ГЕОХИМИЯ, 2003, № 2, с. 225-236
КРАТКИЕ СООБЩЕНИЯ
Geoehemieal and Petrographieal Studies and Origin of the Spheroidal Dolomite in the Upper Cretaceous/Lower Tertiary Maghra El-Bahari Formation at Gabal Ataqa, Suez Canal district, Egypt © 2003 r. Hamdalla A. Wanas
Department of Geology, Faculty of Science, Menoufia University, Sbebin El-Kom, Egypt
E-mail: wanas2000@yahoo.com fax 002 048 235-689
Dolomite mineral is characterized by a variety of morphological forms ranging from perfect rhombs to spherulites with different textures and fabrics (Naiman et al., 1983; Zenger et al., 1983; Gunatilaka et al., 1984; Gunatilaka et al, 1987a; Sibley and Greegg, 1987 and Abu El-hassan and Soliman, 1999). It exhibits a compositional variation that ranges from Ca-rich through stoichiometric to Ca-poor (Katz, 1968; Reeder, 1983 and Gunatilaka et al., 1987b). Dolomite can be developed in a wide variety of environments by different mechanisms (Land, 1982; Machel and Mountjoy, 1986; Hardie, 1987 and Gunatilaka et al., 1984). Despite the widely occurrence of dolomite mineral, a relatively few records of spheroidal dolomite have been found (Lalou, 1957; Von der Borch and Jones, 1976; Garroussi, 1988; Gunatilaka, 1989; Khalaf, 1990; El-Sayed et al., 1991; El-Sayed, 1997; Nielsen et al., 1997 and Abu El-Hassan and Soliman, 1999). The present work introduces an occurrence of spheroidal dolomite in the lithologies of the Mahgra El-Bahari Formation at Gabal Ataqa. It also illustrates the possibility of a direct precipitation of spheroidal dolomite by saline ground-water flow and in sline playa. This is documented by field, petrographical and geochemical studies on the rocks of the Upper Cretaceous/Lower Tertiary Maghra El-Bahari Formation at Gabal Ataqa, Egypt.
GEOLOGICAL SETTING AND LITHOSTRATIGRAPHY
Gabal Ataqa occurs at the northeastern corner of the Eastern Desert of Egypt. It overlooks the junction of the Gulf of Suez and the Suez Canal. It lies between Latitudes 30°00 & 29°47' N and Longitudes 32°16' & 32°28' E (Fig. 1). Gabal Ataqa is a tilted fault block bounded by normal faults dipping in the northwest direction (El-Akkad and Abdallah, 1971). The geology of Gabal Ataqa has been discussed by several authors (e. g., Sadek, 1926; Said, 1962; Ismail and Selim, 1966; El-Akkad and Abdallah, 1971; El-Nozahy, 1972; Swedan and Ibrahim, 1992 and Abdel-Rahim, 1998). The exposed sedimentary sequence at Gabal Ataqa has been subdivided into six formations ranging in age from Cenomanian to Recent (El-Akkad and Abdallah, 1971,
Fig. 1). These rock units are the Cenomanian Galala Formation, Turonian Maghra El-Hadida Formation, Capmanian-Maastrichtian Adabyia Formation, Upper Cretaceous-Lower Tertiary Maghra El-Bahari Formation, Middle Eocene Suez Formation and Upper Eocene El-Ramiya Formation.
The term Mahgra El-Bahari Formation was introduced by El-Akkad and Abdallah (1971) to refer to in-frossiliferous reddish yellow marl and sandstone inter-beds at Wadi Maghra El-Bahari (the southern part of Gabal Ataqa escarpment). Field study revealed that the Moghra El-Bahari Formation varies in thickness along the escarpment of Gabal Ataqa from 80 m at the south at Wadi Moghra El-Bahari, to 36 m at the northward at Wadi Abu Saiyala (Fig. 2). The nature of the lower contact of the Maghra El-Bahari Formation is also variable from one place to another along the escarpment of Gabal Ataqa. At Wadi Maghra El-Bahari, the base of the Maghra El-Bahari Formation is unexposed where it is obscured by Wadi fillings (Fig. 3A). At Wadi Abu Saiyala, the base of the Maghra El-Bahari Formation is exposed and lies unconformably above the dark grey dolostone of the uppermost part of the Campanian-Maastrichtian Adabiya Formation (Fig. 3B). The upper boundary of the Maghra El-Bahari Formation is defined by a sharp lithological change from the evaporites of its uppermost part to the horizontally-bedded, fossil-iferous chalky limestone of its overlying Middle Eocene Suez Formation.
With referring to its lithological characteristics, the Maghra El-Bahari Formation can be subdivided into two informal lithostratigraphic units (lower and upper units, Fig. 3 C and D). The lower unit attains a thickness of about 55 m, and is made up of variegated sandstone intercalated with few reddish marl (Fig. 2. & 3C). The sandstone is partially calcareous and is fine to coarsegrained with randomly distributed granules and pebbles. These lithologies are unfossiliferous, structureless and exhibit brick red colour (Fig. 3C). The upper unit is 35 m thick, and is composed mainly of snow white carbonates and evaporites that are intercalated with reddish siltstone, claystone and greenish marl (Fig. 2
iii
Recent
Upper Eocene
Middle Eocene
LateCretaceous-Early Eocene
Campanian Turonian
Cenomanian
Measured section Asphaltik Road
Fault
29° 54'
29° 50'
32° 25'
Fig. 1. Geological map of Gabal Ataqa, simplified from El-Akkad and Abdallah (1971).
