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SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

BADLANDS AND TALUS FLATIRONS IN THE BARDENAS REALES REGION

G. Desir; C. Marín and J. Guerrero

FIELD TRIP GUIDE - B3

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

BADLANDS AND TALUS FLATIRONS IN THE BARDENAS REALES REGION Gloria Desir, Cinta Marín and Jesùs Guerrero Dpto. Ciencias de la Tierra; Universidad de Zaragoza; C/ Pedro Cerbuna, 12; 50.009 Zaragoza, Spain. E-mail: [email protected] Phone: +34 976 762781; Fax: +34 976 761106 1. Introduction The Bardenas Reales are located in the south-eastern margin of Navarra Province, in the middle-western sector of the Ebro depression (Fig. 1). They conform a big erosional depression with steep slopes at the margins and deeply dissected valleys, where the landscape is a consequence of the climatology and geological framework. The geology is made up of Tertiary and Quaternary sediments. The word “Bardenas” means low land where the sheeps graze. They are called “Reales” because they were Royal property in the past (Leranoz, 1993). In the Bardenas Reales it doesn't exist any human settling, and human presence is restricted to the use of the natural resources. Historically cattle have been the main use, and the recorded highest sheep concentration is 300.000 sheeps. At the present time, its number has been reduced to less than 90.000 and it is the agriculture the main activity of the zone. The cattle use, become fulfilled mainly with ovine, besides cows and goats. The agriculture did not acquire importance until the second half of the 19th century. Nowadays the total cultivated surface is 21.986 has. that represents the 52% of the surface. The main crop of the zone is the cereal, although in the last years the cultivation of rice has increased. This crop helps to combat the present salinity in the soils of Bardenas.

SPAIN

PORT

UGA

L

FRANCE

FRANCE

ARAGON

EBRO RIVER

ZARAGOZA

RIVER

Figure 1. General situation map of the Bardenas Reales area. The Bardenas Reales are included in the semi-arid Mediterranean environment, with scarce and torrential showers and high temperatures which cause the vegetation cover is scarce enough, what enhances erosion processes. Secular overfarming and overgrazing together with stormy rains, strong winds and soft clayed soils have given place to high erosion rates. Also it is necessary to add a substrate highly erodible, topography with steep slopes and the incorrect use of the territory many times, which has caused the existence of zones specially vulnerable to erosion.

Badlands and Talus Flatirons in the Bardenas Reales region

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Due to their singular landscapes the Bardenas Reales became Natural Park by Law 10/1999 of the Navarra Goverment. In November 9th of 2000 the United Nations Educational, Scientific and Cultural Organization (UNESCO) declare the Bardenas Reales World Biosphere Reserve. The Bardenas Reales are an area of agricultural and cattle use, whose economic exploitation is shared by 22 municipalities of Navarra and Aragón. The centre of the erosive depression is at present a Militar Exclusion Zone of 2,244 ha. belonging to the Defense Ministry (Fig. 1), being FORBIDDEN the access. This area does not belong to the Biosphere Reserve, but its restricted use has permitted a natural development of the vegetation and of the steppe fauna. The so called Bardena Reales are composed of three parts or regions well differentiated: - The Bardena Blanca or Erosive Depression

It is the northern zone. It is with an erosive depression excavated in the clay-marl-silty materials of the Ujué facies of Miocene age. The most typical features of this area include an almost complete lack of relief and the presence of many shallow and narrow gullies, whose slopes are affected by piping and incision processes. Nevertheless, a few and small hills still remain in the depression. These hills correspond, on one hand, to small and low sandstone mesas related with the Paleochannel Facies, and on the other hand, with several remains of cover pediments, some of them poorly preserved, where detritic cover protects the tertiary materials from erosion. These hills show steep slopes without vegetation so that slope retreat and evolution is very active. - The Bardena Negra or Southern Relief.

It is located in the most southern sector and it displays more prominent relief than the previous zone. It is constituted by the calcareous materials of the Tudela Formation. The lower materials, more marly, are deeply incised, giving place to sharp badlands. The stratigraphical sequence is slightly tilted towards the south and thus gently dipping cuestas, with low continuity have formed. These hills display a NNE-SSO jointing direction. - The Transition Zone

It is located between the erosive depression and the calcareous relief. In this area, strongly dissected, subhorizontal cuestas, formed over the Monteagudo and Tudela Formation materials. Hillslopes have been strongly incised and show knife-edged badlands. Geologically, the Bardenas Reales are constituted by Tertiary and Quaternary materials. The Tertiary materials, Miocene in age, correspond to 3 different types of lithologies ( Fig. 2 ):

- Lerín Gypsums: (Castiella et al., 1978) They are Aquitanian in age (Early Miocene) and have thickness ranging from 80 to 250 m. They are interpreted as deposits of open playa-lake. These gypsums crop out in the vicinity of Arguedas and Valtierr villages, 5 km. away from the erosive depression. As long as it is a lithology of low consistency, by end of the 19th Century almost 150 troglodytic housings were excavated in this substratum. At the present time, some of these caves are used as tourist lodgings. Gypsums of the Lerín Formation change laterally to the clays of the Tudela Formation - Tudela Formation: (Castiella et al., 1978) It is composed by a 320 m thick sequence of clays of different tonalities (red, gray, beige ), interbedded with lacustrine limestones, sandstones and gypsums (Leránoz, 1993; Murelaga, 2000). They are Burdigalian-Vindobonian in age (Lower -Middle Miocene) (Solé, 1977) and are interpreted as distal facies of alluvial fans of Pyrenean origin. The Tudela Formation is located in the margins of the erosive depression.

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The absence of vegetation and the steep slopes have favoured the formation of a spectacular Badland landscape. - Limestones of Sancho Abarca: Age Vindobonian-Pontian (Middle -Upper Miocene) (Gracia, 1985). It is made up of a thick package of lacustrine limestones, slightly marly, that crown the Gypsum of Lerín and Tudela formations. They are localized in the southern part of the depression and give rise to cuestas or stepped landscape depending on the slope angle. In the upper zone of “La Plana Negra”, a limestone structural platform, it is possible to find a calcrete or calcareous crust (Gracia, 1985).

