The Miocene carbonate Dam Formation is well exposed in the Jebel Al-Nakhash area of southern Qatar. Three sections were measured in a detailed investigation of the Formation’s stratigraphy, micropaleontology, and paleoecology. This biostratigraphic and paleoecologic study was supported by the analysis of microlithofacies and foraminiferal assemblages. Microfossils are predominantly benthic foraminifera represented by 38 species, many of which are milioline and one is a larger form. A Borelis melo melo Local Range Zone of Early Miocene (Burdigalian) age was recognized. The nature and distribution of the benthic foraminiferal assemblage, in association with lithofacies evidence, indicated a general shoaling-upward trend.

The Dam Formation was stratigraphically subdivided into two new formal members. The basal Al-Kharrara Member is made up of limestone, marl, and claystone, and the overlying Al-Nakhash Member is a cyclic assemblage of carbonate, evaporite, and algal stromatolite facies. The lithofacies are grouped into four major types of which limestone, subdivided into six subfacies, is dominant. The Al-Kharrara was interpreted as having been deposited in warm (25°–30°C), clear, shallow waters of the inner neritic zone (0–35 m) that had an elevated salinity (35‰–50‰) and a vegetated substrate. The Al-Nakhash probably formed in an oscillating, very shallow-marine environment (0–10 m deep, including tidal flats), under warm climatic conditions that eventually led to the accumulation of evaporites and algal stromatolites.

Exposures of Miocene rocks in Qatar are confined to the south of the country in the Al-Nakhash, Al-Kharrara, and Abu-Samra areas (Figure 1). They are best exposed at Jebel Al-Nakhash near the southern end of the Dukhan oilfield. The Miocene rocks form irregular, isolated, benched hills that range in height from 15 to 80 m. They are represented by the Dam and Hofuf formations, the latter extending into the Pliocene.

Figure 1:

The Miocene outcrops of southwestern Qatar (shown in brown), and the type location of the Dam Formation (Jabal al Lidam) in eastern Saudi Arabia.

Figure 1:

The Miocene outcrops of southwestern Qatar (shown in brown), and the type location of the Dam Formation (Jabal al Lidam) in eastern Saudi Arabia.

Thralls and Hasson (1956) formalized the Dam Formation, the name having been introduced by Steineke and Koch in an unpublished Aramco report in 1935. The type location of the Dam is at Jabal al Lidam in Saudi Arabia about 60 km west of Dhahran (Figure 1). The total thickness of the Formation at its type locality is about 90 m where it consists mainly of marl, chalky limestone and clay, interbedded with thin beds of sandstone. It is unconformably overlain by the Hofuf Formation.

Figure 2:

Aerial photograph and contoured map (elevation in meters) of the Jebel Al-Nakhash area showing locations of sections I, II, and III.

Figure 2:

Aerial photograph and contoured map (elevation in meters) of the Jebel Al-Nakhash area showing locations of sections I, II, and III.

Previous Work

The Dam Formation of Qatar has been studied by several workers, notably Cavelier (1970) who subdivided it into the lower and upper Dam subformations. Later, Abu-Zeid and Khalifa (1983) modified Cavelier’s work and divided the Formation into members A (lower) and B (upper). Abu-Zeid and Al-Kuwari (1989) investigated the clay components of the Formation in Qatar. Hewaidy (1991) studied the foraminifera of the Dam Formation in the Jebel Al-Nakhash and Al-Kharrara areas and dated them as Burdigalian-Helvetian (Early to Middle Miocene). More recently, Khalifa and Mahmoud (1993) identified three types of algal stromatolites in member B of the Dam Formation at Jebel Al-Nakhash. They proposed a protected tidal environment for the deposition of the Formation.

Objectives and Methods

The objectives of our study of the Dam Formation were as follows:

  1. Describe its lithofacies characteristics;

  2. Present its formal stratigraphy incorporating a full biostratigraphic age interpretation of the foraminifera; and

  3. Establish the paleoecological conditions that prevailed at the time of its deposition.

A total of 121 rock samples were collected from three stratigraphic sections (sections I, II, and III) in the Jebel Al-Nakhash area (Figure 2). From these, 60 thin sections were prepared and qualitatively examined for microfacies and lithofacies analysis, and 97 samples were selected for microfaunal analysis. Standard processing techniques were used. All rock materials, macrofossils, and microfossil slides are curated in the Geology Department, Faculty of Science, University of Qatar.

