Sedimentology, sequential analysis and clay mineralogy of the lower Eocene sequence at Farafra Oasis area, Western Desert of Egypt

Journal of African Earth Sciences 78 (2013) 28–50

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Sedimentology, sequential analysis and clay mineralogy of the lower Eocene sequence at Farafra Oasis area, Western Desert of Egypt Abdalla M. El Ayyat ⇑ Geology Department, Faculty of Science, Assiut University, Assiut 71516, Egypt

a r t i c l e

i n f o

Article history: Received 28 December 2011 Received in revised form 10 September 2012 Accepted 14 September 2012 Available online 5 October 2012 Keywords: Farafra Oasis Facies analysis Depositional model Sequential analysis Stacking pattern Clay mineralogy

a b s t r a c t Integrated sedimentological studies, sequential analysis and clay mineralogy on the lower Eocene rocks in the Western Desert provided important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic conditions. Two formations are recognized; the Esna and Farafra formations, with a gradational contact in-between. The studied sequence exhibits lateral facies changes as revealed from field and microfacies investigations. Eight facies were recognized and summarized in a carbonate ramp model. It represents also a general regressive trend, which records a transition from an outer ramp into a peritidal zone. The facies stacking patterns constitute several kinds of meter-scale, shallowingupward cycles. Two different types of depositional cycles are here defined. The stratigraphic sections show a hierarchical organization of many cycles defined by five depositional sequences. It is suggested that composite eustatic sea level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of the observed hierarchical cyclic organization. Summing up, it is believed that the paleotopography had resulted from the impact of the Syrian Arc Folding System. A confusing additional complication is introduced by syndepositional sedimentary structures, especially during the late Cretaceous/Eocene times, coupled by several tensional forces. Clay mineralogy has revealed the presence of smectite, kaolinite and illite. Their origin may be attributed to the gradual increase in the amount of erosion of the newly elevated crystalline source rocks to the south of Egypt, in areas of moderate rainfall and rapid weathering and/or to reworking processes of soils which presumably developed on basement rocks. Changes in source rocks or climatic influence during the early Eocene may account for the observed differences in clay mineral abundances. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The Farafra depression is an isolated irregular oval-shaped depression in the middle sector of the Western Desert (Fig. 1). This depression is carved out of upper Cretaceous Khoman Chalk. It is the second largest depression in the Western Desert and sits at longitude 27°200 & 28°590 E and at latitude 26°180 & 27°420 N. The escarpment rings the depression on three sides. The eastern and western scarp are both steep-sided, formidable barriers. Since the earliest days of the geological investigation in Egypt; the upper Cretaceous/lower Tertiary outcrops of the Farafra Oasis have been the subject of many studies that focus on general geology, stratigraphy and micropaleontology (Youssef and Abdel Aziz, 1971; Barthel and Herrmann-Degen, 1981; Hermina, 1990; Keheila and Kassab, 2001; Khalifa et al., 2005; Obaidalla et al., 2006). However, little has been done on detailed sequential analysis and depositional history of the lower Eocene rocks exposed west and east of ⇑ Tel.: +20 0882412183 (O), +20 0882271822 (H), +20 01146889094 (M); fax: +20 088 2342708. E-mail addresses: [email protected], [email protected] 1464-343X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2012.09.011

the Farafra Oasis. Moreover, many questions related to some stratigraphical and sedimentological problems remained without satisfactory answers. 2. Geological setting The Farafra Oasis (Fig. 1) lies within an oval shaped depression, which is bounded by scarps from the eastern, northern and western sides, where it is open due to south. The larger axis of the depression is 102 km whereas its eastwest axis is about 90 km. It covers an area of about 980 km2. Its floor is covered by the Dakhla Formation in its southern part. Northwards at latitude 26 45° N, the Dakhla Formation gives place to a coeval chalk unit of Maastrichtian age named Khoman Formation (Hermina, 1990). The eastern part of the depression is covered by the Karawein sand sheets with some seif dune on top. Its altitude is about 144 m above sea level at its southern parts, decreasing in height northward to only 32 m at Ain El Wadi (Wadi Hennis) and Wadi El Maqfi areas. These areas are covered by wet sabkhas. The eastern scarp face extends in an undulating line running generally in a NNW–SSE direction. The scarp abruptly changes its direction to

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Fig. 1. Simple geological map (after Hermina, 1990) for the study area and the measured sections.

ENE–WSW to form the northern Farafra scarp. Most probably this change in direction is due to faulting and folding (Said, 1990). The western scarp is named El Guss Abu Said syncline. It is one of the most important folds in the west central latitudes of Egypt between Libya in the west and the Bahariya high in the northeast. The structure is located between El Maqfi and Ain Dalla anticlines. The axis of this syncline trends N 40° E parallel to the axis of El Maqfi anticline and extends for about 80 km. The syncline covers an area of 400 km2. The axis of this structure is located very close to the western face of El Guss Abu Said plateau. On the other hand, it was cut by two major faults which run parallel to the present eastern and western sides of the plateau. About two thirds of the surrounding scarp faces consist of shales (Esna Formation) which are overlain by hard, fractured, jointed and dolomitic limestone (Farafra Formation). The different lithologies of the scarp in addition to the fracture lines dissecting the area are the main factors controlling the retreat of the scarp mainly by wind and to a less degree by water erosion (Khalifa et al., 2005). The Farafra Oasis was affected by tectonic movements, which extended from late Cretaceous to late middle Eocene (Zaghloul,

1983). Such movements may belong to the Syrian Arc Folding System (NE–SW direction), that had affected the northern part of the Egyptian territory (Moustafa and Khalil, 1995). According to Neev and Hall (1982) the distribution pattern of the lineament swarms across Africa puts the Farafra Oasis within the Pelusium Megashear System or the seismoactive Pelusium line of Said (1971) which extends subparallel to the east Mediterranean shores of Levant and continues inland across the Nile Delta to Niger Delta on the Gulf of Guinea. The shear joints and the majority of the fault directions are both due to the folding system of upper Cretaceous origin. On the other hand, the tectonism during the post deposition led to the formation of a few positive anticlines and negative syncline structures. There are, in fact, evidences that tectonic movements affected the Farafra Oasis at various stages before the end of the Cretaceous to the late middle Eocene. During the Maastrichtian to the early Eocene time, the syndepositional growth of folds were responsible for the variations in the thickness of Khoman Formation on the structural high areas and unconformity between the Maastrichtian and the middle Eocene as recorded in many parts of north Egypt along these arcs. This unconformity was recorded by El Akkad and Issawi

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(1963) in the Bahariya scarp, by Al Far (1966) and Sestini (1984) at Gebel Maghara and by El Akkad and Abdallah (1971) at Gebel Ataqa. Recently, Obaidalla et al. (2006) recorded three syndepositional tectonic events that affected the upper Cretaceous/Paleogene rocks in Farafra and its environs. Therefore, the structure of the study area is a result of the impact of the Syrian Arc Folding System, on lithologies of vastly different competence. A confusing additional complication is introduced by syndepositional sedimentary structures, especially during the late Cretaceous/Eocene times, coupled by several tensional forces which led to the development of many faults dissecting the area (Neev and Hall, 1982). 3. Scope of the present work The objectives of this study are to determine the microfacies properties and the petrographic aspects of the lower Eocene sediments to provide information on the mineralogy and diagenetic alterations; to determine the different sedimentary facies and environments in order to provide a comprehensive view of the sedimentological model for depositional history; to correlate between the sedimentary facies on both sides of the Farafra Oasis aiming to deduce the effect of the Farafra uplift on sedimentation; to analyze sequentially the studied rocks to gain a great deal of insights into the depositional sequences, their stratal geometries and relation to sea level changes; and finally to discuss the paleoclimatic conditions prevailed during the accumulation of Esna Formation shales. 4. Materials and methods Two stratigraphic sections were measured bed by bed, spanning a roughly W–E trending transect of around 90 km in length (Fig. 1). The El Guss Abu Said section (27°030 N and 27°400 E) is located ca 15 km northwest of Farafra Oasis (Fig. 3). The section is approximately 89 m thick and covers Farafra Formation and most of Esna Formation. The section of the eastern scarp (27°010 N and 28°430 E) is located at 75 km east of the Farafra Oasis (Fig. 4). It is approximately 83 m thick and consists of nearly the same stratigraphic sequence. The sections were logged, sampled, and described with respect to the sedimentary structures, textures and biotic components. Textural and compositional characteristics (Fig. 2) of the investigated lithologies were based on transmitted-light microscopy of 60 thin sections and cut slabs. Nearly 35 friable and soft

samples were washed by water, their residue are carefully examined under binocular microscope. The petrographic classification for carbonates is based on Dunham’s (1962) limestone classification. Wilson (1975) and Flügel (2010) facies belts and sedimentary models were also used. Microfacies studies include the analysis of matrix and grains, textural features, fossil content, petrographic and energy index classification (EI), facies zone, standard microfacies zone and model formation, origin, correlation and interpretation criteria as determined from thin sections. Following previous quantitative microfacies studies by point counting (Huelse, 2007), a semiquantitative frequency analysis of the investigated sections were applied by using a point counter set (500 counts for each thin section). The ratio of nummulites A to B-forms was determined, because it provides useful paleoenvironmental information (Hottinger, 1997). Thin sections were stained using the method of Dickson (1966) to distinguish ferron and non-ferron calcite from dolomite. In order to resolve the facies arrangement of the single cycles and overall sequential architecture of the studied sequence, outcrop investigations were combined with detailed microfacies studies. To throw some light on the mineralogy of the shales of the Esna Formation, ten samples were disaggregated using ultrasonic equipment and washed in distilled water until all soluble salts were removed. Each sample was dispersed with Calgon. Separation of the clay size fraction was performed by the use of the pipette method (Galehouse, 1971). Oriented samples were mounted on glass slides. Three X-ray diffractograms were made for each sample: (1) using an untreated sample; (2) using a glycolated sample and (3) using a heated sample at 550 °C for three hours. The samples were investigated using Philips X-ray diffractometer with CuKa radiation, 45 kV, and 35 mA and scanning between 2 and 40 2h at rate 1.2 2h/min. Semi-quantitative estimates of the mineral constituents of the investigated samples were made following the method of Schultz (1964). 5. Lithostratigraphy 5.1. Esna Formation The Esna Shale was firstly used by Beadnell (1905) to define the laminated green and grey shale that exposed at Gebel Oweina, opposite Esna, Nile Valley. Said (1962) amended the term, restrict-

Fig. 2. General legend for symbols used in the present study.