Figure 2. Geological map of the Bardenas Reales Area. Source: Confederación Hidrográfica del Ebro. The quaternary deposits can be found principally in the erosive depression. They are Holocene clays and silts that come from the washing of the surrounding tertiary clays, of the Tudela Formation. Since this material is poorly lithified it can be easily incised. Over these deposits, deep gullies have developed (Fig. 3) that mobilize large quantities of materials in every important rainfall event. For example, the “Barranco Grande”, which is 42 km long, is able to mobilize 12 hm3 of sediments per year to the Ebro river (http://www.bardenasreales.es). The erosive processes act with great celerity on this fill. Quaternary deposits that filled in the endorheic basin during the Early Holocene are undergoing strong erosion and deep gullies have developed. The landscape of the recent quaternary accumulations is characterized by a dense network of flat bottomed valleys that build up a badland landscape, with rounded watersheds, affected by piping processes (Hernández -Pacheco, 1949; Gracia, 1985; Elosegui and Ursúa, 1990; Del Valle and Del Val, 1990; Del Valle et al., 1991; Leránoz, 1993).


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Geomorphologically, the alternation of materials with different resistance to erosion (clay with limestone, sandstone or gypsum) where soft materials clearly dominates (clay and silt), and their horizontal disposition, have allowed erosion acting intensively, giving rise to a great erosive depression, “La Blanca”, surrounded by tabular reliefs as “El Plan” in the North, and a number of stepped plateaus in the south, where outstands “La Plana Negra”. The erosive depression or Bardena Blanca is surrounded by structural reliefs of different origin. In some cases they are made up of tertiary formations with horizontal stratums, like in “La Plana Negra” or “Alfarillo”. In other cases the structural reliefs are crown by quaternary sediments like terraces or pediments (El Plano, Ralla-Rallón, Pisquerra, Cortinar, Castildetierra, etc.) (Fig.3). In these depression, erosion is very accentuated due to the scarcity of hard stratums that protects the clays from erosion. Soils of the Bardenas Reales are clearly tied with landscape and lithology. So in the upper part of structural platforms soils are calcisols with a great development of the petrocalcid horizon. In the slopes of the platforms and in the flat areas, over tertiary clays, regosol and xerosol soils, develop. On the Holocene fill, soils are of fluvisol type. Leránoz (1993) indicates that soils in the Bardenas Reales belong to the D Class whose main features are: low use capacity, limited crops, high erosion risk, salinity and sodicity from high to moderate and water deficit along the main part of the year. On the steep slopes with intense erosion processes soils belong to E Class (high erosion risk, very steep slopes, reduced thickness and high water deficit). Climatically, it is a semi-arid zone with a mean annual precipitation of 350 mm distributed unequally along the year. Rainfall events are of stormy character with two annual maximum, one to the end of spring and another at the beginning of autumn. The values of the mean monthly temperature indicate a clear annual thermal contrast, with temperatures of 10 °C in January and 24 °C in July, being the mean annual temperature 13 °C. TO REMIND: IT IS FORBIDDEN TO GO ACROSS THE MILITAR ZONE.


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Stop 1. Road from Arguedas to the military zone. “Alto de Aguilas”. From “Alto the Aguilas" view point we can have a general perspective of the erosive depression, its landscape, its materials and cuestas, outlets, terraces, pediments, fill up deposits, etc. Also we can see the northern reliefs of the Bardena Negra, like “El Rincón del Bú” ot “El Balcón de Pilatos”. As already mentioned, Bardenas Reales have been traditionally divided in two zones: Bardena Blanca and Bardena Negra. The “Bardena Blanca”, occupies the center of the erosive depression, and is constituted by silty-clay materials with scarce vegetation (Fig. 4). The existence of saline efflorescences contributes to give its typical whitish coloration. The Bardena Negra, in the south of the erosive depression, is formed by the limestone reliefs (Fig.4). This zone of Bardenas is settled of extensively pine and oak trees, which gave origin to the Bardena Negra name. .

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Figure 4. Geomorphological map of the Erosive Depresión or Bardena Blanca. The Bardenas Reales are surrounded in their south and west margins by two rivers, Ebro and Aragon respectively. The Aragon river, of Pyrenean origin, is one of the main Ebro River tributaries. The geographical situation of the Bardenas, between both rivers, has conditioned the presence of deposits of terrace and pediments belonging to both rivers in the margins of the depression. The hermitage of “La Virgen del Yugo”, to the Southwest of the depression, is situated on a terrace level correlative to the T10 level of the Aragon River (Fig.3). It has been assumed that it belongs to this river due to the high percentage of limestones with foraminifer boulders, higher than in the terraces of the Ebro River (Leránoz, 1993). Close to this terrace surface, it is located the T9 terrace surface of the Ebro River. The deposits of both terrace levels display anomalously high thickness, increased due to the subsidence phenomenon, caused by the dissolution of the underlying Tertiary evaporites (Fm. Gypsums of Lerín). It is also possible to recognize some U-shaped dolines (pan-shaped dolines) on their surfaces. The most typical features of the erosive depression is the lack of relief and the presence of many shallow and narrow gullies, whose sidewalls are affected by piping and incision processes (Leranoz, 1993). Nevertheless, a few and small hills can be identified over the depression. In some cases they correspond to sandstone mesas or more frequently, to remains of cover pediments with a thin detritic cover. These cover pediments converge to the centre of the depression. In the North of the depression, on the Tudela Formation wide plioquaternary pediments are developed (Fig.4). One of them, the G9 pediment level, starts from an inferred T10 terrace of the Aragon River, which has been subsequently eroded (Bomer, 1978). The only remain of this terrace surface would be the terrace level where the “Virgen del Yugo” hermitage is placed. The best preserved pediments in the erosive depression (Rallón and Piskerra) are younger than the G9 level. Their slopes dip in opposite senses, what indicates the existence of a palaeorelief among them (Fig.5). This palaeorelief seems to be an ancient pediment level (Leránoz, 1993) whose origin is the same of the T10 level of the Aragon River, because it also presents boulders of Pyrenean origin (Gracia, 1985 ).

Figure 5. General view of the Rallón and Piskerra pediments. Both show an opposite slope angle. Hypothetically, in the central part was placed the level from which they started. Stop 2. Analysis of the Holocene fill up materials. Within the Holocene filling materials, 3 levels can be differentiated: the upper laminated unit (C1), the intermediate massive unit (C2) and the lower laminated unit (C3) (Marín and Desir, 2004) (Fig.6):


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- Upper laminated level: clays with silty-loamy texture. There is an alternance of laminated levels with other thick massive ones. It shows scarce piping processes, popcorn morphologies and rills.

- Intermediate massive level: clays, severely affected by piping processes and popcorn morphologies.

