The role of salt weathering in the origin of the Qattara Depression,

Western Desert, Egypt

M.A.M. Aref a,*, E. El-Khoriby b, M.A. Hamdan a

aGeology Department, Faculty of Science, Cairo University, Giza, EgyptbGeology Department, Faculty of Science, Mansoura University, Mansoura, Egypt

Received 2 January 2001; received in revised form 20 August 2001; accepted 28 September 2001

Abstract

Field studies and petrographic examinations of core samples and of the bedrock of the floor of the Qattara Depression,

Egypt, indicate that salt weathering predominates in its western part in marked contrast to its eastern part. The eastern part of the

depression is covered with more than 120-cm-thick, moist sands with sporadic occurrence of halite and gypsum due to the low

salinity of the groundwater table. At the western part of the depression, the strongly saline, sodium chloride nature of the

groundwater table favors crystallization of halite (and sometimes gypsum) at or near the surface of the outcropping bedrock of

the Moghra clastics and/or Dabaa shale. Crystallization of halite and/or gypsum generates increased pressure that leads to

mechanical disintegration of the bedrock into fine-grained debris. Features related to disintegration include blistering of the rock

surface, splitting, spalling and/or granular disintegration. Alternation of dry and wet cycles favor halite crystallization,

mechanical disintegration of the outcropping bedrock and dissolution of the halite cement, which exposes fine-grained debris to

wind deflation. Removal of the debris from the floor of the depression leads to the accumulation of lunettes and other dunes in

the downwind direction. Therefore, salt weathering provides fine-grained debris that are easily removed by deflation, which

accounts for the topographically lower level of the western part of the depression (134 m below sea level (b.s.l.)). In contrast,

the presence of moist sediments at the eastern part of the depression inhibits deflation and encourages sedimentation by

adhesion of wind-blown sand to the damp surface of the sabkha at an elevation of 45 m below sea level. The disintegration of

the bedrock of the Qattara Depression by salt weathering has been in effect since the onset of aridity in northern Egypt in

Quaternary time. Whereas the initial excavation of the depression started in Late Miocene or Pliocene time by fluvial erosion,

karstic process, mass-wasting and wind deflation. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Halite; Crystallization; Disintegration; Excavation; Qattara Depression; Egypt

1. Introduction

Salt weathering is a potent agent of rock disinte-

gration by salts that have accumulated at or near the

rock surface in coastal, urban, polar and arid environ-

ments. Important indications of the power of salt

weathering come from field observations (Chapman,

1980); field monitoring (Goudie and Watson, 1984;

Goudie et al., 1997) and laboratory simulations (Gou-

die and Viles, 1995; Goudie and Parker, 1998), and

from studies of the decay of ancient structures

(Bromblet and Bocquier, 1985) and of modern engi-

neering structures (Doornkamp et al., 1980).

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0169 -555X(01 )00152 -0

* Corresponding author.

E-mail address: m1aref@yahoo.com (M.A.M. Aref ).

www.elsevier.com/locate/geomorph

Geomorphology 45 (2002) 181–195

The Qattara Depression in the north Western Desert

of Egypt, which is the largest, natural closed depres-

sion of the eastern Sahara is a region where salt

weathering appears to be particularly effective (Fig.

1). The origin of this depression is still a geological

puzzle (Albritton et al., 1990). A common origin by

wind deflation to a base level controlled by the ground-

water table has been the generally accepted explana-

tion (Ball, 1927, 1933; Squyres and Bradley, 1964).

Other explanations include solution, mass-wasting

followed by wind deflation (Said, 1960), or the depre-

ssion was originally excavated as a stream valley,

subsequently modified by karstic activity, and was

further deepened and extended by mass-wasting,

deflation and fluviatile processes (Albritton et al.,

1990). It has been recently suggested that the depres-

sion is of structural control origin (Gindy, 1991).

Although most studies have noted the presence of an

extensive sheet of sabkha that covers the floor of the

Qattara Depression, a salt-weathering origin for the

Qattara Depression had not been proposed.

This paper describes the results of field studies of

evaporite sediments in recent sabkhas and in scattered,

isolated hills at elevations of 25, 75 and 100 m below

sea level (b.s.l.) in the Qattara Depression and petro-

graphic descriptions of core samples. The purpose of

the study was to assess the rate and nature of weath-

ering in this environment. We propose that rapid

erosion of the depression is favored by the high

sodium chloride content of the near surface ground-

water, combined with alternating wetting and drying

cycles caused by sporadic groundwater seepage and

occasional rainfall and evaporation.

2. Topography

The Qattara Depression forms one of the most

significant morphological features of the north West-

ern Desert of Egypt (Fig. 1A and B). The depression

is a closed inland basin that is bounded from the north

and west by steep escarpments, with an average

elevation of about 200 m above sea level (a.s.l.).