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Fig. 3. Vertical distribution of the main sedimentological characteristics, microfacies types and depositional environments of the sedimentary sequence at El Guss Abu Said section (Legend in Fig. 2).

ing its usage for the shale beds above the Tarawan Formation and below the Thebes Formation (Table 1). This unit is of more or less uniform thickness (Plate A1), with 20.8 m thick at eastern scarp and 29.5 m at El Guss Abu Said plateau (Fig. 5).

Lithologically, it consists of shales, mudstones and thin limestone intercalations. The shale is commonly green, olive, and yellow in color, associated usually with planktonic forams and larger foraminifera (nummulites and operculines) with some echi-

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Fig. 4. Vertical distribution of the main sedimentological characteristics, microfacies types and depositional environments of the sedimentary sequence at the eastern scarp section (Legend in Fig. 2).

noids. The Esna formation is characterized by the presence of a dwarfed fauna zone at base (Figs. 3 and 4) and an upper marly limestone with rare megafossils at top.

Upsection, Esna Formation is conformably and gradationally overlain by the Farafra Formation. The contact between Esna and Farafra formations is taken arbitrary where the percent of calcare-

Esna Fm Tarawan Fm Dakhla Fm Esna Fm Abdaila L.S.

Farafra Fm Esna Fm Farafra Fm Farafra Fm

Esna Shale Chalk Tarawan Fm

Dakhla Fm

Farafra Fm

Esna Fm

Tawaran Fm

Thebes Fm

Esna Fm Tarawan Fm Dakhla Fm

Khoman Fm

Farafra Fm

Farafra Fm

Said and Kerdany, 1961, Farafra Hermina, 1990 Western Desert Zaghloul, 1983, Farafra/Ain Dalla Youssef and Abdel Aziz, 1971, Western Desert Khalil and ElYounsy, 2003, Farafra Hendriks et al. 1984 Kharga/ Baris

Obaidalla et al. 2006 Western Desert

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ous content increases at the expense of shales (Plates A1 and 2). The distribution of Esna Formation is controlled to a great extent by the structurally folded areas over which deposition took place. It has been observed that the shale, mudstone and marl beds are usually represented by a maximum thickness in the structurally low areas (e.g. the studied sections) and the thickness decreases due to north and south outside the study area. This may indicate continuous deposition of shales accompanied with continuous subsidence. The rhythmic bedding may be caused by fluctuations in the depositional environment including variations in terrigenous input, surface productivity, current velocities, and water depth (Wilson, 1975). The predominance of planktonic foraminifera over benthonics in this facies type attests to deposition in an open marine environment. 5.2. Farafra Formation Overlying Esna Formation is a 15.5–20.5 m thick bedded, massive, nodular and tan to buff alveolinid limestone (Plate A4). The term Farafra Limestone was first introduced by Said (1962) to describe about 34 m of limestone beds overlying Esna Formation in Farafra Oasis (Plates A1 and 3). The formation is well exposed at El Guss Abu Said plateau. In the study area, Farafra Formation conformably overlies the Esna Formation (Fig. 5). It has a great extension, forming the cap rock of El Guss Abu Said (15.5 m thick) as well as at the eastern Farafra scarp (20.5 m). Lithologically, this formation is notable for its increased number of the intercalating shale beds particularly in its lower part (Plate A3). It is made up of grey limestone, hard, partly crystalline, dolomitic at top, marly at base, thin bedded, highly fossiliferous with alveolines, nummulites, miliolids and green calcareous algae. The miliolids and alveolines with algae association indicate lagoonal facies deposited under certain conditions of restricted water circulation and higher salinity. The formation attains a maximum thickness (29.5 m) at El Guss Abu Said. In north Kharga Oasis, it grades into the coeval facial variant of El Rufuf Formation. The Farafra Formation represents the counterparts and/or the lateral facies changes of the lower Eocene Thebes Formation exposed further eastward at the Nile Valley. This facies change may be largely controlled by the tectonic movements that affected Egypt during the late Cretaceous and before the deposition of the lower Eocene sediments.

Esna Fm Tarawan Fm Dakhla Fm

Dominik, 1985 Farafra Oasis

Farafra Fm

Esna Fm

Farafra Fm

Esna Shale Gunna L.S. Chalk

Eocene

Paleocene

Farafra Fm

Combining outcrop informations with the detailed microfacies studies led to reconstruction of different facies zones. Eight principal facies zones were distinguished (Fig. 6). 6.1. Tidal flat facies 6.1.1. Occurrence It is well represented at El Guss Abu Said locality, where it is interstratified with open marine outer shelf shales of Esna Formation. It crops out along the upper part. It is completely absent at the Eastern scarp (Fig. 6).

Geofizika, 1963 Farafra Oasis

Author & locality

Barthel and HerrmannDegen, 1981 Farafra/ Dakhla

6. Facies analysis and paleoenvironments

Age

Table 1 Correlation chart of the rock units advocated in the present study with those proposed by different authors for the early Paleogene successions at various localities in the Western Desert, Egypt.

The present study

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6.1.2. Description Prevailing rock types are homogeneous, unfossiliferous dolomitic mudstones, yellowish grey, thin to medium bedded, finegrained dolomite including dolomicrite, dolosparite, dolomitic lime-mudstone and calcareous dolomite microfacies with irregular millimeters-scale lamina. The dolomitic facies, which is usually yellowish grey in color, is porous. In thin section (Plate B1) the rock is composed of dolomite rhombs, which can be differentiated into

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Plate A. 1. Field view showing dark grey to greenish, slope forming shale of the Esna Formation at the lower part. It passes gradually into hard, ledge forming limestone of the Farafra Formation. Note the transitional contact in-between. El Guss Abu Said scarp. Looking east. 2. Field view showing the nature of the gradational contact between Esna Formation and the overlying Farafra Formation. Note the interbedded shales and limestones. Eastern scarp. Looking west. 3. Field view showing thick bedded, ledge forming limestones of the Farafra Formation overlying Esna Formation through a gradational contact in-between. Eastern scarp. Looking west. 4. Field view showing hard nodular fossiliferous limestone of the Farafra Formation. Eastern scarp.

Fig. 5. Schematic diagram showing the lateral and vertical distribution of the Farafra and Esna formations and the nature of contact in-between along the W–E correlation trend.

two types. The first type (80% of the rock) is represented by finegrained rhombs ranging from 10 to 50 lm. The matrix is formed of lime mud. Most of rhombs show idiotopic to hypidiotopic fabric with inequigranular texture. They do not develop zoning but have a cloudy appearance due to the presence of very fine opaque inclusions. The second type of dolomite (20–30% by volume) is less abundant. The dolomite rhombs range in size from 90 to 200 lm and are hypidiotopic to xenotopic in fabric. Some rhombs show zoning as dark to brownish cores with thin clear outer rims (Plate B2). Where sediment has been totally dolomitized, the rhombic shape of the dolomite crystals in thin section may no longer be

apparent. These dolomites are closely similar to those recorded in ancient tidal flats by many workers. Khalifa (1982) described dolomite rhombs in the lower Eocene, and attributed these rhombs to contemporaneous dolomitization in supratidal flats (see Plate B3). 6.1.3. Interpretation The unzoned habit is a good proof for syngenetic origin. The presence of finely divided dark spots in all the rhombs indicates the original dissemination in the formation of lime mud in very near shore areas. The first type of fine-grained dolomite rhombs was originally laid down as micrite, deposited in supratidal zone

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Fig. 6. Lateral and vertical distribution of the studied sedimentary facies along W–E correlation trend for the Esna and Farafra formations, Farafra Oasis, Western Desert.

of marginal marine environment. The second type of coarse dolomite rhombs is a probably neomorphic product of the fine ones (Sibley and Gregg, 1987). The petrographic evidences confirm the replacement (secondary) origin dolomite in the study area, these evidences are: (1) the different sizes of the dolomite crystals (in the range 10–200 lm), these grain sizes are typically characteristic of dolomite of secondary origin (Friedman and Sanders, 1987). (2) The predominance of subhedral to euhedral shape of the grains substantiates the secondary origin and obliteration of the original features and structures due to the dolomitization process adds further evidence. It is believed that fine dolomites were formed in tidal flat environment (Tucker, 1990) around El Guss Abu Said basin. Here, it is suggested that, the paleogeographic control and early shallow

subsurface origin of the dolomite, together with an absence of evaporite and a humid climate with moderate to strong seasonal rainfall, point to the meteoric-marine mixing-zone model (Hanshaw et al., 1971). The logic behind this model is that, it is easier to precipitate dolomite from a dilute solution. Mixing zone dolomites can be expected to develop extensively during major regressive periods (Fig. 9), when platform carbonates are being deposited (Tucker, 1990). Potential fluid sources for Mg+2 are seawater, subsurface fluids of marine and/or meteoric origin, and in addition Mg+2 could be released from high Mg-calcite and smectite clays (Tucker, 1990). Similar cases in the geological record are represented by Mississippian Site Genevieve Limestone of the Illinois Basin (Choquette and Steinen, 1980) and tidal flats of southwest Andros Island (Gebelein et al., 1980).