- Lower laminated level: alternation of laminar and massive levels. This level is covered by biocrust. There is also a charcoal level at the upper part of this level, crowning the sequence. It presents little development of piping processes and popcorn morphologies.

The resultant morphologies developed over each level varies widely depending on their structure

and their physico-chemical properties. These variations can be observed not only on the rilling processes but also on crust development, interrill morphologies development and piping processes.

Figure 6. General view of the three different levels identified in the Holocene filling deposits. Results of the Physico-chemical and mineralogical analysis of the three units appear in tables 1

and 2.

Table 1 : Results of the Physico-chemical and mineralogical analysis.

pH ( 1:2.5 )

Ec (MS)

O.M. ( % )

Clay Minerals

Calcite( % )

Quartz( % )

Other ( % )

Dispersion Index

Real density

C1 8.3 6.7 0.4 32 40 21 7 0.67 1.98 C2 8.3 7.2 0.4 44 35 16 5 0.65 1.83 C3 9.1 3.5 0.3 18 45 32 5 0.93 2.22

The C1 unit is composed of clays of silty-loamy texture, with very fine laminar structure. The

present forms are rills of scarce development and low penetration, and interills with tooth-shape forms with rounded or flat pinnacles and wide scouring surfaces covered by crusts (Fig.7).

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Figure 7: C1. Lower laminated level C2 unit is loamy with a masive structure. Its main feature is the presence of a high density of

pipes and popcorn morphologies. Cracking polygons are of centimetric to decametric scale with irregular margins and convex profiles (Fig.8). Moreover, it shows a high density of rills with tributaries that can reach order 4. The rills have steep slopes with smooth and rounded interrills. Some of the minor order rill disappear in the pipe conduct (inlet) to reappear in the lower part of the slope (outlet). These pipes are centimetric in size and they are always related with the rill network.

Figure 8: General aspect of the intermediate massive level, C2, overlaying it the upper laminated level, C3.

C3 level are clays with a silty-loamy texture and laminar structure (Fig.9), where fine laminated

levels alternate with more thick massive levels. These levels can host popcorn morphologies with cracking cells of minor scale than those of C2 level. Rills on this level are less deep that in the previous one. Interrill areas are wide and flat.


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Figure 9: Typical morphologies developed over the upper laminated level, C3, overlaying the intermediate massive level, C2. From the textural viewpoint clear differences between the three levels can be observed. C2 Level,

is composed mainly by clays, constituted up to 44% of clay minerals (basically illite). The other levels show lower clay contents and their texture may be silty-clay. On the other hand, clays from C2 level show an important cracking and are more highly erodible than the laminated levels. Table 2: Chemical analysis from the samples.

SO42-

(meq/l) HCO3- (meq/l)

Cl-(meq/l)

Ca2+ (meq/l)

Mg2+ (meq/l)

Na+ (meq/l)

K+ (meq/l)

SAR ESP

C1 12.7 32.5 739.6 117.5 365.7 509.4 0.3 32.7 31.4 C2 13.7 55.8 643.1 92.5 325.0 477.4 0.6 33.0 31.6 C3 16.6 17.3 442.1 16.6 125.8 392.2 0.3 46.4 39.6

In the three levels the chemical analyses show that all are sediments with moderate dispersion

indexes and high SAR and ESP values (Table.2). However, only C2 level, is prone to piping processes (Fig.10).

Figure 10. Piping processes developed over the C2 or masive intermediate level.


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As already mentioned, one of the main conditioning factors of the resultant morphologies is the mineralogical composition. An important factor favouring the genesis of piping is the presence of swelling clays. If no swelling clays are present, as it happens in the Bardenas Reales, the clay dispersion plays also an important role, since dispersion is the cause of the cracking that generates the loss of cohesion of the clays (Arulanandan and Heinzen, 1977). This loss of cohesion favours the appearance of piping. C3 level shows higher dispersion index, SAR and ESP values than C2 level, however no piping processes have developed on it. This could be due to its smaller sodium content, and its higher silt content.

Finally, crust are one of the present morphologies in this area with greater superficial distribution. These crust types appear fundamentally tied to the C1 and C3 levels (Fig.11). It can be differentiated three crust types developed on the Holocene substratum and its genesis is sue to the biological activity of lichens, funguses and algae. The first one, has rough texture, it is being shading coloured and located in zones with southern exposure. In some cases it can be accompanied of lichens (Fig.11A). A second type, with rounded morphologies and internal lamination, whose origin is due to the activity of lichens and algae (Fig. 11B). Finally, the third type exist on the zones where sheetwash (Fig.11C). It has a very fine internal lamination, with structure similar to that of stromatolith. As already said, crusts are resistant to erosion, decrease soil infiltration capacity and increase runoff, so that in the crust margins retreat could form pedestals. (Fig.11).

Figure 11: Different crust types developed over the Holocene infill deposits.

A B

C D


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Stop 3. Experimental Site Bardenas-1. The experimental plot of Bardenas-1 (BD1) is placed on the Holocene filling deposits, C1, C2 and C3 levels. It is 300 m above sea level, has a surface of 415 m2, and it is on a slope oriented to the NE. The slope profile is convex with a mean slope of 9°. It is characterized by the presence of deep rills with arched interrill areas and piping processes. The plot occupies part of a small natural amphitheatre dissected by slightly meandering rills (Fig.13), that drain into a flat bottomed valley placed few meters southwards of the plot. The network is quite ramified reaching some of the rills a Horton order of 4. Many rills of lower order disappear in subsuperficial pipes (inlet) and reappear at the base of the slope (outlet). Pipe scale is centimetric and they are always related to the rill network.

Figure 12: General view of the experimental plot Bardenas-1. BD1. The plot is surrounded by a metallic fence in order to avoid disturbances. 242 erosion pins have been installed following a square metre grid. Erosion pins have also been placed on fixed points to obtain microtopographic profiles. Alls these profiles, 26 in total, are perpendicular to the rills. On the other hand, outside of the plot, 25 sections have been installed, but in this case parallel to the maximum slope direction, in order to evaluate slope retreat. The erosion studies have been carried out over a 54.4 m2 and 9° slope microbasin. This microbasin has been fitted with a collector device with two Geib type divisors. In order to determine the climatic conditions controlling the erosion processes a weather station was installed on the top of the plot. It is fitted with a pluviograph and a data logger where data were collected every two minutes. The mineralogical composition indicates that clays are the main soil constituent (18-44 % of the fresh rock and 16-39% of regolith). There is a loss of clay constituents in the transformation from fresh rock to regolith. Quartz fluctuates between 16 and 32% and calcite between 35 and 48%. These minerals go together with dolomite, potassic feldspar and gypsum in very low or nonexistent

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quantities. Gypsum in the regolith samples have been almost lixiviated. Difratograms of the clay fraction carried out on orientated aggregates show a clear predominance of the illite (81-85 %), chlorite (11-17 %) and traces of kaolinite.

