Petrography, geochemistry and primary origin of spheroidal dolomite from the Upper Cretaceous/Lower Tertiary Maghra El-Bahari Formation at Gabal Ataqa, Northwest Gulf of Suez, Egypt
✍ Scribed by H.A Wanas
- Book ID
- 104165362
- Publisher
- Elsevier Science
- Year
- 2002
- Tongue
- English
- Weight
- 579 KB
- Volume
- 151
- Category
- Article
- ISSN
- 0037-0738
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✦ Synopsis
Field, petrographical, and geochemical studies of the Maghra El-Bahari Formation at Gabal Ataqa in Egypt indicate an occurrence of spheroidal dolomite as a primary precipitate. Petrographically, this dolomite is microcrystalline (10 -40 Am in diameter) and lacks any direct or indirect evidence of a precursor mineral. Each dolomite spherule is composed of a cluster of coalesced incomplete rhombohedral sub-units exhibiting a globular body with a hollow core. This dolomite is commonly associated with authigenic palygorskite and gypsum. Based on its associations, the spheroidal dolomite occurs in two modes: (i) siliciclastic-disseminated dolomite and (ii) evaporitic-associated dolomite. On the basis of its elemental and stable isotopic signature, the spheroidal dolomite is subdivided into two types: Type A and Type B. Type A is Ca-rich (with average CaCO 3 and MgCO 3 equal to 58.20 and 39.91 mol%, respectively) and with average Sr = 108 ppm, average Na = 656 ppm, average Mn = 72 ppm, and d 18 O ranges from À 0.4 to + 0.9xPDB. Type B, compared with Type A, is nearly stoichiometric (with average CaCO 3 and MgCO 3 equal to 49.32 and 43.75 mol%, respectively) with higher average Sr (508 ppm), average Na (3008 ppm), average Mn (171 ppm), and d 18 O ( + 0.98 to + 1.8xPDB). Slightly positive values of d 13 C ( + 0.18 to + 2.10xPDB) are recorded for the two types of dolomite. Type A belongs to the siliciclastic-disseminated dolomite, whereas Type B is of the evaporitic-associated dolomite. It is interpreted that the spheroidal dolomite directly precipitated from a solution that was supersaturated with respect to dolomite components and did not develop by dolomitization of precursor carbonates. Two main sources are suggested for dolomite precipitation: hypersaline water and mixed saline -fresh water.