleoclimatic conditions and geomorphic context of formation and to determine the main factors controlling their formation. This study emphasizes the paleoclimatic history and leads to represent features
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Fig. 1. Geological map and location of the studied area and calcrete profiles.
and factors that may be used to interpret other developed calcrete profiles worldwide. 2. Study area The Tensift Al Haouz is both a political region and a large watershed including several small sub-watersheds. It is located at the central part of Morocco and occupies an area of about 20,500 km2 extending between 31° and 32°30′ North and between 7° and 10° West (Fig. 1). From a geomorphological point of view, the region is composed of three distinct areas: the piedmont of the High Atlas at the south, the coastal plain of Essaouira at the west and the plain of Marrakech Al Haouz and Chichaoua which constitutes the major part of the region (Chehbouni et al., 2012). The study area lies within a semi-arid climatic region with two tendencies; in the western part of the region where the oceanic influences interfere, the climate is characterized by a sub humid tendency, whereas the eastern part is characterized by clear continental influences. Water is irregularly and unevenly distributed. The Tensift Wadi, which is the main natural water resource, originates from the High Atlas Mountains and flows in to the Marrakech-Al Haouz plain before reaching the Atlantic Ocean in the West (Fig. 1). Two different aquifer systems characterize the Tensift Al Haouz region. In the East, the plain of Marrakech- Al Haouz contains an unconfined groundwater flow system composed of Pliocene-Quaternary alluvium and Neogene formations. While in the west, a multi-aquifer was identified constituted by detrital deposits of the Plio-quaternary and dolomitic limestone of the Turonian. The Plio-quaternary aquifer is unconfined. The Turonian aquifer is confined by the Senonian marls and in direct contact with the Plioquaternary. On the geological side, the studied area offers different types of lithology (Fig. 1). The upstream part of the Atlas Mountain consists of a Precambrian and Paleozoic basement composed of igneous and metamorphic rocks wish form the Atlas chain platform. On the northern side of the chain, these facies are topped with sedimentary formations of varied composition and age. In the western part, the overlying
sedimentary sequences comprises epicontinental and marine sediments of Cretaceous and Eocene dominated largely by limestone, calcareous sandstones and marls (Sinan, 2000). In the central and eastern part of the region, the plain of Marrakech-Al Haouz and Chichaoua is composed by alluvium resulted from the dismantling of the mountain range of Atlas and accumulated in the Neogene and in the Quaternary (Soltanian); these facies are formed by conglomerates, sandstones, silts and clays. The geomorphological environments of these formations are marked by two major groups of forms of deposits: the dejection cones and fluvial terraces. These two units are intimately linked in time and space. Finally, in the coastal zone, the Plio-Quaternary outcrops are composed of calcareous sandstones of coastal and eolian dunes (Weisrock, 1980). The soil cover of the Tensift Al Haouz region is composed of three main types of soils; while the eastern and central parts consist of heterogeneous alluvial soil, the western part had sandy calcareous humus-bearing soil and the coastal zone is composed of sandy soil. 3. Materials and methods Hundred ten soil profiles (Fig. 2) were examined; the following observations and criteria were considered in situ for each profile: the thickness of the profile, the nature of the bedrock, the nature and texture of the soil, the size, the shape and situation of calcretes. The most representative profiles were surveyed and sampled; 4 to 5 samples were collected for each profile (one sample from the bedrock and the others from different types of horizons), collected samples underwent petrographic, mineralogical and chemical studies. The petrographic study was performed on thin sections of 30 μm in thickness using binocular Optical Microscope (Leitz). The scanning electron microscope (SEM) observations were performed using a Zeiss DSM-950 (equipped with a LINK Systems energy-dispersive Xraymicroanalysis system). Samples were prepared for SEM observation by dispersing powdered material onto double sided conductive tape fixed on SEM stub mounts. These were then coated with a film of silver using an Edwards S150B sputter coating unit.
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Fig. 2. Distribution of the studied calcrete profiles in the studied area.