Towards the south and east the floor of the depression

rises gradually from 60 m b.s.l. to the general desert

level at 200 m a.s.l. (Fig. 1A). The depression has an

area of some 19500 km2 and an average depth of 60 m

b.s.l. The lowest point in the depression is 134 m

b.s.l., which lies at the southwestern part of the

depression. The depression is estimated to have an

excavated volume of 3200 km3 (Peel, 1966). Within

the depression, cones, towers, mushrooms and pla-

teau-like hills, ranging in height from 5 to 30 m, are

common, especially, near the western scarp of the

depression. Sinkholes and caves are common in the

northern Diffa Plateau. The northern and western

escarpments are dominated by large mass-wasted

blocks.

3. Geology

The Qattara Depression is excavated into northerly

dipping Miocene and Eocene rocks (Fig. 1B and C).

Sandy and clayey layers of the Lower Miocene

Moghra Formation form the bottom and the surround-

ings of the depression (Fig. 1B), where the elevation

ranges from 50 to 80 m b.s.l. In some areas, the

Moghra sediments occur as small plateau and dis-

sected hills within the sabkhas. Middle Eocene calca-

reous sediments of the Mokattam Formation form the

southern scarp of the depression. The Upper Eocene–

Oligocene Dabaa Formation underlies the southwest-

ern part of the depression, including all areas below

100 m b.s.l. It consists of black shales and contains

abundant gypsum veins and shark bones and teeth.

The northern steep escarpment is associated with the

Middle Miocene calcareous sediments of the Marmar-

ica Formation, with a thickness of a few meters at the

rim of the depression, increasing to several hundred

meters at the coast, where Pliocene carbonate rocks

are exposed.

Over large areas of the floor, the bedrock is

covered by younger deposits including wind-blown

sand, sabkhas and Quaternary evaporite sediments.

The sands that cover most of the depression are

associated with moist sand sheets with adhesion

ripples at the surface in the northeastern part of the

depression and large parallel, longitudinal, lunette,

seif and complex dune belts in the southern part of

the depression. The dune axes trend north–north-

west–south–southeast, parallel with the prevailing

wind direction. The dunes are composed mostly of

quartz sand mixed with minor carbonate, mud, shale

and gypsum fragments. Near the southwestern part of

the depression, the dunes are black (named as El

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195182

Fig. 1. (A) Topography of the Qattara Depression (after Ball, 1933). (B) Geologic map of the Qattara Depression (after El Bassyony, 1990). (C)

Geologic cross-section of the Qattara Depression (after Ball, 1933; El Bassyony, 1990).

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 183

Ghorood El Sood, El Bassyony, 1990), due to their

high content of black shale fragments derived from

the Dabaa Formation.

The sabkha sediments also cover large areas of the

floor and lower slopes of the depression (about 5800

km2 according to Ball, 1933), generally occurring at

or below the elevation of 50 m b.s.l. At the northeast-

ern part of the depression, the sabkha is commonly

moist, and consists of loose wind-blown sand and silt

that contains sporadic halite and gypsum crystals. In

the western and southwestern part of the depression,

the sabkha sediments are wet or dry in many places

and have a rough granular salt crust that grows within

the bedrock of the Moghra and Dabaa Formations.

The Quaternary evaporite sediments cover isolated,

scattered hills, 5–30 m in height, at the lower slopes

of the western scarp as well as isolated plateau at the

ground elevations 25, 75 and 100 m b.s.l. in the

southwestern part of the depression. The hills are

scattered around the wet and dry sabkhas and consist

of the clastic facies of the Pliocene/Pleistocene Kalakh

Formation or the Lower Miocene Moghra Formation

or the shaley facies of the Upper Eocene–Oligocene

Dabaa Formation. These facies are encrusted with an

up to 50 cm thick, hard, indurated gypsum/anhydrite

crust or contain up to 30 cm in size, gypsum/anhydrite

nodules near the top.

4. Climate

The climate of the Qattara Depression is arid. The

average temperature ranges from 36.2 �C during sum-

mer months June, July and August, to 6.2 �C during

winter month January. Annual precipitation ranges

from 25 to 50 mm on the northern rim to less than 25

mm in the southern part of the depression, although

torrential winter rains have been reported (El Ramly,

1967). The humidity value ranges from 71% during

January and December, to 17% during April, May and

June. The average annual evaporation rate is 330 mm

(El Bassyony, 1995), and the daily evaporation rate

ranges from 2.8 to 5.5 mm. This exceeds the average

daily rainfall of 0.06 mm (Ball, 1933). The prevailing

wind is generally northerly varies at times from north

easterly to north westerly (Hamdan and Refaat, 1999).