Figure 13. Sketch of the Experimental Plot BD1. The runoff and sediment production are measured and stored in a collector device. This consits on a first tank, 400 l. in volume connected to a second one with two Geib Type divisors (Albaladejo, 1990; Sirvent et al., 1997; Desir, 2001, 2002). Only a quarter part of the collected runoff rises the second tank wit a capacity of 380 l. The plot ground lowering was obtained by erosion pin measurements and microtopographic profile gauge techniques (Sancho et al., 1991; Benito et al., 1992; Sirvent et al., 1997). The erosion pins are made of steel rods covered by a coat of zinc to avoid oxidation. Pins must be wide and long enough to guarantee a good anchorage without causing any disturbance in soil surface. Two kinds of pins have been installed, one of them is 30 cm long and 4 mm wide, and has been used to take measurements of ground lowering. The other ones are 40 cm long and 6 mm wide. They have been placed at fixed points over which stand the microtopographic profile gauge and let to know the evolution of a slope profile along the time when the measurement is made in the same point over different time periods. Erosion pins must be perpendicular to soil surface. The recordings were made every six months setting a metal washer on the ground, in order to avoid or minimize soil irregularities, and taking measurements with a depth gauge. The erosion pin record was analysed by computer, generating ground lowering contour lines. The microtopographic profile gauge used (Sancho et al., 1991) was based on Curtis and Cole (1972), Mosley (1975) and Benito et al. (1992) profilometers. The device has a 110 cm wide and 90 cm high


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aluminum panel which has black horizontal lines 2 cm apart painted on its front face (Fig.14). Along its lower horizontal edge, a 114 cm long hollow aluminium bar, with holes drilled every 2 cm, is fixed. Through these holes, 51 rods, 4 mm in diameter, can move freely up and down in response to microtopographic variations. The movement of these rods is helped by another drilled horizontal bar located within the frame. In order to obtain absolute values of ground lowering, the profilometer was maintained in a horizontal position by two adjustable vertical tubes mounted on fixed erosion pins. The results were recorded every six months by means of photographs. Data analysis was carried out by computer, giving (x, y) values for each rod (Sancho et al., 1991; Benito et al., 1992).

BARDENAS REALES Profile BD18

01020304050

1 7 13 19 25 31 37 43 49

Low

erin

g (c

m)

. Figure 14. Microtopographic profile gauge and rill cross-section taken by profilometer for Bardenas 1 plot Erosion rates registered in BD1 plot by the collector device and its temporal evolution show how these rates, always depending on annual rainfall amount, are growing up from year to year (Fig. 15). The explanation can be related to rill evolution inside the plot and its position regarding the different Holocene fill up levels. So the exposed surface of the intermediate level, C2, has been growing up as consequence of rill incision and headcut retreat (Fig.16). This is the level that shows higher dispersion indexes and also higher SAR values (Table.3)

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Figure 15. The accumulative graph shows the increasing tendency of the erosion rates with time.

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Figure 16. Sketch of the experimental plot BD1 where the actual contact between C2 and C3 has been overdrawn. Table 3. Measured annual erosion rates in the experimental Plot BD1.

1993 136,918

1994 73,74

1995 78,598

1996 83,905

1997 10,69

1998 13,465

1999 147,955

2000 144,425

2001 8,278

2002 73,138

2003 31,138

Mean 81,126 Erosion pin techniques and microtopographic profiles furnish very valuable data regarding to particle mobilization inside the plots and, in turn, they complement the erosion rates data obtained from the collector technique. By means of these techniques data of accretion as well as ground lowering can be obtained (Sirvent et al., 1996), and thus it is possible to know which is the erosion distribution through the monitored zone. The obtained data of lowering and superficial accretion is given in mm. These data can be turned into Tm/Ha/year knowing the regolith density. Density measurement is carried out every 6 month, at the same time that pins are measured. Periodic measurements are taken because density is a dynamic characteristic of soils and is subject to seasonal and temporal variations.


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The mean values of denudation calculated by means of the microtopographic profile gauge are 7.9 mm and 5.4 mm by means of the erosion pins. When converting it in erosion rates these are 118.5 Tm/Ha/year with microtopographic profile gauge and of 81 Tm/Ha/year with the erosion pins. The comparison between volumetric and dynamic methods, keeping in mind the values of apparent density, shows that the technique of the microtopographic profiles furnishes higher values of erosion rates to those supplied by means of erosion pins and collector devices. The microtopographic profiles were used to measure erosion in rill areas. Since erosion concentrate in these areas the so calculated erosion rates are overestimated. It is important to remark the existence of an intrinsic error of this methodology, of ± 0.5 cm. Assuming a mean bulk density of 2.65 gr/cm3 for all soils (Marshall and Holmes, 1988), the calculated error is ± 130 Tm/Ha (Marín and Desir, 2003). In the erosion pins case, lowering is measured together with accretion, so the obtained data are underestimated. Moreover, the depth gauge used has an error of ± 0.5 mm, which implies a measuring error of 13.2 Tm/Ha. The total events with production of sediments recorded with the collector devices and their temporal evolution indicates that there is only significant erosion during the pluviometric maximums, the end of the spring and the beginning of autumn (Fig.17). There is a clear report between precipitation and runoff and sediment production. In the accumulative curves two clear steps appear related to events of high magnitude that took place at end of spring and beginnings of autumn (Fig.17). The sediment yield recorded in these events was more than the double of the rest. This indicates that the erosion is controlled fundamentally for stormy rains. Between these steps there are stages of scarce runoff and sediment production. These events are localized during the summer, final of autumn, winter and the majority of the spring. Rains during these periods whether have no erosion or correspond to cyclonal rains of low amount and intensity in which erosion is small.

Figure 17. Cumulative sediment yield versus total annual rainfall for one year in the Bardenas Reales experimental plots.

