Facies framework and depositional setting of the Middle-Upper Eocene Hamra Formation, north of Bahariya Oasis, Western Desert, Egypt

The Middle-Upper Eocene Hamra Formation, covering the northern plateau of the Bahariya Oasis, northwestern Desert, comprises two informal (lower and upper) members; the lower member (Upper Middle Eocene, 34.5-m thick) is made up entirely of fossiliferous limestone rocks (Nummulites gizehensis, Nummulites champolioni), while the upper member (Late Eocene, 38-m thick) consists of an intercalation of limestone with thin beds of sandstone, siltstone, and claystone. The upper member is highly fossiliferous with Turrittella, Carolia palcnoids, and Ostrea multicostata. Seven carbonate and three clastic microfacies have been recognized in five measured sections of the Hamra Formation. The carbonate microfacies were deposited on the proximal ramp setting of a warm and shallow sub-tropical environment. The lower member can be correlated with the (Upper Middle Eocene) Sath El Hadid and Fashn formations, while the upper member is correlated with the Upper Eocene Beni Sueif Formation. The reduced thickness of both members of the Hamra Formation compared with the corresponding rock units to the east reflects the structural uplift prevailed in the north of Bahariya Oasis during sedimentation.


Introduction
During the Paleogene, shallow-water carbonates were accumulated in a large swath of the southern margin of Tethys area, accounting for more than 80% of the global neritic carbonate production throughout the Eocene (Hottinger, 1983(Hottinger, , 1997Racey, 2001;Racey et al., 2001;Philips, 2001). Larger benthic foraminifera were the major contributors to the platform and ramp formations during Early and Middle Eocene. Nummulites, alveolines, orbitolites, and assilines are among the Early Cenozoic larger benthic foraminifera. The Paleocene to the upper Eocene sedimentary rocks from the Mediterranean and Arabian Peninsula carbonate platforms are rich in Nummulitic foraminifera (Hadi et al., 2016;Özcan et al., 2022). Also, nummulitic limestones which form hydrocarbon reserves off the coastlines of North Africa and India represent promising exploration possibilities in the Middle East (Beavington-Penney et al., 2005).
Large benthic foraminifera facies of this age are frequently associated with carbonate ramp settings (Flügel, 2004). For instance, nummulitic accumulations off the coast of Tunisia and Libya make up a significant hydrocarbon resource. This concept is investigated in countries such as Egypt and Oman (Racey, 2001;Racey et al., 2001). Nummulitic deposits from inner to outer-ramp origin collected everywhere over Tethys' continental margin (Flügel, 2004). Meanwhile, Nummulites accumulations of the Eocene Jdeir Formation in Tunisia and NW Libya form an excellent reservoir for oil and gas (Flügel, 2004). These accumulations produce 20,000 to 150,000 barrels of oil per day with a potential reserve of up to 700 million barrels (Racey, 1995).
The area of study covers an area around 2400 km 2 and is located north of the Bahariya Oasis up to the Bahr depression (Fig. 1). The Bahariya plateau, which has a prominent flat surface with slight undulations, is the name given to this location (Said and Issawi, 1964). The plateau's base is made up of the grayish brown Naqb Formation while the plateau's midsection is composed of white limestone from the Middle Eocene Qazzun Formation.
The limestones of the Hamra and the clastics of the Qatrani formations cover the majority of the research area (Fig. 1). The Quaternary deposits are characterized by sand dunes with variable extensions that cover a vast region and provide a barrier in the area of study. Sand dunes vary in size from a few meters to many kilometers. For instance, Ghard Abu Muharik and Ghard El Qazzun are the most remarkable sand dunes in the vicinity of the study area.
Several geological and hydrological work have been done on the Bahariya Oasis (e.g., El Akkad and Issawi, 1963;Catuneanu et al., 2006;Afify et al., 2016;Plyusnina et al., 2016;Al-Dhwadi and Sallam, 2019;Hamza et al., 2022). However, the Hamra Formation has not been subjected to sedimentological investigation, except for a few works carried out by some researchers (e.g., Hilmy et al., 1984, Issawi et al., 1999). There is a distinctive lack of environmental explanation about microfacies and their depositional setting. Moreover, no correlation has been done with the relevant rock units in adjacent areas. Thus, the purposes of this paper are to (a) illustrate the characteristics of microfacies types, (b) provide a facies description and interpretation for the Middle-Upper Eocene Hamra Formation, (c) establish the factors that impacted their deposition, (d) demonstrate the facies distribution (lateral and vertical) of the Hamra Formation east of the Bahariya Oasis and south El Fayium Province, and (e) correlate the Hamra Formation with the equivalent rock units in the east of Nile Valley.