The wind speed reaches a maximum of 11.5 m/s in

March and a minimum of 3.2 m/s in December.

This strongly cyclic pattern between dry summer

and relatively wet winter months accounts for evapor-

ite minerals deposition in summer months and partial

dissolution of these evaporites in winter months.

Therefore, these data reflect the generally climatic

conditions under which salt weathering is now pro-

ceeding in the Qattara Depression.

5. Hydrogeology and hydrochemistry

Since the Qattara Depression forms the deepest

point in the Western Desert of Egypt, groundwater

flow in all aquifers bordering this area is consequently

directed to this final base level (Qattara Project

Authority, 1979). Most of the groundwater that evap-

orates in the depression comes from the Moghra

Aquifer system which is recharged from four sources;

the Nubian sandstone aquifer in the south, Nile water

in the east, saline water from the Mediterranean Sea to

the north, and rain water. In contrast, in the western

part of the depression, groundwater seepage is re-

charged from Nubian and Upper Cretaceous–Eocene

aquifer systems (El Bassyony, 1995).

The Moghra aquifer water is of the sodium chloride

type (Qattara Project Authority, 1979), it increases in

salinity from a relatively fresh water zone (1650 ppm

chlorides) at a depth of 1000 m to a relatively saline

water zone (100,000 ppm) at the depth 2200 m (Ezzat,

1974). The estimated amount of groundwater flow to

the depression is 3.2 m3/s, while the total evaporation

from the depression is 7.2 m3/s (Qattara Project

Authority, 1979). Upon evaporation, the groundwater

seepage to the Qattara Depression increases in salinity.

The near surface groundwater ranges in salinity from

3.3 g/l around the Moghra Lake at the east, to 38.4 g/l

at the center to about 300 g/l in the sabkha area to the

west. An exception to this east–west increase in

salinity is found around fresh water springs such as

at Bir Qifar area. El Bassyony (1995) interpreted the

east–west increases in salinity from 43.6 to 421.0 g/l

to the leaching of salts by surface water and ground-

water, and due to excessive evaporation of ground-

water at or slightly below the surface in the lowest part

of the depression.

Results of the chemical analyses performed by the

Qattara Project Authority (1979) and El Bassyony

(1995) of the near surface groundwater in the Qattara

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195184

Depression are shown (Fig. 2). This data can be used

to determine the origin of groundwater. The data show

that most of the water samples are of the chloride type

(MgCl2 and CaCl2) of marine origin. A few samples

are, usually, of the NaHCO3 and Na2SO4 types of

meteoric origin. This indicates either the large influ-

ence of original sea-water invasion, or the dissolution

of the Moghra aquifer water of salts from the host

rocks or preexisting salts. The westward increase in

salinity of the chloride-type groundwater allows

excessive crystallization of a near-surface thick halite

crust in the western part, in contrast with the eastern

part of the depression.

6. Evaporite sediments and salt weathering

location

Evaporite sediments cover large areas of the floor

and lower slopes of the depression. Detailed classi-

fication of these evaporites based on the evaporation

rate has been elaborated by Qattara Project Authority

(1979) and El Bassyony (1995), and on the basis of

the dominant depositional environment has been done

by Aref and Hamdan (1998). For the purpose of

finding out the relation between evaporite sediments

and salt weathering, the evaporite sediments are sub-

divided into three types (1, 2 and 3) based on their

relative age and ground elevation, in relation to the

groundwater level. Distribution and impact of these

types on salt weathering differ between the eastern

and western parts of the Qattara Depression.

At the eastern part of the depression, the level of

the sabkha area is flat, and decreases gradually from

about 45 m b.s.l. in the east to about 80 m b.s.l. in the

center of the depression. The general slope of the

sabkha surface is conformable to the general ground-

water gradient (Ball, 1933; El Bassyony, 1995). No

intervening Miocene or younger hills intersect the

monotonous flat sabkha surface. This sabkha (type

Fig. 2. Sulin graph representing the water genesis in the Qattara Depression (chemical data from the Qattara Project Authority, 1979; El Bassyony,

1990).

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 185

3) is recorded as receptacles of aeolian sediments that

consists of about 120 cm thick, medium to coarse

grained, moist sand (Fig. 3A). Growth of evaporite

minerals in the sabkha area is very sporadic or rare,

due to the low salinity of the near surface water table.

Whenever halite crystals grow within the aeolian

Fig. 3. (A) Adhesion ripple on the surface of moist sand. (B) Disintegration of the Moghra clastics by displacive growth of irregular nodular

gypsum/anhydrite. (C) A pavement surface of nodular anhydrite at the surface of the ‘‘Salt Plateau’’ area, with cone-like hills of the Moghra

clastics. (D) Dry, indurated type 2 halite crust that forms tepee polygonal structure. (E) Dark zone of outcropped Dabaa shale mixed with halite

crust in wet sabkha sediment of type 3. (F) Crenulated and crater-like blistered halite crust on the sediment surface of the Dabaa shale. M:

Moghra clastics, G: Gypsum/anhydrite nodules, Sh: Dabaa shale H: Halite.

