Gypsum karstification in the Middle Miocene Fatha Formation, Mosul area, northern Iraq

ELSEVIER

Geomorphology 18 (1997) 137-149

Gypsum kaxstification in the Middle Miocene Fatha Formation, Mosul area, northern Iraq Saad Z. Jassim a, Antwanet S. Jibril b, Nazar M.S. Numan ’ a Department of Earth Sciences, The University of Leeds, Leeds, UK b Geological Survey of Iraq, Baghdad, Iraq ’ Department of Geology, Mosul University, Mosul, Iraq

Received 1 November 1995; accepted 14 April 1996

Abstract Karstified Middle M.iocene sediments are widely exposed in northern Iraq particularly in the area surrounding the city of Mosul. The unit is dominated by gypsum and exposed in thirteen anticlinal structures within the investigated area of about 1600 square kilometers. Synclines, though containing the same sequence, are not karstified due to a Quatemary cover. Karst features were located from air photos: Over 4000 were recorded, the smallest detectable being two meters in diameter. The majority are sinkholes (dolines), developed in gypsum and manifested in the overlying collapsing limestone. They are singular, in lines or clusters. Shafts and karren are fewer in number and are usually developed in uncovered gypsum. Sinkholes are visibly located along fractures and at fracture intersections over gently inclined limestone beds overlying the gypsum. Two karst systems were identified, an active and recent system characteristic of all the anticlinal structures and an older (Pleistocene) fosG1 karst system characteristic of Alan, Ishkaft, Albu Saif and Hammam structures. The fossil karst system is preserved on remnant elevated old land surfaces and produces characteristic tight undulations in the limestone due to collapse inwards in ,sinkholes and elongated tunnels formed along a series of sinkholes. The fracture study of anticlinal structures reveals that the mean fracture density per area ranges between 4 and 8 (km/km2) and shows a unimodal character for most of the structures. However the distribution of karst in relation to fractures is bimodal for at least half of the structures with mean values ranging from 4.5 to 11 (km/km2). The fractures in the anticlines are thought to have formed due to folding but some are associated with major lineaments cross cutting the structures, which is reflected in the bimodality and the crude unimodal fracture/karst distribution. Karst features are related to the general fracture pattern but are more localized in densely fractured areas. Karst areas were also found to correlate with lower slope gradient and lower drainage density.

1. Introduction Karst regions are important for groundwater exploration and are problematic for major engineering projects. In Iraq, very little work on karst mapping was done in the past; a few large dolines were plotted by field geologists during the regional geo-

logical survey projects mainly due to the regional nature of the work. Other researchers dealt mostly with the origin rather than the spatial distribution. In this work, the authors used a cheap, fast and reasonably accurate method for karst localization, namely remote sensing. An area of about 1600 square kilometers sur-

0169-555X/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PZI SO169-555XC96)00018-9

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S.Z. Jassim et al./Geomorphoiogy

rounding the city of Mosul in northern Iraq was covered by air photo interpretation at a scale of l : 35,000 and 1 : 50,000 to determine the karst locations, photogeologic units and structural elements. Although geological maps of reasonable detail are available, it was thought that all of these elements should be obtained from one source. The area is covered by 20 runs of 240 frames. Additionally, a Landsat 2 image was used to determine the distribution of larger lineaments. Spot Images were not available to the authors at the time; they are believed to be a good tool for locating karst features, structural and photogeological units due to their relatively large scale and stereoscopic nature. The morphology of the area reflects the structure faithfully; anticlinal structures form the hilly terrain whereas the synclines, which are usually wide, form relatively flatter expanses. The highest structure is Butmah reaching 600 m above sea level while the lowest is the Hammam structure with an elevation of 200 m. The major part of the area is part of the Tigris river drainage system while only a small part

18 (1997) 137-149

west of the crest of Adayah-Shiekh Ibrahim drains towards Wadi Tharthar to the southwest. The location and the position of the anticlines and the karst fields is given in Fig. 1. The mean annual precipitation for the Mosul area is about 300-400 mm per year, half of which falls in January and February. The temperature varies between 7°C in winter to 34°C in summer on average. Water springs are distributed in two zones, an eastern zone along the Tigris river associated with sulphur mineralization; they are of mixed brine and meteoric origin (Al Sawaf, 1977). The western zone, on the other hand, is located along the Adayah-Shiekh Ibrahim line, characterized by sulphate water type with relatively uniform TDS of about 2000 ppm (Al Talabani et al., 1986).

2. Stratigraphy The sequence exposed in the area surrounding Mosul is Miocene in age and represented by the

-

Anticline

--.._ Limit of stnxture 4 -

Fig. 1. Location of anticlinal

Karst field Lineament zones

structures with karst fields: Inset shows location.

