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Dolomitization in the Carbonate Rocks of the Upper Turonian Wata Formation, West Sinai, NE Egypt: Petrographic and Geochemical Constraints
Journal of African Earth Sciences 111 (2015) 127e137
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Dolomitization in the Carbonate Rocks of the Upper Turonian Wata Formation, West Sinai, NE Egypt: Petrographic and Geochemical Constraints Tarek Anan a, *, Hamdalla Wanas b a b
Department of Geology, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Geology Department, Faculty of Science, Menoufia University, Shebin El-Kom, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 9 July 2015 Accepted 14 July 2015 Available online 15 July 2015
This study discusses the mechanism of dolomitization of carbonate rocks of the Upper Turonian Wata Formation in west Sinai, Egypt. It has been done in terms of study of textural, mineralogical, and geochemical characteristics of the dolostones and dolomitic limestones of the Wata Formation. The Wata Formation is composed mainly of dolostone and dolomitic limestone intercalated with few shale and sandstone beds. Beside the studied dolostone facies, five associated limestone microfacies have been identified. The limestone microfacies also provide an evidence of sparse dolomitization. Dolomites occur as replacive dolomites with minor dolomite cements. Four textural types of dolomite are distinguished: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The recorded dolomite is nearly non-stoichiometric (CaCO3 ranges from 51 to 56 mol % with an average of 53.5 mol %) and has similar geochemical features. It has d18OVPDB values range from 7.16‰ to þ0.26‰ and d13C VPDB values vary between 0.52‰ and þ5.2‰. Its strontium content lies between 11 and 53 ppm. Petrographic investigations and geochemical data indicated that the dolomitization of the studied carbonates probably took place in a meteoric water-sea water mixing zone at shallow burial depth during the early stage of diagenesis. In this situation, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio took place by two possible ways: (1) through leaching of Mg from high Mg-calcite grains (micrite) and aragonitic shells in the studied limestone (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. Such leaching brought about by freshwater-dominated fluid during meteoric water diagenesis. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dolomitization Wata formation Meteoric water-sea water mixing zone Sinai Egypt
1. Introduction Dolomite [CaMg(CO3)2] can form in two different ways (Machel, 2004): (1) by replacement of CaCO3 by CaMg(CO3)2 (replacive dolomite) and (2) by precipitation of dolomite from aqueous solution in primary or secondary pore spaces (dolomite cement). Dolomite can form as a primary precipitate, a diagenetic replacement, or a hydrothermal phase; all that it requires fluid flow and sufficient supply of magnesium. It may be formed from seawater, lake water,
* Corresponding author. E-mail addresses: [email protected] (T. Anan), [email protected] (H. Wanas). http://dx.doi.org/10.1016/j.jafrearsci.2015.07.015 1464-343X/© 2015 Elsevier Ltd. All rights reserved.
the mixing of hypersaline brine with seawater, and/or the mixing of meteoric water with seawater (Warren, 2000). In addition, it has been found that microbial activity in settings where sulfatereducing species flourish (hypersaline anoxic lake water) may control the formation of an early or a primary precipitate dolomite (Vasconcelos and McKenzie, 1997; Bontognali et al., 2010). In general, five models were issued for the dolomitization process: evaporative (penecontemporaneous; in hypersaline sabkha/supratidal), seepage-reflux, mixing-zone, burial, and seawater models (e. g., Tucker and Wright, 1990; Warren, 2000). The nature of dolomitization in carbonate platforms has been some of the most intensively debated and extensively studied problems (Hardie, 1987; Hass and Demeny, 2002; Machel, 2004;
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Ziya, 2013; Sena et al., 2014). Also, despite the widespread occurrence of dolomite in carbonate platform of the studied formation, there is no previous work on its textural and geochemical characteristics in terms of origin was issued (as the authors are aware). Therefore, this study aims to integrate textural, mineralogical, and geochemical characteristics of dolomite of the Upper Turonian Wata Formation, in order to discuss the mechanism of its dolomitization and nature of the dolomitizing fluid. In addition, a suitable model for dolomitization in the studied sequences is suggested. To achieve these purposes, detailed field, petrographic and geochemical investigations of the recognized dolomites and dolomitic limestones have been done. The studied Upper Turonian Wata 00 Formation is located at Wadi Budra (Lat. 28 530 2.2 N and Long. 00 0 00 0 00 0 33 20 15.4 E), Gabal Ekma (Lat. 28 40 06 N and Long. 33 16 51 00 00 E), and Gabal Qabaliate (Lat. 28 180 11.4 N and Long. 33 330 59.2 E) (Fig. 1).
2. Geological setting and lithostratigraphy During Mid-Cretaceous (Turonian) time, the Sinai Peninsula was a broad shallow shelf situated on the African continental passive margin of the Tethys, where a carbonate platform with siliciclastic intercalations was established (Kuss and Bachmann, 1996; Bauer et al., 2001). This broad shallow shelf extended over 200 km from north to south Sinai without any recognizable regional slope (Bauer et al., 2003; Wanas, 2008). Mid-Cretaceous strata in Sinai have witnessed two main episodes of deformation that are related to the Syrian Arc System tectonic activities (Bartov and Steinitz, 1977; Noweir et al., 2006). These two episodes of deformation are: 1) NE-SW folds that were formed in northern Sinai as a result of convergence between Africa and Eurasia and the closure of the
Neotethys; 2) a narrow belt, termed the “hinge belt” (Shata, 1956) or the “Central Sinai-Negev shear zone” (Bartov and Steinitz, 1977) was formed. During the Cretaceous period, the southern Tethys (Egypt) was characterized by alternating transgressions and regressions (Flexer et al., 1986), providing evidences for multiple small scale frequencies in sea-level change. According to Ghorab (1961), the Upper Cretaceous rocks in the Sinai Peninsula are divided into five formations: the Cenomanian Raha, the Upper Cenomanian-Lower Turonian Abu Qada, the Upper Turonian Wata, the Coniacian-Santonian Matulla, and the Campanian-Maastrichtian Sudr. The studied Wata Formation is composed mainly of grayish white dolostones and dolomitic limestones (Fig. 2A, B) that are intercalated with varicolored calcareous shale beds and a few thin bands of friable sandstone (Fig. 3). It attains a thickness of 39.6 m, 30.5 m, and 12 m at Gabal Ekma, Wadi Budra, and Gabal Qabaliate, respectively (Fig. 3). Shahin and Kora (1991) assigned the Late Turonian age to The Wata Formation due to the presence of benthic foraminifera Discorbis turonicus, Discorbis simplex, Quinqueloculina gussensis, and Phenacofragma elegans. The Wata Formation conformably overlies the Abu Qada Formation (Fig. 2A) and unconformably underlies the Matulla Formation.
3. Material and methods Three stratigraphic sections representing the Wata Formation in the west of Sinai were measured and described in the field (Fig. 1). Ninety-five samples were collected throughout the studied sections. Detailed petrographic study was conducted on forty six thin sections that were stained with Alizarin Red-S and potassium ferricyanide to distinguish between dolomite and calcite based on
Fig. 1. Location map of the studied sections.
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Fig. 2. Field photographs showing: A) The contact between the Abu Qada and Wata formations, Wadi Budra, person for scale is 1.6 m. B) Fractured dolostone, Wadi Budra, geologic hammer for scale is 32.5 cm long.
the method of Dickson (1965). The petrographic investigation allowed the identification of limestone microfacies and dolomite textures. Limestone microfacies were described according to the classification of Dunham (1962) with the modifications of Embry and Klovan (1972). The textural description of dolomite was conducted with terminology following Sibley and Gregg (1987). For dolomite crystal-size classes, the scheme of Folk (1962) is adopted. These classes are as follows: <30 mm is fine-crystalline, 30e150 mm for medium-crystalline, and >150 mm for coarse-crystalline dolomite. Mineral types of representative non-oriented carbonates were identified by an X-ray diffractometer (XRD) with Ni-filter and Cu radiation. Fresh surface samples were investigated using scanning electron microscope (SEM). The SEM analysis was used to illustrate the morphologies of the studied dolomite. Different types of dolomite were sampled for carbon and oxygen isotope using a micro-drilling device which was performed at the Western Michigan University, USA. For C and O isotope analyses, 0.1e0.2 mg of samples was reacted with anhydrous phosphoric acid at 70 C in a Finnigan Kiel-III carbonate preparation device coupled to the inlet of a Finnigan MAT-253 mass spectrometer. The precision measurement is better than 0.02‰ for d13C and better than 0.04‰ for d18O. The d13C and d18O values are reported in per mil relative to the Vienna Peedee Belemnite (VPDB) standard. Iron, manganese, and strontium contents were measured using inductively coupled argon plasma atomic emission spectroscopy (ICP-AES). Chemical analyses were performed in the laboratories of the University of Miami/RSMAS, USA.
dolomites can help greatly to determine the origin and mechanism of dolomitization (Warren, 2000); we shed more light on those subjects for the studied dolomite as will be discussed below. 5.1. Dolomite-rock textures Microscopically, four types of dolomite-rock textures have been recognized based on the crystal-size, distribution (unimodal or polymodal), and crystal-boundary shape of dolomite. These are (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) finecrystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. 5.1.1. Dolomite texture 1: fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite The dolomite of this type is characterized by fine dolomite crystals (<30 mm in size). These crystals are subhedral with straight and rarely curved intercrystalline boundaries (hypidiotopic). Fine dolomite crystals commonly show sharp extinction and could belong to planar-s. This type of dolomite texture occurs as a replacive dolomite to the previously lime-mud (Sibley and Gregg, 1987). It displays pervasive (Fig. 5A) and sparse (partial) dolomitization (Figs. 4B and 5B). It also displays unimodal to bimodal-size distribution. The pervasive dolomite is recorded in some dolostone horizons at Gabal Ekma. The sparse dolomite occurs in the oyster floatstone facies in all the studied sections.