The mineralogical analysis was performed by X ray diffraction (XRD), with a Bruker D8 Advance diffractometer, using Cu Ka radiation on powdered bulk sediment and on the b 2 μm fraction. From the same profiles, samples were chosen to undergo a mineralogical study of their oriented clay powders after air-drying at 25 °C, heating at 500 °C for 2 h, and adding glycol solvated. The chemical composition of representative calcrete samples were determined by X-ray fluorescence spectroscopy in Philips PW 2400 equipment. The total carbonate amount was determinate by Bernad Calcimetry method and confirmed by XRD analysis. 4. Results 4.1. Field description In order to bring out the vertical and lateral evolution of the calcretes within Tensift Al Haouz region, a series of lithostratigraphic profiles were surveyed on North–South and East-West section (Fig. 1). The calcrete forms were grouped into five types based on their size and morphological characteristics. 4.1.1. Incipient calcrete This type of accumulation particularly described in the east and central part of the studied area is represented by two forms (Fig. 2). The first one is characterized by fine particles of carbonate sporadically scattered in the fine soil. They are invisible to the naked eye, but they react differently to the acid (HCl) depending on their nature. These forms are principally encountered on the slopes and river banks. The second one is represented by small and chalky calcareous patches, scattered within the matrix of the host sediments. The color of these patches is white to cream, their shape is irregular and their size range from 1 to 10 cm (Fig. 3.A). These forms are more abundant in lowland profiles with a higher concentration at the base of the profiles. 4.1.2. Pebble and nodular calcrete Carbonate has accumulated as fine and indurate pebbles randomly scattered within the matrix of the host sediments (Fig. 3.A). They are
rounded in shape and have generally a diameter of 1 cm. They are usually white to cream in color. As for the nodules (Fig. 3.A, B), they range in size from 1 to 4 cm, they are spheroidal to elliptical in shape, and their colours vary generally from light brown to white. The core of these nodules can be hard and red in color. 4.1.3. Concretionary calcrete This form is characterized by hard and coalescing nodules with a diameter up to 20 cm (Fig. 3.B). These flattened and irregular calcrete concretions are surrounded by laminated calcrete (Fig. 3.C). They form a layer of about 0.5 to 1.5 m thick and usually occur above the nodular calcrete. 4.1.4. Massive calcrete Massive calcrete, which thickness reaches approximately 2.5 m, are soft to moderately indurated (Fig. 3.D). The color varies from white, brown, beige to gray; this variation of color seems to be in relation with the bed rock color. Very commonly, they form a transitional form between the bedrock and the hardpan. 4.1.5. Hardpan This form consists of hard and relatively impervious layers of indurated carbonate with a thickness ranging from 0.5 to 2.0 m. This form represents the most indurated of the different calcrete morphologies. The hardpan calcrete is exposed in the plains and in the low lying areas. The morphology is complex; we can distinguish pisolithic hardpan (Fig. 3.E) when it covers nodular calcrete and coated gravels hardpan (Fig. 3.F) when it covers massive calcrete. The pisoliths (Fig. 3.E) are more or less rounded grains up to 10 cm across and consist of a nucleus and cortex. The nucleus is mostly reworked fragment sourced from a previous calcrete horizon. The cortex, redder than the nucleus, is usually asymmetric and shows prominent underside development. They are commonly coated with irregular laminae formed of dark and lighter layers. Gravel hardpan (Fig. 3.F) consists mostly of quartz, carbonates and fragments of bedrocks. They are etched by a thin and dark carbonated layer and commonly cemented by calcite.