Data obtained from the collector devices show a clear relationship between precipitation and runoff. Nevertheless, below 6 mm there is no significant runoff (Fig.18). Runoff is considered significant when surpasses 1 l/m2. Runoff increase considerably when precipitation reaches this 6 mm threshold. This threshold may correspond to the soil saturation capacity. The maximum runoff generated was 43.6 l/m2 for a precipitation event of 50 mm and with a maximum intensity of 5.8 mm/h. But, however there is no clear relationship between maximum intensities and runoff (Fig. 19). Whereas runoff and mean intensity show a trend between runoff and precipitation, except events of low

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magnitude and high maximum intensity.

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Figure 18. Relationship between rainfall and runoff. Note that runoff increases above 6 mm of precipitation.

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Figure 19. Graph showing the relationship between maximum intensity and runoff. This direct relationship also appears between precipitation and sediment yield (Fig.20). A threshold is observed about the 6 mm of precipitation that corresponds with the runoff threshold. With values above it, soil loss increases. In general, sediment yield increases with mean intensity (Fig.21), except for events of high intensity and of low precipitation.


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Figure 20. Rainfall and sediment yield relationship. The sediment yield increased above 6 mm of precipitation.

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Figure 21. Maximun intensity and sediment yield relationship. Stop 4. Gully erosion processes and Gully evolution. The Miocene gypsums and clays host a system of infilled valleys that at present are affected by incision processes (Fig.4). Incision also affects the tertiary clays of the Tudela Formation. Gullies generated during the present erosion stage reach more than 10 km long and 10-12 m depth. The most typical features of the erosive depression is the lack of relief and the presence of many low and narrow gullies, on which the sidewalls are affected by piping and incision processes (Leranoz, 1993). The main gullies have a U-shaped section and are only one or two meters deep. The network has a dendritic pattern and the gullies have high sinuosity (Fig.4). The erosive depression is crossed by three main gullies: Grande, Andarraguria and Limos. The two first drain the last one which joints the Ebro River in the Bardena Negra zone.

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Gullies are erosive forms that show steep walls and depths higher than 0.5 m and are only occupied by water under storm conditions. At the Bardenas Reales site, gullies are developed over Holocene silts and are wide spread. It is possible to recognise two filling up stages (Soriano, 1986; Leránoz, 1993; Marín y Desir, 2004). The first one is characterised by a thick silt sequence where alternate levels with ripples and decantation levels (Fig.22). The second one is also composed by silt deposits with cross-stratified gravel deposits at the top of the sequence (Fig.23). The gullies evolution responds to different processes like headcut retreat, scouring, undermining and piping. The extent and evolution of the gullies at the Bardenas Reales site, shows clearly that they are the main paths of sediment loss and exportation.

Figure 22.. Fill up stage composed by a thick silt sequence where alternate levels of ripples and decantation levels (see photo on the right) .

Figure 23. Fill up stage composed by silt deposits with cross-stratified gravel deposits at the top of the sequence.


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The FAO (1978) indicates that gullies evolution takes place by means of several processes, which can act together or separately. These are: bottom deepening and wall and headcut retreat due to undermining and scouring. Schnabel (1997) points out that the evaluation processes of the gullies are the collapse and headcut retreat, channel deepening, undermining and incision of the base of the scarp and walls, and finally the wash out of the walls. All these processes have been recognized in the area of study, varying the magnitude and intensity in function of the gully order. Thus, in the tributaries of minor order the headcut retreat and the channel widening are caused by topples, piping and undermining (Fig.24). In the major order gullies dominates channel deepening (Fig.25).

Figure 24. Gully erosion processes in minor order gully. Headcut retret is caused by undermining, scourring and toples.

Figure 25. Evolution of major gullies in the Bardenas is mainly due to channel deepening. There are also piping processes and topples. One conspicuous example of the erosion processes celerity over the minor order gullies of these area is given by the evolution of one gully placed approximately 100 m away from the erosion plot BD1.This gully drains the plot. Originally it was a man made ditch which acted as drain pipe for the plot. The ditch was dug in April, 1997. During the first years morphological changes, in the plot and the channel, were subtle. The process which triggered its downcutting was the wash out of the accumulated sediments that filled up the plot base and the collector device. Afterwards, wash increased and the surface lowering provoked the concrete base on which the collector device is fixed, to crop out. Up to this moment there were a great increase of erosion, mainly due to the decrease of runoff load. Subsequent increase of the runoff gave place to bottom incision. On the other hand, erosion retreats from the main gully outlet point, which was placed 1.5 m above the bottom level, deepened and widened the ditch.

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As a result of the undermining and piping, the ditch shows an enlargement and headcut retreat that evolve in an unusual speed in comparison with the environment. The sequence of images show the gully evolution (Figs.26, 27, 28). Nowadays, it is 20 to 25 m behind its original head, and the incision of the ditch channel in the final part is 1 to 2 m below the original level.

Figure 26. Dictch on spring 1999, when erosion began to incise the channel and the head of the main gully began its retreat.

Figure 27. Dictch on january 2004, it is clear the channell deepening and the piping processes and topples.

Figure 28. Dictch on March 2005, there is an intense erosion on the gully sides.


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Stop 5. Quaternary evolution of the Bardenas erosive Depression. Gully deposits in these area show two accumulation stages (Soriano, 1986; Leránoz, 1993; Marín and Desir, 2004). Gracia (1985) recognizes the existence of two accumulation stages in the Tudela Gully. Although the presence of a pediment (G3) hung on this gully, suggests that at least there are three accumulative stages in this valley. First the tertiary materials were incised. Afterwards, during the general fill of the depression the gully deposits were uncomformably sedimented. This first stage is constituted by a thick deposit of silts where there is an alternation between ripple-bearing levels and decanted sediments (Figs.22). Locally, in the base of these silts, in contact with the tertiary substratum, a 40 cm thick level of slightly cemented gravels, outcrops. In these stop it can be seen one palaeorelief of tertiary materials. It is covered with a pediment 1 or 2 cm thick which is overlayed by the C2 or intermediate massive level, showing an erosive contact (Fig.29). It is important to remark the lack of the C1 or lower laminated level in these area. It can be assumed that the tertiary remaining relief act as a topographic high which avoid the sedimentation of level C1 in this point. In this area, bellow the C2 level, a sandstone level belonging to the palaeochannel Facies also crops out.

After this first accumulation stage, there was a second one, when fine deposits alternating with cross stratified gravels were deposited (Fig.23). At the present, the dominant dynamics in the area is erosive and deep incisions that affects both the tertiary materials and the filling deposits can be recognized. Some gullies in this area are more than 5 km long, and up to 10 m deep. They form part of a well hierarchically arranged network that goes over the whole Bardenas Reales depression, cutting the prominent reliefs towards the base level: the Ebro River in the proximities of Tudela (Navarra).

