Journal of African Earth Sciences, Vol. 2, No. 4, pp. 341 to 350, 1984 (1731-7247/84 $3.00 + 0.00 Printed in Great Britain © 1984 Pergamon Press Ltd.

Sedimentology of the middle Eocene Minia and Samalut Formations , West Beni Mazar , Western Desert, Egypt

S. A. WAHAB

Department of Geology, Cairo University, Cairo, Egypt

and

M. A. G. KHALIFA

Department of Geology, Manoufia University, Egypt

(Received 1 June 1984)

Abstract--The petrography of the middle Eocene Minia and Samalut Formations is discussed. The formations are subdivided into lithofacies denoting lateral as well as vertical facies changes. An environmental interpretation for these two formations is proposed. The lower portion of the Minia Formation was deposited in a shoal environment whilst the upper portion was deposited in a back reef environment. The Samalut Formation can be differentiated into the following subenvironments from south to north: shelf edge, shelf break mounds and fore slope talus.

INTRODUCTION Table 1

Rock units Age WEST BENI MAZAR area lies in the northeastern part of the Western Desert , 40 km west of the Nile Valley and 50 km south of the Fayoum Province (Fig. 1). The stratigraphic sequence of this area is subdivided into various stratigraphic rock units by Khalifa et al. (in press) as shown in Table 1.

This paper is concerned with the study of the Minia

Gravels and sand dunes Quaternary Recent Volcanic Quatran Oligocene Late-Early Quasr EISagha Late Eocene Bartonian Abu Ali E1Muelhi Middle Upper Lutetian El Saqqal Samalut (Bishay 1961) Eocene Middle Lutetian Minia (Said 1960) Lower Lutetian El Nashfa Early Eocene Upper Ypressian

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341

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

and Samalut Formations (Table 1) which represent a significant event in the depositional history of the area. The Samalut Formation is separated from El Saqqal Formation by a pronounced unconformity marking the end of this event.

STRATIGRAPHY

The stratigraphic relations of these rock units and the nature of their petrographic characteristic and faunal assemblage suggest that several lithofacies may be recog- nized and assist with the recognition of different deposi- tional environments. A model for deposition is proposed.

Minia Formation

Petrographic analyses of this formation (Fig. 2) revealed five Lithofacies.

Echinoid wackestone (We). The rock is snowy-white moderately-hard and poorly-bedded, it attains 7 m in thickness and consists of fine lime mud recrystallized in places into microscopar. There are about 20% allochems: echinoid, mollusc and bryzoa fragments. Some of the crinoids are silicified (Fig. 3).

Nummulitic wackestone (WN). This facies is recorded in the middle and near the top of the Minia Formation. It is fine grained consisting of dense lime mud with some nummulites and ostracods. They show a parallel arrangement and exhibit some degrading recrystalliza- tion which begin from the margin proceeding towards the centre (Fig. 4).

Molluscan nummulitic packestone ( PMN )" This lithofacies interbedded with the wackestone facies attains a thickness of 9.5 m. It is a coarse grained rock formed of oriented skeletal particles (70%) in which mollusc and nummulites are the most abundant. They are set in a lime mud matrix. Some echinoids and bryozoa are also present (Fig. 5).

Algal alveolina packestone (PAv). It forms the top most part of this formation. It is usually associated with dark silicieous concretions. The rock is coarse grained with about 85% allochems: red algae (lithophyllum) with well preserved cellular texture and some micritiza- tion is noticed along these algal remains (Fig. 6), alveolina exhibit micritization which is believed to be due to algal activity (Bathrust 1966). Echinoids and nummulites are less common. The matrix is a dense lime mud with cracks filled with sparry calcite crystals.

Alveolina miliolid grainstone (GAM). This Lithofacies is the most distinct feature of the Minia Formation, making about 1/3 of it. It is a coarse grained rock occurring in steplike outcrops, it contains a conspicuous amount of elongated alveolina and dark grey silicieous

concretions. Miliolids are the most abundant form fol- lowed by alveolina, orbitolites, echinoids and bioclasts (Fig. 7). Most of these particles are affected by micritiza- tion. Sparry calcite cement is observed.

