The stratigraphy of the Middle Eocene–Pleistocene sediments in Kuwait

Journal of Arid Environments (1997) 37: 1–22

The stratigraphy of the Middle Eocene–Pleistocene sediments in Kuwait

E. Al-Awadi*, F. Al-Ruwaih†, Z. Al-Rawdan† & I. Ozkaya† *Kuwait Institute for Scientific Research (KISR), Water Res. Division, Hydrology Dept., P.O. Box 24885, Safat 13109, Kuwait †Kuwait University, Dept. of Geology, P.O. Box 5969, Safat 13060, Kuwait (Received 5 October 1996, accepted 3 January 1997)

The Dammam Formation and the Kuwait Group are the major aquifers containing useable brackish water in Kuwait. The Dammam Formation is a limestone–dolomite sequence of Middle Eocene age. It is underlain by Middle Eocene Rus evaporites and is overlain unconformably by the clastic sediments of the Kuwait Group. It is subdivided into three members. The lower member, A, is a nummulitic limestone with shale interlayers at its base. The middle member, B, is a highly silicified and dolomitized fossiliferous limestone which includes lignite and silty lignite interlayers. The upper member, C, is a friable, white dolomite which is silicified and karstified at the top, beneath the unconformity with the overlying Kuwait Group. Similar silicification and karstification occurs at the top of the B member, suggesting a minor but significant unconformity between the B and C members. The upper part of the Dammam Formation is dolomite of Lutetian age with fragmented and cherty zone on top. Secondary dolomite was formed from early dolomitization of fossiliferous micrites, pelmicites, intramicrites and fossiliferous pelmicrites. Silification was related to percolation of acidic ground-water during the non-depositional period in Oligocene time when the surface of the Dammam Formation was karstified. The Dammam Formation was deposited on a shallow marine shelf experiencing minor fluctuations from lagoon to tidal flat and swamp environments. This tectonically stable period was interrupted by small pulses in the source land and minor fluctuations in the sea level, which caused alternating transgressive and regressive cycles. Stratigraphy of Miocene–Pleistocene Kuwait Group is studied. The work is based on cores and bore hole logs from Umm-Gudair, NW Al-Shagaya, AlSalmi and Al-Wafra water wells, and exposures at Jal Az-Zor escarpment and Ahmadi quarry. The Kuwait Group was subdivided into two or three formations. There is no general agreement on the boundaries of the formations and the number of formations. The confusion has persisted until present mainly because there is no clear cut criteria to delineate formations. There are no markers or distinct lithologic boundaries to set formations apart. The Kuwait Group is subdivided into three formations in the present study in agreement with 0140–1963/97/010001 + 22 $25.00/0/ae970268

© 1997 Academic Press Limited

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previous work. These are Ghar, Jal Az-Zor and Dibdibba Formations from bottom to top. No clear cut stratigraphic clue could be obtained from wells to identify formations. The fossiliferous sandy limestone layer at the lower part of the group was selected as the boundary between Ghar and overlying Jal AzZor Formations. The boundary between the Jal Az-Zor and Dibdibba Formations is hard to locate. The ledge-making, carbonate-cemented sand layers are included in the Jal Az-Zor, and overlying poorly-bedded and weakly-cemented gravels are included in the Dibdibba Formation. The overall stratigraphic sequence of the Dammam limestone and the Kuwait Group bear clues for a gradual regional regression and accompanying increase in clastic material influx. The Kuwait Group starts with marine clays and continues upward as fluvial, river channel sands, and ends with alluvial fan conglomerates. Deposition of the Kuwait Group was interrupted frequently by brief periods of quicence and non-deposition. Pauses in sedimentation are marked by surfaces of carbonate and gypsum cementation in the pore spaces of sands by evaporation of capillary water, and alteration of clays. The lower parts of the Kuwait Group consist mostly of loose sands without cementation. The degree of cementation and frequency of cemented layers increase towards the top of the Kuwait Group. ©1997 Academic Press Limited Keywords: Dammam Formation; Kuwait silicification; karstification; dolomitization

Group;

unconformity;

Introduction The purpose of the present study is to evaluate the stratigraphy of the Dammam Formation of Eocene age and the Kuwait Group of Miocene–Pleistocene age. The Dammam Formation of Eocene age is studied in order to investigate fully the nature of the unconformity between Dammam Formation and overlying Ghar Formation of the Kuwait Group. The work is based on subsurface information from wells of Ministry of Electricity and Water (MEW). The wells are located in Umm-Gudair, AlWafra, NW Al-Shagaya, and Al-Salmi areas (Fig. 1). The bore hole information includes cores, cuttings, composite logs, Gamma Ray, Resistivity and SP logs. Around 47 wells were drilled in Umm-Gudair out of which seven were cored. Only one well was cored in NW Al-Shagaya and Al-Salmi areas. Three wells were cored in Al-Wafra area. The coring is complete only in NW Al-Shagaya well. In other wells, cores samples were obtained only from some intervals. The uppermost 10–20 m of the Dammam Formation and the overlying Ghar Formation are exposed in the Ahmadi quarry. There is a relatively complete exposure of the Kuwait Group at Jal Az-Zor escarpment (Fig. 1). These exposures are also studied in detail. The unconformity between the Dammam and the overlying Ghar Formation and the geological features associated with the unconformity are conspicuous in the Ahmadi quarry. The earlier studies are based mainly on the Ahmadi and Jal Az-Zor exposures and very few drill holes in the vicinity of Kuwait City. The present study made use of new subsurface data. The newly obtained subsurface information improves the understanding of the depositional history and stratigraphic sequence of the Eocene–Recent sediments in Kuwait. Another advantage the present work has over the earlier studies is that the wells are distributed all around Kuwait. It has been possible to obtain a clear picture of the facies and thickness variations of the Dammam Formation and Kuwait Group on the basis of such evenly distributed subsurface information. One additional objective of the present study is to determine the subsurface geometric configuration of the different facies of Miocene–

SEDIMENT STRATIGRAPHY IN KUWAIT

48° 30°

30°

56°

3

ba

r Wa

I AB AR AN G LF U

NW-AL-SHAGAYA NW. SH.EXP.5KL

l-B

ati

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200 km

KUWAIT

af

ARP. SC

BUBIYAN

N

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r ah

-B

Al

KUWAIT BAY

FAILAKA

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Wa d

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iA

IRAQ

E

26° 0

tr

-A

Al

SAUD

I ARAB

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IA

R AI

t ana uss

29°

ARABIAN GULF Al-Ahmdi Ridge

-G M UM

Al-Salmi

l-M di-A Wa

SL-PW.IL

Al-WAFRA

0 10 20 km

47°

48°

Figure 1. Location map of the studied areas.