Figure 29. Palaeorelief of tertiary materials covered overlayed by the C2 or intermediate massive level, showing an erosive contact

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The basin evolution can be described as follows:

1. erosion of the terraces and pediments levels (G9 and T10) 2. accumulation of the basal level of gravels and formation of cover pediments with a thin

detritic cover over the eroded tertiary materials. 3. deposition of the C1, C2 and C3 levels 4. development and hierarchization of the valleys and gullies network. 5. 2nd and 3th filling stages of the flat bottomed valleys 6. actual stage of gullies erosion when channel deepening predominates

Stop6 .The Bardenas Reales Symbol or “Castildetierra” and Talus Flatirons. In the Bardenas Reales there are some factors that make this area very sensitive to erosion. On one hand, a climate with large drought periods together with storms of high intensity, which gives place to an intense water erosion. On the other hand, the high fragility and erodibility of the substrate mainly formed by marls, gypsum, silts and clays. It must to be added a high human pressure on a highly unstable environment that has given place to an irreversible loss of the vegetation cover in wide zones where desertification problems are reaching alarming degrees. Valle de Lersundi (1991) affirms that the magnitude of the erosive phenomenon in the Bardenas Reales is due to the human activity, favoured by a great vulnerability to erosion of soils and rocks as well as to the adverse climatology of the zone. Leránoz (1993) indicates that the Bardenas landscape is a typical erosive one, nearly desert, where badlands and piping are the dominant forms on the filling deposits. On the tertiary materials there are also intense rilling processes that indicate the celerity of the erosive processes. At the same time slope retreat causes caprock undermining that generate block falls and topples and the development of outliers and talus flatirons. An example of it is the Castildetierra (Fig.30) that has been consigned as symbol of the Bardenas Reales. This represents a beautiful example of Dammé Coifeé that result of the differential erosion of the clays and the remains of glacis and sandstones that from the depression. In some particular case we can also find situations where talus flatirons are the only remains of the ancient relief that originated them (Fig.31).

Figure 30. Castildetierra that has been consigned as symbol of the Bardenas Reales is a beautiful example of Dammé Coifeé t resulting from the differential erosion.


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Talus Flatirons: When a sequence of labile sediments is covered by a caprock of resistant material, the slope profile commonly displays an upper segment with a free face and a debris slope in the lower segment. Weathering and erosion of the caprock cause its retreat and provide the detrital deposits that accumulate on the debris slope. Generally, the debris slope has a basal concavity, which may grade to a pediment.

Figure 31. Talus flatarions preserved as only remainder of the ancient relief that originated it. The incision processes acting on the debris slopes, together with the retreat of the caprock, may give place to relict slopes detached from the source area, known as talus flatirons (Koons, 1955), tripartite slopes (Everard, 1963) and triangular slope facets (Büdel, 1982). Talus flatirons are generated when there is a significant alteration in the morphogenesis of the slope involving the change from a domination of accumulation processes to a prevalence of incision processes. These landforms usually have a triangular shape in plan view and are separated form the scarp (Fig.32). The alternation of incision and accumulation periods through time gives place to talus flatirons sequences. The chronological order of the flatiron generations is given by their relative position, being the oldest facets the ones located further away from the scarp.

The important role that lithology and structure play in the genesis of talus flatirons is well known (Schumm and Chorley, 1966; Nicholas and Dixon, 1986; Schmidt, 1987, 1989a; Gutiérrez et al., 1998a, b). The generation of triangular slope facets is influenced by numerous factors like the caprock-free face thickness, the presence of massive sequences of labile materials, joint density of the caprock, cryogenic weathering or limestone dissolution. Besides, small mass movements like rock-falls, rock-topples and rock-slumps, frequently favoured by sapping at the base of the scarp, contribute to talus flatiron formation. On the other hand, rilling and gullying processes affect the soft material underlying the scarp (Schippull, 1980; Gerson and Grossman, 1987).

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Figure 32. Traditional scheme of talus flatiron genesis by

Talus flatirons genesis requires the alternation of aggradation and incision periods. These changes in the morphogenetic regime seem to be related to climatic changes (Everard, 1963; Gerson and Grossman, 1987; Sancho et al., 1988; Schmidt, 1989b, 1994; Arauzo et al, 1996; Gutiérrez and Peña, 1989, 1998; Gutiérrez et al., 1998a, b; Gutiérrez and Sesé, 2001). It is also important to take into account human activity that may have influenced or controlled the generation of flatirons in recent times (Everard, 1963; Gutiérrez and Peña, 1989, 1992, 1998). Overgrazing and overfarming may trigger downcutting processes as a consequence of vegetation cover reduction. Since vegetation cover in arid regions is scarce and vulnerable, low magnitude climatic variations may cause significant variations in the vegetation density and important changes in the morphogenesis of the slopes. A decrease in the vegetation cover due to climatic or human modifications leads to an increase in sediment yield. When rainfall diminishes vegetation cover also decreases. A rise in temperature also implies an increase in evapotranspiration leading to a decrease in soil moisture availability and vegetation cover. Slight changes in the vegetation cover may give place to variations in the morphogenetic processes acting in the slopes (e.g. accumulation/incision). Although talus flatiron formation needs a time response to reach new equilibrium conditions (Bull, 1991), a great number of talus flatirons from different stages could remain preserved in landscape, whereas other ones could be destroyed by erosion.

In these areas due to the slight thickness of the slope deposits it has no been possible to recover datable charcoal in order to establish the absolute chronology of flatirons. In other areas from the Ebro, Duero, Tajo Basin, Gutiérrez et al. (2005) have recognized five stages of flatirons related with cold periods. In the three mentioned Tertiary basins talus flatirons have been found in numerous places and up to five stages of slope evolution have been recognised in some locations. The second youngest stage (S2) has been dated in several places in the Ebro and Duero Basins by means of 14C. The age of this stage ranges from 2,529 ± 52 to 3,590 ± 40 radiocarbon yBP. The third youngest stage (S3) has yielded dates of 27,862 ± 444 radiocarbon years BP in the Ebro Basin, and 28,550 ± 130 14C yBP in the Duero Basin. S4 has been dated in the Ebro Basin as old as 35,570 ± 490 radiocarbon yBP. The S2 slope facets correspond to the Iron and Bronze Age Cold Stages. The S3 and S4 flatirons may be correlated with the Heinrich events H3 and H4. These dates may indicate that the accumulation periods on the slopes correspond to cold global events. The dates obtained for the stages S3 and S4 in the central and north-eastern sectors of Spain and their good correlation with Heinrich events suggest that flatirons could be related to climatic sequences in the Upper Pleistocene and Holocene.


