Introduction

In Beni-Suef Governorate, Egypt, the first occurrence of igneous rocks (Oligocene basaltic plugs) has been observed in the study area between Wadi (W.) Sannur and W. Al Maghaza. However, numerous isolated basaltic outcrops occur along the Cairo-Suez, Fayum-Abu Rowash, and Tihna-El Bahnasa stretches (Said 1990). Some isolated basalts occur east El-Minia area. Egyptian alabaster was also discovered in the study area, further the well-known ancient area around the Cavern of W. Sannur. The present area is characterized by economic natural resources, represented by limestones, clays, and basalts, which can be used for building, roads, construction work, and industry. Basaltic rocks are widely used as a filtering agent, fibers, and concrete aggregate, they can be utilized as the primary raw material for cement clinker manufacturing (El-Desoky et al. 2021). It facilitates the consumption of free lime and enhances burnability, making it higher quality than clay in cement clinker production.

The term “Alabaster” was derived from the Greek "Alabastron,” which was given for the Egyptian material (Lucas 1933), or from the Ancient Roman “Alabastrites” (Harrell 1995). Egyptian Alabaster is a term that was named in Ancient Egypt (Klemm and Klemm 2001) and is known in archaeological literature as “Oriental or Egyptian alabaster “. In general, the term “alabaster” refers to “gypsum alabaster”, but only in Egypt; it refers to “calcite alabaster” because it consists of calcium carbonate (Lucas 1933; El-Hinnawi and Loukina 1972).

There are no detailed geological studies related to the area as it was previously difficult to reach. Some geological studies have been conducted in the north of the study area by (Salama et al. 2021). In addition, some studies have carried out South of the study area in the W. Sannur region by Philip et al. 1991; Klemm and Klemm 1991; Gunay et al. 1997; Gharieb 1998; Dabous and Osmond 2000; Railsback 2002; Rifai 2007; Yahia 2010; Blasy 2014; Sallam et al. 2020; El Mezayen et al. 2020; Amin et al. 2022). The present work aims to study lithostratigraphy, structure, petrography, geochemistry, and thermal behaviors (TGA and DSC) of the studied rock (Egyptian alabaster, recrystallized limestone, and basalt) types. In addition, it aims, to record and study the first existence of basaltic rocks east of Beni-Suef Governorate, Egypt, and clarify the importance of the role played by volcanic activity in the formation of the karst processes affecting the Eocene carbonates in the study area.

Methodology

The study area is located in the northeastern sector of Beni-Suef Governorate, North Eastern Desert, Egypt (Fig. 1). The area is 280 km2 and it is situated between Latitudes 28° 44 and 28° 51 North and Longitudes 31° 15 and 31° 30 East. It can be reached through the Upper Egypt Freeway, then turns East about 30 km through the W. Sannur desert road. Three stratigraphic sections of the Middle–Late Eocene sequences in addition to the Oligocene basaltic plugs in the area between W. Arhab and W. Sannur have been measured, recorded, and sampled. The geologic map of Conoco 1987 was utilized as a basis for the field. The construction of a geological and structural map for the study area was based on field observations. Fourteen thin sections representing the studied sequences were prepared for microscopic investigation to detect their textures, microfacies, and mineral composition. The microfacies have been classified according to the schemes of Dunham (1962).

Fig. 1
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Location map of the study area in the North Eastern Desert, Egypt

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Thermo-Gravimetry Analysis (TGA) and Differential Scanning Calorimetry (DSC) were carried out on three samples, Egyptian alabaster, bucchino (recrystallized limestone), and basalt, using the Linseis Simultaneous Thermal Analysis system (Germany). The heating rate was 10 °C/min., and the heating temperature was up to 1000 °C for TGA and DSC under a nitrogen atmosphere. The mass of the samples and the DSC signal were measured during the heating stages. The measurements provide information on possible phases or dehydration processes in the samples.

X-ray fluorescence (XRF) analysis was performed for the basaltic rock to determine the geochemical characteristics of the major and trace elements. The measurements were made using XRF (X, Unique-II Separator with Automatic Sample Changer PW-1510). The acid-insoluble residue was estimated by grinding a rock sample, then digesting one gram with diluted hydrochloric acid for 30 min, and after that filtering, drying, and detecting the percent of the acid-insoluble residue by weighing the residue. To detect L.O.I (Loss on ignition), 1gm of the sample was ignited in a known crucible weighing 1000 °C in a silencer oven for 2 h. Chemical elements that are not included in the chemical analysis results tables are either below the detection limit (less than 0.001 ppm) or are not present in the samples. XRF analysis were carried out in the Nuclear Material Authority, Ain Al-Sokhna Road, Al-Qattamia, Cairo, Egypt. The TG analysis was carried out in the Faculty of Post Graduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef, Egypt.

Geologic setting and background

Detailed field studies have revealed that most of the study area is composed of sedimentary rocks with a few igneous outcrops. The deposits of the present area have an age range from the late Middle Eocene to the Oligocene. A lot of the Eocene rocks crop out as a lone great level, like the Nile Valley Plateau (Abd El-Aal 2015). The Middle Eocene outcrops are extend from north of Assiut to Beni-Suef (Abdel Shafy et al. 1983).

In the study area, the Eocene successions are exposed around Wadis; Arhab, Sannur, Abu Rimth, and Mulailah (Fig. 2). The sediments were deposited in elongated tectonic basins that formed the arms of the Tethys Sea (Salem 1976; Saber and Salama 2017). According to Said (1962, 1990), the Middle Eocene shore attained a latitude of 27° N in southern Egypt. Structures and tectonics were primarily responsible for the deposition of upper Eocene sediments (Pattonet et al. 1994). On the other hand, the Oligocene rocks represented by basaltic plugs exposed in the southern part of the area of study, between W. Sannur and W. Al-Maghaza (Fig. 2). It lies near W. Mawathil, which contains the oldest and most precious types of Egyptian alabaster that extend to the time of the Pharaohs and is very rich in the manifestations of the karst processes.

Fig. 2
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Geological map of the study area in the northern part of the Eastern Desert, Egypt (modified after Conoco 1987; Saber and Salama 2017)

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According to Said (1981), Egypt experienced many episodes of volcanic activity during the Cenozoic era. The oldest one was from the Paleocene age and was the continuation of the igneous activity during the Late Cretaceous (Orabi et al. 2015). Basaltic outcrops cover a large area below the Nile Delta, Sinai Peninsula accompanied by the opening of the Red, and adjacent areas of the Western Desert (Bayoumi and Sabri 1971; Said 1981; Williams and Small 1984). The rising of the Red Sea Mountains in addition to the progressive retreat of the Mediterranean Sea shore northwards has resulted in the continued swallowing-upward from the Late Bartonian until the Late Oligocene (Sallam et al. 2015a). The Oligocene rocks were mainly controlled by the structural and topographic lows.

In Egypt, lower Tertiary sediments are widely distributed geographically (Bassiouni et al. 1980). Topographically, the study area is low relief but has a medium elevated plateau with a view of the river Nile. It was subjected to three subsequent erosion cycles that began after uplift at the end of the Eocene and continued until the Pliocene age (Philip et al. 1991).