Geological setting
The Bahariya Oasis is situated in the Western Desert of Egypt, near the contact between stable and unstable shelves (Said, 1962;Said, 1990) and/or on the contact between the southern and northern facies belts (Khalifa, 2017). The Bahariya Oasis was affected by tectonic instability during the Cretaceous and the Eocene periods. The Bahariya Oasis is a structural high area, formed by the dominant Syrian Arc System during the post-Cenomanian along the Lower Tertiary (Paleocene and Lower Eocene). It separates among the Beni Sueif basin east and west River Nile and Alamein, Abou Gharadig, Shushan, and Qattara basins, located in the northern area of the Western Desert (Taha and Halim, 1992). According to Moustafa et al. (2003), two different episodes have emerged from Cretaceous to Early Tertiary basin inversion: The first is the Campanian-Maastrichtian while the second is the Post-Middle Eocene. The first phase might result in the creation of synclinal structures in the Bahariya Oasis' northern regions, such as the El Hefhuf, Hammad, Fagget El Hara, and Topog synclines. In contrast, the second phase has resulted in dislocation faulting and eruption of volcanism within or outside the Bahariya Oasis. The Bahariya Oasis is situated between three ENE-oriented structural belts, it is controlled by deep-seated faults. Meanwhile, the central areas are undeformed or slightly deformed (Taha and Halim, 1992). Basins were developed due to the Mesozoic rifting that controlled northern Egypt alongside with the opening of the Neo-Tethys. The northern Bahariya plateau (Upper-Lower Eocene Naqb, Middle Eocene Qazzun, and the Middle-Upper Eocene Hamra formations) was deposited on unconformity surface overlying the Late Cenomanian Bahariya Formation. Hence, this unconformity elucidates the absence of the Paleocene Esna Formation and the Lower Eocene Thebes and their equivalent rock units such as the Rufuf Formation and the Farafra Limestone were not deposited. The deposition of the Eocene plateau took place during a quiet structural event which was restricted between the Syrian Arc System (Late Cretaceous) and the Alpine (Oligo-Miocene) movements. Later on, they were subjected to Oligocene volcanicity producing basalt lavas at several localities covering some parts of the limestone plateau.

Material and methods
Five stratigraphic sections were measured and described from the Hamra Formation (Fig. 2). Fossils have been described and identified. About 85 samples from the Hamra Formation (78 limestone and 7 sandstone samples) were investigated using petrographic microscope to manifest their different microfacies types. In addition, seven samples of shales and claystones were analyzed by the XRD technique. Philips X-ray diffraction equipment model PW/1710 with monochromator, Ni-filter, Cu-radiation, and scan speed 0.02/s at the central laboratories of the Egyptian Mineral Resources Authority (EMRA) have been used for the identification of clay minerals. The clay fraction was separated after the removal of carbonate, iron oxide, organic carbon, and soluble salts. The carbonates were removed by treating the sediment samples with 5% HCl. The organic matter was removed by treating the sediment samples with 30% hydrogen peroxide several times. Soluble salts were washed by distilled water. Iron oxide was removed by stannous chloride then washed by distilled water. Also, sandstone samples were investigated using SEM. Microfacies types of limestones are classified on the basis of Dunham ' s terminology (1962), with the modification of Embry and Klovan (1971). However, the grainstone term of Dunham is replaced herein by the term recrystallized packstone (Re-packstone) instead of the term grainstone. Said and Issawi (1964) used the term Eocene Plateau to characterize the rocks that cover a vast region north of the Bahariya Oasis. In an unconformable way, the Eocene Plateau sits atop of the Lower Cenomanian Bahariya Formation. The presence of gray to pale gray lateritic claystone with occasional iron oxide pebbles that ranges in thickness from 50 cm to 3 m demonstrates the unconformable relationship. These carbonate rocks are unconformably overlain by the Oligocene (Qatrani Formation) which encircles the Bahr depression from the north (Fig. 1). The Bahariya Oasis' northern Plateau is made up of three formations that are distinguished by their lithology and field appearance (Said and Issawi, 1964). From the bottom to the top, they are organized as follows: The Upper-Lower Eocene Naqb Formation, the Middle Eocene Qazzun Formation, and the Middle-Upper Eocene Hamra Formation (Fig. 1). Said and Issawi (1964) used the name Hamra Formation to describe the intercalations of limestone, sandy clays, and thin sandstone rocks, covering the northern Bahariya plateau. They named the formation after the hill of El Qara El Hamra, which has the most complete sequence. The Hamra Formation is distinguished by its scattered small hills that appear north of the Bahariya Oasis (Fig. 3).