The Miocene Dam Formation has unconformable (disconformable) contacts with the underlying and overlying formations. It overlies the Middle Eocene shaly limestone of the Dammam Formation and is overlain by the Late Miocene to Early Pliocene conglomerate and sandstone of the Hofuf Formation (Figure 3). According to the sequence stratigraphic model of Sharland et al. (2001), Maximum Flooding Surface MFS Ng20 occurs within carbonates of the Dam Formation (Tables 1, 2).

Figure 3:

Generalized stratigraphic column of the Tertiary rocks in Qatar. The lower boundary of the Dam Formation is not exposed.

Figure 3:

Generalized stratigraphic column of the Tertiary rocks in Qatar. The lower boundary of the Dam Formation is not exposed.

Table 1

MFS Ng20 (after Sharland et al., 2001)

Reference Section:shales below upper Asmari sandstones, southern Iran
Criteria:influx of open-marine fauna
Age diagnostic fossils:the larger foraminifera Borelis melo and Meandropsina iranica
Environmental indicators:open marine fauna
Age:Neogene (Early Miocene; mid Burdigalian)
Date:18 Ma
Original extent of MFS:subregional
Preservation of MFS:local
Controlling mechanism:subsidence enhanced by eustacy
Correlation confidence:moderate
Date uncertainty:moderate
Reference Section:shales below upper Asmari sandstones, southern Iran
Criteria:influx of open-marine fauna
Age diagnostic fossils:the larger foraminifera Borelis melo and Meandropsina iranica
Environmental indicators:open marine fauna
Age:Neogene (Early Miocene; mid Burdigalian)
Date:18 Ma
Original extent of MFS:subregional
Preservation of MFS:local
Controlling mechanism:subsidence enhanced by eustacy
Correlation confidence:moderate
Date uncertainty:moderate
Table 2

MFS Ng20 plate-wide correlation (after Sharland et al., 2001 

Iran:shales below upper Asmari sandstones, southern Iran; Razak Formation
Oman:within the Taqa and Mughsayl carbonates of southern Oman
Saudi Arabia:?carbonates near base Dam Formation; intra-Wadi Waqb limestones of the Red Sea containing Borelis melo
Qatar:Dam Formation carbonates
UAE:Dam Formation carbonates
Iran:shales below upper Asmari sandstones, southern Iran; Razak Formation
Oman:within the Taqa and Mughsayl carbonates of southern Oman
Saudi Arabia:?carbonates near base Dam Formation; intra-Wadi Waqb limestones of the Red Sea containing Borelis melo
Qatar:Dam Formation carbonates
UAE:Dam Formation carbonates

Rapid vertical and lateral changes occur within the Dam Formation. Figure 4 shows the lithology and thickness of each of the three stratigraphic sections in the Al-Nakhash area and the correlation between them. Limestone, marl, clay and gypsum are the major elements of the Formation in Qatar. It is richly fossiliferous in some units but others are unfossiliferous. The richly fossiliferous units are mainly marly and limestone facies, whereas the poorly fossiliferous or unfossiliferous ones are generally associated with clayey facies. In addition, diagenesis has strongly affected the fossil content.

Figure 4:

Lithology, thickness, and correlation of the studied sections in the Al-Nakhash area. Note that the upper member (Al-Nakhash) attains its maximum thickness in the main Jebel Al-Nakhash, the highest point in Qatar (about 90 m high).

Figure 4:

Lithology, thickness, and correlation of the studied sections in the Al-Nakhash area. Note that the upper member (Al-Nakhash) attains its maximum thickness in the main Jebel Al-Nakhash, the highest point in Qatar (about 90 m high).

The basal part of the Formation is characterized by clay and marl intercalated with limestone that gradually changes upward into chalky limestone. It contains abundant pelecypods, gastropods and burrows. The upper part is composed almost entirely of limestone and gypsum. The gypsum content increases toward the top of the Formation. At least three stromatolitic bands are present in the upper part of the Formation.