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Fig. 3. Photo of calcrete profile within Tensift Al Haouz region: A: Incipient and nodular calcrete profile: (Inc) Incipient calcrete, (Pd) Pebble, (Nd) Nodules. B: Nodular and concretionary calcrete: (Nd) Nodules, (Co) Concretionary calcrete. C: Concretionary calcrete with coalescent nodules. D: Massive calcrete profile: (Ma) Massive calcrete, (Hp) Hardpan. E: Pisolithic hardpan. F: Coated gravel hardpan. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.2. Microscopic features The observations of thin sections by the (OM) and by (SEM) reveal that the hardpan shows various microscopic features. 4.2.1. Cement Calcretes are generally cemented by a micritic or microsparitic calcite. In the pisolithic calcrete, the cement is composed of quartz grains randomly-distributed in micritic calcite groundmass and surrounded by microsparite (Fig. 4.A). In the gravel hardpan, coated grains are an important component which can be very variable in size (Fig. 4.B). The nucleus of the grains can include relics of the host rock, micrite clasts of quartz, feldspar, mica and heavy minerals. Wright and Alonso-Zarza (1990); Alonso-Zarza et al. (1992); Alonso-Zarza (2003) proposed that the formation of these grains requires the generation of the nuclei, either by desiccation or by root activity, and the formation of the coating, which is controlled by roots and associated microorganisms, especially fungal filaments and cyanobacteria. 4.2.2. Microsparitic veins Veins of calcite cut across randomly in thin sections of pisolithic hardpan (Fig. 4.C). These veins are filled with microsparite, and directly linked with the width of the veins. Khadkikar et al. (2000); Moussavi-Harami et
al. (2009) believed that shrinkage cracks are formed through successive wet-dry cycles, which ultimately caused spar/microspar filled veins. 4.2.3. Alveolar septal structures In the studied profiles, networks of alveolar septal structures are shown in pisolithic hardpan (Fig. 4.D). Under the petrographic microscope, they are mostly seen in irregular pores within the calcrete groundmass. The pores also show an irregular network of micritic septa and in some cases, the pores are filled with microspar calcite cement. Alveolarseptal structures are basically interpreted as products of fungal activity formed in fungal mycelia commonly, but not wholly, associated with roots (Wright, 1986; Wright and Tucker, 1991; Meléndez et al., 2011). 4.2.4. Needle-fibre calcite crystals Needle-fibre calcite crystals are common in gravel hardpan (Fig. 4.E). SEM images indicate that micro-rods of calcite are an important component of these calcretes. They are 1–4 μm long and 0.4 μm wide, but can be very variable in size. These needle-fibers are seen around rounded pores where they form an interwoven mass of crystals. The origin of needle-fibre calcite has been the subject of detailed studies, their formation is due to either high levels of supersaturation or microbial activity especially that of fungi or cyanobacteria (Callot et al., 1985; Phillips and Self, 1987). Most other authors attribute these crystals to microbial activity, but not necessarily to that of fungi.
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Fig. 4. A: photomicrograph of pisolithic hardpan showing quartz grains floating in micritic calcite groundmass. B: photomicrograph of gravel hardpan showing coated grains. C: Microsparitic veins. D: Alveolar septal structures. E and F: SEM image showing Needle-fibre calcite crystals. G and H: Palygorskite fibers developed on preexisting minerals: (Ca) Calcite, (Pl) Palygorskite, (Sm) Smectite.
The coated grains as well as the alveolar septal structures and the needle-fibre calcite crystals characterized by biogenic features are considered as beta micro-fabrics, while alpha micro-fabrics are characterized by non-biogenic features (Wright, 1990).
4.2.5. Palygorskite Under Scanning Electron Microscopy (SEM), the palygorskite was clearly noticed within the hardpan. Individual and flexuous palygorskite fibers are 0.5 μm to 2 μm long. Fibre bundles appear generally as overgrowths on pre-existing minerals (Fig. 4.G, H).
4.3. Mineralogical composition and geographic distribution The arrangement of different forms of calcrete allows identifying four types of profiles (Fig. 2) according to their morphology, their carbonate content, their mineralogy and their clay assemblages.