Results

Field observations

Stratigraphically, the study area includes three rock units arranged from base to top; El Fashn Formation (Tem), Sannur Formation (TemSn), and Maadi Formation (Ted) (Fig. 3), in addition to the basaltic rocks. The first three lithostratigraphic units represent the different stages of the Eocene age, while the basaltic rocks belong to the Oligocene. Paleogene and a few Quaternary deposits are covering the floor of the wadis.

Fig. 3
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Lithostratigraphic units of the studied sections (A, B, and C) in the study area

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Eocene successions

During the Eocene age, there was a major development for carbonate platforms over a wide area of Egypt (Said 1962; Abd El-Gaied et al. 2019; Saber and Salama 2017; Salama et al. 2021). It is occasionally associated with variable quantities of siliciclastic (Rifai 2007; Amin et al. 2022). The Eocene deposits are several thousands of meters thick and cover nearly a fifth (21%) of the surface area of Egypt. In general, it reflects the continuous progressive uplift of Africa Craton that resulted from the compressive tectonics between Eurasia and Africa (El Hawat 1997). These sediments are subdivided into late Middle Eocene and Late Eocene rocks in the study area. Detailed field studies of the different rock units are differentiated into three rock units, arranged from oldest to youngest, and described as follows:

El Fashn  Formation (late Middle Eocene)

The El Fashn Formation was first proposed by Bishay (1966) to describe 88 m thick succession of fine-grained, sandy limestone, chalky limestone, sandy shale, and marl intercalations exposed at W. El Sheikh in the El Fashn area. He considered it a separate rock unit from the Observatory series of Farag and Ismail (1959). The thickness of the exposed El Fashn Formation at W. Sannur attains 59 m in the study area (Fig. 3A). It is located at the intersection of Latitude 28° 45.518ʹN and Longitude 31° 24.619ʹE. The base of this formation is thickly bedded, jointed, whitish-yellow, and moderately hard limestone. It contains ferruginous tubes (Fig. 4a). The middle and upper unit is composed of white, hard limestone with chert bands, and nodules (Fig. 4b) overlain by a grayish-white, and hard limestone bed which shows a sinkhole as a result of intense weathering with a diameter of 22 cm and depth of about 45 cm (Fig. 4c).

Fig. 4
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Field photographs showing as follows: a Whitish-yellow limestone containing ferruginated tubes (arrows), b Chert nodules (black arrow), and bands (yellow arrows) within the limestone, c Sinkhole as a result of the effect of strong weathering, d Highly fossiliferous limestone bed at the top part of the El Fashn Fm., e Ferruginous, cavernous limestone within the Sannur Fm., looking SE, f Thinly bed of nodular limestone, looking SE, g Burrowed chalky limestone, h Veinlets of Egyptian alabaster, I alabaster rock sample were taken and polished, j Lenses of sand (arrows) associated the shale bed in the Maadi Fm., k Burrowed, highly fossiliferous limestone at the upper part of the Maadi Fm., and l Oligocene gravel is covering the top part of the Maadi Fm

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The El Fashn Formation consists of yellowish-white, weathered, sandy, and highly fossiliferous limestone at the top (Fig. 4d). Nodular cherts are widespread in shelf limestones, they occur mostly in carbonates as host rocks (Tucker 2001). The El Fashn Formation is belonging to the Middle Eocene age (Omara, et al. 1978; Mansour et al. 1982; Abd El Shafy et al. 1983; Bassiouni et al. 1974) and the Bartonian age of the late Middle Eocene (Saber and Salama 2017; Salama et al. 2021).

Sannur Formation (late Middle Eocene)

The Sannur Formation was first named by Boukhary and Abdelmalik (1983) to describe the 48.4 m thickness of shallow reefal carbonate successions at the latitude of east Beni-Suef City. Akaad and Naggar (1965) introduced this formation for a nummulitic limestone succession at Gebel Sannur. The Sannur Formation was differentiated into two units; The lower unit is coeval to the Qarara Formation (Bishay 1966), while the upper unit is laterally coeval to the Shaiboun Formation that was exposed at the top of Gebel Homret Shaiboun east of Beni-Suef (Gharieb 2003), or to the Observatory Formation (El Fashn Formation in the study area) of Mokattam Group exposed at Gebel Sannur (Abdou Soliman 1980; Sallam 2015b). This formation shows the dissolution and recrystallization effects (Amin et al. 2022). These features led Boukhary and Abdelmalik (1983) to give the term “Sannur Formation” on the limestones at Gebel Sannur around the Protectorate of Wadi Sannur Cavern (South, near the study area) as lateral facies change coeval to the Beni-Suef Formation of Bishay (1966).

The measured lithostratigraphic section of the Sannur Formation in the present area (Fig. 3b) is located at the intersection of Latitude 28° 48.572ʹN and Longitude 31° 24.164ʹE near the intersection of W. Sannur and W. Mulailah, in the southern sector of the study area. Lithologically, it is 29 m thick. It is mainly composed of hard, ferruginous, cavernous (Fig. 4e), nodular (Fig. 4f), and nummulitic limestone at the base. It is bioturbated chalky limestone at the top (Fig. 4g). This rock unit includes a new occurrence of the Egyptian alabaster veinlets (Fig. 4h), which shows the banding structure; honey color, transparent bands alternative with milky white, opaque bands.

The term “Amber” is also usually used to describe the color of the transparent bands (Abed El Tawab and Askalany 2011). The bands occur in the alabaster rocks with variable thicknesses (Blasy 2014; El Mezayen et al. 2020; Amin et al. 2022), and taking place in these rocks may be the physiochemical processes (Craig and Vaughan 1981). Some samples of these alabaster rocks were selected and polished (Fig. 4i). Moreover, the alabaster rocks of this location are very similar to those in the W. Mawathil area, around the Sannur Cave. The Sannur Formation is dated to the Bartonian of the late Middle Eocene age (Soliman and Korany 1980).

Maadi formation (Late Eocene)

This term was first introduced by Said (1962, 1971) to describe a sequence (90 m thick) exposed east of the Maadi area as a unit of the upper Mokattam succession of Zittel (1883). These sediments are composed mainly of clastic with minor carbonate. In the study area, the Maadi Formation conformably overlies the Sannur Formation and unconformably underlies the Oligocene rocks at the type section. Mansour (1982) studied 88 m thick Maadi Formation at Gebel Homret Shaiboun, mainly represented from base to top by; shale, sandstone, and sandy limestone capped by hard, and sometimes fossiliferous limestone.

Lithologically, this rock unit is made up of a sequence mainly of siliciclastics, and carbonate sediments (Fig. 3c), including a little iron concretion, and is rich in macrofossil shells. The lithostratigraphic section of the Maadi Formation was measured west of the study area at the intersection of Latitude 28° 47.687ʹN and Longitude 31° 18.054ʹE. It attains of 36 m thick, mainly composed of laminated, sandy shale at the base, followed by shale with lenses of sand (Fig. 4j), sandy limestone, marly limestone, and highly burrowed, bioturbated, fossiliferous limestone at the top (Fig. 4k). This formation is covered by the Oligocene gravel (Fig. 4l). This formation is dated to the Late Eocene (Said 1962 1971; Bishay 1966; Barakat et al. 1971; Mansour and Philobbos 1983; Abu El-Ghar 2007; Saber and Salama 2017; Salama et al. 2021).