Nature of stratigraphic contacts
There is a conformable relationship between the Hamra Formation and the underlying Qazzun Formation (Said and Issawi, 1964). The conformable relationship was confirmed by Lotfy (1988). However, Hussein-Kamel (1997) argued the presence of an unconformable relationship between the Qazzun Formation below and the Hamra Formation above based on paleontological investigation conducted on a studied section, located 1 km west of El Qara El Hamra. Issawi et al. (2009) confirmed the presence of an unconformity surface between the Qazzun Formation and the overlaying Hamra Formation evidenced by the uneven contact and presence of intraformational conglomerate. In the current study, the lower contact of the Hamra Formation reveals the presence of unconformable boundary with the underlying Qazzun Formation at its type locality (El Qara El Hamra) (Fig. 4A). This contact occurs between the Qazzun Formation's chalky white limestone and the lower Hamra Formation's yellowish-brown limestone (Fig. 4A). The difference in lithology, fossil types, and uneven contact demonstrate the unconformable relationship. The upper boundary of the Hamra Formation displays unconformable relationship with the overlying Oligocene Qatrani Formation (Fig. 4B). The sharp lithological change from the yellowish-brown limestone of the Hamra Formation below to the dark brown, nonfossiliferous, thinly bedded sandstone and siltstone of the above Qatrani Formation confirm this unconformable

The lower member
The lower member is composed of limestones in the five measured sections, except for section 4 (Fig. 2), which includes a 2.5-m-thick shale layer at its base. The limestones in this member are characterized by a variety of colors ranging from yellowish brown, whitish brown, brownish gray, to white (Fig. 5A). Limestones are sandy, glauconitic, massive, thin-bedded, and highly fossiliferous, with calcite pockets (in section three). Nummulites gizehensis and Nummulites champlioni are the common two fossils which are found mainly in the lower member (Fig. 5B). This member is assigned to the Middle Eocene as evidenced by the existence of the above-mentioned fossils, and it can be correlated

The upper member
The upper member is made up of brownish-white and yellowish-brown fossiliferous limestones interbedded with sandy limestones, sandstones, siltstones, mudstones, and shales (Figs. 2 and 6). The limestones are yellow, brown, gray, brownish-yellow, and white gray in color, sandy, thinbedded and fossiliferous with Ostrea clotbeyi and small Nummulites (Fig. 7A), Carolia placunoides, Ostrea sp. and Carolia sp. (Fig. 7B), Lucina, Nummulites sp., and Gisortia sp. The upper part of this member consists of an intercalation of sandstone and claystone. The sandstone is usually gray, red, grayish-white, yellow, and greenish-yellow, usually thin-to thick-bedded or massive, glauconitic, calcareous, and fossiliferous with pelecypods (Fig. 2). The claystone ranges in thickness from 50 cm to 3 m. It is brownish-yellow, green, yellow, gypsiferous, glauconitic, fissile, and laminated. In addition, vertebrate bones, Crinoids, and oyster banks are encountered. Specifically, this member is characterized by Turritella bed (0.5-m thick) (Fig. 8A) and by several vertical and horizontal burrows (Fig. 8B).
In the northeastern part of the study area (section 1), the upper member is made up mainly of limestones, shales, burrowed mudstones, and siltstones interbeds, forming eight cycles, each of which starts by shale at the base and ends at top by limestone. Occasionally, the shale changes to siltstone or mudstone. The limestone beds range in thickness from 20 cm at the uppermost part of the Hamra Formation to 4 m at the middle of the upper member. In section 3, a 30-cm-thick caliche bed is encountered at the middle part of this member.

Age assignment and correlation
The Hamra Formation has been given different ages by several authors: Late Lutetian-Bartonian (Said and Issawi, 1964), Middle-Late Eocene (Bartonian-Priabonian) (Mesaed, 1999;Osman, 2003). In the present study, the Bartonian-Priabonian age is assigned to the Hamra Formation. The presence of Nummulites gizehensis and Nummulites champlioni (Fig. 5BB) in the lower member suggests Late Middle Eocene age and can be correlated with Sath El Hadid Formation at the area west of Beni Mazar and south El Fayium district (Fig. 6). Also, the lower member can be correlated with Upper Middle Eocene El Fashn Formation east of the Nile Valley. Meanwhile, the Upper Member is assigned to Late Eocene and can be correlated with Beni Suef Formation. The correlation is asserted by the presence of the mega fossils such as Bryozoa, Natica longa, Gisortia gigantia, and Lucina Pharonium.