The Dam Formation has been redescribed based on lithological characters, and subdivided into two formal members, the Al-Kharrara Member and the Al-Nakhash Member. The naming of the new members follows the general rules of stratigraphic nomenclature as given by the International Subcommission on Stratigraphic Classification of the IUGS International Commission on Stratigraphy (Murphy and Salvador, 2000). Table 3 shows the relationships of the Al-Kharrara and Al-Nakhash members to the previous informal stratigraphic subdivision of the Dam Formation in Qatar. Figures 5a to 5c are photographs of exposures of the Dam Formation at Sections I, II, and III, respectively.

Figure 5:

Outcrops of the Dam Formation in the Jebel Al-Nakhash area: (a) Al-Kharrara and Al-Nakhash members exposed in Section I; (b) Al-Kharrara and Al-Nakhash members exposed in Section II; (c) Al-Kharrara Member exposed in Section III.

Figure 5:

Outcrops of the Dam Formation in the Jebel Al-Nakhash area: (a) Al-Kharrara and Al-Nakhash members exposed in Section I; (b) Al-Kharrara and Al-Nakhash members exposed in Section II; (c) Al-Kharrara Member exposed in Section III.

Table 3

Stratigraphic subdivision and nomenclature of the Miocene Dam Formation in Qatar.

Rock unitCavelier (1970) Abu-Zeid and Khalifa (1983) Hewaidy (1991) Present study
Dam Formationupper Dam subformationmember Bupper Dam memberAl-Nakhash Member
lower Dam subformationmember Alower Dam memberAl-Kharrara Member
Rock unitCavelier (1970) Abu-Zeid and Khalifa (1983) Hewaidy (1991) Present study
Dam Formationupper Dam subformationmember Bupper Dam memberAl-Nakhash Member
lower Dam subformationmember Alower Dam memberAl-Kharrara Member
Table 4:

Distribution of benthic foraminifera; for location of samples see Figure 4.

Al-Kharrara Member

Definition: Lower part of the Dam Formation.

Stratotype (type section): Jebel Al-Nakhash, 85 km southwest of Doha, between latitude 24°52’ and 24°53’N., and longitude 50°50’30” and 50°54’30”E. (Figures 1 and 2).

Geologic age: Early Miocene; basal part of Borelis melo melo Local Range Zone (see section on Age of the Dam Formation, below).

Etymology: The name was derived from the Al-Kharrara area of southern Qatar (Figure 1).

Characterization: The base of the Member is not exposed and the upper boundary was placed between the dolomitic limestone of the Al-Kharrara Member and the stromatolitic limestone of the overlying Al-Nakhash Member.

Description: The Al-Kharrara Member is about 28 m thick and consists mainly of calcareous claystone interbedded with marls and arenitic limestone capped by dolomitic limestone. In the Al-Nakhash area, the clayey and sandy facies pinch-out toward the south and are generally replaced by a dolomitic limestone or limestone facies. In addition, a wide lateral variation was observed in the Al-Kharrara area where limestone replaces the clastic facies (Hewaidy, 1991).

In the studied sections, molds and casts of bivalves and gastropods (Plate 1), together with fish teeth and ostracods, characterized the Al-Kharrara Member. In addition to a rich miliolid assemblage, large arenaceous foraminifera (Clavuli-noides, Clavulina and Haplophrag-moides) and rare representatives of Elphidium, Ammonia, Cibicides, and Lenticulina were present.

Plate 1:

Gastropods and pelecypods from the Dam Formation; for location of samples see Figure 4. All specimens are approximately natural size.

Plate 1:

Gastropods and pelecypods from the Dam Formation; for location of samples see Figure 4. All specimens are approximately natural size.

Al-Nakhash Member

Definition: Upper part of the Dam Formation.

Stratotype (type section): Jebel Al-Nakhash, 85 km southwest of Doha, between latitude 24°52’ and 24°53”N, and longitude 50°50’30” and 50°54’30”E (Figure 1).

Geologic age: Early Miocene; upper part of the Borelis melo melo Local Range Zone (see section on Age of the Dam Formation, below).

