A study of erosion rates on salt diapir surfaces in the Zagros Mountains, SE Iran

ORIGINAL ARTICLE

A study of erosion rates on salt diapir surfacesin the Zagros Mountains, SE Iran

J. Bruthans Æ N. Asadi Æ M. Filippi Æ Z. Vilhelm ÆM. Zare

Received: 20 May 2006 / Accepted: 6 March 2007 / Published online: 12 April 2007

� Springer-Verlag 2007

Abstract Salt exposures and weathering residuum on

several salt diapirs in different geographic/climatic settings

were studied. Anhydrite, gypsum, hematite, calcite, dolo-

mite, quartz, and clay minerals are the main constituents of

the weathering residuum covering the salt diapirs in vari-

ous thicknesses. Erosion rates of residuum as well as of

rock salt exposures were measured at selected sites for a

period of 5 years by plastic pegs as benchmarks. Recorded

data were standardized to a horizontal surface and to long-

term mean precipitation. For the rock salt exposures the

following long-term denudation rates were determined of

30–40 mm a–1 for coastal diapirs and up to 120 mm a–1 for

mountain salt diapirs. Long-term mean superficial denu-

dation rate measured on weathering residuum of low

thickness reached 3.5 mm a–1 on coastal diapirs. The total

denudation rate estimated for the thin residuum is close to

4–7 mm a–1 based on apparent correlation with the uplift

rate on Hormoz and Namakdan diapirs. Denudation of rock

salt exposures is much faster compared to parts of diapirs

covered by weathering residuum. The extent of salt expo-

sures is an important factor in the morphological evolution

of salt diapirs as it can inhibit further expansion of the

diapir. Salt exposures produce huge amounts of dissolved

and clastic load, thus affecting the surrounding of the

diapir.

Keywords Salt diapir � Weathering residuum � Erosion

rate � Rillenkarren � Zagros Mountains

Introduction

Denudation rates in different catchments on a salt diapir

were studied by Frumkin (1994) on Mt. Sedom, Israel,

where precipitation reaches 50 mm a–1. The calculated

solutional denudation rate was about 0.5–0.75 mm a–1.

Frumkin (1994) demonstrated that a thin film of water

could reach saturation with respect to NaCl along a path

several tens of meters long over a wall of rock salt.

Mottershead et al. (2005) recently measured erosion rate on

a salt exposure in the Cardona diapir, Spain with a

reflectorless total station. They observed erosion rates of up

to 20 mm per 100 mm of rainfall. Rillenkarren as a typical

karst form covering salt exposures were studied by

Mottershead et al. (2000). Glew and Ford (1980) and Ford

and Lundberg (1987) simulated evolution of gypsum and

salt Rillenkarren and explained their development.

Halite, the main constituent of the studied salt diapirs in

SE Iran, is highly soluble. At 25�C, the solubility of halite

is 360 g l–1. Assuming NaCl density of 2.165 g cm–3,

100 mm of rain water can dissolve 17 mm column of salt

(Talbot and Jarvis 1984). Denudation rate can therefore

potentially reach 17% of the annual precipitation total. The

actual denudation rate could be much lower as the rock salt

on diapirs in SE Iran is commonly covered by several

meters to several tens of decimeters thick weathering

J. Bruthans (&) � Z. Vilhelm

Faculty of Science, Department of Hydrogeology,

Engineering Geology and Applied Geophysics,

Charles University in Prague, Albertov 6,

128 43 Praha 2, Czech Republic

e-mail: [email protected]

N. Asadi � M. Zare

Department of Earth Sciences,

Shiraz University, Shiraz, Iran

M. Filippi

Institute of Geology, Academy of Sciences

of the Czech Republic, Rozvojova 269,

165 00 Praha 6, Czech Republic

123

Environ Geol (2008) 53:1079–1089

DOI 10.1007/s00254-007-0734-6

residuum and other sedimentary material, which is covered

by the terms ‘‘cap rock’’ or ‘‘gypcrust’’ in some previous

works (e.g., Bosak et al. 1998; Bruthans et al. 2000). There

are no measurements available of erosion rates on salt

diapirs in SE Iran. The main objectives of this paper are:

(1) to determine the erosion rate on salt exposures and

weathering residuum by direct measurement, (2) to com-

pare the erosion rates on various materials of surface cover

and (3) to estimate the long-term denudation rates of diapir

surfaces made up of different materials. Short comment on

the environmental impact of salt exposures on surroundings

of the diapirs is provided as well.