Lime-mudstone
Lime-mudstone microfacies are encountered in all of the measured sections and display a thickness percentage ranging from 3.6 to 34.6% of the Hamra Formation. Specifically, in the lower member (sections 1, 2, and 3) and in the upper member in all sections of the Hamra Formation (Fig. 9). The beds of these microfacies range in thickness from 0.3-to 20-m thick. These microfacies are massive, thick-bedded, sandy, yellowish-brown, red, gray, and brown in color and have a thickness of 0.3 to 4 m.
Micrite is the main constituent of the lime-mudstone microfacies (97%), with few quartz grains and pelecypod shell fragments accounting for less than 4% (Fig. 10AB). Occasionally, the micrite matrix exhibits neomorphism, resulting in formation of microspar and sparry calcite. Quartz grains are subrounded to rounded medium to coarsegrained. Meanwhile, glauconite and shell debris are scattered randomly within the micrite matrix (Fig. 10B). Glauconite peloids are yellowish-green, olive to green in color, subrounded, and well-sorted, embedded in microsparry calcite cement. Additionally, they are found as patches within the calcite cement invading the skeletal particles giving them the greenish-yellow color. Chalcedony spherulites and microcryslalline silica are encountered in the upper member of the Hamra Formation replacing microsparry to sparry calcite. The replacement of silica to the calcite is marked by the occurrence of lime-mud relics within the microcrystalline silica and chalcedony spherulites.
Interpretation The abundance of micritic matrix in the lime-mudstones, including fine skeletal pieces of pelecypods, suggests deposition in a low energy deep subtidal environment (Wilson, 1975;Lee and Kim, 1992). Besides, the presence of glauconite peloids embedded in micrites and replacing neomorphosed calcite indicate authigenic origin of glauconite, formed in shallow marine middle shelf. Commonly, glauconite is an authigenic mineral which is formed only in marine settings and is frequently accompanied with low-oxygen conditions (Hiscott, 1982;Smith and Hiscott, 1984). The presence of glauconite peloids as solitary pellets indicates low depositional rate environments. In the present time, glauconites are found on shallow-water continental shelf depositional sites (Oddin, 1988). The occurrence of microcrystalline silica may be attributed to biological factors such as dissolution of spongy spicules and other organisms bearing silica (Hesse, 1989).
Interpretation The abundant occurrence of lime-mud indicates a low energy environment of deposition that had resulted in the accumulation of fine matrix (Flügel, 2004). The association of big size pelecypods indicates the inner shelf (Ziegler, 1967;Kulm et al., 1975). Moreover, pelecypods and gastropods shells can be found in depths of 20 to 50 m (Ziegler, 1967). The micritization of pelecypods shells was mostly from boring by algae and fungi in shallow and restricted marine water (Harris et al., 1997).

Glauconitic nummulitic wackestone (GNW)
These microfacies are encountered throughout the Hamra Formation's lower member especially in sections 1, 2, and 3, as well as the top of the lower member of section 4 ( Fig. 9). It reaches a thickness of 2 to 12 m and is distinguished by yellowish-brown limestone that is thickly bedded and fossiliferous. Glauconite is common at the base of the lower member in section 3 and the base and middle of the upper member in section 4. Rocks of these microfacies are massive, brownish-yellow to yellowish-green in color and fossiliferous.
The glauconitic nummulitic wackestone microfacies are made up of nummulitic tests (30%), pelecypod shell fragments (12%), echinoid plates and spines (3%), and glauconite pellets (10%) embedded in a micrite groundmass (55%) (Fig. 12A). Nummulites tests are preserved in their original aragonitic fabric and cemented with micrite (Fig. 12A). Some of these tests are obliterated by transportation and/or affected by aggrading neomorphism where the outer margins replaced by microspar. Glauconite pellets are scattered randomly in the micritic matrix. These pellets are brown and brownish-green in color and rounded in shape. Pelecypod and gastropod debris are frequently present. Most of the pelecypod shell fragments are partially recrystallized and micritized (Fig. 12A). Quartz grains are exceptionally fine in size, rounded, well sorted, and are monocrystalline in type.

Interpretation
The nummulitic wackestone microfacies are commonly deposited in subtidal low energy environments (Hardie and Ginsburg 1977;Shinn, 1986). Whalen (1988) related the wackestone to the shallow subtidal-intertidal zone. The abundance of micrite in this lithofacies tends to suggest low energetic subtidal open marine shelf and shallow neritic water with wide circulation regime for the depositional environment (Wilson, 1975;Flügel, 2004). Nummulites sp. may distinguish warm shallow water or shallow neritic environment (Arni, 1965). Sediments are dominated by nummulitidae and mollusca characterize the middle ramp environment (Pomar, 2001 andCosovic et al., 2004). Larger benthic Foraminifera are supposed to have lived above the Euphotic Zone Depth (Lee et al., 1992). Spatiotemporal variation results from many factors such as the quantity of suspended particulate matter in the water (Kirk, 1994). The well-rounded glauconite grains represent allochthonous origin from nearby environments. Allochthonous glauconite is introduced into sediments from external sources, such as reworking of glauconite grains from older strata (Amorosi, 1997). Glauconite was probably deposited in the modern continental shelf at depth from tens to hundreds of meters (McRae, 1972).

