SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY Bujaraloz... · 2014. 6. 13. · SIXTH...

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

AEOLIAN LANDFORMS AND SALINE LAKES (CENTRAL EBRO BASIN)

M. Gutiérrez; G. Desir; F. Gutiérrez; J.A. Sánchez; C. Castañeda and P. Lucha

FIELD TRIP GUIDE - B7

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY

AEOLIAN LANDFORMS AND SALINE LAKES (CENTRAL EBRO BASIN) M. Gutiérrez; G Desir; F. Gutiérrez and P. Lucha Dpto. Ciencias de la Tierra. Geomorfología. Facultad de Ciencias. Universidad de Zaragoza, 50009 Zaragoza. 1. Introduction to the geology and geomorphology of the Ebro Depression The Tertiary Ebro Depression has a roughly triangular shape and is limited to the north by the Pyrenees and the Basque-Cantabrian mountains, to the south by the Iberian Range, and to the east by the Catalan Coastal Range. It narrows towards the west and is connected to the Duero Basin, through the Bureba Corridor. Towards the north-west, in the Catalan sector, it has a broad appendix and is separated from the Ampurdan Depression by the Catalan Transversal Range (Figs. 1 and 2). In geological literature, this great sedimentary basin filled with Tertiary materials, is commonly named as the Ebro Basin.

Figure 1. Cronoestrigraphic sketch of the continental infill of the Ebro Tertiary Basin (Riba et al., 1983). The Ebro River cleaves the entire depression from the WNW to the ESE, to finally dissect the Mesozoic materials of the Catalan Coastal Range, 30 km before it reaches the Mediterranean Sea. It occupies a slightly southern position in relation to the geometrical axis of the depression, along most of its course. The Pyrenean tributaries have a greater length and mean annual discharge than the Iberian ones. The orography of the basin is controlled by the downcutting of the fluvial network; the highest zones within the depression are the central platforms of the Sierra de Alcubierre (811 m a.s.l.). In the edges of the depression, the resistant conglomeratic deposits give rise to numerous landforms over 1,000 m high (Montserrat: 1,235 m a.s.l.; Riglos: 1,162 m a.s.l.). The lowest point is found where the Ebro River leaves the depression (45 m a.s.l.).

Aeolian Landforms and Saline Lakes

2

Figu

re 2

. Sim

plifi

ed m

ap o

f the

Qua

tern

ary

depo

sits

in th

e Eb

ro D

epre

ssio

n (G

utié

rrez

Elo

rza

et a

l., 2

002)

.

M. Gutiérrez et al.

3

The climate is Mediterranean continental, with major Atlantic influences in the western area. Extremely arid areas are found in the central sector (areas of Zaragoza and Lérida), where the mean annual rainfall is below 350 mm. The precipitation increases towards the surrounding sierras (approximately 600 mm), although towards the Iberian Range, the aridity is still high (between 400 and 500 mm). The rainfall has a great interannual variation, with marked incidence of storm precipitations, which occur mainly in spring and autumn. The temperatures have a high seasonal contrast. The mean annual temperature in the depression is 15°C in the central sector and the mean annual oscillation approximately 18°C. The dominant wind is the so-called Cierzo, whose direction is WNW-ESE and which is channelled along the depression, sometimes reaching speeds faster than 100 km/h. The geology of the Ebro Depression is perfectly synthesized in the studies of Riba et al. (1983), Vera (2004) and Gibbons and Moreno (2002). The Tertiary infill of the Ebro Basin is the result of the action of erosive processes which partly lowered the ranges that form its limits. Throughout the Tertiary, these ranges were the subject of discontinuous uplifts, which were more marked in the Pyrenees, and caused the displacement of the axis of the basin towards the southern zone. The maximum thickness is recorded in the northern sector of the basin and, at the same time, a gradual decrease in thickness is observed towards the South, where there are zones without Tertiary sediments. In the Ebro Depression, the sedimentation of marine deposits began during the upper Eocene and it coincides with the beginning of the Pyrenean Orogeny. Afterwards a regression took place which is recorded by the sedimentation of the sodium and potassium-bearing salt formations of Catalonia and Navarra and the salt-bearing evaporites of the Barbastro Formation. Simultaneously, the molassic sedimentation took place in the margin of the Catalan Coastal Range. Approximately at the end of the Eocene, the Ebro Basin was disconnected from the sea and the continental sedimentation started. The sedimentary environment within the depression was similar during the Oligocene and the Miocene times, with alluvial fans in the mountain borders and a playa-lake system in the centre of the basin. The alluvial fan environments are composed of proximal facies (conglomerates and sandstones) which border the margins of the depression, midfan facies (sandstones and siltstones intercalated with carbonates) and distal facies where evaporitic sedimentation dominated (gypsum, salt, marls and limestones). The lacustrine facies reached their maximum extent during the Miocene, in the central sector of the depression. The upper part of the Tertiary sedimentation is made up of thick carbonate beds which nowadays constitute the dominant reliefs of the central part of the depression. During the Neogene the Ebro Depression entered a predominantly tectonically stable period and sedimentation predominated. By this time, several erosion surfaces developed in the surrounding mountain ranges. In the Iberian Range, the largest and most ubiquitous erosion surface is Turolian-Ruscinian in age. At the borderline of the Ebro Depression it is linked into the upper part of the carbonate sedimentation of the late Neogene. The exact age of these calcareous sediments is unknown. In the Pliocene, a tectonic reactivation probably took place. It implied the uplift of the bordering ranges and the relative sinking of the depression, together with the deformation of the previously generated erosion surfaces and the infill of the basin. The generated topographic gradient


4

triggered a new erosive stage developing a new erosive surface which beveled the terminal carbonate materials. This erosion surface is gently tilted towards the centre of the depression, where the waters of a primitive Ebro River may gathered. Subsequent fluvial incision caused a continuous erosion of the Tertiary sediments of the basin and a progressive hierarchization of the fluvial network. At the same time, the genesis of structural reliefs and the individualization of great carbonate platforms began. The Quaternary depositional sequences, are chiefly composed of glacis and terrace systems (Fig. 2). These accumulations are well developed within the depression, and constitute one of its singularities. It is very difficult to estimate the absolute age of the highest levels. They may correspond to the Early Pleistocene, although they could be even older. These fluvial deposits have been punctually deformed as a consequence of the Quaternary Neotectonics. Moreover, at several points of the depression, halokinetic deformations which affect both, Quaternary erosion surfaces, and alluvial deposits have been recognized. Gravitational deformations have also been identified in Miocene and Quaternary deposits due to the dissolution-induced subsidence of the underlying evaporites. During the Holocene, extensive deposits have covered slopes and infilled valleys. Somewhere these infilled valleys have been cut off by the present fluvial network. Finally, in the central sector of the Depression, numerous closed depressions have been developed due to: (a) solution processes, both in gypsums and limestones, (b) aeolian deflaction processes, and (c) the damming effect of coalescent alluvial fans. 2. The exhumed Bujaraloz Structural Platform This area is located in the central sector of the Ebro Depression, (Fig. 3). It is situated 50 km to the southeast of Zaragoza city and 5 km to the south of Bujaraloz village, forming part of a semiarid region called Los Monegros.

Figure 3. Location of the study areas.


5

Figu

re 5

. Geo

logi

cal s

ettin

g of

the

stud

y ar

ea m

odifi

ed (S

alva

ny e

t al.,

199

6). U

.D.U

.: U

pper

Det

ritic

Uni

t. M

.D.U

.: M

iddl

e D

etrit

ic U

nit.


6

The sedimentary fill in this sector of the Ebro Tertiary Basin is made up of horizontally lying Oligo-Miocene continental sediments. This platform, developed around 320-350 m a.s.l., is located between the Miocene sediments of Santa Quiteria Sierra (574 m) to the north (Fig. 4), and the deeply entrenched meandering Ebro River which flows 200 m below the Bujaraloz platform, 4-15 km to the south, in the Sástago area (Fig. 5).

Figure 4. La Playa saline lake and Santa Quiteria Sierra (574 m) in the background The exhumed Bujaraloz platform is underlain by Lower Miocene gypsum and limestone sediments (Fig. 6) described by Salvany et al. (1994a, b; 1996). These sediments form part of the Bujaraloz-Sariñena Unit (Ramírez, 1997; Solá and Costa, 1997) and the gypsum lithofacies corresponds to the Yesos de Retuerta Unit of the Zaragoza Gypsum Formation (Quirantes, 1978).

Figure 6. Miocene gypsum and sandy limestones, outcroping in La Playa saline-lake border.


7

The mean annual temperature and precipitation in Bujaraloz meteorological station are respectively 14.5°C and 401 mm. A potential evapotranspiration of 778 mm has been estimated for the area (Liso and Ascaso, 1969) indicating semiarid conditions. The wind and aeolian energy have been studied in detail during three years in an anemometric station installed in the proximity of Bujaraloz village (Puircercús et al., 1994). The record shows that the strong wind, locally known as Cierzo, preferentially blows during the spring and winter with a dominant WNW to ESE direction (Fig. 7A). This wind develops most of the kinetic energy, E = 1/2 ρ v2 (ρ is the air density) (Fig. 7B). Mean speeds of 4.5, 5.5 and 6 m/s at 10, 30 and 50 m in height respectively have been recorded in the anemometric station. The maximum speed measured in the Zaragoza Meteorological station reached 38 m/s (136 km/h) with WNW winds, in 1979. The Cierzo is a dry and gusty wind which is channelled in the direction of the Ebro River valley when there is an atmospheric pressure gradient between the Cantabrian (Atlantic) and Mediterranean seas (Ascaso and Cuadrat, 1981).