Geography and geology

The study area is in the southern part of the Islamic

Republic of Iran, in the eastern part of the Zagros Mountain

Range and the Persian Gulf Platform (Fig. 1). The Zagros

Fold Belt is composed of elongated whaleback or box-

shaped anticlinal mountains, which generally trend NW-

SE. The relief is relatively young; the principal folding

started in the Middle Pliocene. About 200 salt diapirs are

known in the Persian Gulf area (Kent 1970). The diameters

of salt diapirs usually vary between about 1 and 15 km

(Kent 1958). Rock salt belongs to the Hormoz Complex,

which was deposited during Upper Precambrian (Riphean-

Vendian) to Middle Cambrian times in extensive evaporitic

basins. Beds of salts and other evaporites (mainly gypsum

and anhydrite) alternate with carbonate, siliciclastic and

volcanosedimentary layers, which now form blocks and

impurities. Salt diapirs are found at many sites differing in

altitude (from sea level to more than 2,000 m a.s.l.) and

consequently in climates (precipitation between 170 mm a–1

on coastal diapirs and 550 mm a–1 at 1,750 m a.s.l.). The

following salt diapirs were visited and studied in the

field: Hormoz, Namakdan, Larak, Khurgu (Bandar Abbas

region), Jahani (near Firoozabad) and Namak (near Busher).

Detailed descriptions of the individual diapirs can be found

in Bosak et al. (1998) and in Talbot and Alavi (1996). Salt

diapirs in SE Iran are covered by various types of deposits

(Bosak et al. 1998).

Precipitation records were obtained from meteorological

stations that are close to diapirs at similar altitudes

(Table 1). The total of precipitation at Jahani in a 5-year

period (March 2000 to April 2005) was 1,474 mm, which

is nearly three times higher than the ca 500 mm of rain at

coastal salt diapirs over the same period. This is due to the

higher altitude and more northwestern location of the

Jahani diapir. Based on the data from Bandar Abbas

observatory, the precipitation reached only about 60% of a

long-term average (1961–1990) during the 5-year study

period (Table 1). The first part (March 2000 to February

2004) was extremely dry (45% of mean rainfall), while in

the second part (February 2004 to April 2005), the average

precipitation was above the long-term average (119%).

Precipitation in the SE Zagros Mountains occurs between

October and March, with a maximum between December

and February. Commonly, rains are of an intense nature (up

to 60 mm day–1 during the study period).

Methods

The extent of salt exposures and areas covered by weath-

ering residuum was determined from aerial images at a

Me hran river

Gowdar river

Kul riv

er

Kandrestan Bandar Abbas

Pe

rs

i

an

g u l f

KhamirQeshm

IR

AN

Namakdan salt diapir

Hormozsalt diapir

Hormoz Island

Qeshm Island Larak Island

Hengam IslandN

25 km

Afghanistan

Turkmenistan

Pakistan

I r a n

Saudi Arabia

Iraq

Oman

P ersia n gulf

Caspian

sea

Kuwait

Unated ArabEmirates

Turkey

Za g r o s

Mt s.

Tehran

12

1

2

Shur river

Kuh-e Jahanisalt diapir

Firoozabad

N

Kuh-e Khurgu salt diapir

3

3

Salt diapirSymbol: 10 km10 km

N

Kuh-e Namaksalt diapir

Mand

river

Fig. 1 Location of study areas

1080 Environ Geol (2008) 53:1079–1089

123

scale of 1:25,000 in combination with field mapping on

Namakdan, Hormoz and Jahani salt diapirs. Topographical

maps 1:25,000 were used for mapping.

A number of X-ray diffraction analyses (XRD) were

performed to identify individual minerals that form the

sedimentary material. The XRD analyses were carried out

using a PHILIPS X¢Pert PW 3710 diffractograph under the

following conditions: CuKa radiation, graphite secondary

monochromator, 35–40 kV, 30 mA, step scanning at 0.01�and 0.05� per 3 s in the range of 2�–60� or 2�–90� 2h.

Semi-quantitative bulk chemical composition was deter-

mined by X-ray fluorescence (XRF). XRF analysis data

were acquired with an ARL 9400 XP + spectrometer.

Finely milled samples of 0.8 g were compacted into

tablets and measured under the following conditions:

voltage 20–60 kV, probe current 40–80 mA, effective

area 490.6 mm2. UniQuant software was used for data

evaluation.

Erosion rates of weathering residuum and rock salt

exposures were measured directly by plastic pegs that were

fixed at selected sites for a period of 5 years as bench-

marks. A similar technique was used by Frumkin and Ford

(1995) in salt caves at Mt. Sedom in Israel for the mea-

surement of erosion rates of a cave stream channel.

Cylindrical pegs with flat tops and sharp tips, 1 cm in

diameter and 11 cm long were used. On rock salt expo-

sures, the plastic pegs were cemented into drilled holes.