Pleistocene sediments in Kuwait as well as the determination of the facies changes, age and environment of deposition of some horizons. Physiography Kuwait lies on the north-western side of the Arabian Gulf. It has a surface area of about 6150 square miles. Iraq borders it from the north, and Saudia Arabia from the west and south. The ground surface of Kuwait is generally featureless, gently undulating, flat desert. It is covered with sands and gravels and rises gradually from the Arabian Gulf shores towards south-west. The most predominant physiographic features of the country are Jal Az-Zor escarpment, Ahmadi Ridge, Wadi Al-Batin, and coastal lines. The Jal Az-Zor escarpment extends from Al-Atraf north-east to Al-Bahra for about 60 miles. It has a maximum height of about 400 feet. Ahmadi Ridge is another prominent topographic feature. It extends parallel to the east coast of Kuwait. The maximum height is about 450 feet. Wadi Al-Batin and Wadi Al-Mussanat are the major depressions in the country. Bubyan, Warba and surrounding muddy flat offshore in the north is composed of silt, clay and few windblown sands (Fig. 1). Regional geology Tectonic setting and pre-Dammam stratigraphy The State of Kuwait is located on the north-eastern extension of the continental crust of the Arabian plate, bordered by Zagros mountains to the east. Zagros is part of the Alpian–Himalayan suture belt which was formed by continental collision following closure of Tethys ocean in Late Cretaceous to Miocene times (Kamen Kaye, 1970;

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E. AL-AWADI ET AL.

Murris, 1980). The Arabian plateform is covered by a thick section of dominantly shallow marine sediments. Sedimentation continued from Cambrian to Recent with occasional breaks. Rate of sedimentation was nearly equal to rate of subsidence (Kamen Kaye, 1970; Murris, 1980). Sedimentary cover becomes thicker towards Zagros belt with a general dip of 1–2 towards north-east. There are gently undulating structures which become smaller, and asymmetrical towards Zagros. They were formed as a result of vertical movements of basement and horizontal compression (Kamen Kaye, 1970; Kassler, 1973; Murris, 1980). Stratigraphy of Kuwait is well known from oil wells which penetrate to about 20,000 feet. The age of the stratigraphic sequence ranges from Triassic to Pleistocene. Paleozoic sediments are not well known in Kuwait. Triassic is represented by sands and shales, Jurassic by evaporite and some carbonates. Cretaceous sediments are predominantly limestone and subordinate shale with two major sandstone Formations; Zubair and Burgan (Owen & Nasr, 1958, Milton, 1967). Paleocene–Eocene epochs are represented by Hasa Group which consists of limestone and dolomite of the Radhuma and Dammam Formations, and evaporites of the Rus Formation. The Miocene–Pleistocene epochs are represented by clastic sediments of Kuwait Group, which includes Ghar, Lower Fars, and Dibdibba Formations. The Recent sediments cover most of ground surface of Kuwait State (Owen & Nasr, 1958; Milton, 1967; Salman, 1979; Arisha, 1984). The major unconformities of the stratigraphic sequence of Kuwait are at the end of Triassic, Middle Cretaceous, end of Late Cretaceous, and Oligocene (Arisha, 1984; Abdulla, 1987).

Stratigraphy of Dammam Formation and Kuwait Group in Kuwait and Arabian Gulf Near surface rock in Kuwait are of sedimentary origin ranging in age from Middle Eocene to Recent (Fig. 2). Middle Eocene is represented by the Dammam Formation, 600–700 feet of limestones and dolomites (Parson Corporation, 1963; Bergstrom & Aten, 1964; Burdon and Al Sharhan, 1968). Clastic sediments of the Kuwait Group cover the Dammam Formation unconformably. The Kuwait Group is subdivided into Ghar, Lower Fars, and Dibdibba Formations (Owen & Nasr, 1958; Parsons Corporation, 1963; Lababidi & Hamdan, 1985). Recent deposits cover most of the surface of Kuwait except Jal Az-Zor escarpment and Ahmadi quarry. Dammam Formation Dammam Dome in Saudi Arabia is the type locality of the Dammam Formation where the complete sequence outcrops (Burdon & Al-Sharhan, 1968). The Dammam Formation starts with a fossiliferous Nummulitic limestone with thin streaks of anhydrites and a green shale. This is followed by a chalky limestone with shelly limestone interlayers (Owen & Nasr, 1958; Steineke et al., 1958; Parson Corporation, 1963; Burdon & Al-Sharhan, 1968; Omar et al., 1981). The topmost layer is a shelly granular to chalky limestone interbedded with discontinuous beds to chert. The uppermost part is highly silicified (O’Brien, 1952; Owen & Nasr, 1958; Steineke et al., 1958; Parson Corporation, 1963; Burdon & Al-Sharhan, 1968; Lababidi & Hamdan, 1985). The Dammam Formation is of Middle Eocene (Lutetian) all over the Arabian Gulf except in the United Arab Emirates where it is of Lutetian–Bartonian age. Fossils in the Dammam Formation include Nummulite sp., Coskinolina sp., Dictyoconoids sp., Alveolina elliptica, Bryozoan, and Peneroplid (O’Brien, 1952; Owen & Nasr, 1958; Lababidi & Hamdan, 1985). There is a major unconformity between the Dammam Formation and the overlying Ghar Formation. This unconformity is exposed in the Ahmadi quarry. It is at a depth of 1800 feet in Bubyan (Omar et al., 1981). The Dammam Formation extends all over Arabian Gulf countries with a slight increase in

SEDIMENT STRATIGRAPHY IN KUWAIT

5

thickness towards the east (Schlumberger, 1972; Labadibi & Hamdan, 1985). According to O’Brien (1952), and Burdon & Al-Sharhan (1968) there is a minor unconformity between the lower and the upper part of the Dammam Formation. A minor diastem may also be present between the Dammam Formation and underlying Rus evaporites (Owen & Nasr, 1958; Arisha, 1984; Al-Awadi, 1988). Kuwait Group

GROUP

Thickness (ft)

KUWAIT

Lithology

Description Beach sands and limestones, wind blown sand; playa silts and clays; wadi alluvium

350

Coarse upland gravels. Gravel and sand; mainly conglomeratic sandstone siltstone; shale

LOWER FARS

550

Fine to conglomeratic calcareous sandstone; variegated shales; fossiliferous; limestone; gypsiferous

GHAR

300

Quartzose sandstone and conglomerate; some shale in lower part

DAMMAM

600– 700

Discontinuous chert cap; chalky and siliceous limestone; dolomite

RUS

250– 400

Anhydrite; limestone; marl

RADHUMA

600– 1400

Marly limestone; dolomite; anhydrite

DIBDIBBA

HASA

MIOCENE–PLIOCENE EOCENE

Formation

RECENT

PLEISTOCENE

RECENT AGE

Clastic sediments of the Kuwait Group are subdivided into Ghar, Lower Fars or Mutla and Jal Az-Zor, and Dibdibba Formations (Owen & Nasr, 1958; Fuchs et al., 1968; Salman, 1979). The Ghar Formation is of Aquitanian–Burdigalian age, and overlies the Dammam Formation unconformably. The Ghar Formation consists of sands and gravels with some rare anhydrite, clays and the sandy limestone interbedded with up to 900 feet thickness (Burdon & Al-Sharhan, 1968). The type locality is Zubair south Iraq and north Kuwait. The Ghar Formation was deposited in a littoral environment (Lababidi & Hamdan, 1985). The lower Fars Formation is up to 350 feet thick deposit of Lanhian–Serravalian (Burdon & Al-Sharhan, 1968; Lababidi & Hamdan, 1985). It consists of evaporites interbedded with clastic red beds and carbonates of shallow marine environment (Beydoun, 1984). The following fossils are reported from Lower

Figure 2. Stratigraphic sequence of the Kuwait Group and Hasa Group (after Bergstrom & Aten, 1964).

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E. AL-AWADI ET AL.