Etymology: The name was derived from the Al-Nakhash area of southern Qatar (Figures 1 and 2).

Characterization: The upper boundary of the Al-Nakhash Member was drawn at the erosive boundary between the gypsiferous limestone of the Al-Nakhash Member and the overlying conglomerate of the Hofuf Formation (see Section I; Figure 4). Gypsiferous layers are characteristic of the Al-Nakhash Member.

Description: The Al-Nakhash Member is about 46 m thick and consists mainly of gypsiferous, chalky and stromatolitic limestone. The stromatolitic facies appear to be restricted to parts of the Al-Nakhash area, and are replaced by marly limestone at Al–Kharrara (Hewaidy, 1991). Gypsum or celestite is common in the uppermost part of the Member.

The foraminiferal assemblage within the Al-Nakhash Member differed from the underlying Al-Kharrara Member in the complete absence of the arenaceous species.

The Dam Formation showed cyclic sedimentation and significant lateral changes between the studied sections. The lithofacies of the Formation were grouped into four major types of which limestone is the most abundant. Several representative lithofacies are illustrated by outcrop photographs (Figure 6) and thin-sections (Plate 2).

Figure 6:

Selected lithofacies of the Dam Formation. (a) Reddish to greenish compact claystone (lithofacies B) lower part of Al-Kharrara Member. (b) Yellowish sandy limestone (lithofacies LA2) interbedded with thin marl bands (lithofacies C) in fining-upward cycle, Al-Kharrara Member. (c) Yellowish, hard stromatolitic limestone (lithofacies LA5), Al-Nakhash Member. (d) Massive white crystalline celestite (lithofacies D), upper part of Al-Nakhash Member.

Figure 6:

Selected lithofacies of the Dam Formation. (a) Reddish to greenish compact claystone (lithofacies B) lower part of Al-Kharrara Member. (b) Yellowish sandy limestone (lithofacies LA2) interbedded with thin marl bands (lithofacies C) in fining-upward cycle, Al-Kharrara Member. (c) Yellowish, hard stromatolitic limestone (lithofacies LA5), Al-Nakhash Member. (d) Massive white crystalline celestite (lithofacies D), upper part of Al-Nakhash Member.

Plate 2:

Thin-sections of representative samples of lithofacies of the Dam Formation.

Plate 2:

Thin-sections of representative samples of lithofacies of the Dam Formation.

Limestone (Lithofacies A; LA)

Limestone is the most abundant lithofacies in the Dam Formation. The classification followed that of Dunham (1962). Plate 2-1 is a thin-section of foraminiferal limestone. The limestone lithofacies were subdivided into the following subfacies based on its components:

Subfacies (LA1): Clayey limestone. Dull-yellow layers of clayey limestone were present throughout most of the Dam Formation and varied in thickness from a few centimeters to about 4 m. The subfacies contained pelecypods and foraminifera. Frequent clay flocculiths were present, probably as bound materials.

Subfacies (LA2): Sandy limestone. This subfacies was particularly well developed in the Al-Kharrara Member (Figure 6b; Plate 2-2) where it varied from 0.35 to 9.0 m thick. It was yellow to brown, and gypsiferous and ferruginous in part. Rare gastropods and pelecypods were present. In thin-section, the sandy limestone consisted of 40 percent detrital quartz grains (medium- to fine-grained), 40 percent micrite matrix, and 20 percent carbonate allochems.

Subfacies (LA3): Chalky limestone. The amount of chalky limestone generally increased toward the top of the Formation. The subfacies varied from dull-white to gray and contained a few pelecypods, gastropods, and foraminifera. Petrographic analysis showed that it was composed of mudstone to wackestone (micritic material with some clay and quartz). The thickness of individual units varied from 2.0 m to about 4.0 m.

Subfacies (LA4): Oolitic limestone. This subfacies consisted of light-brown, medium- to coarse-grained grainstone as thin beds in the Al-Nakhash Member (Plate 2-3). The grains were well sorted and included rounded oolites and pellets. The thickness ranged from 0.75 m to 1.20 m.