Sandy intraclastic wackestone (SIW)
The sandy intraclastic wackestone microfacies occur in the upper member of the Hamra Formation in section 5 with an average thickness of 1.5 m. (Fig. 9). Rocks of this microfacies appear to be massive, thick-bedded, and gray to reddish brown in color.
In thin section, the sandy intraclatic wackestone consists of intraclasts (20%), quartz grains (8%), and glauconite peloids (2%) that are bound by micrite (70%). The intraclasts are present as coarse-grained and fine-grained (Fig. 12B). The coarse-grained types are rounded to subrounded to ovoidal ranging in size from medium-to coarse-grained sand-sized. Intraclasts are made up of lime-mud clasts filled with fine quartz grains. Other intraclasts consist of dark lime-mud with remnants of Mollusca debris. These clasts are coated by bladed calcite in the form of isopachous cement. The fine-grained intraclasts have various sizes ranging from 30 to 50 μm and are composed of well-rounded dark-gray lime-mud without any association of skeletal debris. Quartz grains are exceptionally fine, monocrystalline, moderately sorted, and exhibit unit extinction.
Interpretation Intraclasts consist of fragments of early cemented limestones of local origin. The presence of various rock types (lime-mud, intraclasts, and quartz grains) indicates a fluctuation between low-and high-energy conditions. Intraclasts reflect the fragmentation of sediments in the intertidal zone and re-deposition in the subtidal zone. On the other hand, gravel grade materials are commonly made up of whole disarticulated or broken skeletal fragments together with sand grade material of whole, broken, and disaggregated skeletal debris. The detrital quartz and lime-mud intraclasts occur mainly in restricted zone of inner shelf, especially in the intertidal and subtidal zones, near the shoreline (Thambunya, et al., 2007). These sediments were affected by vadose and meteoric fresh-water phreatic diagenesis as the grains are bounded by meniscus and fine crystalline cements (Longman, 1980).

Nummulitic molluscan packstone (NMP)
The nummulitic molluscan packstone microfacies are recorded in the upper member of the Hamra Formation in sections 1, 3, 4, and 5 (Fig. 9). It ranges in thickness from 0.3 to 5 m with maximum thickness in sections 3 and 5. Such rocks are brown in color and rich in pelecypod fossils (Ostrea clotbeyi and Ostrea multicostata) and gastropod (Turritella sp.). The rocks of these microfacies are sandy in the upper member of the Hamra Formation in section 1, whereas in section 5, they are gray and brown, sandy, massive, and fossiliferous with echinoids and Nummulites.

Bioclastic Nummulitic packstone (BNP)
Bioclastic nummulitic packstone microfacies are encountered in the lower member of the Hamra Formation in section 2 and in the upper member in sections 2, 3, and 4. They range in thickness from 2 to 8 m with maximum thickness in section 2 (10 m). Rocks of these microfacies   are yellowish-brown in color and highly fossiliferous with large Ostrea sp. and Nummulites sp. They are sandy, gray, and pale brown in color with pelecypod shells in the lower member in section 2. At the top of section 4, these microfacies are fossiliferous with echinoids. In thin section, the bioclastic nummulitic packstone microfacies comprise Nummulites tests (10-15%), pelecypod (20%), bioclasts (30%), and echinoid plates (5%) embedded in micrite (30%) (Fig. 13B). Bioclasts were most probably derived from Nummulites, echinoids, and Mollusca. Nummulites tests show partial fragmentation while pelecypod shell fragments are partially micritized. Also, few glauconite pellets and quartz grains (2%) are observed. Most of the shell fragments have the original fibrous structure. No recrystallization of the shells is observed. Few pelecypod shells are heavily micritized Interpretation The presence of Nummulites, molluscan debris, and echiniods indicate shallow marine facies (may be deep subtidal) (Flügel, 2004). The above mentioned fossils may refer to rapid production of carbonate building sediments on submarine mounds up to sea-level and/or sudden falls in relative sea-level (Tlig et al., 2010).

Molluscan packstone (MP)
These microfacies occur in the lower member of the Hamra Formation in section 3 with a thickness of 2 m (Fig. 9). Rocks of this microfacies are brown in color, thick-bedded, and fossiliferous with Lucina sp. and Turritella sp. These micorfacies are composed of pelecypod shell fragments (38%), gastropods (15%), echinoid plates (5%), and glauconite (4%) cemented by micritic matrix (40%) (Fig. 14A). Most of the pelecypods are fragmented and were subjected to micritization processes (Fig. 14B). The micritization process usually took place along the outer margins of the shells.
Interpretation The occurrence of fragmented shells and abundant matrix in the molluscan packstone suggest that they were probably deposited in the high energy shallow subtidal depositional environment (Kumar et al., 2009). The molluscan microfacies indicate a shoal environment above the normal wave-base that was deposited at the shelf margin separating the open marine from the more restricted marine environment (Wilson, 1975;Flügel, 2004). The presence of carbonate mud and abundance of normal marine fauna indicate deposition in relatively shallow marine environment within the zone of wave agitation (Heckel, 1972).