S.Z. Jassim et al./Geomorphology

Euphrates-Jeribe Formation (L. Miocene), Fatha Formation (M. Miocene) and Injana Formation (U. Miocene). The Fatha Formation is fully exposed within the area whereas the others are partly exposed or denuded (Table 1). 2.1. Euphrates-Jeribme

Formation

This formation is of Late Lower Miocene age. The hyphenated name, suggested by Buday (1980) has been widely used in northern Iraq for this sequence due to the Idifficulty of differentiating the Euphrates from the Jeribe Formation. The unit is exposed in some deep gullies in the cores of some anticlines, namely Alan, Sasan and Butmah. It is composed of bioclastic recrystallized, dolomitized limestone, dolostone,, clayey dolostone and sporadic thin gypsum. 2.2. Fatha Formation The name Fatha Formation was introduced by Jassim et al. (1984 and Jassim et al., 1986) to replace the Lower Fars Formation. A new type section in Jabal Makhul about 30 km northwest of Fatha (150 km south of Mosul) was chosen. The formation was divided into two members, lower and upper units or members. In the Mosul area the lower member varies in thickness from 90 m near Mosul to 235 m in Shiekh Ibrahim and 1352 m in Butmah. This member is dominated by carbonate in its lower part and gypsum in its upper part and is topped by a persistent limestone marker (Table 1). The upper member is usually exposed in a narrow belt around the Table 1 Stratigraphic

anticlines or in the synclines. It varies in thickness in this area from 60 m near Mosul to 160 m in the western part. It is dominated by green and red claystone intercalated with thick gypsum and subordinate thin oolitic and sandy limestones. Lithofacies data of Mustafa (1980) and Jassim and Karim (1984) showed that a ridge existed within the basin of the Fatha Formation separating it into two sub-basins, the western Sinjar basin extending into Syria and the eastern basin extending southeastwards into Iran. This ridge is the Mosul Uplift or the Mosul High which was active during the deposition of the Fatha formation. The thickness of the formation is 250 m in the Mosul area and 600 m in Sinjar towards the center of the western basin. Buday and Jassim (1987) indicated that this uplift was active since the Upper Cretaceous and affected the thickness and facies of all the following sedimentation. The carbonates of the Fatha Formation decrease in thickness away from the uplift and change facies from bioclastic limestone with abundant oyster shell fragments into dolomitized n&rite. The reverse trend is registered for the evaporites that increase thickness towards Sinjar and are associated with salt. The sedimentation of the Fatha Formation is cyclic; the cycles are in the form of a triplex of marl, gypsum and carbonates. The formation was deposited in closed lagoons and sabkha interrupted at times by normal marine sedimentation. 2.3. Injana Formation The name Injana Formation was introduced by Jassim et al. (1984), to replace the Upper Fars

units of the .Mosul area

Age

Formation

U. Miocene

Injana (U Fars)

M. Miocene

Fatba (L. Fars)

Euphrates-Jeribe

Description

Member unit

Litharenite and red claystone Top eroded

C

Red and green claystone Thickness 60-160 m Carbonate unit

B A

Gypsum-dominated unit Carbonate-dominated unit

U. Member L. Member (thickness 60-235

L. Miocene

139

18 (1997) 137-149

in alternating

fining-upwards

and gypsum with occasional

m)

Bioclassic

dolomitized

limestone. Top exposed only

cycles.

carbonates.

140

S.Z. Jassim et al./Geomorphology

Formation of Upper Miocene age. The type section is in Jabal Hemrin South at Injana 120 km northeast of Baghdad along the Kirkuk highway. It is mostly denuded in the area of investigation being present mostly in the northern part between Butmah, Alan and Ishkaft structures. The unit is composed of alternating calcarenite and red claystone of fluviatile origin. 2.4. Quaternary Quatemary sediments are either related to the fluvial system of the Tigris river or the polygenetic synclinal filling. The fluvial sediments are restricted to the flood plain valley of the river. Terraces are developed mostly on the eastern side of the river and show a progressive increase in height eastwards indicating the river progressively migrated westwards. The restriction of the terraces to the eastern bank is largely due to structure; this area is devoid of major anticlinal structures and hence has no major barriers to the river. Five terrace stages were identified, the elevation of their tops above river level are: (1) 80-90 m and 15-30 m thickness; (2) 55-65 m elevation and 25 m thickness; (3) 30-35 m and 15 m thickness; (4). 12-18 m and 15 m thickness. The fifth stage is the modem flood plain with sediments about 20 m thick.