4. Facies analysis Although the analysis of dolostone facies is the goal of the present work, the associated limestone facies were also studied to give more light on the nature of dolomitization process in the studied rocks. Five limestone microfacies were identified in the Wata Formation (Fig. 3), and the majority of them have provided evidences of sparse dolomitization. The description and environmental interpretation of the recognized limestone microfacies are listed in Table 1. 5. Results of the studied dolomites The dolomite facies dominates the carbonate succession of the Wata Formation in all the studied sections. At Gabal Qabaliate section, the relevant dolomite strata are dominated by rudists (Fig. 4F). Dolomite facies differs markedly from the limestone facies in their dark gray to yellowish gray color. Because of the study of a combination of petrography, stable isotopes, and trace elements of
5.1.2. Dolomite texture 2: medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomites This type of dolomite texture is mainly recognized in the Wata Formation at Gabal Ekma and Wadi Budra. It is generally composed of bimodal, medium to coarsely crystalline dolomite with size ranging from 65 to 125 mm (Fig. 5C, D). Dolomite crystals are planar euhedral (idiotopic) with straight intercrystalline boundaries and sharp extinction (Fig. 5C). They are zoned with cloudy cores and clear rims. The cloudy cores of the crystals result from the presence of iron oxide inclusions, whereas the clear rims are nearly inclusion-free (Fig. 5C). This type of dolomite texture could belong to planar-e (Sibley and Gregg, 1987). It is replacive dolomite and displays pervasive (Fig. 5C) and sparse dolomitization (Fig. 5D). The pervasive dolomite probably formed through complete replacement of the previously sparry carbonate crystals, where the relics of original fabrics are absent (Fig. 5C). It is characterized by intercrystalline to isolated micro-vuggy porosity (Fig. 5C). The sparse dolomite replaces partially both the lime-mud matrix
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Fig. 3. Columnar sections showing the lithological and microfacies characteristics of the Wata Formation at the three studied localities.
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Table 1 Summary of the recognized limestone microfacies and their related depositional environments. Microfacies types
Field description
Microscopic description
Depositional environment
Oyster floatstone
It is encountered in the three studied sections and occured as oyster banks (Fig. 4A). Its rock is gray to grayish yellow in color, hard, and contains gypsum veins (Fig. 4A).
The common oysters indicate a deposition in less than 50 m depth in low oxygenated, brackish, and nutrient-rich waters (Dhondt et al., 1999; Gertsch et al., 2010).
Skeletal wacke-packstone
It is widespread in the Wata Formation all over the studied localities. The rock of this microfacies is varied in color (gray, yellow, grayish yellow) and it is dolomitic, fossiliferous, hard, nodular, and contains gypsum and calcite veins.
Bivalve wackestone
It is distinguished in both Wadi Budra and Gabal Qabaliate sections. The rock of this microfacies is varicolored (gray, grayish white, yellow), hard, dolomitic and fossiliferous.
Lime-mudstone
It is recorded in all the studied sections. The rock of this microfacies is yellow to grayish white in color, hard, dolomitic, fossiliferous, and contains gypsum veins.
It is composed mainly of large-sized oyster shell fragments that float in a carbonate lime-mud groundmass (Fig. 4A). Most of the oyster shells exhibit their original fibrous structure. The lime-mud groundmass is partially dolomitized (Fig. 4B). It consists of shell fragments and particles of pelecypod, gastropod and echinoderm that embedded in micrite matrix. Small amount of silt-sized quartz grains are disseminated. The skeletal fragments and particles are moderately sorted, packed, and range in size from fine to coarse sand size. They are embedded in micrite matrix, which is partially dolomitized. It is dominated by bivalve fragments, echinoderms, ostracods, and peloids that are embedded in a lime mud matrix (Fig. 4C). The lime-mud matrix shows partial recrystallization to microspars. Some of the shell fragments and micritic matrix are partially replaced by euhedral dolomite rhombs (Fig. 4C). It is dense micrite (95%) with rare shell debris (3e5%). The carbonate mud matrix is partially dolomitized.
Peloidal grainstone
The rock is recorded only in Gabal Ekma. It is grayish white to yellowish gray in color, very hard, dolomitic, and burrowed at its top part.
Oolitic grainstone
The rock is recorded only in Wadi Budra. It is yellowish brown in color, very hard, and dolomitic.
This microfacies is similar to SMF5 and FZ6 of Wilson (1975) and Flügel (2004), respectively. It reflects the deposition in a shallow subtidal environment.
It is equivalent to SMF9 and FZ7 of Wilson (1975) and Flügel (2004), respectively, that suggests accumulation in a restricted shallow lagoon.
Lime-mudstones that consist of a low diversity fauna and rare ostracods and bivalve fragments are commonly recorded in muddy, lagoonal environments (Heckel, 1972). This microfacies is equivalent to SMF9 and FZ7 of Wilson (1975) and Flügel (2004), respectively. It is composed essentially of peloids (70 This microfacies is common in protected e80%) (Fig. 4D) embedded in sparry calcite shallow marine environments with cement (20e30%). The peloids are rounded moderate water circulation (SMF16 and to oval in shape and exhibit sharp contacts FZ8) and in inner ramp settings. Also, with their cement. Silt-sized quartz grains peloids are abundant in low energy shallow are recorded. Some of the peloids and tidal and subtidal carbonates (Flügel, 2004). sparry calcite cement show a partial replacement by euhedral, zoned, dolomite crystals (Fig. 4D). These crystals are varied in size between 65 and 125 mm. It is predominately composed of concentric This microfacies suggests high energy ooids cemented by sparry calcite (Fig. 4E). conditions such as shoal environment and beaches (winnowed edge sands, SMF15 and Thes ooids are varied in size (from 0.5 to 1.0 mm) and contain carbonate mud nuclei FZ6; Wilson, 1975 and Flügel, 2004). Also, ooids indicate warm water environments or (Fig. 4E). tropical settings (Kiessling et al., 2002).
(Fig. 5D, E) and allochems (Fig. 4C, E). It occurs as scattered crystals distributed irregularly through the host lime-mud (Figs. 4D and 5D), bivalve fragments (Fig. 4C) and peloids (Fig. 4D).
ranging from 30 up to 50 mm in size (Fig. 5G). It is precipitated in micro-pores and surrounded by microcrystalline calcite (Fig. 5G). It is volumetrically rare and of minor importance.
5.1.3. Dolomite texture 3: medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomites This type of dolomite texture is distinguished in the dolostone facies at Gabal Qabaliate and Wadi Budra. It is not common and it is characterized by unimodal, medium-crystalline (>100 mm) dolomite mosaic. Dolomite crystals are clear in appearance, having very rare cloudy cores. They are subhedral with curved (locally straight) intercrystalline boundaries (Fig. 5E). They commonly show sharp to slightly undulatory extinction. This type of dolomite texture could belong to planar-s (Sibley and Gregg, 1987). It is generally characterized by low intercrystalline porosity that is filled by iron oxides (Fig. 5E). SEM images reveal the presence of medium-crystalline, planar-s rhombic dolomite; dolomite appears slightly dissolved (Fig. 5F).
5.2. Mineralogy
5.1.4. Dolomite texture 4: fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cement This type of dolomite cement is detected in the lime-mud facies at Gabal Ekma. It consists of planar, euhedral void-filling crystals,
SEM images (Fig. 5FeH) and X-ray diffraction analysis of carbonate rocks of the Wata Formation revealed that they are composed mainly of calcite, dolomite, and a minor amount of quartz with an absence of evaporite minerals (Fig. 6). The dolomite percentage in dolomitic limestone microfacies ranges between 2.9% and 11.8%, whereas in dolostone facies the percentage is 89.6e100 % (Table 2). The Ca % in sedimentary dolomite ranges from 48 to 62.5 %, and can be determined by powder X-ray diffraction analysis (Lumsden, 1979; Jones et al., 2001). This procedure relies up on the fact that the position of the d104 reflection depends linearly on the Ca content in dolostone. Lumsden (1979) established an equation linking molar content of CaCO3 in dolomite to the d104 spacing measured on XRD profiles. The equation is:
NCaCO3 ¼ Md þ B
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Fig. 4. A) Field photograph showing oyster-bearing limestone in the Wata Formation. Notice the presence of post-depositional gypsum veins (arrows), Gabal Qabaliate, marker pen for scale is 12 cm. B) Photomicrograph showing dolomitic oyster (Oy) floatstone microfacies. Notice, the carbonate mud is replaced by fine-crystalline, euhedral dolomite crystal rhombs (arrows) that are cloudy in cores and clear at rims, Wadi Budra, sample B25, PPL. C) Photomicrograph showing bivalve wackestone microfacies. Bivalve fragments are replaced by medium-to coarse-crystalline, euhedral dolomite rhombs (arrows) (100e125 mm), Wadi Budra, sample B19, PPL. D) Photomicrograph showing peloidal grainstone microfacies. Notice the peloids and sparry calcite groundmass (arrows) are replaced by zoned medium-crystalline, euhedral dolomite crystal rhombs (65e125 mm), Gabal Ekma, sample K15, PPL. E) Photomicrograph showing oolitic grainstone microfacies. Ooids are cemented by a sparry calcite, Wadi Budra, sample Bud, PPL. F) Field photograph showing the rudist-bearing dolostone in the uppermost part of the Wata Formation, Gabal Qabaliate, geologic hammer for scale is 32.5 cm long.
where (M) is 333.33 and (B) is 911.99. The standard error is ±0.15% on the mean composition with this procedure. The obtained data is illustrated in (Table 3). The molar content of CaCO3 ranges from 51 to 56 % with an average of 53.5%, whereas the molar content of MgCO3 varies between 44 and 49% with an average of 46.5%. XRD diffractograms of the recorded dolomites show super lattice reflections, indicating a fairly high degree of ordering (Burns and Baker, 1987).
5.4. Major and trace elements The CaCO3 and MgCO3 mole % based on XRD (Table 3) indicate that dolomite of the Wata Formation is nearly non-stoichiometry (Morse and Mackenzie, 1990). Nine samples of dolomite and dolomitic limestone analyzed for strontium. The strontium content of the studied dolostone ranges from 11 to 53 ppm (Table 4). Six dolomite samples were analyzed for manganese and iron. The resultant data are 690e900 ppm for manganese and 1900e4500 ppm for iron Table 4). Manganese content is roughly proportional to iron content in the studied samples (Table 4).