Calcareous quartzarenite microfacies (CQA)
The quartzarenite microfacies association occur in the Hamra Formation at the base of the upper member in sections 3, 4, and 5. In addition to, near the top of the upper member in section 5 (Fig. 9). These microfacies range in thickness from 2 to 8 m. Rocks belonging to this microfacies exhibit variable colors ranging from grayish-white to yellowish-brown and brown. They are hard, cross-bedded, and burrowed.  These microfacies consist of quartz grains (85%) and glauconite pellets (3%) that are cemented by blocky to poikilitopic calcite (13%) (Fig. 15A). Each of poikilotopic calcite crystal encloses several quartz grains. The quartz grains are fine, well-sorted, and subrounded. They display unit extinction and show point to straight grain contacts. Some of the quartz grains display a partial replacement by calcite cement as evidenced by their peripheral corrosions. Glauconite peloids are green and yellowish-green in color. They are subrounded to rounded and show a partial replacement by calcite cement. Frequently, the calcareous quartzarenite microfacies are glauconitic where they consist of quartz grains (76%) and glauconite peloids (10-15%) and are cemented by blocky to poikilotopic calcite. The quartz grains are floated, subrounded, fine-grained, and moderately to well-sorted. Some of the quartz grains are replaced by calcite. Glauconite shows various colors (green, yellowishgreen, and orange). It occurs as pellets as well as matrix that exhibits widely cracking. Few muscovite flakes are noticed.
Interpretation The calcareous quartzarenite is given the name depositional quartzarenite as it was deposited in coastal marine zone (Khalifa, 2017). The depositional environment of these lithofacies indicates a peritidal environment (supra-intertidal flat) (Shinn, 1983). The well-rounded quartz grains indicate that they were transported prolonged distance before their deposition. The depositional environment of these lithofacies suggests a near shore deposition of marine environment. The glauconitic sands in such sediments reflect a high energy marine environment of shoals, sand bars, and barrier islands (Selley, 1976). Fig. 12 A Photomicrograph of the glauconitic nummulitic wackestone manifesting the occurrence of elongated molluscan shell debris, fine-grained quartz, and yellow glauconite pellets embedded in micritic lime-mud matrix. B Photomicrograph of the sandy intraclastic wackestone showing intraclasts (see arrows) and quartzgrains binded by micrite Fig. 13 A Photomicrograph of the nummulitic molluscan packstone manifesting the occurrence of elongated molluscan and nummulite shell debris, fine-grained yellow glauconite pellets embedded in micritic lime-mud matrix. B Photomicrograph of the bioclastic nummulitic packstone manifesting the occurrence of fine-grained bioclasts and nummulite shell debris, embedded in micritic lime-mud matrix

Claystone lithofacies
The claystone lithofacies are recorded in the lower member of the Hamra Formation in section 4 (2.5-m thick), and in the upper member of sections 1 and 2. Claystones are light-brown, gray to dark gray, fissile, and horizontal lamination. Some clay beds display bioturbation with vertical and oblique burrows. In some beds, claystones are fine to very fine, angular to sub-angular quartz grains. The clay minerals are mostly represented by mixed layer illite-smectite and kaolinite. Scarcely, illite-smectite mixed layer is the dominant clay mineral.
Interpretation The abundance of illite and illite-smectite in these lithofacies suggests deposition in a marine environment (Huggett, 1989). Claystone facies indicate low-energy water sedimentation (Reineck and Singh, 1973). The presence of bioturbation in the claystone may suggest deposition in a coastal grass flats environment (Snedden and Kersey, 1981). The illite mineral group typically suggests a marine clay deposits. Commonly, illite mineral in the claystones is derived from the diagenesis of kaolinite which is a byproduct from the chemical weathering of feldspar. It represents a typical marine origin (Haldar, 2020).