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Plate B. 1. Photomicrograph showing bimodal grain size distribution of dolomite rhombs. Most of which show idiotopic to hypidiotopic fabric with inequigranular texture. The matrix is formed of lime mud. El Guss Abu Said locality. PPL. 2. SEM Photomicrograph showing unzoned dolomicrite rhombs (photo center) and zoned dolomicrosparite (photo center on the right). Note euhedral to subhedral crystals. El Guss Abu Said locality. 3. Field photo showing crudely bedded, hard, medium to thick-bedded, nodular and argillaceous limestone of inner lagoonal facies. El Guss Abu Said locality. 4. Photomicrograph showing opertorbitolites foraminiferal alveolinid wacke- to packstone of inner shelf lagoon. El Guss Abu Said locality. PPL. 5. Field view showing the location of the shoal facies on the top of the Farafra Formation (the arrow). Eastern scarp locality. Looking west. 6. Photomicrograph showing well sorted, partially fragmented alveolines of the shoal facies. All components are cemented by sparite. Eastern scarp locality. PPL. 7. Field photo shows regular alternations of hard limestone of outer shelf facies with yellowish grey to greenish calcareous shales beds in a cyclic manner. Eastern scarp locality, looking west. 8. Photomicrograph showing bioclastc wacke- to packstone microfacies of the outer lagoonal facies. Components are diagenetically altered and embedded in micrite.

6.2. Inner shelf lagoon facies 6.2.1. Occurrence It measures 38 m at El Guss Abu Said with only 5 m at the eastern scarp. It caps the studied sequence and covered with shoal bar facies (Fig. 6). 6.2.2. Description It consists of regular alternation of crudely bedded limestone with bands of yellowish marly limestone to marl (Plate B1). Burrow and bioturbation are the most common sedimentary

structures. Wackestones, wacke- to packstone and packstones dominate the sequence, but mudstones are also occurring in certain horizons. Faunal contents are represented by alveolines (20– 50%), miliolids (20–30%), opertorbitolites (7–13%), ostracods (3– 5%), smaller benthonic biserial forams (4–7%), green calcareous algae (10–15%) and pelecypods (2–6%). Peloids of different shapes and nearly same size are scattered in the micrite. Most components show micritization and/or micrite coating and the micrite matrix show patchy neomorphism (Plate B4). They are localized behind the shoal bars, which seems to be semi-restricted from the open marine conditions (Fig. 7). Accordingly, the salinity varies from

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Fig. 7. General biofacies distribution on the lower Eocene platform, Farafra Oasis, Western Desert. Legend in Fig. 2. MSN = Mean sea level, FWWB = Fear weather wave base, SWB = Storm wave base.

Fig. 8. Facies model of the lower Eocene sedimentary sequence in the Farafra Oasis area, Western Desert, demonstrating the distribution of the main ortho- and allo-chemical components, depositional textures and primary sedimentary structures.

normal marine to slightly higher, circulation is very moderate, and water depth is shallow varying from few meters to tens of meters. The biota; occasionally occur in great abundance. The sediments are texturally varied but contain considerable amounts of lime mud. The alveolines and opertorbitolites greater than 2 mm in

diameter are floating in finer groundmass. Alveolines are occasionally exhibit algal coating. Grain size is calcarenitic to calcruditic. Sorting is moderate to good. The water energy is mainly intermittently agitated, and occasionally slightly to moderately agitated (EI = I–III).

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Fig. 9. Representative stratigraphic sections showing the hierarchical arrangement of the different types of depositional cycles, sequences and megasequences according to their evolution through time.

6.2.3. Interpretation The sedimentological characteristics, stratigraphic position and faunal content suggest deposition in semi-restricted environment -

inner lagoon- probably with low current activity, where most of muds were laid down from suspension. Alveolines are, in general shallow water neritic forms (Reichel, 1964). Its preference for re-

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stricted protected shelves; shoals, bars and reefs have been reported by Hottinger (1973). Ghose (1977) concluded that orbitolites had a maximum development in protected areas associated with reefs such as algal or coral pools on the reef flat, and back reef zones near the reef core. Buxton and Pedley (1989) stated that abundant miliolids and alveloinid forms are characterizing the protected embayments in the Tethyan Tertiary carbonate ramps. Restricted platforms (including lagoons) contain relatively high percentages of milioline foraminifera, including larger soritids and peneroplids (Hallock and Glenn, 1986). Geel (2000) mentioned that miliolids prefer low turbulence and soft substrates and when abundant indicate restricted lagoonal and/or nutrient rich backreef. The rather low diversity and type of fauna suggests that deposition was in shallow, warm, and restricted environments. Biofacies comparable to the present assemblage are known in different middle Eocene outcrops on the African and middle East platforms as in; Libya: subsurface Gialo Limestone Formation in Sirte basin (Lehman et al., 1967); Iran: Jahrum Formation in south western Zagros (Kalantari, 1986); Spain; in the Pyrenean Gulf (Caus, 1979). The sediments of this facies were accumulated in intermittently agitated water conditions (EI = II). This facies comprises several microfacies such as bioclastic alveolines miliolids packstone, opertorbitolites alveolines miliolids wacke–to packstone microfacies. 6.3. Shoal bar facies 6.3.1. Occurrence It caps the studied section and recorded mainly at remote areas. It takes the form of isolated hillocks and table lands (Fig. 6). 6.3.2. Description It comprises a succession of mud-free (1.20–1.50 m thick), nonargillaceous well-sorted medium to coarse-peloidal and skeletal grainstones. It is creamy, brown, slope forming and moderately hard to hard limestone. Cross bedding is present in large-scale planar type. It is 3–7 m in set height and 10–15 m in lateral extension, with foreset dip angles ranging from 11° to 27°. Skeletal debris is abundant, which creates a grain-supported fabric (i.e. grainstones). Allochems are well rounded with micrite rinds. Lower contacts are generally sharp and planar or, occasionally erosive. Worn and abraded alveolines (33–55%); opertorbitolites (10–15%); miliolids (7–20%) and fragmented bioclastics (10–15%) are the main microscopic constituents. some of bioclasts show over packing and micritization (Plate B6). Bioturbation is of varying intensity and is generally confined to the tops of beds. Sorting is moderate to good. The estimated water energy is moderately agitated (EI = IV). The different components are cemented with drusy, clear calcite crystals. This facies includes several microfacies such as miliolids alveolines grainstone and bioclastic alveolines packstone/grainstone.

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are very susceptible to mechanical breakage during transport. Consequently, the occurrence of polished, intact and broken alveolines is an indication that the fauna has undergone reworking without substantial transport. During his study on the Thebes Formation in the Red Sea region, Snavely et al. (1979) recorded cross-bedded alveolines lime sand. The coarse grain size and lack of interparticle fines suggests that this facies was deposited in a relatively high-energy environment. Packing of foraminiferal tests is probably a result of early solution. 6.4. Outer lagoon facies 6.4.1. Occurrence It crops out at the middle part of the sequence and shows a widespread distribution from west to east. Upsection, it is interbedded with back bank and fore bank facies. It also drapes over the outer shelf facies and covered by inner lagoon (Fig. 6). 6.4.2. Description It crops out as regular alternations of limestone with yellowish grey to greenish calcareous shales. It displays a cyclic manner forming packages between 20 and 25 m thick (Plate B7). The hard limestone beds are ledge forming, while the soft shales are slope forming. Limestone beds are nodular, slightly dolomitized, moderately to strongly bioturbated. The shale bands are grayish green to olive green and are much thicker compared with alternated limestone beds. They are being more calcareous upwards. Fossil content is represented by fragments of operculines (3–5%), pelecypodal bioclasts (3–10%) long-spired gastropods, ostracods (2–4%), smaller biserial benthonic and planktonic forams (1–5%), miliolids (1–4%), echinodermal fragments (2–5%) and unknown bioclasts. The represented fauna are entombed in a peloidal micrite (Plate B8). 6.4.3. Interpretation The dominance of burrowing echinoids and bivalves, the depositional textures, and the lack of features indicative of emergent conditions, support the interpretation of this association as deposited in a muddy substrate in an open lagoon behind a barrier (nummulites bank/shoal). Deposition has taken place in a quiet, lowenergy regime below fair-weather wave base (upper deep subtidal). This low-energy regime contrasts with the back-bank facies where the wave energy is predominate. It differs from the inner lagoon in that it contains high fossil diversity which reflects normal salinity. It was dominated by matrix-supported sediments with small amounts of nummulites, miliolids and some diverse megafossils. It consists mainly of lime mudstones microfacies, but limemudstone/wackestone is also abundant in certain horizons. It is deposited under quiet to intermittently water conditions (EI = I–II). 6.5. Nummulites back-bank facies