5.3. Oxygen and carbon isotopes 6. Discussion Twelve samples of dolomite and dolomitic limestone have been analyzed for oxygen and carbon isotopes. The recorded values for d18O and d13C values vary from 7.16‰ to þ0.26‰ and 0.52‰ to þ5.20‰, respectively (Table 4, Fig. 7).
6.1. Mechanism of dolomitization Previous studies on the dolomite formation throughout the
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Fig. 5. Photomicrographs showing: A) Dolostone microfacies with fine-crystalline (<30 mm) dolomite crystals, Gabal Ekma, sample K35, XPL. B) Lime-mudstone microfacies is replaced by fine-crystalline euhedral dolomite rhombs (25e65 mm) with small cloudy cores and clear outer rims, Gabal Qabaliate, sample QA15, PPL. C) Dolostone microfacies displays medium-sized (70e100 mm), closely packed, zoned, euhedral dolomite crystal rhombs which have dark brown iron-rich cores with clear outer rims, Gabal Ekma, sample K3, PPL. D) Carbonate mud is replaced by subhedral dolomite crystal rhombs with curved (locally straight) intercrystalline boundaries (arrows). Dolomite crystals are varied in size between 65 and 150 mm, Wadi Budra, sample B8, PPL. E) Medium-sized (60e100 mm), closely packed, subhedral dolomite crystals. Notice low intercrystalline porosity which is filled with iron oxides (arrows), Gabal Qabaliate, sample QA23, PPL. F) Subhedral dolomite rhombs, Gabal Ekma, sample K17. G) Void filled with dolomite crystals (arrows). The void occurs in a sparry calcite cement, Gabal Ekma, sample K17. H) Dolomite rhombs within calcite crystals, Gabal Qabaliate, sample QA23.
geological record indicate that it can form as a result of physicochemical and microbial processes (see Warren, 2000 for review). Previous workers have shown that it is difficult to determine the
origin of dolomite and/or mechanism of dolomitization by using isotopic and chemical analyses only (Hardie, 1987). Thus, in this study we merge both the chemical data with textural types of
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Fig. 6. X-Ray diffraction patterns showing the characteristic peaks of the encountered minerals in the studied carbonate rocks.
dolomite crystals to infer the origin and mechanism of dolomitization as discussed below: 6.1.1. Textural evidences As shown by petrographic investigations, the prevalence of planar (euhedral, subhedral) dolomite texture and the absence of non-planar (anhedral) one indicate that the dolomitization
occurred under low salinity and temperature conditions (Gregg and Sibley, 1984). Planar crystals tend to develop around the so called critical roughening temperature, which appears to be about 50e60 C for dolomites (Gregg and Sibley, 1984). These temperatures are considerably lower than those likely to have been generated during dolomitization of limestones at deep burial temperature (70e90 C) or hydrothermal water (100e220 C)
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Table 2 Semi-quantitative mineral composition of bulk carbonate samples of the Wata Formation (derived from XRD). Lithology
S.No.
Calcite
Dolomite
Quartz
Dolostone
QA23 K35 K13 K17 Bud B8 QA22 K3 K27 B25 Average
0.0 10.4 94.3 95.7 88.2 97.1 97.7 97 100 100 78.0
100 89.6 0.0 4.3 11.8 2.9 0.0 3.0 0.0 0.0 21.2
0.0 0.0 5.7 0.0 0.0 0.0 2.3 0.0 0.0 0.0 0.8
Dolomitic limestone
limestone
Fig. 7. Cross plot of d18O and d13C of the studied Wata Formation. Table 3 The d104 spacing (Ao), Mole % CaCo3, and Mole % MgCo3 of the studied dolostone. S.No.
d(Ao)
NCaCO3%
NMgCO3%
K35 QA23 Bud B8 K3 K17 Average Min Max
2.90355 2.89054 2.89729 2.89464 2.90492 2.89870
55 51 53 52 56 54 53.5 51 56
45 49 47 48 44 46 46.5 44 49
6.1.2. Trace elements evidences The concentrations of Sr in the studied dolomites (11e53 ppm) are lower than those in the hypersaline evaporative dolomites (600e900 ppm) and in the dolomites forming within a marine burial setting (300e500 ppm) (Veizer, 1983; Warren, 2000). Thus, the depletion of Sr content (11e53 ppm) in the studied dolomites would possibly reflect the influence of meteoric water during crystallization of the dolomites in early stage of diagenesis (Veizer, 1983; Warren, 2000). The recorded higher content of manganese (690e900 ppm) and iron (1900e4500 ppm) may be leached from the associated siliciclastic rocks. 6.1.3. Oxygen isotope evidences The d18O values of dolomite precipitated in equilibrium with normal seawater are 1e3 ‰, and those of hypersaline or brine water are more than þ3‰ (Land, 1985; Warren, 2000). Consequently, the oxygen isotopic signature in the studied dolomites (7.16 to þ0.26‰) could indicate that dolomitization was not probably mediated by hypersaline or seawater, slightly modified seawater by evaporation. Thus, the recorded depletion in the oxygen values (7.16 to þ0.26‰) could indicate that the studied dolomitization was probably mediated by seawateremeteoric water mixture (Land, 1985; Warren, 2000).
(Warren, 2000). In addition, the absence of fractures and/or faults within or in the vicinity of the studied dolomite supports the negativity of its hydrothermal origin (Middleton et al., 1993). Worldwide, the previous petrographical studies on dolomite have shown that particular dolomite crystal habits and textures are related to specific physicochemical environments and processes. For example spheroidal, dumbbell and spindle-shaped dolomite crystals have been recorded from microbially influenced settings (Gunatilaka, 1989), groundwater dolocretes (Khalaf, 1990; El-Sayed et al., 1991), and/or direct precipitation from hypersaline lake water (Wanas, 2002). These shapes of dolomite crystals are not recorded in the studied dolomite. Therefore, the microbial origin and/or groundwater dolocrete models do not appear to be appropriate for the studied dolomites. Thin-section studies and XRD analysis revealed that there is no evaporite minerals recorded in the studied dolostones. This documents that the studied dolomite generally is not reconciled with the model of dolomitization which took place in evaporitic supratidal (sabkha style) or brine reflux environments as were discussed by Machel (2004) and Boggs (2009).
6.1.4. Carbon isotope evidences The carbon isotopic composition of carbonates is an indicator of carbon sources incorporated during carbonate formation. The d13C values in dolomites are mainly dependent on the relative amount of CO2 supplied from pore water carbonate ion, dissolution of biogenic carbonate precursors, and degradation of organic matter (Compton et al., 1994; Montanez and Read, 1992). The d13C values vary mostly between 0.52 and þ 5.20‰ for the studied dolomites. These values
Table 4 Elemental and stable isotopic compositions (C and O) of the studied Wata Formation.
Dolostone
Dolomitic limestone
Min Max
S.No.
d13C(VPDB) ‰
d18O(VPDB) ‰
Mn (ppm)
Fe (ppm)
Sr (ppm)
K35 QA23 K25 QA23b B25 K27 B8 QA22 Bud K13 K3 K17
1.44 4.37 5.10 5.20 1.06 0.84 1.43 4.91 0.52 0.73 0.80 0.57 0.52 5.20
1.64 0.26 1.40 1.20 5.40 7.16 6.82 5.45 6.31 6.13 6.31 6.21 7.16 0.26
690 860 770 900 790 820
1900 2800 3000 4500 1950 2900
43 19 53 11 52 16 43 49 15
690 900
1900 4500
11 53
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lie, more or less, within the range of normal marine-derived carbon source of the biogenic carbonate precursors (Coniglio et al., 1988). They indicate that the carbonate in the studied dolomite is possible to be derived from biogenic carbonate precursor as is proved by the presence of fossil molds (Fig. 4B, C). Also, the d13C values of the studied dolostones (0.52‰ and þ5.20‰, Table 4) do not give a trend to organogenic/methanogenic origin of dolomite, in which the d13C values are more positive (about þ15‰) (Warren, 2000). So far, in our foregoing discussion the fundamental textural and chemical requirements to form dolomite from evaporitic supratidal (sabkha style), brine reflux, methanogeneses, hydrothermal phase and microbial activity seem to be unfavorable conditions for the model of dolomitization in the studied dolomites. Therefore, the possibility of its formation by mixing meteoric (fresh) water-sea water under low temperature conditions at shallow burial depth (early stage of diagenesis) becomes more probable (Fig. 8). The occurrence of the investigated dolomite at mixing meteoricsea water zone and low temperature conditions in the early buried sediments is documented by: 1) its low Sr content (11e53 ppm) and light d18O values (from 7.16 to þ0.26‰) (Brand and Veizer, 1980; Choquette and Steinen, 1980; Veizer, 1983; Ward and Halley, 1985; Budd, 1997; Meyers et al., 1997; Warren, 2000), 2) The recorded sea-level regression in the studied carbonate platform during the Upper Turonian Wata Formation (Anan, 2014) which supports the mixing meteoric-sea water model (Tucker and Wright, 1990), 3) the prevalence of planar (euhedral, subhedral) textural type in the studied dolomite which indicates that the dolomitization occurred under low salinity and temperature conditions at shallow burial depth in the early stage of diagenesis (Gregg and Sibley, 1984), and 4) the non-stoichiometry of the studied dolomite (see mineralogy), which could be a result of slightly modification of the dolomitizing fluid by fresh water that favor formation for more calcian dolomites (Folk and Land, 1975). Such an early burial origin of the studied dolomite is also supported by the prevalence of burrows in the encountered dolomite beds (Compton et al., 1994). In this situation where the dolomite occurs in mixing meteoricsea water zone in the early diagenetic stage, it can be suggested that the studied dolomite could be formed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio could be took place by two possible ways: (1) through leaching of Mg from high Mg calcite grains (micrite) and aragonitic shells in the studied limestone. This is evidenced by calcitization of original composition of the aragonitic bivalve shells (aragonite and high Mg calcite), and aggrading neomorphism of the micritic (high Mg calcite) matrix (Fig. 4B; see Anan, 2014), (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons (Figs. 3 and 8). Such leaching was developed by freshwater-dominated fluid during meteoric water diagenesis. In this context, the sparse
dolomitization of some carbonate beds could be a result of less supersaturation state of the dolomitizing fluid with respect to complete dolomitization in those carbonate beds. Although some workers have questioned the validity of a mixing zone model as an agent of dolomitization (e.g., Plummer, 1977; Machel and Mountjoy, 1986; Machel and Burton, 1994), our study supports the model of dolomitization by mixing meteoric-sea water as issued by previous authors worldwide (e. g., Ward and Halley, 1985; Tucker and Wright, 1990; Ye and Mazzullo, 1993; Cander, 1994). 7. Conclusions This study is aimed to clarify the mechanism of dolomitization in the carbonates of the Upper Turonian Wata Formation in west Sinai, Egypt in terms of their textural and chemical data. The beds of Wata Formation are described, measured and collected in the field. The collected samples were thin-sectioned and investigated under the petrographic microscopes. They also subjected to chemical analyses to determine their major and trace elements in addition to their stable carbon and oxygen isotopes. In addition to dolostones, five limestone microfacies have been identified in the carbonate beds of the Wata Formations. These are oyster floatstone, skeletal wacke-packstone, bivalve wackestone, lime-mudstone, and grainstone. Most of these microfacies have provided evidences of sparse to pervasive dolomitization. Four main textural types of dolomite are recognized: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) mediumto coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The prevalence of these planar textures and absence of non-planar ones indicate dolomitization under low temperature and salinity conditions. Both petrographic and geochemical studies declared that the studied dolomite generally is not reconciled with the model of dolomitization which took place in evaporitic supratidal (sabkha style) or brine reflux, methanogeneses, hydrothermal phase and microbial activity. Therefore, the possibility of its formation by mixing meteoric (fresh)-sea water under low temperature conditions at shallow burial depth (early stage of diagenesis) becomes more probable. This is documented by planar texture, low Sr content (11e53 ppm), non-stoichiometry, and light d18O values (from 7.16‰ to þ0.26‰) of the investigated dolomites. In this situation where the dolomite occurs in a mixing fresh water-sea water zone during early buried stage, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. Such increase of Mg/Ca ratio took place by
Fig. 8. Sketch diagram showing the suggested model for the dolomitization in the Wata Formation at the studied localities.