3. Structure The Mosul area is part of the Foothill Zone of the Unstable Shelf of Iraq according to the subdivisions of Buday and Jassim (1987). This zone is an NW-SE trending belt, about 200 kilometers wide. It is bordered from the northeast by the High Folded Zone at the sudden appearance of high anticlines with Cretaceous and Paleogene rocks in their cores. Its boundary to the southwest is located at the last fold before the flat terrain of Mesopotamia and Jezira which are characterized by the absence of folds (Fig. 1). The Foothill Zone is characterized by long and narrow NW-SE oriented anticlines extending over 300 kilometers in length and made of a Miocene sequence, separated by wide and shallow synclines filled by Pleistocene and Quatemary elastics. The Mosul area is different from the rest of the zone due the shorter

18 (19971 137-149

length of its structures of up to 30 kilometers only. These differences were explained by Buday and Jassim (1987) as a result of the shallow basement depth in the Mosul block (5 km compared to over 9 km elsewhere) affecting the folding style. The investigated area is at the intersection of the Zagros NW-SE trend and the Taurus E-W trend. Apart from Butmah and Alan, the rest of the structures all belong to the Zagros trend (Fig. 1). Most of the anticlines in the southern part of the area are single domes whereas those in the north and northwest are of double or multiple dome type. The structural characteristics are given in Table 2. The area is traversed by major lineaments mostly of N-S and NE-SW orientation with less prominent NW-SE and E-W orientations (Fig. 1). The zones presented in this figure are the axes of anomalies of large lineament plots obtained from Landsat imagery and contoured as lineament densities. 3.1. Karst The majority of the published work on karst is related to limestone karst though there has been some work especially in the U.S., Canada, Spain, Italy, Australia and China on the subject of gypsum karsts (Palmer and Palmer, 1989; McRitchie and Voitovici, 1991; Stenson and Ford, 1993; Benito et al., 1995; Soriano and Simon, 1994; Choppy, 1986; Goede et al., 1990; Qian, 1988). Some work on the gypsum and limestone karst of the Mosul area was carried out sporadically on small localities and related to the description and origin. Al Sabti et al. (1988) studied some karsts from the She&h Ibrahim structure and concluded that they were mainly restricted to areas of low dip and that they were mainly the doline type. Adib (1988) studied the geology of Mosul City and concluded that karstification was intense and having great effect on older buildings due to its active nature. He also indicated that the karst features were developed in the gypsum with nodular texture. Generally, the presence of soluble rocks associated with concentrated water circulation along high permeability zones such as fractures and fracture intersections is the basic factor in karst formation (Sweeting, 1972). Karst features are usually developed in the zone of aeration and seasonal fluctuations, where vertical water movement takes place

X2. Jassim et al./Geomorphology Table 2 Karst features distribution Structure

Shiek Ibrahim Muhalabia Adayah Sasan Ishkaft E. Ishkaft W. Butmah Tel el Ashiq Alan Atshan Nuwaigit Qalian Hammam

in structures

and stratigraphic

units

Number of karst features in formations Fatha

Euphrates-Jeribe

U. Member

L. Member

455 28 34 22 97 16 32 19 23

A

B

C

360

1031

76

74 58

305 64

50 55

72 135

83 3 45 3 64

36 6 20 44

59

9 5 5 79

Structure

Ishkaft east Ishkaft west Alan Hanlmanl Nuwaigit Sasan Butmab Sheihk Ibrahim Adayah Atshan Qalian a Main population.

Karst feature density

110 177 124

1922 28 501 151 142 141 286 19 69 124 83 115 214

120

16.0

43 85 21 42 79

11.7 1.8 6.8 3.4 3.6

1.6 1.4 1.5 62 42

1.8 5.1

Movement of water and rock solubility in a fractured system and the enlargement of these fissures is dependant on many factors. These are: Initial fissure width, hydraulic gradient, length of passage, CO2 pressure and temperature (Ford et al., 1988). They indicated that development of a karstic tunnel system takes about 10,000 to 100,000 years dependant on the factors mentioned above. However, these figures are for carbonate rocks; gypsum rocks are 10 to 30 times higher in solubility (Bogli, 1980). The Pleistocene had strongly fluctuating climatic conditions

(Murty, 1988). In limestone terrain karstification takes place in areas of annual precipitation of no less than 300 mm; on an average most limestone karst countries are in the 1000 mm annual precipitation range (Herak and Stringfield, 1972; Sweeting, 1972). However, due to its higher solubility, gypsum is much less dependant on high precipitation. The Mosul area has 300-400 mm precipitation and that is just enough to produce karsts in the carbonates of the Euphrates-Jeribe Formation but abundant for the karstification of gypsum of the Fatha Formation. Table 3 Fracture and karst data of anticlinal