Depositional setting and discussion
The northeastern Bahariya Oasis was subjected to tectonic uplift during post-Cenomanian until the end of Eocene. Such tectonic phase was affected by the Syrian Arc System (NE-SW). Consequently, it led to the development of unconformity between the Cenomanian Bahariya Formation and the Lower Eocene Naqb Formation. Therefore, different rock units (e.g., the Upper Cenomanian El Heiz Formation, Turonian-Coniacian-Santonian El Hefhuf Formation, Campanian-Maastrichtian Ain Giffara Formation, Maastrichtian-Danian Khoman Chalk, Paleocene Esna Formation, and the Lower Eocene Thebes Formation) had not been deposited. Such tectonic movement was extended farther resulting in the reduced thickness of the subsequent Eocene plateau (Naqb, Qazzune and El Hamra formation). For instance, the limestones of the lower Eocene in the Farafra Oasis (Farafra Limestone and the Rufuf Formation) are equivalent to dolostone of the Naqb Formation. This is can be related to the shallowing of the sea over and around Bahariya platform which raised the concentration of Mg ions, resulting in the emergence of dolomitization processes. The nummulitic massive carbonate facies of the Middle Eocene (Minia and Samalut formations) in the west and east of the Nile Valley were also reduced in thickness over the Bahariya Platform and grouped together in one rock unit, termed El Qazzun Formation, as a result of tectonic uplift. Furthermore, the   (Maghagha, Qarara, Fashn, and Beni Sueif formations) are grouped into a single rock unit correlated with the Hamra Formation in the area of study (Fig. 6).
At the type section, El Qara Hamra (section 2), the lower member of the Hamra Formation in the central part of the study region, displays maximum thickness (34.5 m) indicating continuous subsidence throughout deposition, while to the west, at Naqb Siwa, sections 4 and 5 exhibit a decreased thickness (14.5, 19.5 m), respectively, implying modest shoaling or elevating during deposition. The lower member is predominantly composed of carbonate facies, evidenced by the presence of pelecypod and nummlitic wackestones and lime-mudstone. This indicates a maintained dry and warm condition during the deposition of lower member. The absence of wave and current structures and the presence of mud-supported textures imply a low-energy environment under storm wave base (Burchette and Wright, 1992). The presence of pelagic foraminifera and a lime-mud-dominated lithology imply deposition in a low-energy deep water environment underneath storm wave-base (Corda and Brandano, 2003).
Indeed, the presence of larger foraminifers (Nummulites, alveolines, orbitolites, and assilines) in the Hamra Formation is valuable tool in constructing paleoenvironmental models (Adabi and Zohdi, 2008). They live on shallow, oligotrophic, Circum-Tethyan carbonate platforms (Buxton and Pedley 1989). Larger foraminifera occur abundantly in many shelf carbonate platforms. Nummulites are abundant in the Paleocene to the Upper Eocene sediments of the Mediterranean and Arabian Peninsula, where nummulitic limestones form hydrocarbon reservoirs in onshore North Africa and India, and represent a potential exploration target in the Middle East (Beavington-Penney et al. 2005). Scheibner et al. (2005) suggested that the larger foraminifera are common during the Paleocene-Eocene transition and they are closely correlated with the Paleocene-Eocene thermal maximum.
Climate, temperature, salinity, nutrient availability, sealevel fluctuations, and tectonics are all variables that influence carbonate deposition (Tucker, 2003). The presence of lime-mudstone and wackestone texture suggests that the inner platform or ramp is low-energy. Such facies are incomplete cycles which might be the result of a continuous subsidence. Variation in relative sea level has a profound effect on carbonate sediment supply. Carbonate deposition prevailed during high-stand when supply was massive and the progradation was active. While it was less prevalent during low-stand or transgression. Carbonate production catches up with the increase in accommodation as the sea level rises. In contrast, sea-level fall reduces for the carbonate supply in shallow subtidal area. Carbonate sediments have accumulated on platforms along continental margins in numerous areas throughout the world when siliciclastic deposits are lacking. The absence of siliciclastic input, high biogeneic carbonate productivity, and the availability of warm water are the three most significant variables that govern carbonate buildup. The highest sea temperatures of the Cenozoic are seen in the late Paleocene to Early Eocene time span (58 to 49 Ma) which coincides with a protracted period of "greenhouse" climates (Zachos et al., 2001). This was followed by a slowed warming throughout a period of long-term cold in the Middle and Late Eocene (49 to 34 Ma). Eventually, it led to the formation of icehouse climate with extensive polar glaciation by the early Oligocene (Miller et al., 1987;Zachos et al., 1996).
Before the deposition of the upper member, a significant tectonic movement caused the break off carbonate deposition and the successive deposition of coarse clastic facies characterized by quartzarenite facies. A substantial amount of siliciclastic materials were routinely deposited. Therefore, carbonate deposition was inhibited.
The upper member differs somewhat from the lower member in facies, starting with sandstones (quartzarenite) in sections 3, 4, and 5, and mudstone in sections 1 and 2 (Fig. 2). The regression of the sea and the outbreak of climate changes in the Late Eocene may have caused such facies alterations. The predominance of quartz grains and the limited variety of Nummulites sp. fossils indicate a relatively shallow deposition environment (Braun and Friedman, 1969). At El Bahr area (to the north), the quartzarenites change to claystones facies. The lower member of the Hamra Formation is composed of limestone and sandstone; however, in section 1 of the El Bahr depression, clastic rocks constitute the majority with limestone intercalations. With the exception of one intermediate cycle, which has thin claystone at the bottom and substantial limestone at the top.
The following reasons account for the mixing of siliciclastic-carbonate facies in the top member of the El Hamra Formation: (1) eustatic sea level, (2) tectonic pulses, and (3) climatic fluctuations. The vertical distribution of mixed facies can be attributed to eustatic sea level. The claystones that make up the cycles base suggest transgressive deepening whereas the carbonates that make up the cycles top are thought to represent progressive shoaling. Osleger and Montanez (1996) stated that during long-term progradation and formation of HST, siliciclastics were deposited on top of carbonate by aeolian processes while throughout the early phases of the succeeding long-term relative sea-level rise, siliciclastics source area were pushed back toward the craton margin. Climatic changes account for the fine siliciclastics supply; this may happen during humid conditions. This hypothesis was described in the southern Alpine Rhaetic sediments by Burchell et al. (1990).
The structural control can be used to establish a source area for fine siliciclastic. Hence, siliciclastic inflow onto the platform might be a response to cratonic hinterland processes such source-terrain tectonism or climatic variations (Osleger and Montanez, 1996). On the other hand, the occurrence of siliciclastic may be regulated by variations in the rate of carbonate formation on the platform, rather than variation in the base level or source area dynamics (Osleger and Montanez, 1996).
The Middle Eocene-Upper Eocene contact is dominated by a wide range of detrital and authigenic minerals in variable quantities. The increase in the yellow to brownish-yellow color in the Upper Eocene indicates change in facies from Middle to Late Eocene. Therefore, the increase in iron content is a frequent feature throughout Egypt's territory whether in the Western or the Eastern deserts. The following factors may account for the concentration of iron or ferrugination in Upper Eocene rocks. (1) Unconformity surface that divide Middle-Upper Eocene rocks and is typically associated with iron oxide nodules or iron pebbles. This unconformity also occurs at west Beni Mazar between the Middle Eocene Midawra (equivalent to the Qazzun Formation) and Sath El Hadid Formation (equivalent to the lower member of the Hamra Formation). (2) Subsequent flooding during the Late Eocene, the iron oxides were weathered or dissolved and re-distributed vertically. (3) The onset of hydrothermal activity which was accompanied by tectonic uplift that was influenced by the Alpine movement which started at the end of the Upper Eocene and continued until the beginning of the Oligocene Period.
In addition, nonphotosynthetic bacteria are responsible for iron oxidation. Iron oxide precipitation is the consequence of either direct (metabolic absorption for energy generation) or indirect (sorption, microenvironmental alterations) causes (Casanova et al., 1999;Eren and Kadir, 2013). Inorganic iron oxide mineralization proceeded around the nuclei after bacterially driven nucleation until the system achieved equilibrium (Konhauser, 1998). The surface imperfections on the bacteria-shaped particles are indicative of this interpretation. The cyclic reddening in the series also suggests that macroenvironmental changes occurred. In the intertidal flat environment, a lower sea level or low tide resulted in more oxygenated conditions in both the microenvironments and macroenvironments, resulting in widespread oxidation across the bed (Eren and Kadir, 2013). Some microorganisms (e.g., bacteria) may oxidize Fe 2+ in aqueous solutions and are known as iron bacteria (Schwertmann and Taylor, 1989). The possible pathways involved in bacterially mediated iron oxidation were outlined by Fortin and Langley (2005). The passive sorption and nucleation reactions and direct metabolic activity are both behind the formation of biogenic iron oxide. The presence of clastic rocks such as quartzarenites, sublitharenites, and claystones in the upper member indicates that there was shallowing, uplifting, and a decrease in sea level throughout the Late Eocene Epoch, resulting in the advance of fluvial and the progress of the continental facies over the Middle Eocene subtidal marine carbonates.
The term Re-packstone microfacies is utilized in the current work, a "Re" prefix is added to packstone. This word is used instead of grainstone as defined by Dunham (1962) suggesting that the grainstone is diagenetic texture rather than depositional texture. This is because both the cement (lime-mud) and the skeleton grains were susceptible to aggrading neomorphism when the packstone texture was combined with meteoric fresh water. This phenomenon is explained by the absence of granular calcite deposited in the marine condition while lime-mudstone, fibrous calcite, and drusy calcite represent the common cement formed under marine condition. Other than granular calcite, the mentioned cement kinds are made up of calcium and magnesium carbonate which were most likely produced in seawater. Mg 2 + ions were eliminated during percolation with fresh water leaving calcium carbonates which produce granular calcite grains such as microcrystalline, pseudspar, and even blocky.