Figure 7. A) Frequency distribution of wind directions (%). B) Kinetic energy (%). To study the playa-yardang systems, a preliminary geomorphological map of the study area was produced by interpretation of aerial photographs of 1:18,000 and 1:33,000 scales. All the recognisable yardang-like morphologies were represented in this map. Subsequently, a detailed field survey of the area was carried out. All the mapped yardang morphologies were carefully checked and analysed in situ in order to gain information about their geometry, morphometry and spatial relationship with other geomorphological and sedimentary features. Additionally, a


8

detailed geomorphological map of the largest closed depressions was made in the field on a 1:5,000 scale topographic map with contour intervals of 1 m. This map includes a precise representation of the lacustrine terraces and the aeolian morphologies identified in the field. Combining the maps produced in the field with the ones created by stereoscopic analysis of the airborne imagery, a final geomorphological map was delineated on the 1:5,000 topography. A synthetic sketch derived from this map is shown in Figure 8. The grain size and carbonate content of several samples collected from terraces, yardang slopes, corridors and playa bottoms were analysed in the laboratory. 2.1. Main characteristics of playas 2.1.1. Introduction The term playa is commonly used to designate vegetation-free closed depressions in arid and semiarid areas whose flat bottom is subject to sporadic or periodic flooding and the precipitation of salts by evaporation. A large number of examples have been documented from numerous regions. In southern Africa they occur below the 500 mm isohyets (Goudie and Wells, 1995). The size is highly variable, ranging from 0.004 to 100 km2 in southwestern Australia (Killigrew and Gilkes, 1974). They may be clustered reaching a high density, such as in the Orange Free State of South Africa where more than 100 playas have been recognised in an area of 100 km2 (Goudie and Thomas, 1985; Goudie, 1991). There is a very wide terminology referring to closed depressions developed in arid environments, largely due to the use of vernacular names (Tricart, 1969; Cooke and Warren, 1973; Neal, 1975; Cooke et al., 1993; Currey, 1994; Rosen, 1994; Shaw and Thomas, 1997; Briere, 2000; Gutiérrez Elorza, 2001). The term playa has a Spanish origin and was introduced in the English language from the exploration of the Northamerican southwest by the Spaniards (Gutiérrez Elorza, 2001; Gutiérrez Elorza et al., 2002). Recently, Briere (2000) presented a thorough compilation of names used to designate these depressions. This author proposes the use of the term playa for intracontinental depressions with a negative water balance that remain dry for more than 75% of the year, and playa-lake, a transition between playa and lake, for depressions that host water from 25 to 75% of the year. In many cases these definitions may be difficult to apply since the hydrologic regime of the playas may change substantially in successive years. Tchakerian (1999a), in the Encyclopaedia of the Deserts, defines playas as depressions, typical of arid and semiarid zones, affected by flooding by ephemeral superficial waters or fluctuations in the water table. The definition of pan given by Dregne (1999) is very similar to the previous one. Several authors consider both terms as synonyms (Shaw and Thomas, 1997; Goudie, 2004). On the other hand, the word sabkha has a clear geographic meaning restricted to depressions located in coastal plains (Tchakerian, 1999b; Briere, 2000).


9

Figu

re 8

. Geo

mor

phol

ogic

al m

ap o

f the

pla

ya-y

arda

ng s

yste

ms

loca

ted

abou

t 5 k

m to

the

sout

h of

Buj

aral

oz v

illag

e. 1

: Stru

ctur

al p

latfo

rm; 2

: Sca

rps;

3: E

dge

of

clos

ed d

epre

ssio

ns;

4: S

ingl

e ya

rdan

gs; 5

: Slo

pe d

epos

its; 6

: Cor

ridor

s an

d fla

t bot

tom

infil

led

valle

ys; 7

: Allu

vial

fans

and

gul

lies;

8: L

acus

trine

terr

ace

T 3; 9

: La

cust

rine

terr

ace

T 2a;

10: L

acus

trine

terr

ace

T 2b;

11: L

acus

trine

terr

ace

T 1; 1

2: P

laya

bot

tom

; 13:

Bow

l-sha

ped

dolin

es.


10

Playas form preferentially in low relief areas with a poorly defined or non-existent drainage network (Tricart, 1954; Goudie, 1999). These highly sensitive systems are affected by the interaction among surface and underground waters, aeolian activity and chemical and biological processes (Torgerssen et al., 1986). The evaporation increases the concentration of the water leading to the precipitation of different salts depending on the degree of saturation (Eugster and Hardie, 1978; Yechieli and Wood, 2002). Numerous processes may be involved in the generation of playas including aeolian erosion (Torgerssen et al., 1986; Marshall and Harmse, 1992; Goudie and Wells, 1995; Livingstone and Warren, 1996). When the water dries up or the water table declines below the surface, aeolian erosion may affect the sediments located above the capillary fringe causing the deepening of the basin (Yechieli and Wood, 2002). Consequently, there is a dynamic equilibrium between the fluctuations of the water table and the capillary fringe and deflation processes (Rosen, 1994). During dry periods deflation enlarges the size of the playas and may produce the accumulation of particles in the leeward margin generating asymmetrical crescentic dunes called lunettes (Goudie, 1991; Goudie and Wells, 1995). The transport of particles from the playas is favoured by several processes such as the weathering of the bottom sediments or the precipitation of salts. This last circumstance is especially conspicuous in the White Sands of New Mexico, an active dune field formed of gypsum particles derived from the playa of Lucero Lake (Darton, 1920; McKee, 1966). On the other hand, centripetal water flows transport particles from the margins to the centre of the depressions, which ultimately may be evacuated by the wind. This sequence of processes leads to the enlargement of the depressions and eventually to their coalescence (Goudie and Wells, 1995). These groupings grow with time and may survive to successive cycles of erosion (Marshall and Harmse, 1992). 2.1.2. Bujaraloz Saline Lakes/Bujaraloz playas The subhorizontal topography of the exhumed Bujaraloz structural platform is interrupted by over 100 closed depressions (Fig. 5). These closed basins show extremely flat bottoms and may have scarped or gentle margins. Most of the depressions are strongly elongated and preferentially orientated in the WNW-ESE direction. The largest closed depressions (up to 2.49 km2 in area) occur in the western sector of the platform where the gypsum sediments are thicker (around 40 m). Towards the east the gypsum lithofacies, interfingered with limestone are thinner and the number and size of the depressions decrease. The flat bottom of some of the deepest basins host saline lakes or playas, called saladas in the region, with precipitation of evaporite minerals (Fig. 8). There are three playas in the surveyed area called La Playa, El Pueyo and El Pito (Fig. 9), whose bottoms cover respectively 1.72, 0.14 and 0.35 km2 in area. They have a scarped northwestern margin which may be incised by gullies covered by vegetation suggesting a limited water runoff and sediment supply to the lakes. Locally, there are small debris cones at the boundary between the corridors and the lake. The southeastern margins generally show more gentle slopes.


11

Figure 9. General view of the ephemeral saline lakes of La Playa, El Pueyo and El Pito. Note the yardangs in the foreground. These lakes are commonly flooded during the cold periods of the year since the evaporation diminishes considerably. The only limnometric data available have been recorded in La Playa during the period 1993-1997. The depth ranged from 51 to 25 cm and 35 cm was the average depth. The rest of the lakes have shallower depths. Commonly, by the end of the spring and at the beginning of the summer, the lakes are totally dry and covered with an evaporite crust. During these dry periods the particles from the playa bed can be deflated, conferring an abrasive character to the wind. The aerial photographs show how the southeastern sector of the bed of the playas is higher than the rest of the lake bottoms. Probably, in the downwind sector of the lakes there is a higher aggradation rate by the wave action and by aeolian supply. These accumulations show different geometries in the 1956 and 1980 aerial photographs. In the outer fringe of the lake bottoms there is an aureole of halofilous vegetation (Salicornia sp.) which traps wind blown particles. Commonly, on the windward side of these plants there is a preferential accumulation of elongated limestone and gypsum clasts with the major axis oriented perpendicular to the wind direction. On the leeward side of the salicornias the deposit is mainly formed by small lenticular gypsum crystals. In this fringe there are also nebkhas which may reach up to 1 m on the leeward shore of the playas. In the lateral margins of the lakes there are trains of nebkhas locally forming ribbons several tens of meters long (Fig. 10).


12

Figure 10. Nebkhas ribbons in the margin of La Playa ephemeral lake Most of the water and solutes input to the playas, in the Bujaraloz area is supplied by groundwater flows which tend to concentrate along joints and solutionally enlarged discontinuities (Sánchez et al., 1989, 1998, García-Vera, 1996). According to Pueyo (1978/79), the brines of the playas are of the Cl-SO4-Na-(Mg) type (Eugster and Hardie, 1978). In winter the precipitation of mirabilite (Na2SO4·10H2O) predominates. In spring and autumn algal mats 3-4 mm thick form in the bed of the flooded areas (Fig. 11). Sometimes beneath this bed there is a black sapropelic layer rich in decomposing organic matter (Pueyo, 1980). The salts form by desiccation during spring and summer. During dry periods the salt is evacuated by the prevailing WNW-ESE winds (Ibáñez, 1975; Pueyo, 1978/79; Sánchez et al., 1989). The evaporation enhanced by the dry winds gives place to blisters, cracks (Fig. 12) and polygons (Fig. 13) in the lake bottom.

Figure 11. Algal mats in La Playa bed


13

Figure 12. Mud-cracks in La Playa bed

Figure 13. Salt polygons in La Playa bed


14

The playas have a characteristic fauna represented by crustaceans, including Artemia sp., copepods and ostracods as well as protozoans and flagellated algae. On the other hand, some of the playas have an external periphery modified by agricultural practices. The origin of the closed depressions and playas in the Bujaraloz platform has been explained by an evolutionary model established by Sánchez et al. (1998). According to this model, the closed depressions initially formed by the structurally controlled dissolution of the bedrock by infiltrating waters (solution dolines). The infiltration of the rainfall is favoured by the horizontal topography of the platform (Ibáñez, 1975) whereas the karstification of the bedrock is controlled by a fracture set (Quirantes, 1965, 1978; Arlegui and Soriano, 1996, 1998; Arlegui and Simón, 2001). In the other hand, three stepped levels of lacustrine terraces have been differentiated (Fig. 8). They show a broad development in the southeastern sector of the larger playas (La Playa, El Pueyo and El Pito). The oldest level (T3), at 7.5 m above the current playa bed and 4 m thick, is represented by two small surfaces to the southeast of El Pueyo playa. The intermediate terrace (T2), at 3.5-3 m above the playa bed, is the surface with greatest extent. It reaches more than 3 m in thickness. In El Pito playa this terrace (T2) show two levels (T2a and T2b) incised 20 cm one in the other. These levels are also recognised to the Northwest of La Playa. The base of these deposits is not exposed since it is onlapped by either the sediments of the youngest terrace or by the deposits of the lake bottom. The most recent terrace (T1), is located at 0.3-0.5 m above the playa. Its deposit, more than 0.4 m thick, is onlapped by the sediments of the current lake bottoms. The terrace sediments are made up of white-grey coloured, fine-grained deposits showing a crude horizontal stratification. The grain size analysis of the terrace deposits shows that fine sand is the dominant textural fraction (Fig. 14). They are composed of 87% of lenticular gypsum crystals and 13% of carbonate with some calcareous concretions. The T2 and T1 terraces are dominantly formed by fine sand and silt, whereas the T3 terrace is mainly formed by silt with a proportion of relatively coarse-grained particles.

Figure 14. Grain size curves of the deposits of El Pito and La Playa lake terraces and bottoms.