Fars; Ostrea Latimarginata, Osterea Vestita, Lamellibranc (Lababidi & Hamdan, 1985). Dibdibba Formation is 350 feet thick section of fluvial sand and gravels of Pliocene– Pleistocene age (Owen & Nasr, 1958; Milton, 1967; Burdan & Al-Sharhan, 1968). Quarternary sediment overlies Dibdibba Formation unconformably (Salman, 1979). The Neogene clastics were subsequently named as Kuwait Group and were subdivided into Ghar, Lower Fars and Dibdibba Formations (Owen & Nasr, 1958). Fuchs et al. (1968) identified Lower Fars and Dibdibba boundary on the basis of heavy minerals contents. Dibdibba Formation contains fragments of garnet, pyroxene, and amphiboles, while the older Formations contain rutile, zircon and tourmaline. They correlated the Lower Fars in Kuwait with the Fars Group in south-western Iran. There is a 1–2 m thick fossiliferous sand limestone in the lower part of the Jal Az-Zor escarpment with ostrea Lamimaginula and clausinella of Late Miocene (Fuchs et al., 1968). Cox & Rhoades (1935) referred the fossiliferous bed which includes Ostrea latimarginata and Glausinella from Al-Bahra and the central part of Jal Az-Zor escarpment to the lower subdivision of Fars Group in Iran.

Description and correlation of stratigraphic sections The overall stratigraphic sequence of bore hole sections from Umm-Gudair, Al-Wafra, Al-Salmi and NW Al-Shagaya areas are briefly described. A brief description of Ahmadi and Jal Az-Zor outcrops are also included.

Umm-Gudair The observation wells are correlated on the basis of lithologic similarities, position in sequence and on some key horizons such as the lower nummulitic layer which is recognized in all wells. The lignitic zone is another key horizon which has been used in the correlation. The lithostratigraphic correlations of some observation wells are given in Fig. 3. A general stratigraphic sequence for the Dammam Formation can be established on the basis of cores from the observation wells and the lithostratigraphic correlation of these wells. The upward sequence of the generalized stratigraphic section of the Dammam Formation in the Umm-Gudair area is given below (Fig. 4).

Description of members and submembers of the Dammam Formation The Dammam Formation is subdivided into three members, A, B, and C, from bottom to top on the basis of stratigraphic sequence and correlation of the observation wells. The lower member, A, is a nummulitic limestone with some shale interlayers at the base. The middle member, B, is highly silicified and dolomitized fossiliferous limestone, with a silty dolomite interbed and thin seams of lignite. The upper member, C, is a friable dolomite. The stratigraphic subdivisions of Dammam Formation are described from bottom to top as follows. Member A This consists of nummulitic limestone with shale interlayers at the base, grading into fossiliferous limestone at the top. It is of Early to Middle Eocene age. This member is subdivided into two submembers, namely a1 and a2, from base to top.

SEDIMENT STRATIGRAPHY IN KUWAIT

O.W.2 E.

W.10 B

S.W.

O.W.4

B –300

–300

B

B

B

B

A

A

B –400

B

A

A

B

–500

A

A

–600 (A)

(B) Location map

Legend Limestone

Correlation line

Middle member W.

Observation well

Chert

Marl

47°40'

O.W.2

Possible correlation line O.W.

N.E.

Lower member

Dolomite

Lignite

Fossils

Silt

Organic matter Silicification

Vugs

Shale

Production well

Scale

0

1

2

3 km

Core not available

N

W.16 O.W.4

Umm-Gudair

Elevation in feet

B –400

–600

O.W.2

B

B

–500

W.16

O .W W .1 .1 0

O.W.1

28°50'

W.

7

Figure 3. Lithostratigraphic correlations of some observation wells in Umm-Gudair area.

(a1) Shaly limestone. This submember consists of buff, vuggy, medium to hard nummulitic limestone, with greenish shale interlayers and carbonate matrix. Towards the top, dark bands of organic matter start to appear. It has a thickness of 40 feet, and can be traced in O.W.1. Greenish to gray shale occurs as layers within the biomicrite. X-ray analysis revealed presence of palygorskite within the shale, in addition to other common clay minerals. Microscopically, the rock is biomicrite rich in large nummulites. The fossils are mainly foraminiferids, ostracods, corals and mollusc fragments. X-ray diffraction charts of the bulk samples show mainly calcite and clay minerals with a minor amount of feldspar (Fig. 5). The fossils are scattered throughout the fine-grained micritic matrix, which includes impurities of iron oxide and clay minerals. The foraminiferids belong to different species including Nummulites sp., Pararotalia sp., Alveolina sp., Discocyclina sp., Dictyoconoides sp., and Miliolids sp. The nummulites are of various types. In addition to the foraminiferids, there are ostracods (Bairdya sp.), corals, mollusc fragments, foliated brachiopod shells and echinoids. Some of the fossils have dissolved away leaving a biomoldic porosity, and some of the nummulites have been silicified and then replaced by calcite. The vugs are filled with spar. The brachiopod shells have a micrite envelop on them. The matrix is mainly micrite with some sparry calcite cement. The micrite between the shells contains small circular areas of sparite, which are probably calcite casts of fragments. (a2) Fossiliferous limestone. This submember is composed of cream to buff, vuggy, hard, impure fossiliferous limestone, alternating with organic-rich, spotted algal limestone. Microscopically, this submember is a biomicrite rich in nummilities, or, in places, it is an algal micrite or organic-rich biomicrite. The fossils are foraminiferids, algae, corals, ostacods, mollusc fragments and echinoid spines. Some of the fossils are silicified and dissolved away leaving a biomoldic porosity. Intergranular and moldic porosities are recognized as well.

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Member B This member is considered to be the most complex member of the Dammam Formation. It is of Early to middle Eocene age. Member B is subdivided into six submembers. These submembers, from the top, are described below.

Member

Submember

Formation

DAMMAM

Epoch

EOCENE

TERTIARY

Period

(b1) Algal limestone. This submember consists of buff to grey, fine-grained, non-

C

c

Thickness (ft)

Description Porous, friable dolomite. Occasional chert nodules towards top

b6 110–180

Fossiliferous dolomitized limestone, with shell fragments

b5

Silty lignite with dolomite interlayers

5–10

B Fossiliferous limestone. Highly silicified and dolomitized, with some chert nodules. Occasionally silty in lower part

b4 32–90

b3 4–20

Laminated silty dolomite. Silicified, with some organic matter

b2 36–86

Fossiliferous impure limestone. Silicified and dolomitized, with some chert nodules. Occasionally silty

b1

Bituminous algal limestone

4–10

a2 26–66

Fossiliferous limestone— Nummilitic towards the bottom

a1

Nummilitic limestone with shale interlayers

A

Lithology Limestone

Dolomite

Chert

Fossils

Silt

Shale

Vugs

Lignite

Organic matter

Silicification

Figure 4. Generalized stratigraphic section of the Dammam Formation in Umm-Gudair area.

