Journal of African Earth Sciences, Vol. 2, No. 4, pp. 333 to 340, 1984 11731-7247/84 $3.0{I + 0.00 Printed in Great Britain Pergamon Press Ltd.

The Lower Eocene dolomitized El Nashfa Formation in West Beni Mazar area, Western Desert, Egypt

S. A. WAHAB

Depar tment of Geology, Cairo University, Egypt

and

M. A. G. KHALIFA

Depar tment of Geology, Manoufia University, Egypt

(Received 31 May 1984)

Abs t r ac t - -The sedimentary sequence recorded in West Beni Mazar of Egypt is Lower to Upper Eocene in age. The E1 Nashfa Format ion represents the oldest exposed rock unit in the sequence. It was first proposed as an independent rock unit of Lower Eocene age in the stratigraphic sequence of this area by Khalifa et al. (in press). The basal part of this formation is unexposed while the upper part conformably underlies the Minia Format ion (lower Middle Eocene) . The Format ion is made up mainly of dolomitic l imestone at the base and dolostone in the upper part. A detailed petrographic investigation of the dolomites is carried out in order to provide a clearer picture of the mechan i sm of dolomitization as well as the probable envi ronment of deposition.

INTRODUCTION PETROGRAPHY

THIS PAPER describes the petrography of the E1 Nashfa dolomitic Formations (Fig. 1). The formation is pre- dominantly dolomitic although some dolomitic lime- stones are present at the basal portion. These are charac- terized by certain laminations, mud cracks and birdseye structures, which support a supratidal fiat depositional environment also confirmed by a scarcity of marine organisms.

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About 30 sections were studied in the area. Petro- graphic study helped in establishing five lithofacies (Fig. 2):

(i) birdseye dolostone, (ii) intraclastic dolostone,

(iii) dedolostone, (iv) porous dolostone, (v) and dolomitic sparite.

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Fig. 1. Location map of the studied area (shaded).

333

334 S .A. WAHAB and M. A. G. KHALIFA

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Lithologic Descript ion and Main Fossil Content .

. . . Dolostone, bluish grey, hard, hlghlyporOus) uneven on the exp~d surface /

0.50 O D ( / / Dolostone, gray, hard with calcite velnlets and pockets. dr" /

0,50 O T / / / Oolostone) greyi3h whh% hard with few pores. / r"

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I(" /I Ooloslonep whitish grey~hard3mottled with calcite pockets and veinlets.

0.80

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D T ~ ( / ( / / /

Oolostone, dark grey to pale pink, hard, wlth some moldsof 9astropds I( " / I and pelecypods.

- Ooloston% darkjgreyj with some calcite pokers.

Oolostone~ pale pinkp medium hard with spherical calcite pockets.

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0.70

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OY " i / # / / / /

i / / D 0 / ~" / Oolostone, grey to dark grey~ hard,highlly porous,with casts and

0-70 / / / molds of gastropods.

0.60

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D D I r / r l ( / /

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Ooloston%whitlh gray, hard, mottled with calcites pockets and veinlets.

Oolostone, hard grey, very hard, with ovoided calcite pockets 0 "60

DO I/// ~1 Oolost . . . . pale pink~hard,hlghly p . . . . . . ith fine laminat . . . . 0 . 8 0 /

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r ~ Dolostonej dark grey, hard., fine lamination and slightly porous. O T : - ; 0-70 ? / /

I i l 0.60 DT / / / Oolostone3whltrsh grey. medium hard, with few pore spaces.

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Dolostone, breccJated~white~ pink and grey 3 hard.

1 DL r" ; r" Oolostone~ brecciated, dark grey to pale pink> hard. /

0"60

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Oolostone, dark grey~ hard, highly porous; with fine lain ination.

Dolostonej whitish greyjhard~ with calcite pockets and few pore spaces.

Limeston% dotomitlc 3 whitish grey s hard, mottled.

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0 .60 DY r / I I I

I I I

0.60 t I / I / I

1.6o DSP v I / ~ i

I / ~.70 D S P I/I/1~ Limestone s dolomftic, white, with s o m e vertiCal, burrows.

. I / / • 1.60 DSP, ~ ] / ' ] " Limestone, white crystatt;ne, with dark patches of dotom;tzed burrow~

F i g . 2. L i t h o l o g i c l o g f o r E1 N a s h f a F o r m a t i o n a t its type sect ion I.

These rocks are distinguished by their thin bedding and some recognizable sedimentary structures such as fine laminations, mud cracks, brecciation and birdseye structures.

Birdseye dolostone ( D Y)

This lithofacies is recorded along four levels, the rock is grey in colour, fine grained and characterized by the presence of spherical and elliptical calcite birdseye struc- tures filled with sparry calcite. Microscopically, the rock consists of dolomite rhombs (65% of the rock) and sparry calcite filling the birdseye structure (36%) (Fig. 3).