The pegs were installed approximately perpendicular to the

exposed surface of salt. In the case of weathering residuum,

the pegs were impressed vertically to a mark imprinted on

each peg at a height of 23 mm below its flat top. Plastic

pegs were installed in salt and weathering residuum during

a dry period of the year 2000 (March and October). The

distance from the top of the peg to the surface of the salt

exposure was measured by calliper gauge with precision

better than 1 mm. Exposure inclination was measured by

inclinometer with precision of 1�. Because there is a var-

iation of the inclination of the slope even in close prox-

imity to many pegs, a range of values rather than a single

number is indicated. Close-up and broader views of the

plastic pegs were photographed during their installation in

2000 and during subsequent visits of the sites in 2004 and

2005 to determine any changes in Rillenkarren morphol-

ogy. Erosion at the pegs was first measured after 4 years

and then again 1 year later. Some new plastic pegs were

introduced in 2004. In this paper, the term ‘‘denudation’’ is

used only for denudation in a vertical direction. The term

‘‘erosion’’ is used in a more general sense for reduction of

salt mass or sedimentary cover regardless of the inclination

of the different surfaces.

On salt exposures, the plastic pegs were not placed

vertically, but perpendicular to the surface. Hence, the

denudation must be calculated from the erosion measured

at plastic pegs according to Eq. 1:

Dv ¼ R ðcos AÞ�1 ð1Þ

where Dv is (vertical) denudation (in mm), R is slope-

normal erosion measured at plastic pegs (in mm), A is the

inclination of exposure (in degrees) in relation to a hori-

zontal plane.

The measured values of denudation were finally stan-

dardized to the long-term precipitation average. For in-

stance, the denudation was multiplied by the coefficient of

2.2 for the period of 2000–2004 (45% of long-term pre-

cipitation amount).

Table 1 Precipitation totals recorded by observatories close to individual diapirs

Position of station with respect

to individual salt diapirs

Weather station

Bandar Abbas

18 km NW

from Hormoz

Compared to 30-years’

average (%)

(Bandar Abbas)

Queshm airport

51 km NE

from Namakdan

Dehrood

11 km W

from Jahani

Station altitude (m a.s.l.) 10 – 6 880

30-years’ average (1961–1990) (mm) 171 100 – –

9-years’ average (3.96–3.05) (mm) 155 91 136 –

Period 2000–2004 (mm) 310 45 283 1106

Period 2004–2005 (mm) 203 119 184 368a

Total 2000–2005 (mm) 513 60 467 1474

Annual number of rain days 2000–2005 16 – 14 20b

Source Iranian Meteorological Organization, Fars and Hormozgan Regional Water Organizations, 30-year average is based on WMO Clima-

tological normals (CLINO) for the period 1961–1990 (WMO 2006)

– Data not availablea Measured directly on Jahani diapirb Period 2000–2004

Environ Geol (2008) 53:1079–1089 1081

123

Results and discussion

Salt exposures

Based on the analysis of detailed aerial images in combi-

nation with field mapping, the extent of salt exposures is

about 5% for Jahani and less than 1% for Namakdan and

Hormoz. The rest of the diapir surfaces are formed by

weathering residuum.

The rock salt consisted mainly of halite. Even samples

of a macroscopically impure salt had a NaCl content of

about 96% with a minor admixture of other minerals

(Table 2). Characteristics of grain size and relative purity

of individual samples are presented in Table 3. Fine-

grained salt was restricted only to the Jahani diapir. Bare

rock salt occurred only on steep slopes (>50�) of valleys

and hills and at the bottom of some sinkholes, where

insoluble residuum originating from dissolved rock salt

was repeatedly washed out (Bruthans et al. 2000). The only

exception was the Namak diapir, where the salt exposures

were common also on more gradual slopes. This is prob-

ably due to the collecting of large amounts of rainwater on

extensive salt exposures. Strong surface runoff conse-

quently washes out the fine residuum.