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195186

sand, it is readily dissolved by groundwater seepage.

Therefore, the mechanism of salt weathering is not in

effect in the eastern part of the depression.

At the western part of the depression, evaporite

sediments cover some patches of the floor and lower

slopes between 25 and 100 m b.s.l. Three types of

evaporite sediments are well developed, easily sepa-

rated and their impact on salt weathering can be

observed. The outcropping bedrock (Moghra or

Dabaa Formations) beneath the sabkha surface suffer

a rapid breakdown by salt weathering due to the high

rate of evaporation of the highly saline groundwater.

Type 1 evaporite sediment is the oldest that repre-

sents the earliest record of the Quaternary aridity in

the Qattara Depression. It is recorded overlying iso-

lated, 5–30 m high hills, of the Pliocene/Pleistocene

Kalakh Formation and the Lower Miocene Moghra

Formation at the western slope of the Qattara Depres-

sion. It is recorded at levels 25 m b.s.l. at Talh

Fawakheir, Bir Abdel Nabi and Qara Oasis, at 75 m

b.s.l. at Bir Hussain area, and at 100 m b.s.l. at the

‘‘Salt Plateau’’ area, near the deepest point of the

depression (Fig. 1). The evaporite sediments are

present as: (1) random, isolated or dense, 2–15 cm

evaporite nodules that grow displacively within the

top part of the Moghra clastics (Fig. 3B). Differential

erosion has removed the clastic material of the

Moghra Formation leaving a positive relief of the

evaporite nodules (Fig. 3B). (2) Dense evaporite

nodules that form thin, 20-cm-thick crust, that caps

a mesa-like plateau (Fig. 3C) at 100 m b.s.l., extend-

ing E–W for about 35 km and averages 6 km across

near the southwestern margin of the depression (Fig.

1B). The cap-like nature of the evaporite nodules

represents a lag or a pavement surface that was

formed after removal of the friable sand and clay,

and concentration of the indurated evaporite nodules

over or within the Miocene clastics. Type 1 evaporite

sediment is regarded as an erosional remnant of a

sabkha deposit that was formed at a time when the

floor of the depression stood 5–30 m higher than at

present. This type also represents the former level of

the groundwater table which may coincide with the

arid episodes of the Quaternary (Haynes, 1980), or

may have been controlled by the groundwater dis-

charge pattern during the Quaternary.

Type 2 evaporite sediment is recorded as dry,

indurated rough sabkha surface (Fig. 3D) that extends

for hundreds of meters around the terraces of type 1. It

represents a previous stage of lowering of the ground-

water table, since the sabkha surface has no connec-

tion with the present groundwater table. It consists of

either gypsum/anhydrite or halite crusts, 7–20 cm in

thickness, that form tepee polygonal structure with

warped up margins of about 50 cm in height (Fig.

3D). The dry sabkha surface may be white when

dominated with gypsum/anhydrite or halite, or may

be reddish, yellowish, greenish or blackish when the

bedrock of the Moghra sandstone or Dabaa shale have

been exposed to the surface.

Type 3 evaporite sediment is recorded at levels

lower than types 1 and 2 evaporite sediments, as wet,

rough sabkha surface (Fig. 3E) that also extends for

hundreds of meters. It represents the last stage of a

lowering groundwater table, where the sabkha surface

is recorded in the capillary evaporation zone of near-

surface groundwater. The water table is 30–70 cm

below the sabkha surface. It consists dominantly of a

halite crust ( < 30 cm in thickness) that forms buckling

or tepee polygonal structures. Underneath these struc-

tures is a vuggy zone, which is floored with numerous

fragments of sandstone and shale. The color of the wet

sabkha surface, similar to type 2 evaporite sediments,

varies from whitish, reddish, yellowish, greenish or

blackish according to the lithology of the underlying

bedrock.

The process of salt weathering is clearly observed

when the bedrock of the Moghra clastics or Dabaa

shale has been displaced by growth of evaporite

sediments of types 1, 2 and 3. This feature is observed

only in the western part, where great excavations

occur within the floor of the depression, compared

to the eastern part. The evaporite sediments show

features such as blistering, splitting, spalling or gran-

ular disintegration on the surface of the outcropping

bedrock, which gradually grades downward into

undisturbed and fresh bedrock.

7. Nature and texture of debris liberation

Whenever bedrock exposures, impregnated with

gypsum/anhydrite or halite, are examined, clear signs

of salt weathering have been identified. The bedrock

shows extensive development of a whole range of

weathering phenomena such as blisters on the rock

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 187

surface, as well as splitting, spalling and granular

disintegration.