6.3.3. Interpretation This facies is related to the winnowed platform edge sand of Wilson (1975) and the winnowed shelf sand of Tyrrell (1969). The environment of such sands is characterized by high energy (Flügel, 1982) as well as well-oxygenated and normal marine salinity of good circulation. Much of such clean sand is winnowed and deposited by waves or by tidal or long-shore currents. Peloids are derived from periodic reworking of sediments in extremely shallow water environments with restricted sedimentation. Macrobiotic constituents possess high degree of fragmentation and abrasion consistent with relatively high-energy deposition. Drusy cement represents cavity filling with increase in crystal size centripetally (Bathurst, 1976). The low diversity of the fauna is attributed to the effect of stressful environmental conditions such as turbulent water and mobile substrate (Wilson, 1975). Alveolines

6.5.1. Occurrence It is well exposed at the middle part of the studied sequence. There, it interbedded with lagoonal facies. It decreases in thickness due to west at El Guss Abu Said site (Fig. 6). 6.5.2. Description The rock encloses slightly dolomitized, chalky, medium bedded, marly limestones (Plate C1). This limestone is interrupted by 0.5– 1 m thick bands of yellowish marl and brownish calcareous shales. The sediments are characterized by relatively high degree of clasticity and the absence of packstone and grainstone textures (Plate C2). Terrigenous influx of clays and fine sands is notable. Mudstones and mudstones/wackestones dominate the sequence. But wackestones are common at certain levels. Dense concentration

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of packed nummulites as characterizes the nummulites bank are practically lacking here. Bands with higher nummulites content are dominated by strong and inflated A-forms (5–10%), while the larger B-forms are generally rare or absent. Apart from nummulites, smaller benthonic foraminifera (bi-and uniserial benthonic forams, 3–5%), operculines (10–15%), bryozoa (4–6%), miliolids (1–4%) and ostracods (2–4%) are the most abundant and characteristic fossils. The macrofauna is dominated by burrowing echinoids (i.e. Echinolampas sp.). Other frequent faunal elements include high-spired cerrithid gastropods, large and flat oysters (Carolia sp.), some burrowing bivalves, rare echinoids and red algae. Vertical and slightly oblique burrows with diameters up to 1.5 cm, dominate over the horizontal ones. In spite of different intensities of bioturbation, laminated sediments are occasionally interbedded the sequence at certain levels. Peloids of different shapes and nearly same size are scattered in the micrite. Most components show micritization and/or micrite coating. The most conspicuous microfacies types include nummulites bioclastic mudstone/wackestone, operculines bioclastic wackestone, bioclastic mudstone, nummulites operculines wackestone and dolomitized bioclastic mudstones microfacies associations. The sediments are accumulated in quiet water conditions (EI = I). 6.5.3. Interpretation This facies represents a lower energy environment. Thus, relatively quiet background conditions prevail in the sedimentary record of the back bank environment. Planktonic foraminifera are generally rare. Low plankton productivity suggests minimal upwelling, isolation and/or restriction from oceanic circulation. The dominance of nummulites which were capable of attaining much of their energy and nutrient requirements via their algal symbionts suggests that the environment of deposition was nutrient poor (oligotrophic). The fauna indicates generally muddy substrate conditions (dominance of burrowing bivalves and echinoids) in a back-bank environment behind the barrier formed by the nummulites bank. Low diversity biota is a common, important clue to restricted settings, but relatively great numbers of those few organisms adapted to survival. Reduced size of individuals (dwarf fauna) and aberrant growth forms are also clues to adverse conditions. The presence of well-preserved discrete burrows may indicate a lower population of burrowers, or slower rates of burrowing relative to sedimentation (Roads, 1967). Thin shale units represent periodic incursions of clay. Similar lithofacies have been described by many authors (Aigner, 1983; Serra-Kiel and Reguant, 1984). 6.6. Nummulites bank facies 6.6.1. Occurrence It crops mainly out in the eastern scarp (Fig. 6). The nummulites bank developed into two successive stages. The first stage (oldest one) consists of two separated sheets (0.9 m thick) and is interbedded with outer shelf deposits. The second one (the youngest) is interbedded with outer lagoon facies (3 m thick). It is believed that the main body of the nummulites bank may be located due to north outside the studied area. 6.6.2. Description It posses a sheet like or very low amplitude bank-like geometry. Generally, it is medium to thick bedded, nummulites packstones and limemudstone/wackestone averaging between 0.9 and 3 m thick. Nummulites occur in rock-forming quantities (8–40%) in massive, poorly bedded bioclastic limestones (Plate C3). The fauna consists almost exclusively of nummulite tests. The smaller Aforms dominate over the larger B-forms with bimodal grain size distribution (Plate C4). Layers with only A- or B-forms are rare,

but the ratio of both may vary considerably. Faunal elements other than nummulites are rather sparse and include echinodermal fragments (5–10%), operculines (5–20%) and rare planktonic forams. A variety of physically controlled structures can be observed such as erosional and scoured contacts between beds; erosive pockets and pot holes filled with packstones and floatstones; densely packed and edge wise imbricated nummulites accumulation. 6.6.3. Interpretation According to Blondeau (1972), nummulites development is optimal in well-aerated, warm and shallow water. Such conditions are likely to have predominated on top of a submarine swell (represented by isolated local horsts formed as a result of block faulting). Two types of processes: biological (reproduction strategies) and physical (principally winnowing by storms) are thought to be responsible for the formation and fabric of this facies. Postdepositional bioturbation and compaction may significantly modify the original fabric. The nummulites are reworked by physical processes without large-scale lateral transport by in situ winnowing, so, the bank is parautochthonous coquinas. The nummulite tests and fragments show rare evidences of boring, suggesting they were originally deposited in a continuously agitated environment. This bank has a little associated micro- or macrofauna, suggesting that deposition took place in a nutrient-poor (oligotrophic) environment and/or in an environment with significant hydrodynamic sorting. Nummulite accumulations cannot therefore be considered true reefs since they are not characterized by an organic framework (Braithwaite, 1973); nor can they be described as shoals, which are formed purely by physical processes (Fournie, 1975). The absence of encrusting organisms tolerant of oligotrophic conditions, such as corals and calcareous algae, suggesting either that current energy were sufficiently high to have inhibited attachment and growth, or that sediment deposition rates were great to permit colonization. The nummulite shells were deposited in intermittently to slightly agitated (EI = II–III) water conditions. Nummulites banks and depositional models, especially in Tunisia, Libya, Egypt, Italy and Oman, have been the subject of numerous studies over the last decades (Aigner, 1985a,b; Racey, 1995; Beavington-Penny et al., 2006). 6.7. Nummulites fore bank facies 6.7.1. Occurrence It is recorded at the lower part of the Esna Formation, mainly at the eastern scarp (4–5 m thick). It is interstratified with open marine shales of the Esna Formation. Local occurrences in the form of sheet-like geometry (1–1.5 m thick) are sandwiched between the outer lagoonal facies at El Guss Abu said locality (Fig. 6). 6.7.2. Description Prevailing rock types are argillaceous, moderately hard, dolomitized, yellowish grey to reddish creamy limestone (Plate C5). The rock is locally massive, often slightly laminated and bioturbation is common. This facies forms thick, easily eroded beds, which are easily recognized and forms a good visual marker for outcrop correlation. Faunal elements (Plate C6) are represented by operculines (48–65%), discocyclines (15–30%), medium-sized flat nummulites (10–20%), echinodermal fragments (4–7%), skeletal hash (mostly of nummulitic origin) and small benthic foraminifera such as Lenticulines and Textulariides. The matrix is slightly dolomitized and composed of silt sized nummulites debris. The top of this facies is stained with iron oxides. The dolomitization affected both the matrix and faunal elements. Petrographically, the identified microfacies types include nummulites discocyclines operculines wacke-to packstone, discocyclines nummulites operculines packstone and operculines discocyclines wacke-to packstone microfa-

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cies. The estimated water energy is intermittently agitated water conditions (EI = II). 6.7.3. Interpretation The faunal elements, lithology and stratigraphic position indicate deposition in a fore nummulites bank site. The occurrence of thin discocyclines, operculines and small benthonic foraminifera indicate proximity to open marine conditions. Operculines facies characterizes a deeper water depositional environment than nummulite facies. However, in modern environments, the operculines can occur with thin nummulites in deep water environments (Hohenegger et al., 2000). Laminated fabric and planktonic foraminifera suggest the deep water deposition of material reworked from the shallow water nummulites platform. Discocyclines are recorded from Eocene sediments as fore bank deposits in Libya (Jedir Formation) as broken shells associated with assilines and nummulites fragments by Anketell and Mriheel (2000). The Paleogene section in northeast India contains discocyclines as stout and large fore-reef forms, in middle to outer bank; also fore-and back-bank areas near the reef-core and fore-reef (Ghose, 1977). Consequently, it is inferred that the nummulites were transported into this deepwater, mid-ramp setting either by turbidity or storm currents. Evidence is provided by the fact that nummulites are frequently broken and abraded and show some sorting by size. 6.8. Open marine outer shelf facies 6.8.1. Occurrence It is recorded at the lower part of the studied section. It is interstratified with nummulites bank and tidal flat facies. Its thickness increases due to west and changes into outer shelf lagoon due to east (Fig. 6). 6.8.2. Description Generally, it is represented by a sequence of grey to greenish shales, pale greyish to yellowish grey shales and mudstones enriched with planktic forams (Plate C8) with scattered calcareous benthic fauna, smooth-ornamented ostracods and fragments of bivalves (Pectinids). In places, it is less pure and includes some bands of calcareous shale and marls. Lamination is dominant. Sparse fragmented operculines and bioclasts are accumulated in pockets on fissility surfaces (Plate C7). On outcrop, the soft shales are slope forming and alternate with harder carbonate bands in a cyclic manner. 6.8.3. Interpretation The fine-grained character suggests deposition by low energy currents as suspension in deeper marine suite. The laminated nature implies a very low energy depositional environment. The presence of such laminations is indirectly controlled by burrowing organisms. The scarcity of benthic fossils and trace fossils suggest that this facies was formed as a hemipelagic deposit in deep calm water, in an open marine environment, normally below the range of current activity. The high foraminiferal content, relatively high planktonic/benthonic ratio, presence of abundant planktonics with high species diversity, high diversity of common benthonics indicates that the deposition site was an outer shelf. Buxton and Pedley (1989) mentioned that outer ramp facies is dominated with larger forams, pectinid pelecypods, bryozoa, echinoids, and sparse planktonic forams. The few operculines pockets and unknown bioclasts are thought to form in the outer shelf by the reworking of shallower sediments during storms or periods of lowstand of sea level. The deep water conditions of the lower part of the Esna Formation are evidenced by the abundance of planktonic forams and absence of strictly shallow water fossils. Moreover, the paucity of waves that form sedimentary structures, suggests deposition below wave

41

base. Generally, it is believed that this facies was accumulated in quiet water conditions (EI = I).