4.3.1. Profile type 1 It is characterized by incipient calcrete and occurs on slopes and river banks (Fig. 2). The mineralogical study shows that in these types of profiles, the percentages of calcite, clays and quartz decrease
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upwards, counterbalanced by increase of dolomite and plagioclase (Fig. 5). The percentage of calcite doesn't exceed 20%. The clay fraction (b 2 μm) consists mainly of illite, palygorskite, chlorite and kaolinite. Illite is the major component of the clay fraction (N 50%). The palygorskite, identified in all the samples, does not exceed 20% of the clay fraction; it decreases progressively upward. 4.3.2. Profile type 2 This type of profile has a vertically gradual transition from incipient (calcareous patches) to nodular calcrete passing through pebbles (Fig. 3.A). This type of profile is developed mainly in the east-central part of studied region in areas with low slopes (Fig. 2). The thickness varies from approximately 1 m to 1.5 m. The percentage of CaCO3 increases towards the top; calcareous patch and pebbles consist of approximately 30% of CaCO3, while the nodules consist of 60%. 4.3.3. Profile type 3 The profile has a vertically gradual transition from Pebbles, nodular, concretionary, to pisolithic calcrete (hardpan) (Fig. 3.B). This type of profile is usually encountered in the central part of the studied area from Mzouda to Taftechet (Fig. 2). Mineralogical study of the profile (Fig. 6) shows that calcrete samples consist mainly of calcite, clay and quartz associated with minor amounts of plagioclase and K feldspars. The calcite amounts range between 60% in the nodular calcrete and 70% in the hardpan. Palygorskite is the dominant clay mineral within the clay fraction; its amount increases from the surface horizon of soils (40%) towards the nodular calcrete (80%). Palygorskite is associated with illite, kaolinite and chlorite in variable proportions (Fig. 8). A cross section of the large nodule shows that the mineralogical composition is characterized by calcite as the dominant mineral. It increases from the outside (65%) towards the core (75%) (Fig. 7). Quartz and clays show similar content between the interior and the exterior of the nodule (~20%). Palygorskite is the more abundant and ubiquitous clay mineral phase, which increases from 65% in the peripheral part of the nodule to 85% in the core. It is associated with illite and kaolinite (Fig. 7). 4.3.4. Profile type 4 It corresponds to the complete calcrete profile; it is characterized by vertical gradational maturity pattern which is manifested by the occurrence of well differentiated forms that range from massive calcrete to gravel hardpan (Fig. 3.D). This type of profile is usually encountered in
the western part of the studied area, between Taftechet and Essaouira (Fig. 2). The mineralogical composition of calcrete samples is mainly calcite associated with clays, quartz and accessory K-feldspar and plagioclases (Fig. 7). The calcite content increase upward (65 to 85%). Smectite and palygorskite are the dominant clay mineral phases associated with variable amounts of illite, chlorite and kaolinite. Smectite constitutes the major component of the clay fraction of the bed rock (55%) and massive calcrete (65%) (Fig. 8), it decreases progressively towards the top of the profile (30%). Palygorskite increases from 10% in the bed rock to 25% in the massive calcrete (Fig. 7). 4.4. Geochemical analysis Calcrete samples (Table 1) are characterized by high amount of Ca related to calcite identified both by XRD analysis and calcimetry. Clear differences in Ca among the calcrete samples were likely due to variable amount of calcite. Si (2.16 to 45%) is related to quartz and feldspar, the lower quantity of Al (0.60 to 12.50%) and K (0.26 to 4.22%) are ascribed to clay minerals and alkali feldspar, both identified in XRD analysis. Mg might be assigned to dolomite and palygorskite occurrence; nodular calcrete samples have relatively high concentrations of Mg (1.28 to 4.77%, mean: 2.18%), whereas massive calcrete samples have relatively low concentrations (1.12 to 1.15%, mean: 1.14%). 5. Discussion The analysis of calcrete in a rich and varied observation field as the region of Tensift Al Haouz enabled us to specify the important types of accumulations: incipient, nodular, concretionary, massive calcrete and hardpan. The shape of these forms as well as their consistency and hardness is clearly related to the contents of CaCO3. The vertical and lateral distribution indicates a complex evolution of different calcareous forms resulting from a succession of agents and processes. 5.1. Mechanisms of calcrete profiles development The occurrence of calcrete profiles within the studied area indicates two types of vertical evolution each materializing a genetic process. 5.1.1. Pedogenic calcrete In the eastern and central part of the studied area, the calcrete sequence is characterized by alluvial sedimentation followed by calcrete
Fig. 5. Mineralogy and clay composition of profile type 1.