15

2.2. Features of Yardangs 2.2.1. Introduction Yardangs are erosional landforms produced by wind action, and according to McCauley et al. (1977) they are probably one of the least understood landforms of the Earth surface. In the geomorphological literature yardang is generally used to designate elongate streamlined hills developed in different lithologies in numerous deserts of the world (Halimov and Fezer, 1989; Goudie, 1999; Goudie et al., 1999). The term yardang corresponds to a local word introduced by Hedin (1903) in his studies of the Taklimakan Desert in the east of China. The initial investigations of yardangs had a local character and were focused mainly on their description rather than on their genesis. The use of airborne imagery such as aerial photographs (Mainguet, 1968, 1972) and satellite images for the study of yardang fields both in planet Earth and in Mars gave rise to a significant advance in its knowledge (McCauley, 1973; Ward, 1979). These analyses have been complemented by laboratory experiments, field observations and theoretical considerations (Whitney, 1978, 1983; Ward and Greeley, 1984). From the interpretation of satellite images, McCauley et al. (1977) identified the largest yardang fields in the Earth. All of them are located in hyperarid areas and some of them cover as much as 650,000 km2 (Mainguet, 1968), as in the area southeast and southwest of the Tibesti Massif (Borkou, central Sahara). The numerous investigations of the origin of the yardangs reveal that their generation results from the combined effect of several processes: wind abrasion, deflation, gullying and slope movements. Several excellent review works go deeply into different aspects of these peculiar morphologies (Whitney, 1985; Cooke et al., 1993; Laity, 1994; Breed et al., 1997). 2.2.2. Bujaraloz Yardangs The spatial association between the yardangs and the closed depressions in the Bujaraloz area, indicates a genetic interrelation between both landforms. All the yardangs are located at the leeward strip of the ephemeral saline lakes, especially linked to the larger ones: La Playa, El Pueyo and El Pito (Fig. 8). The yardangs are primarily developed in Miocene gypsum beds, although limestone is exposed in some of them. In the slopes of the yardangs the nodules of alabastrine gypsum show well developed solution features (rillenkarren and napfkarren). In the gypsum blocks on the playa surface these karrens are only developed in the leeward sides, whereas the abrading action of the wind dominates in the windward side. Up to 50 yardangs have been identified in the study area. They can be grouped in two types (Table 1). 44 yardangs are developed in the Miocene bedrock (rock yardangs) and 6 have been recognized in the unconsolidated lacustrine deposits. The mean direction of the yardangs, N122E (Fig. 15A), coincides with the most frequent wind direction (Fig. 7A). There is no consistent relationship between the orientation of the yardangs and the strike of the joints measured in four locations within the study area (Fig. 15B). However, several authors reveal the significant role played by the WNW-ESE fractures in the morphogenesis of the central sector of the Ebro Depression (Quirantes, 1965; Gutiérrez et al., 1994; Arlegui and Soriano, 1998: Maldonado et al., 2000). According to Arlegui and Simón (2001) the generation of the WNW-ESE joint sets is related to perturbations in the stress field induced by pre-existing N-S trending joints.

Tabl

e 1.

Mor

phom

etric

ana

lysi

s and

mai

n ya

rdan

g ty

pes.

(* la

cust

rine

depo

sits

)Sl

ope

Cor

ridor

Type

Num

.D

irect

ion

Leng

htW

idth

Hig

htra

tio l/

wW

indw

ardL

eew

ard

Max

. Wid

thSl

ope

Mes

aSa

w to

oth

cres

Con

eK

eel

Rid

g. to

pla

tf.1

123

136

4013

3,4

26,5

3,3

443,

0x

212

534

83

4,3

14,0

4,0

302,

6x

311

540

127,

53,

330

,38,

170

5,7

x4

111

4820

9,5

2,4

33,7

14,0

705,

7x

511

612

020

66,

020

,60,

356

2,3

x6

110

7626

92,

935

,05,

756

2,3

x7

113

4216

72,

656

,314

,012

01,

8x

811

214

026

10,5

5,4

29,1

19,7

120

1,8

x9

128

3014

32,

139

,89,

5-

-x

1011

954

122,

54,

563

,49,

1-

-x

1111

644

144,

23,

121

,826

,610

5,7

x12

130

223

1,2

7,3

29,9

1,9

--

x13

111

1610

21,

645

,014

,030

0,0

x14

124

2610

32,

614

,06,

716

4,8

x15

125

1810

2,5

1,8

29,1

3,7

164,

8x

1612

220

81,

52,

537

,87,

6-

-x

1712

370

102,

57,

026

,52,

612

18,0

x18

136

5620

102,

841

,83,

615

01,

4x

1912

594

229

4,3

16,6

1,6

150

1,4

x20

120

240

2613

9,2

25,7

5,7

722,

1x

2112

192

206

4,6

31,0

10,6

802,

1x

2212

874

186

4,1

39,8

7,1

86,

3x

2312

634

123

2,8

36,9

5,7

144,

1x

2410

620

80,

52,

514

,02,

9-

-x

2511

150

161

3,1

39,8

1,1

--

x26

130

264

3011

8,8

32,0

2,9

363,

2x

2714

248

83

6,0

14,0

1,7

305,

0x

2813

836

103,

53,

620

,62,

630

5,0

x29

134

9020

114,

520

,61,

144

4,8

x30

133

248

23,

026

,62,

720

4,3

x31

131

2410

22,

418

,41,

836

8,4

x32

145

6416

114,

020

,63,

320

8,5

x33

110

4414

3,2

3,1

14,0

9,5

247,

8x

3411

684

169

5,3

30,3

1,0

303,

2x

3512

320

634

156,

118

,40,

930

3,7

x36

115

174

304

5,8

26,6

10,2

--

x37

122

6824

2,6

2,8

14,0

2,6

--

x38

138

5218

11,4

2,9

38,7

3,2

--

x39

118

6224

82,

639

,96,

6-

-x

4012

032

141,

42,

343

,92,

9-

-x

4112

384

248,

93,

517

,72,

340

2,4

x42

125

132

2817

4,7

56,3

1,0

--

x43

112

3614

112,

651

,314

,036

3,3

x44

122

4612

73,

826

,62,

0-

-x

1*15

516

102,

51,

625

,611

,9-

-x

2*99

146

1,8

2,3

36,9

26,6

--

x3*

153

2012

11,

726

,69,

5-

-x

4*10

536

82,

34,

514

,07,

1-

-x

5*14

250

83,

56,

336

,914

,0-

-x

6*10

330

143,

92,

145

,020

,6-

-x

Mea

n12

2,6

72,0

17,2

6,3

4,0

30,2

5,9

48,4

4,4


16


17

Figure 15. A) Frequency of yardang directions. B) Frequency of joint directions measured in four stations of the study area. The maximum length, width and height of the yardangs are respectively 264, 40 and 17 m (Table 1). The average length/width ratio is 4.1:1 (Fig. 16). These proportions are in agreement with the values obtained in yardang forms generated by experimental studies in wind tunnels (Ward and Greeley, 1984) and differ from the ratios calculated by Mainguet (1968, 1972) 10:1, for the yardangs of the Borkou area in the Sahara Desert. The elongated and aerodynamic shape of these aeolian landforms minimises the resistance to the wind flow. The studied yardangs correspond to the meso- and mega-yardangs of Cooke et al. (1993) and to yardangs and megayardangs following the terminology of Livingstone and Warren (1996).

02

46

810

1214

1618

1 2 3 4 5 6 7 8 9 10 11 12l/w

Figure 16. Frequency distribution of width/length ratio of all yardangs.


18

The windward and leeward slopes of the yardangs have mean angles of 30 and 6 degrees respectively (Fig. 17). The slopes generally show a stepped profile controlled by the horizontal bedding of the bedrock, similar to the yardangs described in the coastal desert of central Perú (McCauley et al., 1977). The windward slope is always steep, whereas the incline of the leeward slope shows a progressive decline towards the downwind extreme. Signs of undercutting at the foot of the slopes are very scarce, indicating that, at the present time, sediment accumulation exceeds wind deflation and abrasion (sandblasting). However, the gypsiferous-silts slope deposits locally show aeolian pits and flutes. The pits are small (2-5 cm) irregular cavities which commonly occur on steep slopes. The flutes have a U-like shape in plan view and open in the downwind direction (McCauley et al., 1979; Whitney, 1983; Laity, 1995). These deposits also show flakes of a thin superficial crust detached by the wind, exposing cohesionless sediments to the erosive action of the wind. These features are only observed on broad surfaces devoid of vegetation.

Figure 17. Keel-yardangs developed in the Miocene gypsiferous deposits. At the background, La Playa lake basin, the exhumed Bujaraloz platform and the overlying Miocene sediments in Santa Quiteria Sierra. Arrow indicates wind direction. The sandy limestone outcropping in the windward slopes of the yardangs shows alveoles and occasionally some tafoni. There are also flakes and salt efflorescences produced by capillary rise. Planar and angular clasts whose major axis (2 cm) are preferentially oriented parallel to the wind direction are found in La Playa and El Pueyo ephemeral lakes. Some of these clasts may be suspended above the surface forming micro-pedestal or micro-hoodoo like micromorphologies. On the other hand, on the leeward side of the yardangs there is commonly an accumulation of acicular gypsum crystals. The flanks of the yardangs show convex-concave slopes that are actually covered by sparse shrub vegetation, dominantly composed by Rosmarinus officinalis, Genista scorpius and Thymus vulgaris (Fig. 17). At the present time, the main process on the slopes is sheet flow conditioned by the obstacles that constitute the vegetation. The differential weathering between the gypsum and limestone beds generates undercuts that finally give place to small limestone rock-falls. Very likely, dissolution processes and salt weathering contribute to the erosion of the bedrock. The


19

lower part of the slopes is generally covered by gypsiferous silts (weathering products of the Miocene gypsum) with limestone blocks and regosols. The thickness of these deposits increases towards the corridors and flat-bottom infilled valleys developed between the yardangs. In the elongated depression between La Playa and El Pueyo the slopes of the yardangs located on north and south margins show a high inclination converging towards the axis of the depression. The corridors, with a WNW-ESE orientation (Fig. 8), have a mean width of 48 m and a mean slope towards the closed depressions of approximately 4o (Table 1). The infill of the corridors is composed of gypsiferous silts with scattered angular limestone clasts. In La Playa lake basin some corridors are connected with the surface of the T2.lacustrine terrace. Some corridors have diffuse-edged closed depressions. Very likely they have been generated by subsidence caused by the dissolution of the evaporite bedrock, although aeolian deflation may have also contributed to their formation. Most of the rock yardangs show flat tops controlled by the horizontal structure of the bedrock. The yardangs have been grouped following the morphological classification established by Halimov and Fezer (1989) from their studies in central Asia. Several morphological types have been identified in the study area: ridge-yardangs associated with platforms which constitute elongated appendixes of the platforms (linked in the leeward side to the platform surfaces); mesa-yardangs controlled by runoff incision; cone-yardangs resulting from the parallel retreat of the slopes of the mesas; saw-tooth crest-yardangs generated by the differential erosion of the bedrock in the crest and keel-yardangs which have the shape of an upturned ship. According to Table 1, the dominant morphologic type is the ridge-yardang associated to platforms. The Halimov and Fezer (1989) classification also introduces a cyclic concept in the development of these landforms. It is difficult to share this cyclic idea since it does not consider the structural factor (mesas and hogbacks) in the evolution of the yardangs. The yardangs developed on lacustrine terrace deposits are located in the western margin of El Pueyo playa. These keel-type yardangs have been formed in the two older terraces (Fig. 18). The yardang slopes are locally onlapped by the deposits of the two highest terraces (T3 and T2).

Figure 18. Yardang developed in the deposits of the lacustrine terrace T2 of the La Playa-El Pueyo ephimeral lakes. Arrow indicates wind direction.