The silicieous concretions are common, they range in size from several centimeters to more than 1 m. In some places they coalesce to form discontinuous lenses. They consist of numerous skeletal particles embedded in microcrystalline quartz. Alveoline orbitolites, miliolids and some algae are among the particles which are well preserved and silicified.

Samalut Formation

Samalut Formation is subdivided into three members which are from base to top:

3- - the El Mizallah member 2- - the Kharba member 1-- the E1 Abayad member.

The petrographic log of this formation (Fig. 8) shows three main lithofacies:

Nummulitic bioclastic packestone ( P N ). This lithofacies constitutes the bulk of the El Abayad and the Kharba members. The rock is a coarse grained packe- stone consisting mainly of nummulites (70%), bioclasts, algae and echinoids. The nummulites are well preserved and closely packed with one another (Fig. 9). Some show signs of fragmentation while others are micritized. Bioclasts are more abundant near the top of E1 Abyad member where they form the main framework of the rock, they are essentially derived from the nummulites, occurring usually as nests engulfed within the autochton- ous nummulitic buildings (Fig. 10). Their occurrence is largely due to the action of currents and waves which were attacking these carbonate build-ups.

Algae occur as patches between the nummulite debris, most of them are micritized. Echinoid plates and spines exhibit syntaxial overgrowth. The matrix is a dark dense lime mud probably due to algal activity especially along the lower part of this lithofacies, whereas towards the top, sparry calcite crystals are fillng the fissure and cracks and show gradational contact with the algae; suggesting that they have replaced it.

Bioclastic foraminiferal packestone (PBF). This lithofacies makes up the lowest part of E1 Mizallah member reaching a thickness of 25 m. The rock is medium grained packestone with particles ranging in size from 0.01 to 40 mm. Bioclasts are very abundant with planktonic forams: Globigerina sp. which are recrystallized into microspar (Fig. 11). The bioclasts are well sorted but their size seems to differ from one bed to the other giving the rock a graded bedded appearance. They are most probably derived from the earlier depo- sited carbonates of AI Abyad and Kharba members. The matrix is dense lime mud with clotted texture due to a differential recrystallization of the matrix into microspar.

Minia and Samalut Formations, Western Desert 343

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

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Minia and Samalut Formations, Western Desert 345

Fig. 3. Echinoidal wackestone (WF). Echinoid spines, crinoidal ossicles in lime mud with some microspar patches, Minia Formation, O.L.X. 125.

Fig. 4. Nummulitic wackestone (WN). Nummulites show some orientation and micritization, Minia Formation O.L.X. 67.

Fig. 5. Mollusc nummulite packestone (PMN)' Notice the nummulites and molluscan molds set in lime mud matrix, Minia Formation, O.L.X. 52.

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

Fig. 6. Algal alveolina packestone (PAv)- Heavily micritized algal particles, some of which show cellular structure, Minia Formation, O.L.X. 30.

Fig. 7. Alveolina miliolida grainstone (GAM). Miliolids and alveolina micritized, some show aggrading recrystallization from centre, Minia Formation, O.L.X, 34.

Fig. 9. Nummulitic bioclastic packestone (PN)-

Minia and Samalut Formations, Western Desert 347

Fig. 10. Nummulitic bioclastic packestone (PNB). Well sorted and well preserved nummulites and some bioclasts, Samalut Formation, O.L.X. 52.

Fig. 11. Lithofacies (PNB)' Nummulite debris embedded within lime mud invading the large nummulites, Samalut Formation, O.L.X. 63.

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

Fig. 12. Bioclastic foraminiferal packestone (PBv). Notice the abundance of skeletal debris and some planktonic foraminifera. Samalut Formation, O.L.X. 85.