60

50

40

30

20

6.3838 Clay

10.567 Clay

9

5.4333 Clay

4.1391 Clay 4.2669 Clay 4.4800 Clay

3.6656 Clay

3.0293 Ca 3.2757 Clay

2.4926 Ca

2.2794 Ca

2.0924 Ca

1.8732 Ca 1.9125 Ca

1.5594 Ca 1.6014 Ca 1.6326 Ca

SEDIMENT STRATIGRAPHY IN KUWAIT

10

4

2θ°

Figure 5. X-ray diffraction chart of the sample at depth 1162 ft/O.W.7 (Ca = calcite).

porous, hard, bituminous limestone rich in organic matter. Microscopically this submember includes foraminiferids and algae. (b2) Fossiliferous limestone. This submember is fine-grained, vuggy, fossiliferous limestone with some organic matter concentrations towards the bottom. It grades upwards into fossiliferous dolomitic limestone with chert nodules. There are a lot of fossils scattered in the fine grained, partially dolomitized micritic matrix. Fossils are foraminiferids, mollusc fragments, brachiopods, echinoids, algae and corals. (b3) Dolomite. This submember consists of hard, vuggy, silty dolomite rich in lignite. The fossils are mainly foraminiferids and mollusc fragments, with some benthonic fossils and algal patches. The fossils are recrystallized or dissolved away leaving biomoldic porosity. (b4) Dolomitic limestone. This submember consists mainly of vuggy, silicified and dolomitized fossiliferous limestone. The fossils are identified as foraminiferids, algae, mollusc fragments and corals are present. (b5) Lignite. Submember b5 is a marker of the middle member of the Dammam Formation. This submember consists of Laminated grey to brown silty dolomite containing Lignitic seams. (b6) Dolomitized limestone. This forms the top of the middle member of the Dammam Formation. The submember consists of buff to grey, vuggy, hard, dolomitized fossiliferous limestone. Microscopically this submember includes different types of fossils such as foraminiferids, mollusc and echinoid. This submember deposited in a shallow shelf marine environment, possibly a lagoon (Fichter & Poche, 1979). The upper part of b6 seems to be silicified and with organicrich laminae. The boundary between b6 and C is sharp and manifested in the logs by a rapid passage from high Gamma Ray activity and high Resistivity to low Gamma activity and low Resistivity zones. Member C This is the topmost member of the Dammam Formation and is distinctly different

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from the lower two members. It consists of white, very friable, porous dolomite with thin chert lenses and nodules. There are occasional thin, dense fossiliferous zones. The top of the unit is marked by a karstified and cherty zone below the unconformity. Kuwait Group consists of two distinct parts. The lower part directly above the unconformity consists of loose, white, yellow, reddish sands with red mudstone interlayers. Sands of the Ghar Formation have a characteristic yellowish tint. There is a 1–2 m sandy limestone on the yellow sands. The upper part of Kuwait Group consists mostly of cemented sands and gravels. The red clay–mudstone layer and reddish-yellow sand are missing. There are occasional red-mottled zones in an otherwise uniform grey, beige, white section. There are some loose sands within the cemented layers. There are also a few marly sand intervals which are distinguished by a greyish tint. The loose sand become gradually coarse upward. Conglomerate layers appear more frequently. The boundaries of the Formations are not clear in the bore holes. Apart from the general subdivision, there is no regular, distinct stratigraphic sequence to subdivide into Formations and member. The boundaries are gradational. The red clay layers are too discontinuous to be of any use for marking boundaries. Cementation, which is late diagenetic, is irregular and of little use in identification of formations.

NW Al-Shagaya Only one well in Al-Shagaya is cored (N.W. SH.EXP.5KL) (Fig. 6). The upper part of the Dammam Formation in NW Al-Shagaya is very friable granular or chalky dolomite. The topmost cherty zone is very thin. There is a greenish clay at the base of the Kuwait Group. The clay layer contains lumps of pyrite and becomes sandy upward. Same greenish clay is also present within the uppermost part of the cherty zone. This makes it difficult to position the unconformity. The contact looks gradational. The clay and sand sequence is terminated sharply and followed by yellow, red, white, loose sand with occasional reddish mudstone interlayers. The loose sands and red muds are overlain by carbonate-cemented sands. There are occasional white clean well-sorted sand interlayers. There are also some red-spotted argillaceous sand zones.

Al-Wafra Facies of the Dammam Formation in Wafra are friable, chalky or granular dolomite, capped by a very thick cherty zone. Above the unconformity there is green basal clay which is overlain by light green sandstone. There are no cores from Kuwait Group in Al-Wafra.

Al-Salmi Bore hole sections in Al-Salmi area are more similar to NW Al-Shagaya than UmmGudair. The Dammam Formation is friable granular dolomite. The Kuwait Group is sand and sandy gravel with alternating cemented and uncemented zones and clay interlayers. An important distinction is in the lower part of the Ghar Formation. There are cherty dolomitic interlayers within sands. The boundary between the Dammam and Ghar Formations seems to be interfingering. There may be a reverse fault which causes repetition of the Dammam dolomite. Reverse fault is not known in Kuwait. If

Form

Period Epoch Group

Era

SEDIMENT STRATIGRAPHY IN KUWAIT Thickness

11

DESCRIPTION SANDSTONE: white, grey, calcareous

DIBDIBBA

SANDSTONE: light grey, buff, loose with an interlayer of thin claystone, light green, silty 350–0

T

N R E T A

SANDSTONE: grey, calcareous

SAND: light grey, loose SANDY MUDSTONE: mottled with grey brown

U

P l i o c e n e – P l e s t o c e n e

Y

SAND: beige, loose

A

SANDSTONE: white, grey

R

SAND: white, loose

Q

I

SAND: light brown, loose

A

SANDY MUDSTONE: mottled with red brown

C

? W

SANDSTONE: light grey, white, calcareous

U

SANDSTONE: white, calcareous

K

SANDSTONE to SANDY LIMESTONE

JAL AZ–ZOR

M i d d l e – U p p e r

N

O

Z

O

M i o c e n e

I

SANDY MUDSTONE: mottled with red brown patches SAND: white, loose CLAYSTONE: light grey, silty

350–0

SANDSTONE: light grey, calcareous SANDSTONE: light brown, resistant SANDY MUDSTONE: mottled red brown SANDSTONE to SANDY LIMESTONE: resistant SAND: white, loose

SANDSTONE to SANDY LIMESTONE: highly resistant

E R

Y

SAND: light grey, loose SAND and GRAVEL: loose

SANDSTONE: light grey, calcareous

SAND: red brown, white, yellow, loose GHAR

Lower–Middle Miocene

R

T

I

C A

CLAYSTONE: red brown FOSSILIFEROUS SANDY LIMESTONE SANDY MUDSTONE: mottled red brown

90–9

SANDY FOSSILIFEROUS LIMESTONE SAND and GRAVEL: light brown, loose SANDY MUDSTONE: mottled red brown green SANDSTONE: green with pyrite GREEN CLAYSTONE: with pyrite

T EOCENE HASA DAMMAM

E

Fragmented Cherty Zone FOSSILIFEROUS DOLOMITE: massive alternates with FOSSILIFEROUS DOLOMITE: thinly bedded with an interlayer of thinly irregular cherty lenses and cherty nodules 210–32

FOSSILIFEROUS DOLOMITE

GRANULAR DOLOMITE: alternates with Fossiliferous dolomite

Figure 6. Stratigraphic sequence of the upper part of the Dammam Formation and the Kuwait Group of Exp. 5KL. in NW Al-Shagaya.

12

E. AL-AWADI ET AL.

cherty dolomites within Ghar Formation are interlayers, then the Oligocene unconformity should not be present in Al-Salmi.