20

In the northeastern sector of the bottom of La Playa saline lake, several yardang-like morphologies have been developed on rubbish dump material, constituted mainly by gypsum blocks (Fig. 19). These currently active morphologies show an approximate E-W orientation. The erosion of the rubble has given place to stream-lined deposits consisting of gypsum clasts with a matrix of gypsiferous silts.

Figure. 19. Upturned ship morphology developed on rubbish dump materials in La Salada ephimeral lake 2.2.3. Age of yardangs Some authors have estimated erosion rates in the yardangs. In the Rogers Lake (Mojave Desert), Ward and Greeley (1984), by means of photographs taken in the field at different dates, estimate that the yardang fronts are eroded by abrasion at rates of 2 cm/year, whereas in the flanks the erosion by abrasion and deflation reaches 0.5 cm/year. Halimov and Fezer (1989), in Qinghai province, China, based on the content of pottery remains in silt deposits, estimate that small yardangs may form in 1500-2000 years. Clarke et al. (1996) calculate with IRSL (infra-red stimulated luminiscence) on aeolian sands in the Mojave Desert that 1m-high yardangs formed in < 250 years. Goudie et al. (1999) calculate in the southwest of Egypt a rate of regional deflation of 1.5 km/ky, assuming a constant vertical downcutting. In the study area, some rubbish dump accumulations have been carved into yardangs. The generation of these landforms may have lasted less than 100 years. Yardang formation chronology show an important variation in its generation rates. Aulthough yardangs normally developed in extreme arid climates, yardang fields have been also recognized in arid and semiarid climates. In the last ones, clear climatic changes should have been taken place that explain these forms generation. 2.3. Discussion and evolutionary model of the playa-yardang system The generation of yardangs results from complex interactions between internal factors (lithology and structure) and external factors (one-directional winds, flow systems surrounding the yardangs and supply of abrading particles) (Greeley and Iversen, 1985). There is also a suite of non aeolian processes such as gullying, salt weathering, cracking, slumping, etc., which contribute to their


21

evolution. The yardangs in the southern sector of the Tibesti desert constitute a good example of the influence of the internal factors. Here, the yardangs are restricted to sandstone lithologies (Mainguet, 1968, 1972). Additionally, in this region the dense jointing controls both the orientation of the yardangs and the width of the corridors. In the studied playa-yardang systems, the abrading particles come primarily from the deflation of particles from the bottom of the playas. As it is shown in the geomorphological map, the generation of yardangs requires the existence of large playas, greater than 0.03-0.14 km2 e.g. the playas situated to the northeast of El Pito (Fig. 8). Hence, the size of the playas and the time and proportion that they remain flooded are limiting factors in the development of these erosional aeolian landforms. The geomorphological evolution of the playa-yardang systems in Bujaraloz platform started with the erosion of a Miocene sequence above the platform (around 250 m thick) and the local exhumation of the structural platform (Fig. 4). Most of the surface was exposed prior to the formation of the highest preserved terrace of the Ebro river in the area, since this alluvial level is inset below the platform. During the Pliocene, the Ebro river started to flow into the Mediterranean Sea. Once the soluble sediments of the platform were exhumed, infiltration and downward percolation of meteoric water, to a shallow saturated zone, generated closed depressions (solution dolines) by dissolution processes. Some of the growing dolines coalesced to form elongated uvalas. The solution dolines became progressively deeper by epiphreatic dissolution until they reached the water table, which causes the periodic flooding of the depressions (Sánchez et al., 1989, 1998). The exposure of the saturated zone at the land surface leads to a significant evaporation creating a hydraulic gradient towards the saline lake and groundwater flow cells. This situation causes a reversal in the flow of the depressions with rising groundwater towards the playas, preventing the infiltration of meteoric water. The ascending ground-water, flowing through the low permeability bedrock, supplies solutes to the flat and shallow lakes which precipitate by evaporation (Bowler, 1976: Sánchez et al., 1989, 1998). The closed depressions constitute local base levels for runoff. This topographic relief favours the development of gullies with internal drainage. A large proportion of the incisions is controlled by WNW-ESE trending fractures (Arlegui and Simón, 2001). The enlargement of the bottom of the playas is produced by several processes including dissolution, salt weathering, gullying and lateral undercutting at the foot of the depression margins during flooding periods. The undercutting of the basin edges promotes falls and different types of mass wasting processes. During dry periods the wind mobilises particles from the bed of the playas towards the leeward sectors of the lake basins, sculpturing the yardangs. The gullies in these margins of the depression are widened and deepened by aeolian erosion, giving place to U-shaped in cross section and flat-bottomed valleys or corridors (Laity, 1994). The compression of the flow lines in the corridors causes abrasion in the bottom and slopes (Blackwelder, 1934), whereas the yardangs are located in zones with lower pressure. According to Whitney (1985) and Laity (1994) the corridors are the zones with greatest geomorphic activity in relation with the yardang genesis. The progressive widening of the corridors tends to break up the yardangs into smaller landforms, such as mesas, cones, pyramids, etc. (Halimov and Fezer, 1989). The bowl-shaped dolines recognised in the corridors of the study area may have been generated by subsidence caused by subsurface dissolution and/or aeolian deflation.


22

The yardangs are more frequently developed in plateaus dissected by the drainage net with unidirectional winds (Halimov and Fezer, 1989; Laity, 1994). The gullies parallel to the wind direction are widened (Breed et al., 1997), whereas wind erosion tends to obliterate the gullies of the windward slopes (Ward and Greeley, 1984). Nowadays, the origin of the yardangs is quite well understood as a result of experiments carried out in the field and with wind tunnels (Whitney, 1978, 1983; Ward and Greeley, 1984). These studies emphasise the relevance of the abrasion and deflation as is also supported by McCauley et al. (1977). The abrupt windward side is experimentally generated by erosion on corners and windward slopes (Ward and Greeley, 1984), whereas Whitney (1983) maintains that they are produced by backward flows from the leeward side. According to Whitney (1985), this negative flow is responsible for the most important erosion in the yardang. Sandblasting is confined to the windward slope and the primary wind causes undercutting and subsequent rock-falls keeping the high inclination of this slope. The abrasion is limited to the saltation transport height which may reach 3 m (Pye and Psoar, 1990), and is at a maximum at 2 m (Sharp, 1969). The flow lines surrounding the yardang separate in the leeward side forming turbulences and return flows. This aerodynamic system produces differential pressures giving place to secondary currents which transport particles over the surface in suspension. This particles flow towards vorticity centers that act as erosional tools (Whitney, 1978, 1983, 1985). The vorticity is one of the most frequent and important wind movements responsible for the erosion. It acts on the surface of an object along positive, negative and secondary flow lines (Whitney, 1978). In the extensive Bujaraloz platform, the yardangs are restricted to the leeward margin of the largest playas: La Playa, El Pueyo and El Pito (Fig. 20). This spatial relationship is clearly due to the necessity of having a source of aeolian particles to carve the yardangs and corridors. Yardangs are elaborated on gypsum layers. The superficial outcropping of these layers are strongly weathering, where the resulting material is constituted by grains with a weak adhesion. Consequently, the abrading action of the silts and acicular gypsum particles in suspension is enhanced. The terrace levels give evidence of several morpho-sedimentary episodes in the evolution of the lakes. The relatively thick deposits of the lacustrine terraces record periods of vertical aggradation. These sedimentary units were probably formed when the deposits accumulated in the lake basins could not be deflated due to semi-permanent flooding. Besides, the vegetation developed during the wet period restricts the aeolian erosion. The entrenchment of the system takes place during dry periods. The bottom of the playa is lowered by subsurface dissolution and deflation. The bedrock beneath the lake is transferred to the surface as a solute, precipitates by evaporation and is removed by the wind. Large volumes of sediments can be evacuated from the playas by aeolian deflation through a considerable time span. During the entrenchment episode subsequent to the formation of the most extensive terrace (T2), the large lake that originally linked La Playa and El Pueyo was segmented into two playas. El Pito has also undergone a partial segmentation during this stage (Fig. 8).


23

Figure 20. Aerial view of the yardangs developed in the leeward of La Playa saline lake. There is no doubt that the phases of greater aeolian erosion correspond to the dry periods, when the yardangs are modelled and the corridors are deepened and widened. Numerous researchers indicate that the generation of yardangs requires extremely arid climates, minimal soil and vegetation cover, prevalent one-directional winds and a scarce presence of sand (McCauley et al., 1977; Whitney, 1983, 1985; Ward and Greeley, 1984; Halimov and Fezer, 1988; Laity, 1994; Breed et al., 1997; Goudie et al., 1999). McCauley et al. (1977) consider that the yardangs may not persist if the climate changes to more humid conditions. The existence of yardangs in the studied semiarid area could be due to the presence of dry playas which constitute a good source for particles for the strong local winds. However, at the present time the rock yardangs are covered by shrub vegetation inhibiting or preventing the aeolian erosion. Probably, these landforms originated during dry periods with scarce or non-existent vegetation. The palaeolimnological studies carried out by Schütt and Baumhauer (1996) in the sediments of El Pito playa indicate a change from humid conditions in the Lower Holocene to the current semiarid climate within a humid phase within the arid period. 3. Introduction to the Geology and Geomorphology of the Alcañiz area. This area is located in the southern sector of the Ebro Depression, in the vicinity of the Iberian Range. The substratum in this area is made up of horizontally lying Miocene sediments which onlap the folded materials of the Iberian range. The thickness of the Miocene deposits decreases towards the southern margin of the Ebro Depression. The outcropping materials are mainly constituted by sandstones and siltstones (claystones). The fluvial erosion of the substratum has given rise to an inverted relief where the palaeochannels of calcareous sandstones stand out. Some of them display a meandering pattern and spread through long distances (Figs. 21 and 22) (Riba et al., 1967; Ibáñez, 1973, 1976). Among the geomorphic features of the area, the large Meseta de Vizcuerno structural platform, elaborated on sandstones, also stands out.


24

Figure. 21. Geomorphological scheme of the endhorreic sector located to the W-SW of Alcañiz (Ibáñez, 1973).


25

Figure 22: Aerial view of a sandstone palaeochannel of the Alcañiz area. The actual weathering processes include dissolution, haloclastisme and wetting and drying cycles which have given rise to subcircular micro-morphologies on both, the sides and tops of the sandstone outcrops, known as caverns, gnammas, tafonis and honeycombs (Gutiérrez Elorza and Ibáñez, 1979). The term gnammas include several morphologies such as pits (Fig. 23) (deep, relatively narrow hollows, sub-circular in plan view), pans (Fig. 24) (shallow depressions limited by abrupt or overhanging walls, displaying flat bottoms) and armchair (variable in depth, circular in plan view and developed on inclined surfaces) (Twidale and Corbin, 1963; Gutiérrez and Ibáñez, 1979). In the other hand, the Guadalope River and its tributaries have dissected the palaeochannels, mesas and structural platforms, individualizing this geomorphic features. Other infilled valleys in the area are tributaries of the small closed depressions distributed throughout the area.

