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Table 1. The timing and environment of the diagenetic processes affecting the studied Upper Dammam Subformation. usually to either mixing zone or to hypersaline reflux models (cf. Tucker 1995). Nevertheless, neither the mixing zone nor the sabkha (with reflux) settings is capable for the production of massive replasive dolomites, based on kinetic and thermodynamic considerations (Machel and Mountjoy 1986). Careful integration of stratigraphic, petrographic and isotopic results is used for the investigated dolomites in order to examine their potential diagenetic setting. pH of porewater. This double action of the degradation of organic matter might increase the supersaturation states of dolomite. The emplacement of dolomite within this euxinic sulphate reduction milieu involved two simultaneous processes: the first is the dissolution of the precursor metastable carbonate phases (i.e. aragonite and/or high-Mg calcite), and the second is the concurrent precipitation of dolomite according to the stoichiometric reaction:

Table 1 The timing and environment of the diagenetic processes affecting the studied Upper Dammam Subformation. usually to either mixing zone or to hypersaline reflux models (cf. Tucker 1995). Nevertheless, neither the mixing zone nor the sabkha (with reflux) settings is capable for the production of massive replasive dolomites, based on kinetic and thermodynamic considerations (Machel and Mountjoy 1986). Careful integration of stratigraphic, petrographic and isotopic results is used for the investigated dolomites in order to examine their potential diagenetic setting. pH of porewater. This double action of the degradation of organic matter might increase the supersaturation states of dolomite. The emplacement of dolomite within this euxinic sulphate reduction milieu involved two simultaneous processes: the first is the dissolution of the precursor metastable carbonate phases (i.e. aragonite and/or high-Mg calcite), and the second is the concurrent precipitation of dolomite according to the stoichiometric reaction:

Related Figures (8)

"he Eocene rocks, which cover more than 80% ofthe surface Figure 1. Locat Figure 1. Location map of the studied area. During further shelf progradation the studied rocks have been emplaced within a mixing marine-meteoric zone. During this period the dolomitization process was interrupted by a silicification phase. The precipitation of quartz and the resultant silica fabrics are controlled by variations in the P.,, and the pH of the porewater. At the later stages of the highstand system tract, when sea level started to fall a meteoric lens possibly developed at the shelf margin. The section was subjected to meteoric diagenesis. A late meteoric origin is suggested for calcite mosaics, that fill some secondary pores, based on their extremely depleted 5'°O values (-5.7 to -11.8%o), variable 88C values (-2.6 to -7.8%o0 PDB), low-Mg composition and their equant form.
"he Eocene rocks, which cover more than 80% ofthe surface Figure 1. Locat Figure 1. Location map of the studied area. During further shelf progradation the studied rocks have been emplaced within a mixing marine-meteoric zone. During this period the dolomitization process was interrupted by a silicification phase. The precipitation of quartz and the resultant silica fabrics are controlled by variations in the P.,, and the pH of the porewater. At the later stages of the highstand system tract, when sea level started to fall a meteoric lens possibly developed at the shelf margin. The section was subjected to meteoric diagenesis. A late meteoric origin is suggested for calcite mosaics, that fill some secondary pores, based on their extremely depleted 5'°O values (-5.7 to -11.8%o), variable 88C values (-2.6 to -7.8%o0 PDB), low-Mg composition and their equant form.