Jal Az-Zor The Dibdibba and Jal Az-Zor Formations are well exposed along Jal Az-Zor escarpment. The yellow sands at the skirts of the ridge most probably correlate with the yellow sands near the base of Kuwait Group in Umm-Gudair bore holes. There is a 0·5–1 m thick sandy fossiliferous limestone layer which is assumed to mark the boundary of the Ghar Formation and overlying Jal Az-Zor Formation. The fossiliferous limestone includes unidentifiable shell fragments, and is overlain by loose cross-bedded, red–white sands with red mudstone interbeds. The red sands are overlain by a (230 feet) section of gravelly sands with occasional sandy marl beds. There are also two loose, white, clean, cross-bedded sand interlayers. The well cemented layers stand out as a ridge and can be traced long distances. The ledgemaking calcareous sands are overlain by a (40 feet) section of partially-cemented poorly layered gravels of the Dibdibba Formation. The boundary between the Dibdibba and Jal Az-Zor Formation is hard to locate, because of gradational contact and lithologic similarities. The top of the last prominent ledge is assumed to be the boundary between the two Formations.

Ahmadi quarry The unconformity between the Dammam Formation and Kuwait Group is well exposed in Ahmadi quarry between C-member of the Dammam and Ghar Formations. The Dammam Formation consists of friable porous granular or chalky dolomite with lens-shaped chert interlayers. There are a few fossil-rich zones. Fossils include shell fragments of Gastropods and Pelecypods. The chert lenses seem to be confined to fine-grained thin-bedded dolomite zones. There is a (8 feet) thick fragmented chert breccia right below the unconformity. The Ghar Formation consists of loose red white–yellow sands without any conspicuous layering and cementation. There is a half meter calcareous zone, about a meter above the unconformity.

Subsurface correlation Correlation was performed in two stages. Initially, stratigraphic sections of bore holes with cores were correlated pairwise. Subsequently, all observation wells were correlated. Correlation is based on lithologic similarity and position in sequence.

The geophysical logs characteristics Gamma Ray and Resistivity logs are used for correlation. Gamma Ray log measures natural radioactivity of minerals containing potassium, uranium or thorium. Such radioactive elements are concentrated in shales (Schlumberger, 1972). Therefore, Gamma Ray reflects the shale content of sediments. Clean sands and carbonate give a low Gamma Ray reading (Asquith & Gibson, 1982). Occasionally, clean sandstone with low shale content may also give a high Gamma Ray reading if it contains potassium feldspars, mica, glauconite or uranium rich waters. Limestone rich in organic matter cause large Gamma Ray deflections. The Resistivity log is used to locate hydrocarbon bearing zones (Asquith & Gibson, 1982).

SEDIMENT STRATIGRAPHY IN KUWAIT

13

The SP logs are used to distinguish impermeable zones, such as shale, and permeable zones such as sand, and also to determine formation water resistivity.

General interpretation of the logs of the Dammam Formation and the Kuwait Group On the basis of Gamma Ray log interpretations, the Dammam Formation can be divided into three parts. The Gamma Ray logs shows high and irregular kicks in the lower member, A, of the Dammam Formation due to the presence of frequent shale (Fig. 7). In the middle member, B, the Gamma Ray also shows irregular high kicks, but these kicks do not correspond to shale interlayers but to the radioactive-rich organic matter. However, these middle kicks are not dependable for correlation, because they show lateral variations due to lateral facies change. Gamma Ray intensity is low and has a uniform pattern in the upper member, C, where the formation is a clean dolomite. The Gamma Ray logs mark the boundary between B and C members clearly in all wells by a sharp decline from a high and fluctuating zone into uniform, clean, low Gamma Ray zone, member C. This boundary has been found very useful in correlation as a marker horizon. The Resistivity log runs in an irregular pattern in the lower member, A, of Dammam Formation, and this member is also divisible by the logs into two parts (1A and 2A). The lower part (1A), has a high Resistivity, which indicates tight and non-porous zones; while, the upper part (2A), has a low and smooth Resistivity representing a porous zone. In the middle member B of the Dammam Formation, the Resistivity log shows irregular, sharp and wide fluctuations representing alternating tight and porous zones. The porous zones result from fracturing, or have originally high porosity, while the tight zones are due to silification (Fig. 7). The Resistivity log is smooth and low in the upper member, C, which represents a porous zone. In the topmost part of this member, Resistivity increases and shows sharp fluctuations near the unconformity, corresponding to highly resistant, tight zones with some fracturing, borehole caving and silicification. The unconformity between the Dammam Formation and the Kuwait Group is clearly displayed by the Resistivity log. Towards the unconformity, Resistivity increases and starts to fluctuate. Right above the unconformity, it drops sharply across the porous sands of the Ghar Formation. A panel diagram is presented in Fig. 8. Gamma Ray shows smooth uniform low reading across the C-member of the Dammam Formation. Gamma Ray registers increasing values towards the unconformity. There is a high Gamma Ray kick at the base of the Kuwait Group right above the unconformity which indicates a clay layer. The Resistivity log displays wide fluctuations due to alternating porous and tight zones. However, towards the unconformity Resistivity increase due to the presence of a tight silicified zone (Fig. 9). Gamma Ray is distinguished by a low smooth reading interrupted by very high kicks across the lower part of the Kuwait Group. The low Gamma Ray zones correspond to clean loose sands. High Gamma Ray kicks are due to clay interlayers. A few of the Gamma Ray kicks can be correlated across the wells. Resistivity readings are generally low but irregularly fluctuating across the upper part of Kuwait Group without a distinctive sequence. Kuwait Group does not possess a regular pattern that can be traced across wells. In general, clean sands show a low Resistivity. Cemented zones show relatively higher Resistivity values. Resistivity readings wildly fluctuate within the uppermost (650–900 feet) above the water table.

Elevation in feet

0

20

Shale

Dolomite Lignite

Chert

U?

Organic matter Silicification

50 mV

0

16" Resistivity

Unconfirmity

Core not available

0

0

1

2

3

Correlation of marker horizons

O.W. W.

5 km

A

B

C

D

Observation well Production well

4

NE

N

47°30' W.16 O.W.4

O.W.2

Location map

–500

–400

–300

–200

–100

0

100

4 12 20 28 Ohm.m

16" Resistivity

40 cps

Scale

20

Correlation of member and formation boundaries

Possible unconfirmity

U?

50 mV SP Gamma Ray

O.W.2

AR

Figure 7. Subsurface correlation of Dammam Formation in Umm-Gudair area, Kuwait.

U?

20 40 cps 0 8 16 28 Ohm.m

SP Gamma Ray

W.16

I UD SA

A: B: C: Lower, Middle and Upper members of Dammam Formation, D: Kuwait Group.

Silt

A

B

C

D

40 cps 0 8 16 24 Ohm.m

16" Resistivity

Limestone

Legend

–600

–500

–400

–300

–200

–100

0

100

50 mV

SP Gamma Ray

O.W.4

28°50'

SW

Umm-Gudair

14 E. AL-AWADI ET AL.

AB IA

Elevation in feet

SEDIMENT STRATIGRAPHY IN KUWAIT

15

Criteria and problems of subsurface correlation The unconformity is easily identified on logs and can be traced across wells. Resistivity has a high kick right below the unconformity and drops abruptly to a low and widely fluctuating pattern across Kuwait Group. Gamma Ray log displays a sharp kick above the unconformity due to the distinct presence of a clay layer. The lower boundary of C-member appears as a step on Gamma Ray logs and is identifiable across all wells. Identification of Formation boundaries and correlation of Kuwait Group proved to be difficult, due to lack of distinctive sequence, key beds, and rapid facies change. The fossiliferous limestone layer between the lower yellow and overlying red sandstone is regarded as the top of the Ghar Formation on the basis of earlier classification in literature (Owen & Nasr, 1958; Milton, 1967; Fuchs et al., 1968; Salman, 1979). There is no conclusive age determination and the boundaries seem to be gradational at least in the subsurface sections. The fossiliferous limestone which is assumed to

BI

RA

W.16

.7 W O.