The height of salt exposures was usually expressed in

meters or low tens of meters. The surface of salt exposures

was typically covered by sharp Rillenkarren and dissected

by deep gullies (Bruthans et al. 2000). Rillenkarren were

missing on steeper slopes (over 85�—pegs HD1, NB3,

JA5), which were smooth. Rillenkarren were missing or

were strongly reduced in lower parts of longer salt slopes,

where smooth surfaces drained by gullies occurred. This is

Table 2 Chemical and

mineralogical composition of

impure salt and weathering

residuum based on XRF and

XRD analysis

a Calculated as element

Peg location/oxide

(wt.%)

JB HF3 HF5 HF13

Na2O 40.1 3.7 5.7 0.2

MgO 0.6 3.4 3.5 3.7

Al2O3 0.2 6.1 6.2 7.2

SiO2 0.6 25.8 24.8 24.8

SO3 0.5 20.8 19.8 24.3

Cla 56.2 3.7 5.4 0.2

K2O 0.1 1.2 1.2 1.4

CaO 1.7 18.3 17.0 19.5

Fe2O3 0.0 16.2 15.6 17.9

Mineralogical

composition

(listed based on

relative quantity)

Halite,

anhydrite,

gypsum,

calcite,

dolomite

Anhydrite, gypsum,

quartz, hematite,

dolomite, calcite,

clay minerals

Gypsum, anhydrite,

quartz, hematite,

dolomite, calcite,

clay minerals

Gypsum, anhydrite,

hematite,

dolomite, calcite,

clay minerals

Table 3 Position and

characterization of individual

sites with installed plastic pegs

for measurements of erosion

rate

Diapir, peg

location

Latitude Longitude Altitude

(m a. s. l.)

Annual precipitation

in years

2000–2005 (mm)

Sample characterisation

Hormoz, HB 27�3¢30.5¢¢ 56�25¢43.5¢¢ 20 103 Pure salt; coarse

grained

(over 10 mm)

Hormoz, HC 27�2¢50.6¢¢ 56�28¢5.3¢¢ 70 103 Anhydrite + gypsum block

Hormoz, HD 27�3¢47.6¢¢ 56�26¢13.4¢¢ 40 103 Pure salt; coarse

grained 15 mm)

Hormoz, HF 27�4¢20.3¢¢ 56�27¢23.0¢¢ 150 103 Weathering residuum

Namakdan, NA 26�37¢6.9¢ 55�31¢0.0¢¢ 30 93 Impure salt;

grains (3–8 mm)

Jahani, JA 28�36¢50.6¢¢ 52�29¢10.1¢¢ 1230 294 Impure salt; fine grained

(1–2 mm)

Jahani, JB 28�37¢21.2¢¢ 52�29¢10.5¢¢ 1150 294 Impure salt; fine grained

(1–3 mm)

Jahani, JC 28�37¢29.4¢¢ 52�29¢13.7¢¢ 1070 294 Pure salt; coarse grained

(6–15 mm)

1082 Environ Geol (2008) 53:1079–1089

123

probably the result of a film of water, as even a minor rain

quickly wets the salt exposure and creates superficial run-

off, as the infiltration capacity of cracks in salt is com-

monly very low. The film gradually gains in thickness in

downslope direction and, from a certain point onward it

can hardly be penetrated by falling raindrops. This inhibits

the development of Rillenkarren (Glew and Ford 1980).

Salt Rillenkarren had sharp edges and smooth rounded

floors (Fig. 2). Inclination of Rillenkarren commonly lay

between 40� and 85�. The depth of rills noticeably in-

creased inversely with the Rillenkarren inclination (Fig. 3).

Edges were very sharp in the case of pure coarse-grained

salt (in best developed examples). They were less perfectly

developed in fine-grained impure salt. Rillenkarren evolu-

tion can be seen in the series of successive photographs of

one of the peg sites that were photographed in 2000, 2004

and 2005. It is apparent that the geometry of edges and rills

in a ground plan remains preserved over the time, whereas

the form itself is propagating downwards (Fig. 2).

Weathering residuum

Few decimeter-thick reddish semi-consolidated residuum

(gypcrust), overlies part of the coastal diapirs (Table 2,

samples HF). On Hormoz, it covers the central part of the

diapir (37% of diapir area), on Namakdan only small area

close to SE margin of diapir (1–2% of diapir surface). It is

a result of current subaerial salt dissolution. The area was

covered by large uvalas with many sinkholes (Bruthans

et al. 2000). Thin residuum is not covered by vegetation

and hence not protected against erosion. Earth pinnacles

several-centimeters high cover the surface at some places.

Water easily infiltrates into this crust. A large proportion of

rainwater captured in the residuum is therefore lost by

evaporation.

Most of the surface of Namakdan, Jahani and many

other salt diapirs in SE Zagros is covered by weathering

residuum over 5 m in thickness. The residuum is composed

mainly of gypsum, anhydrite, calcite, quartz and dolomite

based on XRD and XRF analyses. It originated by long-

term subaerial weathering (mountain salt diapirs) of rock

salt or/and by its submarine dissolution and sedimentation

from intermittent streams (Namakdan diapir). Weathering

residuum more than 5-m thick supports the presence of

vegetation cover because it has an ability to store water

even for long dry periods, which may last several years on

coastal diapirs. The presence of vegetation promotes

evapotranspiration, and its roots protect the superficial zone

of soil against erosion. The vegetation is relatively abun-

dant especially on mountain salt diapirs (Fig. 4). The sur-

face of some salt diapirs covered by thick residuum is used

for grazing sheep and is even temporarily inhabited by

native people (Jahani diapir). Water infiltrates very easily,

especially on surfaces covered by grass.