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two possible ways: (1) through leaching of Mg from high Mg calcite grains (micrite) and aragonitic shells in the studied limestone. (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. The d13C values (between 0.52‰ and þ5.2‰) indicated that the carbonate of the studied dolomite did not form in organicrich zone but it is possible to be derived from biogenic carbonate precursor. The recorded higher content of manganese (690e900 ppm) and iron (1900e4500 ppm) may be leached from the associated siliciclastic rocks. Acknowledgments The authors would like to thank Ahmed El Belasy, Department of Geology, Mansoura University for his help during the field work. Lamees Mohamed, Western Michigan University, is acknowledged for her help in chemical analyses. We thank the anonymous reviewers for their critical revision of the manuscript. References Anan, T., 2014. Facies analysis and sequence stratigraphy of the CenomanianTuronian mixed siliciclastic-carbonate sediments in west Sinai, Egypt. Sediment. Geol. 307, 34e46. Bartov, Y., Steinitz, G., 1977. The Judea and mount scopes groups in the Negev and Sinai with trend surface analysis of the thickness data. Israel J. Earth Sci. 26, 119e148. Bauer, J., Kuss, J., Steuber, T., 2003. Sequence stratigraphy and carbonate platform configuration (Late Cenomanian-Santonian), Sinai, Egypt. Sedimentology 50, 387e414. Bauer, J., Marzouk, A., Steuber, T., Kuss, J., 2001. Lithostratigraphy and biostratigraphy of the Cenomanian-Santonian strata of Sinai, Egypt. Cretac. Res. 22, 497e526. Boggs, S., 2009. Petrology of Sedimentary Rocks. Cambridge University Press. Bontognali, T.R., Vasconcelos, C., Warthmann, R.J., Bernasconi, S.M., Dupraz, C., Strohmenger, C.J., McKenzie, J.A., 2010. Dolomite formation within microbial mats in the coastal sabkha of Abu Dhabi (United Arab Emirates). Sedimentology 57, 824e844. Brand, V., Veizer, J., 1980. Chemical diagenesis of a multicomponent carbonate systems: trace elements. J. Sediment. Petrology 50, 1219e1236. Budd, D.A., 1997. Cenozoic dolomites of carbonate islands: their attributes and origin. Earth-Science Rev. 42, 1e47. Burns, S.J., Baker, P.A., 1987. A geochemical study of dolomite in the Monterey formation, California. J. Sediment. Petrology 57, 128e139. Cander, H.S., 1994. An example of mixing-zone dolomite, middle Eocene Avon Park Formation, Floridan Aquifer system. J. Sediment. Res. A 64, 615e629. Choquette, P.W., Steinen, R.P., 1980. Mississippian non-supratidal dolomite, Ste. Genevieve limestone, Illinois Basin: evidence for mixed-water dolomitization. In: Zenger, D.H., Dunham, J.B., Ethington, R.L. (Eds.), Concepts and Models of Dolomitization. Society of Economic Paleontologists and Mineralogists, pp. 163e196. Special Publication 28. Compton, J.S., Hall, D.L., Mallinson, D.J., Hodell, D.A., 1994. Origin of dolomite in the phosphatic Miocene Hawthron Group of Florida. J. Sediment. Res. A 64, 638e649. Coniglio, M., James, N.P., Aissaoui, D.M., 1988. Dolomitization of Miocene carbonates: Gulf of Suez, Egypt. J. Sediment. Petrology 59, 100e119. Dhondt, A., Malchus, N., Boumaza, L., Jaillard, E., 1999. Cretaceous oysters from North Africa: origin and distribution. Bull. Geol. Soc. France 170, 67e76. Dickson, J.A.D., 1965. A modified staining technique for carbonates in thin section. Nature 205, 587. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists, pp. 108e121. Memoir 1. El-Sayed, M., Fairchild, I., Spiro, B., 1991. Kuwait dolocrete: petrology, geochemistry and groundwater origin. Sediment. Geol. 73, 59e75. Embry, A.F., Klovan, J.E., 1972. Absolute water depths limits of late Devonian paleoecological zones. Geol. Rundsch. 61, 672e686. Flexer, A., Rosenfeld, A., Lipson-Benitah, S., Honigstein, A., 1986. Relative sea level changes during the Cretaceous in Israel. AAPG Bull. 70, 1685e1699. Flügel, E., 2004. Microfacies of Carbonate Rocks. Springer, Berlin. Folk, R.L., 1962. Spectral subdivision of limestone types. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks-a Symposium. AAPG, pp. 62e84. Memoir 1. Folk, R.L., Land, L.S., 1975. Mg/Ca ratio and salinity: two controls over crystallization of dolomite. AAPG Bull. 59, 60e68. Gertsch, B., Keller, G., Adatte, T., Berner, Z., Kassab, A.S., Tantawy, A.A., ElSabbagh, A.M., Stueben, D., 2010. Cenomanian-Turonian transition in a shallow water sequence of the Sinai, Egypt. Int. J. Earth Sci. 99, 165e182. Ghorab, M.A., 1961. Abnormal stratigraphic features in Ras Gharib oilfields, Egypt. In: 3rd Arab Petroleum Congress, Alexandria, Egypt, pp. 1e10. Gregg, J.M., Sibley, D.F., 1984. Epigenetic dolomitization and the origin of xenotopic
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dolomite texture. J. Sediment. Petrology 54, 908e931. Gunatilaka, A., 1989. Spheroidal dolomites-origin by hydrocarbon seepage? Sedimentology 36, 701e710. Hardie, L.A., 1987. Dolomitization: a critical view of some current views. J. Sediment. Petrology 57, 166e182. Hass, D., Demeny, A., 2002. Early dolomitization of Late Triassic platform carbonates in the Transdanubian Range (Hungary). Sediment. Geol. 151, 225e242. Heckel, P.H., 1972. Recognition of ancient shallow marine environments. In: Rigby, J.K., Hamblin, W.K. (Eds.), Recognition of Ancient Sedimentary Environments. SEPM, pp. 226e286. Special Publication 16. Jones, B., Luth, R.W., MacNeil, A.J., 2001. Powder X-ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones. J. Sediment. Res. 71, 790e799. Khalaf, F., 1990. Occurrence of phreatic dolocrete within tertiary clastic deposits of Kuwait, Arabian Gulf. Sediment. Geol. 68, 223e239. Kiessling, W., Flügel, E., Golonka, J., 2002. From patterns to processes: the future of reef research. In: Kiessling, W., Flügel, E., Golonka, J. (Eds.), Phanerozoic Reef Patterns. SEPM, pp. 735e743. Special Publication 72. Kuss, J., Bachmann, M., 1996. Cretaceous paleogeography of the Sinai Peninsula and neighboring areas. C.R. Acad. Sçi. II a 322, 915e933. Land, L.S., 1985. The origin of massive dolomite. J. Geol. Educ. 33, 112e125. Lumsden, D.N., 1979. Discrepancy between thin section and X-ray estimates of dolomite in limestone. J. Sediment. Petrology 49, 429e436. Machel, H.G., 2004. Concepts and models of dolomitization: a critical reappraisal. In: Braithwaite, C.J.R., Rizzi, G., Darke, G. (Eds.), The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs, vol. 35. Geological Society Special Publication, pp. 7e63. Machel, H.G., Burton, E.A., 1994. Golden Grove Dolomite, Barbados; origin from modified seawater. J. Sediment. Res. A 64, 741e751. Machel, H.G., Mountjoy, E.W., 1986. Chemistry and environments of dolomitization e a reappraisal. Earth-Science Rev. 23, 175e222. Meyers, W.J., Lu, F.H., Zachariah, J.K., 1997. Dolomitization by mixed evaporative brines and freshwater, upper Miocene carbonates, Nijar, Spain. J. Sediment. Res. A 67, 898e912. Middleton, K., Coniglio, M., Sherlock, R., Frape, S.K., 1993. Dolomitization of middle Ordovician carbonate reservoirs, southwestern Ontario. Bull. Can. Petroleum Geol. 41, 150e163. Montanez, I., Read, J.F., 1992. Fluid-rock interaction history during stabilization of early dolomites, upper Knox Group (lower Ordovician), US Appalachians. J. Sediment. Petrology 62, 753e778. Morse, J.W., Mackenzie, F.T., 1990. Geochemistry of Sedimentary Carbonates, vol. 48. Elsevier Science Publishers, The Netherlands, p. 707. Developments in Sedimentology. Noweir, M.A., Alfy, Z., Fawwaz, E.M., 2006. Wrench deformation syrian arc structures: El Hamraa, Umm Busal and Umm Horeiba En Echelon anticlines, Mitla Pass, west-central Sinai. Egypt. J. Geol. 50, 265e287. Plummer, L.N., 1977. Defining reactions and mass transfer in part of the Floridan aquifer. Water Resour. Res. 13, 801e812. Sena, C.M., John, C.M., Jourdan, A., Vandeginste, V., Manning, C., 2014. Dolomitization of lower Cretaceous peritidal carbonates by modified seawater: constrains from clumped isotopic paleothermometry, elemental chemistry, and strontium isotopes. J. Sediment. Res. 84, 552e566. Shahin, A., Kora, M., 1991. Biostratigraphy of some upper Cretaceous successions in the eastern central Sinai. Egypt. N. Jb. Geol. Palaont. Mh. 11, 671e692. Shata, A., 1956. Structural development of Sinai Peninsula, Egypt. Bull. Inst. Desert Egypte 6, 117e157. Sibley, D.F., Gregg, J.M., 1987. Classification of dolomite rock textures. J. Sediment. Petrology 57, 967e975. Tucker, M.E., Wright, V.P., 1990. Carbonate Sedimentology. Blackwell Scientific Publications, Oxford, London. Vasconcelos, C., McKenzie, J.A., 1997. 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Dolomitization in the Carbonate Rocks of the Upper Turonian Wata Formation, West Sinai, NE Egypt: Petrographic and Geochemical Constraints Tarek Anan a, *, Hamdalla Wanas b a b