Blistering of gypsum or halite (Fig. 3F) is observed

on the wet sabkha surface of type 3 that is charac-

terized by the presence of fine granules, rock flour,

thin and thick flakes clinging to the sabkha surface.

Blistering crusts are less than 1 cm in thickness and

range from 4 cm to about 15 cm across. The crusts

may form a domal closed structure, or may be a crater-

like open structure (Fig. 3F). They are formed due to

surface evaporation of groundwater seepage highly

enriched with calcium sulfate or sodium chloride.

The morphology of the liberated debris that is

produced by splitting, spalling or granular disintegra-

tion is appeared to be influenced by the original

petrological characteristics of the bedrock. Whereas

the sandstone of theMoghra Formation produces debris

composed almost entirely of floating quartz grains or

aggregates of quartz grains formed by granular disinte-

gration. Shale and carbonate rocks of the Dabaa For-

mation produce large angular fragments as well as a

large amount of very fine angular matrix produced by

splitting and spalling. The granular and angular texture

ofmuch of the liberated debris suggests that mechanical

breakdown of the rock surface by evaporite minerals,

mainly sodium chloride took place, similar to the

processes described by Goudie and Viles (1995).

Megascopic examination of the sabkha surface of

types 2 and 3 indicates initial splitting of the shale

layer parallel to primary lamination and cracking of

the shale normal to, or inclined to the lamination,

leading to the formation of angular fragments (Fig.

4A). All of these fractures are a consequence of

precipitation and growth of halite crystals, which lead

to mechanical disintegration of the shale. Clasts in the

debris produced floating in the halite cement are less

than 3 cm in length (Fig. 4A). The dispersion pattern

and morphology of the shale debris have been con-

trolled by the location of initial planes of rupture as

well as the stress direction produced by growth of

halite crystals. It is observed that the size of the clasts

in the debris generally decreases upwards. The lower

part of the halite crust encloses numerous vugs (Fig.

4A) that are produced by dissolution of halite cement

during wetting. At the surface of the halite crust, halite

cement appears to ‘plug’ near-surface pores by surface

crystallization of halite crystals, encouraging surface

sealing. The ‘plugged’ halite crust is observed only in

type 2 dry sabkha surface which is formed during dry

episodes and lowering of the water table. Plugging of

the halite crust has reduced the evaporation rate of the

sabkha surface, a feature that was observed by the

Qattara Project Authority (1979) and El Bassyony

(1995). Beneath the halite crust, a cavity separating

the bedrock from the buckled halite crust is floored

with numerous flakes of shale fragments that are very

similar to those dispersed on the sabkha surface and in

the nearby dune belts.

Microscopic examination of types 2 and 3 sabkha

sediments shows three methods of mechanical disin-

tegration of the host rock. In the first, initial splitting

of shale parallel to lamination took place at the margin

by the growth of clear halite cement (Fig. 4B). After

initial splitting of shale, continuous growth of clear

halite cement produces spalling of shale flakes to form

a tepee-like structure (Fig. 4B). With continuous

growth of halite crystals within the fissility planes

and cracks of shale, different morphologies of shale

fragments are produced, such as rhombic, rectangular,

triangular or irregular forms (Fig. 4C). The morphol-

ogy of shale fragments and their floating pattern and

direction in the halite cement are controlled by the

stress direction produced by halite growth. It ranges

from small bending of shale fragments (Fig. 4B), to

randomly floating fragments (Fig. 4C), to radial

pattern of shale debris in halite cement (Fig. 4D).

Besides the splitting and spalling, producing me-

chanical disintegration of shale by halite growth,

lenticular and nodular gypsum may also grow after

halite (Fig. 4E), which leads to further disintegration

and sporadic distribution of the shale debris. The

ability of the growing gypsum crystals to disintegrate

shale is very limited, compared with the disintegration

produced by halite growth (Fig. 4E).

Growth of halite crystals in the coarse clastic

sediment of the Moghra Formation leads to its break-

down by granular disintegration. The resultant debris

is composed of either single quartz grains (Fig. 4F), or

aggregates of quartz and glauconite grains, cemented

by argillaceous or carbonate cement (Fig. 5A). Con-

tinuous growth of halite crystals leads to separation of

quartz grains from the cementing material (Fig. 4F).

Fracturing of the sandy quartz grains by salt growth

are not observed in the studied samples, in contrast

with the observation made by Goudie et al. (1979).

This is probably related to the low matrix strength and

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195188

high porosity of the Moghra clastics, which favor the

separation of quartz grains rather than their fracturing

by salt growth.