A

–400 b2 b1

b6 –500 b5

–600

.4 W O.

IA

b3

.1 0 W O. W.1 O.W.2 UD SA

Elevation in feet

O.W.2 b6 b5 b4

b6

–300

28°55'

W.10

a2

UMM-GUDAIR

O.W.1

Location map

a2 O.W.4

b4 b3

a1 b2 b1

0

Scale 1 2

3 km

O.W. Observation well W. Production well

28°45' B

A

b6

Fossiliferous dolomitized limestone

b5

Silty lignite

b4

Silicified dolomitic limestone

b3

Laminated silty dolomite

b2

Silty dolomitic limestone

b1

Bituminous algal limestone

a2

Nummilitic limestone

a1

Nummilitic limestone with shale intervals

O.W.7 –300

–400

47°30'

Elevation in feet

Subdivision of Dammam Formation

–500

47°40'

Figure 8. Panel diagram of the Dammam Formation in Umm-Gudair area, Kuwait.

–200

–100

M.S.E.

100

200

300

400

500

N

U -G

UM

M

UD SA

R IA

AB

IA

DA

IR

0

16 Ohm.m

P.W.14 0 20 40 60 cps

0

16 Ohm.m

Gamma Ray 16" Resistivity

?

?

0 20 40 60 cps

0

16 Ohm.m

16" Resistivity

O.W.4 Gamma Ray 0 20 40 60 cps

0

16 Ohm.m

16" Resistivity

P.W.26 Gamma Ray

Figure 9. Subsurface correlation of the Damman Formation and the Kuwait Group: correlation of O.W.1, P.W.14, O.W.4, P.W.26 in Umm-Gudair area.

LOCATION MAP OF WATER WELL IN UMM-GUDAIR AREA

km

0 5

?

0 20 40 60 cps

16" Resistivity

O.W.1

Gamma Ray

NW

DIBDIBBA FM.

JAL AZ-ZOR FM.

GHAR FM.

DAMMAM FM.

16 E. AL-AWADI ET AL.

SEDIMENT STRATIGRAPHY IN KUWAIT

17

mark the top of Ghar Formation is well developed at Jal Az-Zor escarpment. It is identified in Umm-Gudair wells 4, 6 and 7, but is absent in other wells of UmmGudair, NW Al-Shagaya and Al-Salmi area. Correlation suggests that the Ghar Formation has a nearly constant thickness. Wherever the fossiliferous limestone is missing its position is referred from thickness, and position of Gamma Ray. The Jal AzZor Formation is relatively more regular than the overlying Dibdibba Formation, at least in the lower parts. The clay interlayers within loose sands cause large Gamma Ray kicks which can be traced across some wells. The distinctive alternation of low and high Gamma Ray pattern at fixed intervals disappear upward. Gamma Ray becomes irregularly fluctuating and without any pattern that can be traced from well to well. Resistivity is of little value in correlation. Although loose sands have lower Resistivity values than the cemented intervals, regular, correlatable sequences are absent, and it is difficult to carry individual peaks across the wells. Above the water table Resistivity logs become totally useless for correlation. The boundary between Jal Az-Zor and the overlaying Dibdibba Formation could not be identified in subsurface sections and logs. The boundary is assumed to be on top of the last ledge making calcareous sandstone at Jal Az-Zor escarpment on the basis of previous studies (Fuchs et al., 1968; Salman, 1979). The boundary is extrapolated to subsurface sections using constant thickness as a guide. The Gamma Ray log appears to be more wildly fluctuating across the Dibdibba Formation.

Depositional history Silicification and karstification are the diagenetic processes which indicate the existence of the unconformity on the top of the Dammam Formation in Umm-Gudair area. Silicification is partial and it takes the form of selective replacement of the micritic matrix, or replacement of the fossils which are scattered throughout the matrix. Silicification is indicated by a high resistivity and karstification by rapid and high fluctuation in Resistivity. Below the B–C boundary, a similar silicification and karstification must be present and is also indicated in the Resistivity logs. Indeed, from the cores, silicified and karstified zones are known to occur further below. These silicified zones also show fluctuating and high resistivity. The degree of silicification increases upwards towards the top of the B member. A similar increase is observed in Resistivity logs towards the top of the C member. However there is no connection between these silicified zones, because the lower part of C member is of low resistivity, which indicates a high porosity and no silicification. Silicification and karstification of member B cannot be related to the silicification on top of the C member. Silicification of C is related to the unconformity on top of the Dammam Formation. Member B must have been silicified and karstified at an earlier time, most probably prior to deposition of the C member. If silicification and karstification indicate an unconformity, then there must be a minor break between the B and C members. These conclusions are further backed up the nature of dolomitization. Member B is partially dolomitized and dolomite selectively replaces the matrix of fossils. The degree of dolomitization decreases downwards and may be of a late, mixed water origin (Hanshaw et al., 1971; Land, 1973, 1983). On the other hand, dolomitization of member C is complete, and probably took place very early during its deposition by seepage reflux (Adams & Rhodes, 1960; Zenger et al., 1980). Part of member C is primary dolomite. If dolomitization of member B is of mixed-water origin, then it must have taken place before deposition and dolomitization of the C member. Dolomitization of member B must have taken place during a break in sedimentation, prior to deposition of the C member. Two pieces of depositional evidence bear upon the existence of an unconformity

18

E. AL-AWADI ET AL.

within the Dammam Formation: (a) the Dammam Formation is predominantly of open shallow marine shelf facies with small fluctuations from lagoonal to tidal flat environments. The existence of a lignitic coal layer within the upper part of the B member indicates a regressive phase and terrestrial environments, such as swamps. These lignitic layers signal the beginning of an emergence before member C was deposited; (b) there is a distinct difference in the fauna and lithofacies of the A, B and C members. A and B are composed of massive, hard, partially silicified and dolomitized, fossiliferous limestone with few nummulites, whereas the C member is friable dolomite or totally dolomitized limestone with few microfossils. The thickness and frequency of silty lignitic layers increases upwards in B member. There is absolutely no lignite in C member. This change is sharp and conspicuous. On the basis of the diagenetic and depositional indications, it is suggested that there is a minor break between B and C members of the Dammam Formation. Depositional and diagenetic history

Submember

C

c

Transgression and Regression

Member

Formation

Epoch

Period

During the tectonically stable Eocene period, the Dammam Formation was deposited in a shallow marine shelf, with minor fluctuations from lagoon to tidal flat and swamp environments. This tectonically stable period was interrupted by small pulses in the source land and minor fluctuations in sea level, which caused an alteration of transgressive and regressive cycles and intercalations of lignitic layers within the carbonate sequence of the shallow marine shelf. The depositional history of the Dammam Formation is given below (Fig. 10). The Eocene to Recent depositional history of Kuwait is a local reflection of the

Environment of deposition

Dolomite

Lagoon—tidal flat

?