Oligocene succession

The Oligocene succession disconformably overlies the Late Eocene Maadi Formation. The distribution of the Oligocene sediments was judged to a large-scale extent by the volcanic and tectonic activity that influenced the Red Sea regions during the Oligocene age (Strougo 1985). In the study area, the exposed Oligocene rocks are represented by basaltic rocks as plugs.

The basaltic plugs

The Oligocene basaltic plugs were extruded and exposed within the El Fashn Formation, at the intersection of W. Sannur and W. Al Maghaza in the considered area (Fig. 5a, b). These rocks represent the first existence of igneous rocks in the Beni-Suef Governorate. Abou-Khadrah, et al. (1993) described the basalt flows as a top of two units consisting of the Gebel Ahmar Formation of Shukri (1954), while the basal unit consists mainly of dark brown, sands and gravels, and usually silicified wood trunks.

Fig. 5
figure 5figure 5

Field photographs showing as follows: a First Oligocene basaltic plug, looking S, b panoramic views for the first two basaltic plugs (arrows), looking NE, c third basaltic plug, looking SE, d basalt rocks alternating with argillaceous materials, looking NW, e extension of the basalts on the floor, looking SE, f extension of the fifth basaltic plug (arrows) below the El Fashn Formation, looking E, g new excavation within the Sannur Formation, looking SE, h Hematitic, cavernous, and alabaster veinlet (arrows) associated the recrystallized limestone, looking NE, I small and large vugs filled with alabaster within the recrystallized limestone rock (arrows), looking NE, j Paleokarst profile in the study area, looking N, and k Nummulite bank within the Sannur Formation

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Five basaltic plugs are recorded and studied in a fault zone in the present area; it is coeval with Widan Al Faras basalt (20–35 m thick) in the Western Desert at Fayum region (Gingerich 1992) which overlies the Qatrani Formation of Beadnell (1905). The unconformity surface distinguishes the Qatrani Formation from the basalts of Widan El-Faras (Vondra 1974; Nassar et al. 2022).

The first basaltic plug is located at the intersection of Latitude 28° 45.518ʹN and Longitude 31° 24.619ʹE. It is composed of a 9 m thickness of grayish-black, aphanitic basalt (Fig. 5a). Topographically, it is low and shows a strongly deformed and weathered surface. The second one is located at the intersection of Latitude 28° 45.447ʹN and Longitude 31° 24.698ʹE. It is located east of the first one about 200 m distance and reaches 7 m thick (Fig. 5b). The third one is located at the intersection of Latitude 28° 44. 745ʹN and Longitude 31° 25.093ʹE, and the maximum thickness of the basaltic plugs recorded at this location is about 20 m thick (Fig. 5c). The fourth one is located at the intersection of Latitude 28° 44.503ʹ N and Longitude 31° 25.302vE. It attains 15 m thick, grayish-black in color, and has an aplanatic character. It is enriched with argillaceous materials due to the alteration of the mafic minerals (Fig. 5d). The fifth one shows a weathered surface and its extension on the floor reaches below the scarp of the El Fashn Formation (Fig. 5e, f).

These basaltic plugs extrude the surrounding country rocks. Hence, it is noticed that the limestone is slightly metamorphosed and reached semi-marbleized. Mafic minerals such as, hematite was introduced into the surrounding limestones, due to the thermal effect of the basaltic extrusions. The basaltic plugs were accompanied by hydrothermal solutions of meteoric origin that affected the Eocene rocks and caused the formation of karst, which resulted in the formation of the “Egyptian alabaster”. The Egyptian alabaster rocks were formed in two phases; the first one is the dissolving of the Eocene limestone sequences by the hydrothermal solution that came out due to the river Nile fault (Klemm and Klemm 2001) and the rifting of the Red Sea, which was related to the Oligocene age (Harrell et al. 2007). In addition, the meteoric water during the rainy Pleistocene age. Then, the second one is the re-deposited carbonates in the form of Egyptian alabaster (Amin et al. 2022).

A new occurrence of the alabaster veinlets is recorded as a result of karst processes, it is located at the intersection of W. Sannur and W. Al Maghaza in a new excavation location (Fig. 5g) in the same fault zone, unconformably underlies the Oligocene basalts. This shows the role played by the hydrothermal solutions in the formation of the Egyptian alabaster rocks. In this location, the limestone is slightly metamorphosed and rich in hematitic vugs filled with alabaster veins, this feature represents the karst processes (Fig. 5h, i).

A typical paleokarst profile recorded in the mentioned excavation consists of cavernous, recrystallized, and nummulitic limestone which is very similar to that preserved around the protectorate area of W. Sannur Cave (Fig. 5j). This cave represents Egypt’s most significant and beautiful cave is known to be potentially a World Heritage Site (Halliday 2002b, 2003, 2004; Sallam 2020). It was discovered in one of the Egyptian alabaster quarries (No. 54) within Eocene limestone (Amin et al. 2022) and has been quarried since Pharaonic time (Halliday 2004). On the other hand, recrystallized limestone is often associated with Egyptian alabaster rocks (Philip et al. 1991; Amin et al. 2021). Nummulite bank is recorded at this site (Fig. 5k), these nummulitic limestones of the Middle Eocene age are a suitable environment for the formation of the Egyptian alabaster (El-Hinnawi and Loukina1971; Boukhary and Abelmalik1983; Philip et al. 1991; Rifai 2007; Yehia 2010; El Mezayen et al. 2020; Amin 2022). However, this location needs drilling and excavation to expose more economical alabaster rocks.

Field observations indicate that the Eocene sedimentary rocks are extruded by the basaltic plugs, which confirms that the basalts are younger than the Eocene carbonate more likely of Oligocene age. The thermal effect of the basaltic magma on the Eocene carbonate resulted in the formation of the Egyptian alabaster.

Structure

The Eocene sediments represent a transitional period between two various environmental tectonic conditions; the first in the Late Cretaceous which showed paleotectonic variation due to the plate tectonic movement, and the second one is for those of the Oligocene–Miocene period during which the tectonism was accompanied by volcanic activity (Mohammad et. al. 1992). The study area was uplifted during the Late Eocene–Oligocene regression (Philip et al. 1991). It is influenced by numerous faults mainly of normal types cutting through the different rock units. The most of wadis across the area are major fault zones.