Erosion and denudation rates on salt exposures

Erosion rates on salt exposures were measured by 26

plastic pegs, fixed in the salt exposures of three salt diapirs:

Hormoz and Namakdan in an arid climate and Jahani in a

more humid region. To examine different types of the rock

salt material, several sites were selected on each salt diapir.

If present, places with various inclinations of salt exposures

were selected. Exposure of localities is similar; they are

found on steep sides of valleys or hills.

The measured erosion rate is shown in Table 4. Wide

differences in erosion rates were observed at individual

pegs (between 1 and 45 mm a–1). Erosion rates were also

Fig. 2 Two photographs of peg

HB3 cemented into Rillenkarren

exposure on the Hormoz diapir

taken at a 5-year interval: a15.3.2000; b 21.4.2005

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90

o

depth

of karr

en r

ill (c

m)

outcrop inclination ( )

Fig. 3 Depth of Rillenkarren as a function of exposure inclination.

Data were measured on a block of pure coarse-grained salt on the

Jahani diapir. The depth of rill was measured from karren crests to

karren rill bottom perpendicularly to dip of karren crests. The depth of

rill decreases with greater inclination of exposure surface

Environ Geol (2008) 53:1079–1089 1083

123

observed to change over time (cf. locations HB3, JA5), due

to the downward migration of individual Rillenkarren

edges (Fig. 2).

The following two factors are most important for rock

salt erosion:

1. Erosion rate is inversely proportional to the inclination

of salt exposures (compare erosion at HD2 with HD4

and NA3 with NA4 pegs). This is explained by the fact

that subhorizontal exposures obtain the highest amount

of rainwater per unit area of surface, as the rain is

mainly subvertical. Conversely, subvertical exposures

get smaller amount of rainwater, which is spread over

a relatively larger surface of the exposure with rela-

tively more salt to dissolve.

2. Erosion rate through time is directly proportional to the

precipitation total (Jahani vs. coastal salt diapirs).

The purity of salt has probably only a minor effect on

the erosion rate. However, even the ‘‘impure’’ salt con-

tained only a very small amount of non-halite material

(sample JB, Table 2).

To filter out the effect of the inclination of the exposure,

the slope normal erosion rate was recalculated to vertical

denudation rate by Eq. 1. This calculation exaggerates the

uncertainty in measurements on steeper exposures (>75�),

as the coefficient (cos A)–1 rapidly rises with the inclination

of the exposure. Calculated values were compared with real

measurements of denudation on HB3 and JA11 pegs

(Table 4). Denudation on HB3 based on measurements and

photographs reached 120 mm during 5 years, a rate of

24 mm a–1, whereas the calculated value was 17–21 mm a–1.

Denudation measured above the JA11 peg reached 80 mm

compared to a calculated value of 110 mm. This means

that the calculated values of denudation can differ by tens

of percent values from the real values, especially for

steeper exposures.

At the Hormoz diapir, the denudation rate not stan-

dardized to precipitation was about 20 mm a–1 in the per-

iod of 2000–2004 and about 30–50 mm a–1 during years

2004–2005 (Table 4). At Namakdan, the calculated denu-

dation rate reached only 7–10 mm a–1 in the period of

2000–2004, and about 30–50 mm a–1 during years 2004–

2005. The two periods vary in precipitation average

(Table 1). The long-term mean denudation rate standar-

dised to mean precipitation was estimated to 40 mm a–1 for

salt exposures on the coastal salt diapirs.

Different results were obtained for Jahani diapir, where

the precipitation is higher and evaporation lower. The

majority of plastic pegs were washed out completely by

2004. Therefore, some new pegs were installed in 2004.

Only one peg has survived the whole period. The denu-

dation rate not standardized to precipitation ranged from 40

to 80 mm a–1 in the period 2000–2004 and about 60–

140 mm a–1 during the years 2004–2005. The long-term

mean denudation rate standardized to mean precipitation

was estimated to be 80–120 mm a–1 for salt exposures on

the Jahani diapir.

The 170 mm of the long-term mean precipitation on

coastal diapirs can theoretically dissolve 28 mm of salt. For

Jahani, the corresponding value is 50–80 mm of dissolved

salt. The mean calculated values of denudation rate are

higher than the maximum dissolution capacity of the cor-

responding amount of rainfall. The reason is probably the

inclined direction of some rains (strong wind during

storms), which can create surplus water on steep slopes.