Department of Geology, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Geology Department, Faculty of Science, Menoufia University, Shebin El-Kom, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 9 July 2015 Accepted 14 July 2015 Available online 15 July 2015
This study discusses the mechanism of dolomitization of carbonate rocks of the Upper Turonian Wata Formation in west Sinai, Egypt. It has been done in terms of study of textural, mineralogical, and geochemical characteristics of the dolostones and dolomitic limestones of the Wata Formation. The Wata Formation is composed mainly of dolostone and dolomitic limestone intercalated with few shale and sandstone beds. Beside the studied dolostone facies, five associated limestone microfacies have been identified. The limestone microfacies also provide an evidence of sparse dolomitization. Dolomites occur as replacive dolomites with minor dolomite cements. Four textural types of dolomite are distinguished: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The recorded dolomite is nearly non-stoichiometric (CaCO3 ranges from 51 to 56 mol % with an average of 53.5 mol %) and has similar geochemical features. It has d18OVPDB values range from 7.16‰ to þ0.26‰ and d13C VPDB values vary between 0.52‰ and þ5.2‰. Its strontium content lies between 11 and 53 ppm. Petrographic investigations and geochemical data indicated that the dolomitization of the studied carbonates probably took place in a meteoric water-sea water mixing zone at shallow burial depth during the early stage of diagenesis. In this situation, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio took place by two possible ways: (1) through leaching of Mg from high Mg-calcite grains (micrite) and aragonitic shells in the studied limestone (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. Such leaching brought about by freshwater-dominated fluid during meteoric water diagenesis. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Dolomitization Wata formation Meteoric water-sea water mixing zone Sinai Egypt
1. Introduction Dolomite [CaMg(CO3)2] can form in two different ways (Machel, 2004): (1) by replacement of CaCO3 by CaMg(CO3)2 (replacive dolomite) and (2) by precipitation of dolomite from aqueous solution in primary or secondary pore spaces (dolomite cement). Dolomite can form as a primary precipitate, a diagenetic replacement, or a hydrothermal phase; all that it requires fluid flow and sufficient supply of magnesium. It may be formed from seawater, lake water,
* Corresponding author. E-mail addresses: [email protected] (T. Anan), [email protected] (H. Wanas). http://dx.doi.org/10.1016/j.jafrearsci.2015.07.015 1464-343X/© 2015 Elsevier Ltd. All rights reserved.
the mixing of hypersaline brine with seawater, and/or the mixing of meteoric water with seawater (Warren, 2000). In addition, it has been found that microbial activity in settings where sulfatereducing species flourish (hypersaline anoxic lake water) may control the formation of an early or a primary precipitate dolomite (Vasconcelos and McKenzie, 1997; Bontognali et al., 2010). In general, five models were issued for the dolomitization process: evaporative (penecontemporaneous; in hypersaline sabkha/supratidal), seepage-reflux, mixing-zone, burial, and seawater models (e. g., Tucker and Wright, 1990; Warren, 2000). The nature of dolomitization in carbonate platforms has been some of the most intensively debated and extensively studied problems (Hardie, 1987; Hass and Demeny, 2002; Machel, 2004;
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Ziya, 2013; Sena et al., 2014). Also, despite the widespread occurrence of dolomite in carbonate platform of the studied formation, there is no previous work on its textural and geochemical characteristics in terms of origin was issued (as the authors are aware). Therefore, this study aims to integrate textural, mineralogical, and geochemical characteristics of dolomite of the Upper Turonian Wata Formation, in order to discuss the mechanism of its dolomitization and nature of the dolomitizing fluid. In addition, a suitable model for dolomitization in the studied sequences is suggested. To achieve these purposes, detailed field, petrographic and geochemical investigations of the recognized dolomites and dolomitic limestones have been done. The studied Upper Turonian Wata 00 Formation is located at Wadi Budra (Lat. 28 530 2.2 N and Long. 00 0 00 0 00 0 33 20 15.4 E), Gabal Ekma (Lat. 28 40 06 N and Long. 33 16 51 00 00 E), and Gabal Qabaliate (Lat. 28 180 11.4 N and Long. 33 330 59.2 E) (Fig. 1).
2. Geological setting and lithostratigraphy During Mid-Cretaceous (Turonian) time, the Sinai Peninsula was a broad shallow shelf situated on the African continental passive margin of the Tethys, where a carbonate platform with siliciclastic intercalations was established (Kuss and Bachmann, 1996; Bauer et al., 2001). This broad shallow shelf extended over 200 km from north to south Sinai without any recognizable regional slope (Bauer et al., 2003; Wanas, 2008). Mid-Cretaceous strata in Sinai have witnessed two main episodes of deformation that are related to the Syrian Arc System tectonic activities (Bartov and Steinitz, 1977; Noweir et al., 2006). These two episodes of deformation are: 1) NE-SW folds that were formed in northern Sinai as a result of convergence between Africa and Eurasia and the closure of the
Neotethys; 2) a narrow belt, termed the “hinge belt” (Shata, 1956) or the “Central Sinai-Negev shear zone” (Bartov and Steinitz, 1977) was formed. During the Cretaceous period, the southern Tethys (Egypt) was characterized by alternating transgressions and regressions (Flexer et al., 1986), providing evidences for multiple small scale frequencies in sea-level change. According to Ghorab (1961), the Upper Cretaceous rocks in the Sinai Peninsula are divided into five formations: the Cenomanian Raha, the Upper Cenomanian-Lower Turonian Abu Qada, the Upper Turonian Wata, the Coniacian-Santonian Matulla, and the Campanian-Maastrichtian Sudr. The studied Wata Formation is composed mainly of grayish white dolostones and dolomitic limestones (Fig. 2A, B) that are intercalated with varicolored calcareous shale beds and a few thin bands of friable sandstone (Fig. 3). It attains a thickness of 39.6 m, 30.5 m, and 12 m at Gabal Ekma, Wadi Budra, and Gabal Qabaliate, respectively (Fig. 3). Shahin and Kora (1991) assigned the Late Turonian age to The Wata Formation due to the presence of benthic foraminifera Discorbis turonicus, Discorbis simplex, Quinqueloculina gussensis, and Phenacofragma elegans. The Wata Formation conformably overlies the Abu Qada Formation (Fig. 2A) and unconformably underlies the Matulla Formation.
3. Material and methods Three stratigraphic sections representing the Wata Formation in the west of Sinai were measured and described in the field (Fig. 1). Ninety-five samples were collected throughout the studied sections. Detailed petrographic study was conducted on forty six thin sections that were stained with Alizarin Red-S and potassium ferricyanide to distinguish between dolomite and calcite based on
Fig. 1. Location map of the studied sections.