Figure 23: Infilled gnammas of pit type. Alcañiz area.


26

Figure 24: Gnammas of pan type with overflow channel. Alcañiz region. 3.1. The playas of the Alcañiz area The existence of closed depressions in this sector is the result of several factors including: the sub-horizontal layering of the Tertiary formations, the alternance between sandstones and claystones/siltstones which facilitates the differential erosion and the semiarid climate that favours the existence of temporary saline lakes (Ibáñez, 1973, 1976).

Figure 25: Aerial view of Salada Grande playa


27

To the West and Southwest of Alcañiz there are numerous closed depression that host ephemeral saline lakes known as playas. In this fieldtrip, we will visit La Estanca and La Salada Grande (Fig. 25) saline lakes, located in the northern sector of the geomorphological scheme (Fig. 21). The first one, has its major axis oriented in the NW-SE direction. It has an area of 0.87 km2 and it is the only closed depression which contains water the whole year because it has been artificially dammed to preserve the water for irrigation. La Salada Grande is oriented in the N-S direction and has an area of approximately 1 km2 when the water reaches its highest level (Ibáñez, 1973, 1976). The playas in this area are located among sandstones reliefs or within flat areas and they are always developed on argillaceous Miocene substratum. The area of the water surface varies through the year and usually the depressions remain dry during the summer time. To elucidate the causal processes for the excavation of the closed depression bottoms, several samples where taken along a transverse section and the water extract from the saturated paste in the Salada Grande was analysed (Tabla 2) (Richards, 1954). The analysis indicates that the extracts are weakly alkaline displaying SAR values between 90.36 and 115.56 and ESP values between 75.53 and 78.50. The analysis indicate that the claystones are certainly dispersive (Sherard et al., 1972) (Fig. 26). These values confirm that the claystones are prone to pelletization what makes possible its exportation through aeolian deflation. Since these alkaline claystones are transported out of the closed depressions the bottom of the playas is lowered. Conversely the presence of water stops the removal of particles by the wind (Tricart, 1954, 1969; Bowler, 1973, 1986) Table 2. Analysis of water extract from saturate paste of La Salada sediments.

Samples Ph Conductivity mS/cm

HCO3 (meq/l)

Cl (meq/l)

SO4 (meq/l)

Ca (meq/l)

Mg (meq/l)

Na (meq/l)

K (meq/l) SAR1 ESP2

m-0 7,1 87,2 1,8 944,9 479,0 34,3 271,7 1117,8 2,9 90,3 78,5 m-1 7,5 114,7 2,0 1351,2 1003,4 23,0 501,0 1843,4 4,2 113,8 77,8 m-2 7,6 121,9 1,8 1424,5 980,3 23,1 528,1 1803,8 4,1 108,6 76,5 m-3 7,7 124,2 2,0 1393,5 1442,0 14,3 686,1 2162,6 4,6 115,5 75,5 m-4 8,0 125,0 2,2 1452,7 1208,3 18,1 629,3 2040,0 4,4 113,3 75,9 m-5 7,8 120,6 2,2 1373,7 1219,8 19,0 561,9 1949,9 4,5 114,4 77,0

1 Na/[(Ca+Mg)/2]^1/2 2 Na*100/Ca+Mg+Na


28

0

20

40

60

80

100

0,1 1 10 100 1000 10000

Total amount of dissolved salts (Ca+Mg+Na+K)

Sodi

um e

xcha

ngea

ble

perc

enta

ge

(Na*

100/

Ca+

Mg+

Na+

K)

DISPERSION

TRANSITION

FLOCULATIO

m-0m-5

m-1

m-4m-3m-2

Figure 26: Sherard Diagram of the colleted samples. Note that all the plots are within the dispersion field.


29

REMOTE MONITORING OF SALINE WETLANDS IN MONEGROS DESERT, NE SPAIN. C. Castañeda Centro de Investigación y Tecnología Agroalimentaria (CITA). Unidad de Suelos y Riegos. Apartado 727. 50080 Zaragoza In ecological, social and economic terms, wetlands are among the most valuable and productive ecosystems on earth and priority sites in environmental policies. The Monegros Desert in Aragón (Fig.28) includes an arheic area with scattered playa-lakes and other closed saline depressions of high scientific and environmental value, being the natural habitats of endemic microbes, plants and animals. These fragile habitats are isolated saline wetlands, locally named “saladas”, situated in a vulnerable semi-arid territory where agricultural expansion (Fig.29) threatens the natural hydrologic cycle with regular artificial flooding, risking the survival of a valuable natural resource.

Figure 28. Location of the saladas area (from Castañeda and Herrero, 2005). The Spanish Government has launched a project to irrigate over 60,000 ha in southern Monegros, affecting this territory, while the uniqueness of this European region has been already recognized in some patches delineated for bird protection and included in the Natura 2000 network, illustrating a typical conflict between production and conservation. After the irrigation project, this desert and arheic area will receive water conducted from Pyrenean rivers; the project also involves strong landscape transformations due to farm consolidation, and associated works for agricultural intensification.


30

Figure 29. Landsat 5 TM RGB 741 images. Land consolidation and changes in fallow coverage following common agricultural policy are responsible of the different appearance of the dry farmed area in the two dates (Castañeda, 2002, Castañeda et al. 2004b). The newly irrigated lands (NE corner in 1997 image) advance towards the playa-lakes. Most of them are less than 50 ha, except for La Playa (200 ha), in the middle of the images (From Castañeda et al., 2005b).


31

The forthcoming irrigated lands include the half of the area hosting playa-lakes (Fig.30). Meanwhile, hydrological studies (Berga, 1993; Samper and García-Vera, 1998) assert that irrigation will modify the saline groundwater hydrologic balance and that new fresh and polluted water flows will discharge into the depressions. The result, as we can see in other saladas in Aragón could be a permanent and fresher flood in the depressions and playa-lakes, destroying existing habitats and extinguishing valuable endemic organisms. Southern Monegros is a gypsiferous Miocenic platform more than 100 m above the Ebro river, and it has the highest degree of aridity in Europe (Herrero and Snyder, 1997). A constellation of almost 100 topographic closed depressions were inventoried by Balsa et al. (1991), Comín and Sanz (1989) and Pedrocchi (1988), forming the so-called Bujaraloz-Sástago endorheic complex. As a saline system, the saladas can be considered discharge playas and closed saline lakes, depending on the closeness of the groundwater level to the ground surface (Yechieli and Wood, 2002). They are characterized by ephemeral brines alternating with dry conditions and by halophytes distributed according to salt tolerance.

Figure 30. The remotely detected saladas of Monegros and the future irrigated area bordering them (from Castañeda et al., 2005a). Scientists fear that flows from new irrigated districts on conterminous lands will disturb the natural hydrological cycles and the natural salinity, and polluting with agrochemicals. Several saladas are already artificially flooded, but the current degradation affecting most of these wetlands comes from the dumping of boulders removed from plowed fields, land consolidation, heavy machinery traffic, new linear infrastructures for agricultural intensification, mainly irrigation, and more recently from urban debris. The degradation of the landscape and the habitats


32

are easily observed, since some of these playa-lakes have disappeared and others have deteriorated in appearance. Recognizing valuable wetlands is a task of international interest and many studies use remote sensing as a basic tool. Moreover, closed lakes of arid and semiarid regions are of interest because of their sensitivity to regional climate. Remote sensing techniques offer rapid acquisition of data with a generally short turn-around time, at a cost lower than that of ground surveys, and are a useful tool for delineating the fluctuating margins of shallow-water systems like the playa-lakes. Remote data can also provide an increased historical reach, revealing wetlands hydrology patterns in those cases where the historical data such as water levels is limited and allowing us to bring to bear new approaches to field monitoring on old data. The remote sensing investigation of the playas and salt lakes in the Monegros desert demands an approach adapted to their singular characteristics. The first peculiarity is their small size, ranging from 1.8 ha to 200 ha; the second is their irregular and rapid change of appearance (Fig.31), and third, their alteration due to the agriculture intensification. This variability is influenced by the season, the weather and the groundwater dynamics. Thirdly, there is simply a lack of in situ data, which is common for playas. Moreover, to our knowledge, no standard definitions of the playa-lake land-covers applicable to this study are available. Terms such as saline pan, saline mudflat, dry mudflat describe depositional environments useful to interpret geological record (Smoot and Lowenstein, 1991; Rosen, 1991; Pérez et al., 2002) but unsuited for linking field and remote observations.

Figure 31. Changing looks of two saladas La Playa (a and b) and Salineta (from Castañeda et al., 2005a).


33

The saladas have been studied using Landsat images, which have provided worthwhile historical data and additional information about their facies and hydrologic status. Satellite observations overcame the scarcity of ground records about the water bodies in the arid playa-lakes of Monegros (Castañeda, 2002; Castañeda et al., 2005b). This information is essential to know their natural hydrologic regime and to detect their change and any future impact from the newly irrigated lands. A catalog of different land covers, here defined as facies, has been created to describe and monitor the valuable habitats hosted by the playa-lakes (Castañeda et al., 2004b; 2005a). The catalog includes five facies: Water, Watery Ground, Wet Ground, Vegetated Ground, and Dry Bare Ground (Fig.32). Moreover, the definition of facies overcomes the lack of a conceptual framework to assess the ecological status of the saladas, and the adopted criteria make for an easy distinction of these facies either in the field or using the Landsat images.

Figure 32. An example of a thematic map showing the facies distribution during the wet period (from Castañeda et al., 2005a) The playa inundation regime reveals both hydrological and climatic conditions. Remote sensing has yielded excellent synoptic information and quantitative data on water occurrence and the hydrologic behavior of the Monegros playa-lakes (Castañeda and Herrero, 2005). The water presence-absence cycles in the major Monegros playa-lakes have been identified using ground observations and satellite data. Both data sources agree, allowing us to group the playa-lakes according to their common inundation patterns. There is a close relationship between each of these groups and their location on hydrogeological units established by other authors. It has been confirmed the relationship between the water occurrence and both the previous rainfall and ET0,


34

and the role of the aquifers in the hydrological cycle of these arid wetlands. This groundwater connection with the playa-lakes and their water budget has been modeled in some selected saladas obtaining coherent results and updating the hydrologic balance to know their hydrological behavior (Castañeda and García-Vera, 2004). Finally, the changes occurred in the saladas in the last decades were evaluated and their current status was studied using indexes for quantifying the filed observations and making systematic their study. All this information were included in a Geographic Information System, enabling to bind and contrast scattered observations, from different sources, some of them of high historical value (Castañeda et al., 2004a).