The evaporite nodules of type 1 evaporite sedi-

ments, and the gypsum/anhydrite crust of type 2 dry

sabkha sediment are composed essentially of pris-

Fig. 4. (A) Disintegration of shale into angular, irregular fragments due to halite growth. Irregular vugs resulted from dissolution of halite. (B)

Initial splitting and spalling of shale due to crystallization pressure of halite. Plane Light. (C) Disintegration of shale into rhombic and triangular

fragments due to crystallization pressure of halite. Plane light. (D) Radial pattern of shale fragments indicates the stress direction of halite

growth. Plane light. (E) Growth of nodular gypsum/anhydrite lead to the dispersion of shale fragments that are originally broken down by halite

growth. Plane light. (F) Breakdown of the Moghra clastics into individual quartz grains by crystallization pressure of halite. Plane light. G:

Gypsum/anhydrite nodules, Sh: Dabaa shale, H: Halite, Q: Quartz grains, C: Carbonate fragment.

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 189

matic, epigenetic anhydrite crystals that are replacing

clear gypsum crystals. The anhydrite crystals are

aggregated to form a random texture, or are aligned

into preferred orientation. The growth of gypsum

(now anhydrite) also has the ability to disintegrate

the Moghra clastics by granular disintegration (Fig.

5B), or splitting of the carbonate rocks (Fig. 5C). The

anhydrite crystals are observed to corrode and replace

Fig. 5. (A) Breakdown of the Moghra clastics into aggregates of quartz and glauconite by crystallization pressure of halite. Plane light. (B)

Displacive growth of nodular gypsum/anhydrite lead to breakdown of the Moghra clastics. Polars crossed. (C) Breakdown of carbonate by

growth of gypsum/anhydrite. Polars crossed. (D) Corrosion of quartz grains by anhydrite crystals. Polars crossed. (E) Intense vegetation cover

around Moghra Lake inhibits aeolian deflation. (F) Wind-blown quartz grains cemented with clear halite cement. Plane light. M: Moghra

clastics, Q: Quartz grains, An: Anhydrite crystals, C: Carbonate fragments, H: Halite.

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195190

the quartz grains (Fig. 5D). The high crystalline and

durable nature of the anhydrite nodules will allow

them to survive erosion, whereas the relatively friable

Moghra sands are removed by deflation (Fig. 3B).

Therefore, the anhydrite nodules are aggregated after

the removal of the sand to form a pavement of

anhydrite nodules at the ‘Salt Plateau’ area (Fig. 3C).

8. Petrography of halite and gypsum

Evaporite minerals, mainly halite and gypsum,

grow below the sediment surface in brine-soaked

sediments of the Kalakh, Moghra and Dabaa Forma-

tions. The concentrated brines have diffused upwards

from shallow groundwater seepage. Precipitation

would occur at the level where halite or gypsum

becomes supersaturated. The dominant process of

growth of halite and gypsum is by displacive growth,

where the framework materials of the sediment are

pushed aside by the force of crystallization, leading to

the disintegration of the host rock.

Halite and gypsum crystals that nucleate from

brine-soaked sediment range in size from a few

microns up to several millimeters, they are inequigra-

nular anhedral to subhedral clear crystals that enclose

fragments from the host rock within or at crystal

boundaries (Fig. 4). Growth directions of halite and

gypsum crystals are deduced from the dispersion

pattern of the fragments. The free growth of chevron,

cornet and rafted halite, or laminated, prismatic and

arrow-head gypsum in a salina or lagoon, are not

observed in zones of disintegration of the host rock.

This is taken to indicate that disintegration of the

bedrock took place due to displacive growth of halite

and gypsum below the sabkha surface within brine-

soaked sediments.

9. Discussion

9.1. Groundwater

Several authors stressed that the role of groundwater

as an agent influencing the geomorphology of the

Qattara Depression is limited mainly to its impact as

a base level for the maximum extent of aeolian defla-

tion (Ball, 1933; Squyres and Bradley, 1964). How-

ever, the geomorphological impact of groundwater on

basin enlargement and excavation by salt weathering

was disregarded. Similar to the hypothesis of Goudie

andWells (1995), the present authors believe that, since

the onset of Quaternary aridity in Egypt, the excavation

and enlargement of the Qattara Depression have re-

sulted largely due to salt weathering and aeolian

deflation. The groundwater table controlled the depth

of deflation, and contributed to the salt weathering

processes. Groundwater is believed to have played

different roles in the eastern and western parts of the

depression in concern.

In the eastern part, the low salinity of the near

surface groundwater table (stokes surface, Stokes,

1968) acts as a factor limiting aeolian deflation in

three ways. The first, as described by Stokes (1968),

arises from the high cohesion of moist sand in

proximity to the water table, primarily as a result of

increasing the intergranular surface tension. The moist

or wet sand is less easily entrapped by the wind and

can assist in the formation of adhesion ripples (Fig.