Area Total

42 166 104

7 135

141

18 (1997) 137-149

structures

Fracture density for karst population 2nd 3rd 1st

Fracture density for area population

11 a 8 7 7 i’ 6’ 5a 6 ” 5” 5 5” 2

8” 6” 7 7 6 5 6 5 7 1 2

14 10 11 12

12 15 a

9

12

8 7a 10 9a

1st

2nd

vs. karst

difference

7.9 7.0 6.6 6.1 5.0 4.0 4.4 4.5 5.3 3.4 5.2

10.7 8.4 11.0 6.8 5.4 4.8 5.5 4.9 5.7 5.7 7.5

2.8 1.4 4.4 0.7 0.4 0.8 1.1 0.4 0.4 2.3 2.3

3rd

11

48 7a

Mean fracture density vs. area

6 9

142

S.Z. Jassim et al./Geomorphology

and rates of precipitation (Emiliani, 1971) which suggests that the area may have witnessed much wetter periods. The movement of ground water in an anticlinal structure occurs from the axis toward flanks and plunges if the topography permits (Deyin, 1988; Deika, 1988). The morphologies of the anticlines in the investigated area are all structurally controlled which permits the kind of water movement just mentioned. On movement through the gypsum-rich sequence, the water becomes progressively enriched with calcium sulphate especially in slow laminar flow. Hand dug wells and boreholes drilled in the synclines of the area commonly have very high

Karst

18 (1997) 137-149

sulphate concentrations reaching several thousand ppm. Water springs on the western flank of Adayah and She&h Ibrahim have discharges in the range of 1 to 199 liters per second but their salinity is almost uniform at around 2000 ppm (Al Talabani et al., 1986). These springs are all considered karst springs. Karst water is less salinated than normal formation water due to the flow and volume of water involved. Karst has been divided into mantled and bare karsts (Sweeting, 1972) and the bare karsts were further divided into active and inactive karsts (Beck, 1988). Inactive karst have also been referred to askarsts by Sweeting (1972). Karsts of the investigated area are all of the bare type. Sinkholes, and to

density over 10

Fig. 2. (A) Relationship of karst features (dots) to drainage (dashed lines) and lineaments (solid lines), Sheikh Ibrahim structure, 7 km of Tel Afar. (B) Photogeological map of Sheikh Ibrahim showing photolithological units and areas of high karst features concentrations.

S.Z. Jassim et al. /Geomorphology 18 (1997) 137-149

a lesser extent shafts,, are clearly visible on the air photos, appearing as round to oval sharp edged dark tone spots. Using 3 X enlargement on the stereoscopes, sinkholes down to 2 meter diameter were recognizable. Karst feature numbers and density for each structure are given in Table 3.

4. Distribution and form Within the area karst features are located in karst fields usually at the core and flanks of the anticlines (Fig. 2). They are often found on gentle dip slopes of limestone underlain by gypsum, usually with rectangular drainage patterns or on steeply inclined gypsum (Fig. 3B). The majority, however, are of the first type. They are sinkholes (dolines) and may be singular or in groups; the grouped sinkholes are either linear or in clusters. Linear arrangment of dolines produce an elongated collapse that leads to the development of a karst valley (Fig. 3A). Clusters

143

on the other hand usually have a central large and deep sinkhole surrounded by smaller and shallower sinkholes (Fig. 3C). Kemmerly (1986) described such development as contagion karsts. The deeper parent sinkhole treats a local hydraulic gradient that leads to the development of a surrounding set of shallower daughter sinkholes. Fossil karstic features are found on elevated land surfaces in the interdrainage areas. They are characterized by very strong contortion of the limestone beds producing steep dips along tight synclines and circular basins. In deep excavations and building sites around Mosul inactive fossil sinkholes may be visible as funnel-shaped features in gypsum, 2 to 4 meters in diameter and few meters deep, filled with fragments of limestone and red clays representing the collapsing limestone roof and soil infilling. The neck of the funnel leads to a sub-horizontal tunnel and is often blocked by limestone blocks. The fossil karst field is defined on air photos as a dark tone area with strong pock marking. Karst forms in the area can be divided into sinkhole (dolines), karren, shafts, karst valleys and caves. 4.1. Sinkholes Sinkholes are closed basins formed either directly in exposed gypsum or in gypsum overlain by limestone. In bare gypsum they are between 1 m and 25 m, in depth and usually have gentle slopes. In gypsum overlain by limestone they are usually steep sided and up to 20 meters in diameter. The depth of sinkholes depends on the thickness of the gypsum bed.

/--

4.2. Karren and shafts

-.

’ 811

,,_

__

__’ k ’ ._ ._”

k.