Figure 2. Lithostratigraphic section of the Middle Eocene Dammam Formation in Qatar (modified after Abu-Zeid 1991; Hewaidy and Al-Saad 1993). The studied sequence is correlated with the eustatic sea-level curve of Haq et al. (1989). Jeologically, Qatar Peninsula is considered as a part of the Arabian Gulf Basin, which has been receiving sediments since the Paleozoic. The width of this basin extends nore than 120 km from the Arabian Shield, to the west, o the Zagros fold-belt of Iran, to the east. Except for the -ompressional stresses produced by the opening of the Red sea during the Early Tertiary Qatar has not witnessed any of Qatar Peninsula, are encountered from different onshore and offshore wells. Many deliberations have focused on the Eocene stratigraphy of Qatar both in surface and subsurface (e.g. Cavelier 1970; Abu Zeid and Boukhary 1984; Abu Zeid 1991 and Hewaidy and Al-Saad 1993). These studies subdivided the Eocene rocks in Qatar into three formations, these are from base to top: Umm er Radhuma, Rus and Dammam formations. The Dammam Formation is underlain by fine-grained limestone and evaporites of the Early Lutetian Rus Formation and overlain unconformably by shale, limestone and evaporites of the Burdigalian- Helvatian Dam Formation. Middle to Late Lutetian age has been assigned to the Dammam Formation based on its foraminiferal assemblage as it falls within the Morozovella lehneri zone (Hewaidy and Al-Saad 1993). Cavelier 1970) subdivided the Dammam Formation into lower and upper Dammam subformations. The former is made up of imestone and shale while the latter is mainly composed of dolomites and dolomitic limestones. Meanwhile, Abu-Zeid and Boukhary (1984) defined four members within the Dammam Formation. These are, from base to top: Midra, Dukhan, Umm Bab and Abaruq members (Fig. 2). The Midra member is made up of around 10m fissile calcareous shale and marl. The Dukhan member is represented by a
Figure 2. Lithostratigraphic section of the Middle Eocene Dammam Formation in Qatar (modified after Abu-Zeid 1991; Hewaidy and Al-Saad 1993). The studied sequence is correlated with the eustatic sea-level curve of Haq et al. (1989). Jeologically, Qatar Peninsula is considered as a part of the Arabian Gulf Basin, which has been receiving sediments since the Paleozoic. The width of this basin extends nore than 120 km from the Arabian Shield, to the west, o the Zagros fold-belt of Iran, to the east. Except for the -ompressional stresses produced by the opening of the Red sea during the Early Tertiary Qatar has not witnessed any of Qatar Peninsula, are encountered from different onshore and offshore wells. Many deliberations have focused on the Eocene stratigraphy of Qatar both in surface and subsurface (e.g. Cavelier 1970; Abu Zeid and Boukhary 1984; Abu Zeid 1991 and Hewaidy and Al-Saad 1993). These studies subdivided the Eocene rocks in Qatar into three formations, these are from base to top: Umm er Radhuma, Rus and Dammam formations. The Dammam Formation is underlain by fine-grained limestone and evaporites of the Early Lutetian Rus Formation and overlain unconformably by shale, limestone and evaporites of the Burdigalian- Helvatian Dam Formation. Middle to Late Lutetian age has been assigned to the Dammam Formation based on its foraminiferal assemblage as it falls within the Morozovella lehneri zone (Hewaidy and Al-Saad 1993). Cavelier 1970) subdivided the Dammam Formation into lower and upper Dammam subformations. The former is made up of imestone and shale while the latter is mainly composed of dolomites and dolomitic limestones. Meanwhile, Abu-Zeid and Boukhary (1984) defined four members within the Dammam Formation. These are, from base to top: Midra, Dukhan, Umm Bab and Abaruq members (Fig. 2). The Midra member is made up of around 10m fissile calcareous shale and marl. The Dukhan member is represented by a
Figure 3. Core samples of the Upper Dammam Subformation. Notice the calcite cement (C) in the left-hand figure (A) and the opened pores (P) in the right-hand figure (B). Dolomite type (II) is observed within the Abaruq member, the uppermost portion of the Dammam Formation. It is selective, fine textured, non-stoichiometric (Ca,, Mg,, (CO,),) and composed of unimodal dolomite mosaics (10- 40). SEM study reveals that this type of dolomite is made up of equant euhedral rhombs that display unimodal size distribution (Fig. 5D). It affects essentially the matrix of a grain-supported lithofacies (dolomitic packstones) with the skeletal particles (mostly miliolids) are either unaffected The Middle Eocene Upper Dammam Subformation consists of dolomitic limestones and dolomites interbedded with carbonate wackestones. Primary fabrics are obliterated in
Figure 3. Core samples of the Upper Dammam Subformation. Notice the calcite cement (C) in the left-hand figure (A) and the opened pores (P) in the right-hand figure (B). Dolomite type (II) is observed within the Abaruq member, the uppermost portion of the Dammam Formation. It is selective, fine textured, non-stoichiometric (Ca,, Mg,, (CO,),) and composed of unimodal dolomite mosaics (10- 40). SEM study reveals that this type of dolomite is made up of equant euhedral rhombs that display unimodal size distribution (Fig. 5D). It affects essentially the matrix of a grain-supported lithofacies (dolomitic packstones) with the skeletal particles (mostly miliolids) are either unaffected The Middle Eocene Upper Dammam Subformation consists of dolomitic limestones and dolomites interbedded with carbonate wackestones. Primary fabrics are obliterated in
Figure 4. Photomicrographs of dolomite type (1). A. SEM photomicrograph showing the coarse euhedral planar crystals. Photomicrographs B, C & D show the nonplanar, planar (s) and euhedral planar varieties of dolomite type (I respectively.