Also the catchments above peg sites are partly covered by

weathering residuum and, as such, they yield additional

water for salt dissolution.

Erosion on an anhydrite/gypsum block was measured on

the Hormoz diapir, at the site marked HC, by two plastic

pegs (Table 4). The results showed negative erosion,

apparently due to an expansion of material during recrys-

tallization of anhydrite into gypsum (volume growth of

about 30%). Rillenkarren that covered the block were

dissected and rotated on places showing the expansion of

the superficial zone of the block.

Denudation rates on thin weathering residuum

The measuring of denudation rates was carried out on the

Hormoz salt diapir close to its center, on an area of 20 by

10 m. The thickness of weathering residuum covering the

salt varied between 0.5 and 2 m (Fig. 5). Chemical and

mineral compositions of the residuum are shown in

Table 2. The material is cemented by gypsum to the crust

on places. After rains, this hard crust becomes soft and can

be much easily washed away, promoting erosion.

Fig. 4 Thick-weathering residuum covering the southern part of the

Jahani diapir. Almost continuous vegetation cover slows down

erosion. The picture was taken in May 2005

1084 Environ Geol (2008) 53:1079–1089

123

Ta

ble

4E

rosi

on

mea

sure

men

tso

nro

cksa

ltan

dg

yp

sum

(Gy

p)/

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(An

h)

exp

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res

Lit

ho

log

yF

orm

An

nu

al

pre

cip

itat

ion

20

00

–2

00

5

(mm

)

Lo

cali

ty/

Peg

loca

tio

n

Dat

eo

f

inst

alla

tio

n

Incl

inat

ion

of

exp

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re

surf

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(deg

rees

)

Co

effi

cien

t

(co

sA

)–1

Ero

sio

n

20

00

–

20

04

(mm

)

Av

erag

e

ero

sio

n

20

00

–

20

04

(mm

a–1)

Ero

sio

n

20

04

–

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05

(mm

)

Cal

cula

ted

aver

age

den

ud

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n

20

00

–2

00

4

(mm

a–1)

Cal

cula

ted

den

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n

20

04

–

20

05

(mm

a–1)

No

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den

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n

20

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–2

00

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(mm

per

10

0m

m

of

rain

fall

)

No

rmal

ized

den

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n

20

04

–2

00

5

(mm

per

10

0m

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fall

)

Sal

tR

ille

nk

arre

n1

03

HB

3II

I-2

00

06

0–

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1.9

–2

.33

69

21

7–

21

(24

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–5

22

–2

72

Sal

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ille

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n1

03

HB

4II

-20

04

70

–7

52

.9–

3.8

––

15

–4

4–

57

–2

2–

28

Sal

tR

ille

nk

arre

n1

03

HD

2X

I-2

00

08

2–

85

7.1

–1

1.4

11

34

21

–3

42

8–

46

27

–4

41

4–

23

Sal

tR

ille

nk

arre

n1

03

HD

4X

I-2

00

04

5–

50

1.3

–1

.54

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Environ Geol (2008) 53:1079–1089 1085

123

For a period of 5 years, 15 plastic pegs were placed in

different parts of sinkholes: ridges, slopes and drainage

channels (Table 5; Fig. 6a, b). The average denudation rate

reached 2 mm a–1 (10 mm total during the whole period).

The maximum denudation rate was 5 mm a–1 on the slopes

and in the drainage channel. No erosion was detected at

two plastic pegs and aggradation occurred on one peg

(4 mm in 5 years). However, the zero or even negative

erosion value does not necessarily mean the absence of

erosion, but it rather points towards transport of sediment

down the slope. Eroded material is being instantly replaced

by new material from upslope positions.

No significant correlation was found between denuda-

tion and the catchment area above the peg site (rxy 0.38) or

between denudation and surface inclination (rxy –0.06).

Likewise, no apparent relationship was found between

morphology (ridges, slopes, channels) and the denudation

rate (Table 5). The average denudation rate on slopes

compared to ridges was nearly the same (2.2 and

1.9 mm a–1, respectively), but values within individual

types of morphology varied significantly (Table 5). The

main factor affecting the erosion rate at individual pegs is

probably the resistance of the surface crust. This factor

cannot be quantified due to total loss of resistance of the

crust during and after rains.