0

meters

” ;j 1 ;

/-

,_ 10

k/

/

&ft

’ ’ dip7

,’ k -Y

:’

;

-‘@? ’ ,’ ‘ , A’ ’ /

-

/

_A;-

,_’

, / I ! ? metus

’

’ 1p

-’ /

Karren are unidentifiable on air photos. They are elongated narrow holes with solution striations developed in bare gypsum. On further enlargement the Karren become deep holes about 1 to 2 meters wide and up to 10 meters deep.

/

Fig. 3. Karst development. (A) Karst valley development, Hammam. (B) Karst in steeply inclined gypsum, Sheikh Ibrahim. (C) Mother and daughter clwter, Hammam structure. (D) limestone, characteristic of fossil karst system as indicated by dip of limestone bedding, Atshan structure.

4.3. Karst valleys Karst valleys are characteristic features of limestone dip slopes with underlying gypsum. Linear

S.Z. Jassim et al./Geomorphology

144

limestonedip slope

I8 (1997) 137-149

karst valley

old karst system

limestone dip slope

Fig. 4. Block section in northeastern

limb of the Hammam

structure showing active and fossil karst fields on a limestone dip slope underlain

by gypsum.

sinkholes developed along fractures often become joined to each other and result in the development of new drainage. The rectangular drainage system in such localities reflects faithfully the fracture pattern (Fig. 4). 4.4. Caves Cave systems are not well known in the Mosul area apart from the Tel Afar caves and the Qassab caves. The Tel Afar caves are developed at the northwestern plunge of She&h Ibrahim structure and in the vicinity of Tel Afar town. The Qassab caves were visited by the authors and are located on the northern flanks of Qassab anticline and are developed in thick nodular gypsum. They form straight tunnel systems with underground streams a few hundred metres in length, and produce natural arches near points of their exit The tunnel is usually up to 10 metres high; the walls are smooth and striated indicating influence of water flow. Occasionally, the roof is collapsed and chimneys are developed. Tunnels could be traced on the surface by multiple series of sinkholes on top of them. Similar features were described by the geological survey geologists from the Jezira area around Hatra and are well utilized by the local people for water wells.

5. Stratigraphic

and facies control

The number of karstic features within each photogeologic unit for each structure was established and is summarized in Table 2 which clearly shows that over 99% of the karst forms are in the Fatha Forma-

tion, and out of this over 60% fall in the lower member and especially in unit B. This is due to the predominance of gypsum in this unit. Anticlinal structures towards the Sinjar basin to the west have thicker gypsum and are ultimately more karstified (She&h Ibrahim and Adayah structures). The presence of more gypsum beds in the sequence increases the chances of the presence of gypsum available for karstification at the surface at any particular time in comparison with colums of fewer gypsum beds.

6. Structural

control

Karstic features are developed on anticlinal structures especially at the crestal areas and near the plunges. These areas are the most weakened, the former by tension fractures and the latter by shear fractures developed due to movement along deep seated faults. These deep faults can account for the bending of the axes in that region and indeed cause the subdivision into multi-domes. The area of intersection of these long lineaments with the anticlinal structures was found to have high fracture density and consequently they have high density of karstic features too (Fig. 1). Karst features are often visibly linked to fractures in the visited areas in the field and for that reason the generalization of this relation to the whole population needs a statistical approach. Fracture traces were obtained from the air photos for the structures and a fracture density map was constructed by calculating total fracture length for each square kilometer. The areas for each fracture density population were plotted on frequency and cummulative distribution curves. It was found that

S.Z. Jassim et al. /Geomorphology

Butmah, Sasan, Nuvraigit, She&h Ibrahim, Ishkaft (East and West), Hammam and Adayah structures have near normal distributions with clear unimodality. On the other hand Atshan, Alan and Qalian structures have very abnormal distributions. One of each category is illustrated in Fig. 5 which also shows the distribution of fractures with karstic features. The unimodality may be due to fracturing by the folding process. The structures vary in their mean fracture densities probably due to varying folding intensities and additional shearing by major fractures. The distribution of karstic features with the fracture density shows a similar trend to the area1 distribution, but with marked differences in the mean values, being always higher than those for the fracture/area distributions (Table 3). This may indicate that although the main part of the area is fractured at a certain intensity, the karstic features are biased towards the zone of greater fracturing. Of all the structures, Butmah, Sasan, Nuwaigit, Hammam, Ishkaft W. She&h lbrahim and Adayah structures have near normal distribution whereas Alan, Qalian and Atshan structures have very strongly bimodal distribution. The mean values for the fracture density/area distribution and the mean values for the fracture density/karst features distribution, given in Table 3, show that their difference fall into two categories; those with substantial difference ( > 2) are Ishkaft E., Alan and Atshan structures (i.e. those with bimodal

18 (1997) 137-149

145

Fig. 6. Karst features density versus fracture density.

karst feature distributions) and those with small differences (the rest). The former have higher densities of karstic features in small localized areas though the total density of the structure as a whole is small. unlike, for example, She&h Ibrahim structure which is very strongly karstified over a very large area. This is made clear on the plot of fracture density against karst features density (Fig. 6). In this plot all the structures show a tendency for increasing karstification with increasing fracture density and are mostly clustered together apart from She&h Ibrahim and Adayah structure plots, which are both of unimodal distribution. Some of the structure’s graphs in this figure tail to zero at the highest fracture density value. It is possible to have higher fracture density without karst formation, simply because these high values usually cover very small areas and interference from other factors such as unsuitable lithology or high slope gradient may inhibit karst development.