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Fig. 6. Mineralogy and clay composition of profile type 3.
formation. The plain area of Al Haouz is located within a structurally low area, bounded by the High Atlas and Jbilet highlands respectively from the north and south (Fig. 1). These highlands are mostly formed of Precambrian and Paleozoic basement composed of igneous and metamorphic rocks. It has been suggested that the alluvial sedimentation in the Eastern and Central part of the studied area derives from the highlands by drainage systems during wet period, when humid climate prevailed (Weisrock, 1980). This phase was followed by calcrete development, under more arid conditions. The calcrete profile displays upward increasing carbonate content from an incipient calcrete to a nodular and concretionary calcrete and finally to hardpan. Field occurrence and microscopic descriptions suggest that the incipient calcrete represents the earliest stage of calcrete formation, in which calcite has precipitated within the intergranular pores of the matrix of the host rock, forming a calcareous patches. These patches grow by first displacing and then replacing the siliciclastic muddy matrix of the host sediment. The pebbles and nodules are probably formed by replacement of the varied siliciclastic framework grains by authigenic calcite. The calcrete nodules grow laterally until they coalesce and form calcrete concretions. This stage represents the beginning of hardpan formation and by a complete replacement of the parent material, displacement and partial replacement of framework grains. The pisolithic hardpan can be the result of the intense reworking of the underlying calcrete. This can be deduces from the composition of the pisoliths nuclei which are commonly
composed of hardpan fragments. Furthermore, the pisoliths and beta textures suggest a pedogenic origin of calcrete, formed by precipitation of calcium carbonate in the vadose zone (Alonso-Zarza, 2003; Candy et al., 2003; Khalaf and Gaber, 2008). Their occurrence within the calcrete of the eastern and central part of the studied area suggests that they are best interpreted as pedogenic calcrete. 5.1.2. Combination of groundwater and pedogenic calcrete In the western part of the region (between Taftechet and Essaouira) the vertical evolution and thus, the formation process of calcrete seems to be different. Unlike pedogenic calcretes profiles of the eastern and central part, profiles sampled in this zone are thicker and mostly massive lacking any horizonation and differentiation (Fig. 3.D). In this respect, the profiles are similar to many groundwater calcrete profiles described in many regions around the word (e.g. in Australia (Mann and Horwitz, 1979; Arakel and McConchie, 1982) and Spain (Nash and Smith, 1998). These studies suggest that calcretes formed in a phreatic zone are mostly massive in character. Wright and Tucker (1991) also suggest that, in general, thicker horizons are likely to be present in groundwater calcretes when compared to pedogenic varieties. However, coated grains structures (Fig. 3.F), which appear similar to those of pedogenic calcretes (Fig. 3.E), are frequently present in the uppermost section of these profiles. Otherwise, the occurrence of beta fabrics structures (Needles fibre) in these upper levels suggests that they are best interpreted as pedogenic calcrete formed in the vadose zone.
Fig. 7. Mineralogy and clay composition of profile type 4.
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Fig. 8. X-ray diffraction patterns of oriented clay fractions of samples collected from nodular and massive calcrete.
Consequently, on the basis of these morphological and micromorphological observations, it would appears most likely that the calcrete profile in the western part of Tensift Al Haouz have formed by a combination of groundwater and pedogenic calcretes. 5.2. Environmental control on the development of calcretes The occurrence of calcrete profiles in the studied area shows a lateral evolution from south to north and from east to west (Fig. 2). This suggests that these forms of calcrete do not appear randomly but seems to be related to well define environmental conditions. In the literature, a range of factors has been suggested to affect calcrete development (Goudie, 1973; Wright and Tucker, 1991; Alonso-Zarza, 2003; Candy et al., 2006; Wright, 2007; Reith et al., 2009). In the case of Tensift Al Haouz region, four main factors seem to have controlled the variation and evolution of calcrete: topography, bedrock, parent materiel and climate. 5.2.1. Topography The topography of the studied region has not experienced any significant change since the late Pliocene; the major uplift of the western High Atlas chain has occurred since the Oligocene (Giese and Jacobshagen, 1992; Frizon de Lamotte et al., 2000; El Harfi et al., 2006). Thus, in order to visualize the topography in which the calcrete profile lie, actual slope map was produced with ArcGis spatial analyst function using 30 m resolution DEM (digital elevation model). The projection of
Table 1 Chemical composition of calcrete samples from Tensift Al Haouz region.