DAMMAM

EOCENE

TERTIARY

b6

T R

Fossiliferous dolomitic limestone

Lagoon

Lignite—silt—dolomite

Swamp—salt marsh— tidal flat

R

Cherty dolomitic limestone

Lagoon

T

b3

Lignite—silt—dolomite

Swamp—salt marsh— tidal flat

R

b2

Fossiliferous limestone

Lagoon

T

b1

Bituminous algal limestone

Tidal flat

R

a2

Nummilitic limestone

Lagoon

R

a1

Nummilitic limestone with shale

Shelf—lagoon

T b5 B b4

A

Figure 10. Composite stratigraphic column of Damman Formation in Umm-Gudair area.

SEDIMENT STRATIGRAPHY IN KUWAIT

19

regional geological evolution. Middle Eocene is known to be a quiet stable period of shallow warm seas, and low relief. Shallow shelf, lagoon tidal flat carbonates accumulated all around the Arabian continental platform including Kuwait (Murris, 1980). Dammam Formation in Kuwait represents the stable quiet shallow seas which prevailed throughout the Gulf region in Eocene time (Schlumberger, 1972; Lababidi & Hamdan, 1985). Oligocene witnessed a regional uplifting and is generally a period of non-deposition. Deposition in Oligocene is confined only to scarce restricted isolated basins, where either evaporites or lacustrine limestones are accumulated. The regional uplifting in Oligocene time is recorded as a major unconformity which extends from United Arab Emirates up to Kuwait and further north (Schlumberger, 1972; Murris, 1980). Miocene time marks the renewal of tectonic activity along the northern border of the Arabian Plate. Continental convergence and closure of the Tethyan ocean which started in Cretaceous time was finalized in Miocene time (Murris, 1980). The suture is characterized by large scale underthrusting of the Arabian plate under the Zagros suture belt (Kamen-kaye, 1970; Kassler, 1973; Murris, 1980). Compressional tectonic activity along the northern suture belt is coincident with the initial extensional tectonics which gave birth to the Red Sea as the Arabian plate started to break away from the African plate and move northward (Harris & Gass, 1981; Beydoun, 1984). Thrust faulting along the Zagros suture was accompanied by large scale vertical uplifting of the belt. This gave rise to an upsurge of clastic influx from the north, and molasse type clastic sediments accumulated in shallow marine to continental peripheral basins along the northern edge of the Arabian plate, south of the Zagros mountains in Miocene to Pliocene times. Sediments of Ghar and Jal Az-Zor Formations of Kuwait reflect this upsurge in clastic influx from the northern suture zone (Murris, 1980).

Summary and conclusion A general stratigraphic sequence of the Dammam Formation was established by study of cores and logs of water wells. The upwards sequence of the Dammam Formation is as follows. Member A. Nummulitic limestone with shale interlayers. Fossiliferous limestone, nummulitic towards the bottom. Member B. Bituminous algal limestone. Fossiliferous, impure limestone, silicified and dolomitized; occasionally silty with some chert nodules. Laminated silty to dolomite; silicified, with some organic matter and phosphate concentrations. Vuggy, fossiliferous limestone; highly silicified and dolomitized with some chert nodules; occasionally silty in the lower part. Silty lignite with dolomite interlayers. Fossiliferous, dolomitized limestone with shell fragments. Member C. Porous, friable dolomite. The fossils include Nummulites sp., Discocyclina sp., Alveolina sp., Pararotalia sp., Dictyoconoides sp., Orbitolites sp., Ammonia sp., Miliolids ssp., Rotalids sp., ostracods, corals, brachiopods, echinoids, algae, bryozoa and rate sponges. Environments of deposition varied between lagoon, tidal flat, salt marsh and swamp. Some indirect diagenetic and depositional indications suggest the presence of a minor unconformity between B and C members. Silicification, karstification and dolomitization are the diagenetic processes which reveal the existence of this unconformity. Silicification is indicated by high resistivity and karstification by rapid and high fluctuations in Resistivity logs. Silicification is partial and increases upwards

20

E. AL-AWADI ET AL.

towards the top of B and C members, but there is no connection between these silicified zones, because the lower part of C is not silicified. Silicification and karstification on top of C indicate an unconformity. There must be another minor break on top of B. Dolomitization of member B is partial and of mixed-water origin, (Hanshaw et al., 1971; Land, 1973, 1983) while dolomitization of member C is complete and it is probably of seepage-reflux origin (Adams & Rhodes, 1960; Zenger et al., 1980). Member B must have been silicified, karstified and dolomitized during a break in sedimentation prior to deposition of C. The depositional evidence is represented by the existence of lignitic coal layers within the upper part of the B member, which indicates an emergence before deposition of C. Another piece of evidence is the difference in fauna and lithofacies between A, B and C members. The Dammam limestones and dolomites accumulated during a quiet, stable period of the Eocene with small pulses in the sources land and minor fluctuations in the sea level causing alternating transgressive and regressive cycles. Deposition ended with a major regression in Late Eocene. Stratigraphy of Miocene–Pleistocene Kuwait group is studied on the basis of core samples and bore hole logs from Umm-Gudair, NW Al-Shagaya, Al-Salmi and AlWafra water wells, and exposures at Jal Az-Zor escarpment and Ahmadi quarry. The upper part of the Dammam Formation is also included in the study in order to investigate the unconformity. The upper part of the Dammam Formation is dolomite with a fragmented and cherty zone on top. Dolomite is either primary or secondary. Secondary dolomite was formed from early dolomitization of fossiliferous micrites, pelmicrites, intramicrites and fossiliferous pelmicrites. Fossils include Praerhapydionina Huberi HENSON, Dendritiana cf. dusen burgi HENSON, Praerhapydionina cf. delicata HENSON, Dendritina cf. Glynnjones, cyclogyra sp., Spirolina, Cylindracea LAMARCK, Spirolina sp. ?Coskinolina sp., ?Discorinopsis sp. and Miliolidae. Age range is from Middle Eocene to Oligocene. A more definite age would have helped greatly in delimiting the spatial and time boundaries of the Oligocene unconformity. Facies indicate that the Dammam Formation was deposited in a shallow lagoon to tidal flat environment. There are at least two or more stages of dolomitization and one or more stage of silicification. The final stage of silicification was related to percolation of acidic ground-water during the non-depositional period in Oligocene time when the surface of the Dammam Formation was karstified. Dissolution of carbonate minerals left fragments of chert and silica which collapsed into cavities and sinkholes forming the chert breccia cap along the unconformity. The dolomite near the unconformity surface was also calcitized. Collapsed brecciated chert caping the unconformity surface was mixed with sand filling karstic cavities. The extent of width of the karstic zone reflects the duration of exposure of the Dammam Formation to atmospheric conditions. The Tertiary topography of Kuwait was greatly influenced by structural highs and depressions, exemplified by southern Burgan area, and Northern Kuwait. The karstic features are well developed at Ahmadi rock quarry which is located on the Burgan high. There is a very thin chert breccia on top of the Dammam Formation at NW AlShagaya well which is near the depressions of northern Kuwait. The erosional period commonly recognized in the Oligocene time might have been confined to only topographic highs. Sedimentation either continued in the northern physiographic depressions or else the break in sedimentation was of a comparatively short duration. Clastic sediments of the Kuwait Group are mostly regarded as continental deposits. NW Al-Shagaya well reveals that the lower part of the Kuwait Group is possibly of marine origin at this locality. Grey–green pyrite-bearing clays and overlying green sands constitute the lowermost portion of the Kuwait Group at NW Al-Shagaya well.