& 3D). Mottling with brown, creamy and yellow patches was observed in the lower unit (Fig. 3E). The mottling commonly occurs as oblique to vertically aligned columns or pipes (Fig. 3F).
The age of the Maghra El-Bahari Formation is not precisely known and is still controversial because of the absence of body fossils. With regard to this, Sadek (1926) and Ismail and Selim (1996) attributed the Maghra El-Bahari Formation to the Senomanian age, whereas Swedan and Ibrahim (1992) assigned the Pale-ocene age to it. The present author suggests that the Maghra El-Bahari Formation could be deposited in the period from the end of Late Cretaceous to the Early Middle Eocene on the basis of its stratigraphic position (Fig. 2). This in agreement with that reported by El-Akkad and Abdallah (1971).
SAMPLING AND METHODOLOGY
Field work has been done along the escarpment of Gabal Ataqa where excellent ourtcrops occur. Two stratigraphic sections were measured: the first section at Wadi Maghra El-Bahari and the second at Wadi Abu Saiyala (Fig. 1). Thirty samples were collected to represent the vertical and lateral lithologies of the Maghra El-Bahari Formation (Fig. 2). Fifteen thin sections of indurated samples were prepared and examined under the polarized microscope. Approximately ten gold-coated fresh surface samples were investigated using scanning electron microscope (SEM, type-JSM-840) equipped with an energy-dispersive X-ray analyzer (EDX). The SEM analysis was done to illustrate the morphology of minerals present. Carbon-coated polished thin sections were examined by electron probe micro-analysis (EPM). Operating conditions for EPM
Wadi Maghra El-Bahari
The vertical scale r 8 m
- 4
- 0
Legend
S V V •• / V ■•■•■ ■
6
Limestone Dolostone
Dolomitic limestone
Dolomitic limestone, sandy and argillaceous
Pebbly medium to coarsegrained sandstone.
Argillaceous fine - grained sandstone
Siltstone
Evaporites
Calcareous Mottling
Fossiliferous
Fig. 2. Lithostratigraphic sections of the Maghra El-Bahari Formation at Wadi Maghra El-Bahari and Wadi Abu Saiyala, Gabal Ataqa.
were 15 kv accelerating voltage and 10 nA specimen current with 10 pm spot size. Electron probe microanalysis (EPM) was performed to determine the major elemental composition. Mineral types of representative non-oriented carbonate and oriented clay samples were delineated by using X-ray diffractometer with Ni-fil-tered, Cu radiation (XRD, type - Philips 1980). Prior to XRD analysis, the clay fractions were extracted from the carbonate rocks using the following experimental protocol: 50 gm. of crushed carbonate samples were treated with 1 N sodium acetateacetic acid buffer solution (pH = 5), and the acid-insoluble residue was washed repeatedly with distilled water.The silt (2-20 pm particle size) and clay (<2 pm particle size) fractions were separated by centrifugation, and three oriented clay aggregates (air-dried, glycolated and heated to 550°C) were prepared for each sample.
Quantitative major and trace elemental analysis of dolomite-rich samples (whith less than 8% IR) were measured by atomic absorption. This was performed after dissolution of the <63 pm fraction in HCl3 and HNO3. Stable isotope analysis on dolomite samples has been carried out. To perform this analysis, the dolomite micro-samples were drilled from dolomite-rich core slabs with a dental micro-drill. Organic carbon was removed prior toacid treatment by oxidizing the sample powder in a plasma furnace. The powder was then reacted with phosphoric acid at 90°C and the resultant gas (CO2) was collected after 72 hr, and measured with multi-collector FINNIGAN 252 mass spectrometer (flame AAS). This analysis gas been done according to the method of McCrea (1950). The isotopic results were expressed as 8 value in per mil (%%) relative to
rEOXHMM № 2 2003
7*
Fig. 3. Field photographs showing: A - the lower (Ma) and upper (Mb) units of the Maghra El-Bahari Formation and their overlying the Suez Formation (S) at Wadi Maghra El-Bahari. The arrows mark the boundaries between these rock units. B - the Maghra El-Bahari Formation (M) brackets by the Suez Formation (S) and the Adabiya Formation (D). The dotted lines mark the boundaries between these rock units. This photographs has been taken at midway between Wadi Abu Saiyala and Iron Steel Quarry. C - a close-up on the lower unit of the Maghra El-Bahari Formation at Wadi Maghra El-Bahari. D - a close-up on the upper unit of the Maghra El-Bahari Formation at Wadi Maghra El-Bahari. It is composed mainly of alternating beds of chalky-textured carbonates and evaporites. E - mottled appearance in the lithologies of the lower unit of the Maghra El-Bahari Formation at Wadi El-Bahari. F -vertically aligned mottling in the lithologies of the lower unit of the Maghra El-Bahari Formation at Wadi Abu Sailyala.
Table 1. Microprobe analysis data of the palygorskite of the Maghra El-Bahari Formation at Gabal Ataqa, Egypt
Rock unit Sample No. SiO2 % Al2O3 % MgO % Fe2O3 % CaO % K2O % Na
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