Massive calcrete
Nodular calcrete
Samples
Ca
Si
Al
K
Mg
Mn
Fe
SK_2 SK_3 SK_4 SK_5 AO_2 AO_3 VON_Z Car 2 Car 3 Car 4 SIM2 SIM3 MZ_2 MZ_3 OMNII D OMNII E TMSLT D PMTG D AV CHI D AVCHI E TIGMI D
37.26 38.17 34.06 40.23 31.07 18.87 42.52 39.29 38.49 38.45 66.18 60.60 31.99 32.42 82.53 70.30 68.39 89.14 81.58 26.86 71.79
5.49 4.71 7.31 2.72 7.90 16.21 2.16 3.90 4.32 4.74 19.55 23.43 8.66 8.75 10.68 18.12 15.81 4.23 9.91 45.01 15.57
0.64 0.70 1.29 0.57 1.22 2.49 0.64 0.85 1.04 0.91 6.08 6.97 1.99 1.79 2.77 5.10 6.37 2.21 2.32 12.53 5.02
0.27 0.26 0.47 0.31 0.39 0.77 0.39 0.32 0.33 0.41 2.85 3.17 0.61 0.52 0.90 1.58 2.53 0.40 0.52 4.22 1.84
– – – – – – – – – – 1.15 1.12 4.77 – 1.28 1.57 1.71 2.25 1.72 2.65 1.45
0.03 0.01 0.02 – – 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 – 0.01 0.02 0.01 – 0.06 0.02
0.43 0.62 1.10 0.41 0.79 1.39 0.42 0.45 0.59 0.47 2.81 3.16 0.87 0.91 1.73 3.08 4.58 1.69 1.63 7.03 4.12
calcrete profiles on the slope map (Fig. 2) reveals that the calcretes developed on slopes (piedmont of High Atlas) lack the hard forms and occur as powdery patches and friable nodules, whereas down slope, the calcretes are thicker and show well developed features (concretionary calcrete and hardpan). In high slope areas, the rate of erosion is higher than the rate of soil formation (and CaCO3 accumulation) which explain the presence of more developed soils on the flat areas. In addition, in the low topography, the solutions infiltration can reach deeper horizons which enable the formation of thick and mature calcrete profiles. 5.2.2. Bedrock and source of carbonate In the central and eastern part of the studied area, the bedrock is mainly composed of alluvial deposits (conglomerates, sandstones, siltites and clays) from the High Atlas. The formation of mature nodular calcrete profiles in the absence of calcareous bedrock is related to the fact that the origin of calcium seems predominantly allochthonous. In this context, calcrete may have been formed through detrital input or by lateral transfer of Ca at landscape scale from High Atlas Ca rich source rocks from the south. However, as suggested by several authors (Goudie, 1973; Reeves, 1976; Monger and Gallegos, 2000 and Eren et al., 2004), CaCO3 dust and the water-soluble Ca in that dust may be considered as another source of calcium carbonate. The types of clays and mineral assemblages found in the studied profiles, with a significant presence of quartz, feldspars, illite and kaolinite show that detrital supply by either wind or water transport seems to be a fundamental mechanism during the accumulation of these materials. However, in the western part, massive calcrete formation is due to interstitial cementation, displacement and replacement of sediment bodies by carbonates within shallow aquifer systems. Calcrete in this case have developed in situations where CaCO3 was sufficiently available due to the presence of cretaceous calcareous aquifer. 5.2.3. Parent material The morphology of calcretes seems to depend on the texture of parent material. Calcrete developed over gravelly sediments in the central and eastern part of the study area display a different morphological sequence; the authigenic calcite replaced the siliciclastic framework grains and therefore allows the formation of pebbles, nodules and concretionary calcrete. However, in the west, on sandy calcareous humusbearing soil, cementation, displacement and replacement of sediment bodies by carbonates are interstitial allowing the development of mostly massive calcrete lacking any horizonation and differentiation. 5.2.4. Climate Precipitation regime is one of the main factors that control calcrete formation (Candy and Black, 2009). Data on average rainfall may be obtained from calcretes by studying the mineralogy of the clays they contain (Alonso-Zarza, 2003). Khadkikar et al. (2000) show that calcrete,