Subfacies (LA5): Stromatolitic limestone. This subfacies occured only in the Al-Nakhash Member and consisted of pinkish to creamy limestone interbedded with thin bands of stromatolites (Figure 6c). Internal molds of gastropods were present in some places (see Plate 1-1). Stromatolitic bands occurred in at least three levels. The lithofacies represented the base of the Al-Nakhash Member in all of the studied sections. The thickness of the stromatolite bands ranged from 0.20 to 0.60 m whereas the total thickness of this unit was about 1 m. Petrographic studies identified algal and molluscan debris in the mudstone/wackestone matrix.

Subfacies (LA6): Dolomitic limestone. The subfacies consisted of massive gray dolomitic limestone about 2 m thick intercalated with thin laminae of gypsum (Plate 2-4). In places it was the topmost bed of the Al-Kharrara Member. Microscopic analysis identified rhombic dolomite in a wackestone/packstone matrix.

Claystone (Lithofacies B; LB)

This lithofacies consisted of fine- to very fine-grained, thinly laminated, soft, red-brown and greenish ferruginous clay (Figure 6a; Plate 2-5). It was weathered throughout. Small pelecypods, gastropods, and rare foraminifera were its main faunal elements. Thin bands of gypsum on a scale of 2 mm to 5 mm were generally interbedded with the claystone. The facies was mainly confined to the Al-Kharrara Member and varied in thickness from 1.5 to 4.2 m according to the locality.

Marl (Lithofacies C; LC)

This lithofacies was composed of compact creamy to yellowish marl (Figure 6b; Plate 2-6). Some beds contained small pelecypods. It was present at various levels in the Dam Formation and was intercalated with clays and sands, particularly in the Al-Nakhash Member. In some places, the marls were replaced laterally by the clayey limestone lithofacies (LA1). The thickness of the individual units of lithofacies C ranged from 0.35 to 2.0 m.

Evaporites (Lithofacies D; LD)

The evaporites in the Dam Formation were represented by dull-white to gray weathered gypsum and celestite (Figures 6d; Plates 2-7 and 2-8). Lithofacies D was mainly present in the Al-Nakhash Member and was between 0.80 m and 1.20 m thick.

Foraminiferal analysis of the Miocene succession at Jebel Al-Nakhash area identified 38 species of benthic foraminifera that belong to 29 genera representing 14 families. The classification was based mainly on that of Loeblich and Tappan (1988). Planktonic foraminifera are absent because of paleoecological factors. The distribution of the benthic species in sections I, II, and III is shown in Table 4, and they are illustrated in Plates 3a to 3c.

Plate 3a:

Foraminifera of the Dam Formation; for location of samples refer to Figure 4.

Plate 3a:

Foraminifera of the Dam Formation; for location of samples refer to Figure 4.

Plate 3b:

Foraminifera of the Dam Formation (continued).

Plate 3b:

Foraminifera of the Dam Formation (continued).

Plate 3c:

Foraminifera of the Dam Formation (continued).

Plate 3c:

Foraminifera of the Dam Formation (continued).

The foraminiferal assemblage recovered from the Dam Formation during the present study was characterized by the dominance of a milioline fauna (including Quinqueloculina, Triloculina, Peneroplis, Dendritina, Sigmoilina, Agglutinella, Pyrgo, Spirolina, Archaias) that predominated over the textulariid and rotaliid fauna. Most of the benthic foraminiferal species recovered from the Formation had already been found in the Miocene of Qatar (Hewaidy, 1991) and the Mediterranean region. They are also recorded from other areas of Miocene strata; for example, from Egypt (Souaya, 1963a, 1967; Sadek, 1999); Jamaica (Robertson, 1998); and northeast and northwest Libya (Sherif, 1991; Abuserwil, 1996; Abdulsamad and Barbieri, 1999).

The larger benthic foraminifera Borelis melo melo was found in the Dam Formation in the Al-Nakhash area during the present investigation. According to Hewaidy (1991), it is not present at Jebel Al-Nakhash, although he recorded it from the Al-Kharrara section. Because of the lack of other planktonic foraminifera in the material collected from the Al-Nakhash area, Borelis melo was used as a marker species for biostratigraphic zonation (Borelis melo melo Zone) of the Dam Formation succession.