35

COMBINED EFFECTS OF GROUNDWATER AND AEOLIAN PROCESSES IN THE FORMATION OF THE CLOSED SALINE DEPRESSION THE EBRO BASIN Sánchez-Navarro, J.A. Dpto. Ciencias de la Tierra. Geomorfología. Facultad de Ciencias. Universidad de Zaragoza, 50009 Zaragoza. Introduction This paper explains the formation of the closed depressions of the Bujaraloz area, where both, groundwater flows and aeolian deflation processes interact. This model gives a satisfactory explanation for the origin of the closed depressions and their evolution into salt lake basins elaborated on low permeability substratum (detrital fine grained sedimentary rocks and evaporites). The large number of small water-bearing basins that exist in the study area allows the observation of all the intermediate stages of the proposed evolutionary model. Saline depressions formed on detrital-evaporitic materials In the centre of the Tertiary Ebro Depression, between the Sierra de A1cubierre in the North (700 m above sea level) and the Ebro River in the South (150 m above sea level), there is an extensive platform without superficial drainage to the Ebro River, 250 km2 in area, lying more than 200 m above this river bed which is known as the Monegros platform (Sanchez et al., 1989a). This surface hosts around 100 basins or closed depressions of up to several square kilometres. Some of them are permanent lakes in an area where the rainfall is low (300 mm/year), and with long dry periods which last several months, when there is often a total lack of rain (Sanchez et al., 1989b). The closed depressions are located on gypsum alternating with silts and limestone of the Bujaraloz Member of the Zaragoza Gypsum Formation, which have a total thickness of 80-100 m (Quirantes, 1978). Hydrogeologically, the whole area acts as a low permeability medium. Its saturation surface is close to, and sometimes coincides with, the surface of the ground, with steep hydraulic gradients that reflect the topography of the terrain in an attenuated form. Some of the depressions are funnel-shaped and resemble the “infiltration dolines” of karst landscapes, while others are real playa-lakes (Sánchez et al., 1998). There is a considerable variation in the chemical composition of the water in the small saline lakes, with mineralization varying from about 10 g/l to more than 300 g/l. These differences do exist between lakes, and also within every lake, depending on the volume of water in it. This is a consequence of the concentration of the water as a result of the evaporation (Pueyo, 1978-1979), and the discharge of groundwater flows that have covered long distances underground, with long residence time (Sanchez et al., 1989a). In Figure 33, the different evolutionary stages of the closed depressions are set out. Examples currently exist of the four situations shown. Initially (1), the meteoric water infiltrates through the most permeable areas (dissolution-enlarged fractures), dissolving the carbonates and gypsum. The water carrying dissolved salts penetrates down through the unsaturated zone until it reaches the water table, and is incorporated into the general groundwater flow. This process creates funnel-shaped dolines (2). The deepening of the depression continues until the topographic surface


36

reaches the water table. At this point the periodic flooding of the depression begins; in other words, a small ephemeral lake is formed.

Figure 33.-Evolutionary model of the saline depressions formed on detrital-evaporitic materials: 1.- Dissolution of gypsum and limestone by meteoric water. 2.- Formation of funnel-shaped closed depressions. 3.-The intersection of the saturated zone with the topographic surface, allows the evaporation to act, creating a gradient in the water table towards the lake. 4.-Evacuation of the salts by the wind. The exposure of the saturated area at the surface leads to higher evaporation, which creates a gradient in the water table towards the saline lake (3) (a situation that is similar to the cone of depression produced around a pumping well). This gradient in the water table towards the lake basin triggers the ascent of groundwater flows that prevent the infiltration of meteoric water, creating the situation known as “obstructed drainage”. Thus the water can not evacuate the existing salts and the ascending groundwater flows provide even more salts to the shallow lake. Subsequently, the evaporation of the water from the ephemeral lake leads to the accumulation of salts and organic matter in the lake bottom, forming the extremely flat surfaces that are typical of playa-lakes (4). The lake extends laterally, and increasingly deeper groundwater flows reach the lake. The only possible evacuation of the salts is by the wind, which carries away clouds of salt under dry conditions (Wood and Sanford, 1995). Saline depressions formed on detrital materials In the south-eastern sector of the Ebro Depression there are approximately 30 closed depressions, known as saladas, which constitute the saline depressions of Bajo Aragón. The saladas are formed on Lower Miocene deposits, made up of siltstones, alternating with small lenses of sandstone that have the form of isolated meandering palaeochannels among the siltstone facies. Hydrogeologically, the materials that constitute the surroundings of the saladas are characterized by their high porosity and their low permeability, which results in a very low hydraulic diffusivity, and, therefore, discharge from the water table never gives rise to springs, and it occurs always in a diffuse way. The Tertiary lithologies behave as low permeability mediums (aquitards) with saturation levels near and occasionally intersecting the surface, determined by the proximity of the Iberian Range, which is the source of the regional groundwater flow. The present situation


37

of these saline lakes is possibly similar to that which existed during the Miocene in a large part of the central Ebro Tertiary Basin, where a large lacustrine system like a playa-lake was formed (Salvany el al., 1994). The waters in the saladas are real brines, with average values of 80 g/l of dissolved salts, composed predominantly of sodium chloride and sulphate. These waters may reach 200 g/l of dissolved salts due to the increase in concentration by evaporation, and their sodium chloride becomes predominant. Figure 34 shows, in a diagrammatic manner, the origin and evolution of the saline depression under consideration.

Figure 34.-Evolutionary model of closed depressions formed on detrital rocks: 1.-The ascending groundwater flows brings about differential weathering of the materials. 2.-Erosion and transport of detrital materials by the wind. 3.-Precipitation of salts and their evacuation by the wind. In the initial situation (1), ascending groundwater flows or the infiltration of rainwater bring about differential weathering; the sandstone of the palaeochannels forms a non-deflationary coarse disintegration product like breadcrumbs, whereas the fine particles coming from the disintegration of the siltstone can be carried away by the wind. This leads to differential wind erosion (2), creating small, closed depressions. In these endorheic areas there is a greater accumulation of water, and therefore the weathering of the siltstone increases, and consequently so does its erosion by the wind. At the same time, the palaeochannels remain elevated and are scarcely weathered. Deepening to the water table by aeolian erosion leads to the formation of saline lakes. Finally (3), the accumulation of groundwater and surface water (rainwater) in the depressions, and its subsequent evaporation, leads to the precipitation of salts, and therefore the deepening of the basin ceases. The accumulation of salts, levels out the land, thus creating the typical flat salt plains. In these saline lakes the only evacuation agent of the salts is the wind.


38

Genetic model of the saline depressions It has been possible to establish a genetic and evolutionary model for saline lake basins in continental areas, based on the following basic conditions in the area: -The average annual rainfall (approx. 350 mm) is clearly lower than the average annual evaporation (approx. 1,400 mm). -There are no surface streams reaching the lake basin, or if there are any, their contribution is c1early lower than the average annual evaporation. -The saline lakes develop on low permeability geological media, or aquitards (K < 10 mm/day), with a wide surface extension and great depth (sedimentary basins). The low permeability of the materials implies that the water table is an attenuated replica of the ground surface and sometimes both surfaces coincide. As a result of the low permeability of the materials, the intersection of the water table does not give rise to springs, but rather to diffuse discharges and seepage, which, when they are widespread, prevent the infiltration of rain water, enabling waterlogging to occur. The evolution model takes into account four stages, which are those that appear in the diagrams in Figures 33 and 34, as follows: l. Beginning of the closed depression. Taking advantage of the most permeable areas, the meteoric water filters down, dissolving and evacuating soluble materials. In non-soluble materials, the water produces differential weathering for the sandstone and siltstones, resulting in non-deflationary coarse disintegration of sandstone, and fine disintegration of siltstone, which may be eroded by the wind. These processes lead to the formation of closed depressions. 2. Deepening. Dissolution by infiltrating water, or the aeolian deflation of the siltstone materials, evacuate material and therefore deepen the closed depression. Thus more and more water reaches the area, which increases the infiltration and the weathering, feeding the deepening of the depression. 3. Formation of the saline lake and end of the deepening. When the surface of the depression intersects the water table, the depression remains flooded for long periods of time. Exposure of the water table causes direct evaporation of groundwater, and, therefore, (in the same way as when water is pumped from a well) a cone of depression is formed in the water table, allowing a final slight deepening of the depression. The existence of this cone of depression (caused by direct evaporation) leads to the establishment of ascending groundwater flows towards the lake basin, which prevent the infiltration of meteoric water and also bring large quantities of dissolved salts to the lake. 4. Levelling and lateral extension. Evaporation of the groundwater charged with salts in the shallow lake leads to accumulation of these salts, forming a deposit that levels out the land, creating flat plains. This plain extends laterally, and the groundwater flows that reach the lake area are increasingly deeper. In this situation, the only agent able to evacuate for the salts is the wind, which in dry atmospheric conditions carries away clouds of salts.


39

Figure 35.-Water balance and material balance in the depressions and saline lake. A1.-Balance of water in closed depressions. A2.-Balance of water in saline lakes. B1.-Balance of materials in closed depressions. B2.-Balance of material in saline lakes. P: rainfall. E: evaporation. ET: evapotranspiration. I: infiltration of water and dissolved salts. G: flow of groundwater and dissolved salts. W: material evacuated by the wind. The four stages can be grouped into two phases; the first phase corresponds to the formation and evolution of the closed depressions (stages l and 2), and the second phase (stages 3 and 4) corresponds to the formation and evolution of the saline lakes. These two phases are characterized by having very different components, both in their water balance and in their material balance, (see Figure 35). Water balance: Thus, for the water balance in the first phase (closed depression), the main input is from rainfall (P), whereas the output is from evaporation (E), evapotranspiration (ET) (since plant life is possible because the area is not permanently flooded) and especially from infiltration. The balance of water for the second phase (saline lake) has both rainfall (P) and the discharge of groundwater (G) as inputs, whereas the only output is produced by evaporation (E); the saline lake is flooded for long periods of time and the ground is permanently saturated. Material balance: In the first phase (closed depression), there is no significant input of materials, whereas the output is through materials dissolved in the water that infiltrates (if the materials are soluble) or if insoluble material is carried away by the wind.


40

For the balance of materials in the second phase (saline lake), there is an important input of materials owing to the contribution of groundwater charged with salts (G), whereas the only output of materials is produced by the wind (W), which carries away the salts. Conclusions The combined effects of groundwater flows and aeolian action offer a coherent explanation for the origin-and evolution of the closed depressions found in the Ebro River Valley, and for the origin and evolution of the small saline lakes that form subsequently. The processes described tend towards practically identical morphologies (flat bottom and oval shape with the main axis parallel to the direction of the wind), although acting on geologically different materials; in one case gypsum and siltstone (with subordinate limestone), in the other case on siltstone and sandstone. We believe that the model under consideration is valid throughout the world for low permeability mediums and arid or subarid climatic conditions. In the proposed model, the phenomenon of evaporation is fundamental, since it is the basic determinant of the presence (or lack) of a free nappe in the lake basin, and also for the depression of the phreatic plane, which causes the inflow of groundwater towards the saline lake. The balance of water and salts for the different evolutionary phases of the depressions and saline lakes is particularly indicative of the processes that are taking place in them.


41

References

Arlegui, L. and Simón, J.L., 2001. Geometry and distribution of regional joint sets in a non-homogeneous stress field: case study in the Ebro basin (Spain). Journal of Structural Geology, 23, 293-313.