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when associated with palygorskite, suggest an arid climate (mean annual rainfall = 50–100 mm) and when associated with smectite it indicates a semi-arid climate (mean annual rainfall = 100–500 mm). In the Tensift al Haouz region, mineralogical analyses show that most of the analyzed samples are characterized by the presence of variable amount of palygorskite. The content of this fibrous clay varies between 20% of the clay fraction of the incipient calcrete, 40% in the pebbles to 85% in the nodules. However, in the massive calcrete profiles (western part of the studied area); palygorskite doesn't exceed 25%, whereas smectite represents the abundant clay mineral phase (up to 65% of the clay fraction) (Figs. 7 and 8). Accordingly, the abundance of palygorskite in the nodular calcrete in central part of the studied area suggests their formation under arid climatic conditions. On the other hand, the occurrence of smectite in the massive calcrete in western part may indicate that they formed under semi-arid climate. Furthermore, geochemical study can clarify the position of carbonate accumulation with respect to the water table. Khadkikar et al. (2000) and Khalifa (2005) show that during dry periods that are related to high evaporation rates, Mg concentration in meteoric waters increases, however, when the water table rises and under low evaporation rate conditions, fresh water is capable of leaching Mg ions from calcite crystals. Mg values in samples collected from massive calcrete profiles of the studied area are lower than those of nodular group samples; this confirms that massive calcrete reflects the rising groundwater phase. 5.3. Correlation with regional paleoclimate data Calcrete features described in the Tensift Al Haouz region reflect the sequence of events enabling the reconstitution of paleoclimate history. In the eastern and central parts of the studied area, the deposition of Soltanian silts and alluvium ended with the formation of soil around 17,300 ± 240 and 22,400 ± 400 yr B.P. (Rognon et al., 1984). These silts and alluvium were covered by incipient and calcrete nodules (Weisrock and Rognon, 1977). This could explain the formation of incipient and nodular calcrete in the wetter climate during upper Pleistocene. Weisrock and Rognon (1977) describe a gradual trend to a more arid climate. This would have caused the formation of large and flattened nodules at the top. With sustained aridity, these large nodules became coalescent. This stage represented the beginning of hardpan formation. The thick hardpan requires a significant time to form and reflects the absence of any erosion events. This may indicate the formation during the Lower Holocene under a drier climate than that of the present. The abundance of rotated and re-incorporated petrocalcic fragments in the hardpan (Fig. 3E) suggests probably an increase in aridity. Brock and Buck (2009) explained that the climate was not wet enough for the dissolution of the fragments that were exposed at the surface. As regards to calcretes developed in the western part, after a major pluvial period of the Soltanian that lasted throughout the Upper Pleistocene, the rainfall regime seems to have been more irregular and climatic fluctuations more frequent Rognon (1987). The warm climate prevailed and the formation of massive groundwater calcrete may be promoted by intense evaporation from a water table close to the surface supplied by cretaceous and plio-quaternary aquifers. The increasing aridity in the lower Holocene would have caused the formation of hardpan. The gravel hardpan could be formed in the same conditions as pisolithic hardpan by erosion, deposition and recementation of fragment under arid climate during the Lower Holocene. 6. Conclusion This study is a result of an extensive survey of soil profiles with calcium carbonate accumulations. Field and microscopic observations as well as mineralogical and geochemical studies, were used to reveal the calcrete formation mechanism. The calcrete features offer valuable