Adams (1970) showed that the range base of Borelis melo melo is within the lower Tf zone of the Far East. This was confirmed by recent work of BouDagher-Fadel and Banner (1999) who noted that the lower Tf is as old as planktonic zone N6 (Burdigalian; Early Miocene). Thus, although Borelis melo can occur in Middle Miocene rocks of the Mediterranean region (Bellini, 1969; Bassiouni et al., 1975; Hammad et al., 1976; Abuserwil, 1996; Ouda, 1998; Sadek, 1999; and Abdulsamad and Barbieri, 1999), it could also be as old as Early Miocene.

The age of the Dam Formation has been controversial. In Qatar, it is unconformable on the Lower to Middle Eocene Dammam Formation and is unconformably overlain by clastics of the Hofuf Formation of Late Miocene to Early Pliocene age. Cox (1936) dated the Dam Formation as late Early Miocene based on the occurrence of Ostrea latimarginata (that extends upward through the Neogene) in southern Iran and Bahrain. Powers et al. (1966) and Powers (1968) assigned a Middle Miocene age to the Formation because of the presence of O. latimarginata, Echinocyamus sp., and Archaias angulatus (foraminifera) and other invertebrate fossils in Saudi Arabia. Nevertheless, Cavelier (1970) proposed a Middle to Late Miocene age for the Dam Formation in Qatar. More recently, Hewaidy (1991) and Khalifa and Mahmoud (1993) studied foraminifera from the Formation at Al-Kharrara and Jebel Al-Nakhash and dated the Formation as Burdigalian-Helvetian (Early to Middle Miocene). This age bracket would include the maximum flooding surface MFS Ng20 and Ng30 of Sharland et al. (2001).

In the United Arab Emirates, the Formation is demonstrably Early Miocene (Burdigalian) as shown by strontium isotope dating (Peebles, 1999), and an Early Miocene age is also indicated in Saudi Arabia (Greenwood, 1987; Whybrow et al., 1987; Otero and Gayet, 2001). A Borelis melo-bearing limestone/shale is common in many parts of the Arabian Plate (including the Red Sea margin) and has been given a Burdigalian age (Sharland et al., 2001). The Formation represents a distinct flooding event that culminated in MFS Ng20 (mid-Burdigalian) of Sharland et al. (2001), whereas the later flooding event of MFS Ng30 (Middle Miocene, late Langhian) occurred within the Jeribe Formation of Saudi Arabia. An Early Miocene (Burdigalian) age is therefore assigned to the Dam Formation and the Borelis melo melo Zone is defined as follows:

  • Borelis melo melo Zone

    • Category: Local Range Zone

    • Age: Early Miocene (Burdigalian)

    • Author: This study

    • Definition: Range of the index taxon, from local first appearance to local last occurrence.

Regional shoaling in the Miocene occurred at about the Borelis melo datum and was apparent in the upward succession from the Al-Kharrara Member to the Al-Nakhash Member. Paleogeographically, the Miocene foraminiferal assemblage in the Al-Nakhash area showed some degree of similarity with assemblages from the Caribbean and Indo-Pacific.

Al-Kharrara Member

The fossils identified in the Al-Kharrara Member included Echinocyamus sp. (echinoides), Ostrea latimarginata, molds and casts of bivalves and gastropods, fish teeth, and ostracods, as well as a shallow-water benthic foraminiferal assemblage of the inner-shelf type. The foraminiferal fauna contained an abundance of the genera Quinqueloculina, Triloculina, Peneroplis, Pyrgo, Sigmoilina, Spirolina, Amphisorus, Borelis, Spiroloculina, and rare representatives of Elphidium, Ammonia, Cibicides, Lenticulina, Clavulinoides, Clavulina, and Haplophragmoides.

In the Caribbean, species of Borelis presently inhabit clean, washed sands and oolites of the carbonate banks and inter-reef channels (Brasier, 1975). According to Bignot and Guernet (1976), Borelis melo curdica from the Miocene of Kos in Greece, indicates very shallow, clear water about 1 m deep, temperatures of from 25° to 30°C, strong currents, and an elevated salinity (35‰–50‰). Haynes (1981), Walton and Sloan (1990) and Sherif (1991) noted that the larger miliolid Borelis melo inhabits sandy and oolitic substrates in shallow, warm, clear hypersaline conditions in the inner neritic zone (0–33 m).