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Stop 7. Erosion processes on Tertiary materials. The area is constituted by tertiary clays crowned by quaternary fluvial deposits corresponding to different pediment levels that are deeply dissected. The most ancient pediment bears a calcic crust in its upper part. Bellow these quaternary pediments there is a wide depression developed on quaternary-fill deposits with an intricate network of rills and gullies. Therefore, badlands are principally located on the slopes of the Miocene clays where they generate spectacular morphologies of knife-edged interrill areas, separated by a dense rill network and some monoliths of high altitude. The higher reliefs of the depression are constituted by pediments like Rallón (470 ms) and Pisquerra (460 ms). These pediments show opposed slope direction (Fig.5) and in spite of their relative altitude from the bottom of the depression they correspond to recent accumulative levels. The rock fragments from the pediment deposit show a Pyrenean origin, however, nobody knows the source area. A supposition is that it was a terrace from the Aragon River. However, nowadays, the river flows very far to the NW. Other hypothesis is that the source area was one more ancient pediment that at the present time is dismantled, whose source area would have been a terrace of the Aragon River. Five stratigraphic levels have been differentiated within the Tertiary sequence (Gutiérrez et al, 1995). The results of the physico-chemical and mineralogical analyses carried out are shown in tables 3 and 4 .

Table 3: the Physico-chemical and mineralogical analyses

pH

( 1:2.5 ) CE

(MS) O.M. ( % )

Clay Mineral|

Calcite ( % )

Quartz ( % )

Other ( % )

Dispersion index

Real density

R1 9.2 3.7 0.2 27 42 24 7 0.76 2.27 R2 8.9 4.1 0.2 38 39 20 3 0.35 2.26 R3 8.7 2.9 0.2 13 48 36 3 0.60 1.99 R4 8.4 5.0 0.3 31 38 18 13 0.51 2.27 R5 8.7 3.8 0.2 36 37 20 7 0.61 2.17

Tertiary sediments display massive structure, alkaline pH, high electric conductivity and an almost total absence of organic matter (Table 3). Mineralogical analyses by means of X ray diffraction show a high content in calcite, with quartz and clay minerals in smaller proportion. The clay minerals analyzed on orientated aggregates indicate a high content of illite, 82-86%, chlorite, 9-16%, and traces of kaolinite.

Table 4: Result of the chemical analysis of the saturated extract.

SO42-

(meq/l) HCO3

- (meq/l)

Cl-

(meq/l) Ca2+

(meq/l) Mg2+

(meq/l) Na+

(meq/l) K+

(meq/l) SAR ESP

R1 15.6 13.9 398.2 11.8 88.1 315.4 0.6 44.6 38.6 R2 11.7 62.3 637.7 34.9 161.3 509.4 0.9 51.4 42.1 R3 17.6 78.5 209.0 26.9 127.5 211.4 0.8 24.0 25.0 R4 17.6 89.6 466.2 37.9 165.6 392.2 0.9 38.8 35.3 R5 101.9 9.7 224.5 23.9 99.2 264.3 0.6 33.6 32.0

In absence of swelling minerals, the presence of high sodium contents can give place to swellings (Jones, 1981; Imeson et al., 1982) and piping processes (Benito et al., 1993; Gutiérrez et al., 1995).

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The main factor that controls the development of piping is the amount of dissolved sodium in the water of the pores, related with Ca2+ and Mg2+ (Shepard and Decker, 1977). All the analyzed samples show high SAR and ESP values (Table 4). Clays with ESP values above 15 (McIntyre, 1979) or SAR values above 5 (Aitcheson and Word, 1965) are very susceptible to piping. However, in this area, the stratigraphical levels are not affected by piping processes despite the values of SAR and ESP. This is due to the high slope (34o), and the slight development of the regolith thickness, (2-5 cm) that avoid infiltration to go deeper than this level of alteration. The infiltration takes place mainly through cracks, which display a good spacing and high density. The development of the cracking is uniform and regulate. On the other hand, the soils swelling capacity is evidenced by the presence of popcorn morphologies with polygons of centimetric size (Fig.33), irregular sides and non orthogonal orientation.

Figure 33. Popcorn structures developed on the tertiary clays of the Tudela Formation. One of the main characteristics of this sector is the intense rill development that individualize small drainage basin with knife-edged watersheds and little fans at their outlet (Fig.34). The rills of minor order present slopes about 60° starting close to the watershed divisor. Rills have narrow and flat bottoms. As explained before, piping does not reach a good development, but some pipes can be recognised in the middle and upper part of the slopes.

Figure 34. A typical small drainage basin with knife-edged watersheds where little fans develop on it foot.


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The presence of mud flows is quite common due to the steep slopes of these badlands, the scarce regolith thickness and the high cracking density (Fig.35). They form mainly during the months of winter, when the rains are of low intensity and runoff infiltration is higher. On the other hand, evapotraspiration is smaller in winter times so that higher soil humidity rates can be reached. This causes regolith flow as mud flows, once the plastic limit is reached and the friction angle surpassed. Most of the mudflows identified in the area are found channelled along the main rills (Fig.35) and some of them are also located in the interrill areas, especially in those which display northern exposure. These forms are no perennial; in general they disappear with the firsts spring rainfall events. These rains are of stormy character with high intensity and erosivity. Runoff is concentrated along rills increasing it erosion capacity and shear strength so rill bottom became incised and the mud flow washed up. So, in this area they are temporary forms that sometimes, if rainfall trough the year is scarce, can be preserve. An example are those of the lateral gully in the base of the Piskerra pediment. It can be appreciated the mud flow covering the rill and the formed lobe in the rill outlet. It is curious to see the different response of both materials (mud flow and substrate) to erosion and desiccation (Fig.35).

Figure 35. An old Mud flow covering the rill and the formed lobe in the rill outlet. Actually rilling processes are eroding and inciding it.

From the morphological viewpoint on the tertiary materials a very homogeneous landscape develops with continuity through all the differentiated levels (Fig.36). This tendency seems to be interrupted for the presence of milimetric levels of secondary fibrous gypsum and some small palaeochannels of sandstones, which give place to the presence of small steps that truncate the slope

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profile. On these more or less horizontal surfaces develop small pedestals of crystals of secondary gypsum and sandstone. Also it is possible to observe gnammas, tafonis, alveolos and honey combs over the palaeochannel sandstones (Fig.)