Calculated long-term mean denudation rates normalized

to mean precipitation are about 1.7-times higher than the

measured values (mean value of 2 mm a–1; Table 5), as in

the period 2000–2005, the precipitation amount was only

60% of long-term mean. The long-term mean denudation

rate can be thus estimated to 3.5 mm a–1. Denudation of

the surface is, however, only a portion of the total denu-

dation. Rainwater infiltrates through the weathering resid-

uum and dissolves the underlying salt. Additionally, some

of the rainwater after intensive rains is drained by overland

flow into sinks in doline bottoms. It creates shafts and

Table 5 Erosion measurements on weathering residuum

Position of peg Slope

inclination

(degrees)

Catchment area

upslope

to individual

pegs (m2)

Peg mark Erosion over 5 years Normalized

erosion (mm

per 100 mm

of rainfall)

Minimum

(mm)

Maximum

(mm)

Average

(mm)

Average

per year

(mm a–1)

Ridge between sinkholes 0 0.1 HF11 3 5 4 0.8 0.8

Ridge between sinkholes 0–10 0.01 HF01 5 6 6 1.2 1.2

Ridge between sinkholes 10 0.01 HF15 14 17 15 3.3 3.2




Sinkhole slope 0–10 0.2 HF03 22 27 24 5.3 5.1

Sinkhole slope 15 1 HF14 –6 –3 –4 –0.9 –0.9

Sinkhole slope 30 1 HF09 2 8 6 1.2 1.2

Sinkhole slope 25–40 1 HF10 21 29 24 5.2 5.0

Sinkhole slope 40–50 0.1 HF13 –10 8 1 0.1 0.1

Channel bed 2–5 200 HF05 20 23 22 4.8 4.7

Abandoned channel 5 0.2 HF04 –2 3 0 0.0 0

Total average 10 2.1 2.0

Total maximum 24 5.3 5.1

Total minimum –4 –0.9 –0.9

Minima, maxima and averages of four values gauged at each peg (upslope, downslope, left and right sides) are presented

Fig. 5 Thin-weathering residuum with no vegetation cover in the

central part of the Hormoz diapir (close to HF locality). Rock salt

exposures are developed only on steep slopes where the weathering

residuum is washed off

1086 Environ Geol (2008) 53:1079–1089

123

hollows in salt that become partly filled with eroded

material (Bosak et al. 1999). The total denudation rate is

therefore higher than the measured erosion rate on plastic

pegs.

At many places covered by thin weathering residuum,

downcutting of uvala-like depressions progresses at rates

comparable to those of a salt mass uplift (Bosak et al.

1999). Due to rapid lowering of their floors, the base of the

uvala-like forms reached the water table, and the cave

drainage system was consequently replaced by superficial

streams on the Hormoz diapir. The long-term uplift rate of

the Hormoz and Namakdan diapirs was calculated using

the age of Holocene marine terraces and their current

altitude. The uplift rate for the areas covered by thin

weathering residuum was calculated at 4–7 mm a–1 on the

two diapirs during Holocene (Bruthans et al. 2006).

Assuming a constant uplift rate of the salt diapirs, the long-

term mean incision rate of uvala bottoms is close to

4–7 mm a–1 to keep pace with the uplift rate.

Denudation rate on weathering residuum and sediments

of moderate and large thickness

Surface denudation rate on weathering residuum and sedi-

ments with thickness exceeding 5 m is too slow to be

measured by pegs within a 5-year period. Places, where thin

and moderately thick residuum exist together, show clearly

more rapid surface denudation rate on thin residuum.

Broader consequences

Rock salt exposures are very rapidly evolving surface

environments. Denudation of larger exposures will not

surpass the 17% of the column of mean annual precipita-

tion. The maximum denudation rate was estimated as 30 m

per 1,000 years for coastal diapirs and about 90 m per

1,000 years for Jahani and other inland diapirs in similar

climate. The erosion rate of salt exposures is strongly

dependant on surface gradient of exposures. Vertical salt

walls protected by capping sediment show nearly no

erosion and can hence survive for very long time periods.

Indeed, it was observed in the field that the more thick and

resistant the residuum capping of the salt exposure is, the

steeper salt exposure develops. Inclined walls (common

slope about 70�) will propagate horizontally at a rate of

about 10 m per 1,000 years for coastal diapirs and about

30 m per 1,000 years for the Jahani diapir. On very

extensive salt exposures the erosion rate is markedly lower,

mainly because the dissolved salt precipitates back in the

form of salt sinters after minor rainfalls and thus avoids

being washed out (c.f. Talbot and Jarvis 1984).

The extent of salt exposures is therefore an important

factor in the morphological evolution of a salt diapir.

Salt exposures occur only on steep slopes (>50�), where

insoluble residuum has been washed off. On gentle

slopes the residuum will accumulate, which bury the salt

and protects it from rapid dissolution (Fig. 5). This is

because the presence of the cover promotes the evapo-

ration of rainwater captured in the pore spaces, and thus

less water can infiltrate and dissolve the underlying salt.