In the far southeast of the study area, the major fault striking N43°W was measured at the intersection of the W. Sannur and W. Al Maghaza, at the contact between the El Fashn Formation and five Oligocene basaltic plugs (Fig. 6a). The extension of this fault attains more than 2.5 km in length and about 80–90 m in width. In this fault zone, the El Fashn and Sannur formations of the Middle Eocene age show the karst features. All the basaltic plugs are striking N43°W in the study area. The NW–SE faults trend extends in the area under study and southward in the W. Mawathil area shown as sharp cut straight faults and major fractures, particularly, at the landscape of the Wadi Sannur Cave. It is parallel to the Red Sea trend. In addition, according to Badawy and Abdel Fattah (2002), these faults trend is largely affecting the topography of the eastern part of the river Nile. The measured faults show slickensides on major fault planes (Fig. 6b). This fault striking N45° W is characterized by a downthrown toward SW with a plane dip of 65° toward SW. The fault plane is distinguished by manganese oxides (Fig. 6b) and enriched with Nummulites (Fig. 6c).

Fig. 6
figure 6

Field photographs showing: a Major normal fault striking N43°W, looking, NE, b Fault shows slicken-sides on major fault planes (red arrow) distinguished by manganese oxides (white arrow) looking, NW, and c The enrichment of Nummulites on the fault plane surface

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The cavernous and veinlets of alabaster within the recrystallized limestone are recorded in the Sannur Formation along this fault zone (Fig. 5h–j), which indicates that the faults of the NW–SE trend play an important role in the origin of the Egyptian alabaster (Philip et al. 1991) and Amin et al. (2022). According to Abou Elenean and Hussein (2008), this faults trend is also, pre-existing as a result of span deformation from the northern rifts of the Gulf of Suez and Red Sea.

Two mesoscopic faults striking N15°E are recorded in the upper part of the Sannur Formation forming a graben system on a small scale (Fig. 7a). In addition, an over-turned fold of alabaster rocks is frequently recorded (Fig. 7b), and the fold axis is striking N65°E. It is encountered with mesoscopic fault striking N15°E. These types of folds in alabaster rocks are common.

Fig. 7
figure 7

Field photographs showing as follows: a Mesoscopic faults (arrows), associated with shearing within the Sannur Formation, looking SE and b over-turned fold (arrows) consisting of the hematitized alabaster rocks, looking E

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Petrography

Nineteen thin sections were prepared and microscopically examined for their textures, mineral composition, and diagenetic processes of the studied rock samples of the Eocene and Oligocene. The microfacies type of the studied carbonate rocks has been classified according to Dunham’s (1962) scheme based on the allochemical and depositional textures. A petrographic examination of the studied rock samples showed the existence of numerous microfacies types. The integrated analysis of lithofacies, microfacies, and biofacies is considered to recognize and interpret the depositional environments of the studied formations.

Petrography of the Eocene rock successions

Ten microfacies types (F) are recognized in the Eocene carbonate rocks, as represented by the following:

Microfacies associations of the El Fashn Formation

Four microfacies types are recognized in this rock unit, as represented by the following:

Ferruginous bioclastic wackstone (F1): This microfacies type is recorded at the lower part of the El Fashn Formation. It is composed of whitish-yellow, fine-grained, burrowed, ferruginated nodules, and medium-hard limestone. Microscopically, the allochems of this microfacies compose about 15–25% of this rock and are represented by recrystallized Nummulite in micrite (Fig. 8a). The allochems are partially recrystallized microcrystalline calcite and show a micritized envelope. The iron oxides (hematite) are dispersed in the micrite matrix (cryptocrystalline calcite) (Fig. 8b).

Fig. 8
figure 8

Photomicrographs showing the microfacies types of the El Fashn Formation as follows: a Recrystallized Nummulite in micrite “F1”, b iron oxides (hematite) are spreading in micrite, c Nummulite shell fragments (arrow) are filled with recrystallized fibrous calcite “F2”, d Chambers of gastropod fossils in microspar calcite with quartz grains (arrows), e chambers of foraminifera shell is filled with microsparite calcite “F3”, f iron oxides (arrow) are a substitution for the shell skeletal “F3”, g clasts of amorphous silica are observed within crypto/microcrystalline calcite cement in “F4” (arrow), h and i silica and iron oxides are substitute for some the chambers of shell fragments of Nummulite (F4), and j recrystallized cacite within micrite are enveloped by phosphate pellets and recrystallized into macrosparite “F4”, XPL

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Interpretation: The low density of Nummulites permits the shells to accumulate in a low-relief shelf by storm action (Jorry et al. 2006). The association of the quartz grains with Nummulites may indicate that these siliciclastics inputs in the Nummulites banks derived from the surrounding land area (Saber and Salama 2017). On the other hand, the enrichment of lime wackstone (micritic matrix) and restricted faunal content represent a lower-energy, quite open marine depositional setting and outer shelf ramp environments beneath fair-weather wave base with low circulation conditions.

Nummulitic grainstone (F2): This microfacies type is observed in the middle part of the El Fashn Formation. It is composed of yellowish-white, fine-grained, burrowed, sandy, and marly limestone. Microscopically, the allochems of this microfacies up to 40% of this rock and are represented by nummulite shell fragments (Fig. 8c). The Chambers of gastropod fossils in microspar calcite; some of these chambers are substituted by hematite. Quartz grains are associated with shell fragments in the calcite matrix (Fig. 8d).

Interpretation: The foraminiferal shells often consist low/high-Mg calcite, rarely aragonite (Tucker 2001). The enrichment of larger foraminifera (Nummulites) represents moderate to high-energy, shallow warm, and open marine depositional environment. The Nummulites accumulate between storm wave base and fair weather with well-oxygenated waters (Romero et al. 2002; Jorry et al. 2006). According to (Aigner 1982), the Nummulites were considered one of the carbonate-producing benthos in the Eocene carbonate, then they were deposited to form bioherms or banks.

Foraminiferal bioclastic grainstone (F3): This microfacies type also is recorded in the middle part of the El Fashn Formation. It is composed of grayish brown, highly jointed, and slightly metamorphosed with some scattered black and pale brown spots, and hard limestone. Microscopically, this microfacies is made up of microcrystalline calcite (sparite). Allochems of this microfacies are represented by foraminiferal tests forming 50–60% of this microfacies (Fig. 8e, f). Iron oxides are a substitution for the shell skeletal. The chambers of shell fragments are filled with silica grains. The foraminiferal shells are recrystallized into microsparite and embedded in sparite cement.

Interpretation: This microfacies is distinguished by the accumulation of autochthonous foraminiferal fragments as indicated by the good preservation of the foraminiferal shells that reflect high energy conditions. It may be deposited in a shallow restricted marine platform that is continuously submitted to short storm events. In general, the bioclastic, including foraminifera, as well as the grainstone matrix reflects the moderate to high-energy shallow subtidal environment.

Sandy peloidal bioclastic grainstone (F4): This microfacies type is recorded at the top of the El Fashn Formation. It is composed of brown, fine-grained, flint nodules, and hard limestone. Microscopically, the allochems of this microfacies up to 60% of this rock and are mainly represented by bivalve, foraminiferal shell fragments, and peloids (Fig. 8g–j). Clasts of amorphous silica are observed with recrystallized calcite (Fig. 8g). Silicification and iron substitute for some chambers of shell fragments of Nummulite are recorded in this microfacies as a result of the diagenesis processes (Fig. 8h, i) and the later filled with sparite cement.

