Stratigraphy, Micropaleontology, and Paleoecology of the Miocene Dam Formation, Qatar
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GeoArabia, Vol. 7, No. 1, 2002 Gulf PetroLink, Bahrain Miocene Dam Formation, Qatar Stratigraphy, micropaleontology, and paleoecology of the Miocene Dam Formation, Qatar Hamad Al-Saad and Mohamed I. Ibrahim University of Qatar ABSTRACT 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. INTRODUCTION 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. Thralls and Hasson (1956) formalized the Dam Formation, the name having been introduced by Steineke and Koch in an unpublished Aramco report in 1935.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. 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. 9 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/1/9/4559944/alsaad.pdf by guest on 29 September 2021 Al-Saad and Ibrahim 50° 51° 52° Al-Rayyan Al-Shaheen Dammam BAHRAIN Jabal Dhahran Manama al Lidam North Field Abqaiq 26° 26° Awali QATAR Shedgum Arabian Gulf Gulf of ’Uthmaniyah Salwah Doha Dukhan SAUDI ARABIA Al-Nakhash 25° 25° Al-Kharrara Figure 2 Salwah Abu-Samra Miocene outcrops Jafura Gas field Oil field Harmaliyah N 0 50 km 50° 51° 52° 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. 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. GENERAL STRATIGRAPHY 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 10 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/1/9/4559944/alsaad.pdf by guest on 29 September 2021 Miocene Dam Formation, Qatar Jebel Al-Nakhash 90 80 70 90 N 60 50 40 80 30 70 S 60 20 e c 50 t i o n 40 I to Doha 30 10 20 10 d 10 20 a ro II lt I ha n sp I tio A I c n I Se 20 io ct 10 30 Se o Doha TTo Doha 10 0 500 m II III r Gulf of Salwah de or i B d u a /S h a w l a S o T 0 1,000 m 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 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). 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 11 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/1/9/4559944/alsaad.pdf by guest on 29 September 2021 Al-Saad and Ibrahim 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 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. 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. STRATIGRAPHIC SUBDIVISIONS OF THE DAM 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. Al-Kharrara Member Definition: Lower part of the Dam Formation. Stratotype (type section): Jebel Al-Nakhash, 85 km southwest of Doha, between latitude 24º52’ and 24º53’N., and longitude 50º50’30” and 50º54’30”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). Table 3 Stratigraphic subdivision and nomenclature of the Miocene Dam Formation in Qatar. Rock unit Cavelier Abu-Zeid and Hewaidy Present (1970) Khalifa (1983) (1991) study upper Dam upper Dam Al-Nakhash member B Dam subformation member Member Formation lower Dam lower Dam Al-Kharrara member A subformation member Member 12 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/7/1/9/4559944/alsaad.pdf by guest on 29 September 2021 Miocene Dam Formation,