7. The depositional model and facies development The geological history of the Egyptian Paleogene is dominated by tectonic events, which began as late Cretaceous tectonism and continued steadily or episodically in the Paleogene (Said, 1990). Boukhary and Abdel-Malik (1983) believed that the alternation of shallow and comparatively deeper facies exhibited by the Eocene deposits in Egypt is possibly related to the relief of the depositional basin in which they accumulated. Abu Khadra et al. (1987) recorded syn-tectonic lower and middle Eocene limestone facies in the Southern Galala and suggested that the central part of Gulf of Suez was tectonically active site during the Eocene period. In the northeast of Assiut (Nile Valley), Keheila (2000) believed in the presence of northeast elongated submarine paleohigh, separated by a synform trough during late Ypresian. By analogy, and also from the paleogeographical relationships between the different facies types under study, it is assumed that the Syrian Arc System tectonism had its impact on the sedimentary facies and paleogeography of the lower Eocene sedimentary basin in Farafra Oasis. It became clear that tectonism has had a significant control over sedimentation, although tectonism is only superimposed locally onto more regional sea-level changes. The study area was developed on a passive margin basin during the intermediate closing phase of the Neotethys. The studied sections show a gradual passage from shallow marginal marine facies to relatively deeper marine, without evidence for a talus slope or significant slope break, suggesting deposition on a gently inclined ramp (Fig. 7). The lateral and vertical evolution of facies of the lower Eocene reflects repeated transgressions and regressions due to the combination of tectonic movements and sea level changes. Contacts between each facies are gradational, suggesting migration of the depositional environments. Sedimentological and paleontological studies have shown that a standard Bahamian-type platform model cannot be fully applied to this ancient siliciclastic-carbonate succession. The gentle dip of the sequence suggests that the sediments were deposited on a homoclinal ramp. The scarcity of reef building organisms in subtidal areas despite the favorable conditions is likely characteristic of ramps. Louks et al. (1998) put forward a ramp model which is favored in this paper. A ramp interpretation predicts that facies will shift laterally as a result of only a modest rise in sea level, and will generally interfinger with each other. This contrasts with a shelf/ platform model, which predicts that a modest sea level rise would cause the various facies to aggrade with less interfingering. This ramp (Fig. 8) exhibits two regional paleoenvironmental realms: (1) tectonically stable hemi-pelagic outer ramp, which is characterized by hemipelagic, planktonic rich, fine-grained siliciclastic shales. It reflects a gradational conformable relationship between Farafra and Esna formations. The absence of dasycladacean algae suggests a deeper ramp setting below fair-weather wave base. True storm structures are absent; probably the area lay on the outer limits of wave action and in addition the area has been extensively bioturbated. The fine-grained terrigenous lithologies may represent a deposit of a low energy, deep subtidal, distal ramp setting well below fair weather wave base. This is based on the following characteristics: the dominance of fine-grained siliciclastics and micrite; the excellent lateral continuity of bedding; macro-and microfauna; the absence of shallow-water sedimentary structures and in situ benthonic fauna. The presence of burrows, attest to normal oxygenic conditions prevailing at times in the surface sediment layer, but conditions did not allow benthic colonization. The water energy levels is interpreted as having been very low to low, result-

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Plate C. 1. Field view showing moderately bioturbated, marly limestone of the back bank facies. This facies is covered by hard thick bedded lagoonal facies. Eastern scarp locality. 2. Photomicrograph showing nummulites bioclastic wackestone of back bank origin. Note the high degree of clasticity. Eastern scarp locality. PPL. 3. Rock sample showing nummulites bank facies. Note that the fauna consists of nummulite tests. The smaller A-forms dominate over the larger B-forms with bimodal grain size distribution. Eastern scarp. 4. Photomicrograph of bioclastic nummulites wacke-to packstone microfacies. Bioclasts, nummulites and operculines are embedded in dense micrite. Eastern scarp. PPL. 5. Field view showing dense accumulations of larger foraminifera of fore bank facies. Shells of larger foraminifera are well exposed on the outcrop due to weathering. Eastern scarp. Looking west. 6. Photomicrograph showing discocyclines operculines nummulites packstone microfacies of fore bank facies. The matrix is slightly dolomitized and the allochems were fragmented. Eastern scarp. PPL. 7. Rock sample of Esna shales showing patchy accumulation of the fragmented larger foraminifera on the fissility planes. Most of shells are operculines. El Guss Abu Said section. 8. Photomicrograph showing planktonic wackestone of open marine outer shelf within Esna Formation. Note the enrichment of the matrix with argillaceous matter. El Guss Abu Said locality. PPL.

ing in sedimentation due to the settling of carbonate mud and argillaceous shales, muds and fine silt from suspension, with the exception of occasional high-energy storm events which introduced sediments from shallower water. (2) Tectonically active (mobile) inner and middle ramp, which is classified into tidal flats, inner lagoon, shoals, outer lagoon, back bank, nummulites bank and fore bank facies zones. The characteristic ortho- and allochemical components as well as the carbonate depositional textures of the identified facies are represented in Figs. 3, 4 and 8.

The pattern of sedimentation as well as facies changes may be chiefly controlled by paleotopography (intra-shelf anticlinal and synclinal sub-basins) that were inherited from the late Paleocene/early Eocene tectonism, which resulted from the rejuvenation of the Syrian Arc Fold System. The wide variation in thickness of the different facies indicates that deposition took place on a highly undulating sea bottom of the shelf setting and also indicates continuous sedimentation in a subsiding basin. The paleostructural relief, in addition to repeated sea level changes and clastic supply,

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have controlled the sedimentation pattern and, hence, types and thickness of the deposits accumulated in the studied basin at various stages of its development. At the end of late Cretaceous and beginning of early Paleogene, a possibly pre-existing structural paleohighs (swell-like topography), paved the way for initiation of nummulites bank and shoal areas (Aigner, 1985a,b). Afterwards, and during the early Eocene, the nummulites bank and shoal bars appear to have developed on these local paleohighs when paleoenvironmental conditions are suitable. The accumulation of the nummulite coquina is controlled principally by two integrated factors. The first factor is the suitable ecological conditions and the second one represents the physical concentration of the coarse grained skeletal elements and winnowing of muddy and silty matrix, probably during storms (?). Interaction of these processes resulted in the progressive growth and building up of a bioclastic nummulites body as well as in the differentiation into fore bank, nummulites bank and back bank settings. The nummulites bank probably started to develop below the reach of average storm waves, but subsequently grew into shallower water. In the front of the nummulites bank and exactly on the windward side (eastward), a fore bank facies took place. As the barrier-like effect of the nummulites bank became more effective during later stages, a distinct back bank and restricted lagoonal environment (outer and inner) developed on the landward side. A low energy back bank lithofacies consists principally of mudstones and mudstones/wackestones interbedded with lagoonal facies. Close to the land direction, tidal flats and shoal facies were formed. A shoal (bioclastic bars) may probably accumulated onto a submerged paleohigh. This facies was recorded at the top of the studied sequence. It is believed that the presence of this facies increased the degree of isolation and restriction inside the lagoon and back bank environments, which led to isolation of a part of the lagoon forming a more restricted inner lagoon environment (Flügel, 2010). Accordingly, fine detrital siliciclastic sediments (shales) shed from landmass, or open marine fauna are nearly absent. The geometrical relationships of the previously described facies provide the initial basis for an overall depositional model. This model shares much similarities with other models constructed by earlier authors (Beavington-Penny et al., 2006), but it also has lithological, sedimentological and faunal characteristics of its own which are derived from the development of the basin. The nummulites bank in the study area appears to parallel the shelf margin similar to that recognized by Aigner (1985a,b) in the Eocene Mokattam Formation of Egypt. In terms of fossil content, the depicted model (Fig. 7) can be divided into a succession of depth-controlled communities, which are, from shallowest to deepest: (1) porcellaneous foraminifera and green calcareous algal community. This community flourished and dominated throughout the inner shelf settings. Its fossil content played an important role to build the restricted shelf lagoon, shoals and outer shelf lagoon facies. Fossil assemblages include benthic foraminifera such as textularids, miliolids, opertorbitolites and alveolines. Associated with them is a rich assemblage of calcareous green algae. (2) Larger perforate foraminifera and smaller planktonic and benthonic foraminiferal community. This community dominated the back bank, the nummulites bank, fore bank and outer shelf facies occupying the middle to outer shelf settings.