7. Slope control

0.0

4.0

12.0

8.0

15.0

Fracture density(km/sq.km) Fig. 5. Frequency distribution with karst features.

of fracture

density

with area and

Herak and Stringfield (1972) and Beck (1988) indicated that karst features are restricted to areas of low slope gradient. On this basis two structures in the area were chosen for slope study, these were Hammam and She&h Ibrahim structures. Topographic maps of 1: 25,000 scale were used after superimposing an overlay of the karst map suitably enlarged with a cartographic camera. The slope gra-

146

S.Z. Jassim et al./Geomotphology

18 (1997) 137-149

dient was measured for each kilometer grid as the difference between the highest and lowest contour values. The slope shows unimodal, near normal distribution with both total area and karst features (Fig. 7). The distribution curve shows that the main part of the area has slope gradient of more than 50 m/km whereas karst features are mostly found in areas of lower slope gradient (about 35 m/km). Lower slope gradient increases infiltration relative to surface runoff.

00

8. Drainage control

10 0

20 0

300

400

Drainage bifurcations per km’

Some sinkholes are located directly on drainage courses but the majority are outside them (Fig. 2). Karsts were clearly seen developing new karst valleys but once they did so, their surface manifestation is replaced by the drainage course. The drainage within the anticlinal areas is dendritic where softer strata of the sequence are exposed and trellised where such a sequence is steeply inclined as a result of alternation between hard and soft beds. However, on dip slopes of hard limestones of the lower member, the drainage is rectangular. The southeastern part of the Atshan structure was chosen to determine the statistical relations between drainage and karst. For easy counting, drainage bifurcations were used; they are usually related to drainage density. Drainage bifurcations per square kilometer were obtained from the air photos along

Fig. 8. Frequency and karst features.

distribution

of drainage

bifurcations

for area

with karst distributions. The drainage bifurcation classes were plotted against area and karst features (Fig. 8). The plot against area shows two populations with values at 15 and 25 whereas karst feature/drainage have three modal values at 15, 25 and 35; that at 15 being the largest (over 60%). This indicates that although some correlation exists between karst and drainage, the overwhelming majority is biased towards lower drainage bifurcation values and ultimately lower drainage density. Karstification lowers surface runoff by infiltrating water downwards thus depriving development of denser drainage systems. Less dense, karst-controlled drainage eventually evolves from karstification.

100.0

9. Human activities and karst Bo.0

se60.0

s

Karst features

Area

140.0 20.0

50.0

Slope Fig. 7. Frequency karst features.

distribution

100.0

gradient (m/km2 ) of slope gradient with area and with

Gypsum karstification and its interaction with human activities from the Zaragosa area in Spain has been well documented by Soriano and Simon (1994). They investigated sinkholes and dolines mantled by alluvial sediments and established hazard maps through the study of karst forms, depth to gypsum, thickness of alluvium, water table fluctuations, water chemistry, lineaments and fracture distribution etc. They have also shown that within the last fourty years disappearance and appearance of karst forms proceeds at a rate of about 5 per year; the total number of recorded karstic features has increaed from 277 in 1946 to 286 in 1986. The influence of industrialization on karstification was also discussed.