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data to determine the main environmental factors that operate in the calcrete formation and distribution and to interpret the geomorphic and climatic major events. Pedogenic calcrete profiles (eastern and central part of the studied area) were developed as a result of two main sequential processes, namely; distal alluvial deposition and calcrete formation. This sequence started by the deposition of alluvial sediments during a humid period in the topographically low area, was followed by pedogenic calcrete formation. It is further suggested that incipient calcretes followed upward by nodular calcretes and hardpan may indicate an episode of low sedimentation rate followed by a period of stagnation in a semi-arid climate. The early stage of calcrete development (incipient and nodular calcretes) was probably developed in the vadose zone within floodplain during periods of low terrigenous supply under semi-arid climatic conditions. Cementation of the relatively thick hardpan may correspond to the relatively more arid climate. Whereas, combined groundwater and pedogenic profiles (western part of the studied area) shows the occurrence of groundwater calcrete at the base, it reflects wet to semi-arid climatic conditions, rising of groundwater, and the pedogenic calcrete at the top of the sequence which reflects arid climate. The differentiation and evolution of calcrete throughout the study area is under the influence of three major factors: 1) the climate which is responsible for the evaporation and crystallization of carbonate; 2) the availability of carbonate rich solution which is controlled by the topography, the nature of underlying bedrock and the detrital processes; 3) the texture of the parent materiel which controlled also the morphologies of calcrete. Acknowledgements The financial support is provided by the PPR-CNRST program (grant: PPR1/2015/63) and by the “Projet de Coopération Bilatérale Wallonie Bruxelles-Maroc” (grant 2.7) that are all gratefully acknowledged. The first author acknowledges the financial support provided by the CNRST as part of scholarship program (009UCA2014). References Alonso-Zarza, A.M., 2003. Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record. Earth Sci. Rev. 60, 261–298. Alonso-Zarza, A.M., Calvo, J.P., Garcia del Cura, M.A., 1992. Palustrine sedimentation and associated features —grainification and pseudo-microkarst—in the middle Miocene (Intermediate Unit) of the Madrid Basin, Spain. Sediment. Geol. 76, 43–61. Alonso-Zarza, A.M., Wright, V.P., Alonso-Zarza, A.M., Tanner, L.H., 2010. Calcretes. Carbonates in Continental Settings: Facies, Environment, and Processes: Developments in Sedimentology. 61, pp. 225–267. Arakel, A.V., McConchie, D., 1982. Classification and genesis of calcrete and gypsum lithofacies in palaeodrainage systems of inland Australia and their relationship to carnotite mineralization. J. Sediment. Petrol. 52, 1149–1170. Ballais, J.L., Ben Ouezdou, H., 1991. Forms and deposits of the continental quaternary of the Saharan margin of eastern Maghreb (tentative synthesis). J. Afr. Earth Sci. 12 (1/2), 209–216. Brock, A.L., Buck, B.J., 2009. Polygenetic development of the Mormon Mesa, NV petrocalcic horizons: geomorphic and paleoenvironmental interpretations. Catena 77, 65–75. Callot, G., Guyon, A., Mousain, D., 1985. Interrelations entre aiguilles de calcite et hyphes mycéliens. Agronomie 5, 209–216. Candy, I., Black, S., 2009. The timing of quaternary calcrete development in semi-arid southeast Spain: investigating the role of climate on calcrete genesis. Sediment. Geol. 218, 6–15. Candy, I., Black, S., Sellwood, B.W., 2004. Quantifying time scales of pedogenic calcrete formation using U-series disequilibria. Sediment. Geol. 170, 177–187. Candy, I., Black, S., Sellwood, B.W., Rowan, J.S., 2003. Calcrete profile development in Quaternary alluvial sequences, southeast Spain: implications for using calcretes as a basis for landform chronologies. Earth Surf. Process. Landf. 28, 169–185. Candy, I., Rose, J., Lee, J.R., 2006. A seasonally ‘dry’ interglacial climate in Britain during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK. Boreas 35, 255–265. Chehbouni, A., Escadafal, R., Duchemin, B., Boulet, G., et al., 2012. An integrated modelling and remote sensing approach for hydrological study in arid and semi-arid regions: the SUDMED programme. Int. J. Remote Sens. 29 (17–18), 5161–5181 Taylor & Francis: STM, Behavioural Science and Public Health Titles, 2008. Ducloux, J., Laouina, A., 1986. L'encroûtement calcaire des formations caillouteuses; faciès et types d'évolution. Observations sur formations alluviales et colluviales dans le Maroc nord-oriental. Rev. Géogr. Maroc 1-2, 97–114.
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