3A) due to wind sculpturing. The second influence is

the establishment of a permanent vegetation cover

(Fig. 5E), such as around the Moghra Lake, which

acts as a base level for wind erosion. The third

influence is through the sporadic cementation of loose

sands (Fig. 5F) by precipitation of halite which acts as

a base limit to aeolian deflation and creation of

extensive planar surfaces. Therefore, the low salinity

nature of the groundwater has limited aeolian defla-

tion and deepening of the eastern part in comparison

to the western part of the depression. The presence of

moist sediments in the east Qattara inhibits deflation

and encourages sedimentation by the adhesion of

wind-blown sands to the damp surface of the sabkha

(Fig. 3A).

On the other hand, the high saline, sodium chlor-

ide-enriched groundwater at the western part of the

Qattara Depression favors the formation of a salt crust

on the sabkha surface (Fig. 3D and E). Where the

bedrock is close to, or outcrops on the sabkha surface,

the high rate of evaporation favors interstitial crystal-

lization of halite and rapid breakdown by salt weath-

ering (Fig. 4). The ingress of more saline water by

pumping evaporation leads to more precipitation of

halite which ‘plug’ near surface pores and increases

the rate of breakdown of the bedrock. This hard salt

crust also acts as a base level for deflation unless the

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 191

halite crust is dissolved by an outflow of less saline

groundwater or rainwater during the wet period. After

dissolution of halite, the liberated debris is highly

susceptible to wind deflation. Therefore, the cyclic

pattern of dry and wet periods allows the disintegra-

tion of the bedrock, dissolution of halite cement and

the removal of the susceptible sediments by deflation.

9.2. Causes of rock breakdown

The possible causes of the observed breakdown of

the bedrock do not include frost weathering, as the

Qattara Depression lies in the hot, arid zone of north

Africa. It is likely that karst dissolutional attack

caused the rapid rate of erosion of the Marmarica

carbonates during humid climates of the Quaternary

(Glennie, 1970; Haynes, 1980). Whereas during the

arid phases of the Quaternary, crystallization of halite

near the ground surface led to its high disintegration

by salt weathering. Since sodium chloride, which

cannot be hydrated, is the abundant salt, with sporadic

occurrence of gypsum, it is obvious that most of the

stresses in the salt impregnated bedrock would be

caused by the crystallization of halite and/or gypsum

which leads to breakdown of the bedrock.

9.3. Mechanism of salt weathering

The most cited cause of mechanical weathering is

by salt crystal growth from solutions in rock pores and

cracks because of the substantial pressure that may be

exerted by the growing crystals (Winkler and Singer,

1972). Of the natural salts, sulfates (Na2SO4, MgSO4)

exert the highest pressure, and are more effective than

sodium carbonates. They are also more effective than

halite and gypsum in causing rock breakdown (Gou-

die, 1994). The groundwater in the Qattara Depression

is of the sodium chloride type. Evaporational concen-

tration of this water leads to the precipitation of the

highly soluble salt halite which generates a crystal-

lization pressure ranging from 554 to 3737 kbar/cm3,

when precipitating over a range of temperature

between 0 and 50 �C (Winkler and Singer, 1972).

Such crystallization pressure exceeds the strength of

almost all rocks (Bloom, 1998). On the other hand,

precipitation of gypsum at a temperature range

between 0 and 50 �C generates crystallization pressure

of 282 to 1900 kbar/cm3 (Winkler and Singer, 1972).

Therefore, the pressures generated from the crystal-

lization of halite and gypsum are responsible for the

observed disintegration of the Moghra and Dabaa

sediments (Figs. 4 and 5A,B and C).

9.4. Time of salt weathering

The time and the processes of the initial excavation

of the Qattara Depression have been the subject of

mush controversy for a long time. Said (1960)

believed that the initial dissolution of the Marmarica

limestone cap rock started in post-Middle Miocene

time, followed by wind erosion and mass-wasting.

Albritton et al. (1990) proposed that the initial exca-

vation of the depression started during the Late

Miocene time by fluvial erosion, followed by karstic

processes, wind deflation and mass-wasting. Ball

(1927, 1933) mentioned that the depression was

excavated by wind erosion in Pleistocene and post-

Pleistocene time.