SEDIMENT STRATIGRAPHY IN KUWAIT

21

The sands are overlain by a thin layer of fossiliferous sandy limestone. Loose, gravelly, fluvial sands overlie Dammam Formation unconformably at the Ahmadi rock quarry. Apparently, marine conditions prevailed in northern physiographic depressions during Late Oligocene–Early Miocene times while topographic highs such as the Ahmadi ridge remained above sea level. The basal clay layer is widespread. It is encountered at Umm-Gudair, Al-Wafra and Al-Salmi bore holes as well. The overall stratigraphic sequence of the Kuwait Group bear clues for a gradual regional regression and accompanying increase in clastic material influx. The Kuwait Group starts with marine clays and continues upward as fluvial, river channel sands, and ends with alluvial fan conglomerates. Deposition of the Kuwait Group was interrupted frequently by brief periods of quiecence and non-deposition. Pauses in sedimentation are marked by surfaces of carbonate and gypsum cementation in the pore spaces of sands by evaporation of capillary water, and alteration of kaolinitic clays to polygorskite. The lower parts of the group consist mainly of loose sands without cementation. The degree of cementation and frequency of cemented layers increase towards the top of the Kuwait Group. The Kuwait Group was tentatively subdivided into three Formations. There are no markers or distinct lithologic boundaries to set Formations apart. The Kuwait Group is subdivided into three Formations in the present study confirming most previous work. These are Ghar, Jal Az-Zor and Dibdibba Formations from bottom to top. A fossiliferous sandy limestone layer at the lower part of the Kuwait Group is taken to mark the boundary between Ghar and the overlying Jal Az-Zor Formations. The boundary between the Jal Az-Zor and Dibdibba Formations is hard to locate. The ledge-making, carbonate-cemented sand layers are included in the Jal Az-Zor, while overlying poorly-bedded and weakly-cemented gravels are included in the Dibdibba Formation. Subsurface correlation suggests that the thickness of Ghar and Jal Az-Zor Formations is uniform, but Dibdibba Formation change thickness from north to south. There is a thick section of gravel to the north of Jal Az-Zor escarpment, but the gravels are either absent or occur as a thin surface veneer to the south. Thinning or absence of the gravels in some places is most probably due to non-deposition rather than erosion. Depocenter of the Dibdibba Formation was to the north of Jal AzZor. Fossils are rare or absent in Kuwait Group clastic sediments. The only layer with some broken fragments of macro fossils is the sandy limestone between Ghar and Jal Az-Zor Formations. Fossils are mostly unidentifiable and yield a wide age range. Kuwait Group is regarded as of Miocene to Pleistocene age on the basis of earlier works.

References Abdulla, F.H. (1987). Computer simulation of source rock maturation in Kuwait. M.Sc. thesis, University of Kuwait (unpublished). Adams, J.E. & Rhodes, M.L. (1960). Dolomitization of seepage refluxion. AAPG Bulletin, 44: 1912–1921. Al-Awadi, E.A. (1988). Stratigraphy of the Dammam Formation in the Umm-Gudair Area, Kuwait. M.Sc. thesis, University of Kuwait (unpublished). Arisha, H. (1984). Stratigraphic Column of Kuwait. Kuwait: Ministry of Oil (unpublished). Asquith, G. & Gibson, C. (1982). Basic Well Log Analysis for Geologists. Oklahoma: AAPG. 215 pp. Bergstrom, R.E. & Aten, R.E. (1964). Natural recharge and localization of fresh groundwater in Kuwait. Journal of Hydrology, 2: 213–231.

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Beydoun, Z.R. (1984). The geological setting and tectonic framework of the Middle East. In: Proceedings of the Seminar on Source and Habitat of Petroleum in the Middle East Arab Countries, pp. 5–72. Technical papers. Kuwait: Department of Geology, American University of Beirut. Burdon, D.J. & Al-Sharhan, A. (1968). The problem of the Paleokarstic Dammam limestone aquifer in Kuwait. Journal of Hydrology, 6: 385–404. Cox, P.T. & Rhoades, R.O. (1935). A report on the geology and oil prospects of Kuwait Territory: Geological Report No.1. The Kuwait Oil Company Limited (unpublished). Fichter, L.S. & Poche, D.J. (1979). Ancient Environments and Interpretation of Geologic History. Minneapolis: Burgess Publishing Company. 168 pp. Fuchs, W., Gattinger, T.E. & Holzer, H.F. (1968). Explanatory Text to the Synoptic Geologic Map of Kuwait. Vienna: Geological Survey of Austria. 87 pp. Hanshaw, B.B., Back, W. & Deike, R.G. (1971). A geochemical hypothesis for dolomitization by groundwater. Economic Geology, 66: 710–724. Harris, N.B.W. & Gass, I.G. (1981). Significance of contrasting magmatism in North East Africa and Saudi Arabia. Nature, 289: 394–396. Kassler, P. (1973). The structural and geomorphic evolution of the Persian Gulf. In: Purser, B.H. (Ed.), The Persian Gulf, pp. 11–32. Berlin: Springer-Verlag. Kamen-kaye, M. (1970). Geology and productivity of the Persian Gulf Synclinorium. AAPG Bulletin, 45: 2371–2394. Lababidi, M.M. & Hamdan, A.N. (1985). Preliminary Lithostratigraphic Correlation Study in OAPEC Member Countries. Kuwait: Organization of Arab Petroleum Exporting Countries, Energy Resources Department. 171 pp. Land, L.S. (1973). Holocene meteoric dolomitization of Pleistocene limestone, North Jamaica. Sedimentology, 20: 411–422. Land, L.S. (1983). Dolomitization. AAPG Education Course Note Ser. No. 25. Milton, D.I. (1967). Geology of the Arabian Peninsula “Kuwait”. USGS Prof Paper. 560-F. 7 pp. Murris, R.J. (1980). Middle East: stratigraphic evolution and oil habitat. AAPG Bulletin, 64: 597–618. O’Brien, C.A.E. (1952). Stratigraphical Study of the Eocene Limestone in Southern Kuwait. The Kuwait Oil Company (unpublished). Omar, S.A., Al-Yaqubi, A. & Senay, Y. (1981). Geology and groundwater hydrology of the State of Kuwait. In: First Symposium on the Future Development of Water Resources. Journal of Gulf and Arabian Peninsula Studies, Kuwait University, 1: 5–30. Owen, R.M.S. & Nasr, S.N. (1958). Stratigraphy of Kuwait-Basra Area. Habitat of Oil. AAPG Bulletin, 42: 1252–1278. Parson Corporation. (1963). Report to Government of Kuwait, Groundwater Resources of Kuwait: V.I. and II (unpublished). Salman, A.M.S. (1979). Geology of the Jal Az-Zor -Al-Liyan Area, Kuwait. M.Sc. thesis, Kuwait University (unpublished). Schlumberger. (1972). Log interpretation, Vol. I — Principles. Houston: Schlumberger Ltd. 113 pp. Steineke, M., Bramkamp, R.A. & Sander, N.J. (1958). Stratigraphy of Eastern Saudi Arabia. In: Weeks, L.G. (Ed.), Habitat of Oil: A symposium, pp. 1294–1329. Tulsa: AAPG. Zenger, D.H., Dunham, J.B. & Ethington, R.A. (1980). Concepts and models of dolomitization. SEPM Sp. Pub., 28: 23 pp.