8. Sequential analysis Several geologists have discussed the sedimentary cycles and their mechanism of formation (Einsele et al., 1991). According to the concept of weathering boundary of Einsele (1982), there is a certain carbonate content (around 65–85%) above which the car-

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bonate rock is resistant and appears as limestone and below it the rock is more easily disintegrated and appears as marl. The carbonate oscillations above and below the weathering boundary are difficult to detect in the field exposures, because they do not show any distinct lithological change and weathers either into beds of hard limestone or as beds of less resistant marl (Einsele et al., 1991). Accordingly, moving up section, the sedimentary sequence is looking as hard protruding limestone ledges alternating with soft retreating calcareous shale or marl bands (Plate A3). This is often the case in some repeatedly hard limestone ledges at the upper part of the studied sequence. Here, the rhythmic sequence can be recognized in the field only if their alternating beds have a carbonate content coincided with subtle sediment color changes, whereas high carbonate content marks white to creamy beds, while lower carbonate content refers to darker, brown, buff and yellowish brown interbeds. This studied rhythmic bedding is strongly related to the ratio of carbonate and clay in the marine environments. It occurs in regions with carbonate deposition three to four times higher than terrigenous silt/clay input and has different thickness (Plate B5). They yield a great number of small-scale, shallowing upward rhythms. All rhythms document an upward change from lower/middle shallow subtidal, rarely upper deep subtidal to upper shallow subtidal and rarely lower intertidal zonal depths. The upward-shoaling is the most common type of bedding feature marking those rhythms. 8.1. Types of shallowing-upward rhythms (cycles) Detailed stratigraphic and petrographic studies have revealed that the studied sequence can be divided into several meter-scale cycles (Fig. 9). Two different types of cycles are here defined: shale-based and marly limestone-based cycles. So, two alternating bed types (succession AB, AB, etc.) which can be called rhythmic or cyclic bedding or rhythmic sequences could be distinguished. In an alternation of different beds, weathering resistant beds (e.g. limestones) are described as beds or layers, while less resistant beds (e.g. shales and marls) may be called interbeds or interlayers. A layer and a subsequent interlayer form a bedding couplet (Fischer and Schwarzer, 1984), which measures up to 20 m thick at El Guss Abu Said site and 2–9 m at the eastern scarp. Bedsets (Campbell, 1967) or bundles (Schwarzacher, 1975) represent several bedding couplets. Shallowing upward rhythms (cycles) are bounded by flooding surfaces (characterized by increase in bed thickness, textural change, greater fossil diversity, high percentage of fine quartz and evidence for reworking such as lithoclasts) are dominant. They show shallowing up of facies evolution, which becoming more restricted toward the top. The first type (shale-based cycles) crops out at the lower and middle parts of the studied sequence. Each cycle starts with calcareous shales at the base and capped with hard limestone (Figs. 9 and 10). The limestone cap is generally nummulitic (nummulites bank or back bank). Up-section, this type of cycles shows a remarkable change in thickness. The thickness of the basal shales decreases upward (from 18 m to 0.5 m), but the limestone cap increases in the same direction. In addition, there is a cyclic variation in color (grey to green shales to white and creamy limestone). The two alternating bed types are laterally extent over long distances. Bed contacts are either gradational or wavy undulated surfaces. Rare bioturbation is resulted in a well preserved bedding nature. The marked variation in cycle thickness in different parts of the basin reflects different rates of subsidence. The second type (marly limestone-based cycles) occupies the upper part of the Farafra Formation at El Guss Abu Said locality (Plate A3). It is believed that this kind of rhythms is originated from the previous one by the increase in the carbonate percent in the ba-

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Fig. 10. Sketch diagram illustrates the different types of shallowing upward cycles and their component beds. Note the difference in the nature of rock types and fossil content (legend in Fig. 2).

sal part of each cycle at the expense of clay. Here, the lithological rhythmicity is closely associated with fluctuations in faunal content due to varying of carbonate content. The marly limestone contains more diversified populations due to high nutrient supply. In addition, the faunal assemblage in the upper limestone part of the rhythms is generally of smaller grain size than in their lower parts, creating a normal fining-upward grading. Distinguished changes in the nature and diversity of the fauna as well as the thickness of the cycles differentiate this type from the previous one. The bounding surfaces of the cycles are defined by a facies change between the lower and the overlying cycle and reflect a break in marine sedimentation due to the emersion of the uppermost strata of the carbonate platform within extended areas. In a cyclostratigraphic view, the surfaces represent transgressive surfaces or marine flooding surfaces as they document the onset of marine sedimentation with the renewed flooding of the platform surface and subsequent increasing water depths due to a relative rise in sea level. These two types of rhythms (cycles) correspond to the parasequences of the Vail-Haq nomenclature which refer to repetitive transgressive/regressive events. They have been developed in shallow-water depths, in area receiving low, but oscillating terrigenous sediment influx from the continent; thus they are slightly affected by terrigenous dilution. The rhythmic limestone/marly limestone alternations appear to be diluted in relation to the climatic conditions in land, and to small-scale sea-level variations, both influence the carbonate/clastic ratio of the sediments. As a result, the minor periodic change in terrigenous dilution due to periodic orbital-climatic variations is believed to be the main process controlling the fluctuating carbonate contents in the lower Eocene rhythmic sequences. Most of the small-scale, shallowing-upward cycles are formed during sea-level oscillations of less than 10 m (Grotzinger, 1986). These rhythms are primarily controlled by cyclic variation in Earth orbital parameters (Milankovitch climatic cycles). Milankovitch cycles affect the global climate, small-scale sea level variations and

carbonate production. These orbital cycles have dominant periods of about 20, 41 and 100 ka. In shelf seas, the orbital-climatic variations are reflected by changes in temperature, current velocity and direction, distribution of water masses, recycling and supply of nutrients, carbonate production, amounts of terrigenous sediment and dissolved chemicals enter the sea and input rate of fresh water (Einsele et al., 1991). This denotes that the orbital rhythms in rhythmic sequences record a series of short lived cycles of sea level fluctuations (short period Milankovitch type oscillations). The small scale cyclic changes in relative sea level caused the rhythmic vertical stacking patterns in rhythmic sequences and the stacked depositional cycles or parasequences in sequence stratigraphy. As a matter of fact, if the orbital signals of the Milankovitch cycles are transformed into climatic variations causing minor oscillations of sea level, then the third order variations in sea level are superimposed by a great number of high frequency but low amplitude eustatic oscillations (4th and 5th order cycles). In west central Sinai, upper Lutetian/lower Bartonian rhythmically bedded sequence exposed at Wadi Mheiherrat also exhibits a high frequency allocyclic organization in its stratal pattern which has been interpreted by Abdel Fattah (2000) to record orbitally forced climatic cyclicity. The presence of orbitally forced cyclicity though is mostly probable, cannot be clearly documented by the available data. Analogous small-scale shallowing upward cycles (James, 1984) were reported from many shallow water platform carbonate settings throughout the geological record. The absence of transgressive sediments and the presence of only shallowing up facies evolution create strongly asymmetrical cycles (Wright, 1984). Asymmetric small scale, shallowing upward peritidal successions/cycles are common on many ancient carbonate. 8.2. Depositional sequences and megasequences In the study area, considering the vertical evolution of the recognized facies, the stratigraphic sections show a hierarchical organization of 16–17 rhythmic bedding (cycles) defined by five

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Fig. 11. Diagram exhibiting an allocyclic model. The stratigraphic record shows two types of shallowing upward cycles. Points on water surface, sea floor, and stratigraphic record mark equal time increments.

depositional sequences (Fig. 11). The sequences are grouped in one shallowing upward megasequence. The megasequences range in thickness from 82 m (Eastern scarp) to 87 m (El Guss Abu Said). Vertically, the lithofacies indicates a progressive variation from open marine to restricted inner ramp setting. The following variations in the vertical facies sequence occur along each megasequence: (1) An overall decrease in shale content. (2) A general upward increase on the carbonate content and a corresponding increase in the thickness of limestone beds. (3) An increase in the degree of dolomitization. 8.3. Possible causes of cycles Causes of cyclic sedimentation on platforms have been amply discussed (Grotzinger, 1986). Upward shallowing cycles in platform sequences can be produced by autocyclic or allocyclic mechanisms (Aigner, 1985a,b; Strasser, 1991). Autocyclic processes operating within the sedimentary basin involve progradation of tidal flat or lateral migration of tidal channels. Allocyclic control mechanisms are independent of the depositional processes and include eustatic sea level or repeated synsedimentary tectonic downfaulting events, e.g. yo-yo tectonics model by Cisne (1986). They are linked to basin-wide or global processes, such as orbitally induced sea level oscillations lead to Milankovitch-type cycle formation. Small scaled glacio-eustatic sea level change is considered to be the most important mechanism for generating platform cyclic sequences. Aigner (1985a,b) maintains that glacio-eustatic sea level change is unlikely in the Triassic, while Goldhammer et al. (1987) sustain that glacio-eustasy can occur in geological periods such as the Middle Triassic, generally considered as non-glacial. The studied sequence has been interpreted as a carbonate ramp sequences that formed in a foreland basin. Subsidence rates may well have been high enough to overcome the lag depth (Schlager, 1981) in a short period of time without the formation of erosional surfaces, extensive hardgrounds or transgressive lag deposits; this would have been followed by the progradation of tidal flats. However, several cycles do not contain peritidal facies at the top, which is a prerequisite for the Ginsburg (1971) model, although the presence of such cycles is not inconsistent with the vertical accretion model of Pratt and James (1986). Hardie (1986), in his discussion of the early Proterozoic Rocknest cycles emphasized the point that offshore environments preserved in the lower part of shale based cycles and represented by siliciclastic muds could not have been the carbonate factory for the establishment and progradation of carbonate tidal flats as suggested by autocyclic models. The same holds true for the Farafra cycles; the offshore deposits (shale) lack the organically and inorganically produced carbonate muds typical of Phanerozoic marine-