Figure 4. Photomicrographs of dolomite type (1). A. SEM photomicrograph showing the coarse euhedral planar crystals. Photomicrographs B, C & D show the nonplanar, planar (s) and euhedral planar varieties of dolomite type (I respectively.
Figure 5. A. Photomicrograph showing micro-voids and calcite relicts within the cores of some dolomite crystals (arrows) in dolomite type (1). B. Mimic dolomitization of ooids in dolomite type (I). C. Coarse dolomite clast (C) floats on dolomite matrix in dolomite type (1). D. SEM photomicrograph of dolomite type (II) showing the fine euhedral planar crystals.
Figure 5. A. Photomicrograph showing micro-voids and calcite relicts within the cores of some dolomite crystals (arrows) in dolomite type (1). B. Mimic dolomitization of ooids in dolomite type (I). C. Coarse dolomite clast (C) floats on dolomite matrix in dolomite type (1). D. SEM photomicrograph of dolomite type (II) showing the fine euhedral planar crystals.
Figure 6..A. Photomicrograph of dolomite type (II) showing opened vugs (black). B. Photomicrograph showing void filling microquartz and fibrous spherulitic chalcedonic quartz. C. Photomicrograph showing chalcedonic quartz (Q) enclosing calcite (C) and dolomite remnants. D. Photomicrograph showing the equant calcite mosaics with non-planar crystal boundaries.
Figure 6..A. Photomicrograph of dolomite type (II) showing opened vugs (black). B. Photomicrograph showing void filling microquartz and fibrous spherulitic chalcedonic quartz. C. Photomicrograph showing chalcedonic quartz (Q) enclosing calcite (C) and dolomite remnants. D. Photomicrograph showing the equant calcite mosaics with non-planar crystal boundaries.
Figure 7. 6'°O/6"C cross plot of the isotope data showing the close carbon and oxygen values of dolomite types I and II and the equant calcite cement.
Figure 7. 6'°O/6"C cross plot of the isotope data showing the close carbon and oxygen values of dolomite types I and II and the equant calcite cement.
Figure 8. Stable oxygen and carbon isotope compositions of other Eocene dolomites formed from marine and marine- dominated fluids compared with the isotopic composition of the studied dolomites. Field of Gebel Rewagen, Egypt is from Shaaban (2004). Fields of Gebel Ataga and the nummulitic bank of Giza pyramids, Egypt are from Holail (1994a,b).
Figure 8. Stable oxygen and carbon isotope compositions of other Eocene dolomites formed from marine and marine- dominated fluids compared with the isotopic composition of the studied dolomites. Field of Gebel Rewagen, Egypt is from Shaaban (2004). Fields of Gebel Ataga and the nummulitic bank of Giza pyramids, Egypt are from Holail (1994a,b).
Related topics:
GeologyCarbonate PlatformOxygen minimum zoneSediment-Water Interface
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