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Fig. 2. Field photographs showing: A) The contact between the Abu Qada and Wata formations, Wadi Budra, person for scale is 1.6 m. B) Fractured dolostone, Wadi Budra, geologic hammer for scale is 32.5 cm long.
the method of Dickson (1965). The petrographic investigation allowed the identification of limestone microfacies and dolomite textures. Limestone microfacies were described according to the classification of Dunham (1962) with the modifications of Embry and Klovan (1972). The textural description of dolomite was conducted with terminology following Sibley and Gregg (1987). For dolomite crystal-size classes, the scheme of Folk (1962) is adopted. These classes are as follows: <30 mm is fine-crystalline, 30e150 mm for medium-crystalline, and >150 mm for coarse-crystalline dolomite. Mineral types of representative non-oriented carbonates were identified by an X-ray diffractometer (XRD) with Ni-filter and Cu radiation. Fresh surface samples were investigated using scanning electron microscope (SEM). The SEM analysis was used to illustrate the morphologies of the studied dolomite. Different types of dolomite were sampled for carbon and oxygen isotope using a micro-drilling device which was performed at the Western Michigan University, USA. For C and O isotope analyses, 0.1e0.2 mg of samples was reacted with anhydrous phosphoric acid at 70 C in a Finnigan Kiel-III carbonate preparation device coupled to the inlet of a Finnigan MAT-253 mass spectrometer. The precision measurement is better than 0.02‰ for d13C and better than 0.04‰ for d18O. The d13C and d18O values are reported in per mil relative to the Vienna Peedee Belemnite (VPDB) standard. Iron, manganese, and strontium contents were measured using inductively coupled argon plasma atomic emission spectroscopy (ICP-AES). Chemical analyses were performed in the laboratories of the University of Miami/RSMAS, USA.
dolomites can help greatly to determine the origin and mechanism of dolomitization (Warren, 2000); we shed more light on those subjects for the studied dolomite as will be discussed below. 5.1. Dolomite-rock textures Microscopically, four types of dolomite-rock textures have been recognized based on the crystal-size, distribution (unimodal or polymodal), and crystal-boundary shape of dolomite. These are (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) finecrystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. 5.1.1. Dolomite texture 1: fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite The dolomite of this type is characterized by fine dolomite crystals (<30 mm in size). These crystals are subhedral with straight and rarely curved intercrystalline boundaries (hypidiotopic). Fine dolomite crystals commonly show sharp extinction and could belong to planar-s. This type of dolomite texture occurs as a replacive dolomite to the previously lime-mud (Sibley and Gregg, 1987). It displays pervasive (Fig. 5A) and sparse (partial) dolomitization (Figs. 4B and 5B). It also displays unimodal to bimodal-size distribution. The pervasive dolomite is recorded in some dolostone horizons at Gabal Ekma. The sparse dolomite occurs in the oyster floatstone facies in all the studied sections.
4. Facies analysis Although the analysis of dolostone facies is the goal of the present work, the associated limestone facies were also studied to give more light on the nature of dolomitization process in the studied rocks. Five limestone microfacies were identified in the Wata Formation (Fig. 3), and the majority of them have provided evidences of sparse dolomitization. The description and environmental interpretation of the recognized limestone microfacies are listed in Table 1. 5. Results of the studied dolomites The dolomite facies dominates the carbonate succession of the Wata Formation in all the studied sections. At Gabal Qabaliate section, the relevant dolomite strata are dominated by rudists (Fig. 4F). Dolomite facies differs markedly from the limestone facies in their dark gray to yellowish gray color. Because of the study of a combination of petrography, stable isotopes, and trace elements of
5.1.2. Dolomite texture 2: medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomites This type of dolomite texture is mainly recognized in the Wata Formation at Gabal Ekma and Wadi Budra. It is generally composed of bimodal, medium to coarsely crystalline dolomite with size ranging from 65 to 125 mm (Fig. 5C, D). Dolomite crystals are planar euhedral (idiotopic) with straight intercrystalline boundaries and sharp extinction (Fig. 5C). They are zoned with cloudy cores and clear rims. The cloudy cores of the crystals result from the presence of iron oxide inclusions, whereas the clear rims are nearly inclusion-free (Fig. 5C). This type of dolomite texture could belong to planar-e (Sibley and Gregg, 1987). It is replacive dolomite and displays pervasive (Fig. 5C) and sparse dolomitization (Fig. 5D). The pervasive dolomite probably formed through complete replacement of the previously sparry carbonate crystals, where the relics of original fabrics are absent (Fig. 5C). It is characterized by intercrystalline to isolated micro-vuggy porosity (Fig. 5C). The sparse dolomite replaces partially both the lime-mud matrix
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Fig. 3. Columnar sections showing the lithological and microfacies characteristics of the Wata Formation at the three studied localities.
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Table 1 Summary of the recognized limestone microfacies and their related depositional environments. Microfacies types
Field description
Microscopic description
Depositional environment
Oyster floatstone
It is encountered in the three studied sections and occured as oyster banks (Fig. 4A). Its rock is gray to grayish yellow in color, hard, and contains gypsum veins (Fig. 4A).
The common oysters indicate a deposition in less than 50 m depth in low oxygenated, brackish, and nutrient-rich waters (Dhondt et al., 1999; Gertsch et al., 2010).
Skeletal wacke-packstone
It is widespread in the Wata Formation all over the studied localities. The rock of this microfacies is varied in color (gray, yellow, grayish yellow) and it is dolomitic, fossiliferous, hard, nodular, and contains gypsum and calcite veins.
Bivalve wackestone
It is distinguished in both Wadi Budra and Gabal Qabaliate sections. The rock of this microfacies is varicolored (gray, grayish white, yellow), hard, dolomitic and fossiliferous.
Lime-mudstone
It is recorded in all the studied sections. The rock of this microfacies is yellow to grayish white in color, hard, dolomitic, fossiliferous, and contains gypsum veins.
It is composed mainly of large-sized oyster shell fragments that float in a carbonate lime-mud groundmass (Fig. 4A). Most of the oyster shells exhibit their original fibrous structure. The lime-mud groundmass is partially dolomitized (Fig. 4B). It consists of shell fragments and particles of pelecypod, gastropod and echinoderm that embedded in micrite matrix. Small amount of silt-sized quartz grains are disseminated. The skeletal fragments and particles are moderately sorted, packed, and range in size from fine to coarse sand size. They are embedded in micrite matrix, which is partially dolomitized. It is dominated by bivalve fragments, echinoderms, ostracods, and peloids that are embedded in a lime mud matrix (Fig. 4C). The lime-mud matrix shows partial recrystallization to microspars. Some of the shell fragments and micritic matrix are partially replaced by euhedral dolomite rhombs (Fig. 4C). It is dense micrite (95%) with rare shell debris (3e5%). The carbonate mud matrix is partially dolomitized.
Peloidal grainstone
The rock is recorded only in Gabal Ekma. It is grayish white to yellowish gray in color, very hard, dolomitic, and burrowed at its top part.
Oolitic grainstone
The rock is recorded only in Wadi Budra. It is yellowish brown in color, very hard, and dolomitic.
This microfacies is similar to SMF5 and FZ6 of Wilson (1975) and Flügel (2004), respectively. It reflects the deposition in a shallow subtidal environment.
It is equivalent to SMF9 and FZ7 of Wilson (1975) and Flügel (2004), respectively, that suggests accumulation in a restricted shallow lagoon.
Lime-mudstones that consist of a low diversity fauna and rare ostracods and bivalve fragments are commonly recorded in muddy, lagoonal environments (Heckel, 1972). This microfacies is equivalent to SMF9 and FZ7 of Wilson (1975) and Flügel (2004), respectively. It is composed essentially of peloids (70 This microfacies is common in protected e80%) (Fig. 4D) embedded in sparry calcite shallow marine environments with cement (20e30%). The peloids are rounded moderate water circulation (SMF16 and to oval in shape and exhibit sharp contacts FZ8) and in inner ramp settings. Also, with their cement. Silt-sized quartz grains peloids are abundant in low energy shallow are recorded. Some of the peloids and tidal and subtidal carbonates (Flügel, 2004). sparry calcite cement show a partial replacement by euhedral, zoned, dolomite crystals (Fig. 4D). These crystals are varied in size between 65 and 125 mm. It is predominately composed of concentric This microfacies suggests high energy ooids cemented by sparry calcite (Fig. 4E). conditions such as shoal environment and beaches (winnowed edge sands, SMF15 and Thes ooids are varied in size (from 0.5 to 1.0 mm) and contain carbonate mud nuclei FZ6; Wilson, 1975 and Flügel, 2004). Also, ooids indicate warm water environments or (Fig. 4E). tropical settings (Kiessling et al., 2002).
(Fig. 5D, E) and allochems (Fig. 4C, E). It occurs as scattered crystals distributed irregularly through the host lime-mud (Figs. 4D and 5D), bivalve fragments (Fig. 4C) and peloids (Fig. 4D).
ranging from 30 up to 50 mm in size (Fig. 5G). It is precipitated in micro-pores and surrounded by microcrystalline calcite (Fig. 5G). It is volumetrically rare and of minor importance.
5.1.3. Dolomite texture 3: medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomites This type of dolomite texture is distinguished in the dolostone facies at Gabal Qabaliate and Wadi Budra. It is not common and it is characterized by unimodal, medium-crystalline (>100 mm) dolomite mosaic. Dolomite crystals are clear in appearance, having very rare cloudy cores. They are subhedral with curved (locally straight) intercrystalline boundaries (Fig. 5E). They commonly show sharp to slightly undulatory extinction. This type of dolomite texture could belong to planar-s (Sibley and Gregg, 1987). It is generally characterized by low intercrystalline porosity that is filled by iron oxides (Fig. 5E). SEM images reveal the presence of medium-crystalline, planar-s rhombic dolomite; dolomite appears slightly dissolved (Fig. 5F).