lagoonal environments. The carbonate intercalated with shale was derived from nearshore areas during storm instead. The carbonate factory of the Farafra was represented by the shallow subtidal and tidal flats. In the latter environment microbes probably aided in the precipitation of carbonate mud and aragonite crystals formed from supersaturated waters on supratidal flats. As a result, the classic autocyclic models cannot be applied to the origin of the Farafra cyclicity. In many studies, the cyclicity recorded in carbonate platform successions has been attributed to small scale (less than 10 m), high frequency eustatic sea level changes (Osleger and Read, 1991). The forcing mechanism behind sea level fluctuations is attributed to cyclic climatic changes in the Milankovitch frequency band. The Farafra sedimentary cycles show a systematic, hierarchical arranged stacking pattern, which is strikingly similar to cycle hierarchies in younger sedimentary sequences. These cycle hierarchies can best be explained by combined effects of several orders of eustasy driven relative sea level oscillations. All the above considerations suggest that composite eustatic sea level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of the observed hierarchical cyclic organization of the studied sequence. Taking into account this working hypothesis, the small scale cycles (rhythms) would represent the precession cycle of the equinoxes (c. 20 ka), the depositional sequences and megasequences the short (c. 100 ka) and the long (c. 400 ka) eccentricity signal, respectively. However, it is not excluded that some of the small scale cycles (rhythms) may have been occasionally influenced by intrinsic processes, such as shifting of shoal bars in response to wave reworking. Hierarchical stacking of 2–8 cycles into a depositional sequence and of 1–5 depositional sequence into a megasequence suggest that superimposed high frequency sea level fluctuations dictated by the Earth’s orbital perturbation in the Milankovitch frequency band could have been the main engine driving the formation of the Farafra section. An allocyclic model illustrating the mechanism of cycle’s formation in the studied sequence is proposed in Fig. 11. The cyclic depositional nature of the sediments developed on Farafra Oasis points to a slight change in the bathymetry of the sea due to minor oscillation in eustatic sea level. A slow rise in relative sea level causes the initiation of bank-margin and back-bank lagoonal facies on the shallow marine carbonate bank. Normally, carbonate production is highest when the bank tops are flooded. During the relative sea level rise more sediment are commonly produced than can be accumulated during low eustatic sea level. However, the sediment in the study area is slightly influenced by the low detrital input coming during the sea level rise and thus producing a marly limestone on the flooded carbonate bank. The low detrital input influenced the amount of carbonate production,

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and thus slightly decreased the carbonate productivity. With a slight drop in relative sea level the depositional surface raises to upper shallow subtidal, rarely lower intertidal depth, bringing a change into limestone with higher carbonate content. Back bank shoal (carbonate sand bars), often mark the upper shallow subtidal oscillations. Renewed slow subsidence or a rise in relative sea level caused flooding and new deposition of a marly limestone. Accordingly, the development of limestone and marly limestone is widely dependent on the position of the relative sea level. In fact, the carbonate banks are sensitive recorders of changes in relative sea level, induced by eustatic fluctuations or vertical tectonics (Strasser, 1991). Adding to the lithological change, such cyclic environmental changes were accompanied by minor shifts in the faunal content, their diversity, abundance and population density (species dominance). The transgressive periods imply the flourishing of the faunal community due to major import of the detrital food (nutrient supply). This allows the establishment of sessile epifaunal organisms (e.g. oysters), who achieve stability through their body size. 8.4. Stacking pattern One of the most peculiar features in the lower Eocene sequence is the rhythmic stratification. This well developed rhythmicity is relied upon the bed types, ordering of beds and cyclic color changes. The rhythmic bedding variations are caused by repeated slow gradual variations in the primary composition of the sediments associated with changes in sedimentation rates due to orbital climatic variations, global sea level variations (allocyclic sequence of Einsele et al., 1991). Such allocyclic processes tend to generate cyclic phenomena of a large lateral continuity with a smoothly changing, vertical sediment buildup with time. The repeatedly alternating beds are grouped giving rise to a repetition of larger units, thus designates the rhythmic sedimentary sequences or the depositional sequences in the sense of Vail et al. (1977) and Haq et al. (1988). In the study area, the fossiliferous ramp strata are characterized by a pronounced cyclicity formed by stacked parasequences, which consist of small-scale rhythms reflecting a general shallowing of the depositional area. Up to 17 of these shallowing upward cycles, bounded by distinct discontinuity (marine flooding) surfaces due to the recurrent submersion and subsequent flooding of the platform surface, have been recognized within the ramp strata (Fig. 12). The cyclic platform strata show a general variation in thickness upward. Cycles from the lower part of the succession often exhibit thick, claystone interbeds and greyish thin to mediumbedded limestones. The sharp boundaries at the top of the single cycles are generally characterized by conspicuous color and facies changes of the sedimentary strata, often accompanied by the occurrence of thin, distinct red mudstone or color staining. Shales at the base of the cycles are generally explained by the deposition of very fine, suspended sediment particles under quiet water conditions. The occurrence of mudstones at the base of individual cycles took place subsequent to the renewed flooding of the platform surface, when the carbonate factory was about to start up again. Hence, the facies probably formed under sediment starved conditions when the production of biotically controlled sediment material was still low. In this position, the facies equates to the micritic facies of Morin et al. (1994) reflecting quiet water, deeper mid shelf environment below the mean fair weather wave base. At the top of the sequences, mudstones formed within protected, very shallow marine to intertidal, lagoonal or mudflat areas. The nearly complete absence of skeletal fragments may imply restricted marine, probably hypersaline conditions. Dolomites and dolomitized limestone beds commonly occur in the upper part and at the top of the individual cycles. The sedimentary successions of the cycles are arranged into facies sets of units, which comprise specific microfa-

cies associations and thus reflect different depositional areas (inner, middle and outer ramp) or sedimentary processes (flooding and submersion) of the ramp depositional system as follows: 8.4.1. Outer ramp facies sets It consists mainly of parasequences, which displaying thinning upward. The basal fine-grained claystones and marls are interbedding with nodular mudstones at the top of single sequences (Fig. 12). Sediments are formed by accumulation of fine grained, suspended particles under quiet water, probably sediment starved conditions, which must have prevailed subsequent to the renewed flooding of the platform surface. They reflect a time period before the start up of the carbonate factory and may also represent a deepening of the depositional area a certain lag depth (depth needed before water circulation becomes affective enough to allow sediment production), and sediment composition changed due to increasing amounts of bioclasts, which were produced with the reestablishment of the carbonate factory. The basal fine grained siliciclastic (shales) of open marine conditions decreased in thickness upward and the limestone caps adversely increase. As a whole, the parasequences display thinning upward as a result of relative sea level fall and/or limiting accommodation space may be a consequence of the general upward shallowing. The absence of shallow water sedimentary structures and in situ benthic fauna, together with the high proportion of terrigenous mud suggest deposition on the distal part of the ramp including much terrigenous material which settled from suspension. The burrows, both lateral and vertical, attest to normal oxygenic conditions prevailing at times in the surface sediment layer, but conditions did not allow benthic colonization. The shallowing upward trend of each deep subtidal cycle is interpreted from upward increases of grain size, carbonate content, bioturbation, thickness and frequency of associated limestone beds. 8.4.2. Mid ramp facies sets It is represented by thickening upward parasequences, where the basal fine siliciclastics decrease in thickness and frequency. On the other side, the fossiliferous limestone caps predominate in the same direction. This resulted in gradual absence of the basal shales and deposition of fossiliferous marl. The deposits probably formed below the mean fair weather wave base when sea level was highest and show a gradual transition into the inner-platform facies sets by a general increase of the bed thickness and/or the proportion and diversity of skeletal fragments upwards. The lower part of complete cycles mainly consists of continuous, darker grey to olive grey shales of shallow subtidal to lagoonal origin. The limestone caps are mainly horizontal, thick to medium bedded fossiliferous lime mud/wackestones and minor packstones. The allochthonous sediments comprise the mixed bioclastic, nummulites and alveolinid foraminifer microfacies types, which vertically and laterally gradually merge into each other. Sporadically occurring, thin horizons or lenses, characterized by the accumulations of skeletal fragments (bioclastic packstones) may represent storm deposits. 8.4.3. Inner ramp facies sets It consists of thick to medium bedded, light grey to brownish or multicolored limestone and dolomite beds. It is alternated vertically with soft thick, marly limestone and marl. As a whole, the parasequences are thin and keep a steady and uniform thickness on the scale of individual units. Allochthonous deposits are mainly formed of fossiliferous wackestones to packstones with minor grainstones and mudstones. A considerable part of the limestones is replaced by dolomites. These deposits probably formed during general decreasing water depth, when the increase of accommodation space was less than the rate of sediment supply of the carbonate factory.

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Fig. 12. Sketch diagram showing stacking pattern (A) and fashion of accumulating geometry (B) under a decreasing relative sea level rise during early Eocene time (Ypresian) in Farafra Oasis, Western Desert.