S.2 Jassim et al. / Geomophology

During construction of large building projects around the city of Mosul, close site investigation boreholes were drille’d. These boreholes usually show two features: (1) cavities in gypsum and (2) presence of deep red-coloured clays with rock fragments. Site geologists often mistake the material for lenticular intercalations of claystone in gypsum. However, neither the facies nor the regional understanding of the lithology of the formation permits this interpretation. The red claystone is not a lithologic component of the Lower Member. 0n subsequent excavation of the site, they were found to be karst fillings in gypsum. Fossil karst forms especially sinkholes are well exposed in excavations in and around Mosul City. They appear at the surface as basins and syniforms in limestone with broken elongated slabs heaved upwards at high angles but when excavated reveal an underlying mixture of red clay and rock fragments filling cavities in gypsum. Some cavities are funnelshaped with their necks restricted by large blocks and are connected to sub-horizontal tunnels normally not more than 50 cm in diameter. These tunnels extend to the base of the gypsum layer and are locally enlarged to produce steep-walled filled cavities. Mosul City is an old city, the main part of its old quarter is over century old and some buildings are a few hundred years old. It is situated on the northeastern flank of the Albu Saif structure and near to its northern plunge. It is built over the dip slope of the limestone of unit C which is directly underlain by gypsum then green marl. Houses were built on what seemed to be at the time as a very sound rock foundation. In the early part of this century water distribution in the city was done on mule back and the estimated water consumption did not exceed 10 liters per person per day. Discharge from households was partly to surface drainage and partly to shallow and small septic tanks. The modem water distribution did not start until the 1940’s resulting in a sudden increase in water consumpti’on (presently at 200 liters per person per day) and was not associated with a sewer system. Larger and deeper septic tanks were dug at the perimeter of buildings (which never seemed to fill) resulting in a dramatic increase in water percolating downwards, water which is also more corrosive due to the increased use of detergents and

18 (1997) 137-149

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chlorination. This water passes through the permeable and fractured limestone to the underlying gypsum, creating new dissolution cavities but eventually finding the older karstic system which is then used and further widened. As the old karstic tunnels are widened the blockages by limestone boulders are removed and sudden movement of karst filling occurs. The tunnel is also locally enlarged and causes roof collapse. Since the 1970’s more buildings have been fractured and many have suddenly collapsed. The problem was further intensified due to the expansion of the city in the up-dip direction (west and southwest) including construction of industrial, water dependant centres. Water from the up-dip areas eventually passes under the old city before discharging in the Tigris river. The process was slightly arrested in the 1980’s by the completion of the drainage system for the city.

10. Summary: Karst development (1) The Fatha Formation, rich in evaporites, was deposited in a lagoonal sea segmented into two parts by the Mosul Uplift. The western Sinjar basin shows a general increase in gypsum basinwards (westwards). This lithofacies variation and the cyclic nature of the lithology played an important role at later stages in karst development. (2) Folding in the area started in the Pliocene (possibly Late Pliocene). The Tigris river and its tributaries incised into the area and the structures changed the hydraulic gradient considerably. A deeper and faster rate of incision was probably enhanced by active uplift and higher precipitation during the climatic fluctuations of the Pleistocene. The softer parts of the lithologic cycles were progressively exposed from the core of anticlines towards the flanks and plunge areas. An intricate dendritic drainage pattern was developed which continuously denuded the softer sequences laterally due to its inability to cut through the underlying hard sequences. At this stage there were no large concentration of karstic features; they were mainly restricted to water courses of the drainage. (3) The dendritic drainage ended once the underlying hard limestone dip slope dominated the morphology. The then freshly exposed limestone dip slopes were devoid of significant drainage. Water

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began to seep through fractures to the underlying gypsum, dissolving it, causing roof collapse and formation of sinkholes. Those that were linearly arranged joined together and produced karst drainage. The rectangular drainage pattern began to develop. Karstification moved laterally from fold axes outwards as the softer overburden was progressively removed in the same manner. (4) Some drainage courses established themselves firmly by cutting down towards the older underlying limestone, resulting in the formation of sinkholes in the base of the water courses causing further incision in the sequence. In the interdrainage areas, the sinkholes were gradually filled in with limestone debris and soil. Eventually these were completely mantled and inactive. They became part of fossil karst fields. Fossil karst fields were progressively produced from the axes outwards and once truncated they became remnants as mesas in the axial area or gently dipping cuestas in the interdrainage areas at the flanks. The infilling of sinkholes may have taken place during periods of low precipitation. (5) Fossil karsts were periodically reactivated, probably during periods of high precipitation but mostly due to local human activities. Most of anticlinal structural terrain in the area is used for wheat crops; continuous ploughing affects infiltration and surface runoff. In urban areas, such as Mosul City, since the turn of this century the water consumption per head increased twenty fold as well as the five fold increase in population accentuated karst development.

Acknowledgements This research was sponsored by the Geological Survey of Iraq and Mosul University. The authors wish to thank Drs. Rob Butler, Rob Raiswell, Department of Earth Sciences, and Mark Macklin, Geography Department of Leeds University for reading the manuscript.