Despite the different processes and times of the

initial excavation of the Qattara Depression, we sug-

gest that the depression is further deepened by salt

weathering when: (1) The above-mentioned processes

excavate the bedrock of the depression to a level that

meets the groundwater base level of erosion. (2) The

onset of aridity in northern Egypt in the Quaternary

time. During the wet periods of the Quaternary,

groundwater and rainfall resulted in the rise of the

water table, which fed the inland sabkhas. During the

dry periods of the Quaternary, evaporation processes

predominated, where salt weathering disintegrated the

bedrock of the Moghra or Dabaa Formations. Also,

the wind deflated and lowered the surface of the

depression. It seems that the Qattara Depression was

subjected to at least three alternating long wet–dry

phases in the Quaternary, which coincided with the

formation of the evaporites at the elevation 25, 75 and

100 m b.s.l. In addition to these aridity phases, the

present-day aridity allows the continuation of the salt

weathering mechanism and deflation of the depression

to the present time.

9.5. Deflation

The proposed hypothesis of wind as a powerful

agent of excavation of large closed depressions of the

Western Desert of Egypt has been widely accepted

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195192

(Ball, 1927, 1933; Squyres and Bradley, 1964; Glennie,

1970; Haynes, 1980). Despite the different interpreta-

tions regarding the origin of the Qattara Depression,

there is a general agreement on a strong aeolian over-

print on all assumed mechanisms. The estimated exca-

vated volume of the depression is 3200 km2 (Peel,

1966), and the presumed aeolian denudation rate in the

last 2 million years is 9 cm/1000 years, which is

impressively comparable to reported rates of fluvial

conditions (Bloom, 1998). The effective deflation

process in the Qattara Depression is reinforced by the

lack of a protective vegetation cover and the suscept-

Fig. 6. Schematic block diagram showing the role of salt weathering in the late excavation history of the origin of the Qattara Depression.

M.A.M. Aref et al. / Geomorphology 45 (2002) 181–195 193

ibility of sediments derived from salt weathering to

wind removal, similar to that described by Haynes

(1982). Similar to the criteria mentioned by Goudie

and Thomas (1985), the following evidence indicates

the role of deflation in the origin of the Qattara

Depression:

1. TheQattaraDepression has an elongated kidney

shape, transverse to the prevailing winds.

2. The presence of a relatively steep cliff on the

upwind side and a more gentle slope on the

downwind side.

3. The occurrence of crescentic dunes (lunettes)

and other dunes on the downwind side.

4. Grain composition of the dunes is closely

similar to that of the nearby bedrock.

10. Conclusions

Although salt weathering was first recognized

more than a hundred years ago, prior to this study it

was not perceived in the Qattara Depression as a

weathering mechanism of great importance. Probably

this can be attributed to two reasons: (1) a tendency of

most authors to overemphasize the importance of

deflation in rock destruction, and (2) their failure to

recognize the characteristics of weathered rock surfa-

ces that are diagnostic of salt weathering. It seems

that the high salinity and the sodium chloride nature

of the groundwater table and the high rate of evapo-

ration lead to a highly aggressive nature of the

extensive sabkha in the western part of the Qattara

Depression (Fig. 6). Crystallization of halite at or near

the surface of the outcropping bedrock generates

increased pressures, which disintegrate the bedrock.

Dissolution of halite during the wet episode leaves

fine-grained debris that is easily removed during dry

episodes by the strong northerly winds. The winds

carry the fine-grained debris to form a downwind

extensive dune fields of composition very similar to

that of the nearby Moghra clastics and Dabaa shale.

Therefore, the salt weathering mechanism provides

fine-grained debris that are easily removed by defla-

tion, with the groundwater table both controlling the

base level of deflation and contributing to salt for-

mation, with its weathering disintegrating mechanism.

In contrast, the eastern part of the depression (Fig. 6),

where there is low salinity-groundwater table, salt

weathering is not effective and the deflation process is

relatively inhibited due to the high cohesion of moist

sediment, a permanent vegetation cover and precip-

itation of halite cement between the wind-blown

sandy sediment.

The existence of the evaporite sediments overlying

the Pliocene/Pleistocene Kalakh Formation, the Lower

Miocene Moghra Formation and the Upper Eocene–

Oligocene Dabaa Formation in the western part of the

Qattara Depression at elevation 25, 75 and 100 m b.s.l.

indicates: (1) former floors of the Qattara Depression

that stood higher than present. These floors are com-

posed of sabkha sediments similar to the modern

sabkhas. (2) These evaporite sediments probably coin-

cide with Quaternary aridity phases in northern Egypt.

(3) The salt weathering mechanism is more effective as

a disintegration mechanism at the elevations 25, 75 and

100m b.s.l. in the western part of the depression, where

remnants of evaporite sediments still exist.

Acknowledgements

We wish to thank the journal reviewer Prof. Leslie

McFadden and an anonymous reviewer as well as the

journal editor Prof. Adrian Harvey for their construc-

tive comments and improvement of our language. We

also thank Prof. G. Philip and Prof. Z. Zaghloul for their

reading of the early draft of the manuscript. The first

author is grateful to Prof. Andrew Goudie who

provided him with his relevant articles.

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