The genus Peneroplis in the Quseir area of Egypt is indicative of the inner to middle neritic (0–152 m) vegetated carbonate substrate (Souaya, 1963a). It was very common and dominant at shallow depths down to 18 m. Quinqueloculina spp. could flourish in open, shallow-marine conditions, also with a vegetated substrate. It was slightly tolerant to hypersaline water and had a depth range of from 25 to 35 m in the inner neritic zone (Haynes, 1981). Small miliolids, such as Spiroloculina and Pyrgo, inhabited warm, clear, shallow-marine or hypersaline waters (Haynes, 1981; Walton and Sloan, 1990).

Ammonia beccarii is abundant in shallow waters of present-day seas but, as a fossil, it is commonly found in lagoonal, brackish, or shallow brackish-lagoonal environments (Martin, 1952; Murray, 1989). The presence of various species of Cibicides, Lenticulina and Elphidium is probably indicative of shallow-marine conditions comparable to those described from shallow areas of the present-day Mediterranean littoral (Said and Kamel, 1956). Arenaceous forms (Clavulina and Clavulinoides) are represented by very few individuals. This rarity could be explained by their preference for deeper-water environments. The occurrence of Haplophragmoides sp. could support either a deep-water or shallow-water environment (Murray, 1989).

In summary, sediments of the Al-Kharrara Member contained benthic foraminifera. These indicated deposition in warm (25°–30°C), clear, shallow water of the inner neritic zone (0–35 m deep) that had an elevated salinity (35‰ to 50‰), and a vegetated substrate.

Al-Nakhash Member

The Member is composed of alternating layers of carbonate, marl, evaporites (gypsum and celestite), and stromatolitic limestone. In addition to foraminifera, molds and casts of pelecypods and gastropods, fish teeth, and some bryozoa were present. The benthic foraminiferal assemblage was similar to that of the Al-Kharrara Member except for the absence of the arenaceous species Haplophragmoides, Clavulinoides, and Clavulina. Barren intervals were more common than in the underlying member and may reflect poor fossil preservation. Only 11 samples contained fossils, and the samples had a low species dominance and low diversity.

Khalifa and Mahmoud (1993) recognized three types of algal stromatolites in the main Jebel Al-Nakhash section (Section I in this study). They are stratiform cryptalgal laminites at the base, laterally linked hemispheroids (type LLH) in the middle, and vertically stacked hemispheroids (type-SH) at the top. Stromatolites are good environmental indicators. Algal stromatolites are preferentially formed under warm climatic conditions in a littoral environment and may be restricted to tidal flats (Souaya, 1963b; Keheila et al., 1986). Similarly, the presence of the evaporite minerals gypsum and celestite at various stratigraphic levels in the Al-Nakhash Member suggested sabkha deposits (littoral zone tidal flats) that formed in arid, saline conditions. In view of these environmental indicators, it is probable that sediments of the Al-Nakhash Member accumulated in a very shallow-marine environment (tidal flats), under warm climatic conditions.

Paleoenvironmental summary

The Early Miocene rocks of the Al-Nakhash area were deposited in a restricted carbonate platform environment. The nature and distribution of the benthic foraminiferal assemblage, in association with lithofacies evidence, indicated a general shoaling-upward trend. Similar conditions prevailed contemporaneously in the Mediterranean region; for example, eastern Cyrenaica (Bellini, 1969); the Al Khums area of northwest Libya (Sherif, 1991); subsurface in offshore northwest Libya (Abuserwil, 1996); Jabal Al-Akhdar in northeast Libya (Abdulasamad and Barbieri, 1999); south Quseir and Gulf of Suez, Egypt (Souaya, 1963a, 1967); and the northern Western Desert, Egypt (Bassiouni et al., 1975; Hammad et al., 1976; Ouda, 1998; Sadek, 1999).