References Adib, H.G.M., 1988. Structure and stratigraphy of Most11 City, the bank. Unpublished M.Sc. Thesis, Most11 University (Iraq). Al Sabti, G.M., Ibrahim, IS. and Amin, S.T., 1988. Karst mor-

18 (1997) 137-149

phology of the Sheikh Ibrahim area, northern Iraq and its relation to structure and hydrology. In: Proc. of the Intemational Symposium on the Geology of Deserts and Desert Environments, Baghdad, Vol. 1, pp. 146-157. Al Sawaf, F.D., 1977. Sulfate reduction and sulfur deposition in Lower Fars formation northern Iraq. Econ. Geol., 72: 608-618. Al Talabani, N.J., Mustafa, S.A. and Askar, S.R., 1986. Water quality as index of chemical erosion at karstic springs of Ishkaft structure. J. Water Resour. Iraq, 5. Beck, B.F., 1988. Environmental and engineering effects of sinkholes - the processes behind the problem. Environ. Geol. Water Sci., 12: 71-78. Benito, G., Perez, D.C.P., Gutierrez, E.M. and Sancho C., 1995. Natural and human induced sinkholes in gypsum terrain and associated environmental problems in northern Spain. Environ. Geol., 25: 156-164. Bogli, Al, 1980. Karst hydrology and physical speleology. (Translated by June C. Smith.) Springer-Verlag, Berlin, Heidelberg, New York, 270 pp. Buday, T., 1980. The Regional Geology of Iraq, Vol. 1, Stratigraphy and Paleogeography. Geological Survey of Iraq, 352 pp. Buday, T. and Jassim, S.Z., 1987. The Regional Geology of Iraq, Vol. 2, Tectonism, Magmatism and Metamorphism. Geological Survey of Iraq, 492 pp. Choppy, J., 1986. Les karste de gypse italiens. Karstologia, 8: 39-46. Deika, G.H., 1988. On the karst environment system. In: Proc. of the IAH, 21st Cong. on Karst Hydrogeology and Karst Environment Protection, China, Vol. 1, pp. 394-400. Deyin, X., 1988. The karst hydrogeological conditions and its characteristics in the central-south Shandong Province. In: Proc. of the IAH, 21st Cong. on Karst Hydrogeology and Karst Environment Protection, China, pp. 501-508. Emiliani, C., 1971. Pleistocene climatic cycles at low latitudes. In: Turkekian (Editor), Late Cenozoic Glacial ages. Yale University Press, New Haven and London, 606 pp. Ford, D.C., Palmer, A.N. and White, W.B., 1988. Landform development; karst. In: Hydrogeology. Geological Society of America, pp. 401-412. Goede, A., Harmon, R.S., Atkinson, T.C. and Rowe, P.T., 1990. Pleistocene climatic changes in southern Australia and its effect on speleothem deposition in some Nullarbor caves. J. Quat. Sci., 5: 29-38. Herak, M. and Stringfield, V.T., 1972. Karst. Amsterdam, London, 551 pp. Jassim, S.Z. and Karim, S.A., 1984. Final report of the regional geological survey of Iraq, Vol. 4, Paleogeography. Geological Survey of Iraq. Jassim, S.Z., Karim, S.A., Basi, M., Al Mubarek, M.A. and Munir, J., 1984. Final Report of the Regional Geological Survey of Iraq, Vol. 3, Stratigraphy. Geological Survey of Iraq. Jassim, S.Z., Hagopian, D. and Al Hashimi, H., 1986. The Geological Map of Iraq, scale 1: l,OOO,OOO.Geological Survey of Iraq. Kemmerly, P.R., 1986. Exploring a contagion model for karst terrane evolution. Geol. Sot. Am. Bull., 97: 619-625.

S.Z. Jassim et al./Geomorphology McRitchie, W.D. and Voitovici, P., 1991. Karst landforms in the gypsum lake area; character and distribution. Report of activities NTS 620/16, Manitoba Energy and Mines, Winnipeg, Man. Murty, KS., 1988. Karst hydrogeology, India. In: Proc. of IAH 21st Cong. on Karst Hydrogeology and Karst Environment Protection, China, pp. ,a6-450. Mustafa, A.M., 1980. Sedimentological studies of the Lower Fars Formation in Sinjar tmasin, Iraq. Unpublished M.Sc. thesis, Mosul University (Iraq). Palmer, A.N. and Palmer, M.V., 1989. Geological history of the Black Hills caves, South Dakota. In: J. Pisarowicz (Editor), Black Hills Symposium. NSS Bull., 5 1: 72-99.

18 (1997) 137-149

149

Qian, X., 1988. The formation of gypsum karst collapse column and its hydrogeological significance. Karst hydrogeology and karst environmental protection proceeding. IAHS-AISH Publ., 2: 1186-l 193. Soriano, M.A. and Simon, J.L., 1994. Alluvial dolines in the central Ebro basin, Spain: a spatial and developmental hazard analysis. Geomorpholgy, 11: 295-309. Stenson, R.E. and Ford, D.C., 1993. Rillenkarren on gypsum in Nova Scotia. Geogr. Phys. Quat., 47: 239-243. Sweeting, M.M., 1972. Karst Landforms. Macmillan, London, 362 pp.