5.2. Mineralogy
5.1.4. Dolomite texture 4: fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cement This type of dolomite cement is detected in the lime-mud facies at Gabal Ekma. It consists of planar, euhedral void-filling crystals,
SEM images (Fig. 5FeH) and X-ray diffraction analysis of carbonate rocks of the Wata Formation revealed that they are composed mainly of calcite, dolomite, and a minor amount of quartz with an absence of evaporite minerals (Fig. 6). The dolomite percentage in dolomitic limestone microfacies ranges between 2.9% and 11.8%, whereas in dolostone facies the percentage is 89.6e100 % (Table 2). The Ca % in sedimentary dolomite ranges from 48 to 62.5 %, and can be determined by powder X-ray diffraction analysis (Lumsden, 1979; Jones et al., 2001). This procedure relies up on the fact that the position of the d104 reflection depends linearly on the Ca content in dolostone. Lumsden (1979) established an equation linking molar content of CaCO3 in dolomite to the d104 spacing measured on XRD profiles. The equation is:
NCaCO3 ¼ Md þ B
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Fig. 4. A) Field photograph showing oyster-bearing limestone in the Wata Formation. Notice the presence of post-depositional gypsum veins (arrows), Gabal Qabaliate, marker pen for scale is 12 cm. B) Photomicrograph showing dolomitic oyster (Oy) floatstone microfacies. Notice, the carbonate mud is replaced by fine-crystalline, euhedral dolomite crystal rhombs (arrows) that are cloudy in cores and clear at rims, Wadi Budra, sample B25, PPL. C) Photomicrograph showing bivalve wackestone microfacies. Bivalve fragments are replaced by medium-to coarse-crystalline, euhedral dolomite rhombs (arrows) (100e125 mm), Wadi Budra, sample B19, PPL. D) Photomicrograph showing peloidal grainstone microfacies. Notice the peloids and sparry calcite groundmass (arrows) are replaced by zoned medium-crystalline, euhedral dolomite crystal rhombs (65e125 mm), Gabal Ekma, sample K15, PPL. E) Photomicrograph showing oolitic grainstone microfacies. Ooids are cemented by a sparry calcite, Wadi Budra, sample Bud, PPL. F) Field photograph showing the rudist-bearing dolostone in the uppermost part of the Wata Formation, Gabal Qabaliate, geologic hammer for scale is 32.5 cm long.
where (M) is 333.33 and (B) is 911.99. The standard error is ±0.15% on the mean composition with this procedure. The obtained data is illustrated in (Table 3). The molar content of CaCO3 ranges from 51 to 56 % with an average of 53.5%, whereas the molar content of MgCO3 varies between 44 and 49% with an average of 46.5%. XRD diffractograms of the recorded dolomites show super lattice reflections, indicating a fairly high degree of ordering (Burns and Baker, 1987).
5.4. Major and trace elements The CaCO3 and MgCO3 mole % based on XRD (Table 3) indicate that dolomite of the Wata Formation is nearly non-stoichiometry (Morse and Mackenzie, 1990). Nine samples of dolomite and dolomitic limestone analyzed for strontium. The strontium content of the studied dolostone ranges from 11 to 53 ppm (Table 4). Six dolomite samples were analyzed for manganese and iron. The resultant data are 690e900 ppm for manganese and 1900e4500 ppm for iron Table 4). Manganese content is roughly proportional to iron content in the studied samples (Table 4).
5.3. Oxygen and carbon isotopes 6. Discussion Twelve samples of dolomite and dolomitic limestone have been analyzed for oxygen and carbon isotopes. The recorded values for d18O and d13C values vary from 7.16‰ to þ0.26‰ and 0.52‰ to þ5.20‰, respectively (Table 4, Fig. 7).
6.1. Mechanism of dolomitization Previous studies on the dolomite formation throughout the
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Fig. 5. Photomicrographs showing: A) Dolostone microfacies with fine-crystalline (<30 mm) dolomite crystals, Gabal Ekma, sample K35, XPL. B) Lime-mudstone microfacies is replaced by fine-crystalline euhedral dolomite rhombs (25e65 mm) with small cloudy cores and clear outer rims, Gabal Qabaliate, sample QA15, PPL. C) Dolostone microfacies displays medium-sized (70e100 mm), closely packed, zoned, euhedral dolomite crystal rhombs which have dark brown iron-rich cores with clear outer rims, Gabal Ekma, sample K3, PPL. D) Carbonate mud is replaced by subhedral dolomite crystal rhombs with curved (locally straight) intercrystalline boundaries (arrows). Dolomite crystals are varied in size between 65 and 150 mm, Wadi Budra, sample B8, PPL. E) Medium-sized (60e100 mm), closely packed, subhedral dolomite crystals. Notice low intercrystalline porosity which is filled with iron oxides (arrows), Gabal Qabaliate, sample QA23, PPL. F) Subhedral dolomite rhombs, Gabal Ekma, sample K17. G) Void filled with dolomite crystals (arrows). The void occurs in a sparry calcite cement, Gabal Ekma, sample K17. H) Dolomite rhombs within calcite crystals, Gabal Qabaliate, sample QA23.
geological record indicate that it can form as a result of physicochemical and microbial processes (see Warren, 2000 for review). Previous workers have shown that it is difficult to determine the
origin of dolomite and/or mechanism of dolomitization by using isotopic and chemical analyses only (Hardie, 1987). Thus, in this study we merge both the chemical data with textural types of
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Fig. 6. X-Ray diffraction patterns showing the characteristic peaks of the encountered minerals in the studied carbonate rocks.
dolomite crystals to infer the origin and mechanism of dolomitization as discussed below: 6.1.1. Textural evidences As shown by petrographic investigations, the prevalence of planar (euhedral, subhedral) dolomite texture and the absence of non-planar (anhedral) one indicate that the dolomitization
occurred under low salinity and temperature conditions (Gregg and Sibley, 1984). Planar crystals tend to develop around the so called critical roughening temperature, which appears to be about 50e60 C for dolomites (Gregg and Sibley, 1984). These temperatures are considerably lower than those likely to have been generated during dolomitization of limestones at deep burial temperature (70e90 C) or hydrothermal water (100e220 C)
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Table 2 Semi-quantitative mineral composition of bulk carbonate samples of the Wata Formation (derived from XRD). Lithology
S.No.
Calcite
Dolomite
Quartz
Dolostone
QA23 K35 K13 K17 Bud B8 QA22 K3 K27 B25 Average
0.0 10.4 94.3 95.7 88.2 97.1 97.7 97 100 100 78.0
100 89.6 0.0 4.3 11.8 2.9 0.0 3.0 0.0 0.0 21.2
0.0 0.0 5.7 0.0 0.0 0.0 2.3 0.0 0.0 0.0 0.8
Dolomitic limestone
limestone
Fig. 7. Cross plot of d18O and d13C of the studied Wata Formation. Table 3 The d104 spacing (Ao), Mole % CaCo3, and Mole % MgCo3 of the studied dolostone. S.No.
d(Ao)
NCaCO3%
NMgCO3%
K35 QA23 Bud B8 K3 K17 Average Min Max
2.90355 2.89054 2.89729 2.89464 2.90492 2.89870
55 51 53 52 56 54 53.5 51 56
45 49 47 48 44 46 46.5 44 49
6.1.2. Trace elements evidences The concentrations of Sr in the studied dolomites (11e53 ppm) are lower than those in the hypersaline evaporative dolomites (600e900 ppm) and in the dolomites forming within a marine burial setting (300e500 ppm) (Veizer, 1983; Warren, 2000). Thus, the depletion of Sr content (11e53 ppm) in the studied dolomites would possibly reflect the influence of meteoric water during crystallization of the dolomites in early stage of diagenesis (Veizer, 1983; Warren, 2000). The recorded higher content of manganese (690e900 ppm) and iron (1900e4500 ppm) may be leached from the associated siliciclastic rocks. 6.1.3. Oxygen isotope evidences The d18O values of dolomite precipitated in equilibrium with normal seawater are 1e3 ‰, and those of hypersaline or brine water are more than þ3‰ (Land, 1985; Warren, 2000). Consequently, the oxygen isotopic signature in the studied dolomites (7.16 to þ0.26‰) could indicate that dolomitization was not probably mediated by hypersaline or seawater, slightly modified seawater by evaporation. Thus, the recorded depletion in the oxygen values (7.16 to þ0.26‰) could indicate that the studied dolomitization was probably mediated by seawateremeteoric water mixture (Land, 1985; Warren, 2000).
(Warren, 2000). In addition, the absence of fractures and/or faults within or in the vicinity of the studied dolomite supports the negativity of its hydrothermal origin (Middleton et al., 1993). Worldwide, the previous petrographical studies on dolomite have shown that particular dolomite crystal habits and textures are related to specific physicochemical environments and processes. For example spheroidal, dumbbell and spindle-shaped dolomite crystals have been recorded from microbially influenced settings (Gunatilaka, 1989), groundwater dolocretes (Khalaf, 1990; El-Sayed et al., 1991), and/or direct precipitation from hypersaline lake water (Wanas, 2002). These shapes of dolomite crystals are not recorded in the studied dolomite. Therefore, the microbial origin and/or groundwater dolocrete models do not appear to be appropriate for the studied dolomites. Thin-section studies and XRD analysis revealed that there is no evaporite minerals recorded in the studied dolostones. This documents that the studied dolomite generally is not reconciled with the model of dolomitization which took place in evaporitic supratidal (sabkha style) or brine reflux environments as were discussed by Machel (2004) and Boggs (2009).
6.1.4. Carbon isotope evidences The carbon isotopic composition of carbonates is an indicator of carbon sources incorporated during carbonate formation. The d13C values in dolomites are mainly dependent on the relative amount of CO2 supplied from pore water carbonate ion, dissolution of biogenic carbonate precursors, and degradation of organic matter (Compton et al., 1994; Montanez and Read, 1992). The d13C values vary mostly between 0.52 and þ 5.20‰ for the studied dolomites. These values
Table 4 Elemental and stable isotopic compositions (C and O) of the studied Wata Formation.