Quartz grains are mainly in medium to fine sand size forming up to 15% of the rock. The shell fragments are recrystallized into macrocrystalline calcite (sparite). These shell fragments are enveloped by Phosphate pellets (Fig. 8j). The allochems are embedded in microcrystalline calcite and occasionally, drusy calcite spar.

Interpretation: The bioclastic including bivalve and foraminifera fragments, as well as the grainstone fabric that is associated with high ratios of quartz grains reflects the moderate to high-energy, shallow subtidal environment with siliciclastics influx.

Microfacies associations of the Sannur Formation.

Three microfacies types are recognized in this rock unit, as represented by the following:

Molluscan bioclastic packstone (F5): This microfacies type is detected at the base of the Sannur Formation. It is composed of various colors; yellowish, grayish-white to pink color, compact, hard, and fossiliferous limestone. Microscopically, there is variation in the size of the allochems of this microfacies. The allochems of this microfacies compose about 65–70% of this rock and are represented mainly by bivalve, echinoid, Molluscan, and foraminifera shell fragments, in addition to little siliciclastics (Fig. 9a–c).

Fig. 9
figure 9

Photomicrographs showing microfacies types of the Sannur Formation as follows: a Bivalve shell fragments are recrystallized into microsparite within micrite cement, “F5”, b Molluscan shell fragments show an isopachous type (arrow), c Iron oxides are substitutions for shell fragments (blue arrow) with some siliciclastic grains (yellow arrow), d Coarse- grained quartz (arrows) embedded in the grainstone texture, “F6”, and e Microsparite to sparite including a large vug, iron oxides (red circle), and glauconitic pellets (White arrow) associated with recrystallized shell fragments (black arrow), and f Elongate crystals of calcite formed stalactitic texture showing interlocked coarse calcite crystals in the alabaster facies (arrow) “F7”, XPL

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Deformed shell fossils are recrystallized to microspar embedded in micrite (Fig. 9a). Most of the allochems are recrystallized into microcrystalline calcite. It shows the isopachous structure on the outer edges of some bioclastic grains (Fig. 9b). Iron oxides (hematite) are substitutions for some shell fragments. Quartz grains are associated with test fragments in the micrite matrix (Fig. 9c). These bioclastic grains are embedded in a cryptocrystalline calcite matrix (micrite).

Interpretation: The enrichment of micritic matrix and bioclastic diversity reveals the quit marine or low-energy, shallow subtidal depositional environment. The bivalve fossils represent a large group with other species occupying most brackish, marine, and freshwater environments. It has been a significant contributor to marine carbonate sediments, especially since the Tertiary (Tucker 2001).

Sandy ferruginous bioclastic grainstone (F6): This microfacies type is detected in the middle and upper parts of the Sannur Formation. It is composed of reddish-brown to rose, hematitic, cavernous, a network of calcite along the joints, hard, nodular, and cherty limestone. Microscopically, the microfacies made up of poorly-sorted, rounded to subrounded, fine to very coarse quartz grains(chert), and iron oxides are replacing calcite grains (Fig. 9d). Most quartz grains are monocrystalline type, which may represent an igneous source (Abou El-Anwar et al. 2018). The bioclasts in this microfacies up to 20% of this rock and are embedded in microsparite calcite. It is graded from microsparite to sparite cement. Clasts of quartz, iron oxides, and glauconitic pellets are observed (Fig. 9e).

Interpretation: The enrichment of detrital influx, and these grainstones reflect as having been deposited by storm-wave erosion and in shallow-marine environments (Flügel 2004).

Alabaster facies (F7): The Egyptian alabaster rocks in the present area are similar to those found south of the study area at the W. Mawathil area, it is shown as veins in the form karst-system within the lower part of Sannur Formationin in the present area. It consists of honey, whitish-brown, cavernous, relatively hard, and banded alabaster. Microscopically, it is made up mainly of sparry calcite cement. This cement is formed as porous-filling calcite, it shows stalactitic texture showing interlocked coarse calcite crystals (Fig. 9f) due to recrystallization processes. These types of diagenetic calcites show a gradation of the growth between the Egyptian alabaster bands and those recrystallized in micrite to microsparite cement (El Mezayen et al. 2020). This microfacies shows that the precipitation of the Egyptian alabaster is related to hydrothermal waters within open cavities or karstic systems in the Eocene carbonate (Amin et al. 2022).

Microfacies associations of the maadi formation

Three microfacies types are recognized in this formation, as represented by the following:

Sandstone facies (F8): This microfacies type is detected at the lower part of the Maadi Formation. It is composed of brownish-yellow, sandy, and medium-hard limestone. Petrographically, phosphatic materials and iron oxides are substituted for the shell, including clasts of fine-grained quartz grains (Fig. 10a), surrounded by poorly sorted, surrounded to subangular, and fine to coarse quartz grains. The bioclasts are embedded in ferruginous macrosparite calcite.

Fig. 10
figure 10

Photomicrographs showing microfacies types of the Maadi Formation as follows: a Phosphatic materials and iron oxides substituted for the shell fragments in sandstone facies “F8”, b Muscovite grain (arrow) surrounded by iron oxides and coarse-grained quartz “F9”, c Chlorite grain (arrow) surrounding by coarse-grained quartz, d Muscovite crystals (arrow) are associated with surrounded to elongated, coarse-grained quartz, e Bee-like shape; iron oxides substitution for calcite cement and bone fragments (arrows) with isopachouse calcite “F10”, f walls of the shell fragments are micritized and enriched in phosphatic composition (arrows) and iron oxides, XPL

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Interpretation: The enrichment of detrital influx reflects a shallow subtidal environment. The sandstone facies with a few marine fossils may indicate that the environment is near shore (Flügel 2004). However, the low diversity of fauna in this facies may reflect an inappropriate ecological condition in a limited environment. Thus, it is interpreted to have been deposited in the environment of the sand flat (Saber and Salama 2017).

Quartz arenite microfacies (F9): This microfacies type also is detected in the lower part of the Maadi Formation. It is composed of brownish yellow, grayish white, hard, burrowed, black batches, veinlets, and sandy limestone. Petrographically, this microfacies consist mainly of quartz arenite and iron oxides which are cemented by sparite. Biotite or muscovite grain is observed, and surrounded by iron oxides, angular to subangular, and coarse quartz grains (Fig. 10b). In addition, chlorite grain surrounded by medium to coarse quartz grain is recorded (Fig. 10c). In addition, muscovite crystals are surrounded by elongated coarse monocrystalline quartz grains (Fig. 10d). These clastics are embedded in hematitic calcite cement.

Interpretation: The enrichment of detrital influx and this texture reflect an intertidal depositional environment. However, the low variety of the bioclastics indicates that this microfacies have been deposited in of sand-flat environment.