Figure 36. General aspect of badlands develop over the tertiary clays of the Tudela Formation. When studding the forms developed on Tertiary substratum, two ideas must be kept in mind: - the forms develop homogeneously over all the tertiary levels, and the only appreciable

differences are due to the presence of fine levels of secondary gypsum and of sandstone, and - the unquestionable paper of the slopes when they are high, controlling the magnitude of the

processes independently of the physico-chemical properties of the material.

The active processes on the badlands of the tertiary materials of the Tudela Formation are principally rilling processes and mud flows. In the experimental plot of Bardenas-2 situated over Miocene clays, in the occidental extreme of the Military zone, there have been measured erosion rates of 33,08Tm/Ha/year (Table5). The plot is placed on Miocene clays affected by rilling processes and minor mud flow processes. The plot is 400 m2 in surface and has a mean slope of 34º but erosion rates were only measured in a 42 m2 microcatchment. The chemical analyses show SAR values above 15 and clay minerals as illite (70%) and Chlorite (30%). Measured swelling values were 10.3%. Stop 8. Badlands, Slope evolution and Talus Flatirons.

Slope evolution in the Bardenas Reales is strongly dependent on lithology and climate. Due to the high erodibility of the Tudela and Monteagudo Formations, erosion processes act very fast and strongly on them, so the main features of this area are steep slopes with knife-edged watershed


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badlands and composite slopes in the northern limit. Alberto et al. (1984) indicate that slopes in the Ebro River valley show a complex evolution along the Quaternary. Some steps of this evolution have been recorded by the slope debris deposits. These changes are due to lithologic, climatic and human factors. Table 5. Erosion rates measured by collector devices in Bardenas-2 experimental plot.

1993 89,50

1994 55,68

1995 25,20

1996 35,57

1997 21,89

1998 6,52

1999 32,63

2000 41,35

2001 1,70

2002 38,63

2003 15,49

Mean 33,08

In the Bardena Blanca, appear in the northern part, over the scarps of the high quaternaries pediments, and in general in the scarps of the terraces and pediments overlaying tertiary lithologies. The detrital cover, when is hard cemented acts as resistant level and gives rise to this kind of slopes. In the Bardena Negra, uncovered slopes developed over limestones and marls are very large, as the ones of “La Plana Negra”, “Balcón de Pilatos”, “Plana de la Bandera”, etc.

Composite slopes are constituted by a rocky cliff in the upper part overlying easily erodible layers. The components of those slopes coincide with the ones defined by Wood (1942) and Fair (1947, 1948) in their studies in South Africa, that were deeply analysed and renamed by King (1957). The parts that he differentiated in the slopes were: (1) the crest, which is the upper part, sometimes displaying a convex profile, as a consequence of weathering and creep. (2) The scarp, that constitutes the outcrop of the hardest rock, whose retreat is due to erosive processes. The materials resulting from the destruction of the scarp form the (3) debris slope, which are basically mobilized by water erosion. Finally, (4) the pediments constitute an important concave element that connects with the alluvial plain. In the Bardenas Reales the most spectacular composite slopes are those developed on pediments, like Piskerra o Rallón (Fig.37). The thickness of the layer that constitutes the scarp is an important factor because it controls the cliff height and the length of the debris slope. Average thickness of the pediment deposits are up to 2 m, and they are made up of cemented gravels (Fig.38). Debris of the slopes come from the breakage of the resistant rock that constitutes the scarp. The hard caprock supplies most of the cover of a debris slope, although some fractions can come from the underlaying substratum. These debris can cover total or partially the slope and, at the same time, be affected by rilling and gullyging that gradually provokes the outcrop of the rock substratum. The extreme case results when the slope cover disappears. If the exhumed slope is made up of clays, like in the Bardenas area, a badland landscape is generated. The fragments of the debris slope show a poor

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classification. They are affected by sheet erosion, which exports the fine particles to the lower part of the slope, standing out the larger fragments. These boulders can also suffer an important weathering, with development of alveoles and tafonis in their walls. In the Bardenas Reales, scarp retreat of the pediment composite slopes is affected by water erosion, rilling, gullying but also by piping. This last process can generate natural bridges and pipeoutlets of big dimension (Fig.37).

Figure 37. Scarp retreat of the pediment composite slopes is affected by water erosion, rilling, gullying but moreover by piping that generate natural bridges and pipeoutlets of big dimension. But even more interesting and spectacular than the composite slopes are badlands. They occur in areas of very intense water erosion, high density of drainage (125-350 Km/km2 ), lack of vegetation, and steep slopes. In this area they develop on the Miocene clays of the Tudela Formation and usually show knife-edged watershed, sometimes rounded when the slope degree is low.

Figure 38. Detail of the upper part of the composite slope ann the scarp formed by a thick pediment deposits in the Rallón area.


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At the foot of these reliefs, a large number of talus flatirons are developed on recent pediments. In some cases more than one talus stage can be observed (Fig.39).

Figure 39. Multiple Talus flatirons developed over the tertiary clays of the Tudela Formation. It can be observed three stages.

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ROAD LOG:

Departure from the Conference Hall at 8:00.

Starting from Zaragoza we continue 70 km following the N-232 to Tudela.

In Tudela we take the Road NA-134 to Arguedas. About 1 km before the village and 200 crossing the petrol there is a small road that will take us to the Military Base after 10.8 km.

Stop.1.- About 3 km before the military base, in the Alto de Aguileras.

Stops. 2, 3 and 4.- Continuing to the Military Base. There we will turn right and take the perimetral road about 2.5 km.

Stop. 5.- Return to the Military base and 500 m before arriving, we will stop in the “Las Hermanas” Gully.

Return to Arguedas to have lunch.

Stop.6. After 10.8 km from Arguedas to the Military base, turn left and take the perimetral road 2.5 km we will reach the Castildetierra or Bardenas Simbol.

Stop 7. Continuing on the perimetral road 13.3 km we will reach the car park of “ Barranco de Cambrones”. From this point we have to walk about 700 m to reach the badlands of the “Piskerra”.

Stop 8. About 2 km away from the last stop we arrive to the “Rincón de las Rallas”

Return to Arguedas, Tudela and Zaragoza.

Expected arrival time to the Conference Hall: 19:30.

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