Arlegui, L.E. and Soriano, M.A., 1996. Lineamientos y su influencia en los modelados del centro de la Cuenca del Ebro. In: Grandal, A., Pagés, J. (Eds.). IV Reunión Nacional de Geomorfología. O Castro (Coruña), pp. 11-21.

Arlegui, L.E. and Soriano, M.A., 1998. Charaterizing lineaments from satellite images and field studies in the central Ebro basin. International Journal of Remote Sensing, 19, 3169-3185.

Ascaso, A. and Cuadrat, J.M., 1981. Geografía de Aragón: El Clima, Ed. Guara, vol. 1, Zaragoza, pp. 93-140.

Balsa, J., Guerrero, C., Pascual, M.L. and Montes, C., 1991. Las saladas de Bujaraloz-Sástago y las saladas de Chiprana: riqueza natural de Aragón. Empelte, 7. Grupo Cultural Caspolino. Caspe, Zaragoza.

Berga, A., 1993. Relaciones clima-agua-suelo-subsuelo en Monegros II. PhD Thesis, Universidad de Lérida, Spain, pp.392 + annexes.

Blackwelder, E., 1934. Yardangs. Geological Society America Bulletin, 45, 159-166.

Bowler, J.M. 1986. Spatial variability and hydrologic evolution of Australian lake basins: analogue for Pleistocene hydrologic change and evaporite formation, Palaeogeography, Palaeoclimatology, Palaeoecology, 54, 21 -42.

Bowler, J.M., 1973. Clay Dunes: Their occurrence, formation and environmental significance. Earth Science Reviews, 9, 315-338

Bowler, J.M., 1976. Aridity in Australia: Age, origins and expresion in aeolian landforms and sediments. Earth Science Review, 12, 279-310.

Breed, C.S., Mc Cauley, J.E., Whitney, M.I., Tehakarian, V.P. and Laity, J.E., 1997. Wind erosion in drylands. In: Thomas, D.S.G. (Ed.). Arid Zone Geomorphology. Process, Form and Change in Drylands. Wiley, Chichester, pp. 437-464.

Briere, P.R., 2000. Playa, playa lake, sebkha. Proposed definitions for old terms. Journal of Arid Environments, 45, 1-7.

Castañeda, C. and García-Vera, M.A., 2004. Balance de agua en medios semiáridos: aplicación a tres saladas de Monegros. In: Fernández Uría, A. (Ed.) Hidrogeología y Recursos Hidráulicos, Tomo XXVI: 205-215.

Castañeda, C. and Herrero, J., 2005. The water regime of the Monegros playa-lakes established from ground and satellite data. Journal of Hydrology, in press.

Castañeda, C. Asensio, M.A. and Herrero, J., 2004a. Quantifying visible land degradation of saline wetlands in a arid region of NE Spain. In: Faz, A., Ortiz, R. and Gracia, G. (Eds.) Extended abstracts, edited in compact disc ISBN 84-95781-40-9.

Castañeda, C., 2002. El agua de las saladas de Monegros sur estudiada con datos de campo y de satélite. Consejo de Protección de la Naturaleza en Aragón, Zaragoza, 158 pp.

Castañeda, C., Herrero, J. and Casterad, A., 2004b. La teledetección en la catalogación de las coberturas de las saladas de Monegros. Revista de la Asociación Española de Teledetección, 21, 29-34.

Castañeda, C., Herrero, J. and Casterad, A., 2005a. Devising facies within the playa-lakes of the Monegros Desert, Spain, with field and satellite data. Catena, in press.

Castañeda, C., Herrero, J. and Casterad, M.A., 2005b. Landsat monitoring of playa-lakes in the Spanish Monegros Desert. Journal of Arid Environments, in press.


42

Clarke, M.L., Wintle, A.G. and Lancaster, N. 1996. Infra-red stimulated luminescence dating of sands from the Cronese Basins, Mojave Desert. Geomorphology, 17, 199-205.

Comín, F.A. and Sanz, M.A., 1989. Limnología de las lagunas del polígono Monegros II. In: Pedrocchi, C. (Ed.) Evaluación preliminar del impacto ambiental de los regadíos en el polígono de Monegros II. Tomo II. Instituto Pirenaico de Ecología (CSIC). Funded by the Dirección General del Medio Ambiente, Ministerio de Obras Públicas y Urbanismo. Madrid.

Cooke, R., Warren, A. and Goudie, A., 1993. Desert Geomorphology, UCL Press, London, 526 pp.

Cooke, R.V.and Warren, A., 1973. Geomorphology in Deserts. Batsford, London, 394 pp.

Currey, D.R., 1994. Hemiarid lake basins: hydrographic patterns. In: Abrahams, A.D., Parsons, A.J. (Eds.), Geomorphology of Desert Environments, Chapman and Hall, London, pp.405-421.

Darton, N.H., 1920. White Sands. U.S.G.S. Bulletin, 697, 184-186.

Dregne, H.E., 1999. Pan. In: Mares, M.A. (Ed.), Encyclopedia of Deserts, University of Oklahoma Press, Norman, pp.499-500.

Eugster, H.P. and Hardie, L.A., 1978. Saline lakes. In: Lerman, A. (Ed.). Lakes: Chemisty, Geology, Physics, Springer-Verlag, Nueva York, pp. 237-293.

García Vera, M.A., 1996. Hidrogeología de Zonas Endorreicas en Climas Semiáridos. Aplicación a Los Monegros, Publicaciones del Consejo de Protección de la Naturaleza de Aragón, Zaragoza, 3, 297 pp.

Gibbons, W. and Moreno, T. (Eds.) 2002. The Geology of Spain. The Geological Society, 649 p. London

Goudie, A., Stokes, S., Cook, J., Samieh, S. and El-Rashidi, O.A., 1999. Yardangs landforms from

Kharga Oasis, south-western Egypt. Zeitschrift für Geomorphologie, Suppl.-Bd., 116, 97-112.

Goudie, A.S. and Thomas, D.S.G., 1985. Pans in Southern Africa with particular reference to South Africa and Zimbabwe. Zeitschrift für Geomorphology, 29, 1-19.

Goudie, A.S. and Wells, G.L., 1995. The nature, distribution and formation of pans in arid zones. Earth-Science Reviews, 38, 1-69.

Goudie, A.S., 1991. Pans. Progress in Physical Geography, 15, 221-237.

Goudie, A.S., 1999. Wind erosional landforms: Yardangs and Pans. In: Goudie, A.S., Livingstone, I. and Stokes, S. (Eds.). Aeolian Environments, Sediments and Landforms, Wiley, Chichester, pp. 167-180.

Goudie, A.S., 2004. Pans. In: Goudie, A.S. (Ed.), Encyclopedia of Geomorphology, Routledge, London, vol. 2, pp. 758-759.

Greeley, R. and Iversen, J.D., 1985. Wind as a geological process on Earth, Mars, Venus and Titan, Cambridge University Press, Cambridge, 333 pp.

Gutiérrez Elorza, M. 2001. Geomorfología Climática. Ed. Omega. Barcelona, 642 pp.

Gutiérrez Elorza, M., Desir, G. and Gutiérrez Santolalla, F., 2002. Yardangs in the semiarid central sector of the Ebro Depression (NE Spain). Geomorphology 44, 155-170.

Gutiérrez, F., Arauzo, T. and Desir, G., 1994. Deslizamientos en el escarpe en yesos de Alfajarín. Cuaternario y Geomorfología, 8, 57-69.

Gutiérrez, M. and Ibáñez, M.J. 1979. Las "gnammas" de la región de Alcañiz. Estudios Geológicos, 35, 193-198.

Halimov, M. and Fezer, F., 1989. Eight yardang types in Central Asia. Zeitschrift für Geomorphologie, 33, 205-217.


43

Hedin, S., 1903. Central Asia and Tibet, 2 vols., Charles Scribner and Sons, New York, 608 pp.

Herrero, J. and Snyder, R.L., 1997. Aridity and irrigation in Aragón, Spain. Journal of Arid Environments, 35, 55-547.

Ibáñez, M.J., 1973. Contribución al estudio del endorreísmo de la Depresión del Ebro: el foco endorreico al W y SW de Alcañiz (Teruel). Geográfica, 1,21-32

Ibáñez, M.J., 1975. El endorreismo del sector central de la depresión del Ebro. Cuadernos de Investigación, 1, 35-48. Logroño.

Ibáñez, M.J., 1976. El Piedemonte Ibérico bajoaragonés. Estudio Geomorfológico. Instituto de Geografía Aplicada, C.S.I.C., 523 p.

Killigrew, L.P. and Gilkes, R.J., 1974. Development of playa lakes in South Western Australia, Nature, 247, 454-455.

Laity, J.E., 1994. Landforms of aeolian erosion. In: Abrahams, A.D., Parsons, A.J. (Eds.). Geomorphology of Desert Environments, Chapman and Hall, London, pp. 506-537.

Laity, J.E., 1995. Wind abrasion and ventifact formation in California. In: Tchakerian, V.P. (Ed.). Desert Aeolian Processes, Chapman and Hall, London, pp. 295-321.

Liso, M. and Ascaso, A., 1969. Introducción al Estudio de la Evapotranspiración y Clasificación Climática de la Cuenca del Ebro, Anales de la Estación Experimental de Aula Dei, C.S.I.C., 10, Zaragoza, 505 pp.

Livingstone, I. and Warren, A., 1996. Aeolian Geomorphology: An Introduction. Longman, Harlow, Essex, 211 p.

Mainguet, M., 1968. Le Borkou, aspects d’un modelé éolien. Annales de Géographie, 77, 296-322.

Mainguet, M., 1972. Le Modelé des Grès. Problèmes généraux, Institute Géographique Nationale, Paris, 657 pp.

Maldonado, C., Gutiérrez-Santolalla, F., Gutiérrez-Elorza, M. and Desir, G., 2000. Distribución espacial, morfometría y actividad de la subsidencia por disolución de evaporitas en un campo de dolinas de colapso (valle del Ebro, Zaragoza). Cuaternario y Geomorfología, 14, 9-24.

Marshall, T.R. and Harmse, J.T., 1992. A review of the origin and propagation of pans. South African Geographer, 19, 9-21.

McCauley, J.F., 1973. Mariner 9 evidence for wind erosion in the equatorial and mid-latitude regions of Mars. Journal of Geophysical Research, 78, 4123-4137.

McCauley, J.F., Breed, C.S., El-Baz, F., Whitney, M.I., Grolier, M.J. and Ward, A.W., 1979. Pitted and fluted rocks in the Western Desert of Egypt-Viking comparisons. Journal of Geophysical Research, 84, pp. 8222-8232.

McCauley, J.F., Grolier, M.J. and Breed, C.S., 1977. Yardangs. In: Doehring, D.O. (Ed.). Geomorphology in arid regions, Allen & Unwin, London, pp. 233-269.

McKee, E.D., 1966. Structures of dunes at White Sands National Monument, New Mexico (and comparison with structures of dunes from other selected areas). Sedimentology, 71, 1-69.