Fig. 13. Bioclastic packestone (PB). Rounded and well sorted bioclasts derived from fragmented nummulites, Samalut Formation, O.L.X. 30.

Minia and Samalut Formations, Western Desert 349

Bioclasticpackestone (PB). This lithofacies constitutes the top most part of the mizallah member with an average thickness of 11 m. This rock consists essentially of bioclasts, some nummulites and echinoids. The bio- clasts differ from those recorded in the underlying lithofacies (PBF) in being coarser in size and stained with iron oxides, indicating that they were deposited in shal- lower conditions near the carbonate build-ups of the A1 Abyad and Kharaba members from which they were derived (Fig. 12).

ENVIRONMENT OF DEPOSITION

A deposition model for the Minia Formation and Samalut Formation is proposed.

(A) The Minia Formation

The basal part of this formation was presumably deposited in a shoal environment on the shelf edge. The presence of miliolids which live in a restricted marine environment (Griffith et al. 1969, Wilson 1975) together with their very good sorting and roundness suggested that they were in a restricted environment and then transported to the shelf edge by high energy currents on a shoal. The deepening of the sea with more quiet conditions must have prevailed in order to cause the deposition of the packestone, wackestone facies of the Minia Formation. Complete shells of the allochems and presence of benthonic foraminifera and ostracods embedded in a lime mud matrix indicate quiet, deeper marine water.

Moreover, many of the benthonic forams are highly micritized as a result of algal boring in somewhat restricted marine platform with slow rate of sedimenta- tion. The encrusting of coralline algae around alveolina along the upper part of the sequence may be similar to that described by Krebs (1974) in the back reef facies of

the Devonian carbonates complexes of central Europe. Similar examples of the same age (Eocene) are reported in Iraq and the Mediterranean region supporting a restricted marine platform; back reef facies ? of the Minia formation. The back reef facies, recorded in Northern Iraq, is dominated by miliolids and peneroplid foraminifera whereas the fore reef is dominated by large nummulites (Henson 1950). Arni (1965) found in the Eocene of the Mediterranean region that large species of nummulites form the extensive carbonate build-up along the shelf edge. They separate the water facies from the restricted shelf facies that is dominated by alveolina and miliolid. By comparison, the Eocene Minia Formation showing identical facies rich in alveolina and miliolids may have been associated with Samalut carbonate build up, being the site of a back reef, restricted subenviron- ment of deposition (Fig. 13).

(B) Samalut Formation

This formation displays lateral facies changes towards the north (seaward). It can be divided into three subenvi- ronments: shelf edge, shelf break mounds and a fore slope carbonate depositional environment.

Shelf edge mound. This subenvironment is believed to belong to the lower part of the Samalut Formation (A1 Abyad member). These shelf edge mounds are recorded around the structural highs as encountered at El Nashfa, Dur El Ghada and Dur El Abyad anticlines. These mounds separate the back reef facies of the Minia For- mation in the south from the shelf break mounds of the Kharba member in the north (Fig. 14). These carbonate mounds are massive and convex upwards and stand topographically higher than the surrounding sediments. They are formed exclusively of nummulitic bioclastic packestones in which the organic build-ups are large sized nummulites with algae and echinoderms. The dense nummulitic population, their good sorting and

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Patch reef

Fig. 14. Schematic cross-section showing the facies distribution and depositional environments of the Minia lithofacies and their relationship with the carbonate build up of Samalut Formation.

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

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Fig. 15. Schematic cross-section illustrating the facies distribution and depositional environments of different zones of the Samalut lithofacies.

good preservat ion suggest that they were most probably growing up in the form of colonies in high energy conditions of the shelf edge such as good water circula- tion, nutritent and oxygen replenishment and waste removal. The presence of nummuli te debris, derived f rom these carbonate mounds and redeposited in the form of lenses and pockets within the originally large sized nummulites, indicate the effect of waves along the shelf edge. A similar Eocene deposit has been repor ted by Arni (1965) in the Medi terranean region. He found that large sized nummulites form the extensive build up along the shelf edge separating the deepest water facies (dominated by globigerina) f rom the shelf facies (domi- nated by alveolina and miliolid).