Dolostone
Dolomitic limestone
Min Max
S.No.
d13C(VPDB) ‰
d18O(VPDB) ‰
Mn (ppm)
Fe (ppm)
Sr (ppm)
K35 QA23 K25 QA23b B25 K27 B8 QA22 Bud K13 K3 K17
1.44 4.37 5.10 5.20 1.06 0.84 1.43 4.91 0.52 0.73 0.80 0.57 0.52 5.20
1.64 0.26 1.40 1.20 5.40 7.16 6.82 5.45 6.31 6.13 6.31 6.21 7.16 0.26
690 860 770 900 790 820
1900 2800 3000 4500 1950 2900
43 19 53 11 52 16 43 49 15
690 900
1900 4500
11 53
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lie, more or less, within the range of normal marine-derived carbon source of the biogenic carbonate precursors (Coniglio et al., 1988). They indicate that the carbonate in the studied dolomite is possible to be derived from biogenic carbonate precursor as is proved by the presence of fossil molds (Fig. 4B, C). Also, the d13C values of the studied dolostones (0.52‰ and þ5.20‰, Table 4) do not give a trend to organogenic/methanogenic origin of dolomite, in which the d13C values are more positive (about þ15‰) (Warren, 2000). So far, in our foregoing discussion the fundamental textural and chemical requirements to form dolomite from evaporitic supratidal (sabkha style), brine reflux, methanogeneses, hydrothermal phase and microbial activity seem to be unfavorable conditions for the model of dolomitization in the studied dolomites. Therefore, the possibility of its formation by mixing meteoric (fresh) water-sea water under low temperature conditions at shallow burial depth (early stage of diagenesis) becomes more probable (Fig. 8). The occurrence of the investigated dolomite at mixing meteoricsea water zone and low temperature conditions in the early buried sediments is documented by: 1) its low Sr content (11e53 ppm) and light d18O values (from 7.16 to þ0.26‰) (Brand and Veizer, 1980; Choquette and Steinen, 1980; Veizer, 1983; Ward and Halley, 1985; Budd, 1997; Meyers et al., 1997; Warren, 2000), 2) The recorded sea-level regression in the studied carbonate platform during the Upper Turonian Wata Formation (Anan, 2014) which supports the mixing meteoric-sea water model (Tucker and Wright, 1990), 3) the prevalence of planar (euhedral, subhedral) textural type in the studied dolomite which indicates that the dolomitization occurred under low salinity and temperature conditions at shallow burial depth in the early stage of diagenesis (Gregg and Sibley, 1984), and 4) the non-stoichiometry of the studied dolomite (see mineralogy), which could be a result of slightly modification of the dolomitizing fluid by fresh water that favor formation for more calcian dolomites (Folk and Land, 1975). Such an early burial origin of the studied dolomite is also supported by the prevalence of burrows in the encountered dolomite beds (Compton et al., 1994). In this situation where the dolomite occurs in mixing meteoricsea water zone in the early diagenetic stage, it can be suggested that the studied dolomite could be formed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio could be took place by two possible ways: (1) through leaching of Mg from high Mg calcite grains (micrite) and aragonitic shells in the studied limestone. This is evidenced by calcitization of original composition of the aragonitic bivalve shells (aragonite and high Mg calcite), and aggrading neomorphism of the micritic (high Mg calcite) matrix (Fig. 4B; see Anan, 2014), (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons (Figs. 3 and 8). Such leaching was developed by freshwater-dominated fluid during meteoric water diagenesis. In this context, the sparse
dolomitization of some carbonate beds could be a result of less supersaturation state of the dolomitizing fluid with respect to complete dolomitization in those carbonate beds. Although some workers have questioned the validity of a mixing zone model as an agent of dolomitization (e.g., Plummer, 1977; Machel and Mountjoy, 1986; Machel and Burton, 1994), our study supports the model of dolomitization by mixing meteoric-sea water as issued by previous authors worldwide (e. g., Ward and Halley, 1985; Tucker and Wright, 1990; Ye and Mazzullo, 1993; Cander, 1994). 7. Conclusions This study is aimed to clarify the mechanism of dolomitization in the carbonates of the Upper Turonian Wata Formation in west Sinai, Egypt in terms of their textural and chemical data. The beds of Wata Formation are described, measured and collected in the field. The collected samples were thin-sectioned and investigated under the petrographic microscopes. They also subjected to chemical analyses to determine their major and trace elements in addition to their stable carbon and oxygen isotopes. In addition to dolostones, five limestone microfacies have been identified in the carbonate beds of the Wata Formations. These are oyster floatstone, skeletal wacke-packstone, bivalve wackestone, lime-mudstone, and grainstone. Most of these microfacies have provided evidences of sparse to pervasive dolomitization. Four main textural types of dolomite are recognized: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) mediumto coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The prevalence of these planar textures and absence of non-planar ones indicate dolomitization under low temperature and salinity conditions. Both petrographic and geochemical studies declared that the studied dolomite generally is not reconciled with the model of dolomitization which took place in evaporitic supratidal (sabkha style) or brine reflux, methanogeneses, hydrothermal phase and microbial activity. Therefore, the possibility of its formation by mixing meteoric (fresh)-sea water under low temperature conditions at shallow burial depth (early stage of diagenesis) becomes more probable. This is documented by planar texture, low Sr content (11e53 ppm), non-stoichiometry, and light d18O values (from 7.16‰ to þ0.26‰) of the investigated dolomites. In this situation where the dolomite occurs in a mixing fresh water-sea water zone during early buried stage, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. Such increase of Mg/Ca ratio took place by
Fig. 8. Sketch diagram showing the suggested model for the dolomitization in the Wata Formation at the studied localities.
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two possible ways: (1) through leaching of Mg from high Mg calcite grains (micrite) and aragonitic shells in the studied limestone. (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. The d13C values (between 0.52‰ and þ5.2‰) indicated that the carbonate of the studied dolomite did not form in organicrich zone but it is possible to be derived from biogenic carbonate precursor. The recorded higher content of manganese (690e900 ppm) and iron (1900e4500 ppm) may be leached from the associated siliciclastic rocks. Acknowledgments The authors would like to thank Ahmed El Belasy, Department of Geology, Mansoura University for his help during the field work. Lamees Mohamed, Western Michigan University, is acknowledged for her help in chemical analyses. We thank the anonymous reviewers for their critical revision of the manuscript. References Anan, T., 2014. Facies analysis and sequence stratigraphy of the CenomanianTuronian mixed siliciclastic-carbonate sediments in west Sinai, Egypt. Sediment. Geol. 307, 34e46. Bartov, Y., Steinitz, G., 1977. The Judea and mount scopes groups in the Negev and Sinai with trend surface analysis of the thickness data. Israel J. Earth Sci. 26, 119e148. Bauer, J., Kuss, J., Steuber, T., 2003. Sequence stratigraphy and carbonate platform configuration (Late Cenomanian-Santonian), Sinai, Egypt. Sedimentology 50, 387e414. Bauer, J., Marzouk, A., Steuber, T., Kuss, J., 2001. Lithostratigraphy and biostratigraphy of the Cenomanian-Santonian strata of Sinai, Egypt. Cretac. Res. 22, 497e526. Boggs, S., 2009. Petrology of Sedimentary Rocks. Cambridge University Press. Bontognali, T.R., Vasconcelos, C., Warthmann, R.J., Bernasconi, S.M., Dupraz, C., Strohmenger, C.J., McKenzie, J.A., 2010. Dolomite formation within microbial mats in the coastal sabkha of Abu Dhabi (United Arab Emirates). Sedimentology 57, 824e844. Brand, V., Veizer, J., 1980. Chemical diagenesis of a multicomponent carbonate systems: trace elements. J. Sediment. Petrology 50, 1219e1236. Budd, D.A., 1997. Cenozoic dolomites of carbonate islands: their attributes and origin. Earth-Science Rev. 42, 1e47. Burns, S.J., Baker, P.A., 1987. A geochemical study of dolomite in the Monterey formation, California. J. Sediment. Petrology 57, 128e139. Cander, H.S., 1994. An example of mixing-zone dolomite, middle Eocene Avon Park Formation, Floridan Aquifer system. J. Sediment. Res. A 64, 615e629. Choquette, P.W., Steinen, R.P., 1980. Mississippian non-supratidal dolomite, Ste. Genevieve limestone, Illinois Basin: evidence for mixed-water dolomitization. In: Zenger, D.H., Dunham, J.B., Ethington, R.L. (Eds.), Concepts and Models of Dolomitization. Society of Economic Paleontologists and Mineralogists, pp. 163e196. Special Publication 28. Compton, J.S., Hall, D.L., Mallinson, D.J., Hodell, D.A., 1994. Origin of dolomite in the phosphatic Miocene Hawthron Group of Florida. J. Sediment. Res. A 64, 638e649. Coniglio, M., James, N.P., Aissaoui, D.M., 1988. Dolomitization of Miocene carbonates: Gulf of Suez, Egypt. J. Sediment. Petrology 59, 100e119. Dhondt, A., Malchus, N., Boumaza, L., Jaillard, E., 1999. Cretaceous oysters from North Africa: origin and distribution. Bull. Geol. Soc. France 170, 67e76. Dickson, J.A.D., 1965. A modified staining technique for carbonates in thin section. Nature 205, 587. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists, pp. 108e121. Memoir 1. El-Sayed, M., Fairchild, I., Spiro, B., 1991. Kuwait dolocrete: petrology, geochemistry and groundwater origin. Sediment. Geol. 73, 59e75. Embry, A.F., Klovan, J.E., 1972. Absolute water depths limits of late Devonian paleoecological zones. Geol. Rundsch. 61, 672e686. Flexer, A., Rosenfeld, A., Lipson-Benitah, S., Honigstein, A., 1986. Relative sea level changes during the Cretaceous in Israel. AAPG Bull. 70, 1685e1699. Flügel, E., 2004. Microfacies of Carbonate Rocks. Springer, Berlin. Folk, R.L., 1962. Spectral subdivision of limestone types. In: Ham, W.E. (Ed.), Classification of Carbonate Rocks-a Symposium. AAPG, pp. 62e84. Memoir 1. Folk, R.L., Land, L.S., 1975. Mg/Ca ratio and salinity: two controls over crystallization of dolomite. AAPG Bull. 59, 60e68. Gertsch, B., Keller, G., Adatte, T., Berner, Z., Kassab, A.S., Tantawy, A.A., ElSabbagh, A.M., Stueben, D., 2010. Cenomanian-Turonian transition in a shallow water sequence of the Sinai, Egypt. Int. J. Earth Sci. 99, 165e182. Ghorab, M.A., 1961. Abnormal stratigraphic features in Ras Gharib oilfields, Egypt. In: 3rd Arab Petroleum Congress, Alexandria, Egypt, pp. 1e10. Gregg, J.M., Sibley, D.F., 1984. Epigenetic dolomitization and the origin of xenotopic
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