The dolomite rhombs range in size from 12 to 30/~m, they are usually xenotopic and exhibit an equigranular texture. They do not show any zoning but have some

dark spots scattered within the rhombs. The most strik- ing feature in this lithofacies is the presence of the birdseye structures which are filled with blocky sparry calcite crystals. They are mainly of ovoid to elliptical shape. They might represent algal mat vugs formed by the escaped gas bubbles produced by decaying organic matter (Shinn 1968). These structures have been inter- preted as being a common feature in the supratidal flat sediments (Friedman and Sanders 1978).

It is noticeable that some dolomite rhombs of larger size (80/zm) and of idiotopic fabric are present within the blocky calcite crystals filling the birdseye. They are similar to the limpid dolomite of Folk and Land (1975), replacing the calcite crystals along the periphery of the birdseye (Fig. 4).

El Nasha Formation 335

Fig. 3. Birdseye dolostone (DY). Inequigranular idiotopic dolomite rhombs replacing the calcite crystals of the birdseye structures (O.L.X.: 125).

Fig. 4. DY lithofacies, ldiotopic dolomite rhombs replacing blocky calcite, advancing towards the centre of the birdseye (O.L.X.: 125).

Fig. 5. lntraclastic dolostone (DT). Equigranular dolomite rhombs, notice the rounded dark black intraclast (O. L. X. : 153).

336 S . A . WAHAB a n d M . A . G . KHALIFA

Fig. 6. Dolostones with fine laminations, extending laterally with parallel and wavy orientation, core of El Nashfa anticline.

Fig. 7. Dedolostone lithofacies (DD). Blocky, massive calcite crystals enclosing dark xenotopic dolomite rhombs showing poikilotopic texture due to dedolomitization (O.L.X. : 63).

Fig. 8. Irregular polygonal mud cracks on the bedding planes of the dolostones, notice the calcite filling the cracks. Core of the El Abyad anticline.

El Nasha Formation 337

Fig. 9. Porous dolostone (DO). Dark xenotopic dolomite rhombs form the main part of the rock, while the clear idiotopic limpid dolomite rhombs are lining the walls of pore spaces (O.L.X.: 83).

Fig. 10. Dolomitic sparite (DSp). Dark grey dolomite patch in dolosparite (O.L.X. : 70).

Fig. 11. Dedolostone (DD). The replacement of dolomite by calcite decreases away from the fissures. Notice the sinuous appearance along the fissure representing the pathway of the solution (O. L.X.: 67).

El Nashfa Formation 339

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Fig. 12. Schematic diagram illustrating the facies distribution and depositional environments of the El Nashfa lithofacies with its counterparts.

Intraclastic dolostone ( D T)

These dolostones are dark grey in colour and consist of dolomite rhombs (90%) with some intraclasts diag- nostic of this lithofacies. They have a rounded outline (Fig. 5) and their sizes are variable ranging from 0.90 to 1.50 ram. They are composed of finer dolomite rhombs than those forming the rock itself and exhibit xenotopic to hypidiotopic fabric. The texture is equigranular.

Fine laminations are prominent in this lithofacies (Fig. 6). They are probably the rest of the algal mats separating the dolomite laminae (Laporte 1967). These laminations are one of the diagnostic features of suprati- dal flats since they may be caused by numerous repeated periods of flooding (Roehl 1967).

Dedolostone ( D D )

This dedolostone is the most common lithofacies in the E1 Nashfa Formation. It usually overlies the two previous lithofacies and underlies the porous dolostone. It is characterized by its dark grey colour with numerous light patches (calcite) with mud cracks on the exposed surface. In thin section, the rock consists of dolomite (50-60%) and blocky calcite crystals the size of which varies from 0.70 to 3 mm exhibiting a poikilotopic texture (Fig. 7) characteristic of dedolomitization pro- CCSS.

These dedolostones are usually associated with poly- gonal mud cracks on the bedding planes (Fig. 8). Most of the mud cracks have been filled with sparry calcite like those described by Matter (1967) in the supratidal fiats.

Porous dolostone (DO)

This lithofacies contains abundant pores and usually is found overlying the dedolostone (DD) and the birdseye dolostone (DY). The dolomite rhombs forming this lithofacies are analogous to those forming the birdseye dolostone with pores and voids which constitute more

than half of the total rock. These pores have been generated by the dissolution of the calcite crystals that were occupying the birdseye structures (Fig. 9) as well as those calcite resulting from the replacement of dolomite in the dedolomitized rock.

Dolomitic sparite ( DSp )

The dolomitic sparite is recorded at the basal part of the El Nashfa Formation. The rock is white in colour with numerous dark vertical burrows. On the exposed surface, they appear as rounded dark grey patches giving the rock a mottled appearance. Petrographically, the rock consists of clear crystalline sparry calcite enclosing the dark particles of dolomite rhombs (Fig. 10). The mottling is due to the selective dolomitization of the patches resulting from the activity of the burrowing organisms (Osmond 1956).

DOLOMITIZATION

The petrographic study of the dolostones revealed the possibility of two stages of dolomitization: an early syngenetic process and a later diagenetic one. The majority of the rocks in this formation have suffered a very early or penecontemporaneous dolomitization whereby the very fine dolomite crystals (10-30/zm) with cloudy appearance and xenotopic fabric are closely simi- lar to the early dolomite "protodolomite" recorded in ancient and recent supratidal flats (Alderman and Skin- ner 1957, Deffeyes etal. 1965, Mossier 1971 and others).