S h e l f b reak m o u n d s . These belong to the Kharba m e m b e r occurring as small isolated hills or knoll build- ups (5-15 m in height). These mounds separate the shelf edge mounds of A1 Abyad m e m b e r in the south f rom the fore slope deposits of E1 Mizallah in the north. Knoll carbonate build-ups are characteristic features of many recent continental shelves especially along the upper slope and shelf break. They are recorded in the south- eastern United States (Macintyre and Milliman 1970) and in the Gulf of Mexico (Logan 1969). The shelf break mounds consist of large sized nummulites with coralline algae, they were generally growing up so as to maintain a shallow water level on the shelf slope. This is clearly indicated by their presence down dip (seaward).

Fore s lope deposi ts . The fore slope deposits belong to the El Mizallah, it is recorded at the northern extremes of AI Abyad area and occur in a seaward direction to the shelf b reak mounds (Fig. 15). Fore slope sediments are distinguished by the abundance of reworked and frac- tured nummulites. Its basal part is graded bedded while the top is unbedded. The two lithofacies of the fore slope deposits are the bioclastic planktonic foraminifera and

the bioclastic packestone. The former, in the basal part of the fore slope, is formed mainly of nummulite frag- ments and planktonic forams and lime mud matrix. Most of these fragments are derived from the shelf edge mounds as well as the shelf break mounds. The presence of graded bedding in this part of the fore slope and their association with planktonic forams suggest that these facies have been deposited in the deeper part of the shelf. The top-most part of the fore slope deposits are bioclastic packestone and the disappearance of the plank- tonic forams with the occurrence of iron oxides between the particles imply the shallowing of the sea at the end of this depositional event.

REFERENCES

Arni, P. 1965. L'evolution des Nummulitinae en tant que facteur de modification des depots littoraux. France Bur. Rech. geologie, Mineralogie, M. 32 pp. 7-20.

Bathrust, R. G. C. 1966. Boring algae, micrite envelop and lithification of molluscan biosparites. J. Geol. 5, 15-32.

Bishay, Y. 1961. Biostratigraphical study of the Eocene in the Eastern Desert between Samalut and Asyut by large foraminifera. Third Arab Petrol. Cong., Alexandria, pp. 2-13.

Griffifth, L. S., Pitcher, M. G. and Rice, G. W. 1969. Quantitative environmental analysis of a lower cretaceous reef complex. In: Depositional Environments in Carbonate Rocks (Edited by Fried- man, G, M.). Soc. Econ. Paleo. Mineralogists, Special Publication No. 14, pp. 120-138.

Hanson, F. R. S. 1950. Cretaceous and Tertiary reef formations and associated sediments in Middle East. Bull. Am. Ass'. Petrol. Geol. 34,215-238.

Krebs, W. 1974. Devonian carbonate complexes of central Europe. In: Reef in Time and Space (Edited by Laporte, L.F.). Soc. Econ. Paleo. Mineralogists, Special Publication No. 18, pp. 153-808.

Logan, B. W. 1969. Coral reefs and banks, yucatan shelf, Mexico. In Carbonate Sediments and Reefs (Edited by Logan, B. W. et al.). Yucatan shelf, Mexico. Am. Assoc. Petrol. Geologists mere. 1 i, pp. 129-198.

Macintyre, I. G. and Milliman, J. D. 1970. Physiographic features on the outer shelf and upper slope, Atlantic continental margin south- eastern United States. Bull. geol. Soc. Am. 81,2577-2598.

Said, R. 1960. The Geology of Egypt. Elsevier, Amsterdam. Wilson, J. L. 1975. Carbonate Facies in Geologic History. Springer,

Berlin.