The relative abundance of vegetation, especially on

mountain salt diapirs, effectively protects the surface

against erosion and hence considerably decreases the

denudation rate (Fig. 4).

The origin of extensive salt exposures is a consequence

of the elevation of a diapir into a distinct area of high relief

with consequent erosion of the protective cover (e.g., the

Namak diapir). Climatic changes affecting the vegetation

cover could play an important role as well. The extent of

salt exposures has also strong implications for the sur-

roundings of a salt diapir, since catchment areas formed on

salt exposures produce large amounts of residual material

from the dissolved salt. Large and quickly aggradating

alluvial fans are consequently developed below these

catchments (e.g., the Jahani diapir, see Fig. 7).

In addition, the extent of the salt exposures to a great

degree determines the total amount of salt that is dissolved

by rain and carried off the diapir. A salt exposure of area

1 km2 with precipitation of 300 mm a–1 can annually

produce about 110,000 tons of dissolved NaCl. For areas

covered by residuum this number would be many times

lower as most of water is lost by evaporation. On the

Fig. 6 a Peg site marked HF on

Hormoz diapir covered by thin

weathering residuum. This

surface is typical of high

number of smaller sinkholes

(several meters in diameter)

separated by narrow ridges,

b Peg HF3 installed in the thin

weathering residuum on a slope

of a sinkhole on a

Environ Geol (2008) 53:1079–1089 1087

123

contrary a markedly lower amount of dissolved solids can

be expected in the vicinity of diapirs covered by thick

residuum, where superficial drainage occurs but sinks in

salt are not present. Low TDS waters (4 g l–1) drain from

such areas on Mt. Sedom diapir (Frumkin 1994).

Solutional denudation rates of 0.5–0.75 mm a–1 re-

ported by Frumkin (1994) on Mt. Sedom diapir, Israel, are

rather low compared to the data provided in this study. The

reason is probably a very low amount of precipitation on

Mt. Sedom (50 mm a–1

) compared to Iranian sites.

Conclusion

Erosion rates of various types of salt exposures and

weathering residuum were measured. Long-term denuda-

tion rates on salt exposures are estimated between 30 and

40 mm a–1 for coastal salt diapirs and up to 120 mm a–1

for mountain salt diapirs. Erosion rates are proportional to

the volume of precipitation and inversely proportional to

the inclination of salt exposure.

The long-term mean superficial denudation rate on thin

weathering residuum on coastal diapirs is estimated as

3.5 mm a–1. The average denudation rate on slopes of

dolines compared to ridges was nearly the same, but values

within individual types of morphology varied significantly.

Total denudation rate on thin residuum is close to

4–7 mm a–1 based on similar rates of denudation and uplift

on Hormoz and Namakdan diapirs. The denudation rate of

salt exposures is much faster than parts of diapirs covered

by weathering residuum. The extent of salt exposures is an

important factor in the morphological evolution of salt

diapirs as it can inhibit the further expansion of diapir.

The extent of salt exposures also strongly affects the

surroundings of salt diapirs, as catchment areas developed

on salt exposures produce large amount of clastic material,

which may aggradate as large alluvial fans below these

catchments. Salt exposures can be delineated on large-scale

aerial photographs. This may help to distinguish areas with

a lower probability of contamination of groundwaters

by dissolved NaCl, in this region of very scarce water

supplies.

Acknowledgments Many thanks to the staff of the Department of

Earth Sciences of Shiraz University, F. Anwar and B. Dareshouri,

from Qeshm Free Zone Organization and S. Zebarjad from the Shiraz

University in Qeshm Island for their support during the field work.

The authors also thanks to O. Jager, M. Kolcava, J. Kukacka, L.

Palatinus, T. Svoboda, and J. Smıd for assistance with field mea-

surements. Precipitation data were kindly provided by Iranian Mete-

orological Organization, Fars and Hormozgan Regional Water

Organizations. Research was supported by project no. KJB301110501

of the Grant Agency of the Academy of Sciences of the Czech

Republic, by Institutional Research Plan no. AV0Z30130516 (Insti-

tute of Geology, AS CR) and research project MSM0021620855

(Charles University in Prague). Authors are indebted to D. Motters-

head and A. Frumkin for valuable suggestions and improvement of

the manuscript. The authors appreciate very much the financial sup-

port from Ceskomoravsky cement Co. (Heidelberger Cement Group).

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http://www.worldweather.org

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A study of erosion rates on salt diapir surfaces in the Zagros Mountains, SE Iran

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