In contrast, the Late Miocene (Tortonian-Messinian) was a period of emergence and subaerial erosion in most parts of the Mediterranean and Indo-Pacific regions (Ouda, 1998). The same conditions prevailed in southern Qatar so that the Late Miocene to Pliocene clastic rocks of the Hofuf Formation that overlie the Dam Formation accumulated under shallow-marine or subaerial conditions.

Three surface sections of the Dam Formation in the Al-Nakhash area of southwestern Qatar were logged, measured, and sampled for microlithofacies and micropaleontologic analyses. The Formation consists of four major lithofacies units. They are limestones, marls, clays and evaporites, of which limestone is the most important in terms of thickness and faunal content. The limestone is subdivided into six subfacies as follows: clayey limestone, sandy limestone, chalky limestone, oolitic limestone, stromatolitic limestone, and dolomitic limestone.

The Dam Formation is subdivided into the two new formal units of the Al-Kharrara Member (lower) and Al-Nakhash Member (upper). The Al-Kharrara is about 28 m thick and consists mainly of calcareous claystone interbedded with marl, arenitic limestone, and dolomitic limestone. The Al-Nakhash Member is about 46 m thick and is mainly composed of gypsiferous, chalky and stromatolitic limestones.

The faunal constituents are for the most part foraminifera, pelecypods, gastropods, and stromatolites, together with burrows. The foraminiferal assemblages consist of 38 species of benthic foraminifera belonging to 29 genera that represent 14 families. The larger benthic foraminifera Borelis melo melo is used as a marker species for the Dam Formation in Qatar. It is assigned an Early Miocene (Burdigalian) age by comparison with the age of the Dam Formation in the United Arab Emirates and Saudi Arabia, and by the presence of B. melo melo.

The lithofacies and associated faunal assemblages indicate that the sediments of the Dam Formation were deposited in a very shallow tidal-flat setting under warm climatic conditions. In detail, sediments of the Al-Kharrara Member were deposited in warm (25°–30°C), clear, shallow water of the inner neritic zone (0-35 m deep) with salinity levels of from 35 to 50 parts per thousand. Studies of the Al-Nakhash sediments suggest a warm littoral to sabkha environment of deposition.

The authors express their gratitude to Dr Aisha Al-Thani (SEM Unit, Qatar University) for her kind permission to use the SEM and to David B. Devera for the SEM photomicrography. They thank Dr Abdul-Ali Sadek (Geology Department, Qatar University) for providing the aerial photograph of the Al-Nakhash area, and to Adel M. Ali (Geological Laboratory, Qatar University) for helping in the preparation of the samples. We are obliged to the anonymous referees and to the editorial staff of GeoArabia for their valuable comments and suggestions. We also thank the staff of Gulf Petrolink for the final design and drafting of the graphics.

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ABOUT THE AUTHORS

Hamad Al-Saad is Chairman of the Geology Department at the University of Qatar. He has an MSc in Geology from the University of South Carolina, USA, and a PhD from Ain Sims University, Cairo in 1997. His publications have concentrated on the Middle Jurassic of Qatar. Hamad’s current research interests include the sequence stratigraphy of the Jurassic of eastern Arabia. He is a member of SPE and BMS.

Corresponding author. E-mail: hamadsaad@qu.edu.qa

Mohamed I.A. Ibrahim has recently rejoined the staff of Alexandria University, Egypt (Department of Environmental Sciences) following secondment to the Geology Department of Qatar University as an Assistant Professor from October 1996. Mohamed has a BSc (Honors, 1981) in Special Geology and a MSc (1986) in Palynology and Micropaleontology from Alexandria University. He obtained his PhD in Micropaleontology and Paleoecology in 1993 from Alexandria University and the Technical University of Berlin through a combined-channel program. He taught Micropaleontology and Paleoecology from at Alexandria University (1993–95) and was a Palynostratigraphic Consultant for GEOEX-Egypt (1994–96). He is the author of about 25 research articles on the palynology, micropaleontology, and paleoecolgy of Egypt, Libya, and Qatar. He is National Coordinator and Member of the IGCP 831, ’South Atlantic Mesozoic Correlation Program’, and a member of AASP, BMS, GSE, and ESQUA.

E-mail: m.ibrahim@qu.edu.qa