Sandy bioclastic wackestone (F10): This microfacies type is recorded in the middle part of the Maadi Formation. It is composed of smoky white, brownish gray in color, flaky, sandy, enriched in fossil skeletal fragments, bioturbated, and highly fossiliferous limestone. Petrographically, the allochems in this microfacies up to 75% of this rock and are represented mainly by bivalve shell fragments. It is enriched in mainly moderately sorted, rounded to subrounded, and medium quartz grains (Fig. 10e). Most of these allochems are filled by sparry calcite cement, and the walls of shell fragments are micritized and enriched in phosphatic composition forming a micrite envelope (Fig. 10f). All these components are embedded in the micrite matrix; silica is substituted for the calcite cement (silicification). In addition, iron oxide patches also substitutions for the bone fragments and the Bee-like shape bioclast.

Interpretation: The micrite envelope of the bioclastics is produced mainly by endolithic bacteria that bore into the bioclastic fragments. Then, following vacation, they are filled with micrite (Tucker 2001). The enrichment of calcite cement and the macrofaunal content as bivalve reflect moderate-energy conditions, a quite shallow open marine platform environment.

Diagenetic processes

Diagenetic processes are encountered on most sedimentary rocks. They relate to the interaction between one mineral and another or between minerals and fluids within sedimentary rocks after deposition. The diagenesis of carbonate rocks involves various processes and occurs in meteoric and near-surface marine environments, which also affects the porosity and permeability of deposits. It mostly includes carbonate minerals, calcite, aragonite, and dolomite. Other minerals such as quartz, clays, feldspar, evaporates, phosphates, and iron oxides are also perhaps involved (Tucker 2001).

In general, diagenesis means any process affecting sediments after deposition (El-Hifnawi et al. 2010). The petrographical studies of the studied samples revealed various processes such as cementation, neomorphism (recrystallization), dissolution, phosphatization, and silicification. These processes affected the deposits due to physicochemical processes (El Mezayen et al. 2020).

Cementation

This is the main diagenetic process producing compact and solid rocks. The microscopic investigation of the studied samples shows that the cement materials are calcium carbonate in different forms as; cryptocrystalline calcite “micrite”, microcrystalline calcite “microsparite”, and crystalline sparry calcite. The allochems were cemented by micrite and sparite, indicating that this is what happens when the porous fluid is supersaturated relative to the cement phase (Tucker and Wright 1990). The cement is represented by aragonite and high Mg–calcite in shallow marine environments (Pettijohn 1975; Longman 1980; Tucker 1981; McLane 1995). On the other hand, it represents the low-magnesium calcite in meteoric conditions (Tucker 1981; Blatt 1992; McLane 1995), which may show as coarse to very coarse crystals of calcite (Folk and Land 1975; Al-Hashimi 1977; Abu Al-Ghar and Hussein 2005).

In the study area, the source of cement is perhaps endogenic (the site of deposition), or exogenic (outside the depositional basin). Silica and Iron oxide contents are recorded in the microfacies type as cement. Micrite represents the early diagenetic marine origin (Longman 1980). The post-depositional diagenetic alterations took place in marine and freshwater phreatic conditions (Harris et al. 1997). Early cement can be seen as isopachous cement (a thin edge of equigranular calcite crystals) around the allochem grains (Fig. 10f). In marine diagenetic conditions, isopachous cement predominates (Sam Boggs 2009). Calcite is the main pore-filling component of late cement, which can be seen in a variety of textures such as drusy calcite cement (Fig. 8c). Micrite cement is recorded in the studied carbonate samples in wackstone, wackestone, and packstone facies with peloidal structures.

Neomorphism

The term “Neomorphism” is used by Folk (1965) to describe all transformations of one mineral into itself (recrystallization) or a polymorph (inversion), such as aragonite to calcite without any chemical change (calcitization) and causing the enlargement of the grains (aggrading neomorphoism). In addition, it refers to recrystallization and replacement. Recrystallization is very common and plays an important role in the diageneses in the Eocene rocks outcropping in the present area.

In general, the term “recrystallization” is used when a significant alteration in the size and/or shape of calcite crystals is seen (Land and Moore 1979). The aragonitic shell fragments may dissolve to produce full or partial molds which can be filled with cement, or they may be changed into low-Mg calcite during recrystallization by the simultaneous volume-per-volume dissolution of aragonite and precipitation of calcite. In the studied samples, the lime mud (wackstone matrix) shows partial recrystallization to sparry calcite, and alsosome allochems walls had transformed into crystalline sparry calcite as a polymorph neomorphism (Fig. 10e).

Foraminifera is mainly represented by Nummulites (Fig. 8c, i) and miliolids (Fig. 8e) in the present study which is segmentable to aggrading neomorphism. All nummulites in the Middle and Late Eocene succession have been subjected to either partial or complete recrystallization into microspar and pseudospar which seem to be distributed through the matrix as a porphyroid crystallization (Gharieb 1998). It shows a micrite envelope which may refer to deposition in a shallow water condition at an early event of diagenesis (Bathurst 1975).

Silicification

The diagenetic process of silicification involves the replacement of carbonate rocks with silica. Cherts are produced during the silicification process, and parts of the limestone are replaced only in specific places. The El Fashn and Sannur formations of the examined areas contain chert in several of their strata. It is made up of silica minerals; microquartz (quartz grains of < 20 μm), mega quartz, chalcedonic quartz, and amorphous silica (Figs. 8g, 9d, and 10), with calcite and/or iron oxides as minor impurities in some occurrences. The micro/ megaquartz is exhibited in the studied thin sections in which it replaces the matrix (Fig. 10b–f) or the interior of the test fragments (Fig. 10a). The mega quartz is defined as a grain with a diameter greater than 20 μm (Folk and Pittman 1971).

Dissolution

This process is recorded in the studied carbonate samples of the Sanur Formation where some parts of calcite cement were leached. The calcite cement has been dissolved forming large vugs which are perhaps formed by dissolution processes. Most of the Sannur Formation around the Cavern of Wadi Sannur exhibits cavernous and vuggy textures due to the dissolution processes (Amin et al. 2022).

Ferrugination

Ferrugination is accomplished by substituting carbonates with iron oxides (Mansour and Kenawy 1977; El Gindy et al. 1998). In the study area, the ferrugination is highly marked in the studied three rock units with varying degrees. It stains the carbonates causing a reddish-brown to dark-red color in the studied rocks. Microscopically, the ferrugination process is represented as opaque patches and cement of iron oxide (hematite) with dark red colors. Iron oxides replace the bioclastic particles (Figs. 8c, f, h, i and 10a, e, f) or calcite matrix (Figs. 8b, 9c, d, f, and 10b–d) which is the most very common variety that is affected by the ferrugination in the studied rock samples. In general, ferrugination may occur due to the oxidation of iron-bearing minerals such as glauconite during weathering (Mansour and Kenaway, 1977; El Gindy et al. 1998; Lee 2000).

In our study, the present authors believe that these iron oxides in most rock units of the studied area are related to the hydrothermal solutions associated with the volcanic activity in the present area. These hydrothermal solutions percolate passing through the weak spots in the rocks’ planes and causing the production of new minerals by the substitutions and precipitation process (Abd El Aal and El Gindy 1989; Kamel et al. 1998; Abdel Fattah 1998; Sturesson et al. 1999). It is also strongly affecting the investigated area and its neighbors from the Eocene limestone causing the karst process and its products such as Egyptian alabaster and recrystallized limestone which is commercially known as bucchino rock (Gharieb 1998; Amin et al. 2022).