Conclusions
The Middle-Upper Eocene Hamra Formation that constitutes the upper limestone plateau north of the Bahariya Oasis (Western Desert of Egypt) represents the western ramp margin of the Middle-Upper Eocene Nile basin. It is distinguished into two informal members the lower carbonate and the upper mixed carbonate-siliciclastic members. The lower member is assigned to Late Middle Eocene (Bartonian Age) as it is fossiliferous with Nummulites gizehensis and Nummulites champlioni. It is made up of carbonate lithofacies, including lime-mudstone, wackestone, and packstone. These lithofacies were deposited on platform during warm water and enrichment of benthonic foraminifera. The lower member can be correlated with the Late Middle Eocene Sath El Hadid Formation in west of Nile basin (Beni Mazar-Maghagha stretch) and with the Late Middle Eocene Fashn Formation in east of the Nile Basin (east Beni Sueif). The upper member of the Hamra Formation is assigned Late Eocene (Priabonian Age) as it is fossiliferous with Carolia placunoides, Ostrea multicostata, Gisortia depressa, Bryozoa, Natica longa, Gisortia gigantia, and Lucina Pharonium and can be correlated with the Beni Sueif Formation. It includes quartzarenites, quartzwackes, and claystone microfacies. The claystones are mixed layer illite-smectite and kaolinite with a subtle amount of illite.
Herein, the reddening of lithofacies of the upper member due to the presence of iron oxides is attributed to lowering in sea-level and input of iron oxides from the uplifted hinterland, or due to the effect of bacteria. We introduced the term recrystallized packstone (Re.packstone) instead of the term grainstone texture of Dunham (1962). Because the term grainstone does not express depositional texture but it indicates post-depositional texture during its subaerial exposure.