The presence of several primary sedimentary struc- tures as fine laminations, mud cracks, birdseye, mottling together with the rareness of marine organisms in these dolomites strongly support the contemporaneous origin of the El Nashfa dolomite and their formation on supra- tidal flats. The requisite hypersaline conditions might have prevailed on the elevated sites on and around the standard highs as the Nashfa and Dur E1 Abyad anti- clines.

340 S.A. WAHAB and M. A. G. KHALIFA

In a phase of dolomitization, the sparry calcite crystals filling the birdseye were affected, creating idiotopic larger and clearer dolomite rhombs. These rhombs replaced the calcite along the birdseye structure, the spread towards the centre proves that the Mg ions needed for dolomitization have come from outside the birdseye structures, perhaps from the undolomitized matrix. This can be due to the action of percolating fresh meteoric water during subaerial diagenesis. Similarly, the dedolomitization process, conspicuous around the fissures and cracks in the dedolostone lithofacies on the exposed surfaces, might be caused by the flushing of fresh meteoric water through the dolostones. It is observed that the dolomite rhombs in contact with the fissures are more susceptible to replacement (Fig. 11) being on the pathway of the meteoric water. Away from the fissures the replacement of dolomite by calcite decreases. So the dolomitization marks weathering and erosion surfaces proceeding at or near unconformity surfaces (Folkman 1969, Braun and Friedman 1970). The repeated occurrence of five dedolostone beds throughout the E1 Nashfa formation sequence suggest that they represent weathering surfaces at the end of repeated cycles of deposition.

ENVIRONMENTAL INTERPRETATION

The stratigraphic set-up of the E1 Nashfa Formation as well as its lithology may elucidate to a great extent the depositional environment of this rock unit. The dolo- stones are confined to the structural highs in the studied area being only recorded in the core of El Nashfa and Dur El Abyad anticlines. Wilson (1975) proposed that the stratigraphically controlled dolomite that formed in the positive platform, have an early diagenetic ori- gin.Thus the El Nashfa Formation dolostones formed on and around the structural highs which were the sites of supratidal flats as evidenced by the petrographic and sedimentary structures.

The occurrence of dolomitic limestone at the base, and pure dolomites along the upper part of this forma- tion, strongly support the view that these rocks were deposited in intertidal and supratidal environments respectively (Fig. 12).

REFERENCES

Alderman, A. R. and Skinner, H. C. W. 1957. Dolomite sedimenta- tion in the southeast and south Australia. Am. J. Sci. 255,561-562.

Braun, M. and Friedman, G. M. 1971. Dedolomitization fabric in Peels: a possible clue to unconformity surfaces. J. sedim. Petrol. 40, 417-419.

Deffeyes, K. S., Lucia, F. G. and Weyl, P. K. 1965. Dolomitization of Recent and plio-Pleistocene sediments by marine evaporite water on Bonaire, Netherlands Antilles. In: Dolomitization and Limestone Diagenesis Dragenisy (Edited by Pray, L. C. and Murray, R. C.), pp. 71-88. Soc. Econ. Paleont and Mineralogists, Special Publica- tion No. 13.

Folk, R. L. and Land, L. S. 1975. Mg/CA ratio and salinity: two controls over crystallization of dolomite. Bull. Am. Ass. Petrol. Geol. 59, 60-68.

Folkman, Y. 1969. Diagenetic dedolomitization in the Albian Cenomenian Yagur Formation on Mount Carmel (North Israel). J. sedim. Petrol. 39,380-385.

Friedman, G. M. and Sanders, J. E. 1978. Principles and Sedimentol- ogy. Wiley, New York.

Khalifa, M. A., Abdallah, A. M. and Abu El Ela, N. Contributions to the geology of West Beni Mazar area, Western Desert, Egypt. In press.

Laporte, L. F. 1967. Carbonate deposition near mean sea level and resultant facies mosaic Manlius Formation (Lower Devonian) of New York State. Bull. Am. Ass. Petrol. Geol. 51, 73-101.

Matter, A. 1967. Tidal flat deposits in the Ordovician of western Maryland. J. sedim. Petrol. 37,601-609.

Mossier, J. H. 1971. Diagenesis and dolomitization of Swope Forma- tion (Upper Pennsylvanian) southeast Kansas. J. sedim. Petrol. 41, 962-970.

Osmond, J. C. 1956. Mottled carbonate rocks in the Middle Devonian of eastern Nevada. J. sedim. Petrol. 26, 32-41.

Roehl, P. W. 1967. Stony Mountain (Ordovician) and lnterlake (Silurian) facies analogy of recent low-energy marine and subaerial carbonates, Bahamas. Bull. Am. Ass. Petrol. 51, 1979-2032.

Shinn, E. A. 1968. Practical significance of birdseye structures in carbonate rocks: J. sedim. Petrol. 38,611-616.

Wilson, J. L. 1975. Carbonate Facies In Geologic History. Springer, Berlin.