Phosphatization (Phosphatic fossilization)

This process is clearly shown to a significant extent among the studied thin sections. The phosphate may be produced in large quantities, either by being replaced by calcium carbonate in the marine environment or by decaying the organism’s tissues. The CaCO3 is less likely to precipitate when the PH is low, which makes it easier for phosphate to be deposited, especially in the absence of oxygen in the decaying organism (Briggs et al. 1994). The phosphatization process is distributed on the outer and inner parts of the allochems (Figs. 8j and 10a, e, f).

The microscopic investigations revealed that the mineral composition of late Middle–Late Eocene rocks is mainly carbonate and siliciclastic sediments. The predominant mineral in the El Fashn Formation and the Sannur Formation is mainly calcite, while the Maadi Formation contains more abundant siliciclastics (quartz) besides the calcite mineral. In general, calcite is the predominant mineral in these Middle–Late Eocene sediments with less abundant hematite, as well as quartz, gypsum, muscovite, glauconite, phosphate, chlorite, and clay minerals.

Petrography of the Oligocene basaltic plugs

Five Oligocene basaltic plugs recorded in the present area are extruded within the El Fashn Formation at the intersection of W. Sannur and W. Al Maghaza in the considered area. These basalt plugs are composed of grayish-black, aphanitic basalt (Fig. 5a). They are strongly deformed, weathered surfaces in the field and of low topography; ranging from 7 to 20 m above Wadi level. The extension of the five basaltic plugs attains more than 2.5 km in length within the fault zone at intervals of 200–300 m between each of them. Five samples were taken to represent the five basaltic plugs and then, prepared and studied microscopically. The present basalt is mainly composed of plagioclase, pyroxene, and olivine as essential minerals. While the accessory minerals are represented by hematite and sericite.

Plagioclase

Plagioclase is the most abundant mineral forming the basaltic plugs (∼20%), they show euhedral to subhedral phenocrysts more likely prismatic and elongated attaining 400 µ size (Fig. 11a). Plagioclase aggregates are often zoned but also twined (Fig. 11b), their surfaces are covered with very fine sericite flakes, indicating that they are sericitized. Plagioclase can also occur as microlaths forming the groundmass.

Fig. 11
figure 11

Photomicrograph showing: a basalt porphyry with phenocrysts crystal of plagioclase “Pl” forming poikilitic texture, b Pyroxene “Px” showing pseudomorphs grain, contains zoned, associated elongated, and anhedral crystals of olivine, c oval crystal of the olivine “Ol” surrounded by scattered porphyry with phenocrysts of plagioclase in cryptocrystalline groundmass (aphanitic textures), and d olivine is partially altered to iddingsite in the groundmass

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Pyroxene

Pyroxene mineral forms about 15% in the basaltic plugs. It shows of euhedral form, with phenocrysts attaining 700µ length and 300µ (Fig. 11a). Pyroxene has more modest relief and frequently exhibits a cleavage. Some pyroxenes belong to the orthorhombic system (orthopyroxenes), others to the monoclinic system (clinopyroxenes).

Olivine

This mineral represents the early crystallization in the rock and is usually serpentinized along microcracks. It reaches (∼5%), and has various forms; oval, subhedral, and phenocrysts attaining 600µ size, and up to 5% of the basaltic plugs embedded in porphyritic textures (Fig. 11c). The iron oxides occur as a dark brown mixture, opaque, and typically cryptocrystalline distributed accompanying the other mineral constituents. The olivine minerals are partly altered to iddingsite which occurs as subhedral grains attaining 800µ length and 450µ width (Fig. 11d). It occurs as aggregated between the plagioclase laths in the intergranular gaps.

Geochemistry

The basaltic plugs are similar in all the surficial features and the mode of extrusions thus, a sample of these basaltic plugs (representing the second basaltic plugs) was taken and analyzed by XRF analysis. The results of the chemical analyses are listed in Table 1. The SiO2 versus (Na2O + K2O) classification diagram TAS of Le Bas et al. (1986) shows that the sample falls in the field of basalt (Fig. 12a). Nb/Y versus SiO2 and the Zr/TiO2 versus Nb/Y classification diagrams of Winchester and Floyd (1977) are used and show the sample falling in the field of alkali basalt (Fig. 12b, c). The alkali basalts are produced at higher depths and lower melting degrees than the tholeiitic type (Hirschmann et al. 1998; Johnson et al. 2005).

Table 1 Major oxides (%) and trace elements (in ppm) concentrations of the Oligocene basaltic plugs of the studied area
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Fig. 12
figure 12

Showing as follows: a SiO2 and (Na2O + K2O) classification diagram TAS (Le Bas et al. 1986) shows that the sample is falling in the field of basalt; and b, c Nb/Y versus SiO2, and Zr/TiO2 versus Nb/Y, respectively classification diagrams of (Winchester & Floyd 1977) show the sample falling in the field of alkali basalt

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Thermal behavior (TGA and DSC)

The Egyptian alabaster of the Sannur Formation is exposed to a thermal treatment using TGA and DSC, as shown in Fig. 13a. The TGA curve shows two stages of mass loss. The first stage started with an initial temperature (Ti) of 350°C and ended at a final temperature (Tf) of 390 °C, with an inflection temperature (Tin) of 420 °C and a mass loss of 2.3%. This is attributed to the escape of moisture (H2O) as a result of the dehydroxylation of the brucite mineral Mg (OH)2 (Bruni et al. 1998; Suneetha et al. 2007; Blasy 2007). The brucite mineral is considered a component of the Egyptian alabaster besides calcite and hydro-magnesite minerals (Blasy 2007). The inflection temperature represents the maximum rate of removing the bound water molecules. According to Young et al. (2019), strongly bound water escapes below 400 °C. The internal moisture continuously escapes from the carbonates and thermal stress appears as increasing the temperature, increasing microcracks, and changing the internal composition of the rock.

Fig. 13
figure 13

Showing as follows: a TG and DSC curves of Egyptian alabaster, b TG and DSC curves of recrystallized limestone “Bucchino” (pure calcite, CaCO3), and c TG curve of basaltic rock in the study area

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Ti, Tin, and Tf of the second stage are 695 °C, 870 °C, and 965 °C, respectively, with a mass loss of 38.365% due to the decarbonization of the calcite (CaCO3) into CaO and CO2. Calcite starts to decompose into CaO and CO2 at 700 °C and completely decomposes at 870 °C, while the dolomite CaMg(CO3)2 decomposes into MgO, CO2, and CaCO3 at about 750 °C (Engler et al. 1989), and its internal porosity increased (Young et al. 2019; Tufail et al. 2017; Zhao and Chen 2011). The gradual decomposition of calcite began at a temperature of about 700 °C. Once the temperature reached 750 °C, the decomposition began to occur quickly (Karunadasa et al. 2019).