Some authors suggest that the formation of the stacked cycles within the marine strata is caused by synchronous, global eustatic sea level changes of Milankovitch periodicities (Ross and Ross, 1985), driven by changes in the ice volume. Such glacio-eustatic, high-frequent sea level fluctuations are supposed to have reached amplitudes of more than 100 m (Joachimski et al., 2006). Within the strata of the Farafra Oasis these glacio-eustatic sea level fluctuations are represented within the stacked successions, which are interpreted as shallowing upward cycles, as they reflect a general shallowing of the depositional area. Considering the entire platform strata, systematic changes of the overall stacking pattern become apparent. In field, these systematic changes are often geomorphologically emphasized, as the generally more muddy mid shelf deposits are generally less resistant against weathering compared to the grainy limestones and dolomites of the platform interior. As a result, escarpments comprising the platform strata often show a terraced, step like appearance formed by the stacked sequences within the lower parts of the platform strata. Further upwards, this gemorphological feature gradually disappears, as the less resistant mid shelf facies sets are missing and the platform

strata are more consistent (Plate A3). The systematic changes in cycle thickness and their internal composition could result from continuously decreasing amplitudes of the glacio-eustatic sea level changes during the platform development. 9. Clay mineralogy Mudrocks are the main carrier of clay minerals. In addition to the economic importance of mudrocks and clay minerals in industrial technology and ceramics, clay minerals are useful as provenance and paleoclimate indicators. Moreover, the clay minerals have been used to deduce the tectonic setting during the accumulation of ancient sedimentary succession. 9.1. Results and discussions X-ray diffraction analysis for the clay-fractions of the selected samples has revealed that they are characterized by the presence of, smectite, kaolinite and illite (Table 2 and Fig. 13) arranged in

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Table 2 Relative abundances of the identified clay minerals in the Esna Formation, west and east of the Farafra Oasis, Western Desert, Egypt. Locality

Sample No.

Smectite

Lllite

Kaolinite

Sum%

El Guss Abu Said

4 19 21 28

44.23 59.52 40.21 55.56

22.12 17.46 23.92 14.81

33.65 23.02 35.87 29.63

100.0 100.0 100.0 100.0

Eastern Scarp

45 37 33 29 28

62.22 33.34 37.77 59.39 39.84 48.01

11.11 25.25 11.12 16.36 15.63 19.99

26.67 41.41 51.11 24.25 44.53 34.4

100.0 100.0 100.0 100.0 100.0 100.0

Average

Fig. 13. X-ray diffraction pattern of a representative sample showing the effect of various treatments.

a decreasing order of abundance. Smectite is the more abundant clay mineral in the investigated samples. It ranges from 33.34% (sample No. 37, Eastern scarp) to 62.22% (sample No. 45, Eastern scarp) with an average value of 40.21% (Table 2). Kaolinite is the second abundant clay mineral ranging in percentages between 23.02% (sample No. 19, El Guss Abu Said) and 51.11% (sample No. 33, Eastern scarp) with an average value of 35.87%. Illite is the third abundant clay mineral, where it occurs with percentages ranging from 11.11% (sample No. 45, Eastern scarp) to 25.25% (sample No. 35, Eastern scarp) with an average value of 17.46% (Table 2). The origin of smectite was attributed by Gindy (1983) to the gradual increase in the amount of erosion of the newly elevated crystalline source rocks to the south of Egypt, particularly basic and ultrabasic rocks or their metamorphic equivalents, in areas of moderate rainfall and rapid weathering, which was followed by rapid transportation of the product debris into the sea. The dominance of smectite in the lower Tertiary sediments in SE Egypt was described by Hendriks et al. (1990). Its origin is believed to be continental pedogenesis as well as marine neoformations. Terrestrial smectite developed under warm and humid to seasonally humid conditions by a degradation of chlorite and illite or by crystallization from ion-enriched hydrologic solutions in badlydrained alkaline soils. Marine smectite crystallization indicates, on the other hand, environments with a high concentration of aluminum and silicon. The origin of kaolinite was attributed by Gindy (1983) to the extensive chemical weathering and leaching of rocks which occurred especially on the exposed granite-metamorphic basement areas to the south of Egypt. As cited by Hendriks et al. (1990), the kaolinite within the marine deposits of the early

Tertiary age are products of terrestrial weathering and represent continental recrystallization of a warm and at least seasonally humid climate, being eroded and transported toward the sea by rivers. The origin of illite in SE Egypt was attributed by Hendriks et al. (1990) to reworking processes of soils which presumably developed on basement rocks. The illite can be of metamorphic origin, its formation was promoted by the heat of hydrothermal fluids and exhalations associated with the volcanic and tectonic activity. The metamorphic origin for illlite is not accepted here and the detrital origin is more plausible. The following reasons account for the detrital origin. Detrital illites are generally associated with terrigenous minerals and reflect physical alteration of continental land masses (Millot, 1964). The illite mineral under study is associated with terrigenous sediments (shales of the Esna Formation), which probably confirms the detrital origin of the associated illite (Domonik and Strasser, 1987). In contradiction, the high Fe-content of illite does not imply a detrital origin. Detrital sediments contain aluminous illite or muscovite produced by erosion of older rocks. Fe-illite probably replaced smectites resulting from soil erosion in downstream land masses under hot and humid climate (Paquet, 1970). The dominance of aluminous illite and lack of Fe-illite under study is another indicator for its detrital origin. The necessary K+ concentration and conditions which favor transitions of smectite to illite are common in semi-closed basins being temporarily in contact with marine water and submitted to intense evaporation (Domonik and Strasser, 1987). This transition is accompanied by the production of chalcedony or opal (Millot et al., 1963). The dominance of normal marine conditions during deposition of Esna Formation and the lack of chalcedony and opal are further evidences confirm the detrital origin. The predominance of authigenic clays (smectites) relative to the detrital constituents (kaolinite and illite), suggests the general prevalence of chemical precipitation during the deposition of the studied sequence. The smectite–kaolinite association does not suggest the formation of smectite is compatible with alkaline reducing conditions, whereas the formation of kaolinite is generally thought to require an acid condition, and it may be unstable in alkaline environments (Grim, 1968). Changes in source rocks or climatic influence related to repeated transgressive and regressive events due to the interaction of tectonic movements and sea level fluctuations during the early Eocene may account for the observed differences in clay mineral abundances. This agrees with the deduction of Millot (1970). In North Africa, clay mineral assemblages of upper Cretaceous-lower Tertiary were described by him to be formed in a number of shallow marine basins.

10. Conclusions In the Western Desert, at Farafra Oasis, the lower Eocene succession is well exposed bordering the Farafra Oasis. The stratigraphic section comprises Esna Formation at base and Farafra Formation at top, with a remarkable regional gradational contact in-between. Detailed field and microfacies analyses revealed the predominance of eight sedimentary facies, on the basis of grain size, physical sedimentary and biogenic structures, forming a gently dipping ramp depositional system. This ramp developed on a passive margin basin during the intermediate closing phase of the Neotethys Sea. It could be classified into two major structural and sedimentological provinces: (1) a distal outer ramp, which is characterized by hemipelagic planktonic rich, fine grained siliciclastic shale facies, (2) and an unstable mid- and inner ramp. On this part of the ramp, the thickness and biofacies of the lower Eocene changed widely in areas geographically near to one another. The arrangements of these facies also take nearly the form of E–W oriented parallel belts. The depositional system was recon-

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structed as a ramp rather than a shelf based on the gradational relationships between facies, the absence of shelf edge barriers or reefs, and absence of a steep slope break and associated mass flow deposits. This pattern of sedimentation was controlled chiefly by the paleorelief inherited from the early Ypresian tectonism due to the rejuvenation and growth of the Syrian Arc System, as well as to collision of Afro-Arabian plates and closing of the Tethys Sea. The fossiliferous strata of the early Eocene in Farafra Oasis are marked by a pronounced cyclicity formed by stacked parasequences with thicknesses up to several meters. These are characterized by succession of defined microfacies sets reflecting specific depositional areas and sedimentary processes (flooding, emergence) of the carbonate platform. Continuous development of depositional cycles across the sedimentary ramp and arrangement of meterscale, shallowing-upward cycles indicate that the cycle formation is the result of global, glacio-eustatic, high-frequent and highamplitudinal sea-level fluctuations caused by volume changes of polar ice caps and probably reflect Milankovitch eccentricity periodicities. The cyclicity recorded in the Farafra sequence reflects high frequency eustatic sea level changes, on the basis of the absence of an offshore carbonate factory in the Farafra sedimentary system, negating an autocyclic model of tidal flat progradation; the linear accumulation history of the cyclic sequence and the relationship between cycle thickness and inferred paleowater depth not favoring tectonic subsidence cycles; and the hierarchical arrangement of various orders of cycles. The rhythmic sediments comprise cyclic alternations of shale-limestone and marly limestone–limestone, which were probably formed due to minor periodic variation in the supply of terrigenous sediments (dilution cycles). They are grouped into five depositional sequences with several small-scale, shallowing upward stacked rhythms. These 4th order rhythms clarify that the 3rd order sea level variations are superimposed by several short period high frequency eustatic oscillations. Mineralogically, smectite, kaolinite and illite are the most abundant clay minerals. Their origin may be attributed to the gradual increase in the amount of erosion of the source rocks to the south of Egypt, in areas of moderate rainfall and rapid weathering and/or to reworking processes of soils which presumably developed on basement rocks. Little differences in abundances might be related to changes in source rocks or climatic influence related to repeated transgressive and regressive events during the early Eocene.

Acknowledgments Special thanks are due to Prof. Dr. Mohamed. A. Khalifa, Menofyia University, Egypt, for field assistance and guidance. I would like to appreciate the help of my colleague Dr. Mahmoud A. Essa, Assiut University, Egypt, in X-ray analysis. The author is greatly indebted to the editorial board and anonymous referees for valuable comments which have greatly improved the text.

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