Neal, J.T., 1975. Playa surface features as indicators of environment. In: Neal, J.T. (Ed.), Playas and Dried lakes, Benchmark Papers in Geology. Halsted Press, Strousbourg, pp.363-388.

Pedrocchi, C. (Coordinador), 1998. Ecología de Los Monegros. Instituto de Estudios Altoaragoneses, Huesca, 430 pp.

Pérez, A., Luzón, A., Roc, A.C., Soria, A.R., Mayayo, M.J. and Sánchez, J.A., 2002. Sedimentary facies distribution and genesis of a recent carbonate-rich saline lake: Gallocanta Lake, Iberian Chain, NE Spain. Sedimentary Geology, 148, 185-202.

Pueyo, J.J., 1978/79. La precipitación evaporítica actual en las lagunas saladas del área: Bujaraloz, Sástago, Caspe, Alcañiz y Calanda (provincias de


44

Zaragoza y Teruel). Revista del Instituto de Investigaciones Geológicas, 33, 5-56. Barcelona.

Pueyo, J.J., 1980. Procesos diagenéticos observados en las lagunas tipo playa de la zona de Bujaraloz-Sástago (provincias de Zaragoza y Teruel). Revista del Instituto de Investigaciones Geológicas, 34, 195-207. Barcelona.

Puircercús, J.A., Valero, A., Navarro, J., Terrén, R., Zubiaur, R., Martín, F. and Iniesta, G., 1994. Atlas Eólico de Aragón. Gobierno de Aragón, Zaragoza, 127 pp.

Pye, K. and Tsoar, H., 1990. Aeolian Sand and Sand Deposits, Unwin Hyman, London, 396 pp.

Quirantes, J., 1965. Nota sobre las lagunas de Bujaraloz-Sástago. Geographica, 12, 30-34.

Quirantes, J., 1978. Estudio Sedimentológico y Estratigráfico del Terciario Continental de los Monegros, Institución Fernando el Católico, C.S.I.C. Zaragoza, 200 pp.

Ramírez, J.I., 1997. Mapa Geológico de España a Escala 1:50.000, Gelsa (nº 413), Instituto Tecnológico Geominero de España, Madrid.

Riba, O., Reguant, S. and Villena, J. 1967. Nota preliminar sobre la sedimentación en paleocanales terciarios de la zona de Caspe-Chiprana. Anales de Edafología y Agrobiología, 26, 617-134

Riba, O., Reguant, S. and Villena, J. 1983. Ensayo de síntesis estratigráfica y evolutiva de la Cuenca Terciaria del Ebro. Libro Jubilar J.M: Ríos, IGME, Madrid, 2, 131-159.

Richards. L.A. 1954. Saline and alkaline soils. United States Department of Agriculture. Agriculture Handbook, 60, 160 pp.

Rosen, M.R., 1991. Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, USA. Paleogeography, Palaeoclimatology, Palaeoecology, 84: 229-257.

Rosen, M.R., 1994. The importance of groundwater in playas: a review of playa classifications and the sedimentology and hydrology of playas. In: Rosen,

M.R. (Ed.), Paleoclimate and Basin Evolution of Playas Systems. Geological Society of America, Special Paper, 189, pp.2-18.

Salvany, J.M., García Vera, M.A. and Samper, J., 1994. Influencia del sustrato terciario en el emplazamiento del complejo lagunar de Bujalaroz (Monegros, Cuenca del Ebro). Actas II Congreso Español del Terciario, Jaca, pp. 271-274.

Salvany, J.M., García Vera, M.A. and Samper, J., 1996. Geología e hidrogeología de la zona endorreica de Bujaraloz-Sástogo (Los Monegros, provincia de Zaragoza y Huesca). Acta Geológica Hispánica, 30, 31-50.

Salvany, J.M., Muñoz, A. and Pérez, A., 1994b. Nonmarine evaporitic sedimentation and associated diagenetic processes of the south-western margin of the Ebro Basin (Lower Miocene), Spain. Journal Sedimentary Research, A64, 190-203.

Samper-Calvete, F.J. and García-Vera, M.A., 1998. Inverse modelling of groundwater flow in the semiarid evaporitic closed basin of Los Monegros, Spain. Hydrogeology Journal, 6, 33-49.

Sánchez, J. A., Martínez, F. J, De Miguel, J. L., and San Román, J. 1989a. Hidrogeoquímica de la zona endorreica de las lagunas de Monegros, provincias de Zaragoza y Huesca, Boletín Geológico y Minero, 100, 876-885.

Sánchez, J. A., Martínez, F. J., De Miguel, J. L., and San Román, J. 1989b. Algunos planteamientos básicos para la previsión del impacto ambiental de los regadíos proyectados en el área de las lagunas de Monegros, VIII Conferencia sobre Hidrología General y Aplicada. Febrero 1989. Zaragoza, pp. 67-81.

Sánchez, J.A., Pérez, A., Coloma, P. and Martínez Gil, J., 1998. Combined effects of groundwater and aeolian processes in the formation of the northernmost closed saline depressions of Europe: north-east Spain. Hydrological Processes, 12, 813-820.

Schütt, B. and Baumhauer, R., 1996. Playa sedimente aus dem Zentralen Ebrobecken/Spanien als Indikatoren für holozäne klimaschwankungen-ein


45

vorlaüfige Bericht, Petermans Geograpische Mitteilungen, 140, 33-42.

Sharp, R.P., 1969. Wind–driven sand in Coachella Valley, California. Geological Society of America Bulletin, 75, 785-804.

Shaw, P.A. and Thomas, D.S.G., 1997. Pans, playas and salt lakes. In: Thomas, D.S.G. (Ed.), Arid Zone Geomorphology. Process, Form and Change in Drylands, Wiley, Chichester, pp.293-317.

Sherard, J.L.; Decker, R.S. and Ryker, N.L. 1972. Piping in earth dams of dispersive clays. Proceedings ASCE Special Conference on Permomance of Earth and Earth-supported Structures, 1, 589-626.

Smoot, J.P. and Lowenstein, T.K., 1991. Depositional environments of non marine evaporates. In: Melvin, J.L. (ed.), Evaporites, petroleum, and mineral resources. Developments in Sedimentology, 50, 189-347.

Solá, J. and Costa, J.M., 1997. Mapa Geológico de España a Escala 1:50.000, Bujaraloz (nº 414), Instituto Tecnológico Geominero de España, Madrid.

Tchakerian, V.P., 1999a. Playa. In: Mares, M.A. (Ed.), Encyclopedia of Deserts. University of Oklahoma Press. Norman, pp.443-444.

Tchakerian, V.P., 1999b. Sabkha. In: Mares, M.A. (Ed.), Encyclopedia of Deserts, University of Oklahoma Press. Norman, p. 485.

Torgerssen, T., De Decker, P., Chivas, A.R., Bowler, J.M., 1986. Salt lakes: A discussion of processes influencing palaeoenvironmental interpretation and recommendations for future study. Palaeogeograpy, Palaeoclimatology, Palaeoecology, 54, 7-19.

Tricart, J. 1954. Influence de soils salés sur la déflation éolienne en Basse Mauritanie et dans le Delta du Sénégal. Révue de Geomorphologie Dynamique, 5, 124-132

Tricart, J. 1969. Le Modelé des Régions Sèches. S.E.D.E.S, 472 p. Paris.

Twidale, C.R. and Corbin, E.M. 1963. Gnammas. Revue de Géomorphologie Dynamique, 14, 1-20.

Vera, J.A. (Ed.) 2004. Geología de España. SGE-IGME, Madrid, 890 p.

Ward, A.W. and Greeley, R., 1984. Evolution of the yardangs at Rogers Lake, California. Geological Society of America Bulletin, 95, 829-837.

Ward, A.W., 1979. Yardangs on Mars: Evidence of recent wind erosion. Journal of Geophysical Research, 84, 8147-8166.

Whitney, M.I. 1978. The role of vorticity in developing lineation by wind erosion. Geological Society of America Bulletin, 89, 1-18.

Whitney, M.I., 1983. Eolian features shaped by aerodynamic and vorticity processes. In: Brookfield, M.E. and Ahlbrandt, T.S. (Eds.). Eolian Sediments and Processes, Elsevier, Amsterdam, pp. 223-245.

Whitney, M.I., 1985. Yardangs. Journal of Geological Education, 33, 93-96.

Wood, W. and Sanford, W. 1995. Eolian transport, saline lake basin, and groundwater solutes, Water Resources Research, 31, 3121-3129

Yechieli, Y. and Wood, W.W. 2002. Hydrogeologic processes in saline system: playas, sabkhas and saline lakes. Geomorphology, 58, 343-365.


46

Road Log

Departure from the Conference Hall at 9.00 h

9.00-10.15 h A-2 (E-90) Motorway from Zaragoza up to Bujaraloz Exit (Exit 3) (73 km). Afterwards we will follow the A-230 road for 6 km up to the junction with the regional road which goes to La Playa Lake. From this junction there are 8 km to La Playa Lake western margin (stop 1).

10.15-12.45 h Stop 1: 2 hours and a half of slow walk along the northern border of La Playa Lake up to El Pueyo Lake. The hike goes through a horizontal topography and it will not pose difficulties to those people with regular physical conditions. In the first part we will see desiccation micromorphologies such as algal mats (Fig. 11), blisters, mud cracks (Fig. 12) and salt polygons (Fig. 13). Later on, we will pass next to a possible yardang developed on rubbish materials (Fig. 19). When we get to the eastern margin of La Playa Lake we will walk up to the T1 lacustrine terrace level which divides la Playa and El Pueyo saline lakes (Fig. 8). From this surface, the yardangs developed on the Miocene evaporitic substratum in the southeastern margin of La Playa are already visible. Afterwards we will walk down El Pueyo Lake where it is possible to observe a yardang developed on the T2 lacustrine terrace deposits. At this point we will turn back to the coach.

12.45-13.00 h 15 minutes drive (20 km) along a regional road to the 2nd stop (stop 2)

13.00-13.15 h Stop 2: Brief stop in a viewpoint to see the tight meanders drawn by the Ebro River in the surroundings of Sastago village.

13.15-14.00 h A-221 road for 32 km up to the town of Caspe and afterwards N-211 for 29 km up to Alcañiz. The coach will leave us in the Alcañiz National Parador for lunch.

14.00-16.00 h Lunch

16.00-16.15 h N-232 for approximately 3 km up to the junction of this road with the track which goes to La Salada Grande saline Lake. We will follow the track by coach for several hundreds of meters up to the margin of La Salada Grande saline Lake (stop 3).

16.15-16.45 h Stop 3: Visit to the margin of La Salada saline Lake and its surroundings where it will be possible to observe small features developed on the sandstone layers such as gnammas (Figs. 22 and 23), tafonis and honeycombs.

16.45-18.00 h A-232 for 101 km up to the Zaragoza Conference Hall.


47

SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY Bujaraloz... · 2014. 6. 13. · SIXTH...

Documents

Transcript of SIXTH INTERNATIONAL CONFERENCE ON GEOMORPHOLOGY Bujaraloz... · 2014. 6. 13. · SIXTH...