In general, when the carbonate rocks (calcite and dolomite) are exposed to a temperature above 600°C, they decompose to (CaO) and (CaO& MgO), respectively (Keppert et al. 2017). The DSC curve displayed two endothermic peaks (Fig. 13a) corresponding to the second decomposition stage in the TG curve, suggesting that due to dolomitization processes affected the Egyptian alabaster. A few dolomitized calcite crystals are recorded in the alabaster facies, as well as the contents of MgO are higher in the alabaster of W. Mawathil than in the bucchino rocks “recrystallized limestone” (Amin et al. 2022).

Figure 13b shows the TGA and DSC curves of the recrystallized limestone of the Sannur Formation. The TG curve exhibited a single-stage decomposition and is marked by one endothermic peak in the DSC curve. In this stage, the Ti starts at 700 °C, reaches Tin at 890°C, and finishes at Tf of 911 °C with a loss mass equal to 42.084% as a result of the decarbonization of calcite (CaCO3). According to El Mezayen, et al. (2020), Amin et al. (2022), and the present study, the recrystallized limestone is composed mainly of pure calcite. The maximum rate of decomposition of the present rock is consistent with Maitra et al. (2005), in which the pure calcite (Bucchino) is decomposed between 900  C and 960 °C into CaO.

The basaltic rocks do not exhibit any thermal events when exposed to thermal influences by DSC between 30 and 1000 °C. The thermogravimetry (TG) showed a mass loss of about 3.70% in two stages due to escaping moisture. The first stage started from 50–200 °C with 1.6% mass loss, and the second one (1.1% mass loss) in the range of 272–670 °C (Fig. 13c). According to Hartlieb. Et al. (2016) the thermal behavior of basalt samples shows a loss of mass at about 100 °C, in addition to about 2.5% of the original mass of the basalt was lost and two noticeable stages when the basalts were heated between 100 °C and about 1000 °C. This is attributed to the escape of remaining water within pores or mineral phases that contain water as a result of heating.

The TG results by Mostafa et al. (2004), indicate that all basalt samples contain small amounts of two different types of water. The first is the free water, which is released in the first temperature zone of the study. The second is the structurally restricted water released through the second area. A little amount of mass is lost in this temperature range (RT-900 K). In other words, these samples are regarded as dry rock samples. Geiger (1984) performed seven thermal cycles for 21 h at 550 C to study both quartzite and basalt. After just one cycle, the quartzite was broken, while there was no macroscopically noticeable damage to the basalt.

Discussion

Interpretation of the field observations

Stratigraphically, the area includes three rock units arranged from base to top; El Fashn Formation (Tem), Sannur Formation (TemSn), and Maadi Formation (Ted) in addition to the basaltic rocks. In general, the first three lithostratigraphic units represent the different stages of the Eocene age, while the basaltic rocks belong to the Oligocene. The middle and upper part of the El Fashn Formation is composed of white, hard limestone with chert bands, and nodules overlain by a grayish-white, and hard limestone bed which shows a sinkhole as a result of the effect of strong weathering with a diameter of 22 cm and depth of about 45 cm.

The Sannur Formation is mainly composed of hard, ferruginous, cavernous, nodular, and nummulitic limestone at the base. It is bioturbated chalky limestone at the top. This rock unit includes a new occurrence of the Egyptian alabaster veinlets, which shows the banding structure; honey color, transparent bands alternative with milky white, opaque bands.

The Maadi Formation is made up of a sequence mainly of siliciclastics, and carbonate sediments, including a little iron concretion, and is rich in macrofossil shells. The lithostratigraphic section of the Maadi Formation was measured west of the study area and attains 36 m thick, mainly composed of laminated, sandy shale at the base, followed by shale with lenses of sand, sandy, marly limestone, and highly burrowed, bioturbated, fossiliferous limestone at the top. This formation is covered by the Oligocene gravels.

The Oligocene succession disconformably overlies the Late Eocene Maadi Formation. The distribution of the Oligocene sediments was judged to a large-scale extent by the volcanic and tectonic activity that influenced the Red Sea regions during the Oligocene age (Strougo1985). In the study area, the exposed Oligocene rocks are represented by basaltic rocks as plugs. Five basaltic plugs are recorded and studied in a fault zone coeval with Widan Al Faras basalt in the Western Desert at Fayum region (Gingerich 1992) which overlies the Qatrani Formation of Beadnell (1905).

It is noticed that the trend of the measured fault in the present area is striking NW–SE. this trend is parallel to the alabaster quarries trend in the W. Mawathil area, which is considered the predominant trend and control in most of these quarries. In general, many faults of this trend measured in the eastern sector of Beni-Suef are mainly of normal type. The NW–SE trending graben-like basin was recorded by a seismic cube in the east Beni-Suef basin, this NW–SE structural trend is related to the Syrian Arc System (Red Sea trend), and by the subsequent subsidence of this trend, the Beni-Suef basin has been formed (Salem and Sehim 2017).

Interpretation of petrography

According to the field description and petrographical studies, the Eocene carbonates involve ten microfacies which are; Microfacies associations of the El Fashn Formation comprising; Ferruginous bioclastic wackstone (F1), Nummulitic grainstone (F2), Foraminiferal bioclastic grainstone (F3) and Sandy peloidal bioclastic grainstone (F4). Microfacies association of the Sannur Formation include; Molluscan bioclastic packstone (F5), Sandy ferruginous bioclastic grainstone (F6), and Alabaster facies (F7). Microfacies associations of the Maadi Formation comprise; Sandstone facies (F8), Quartz arenite microfacies (F9), and Sandy bioclastic wackestone (F10).

Foraminifera are biostratigraphically important zonal fossils; their appearances are often related to periods of global warming, relative drought, raised sea levels, expansion of tropical and subtropical habitats, and reduced oceanic circulation (Hallock and Glenn 1986). During such times, nutrient recycling to surface waters was dramatically reduced and organic productivity in the ocean dropped by two orders of magnitude (Bralower and Thierstein 1984). Hence, they occur in shallow tropical carbonate environments (Penney and Rocey 2004).

In general, the above-mentioned microfacies indicate that deposition of Eocene carbonates in the subtidal shallow marine environment except for microfacies of the Egyptian alabaster shows that the precipitation is related to hydrothermal waters within open cavities or karstic systems in the Eocene carbonate. In addition, the alabaster formed due to the basaltic thermal effect and related hydrothermal waters.

Interpretation of geochemistry

Basaltic plugs

According to the XRF analysis of the represented sample of basalt in the study area, it is noticed that there is enrichment in Ba and Sr, indicating that the plagioclase is calcic type. It could be labradorite, bytownite, or anorthite. The Sr content is like those in the ancient limestone which is below 500 ppm (Bathurst 1975). The second predominant elements are Zr, Cr, and Zn, which are usually associated with the basic rocks. Nb, V, Rb, and Y are less predominant in the basaltic rocks, while they enrich the acidic rocks. The lack of the Zr content in basalt is due to dissolved in nearby crust minerals that come into contact with mantle-derived basalt magma (Shao, et al. 2019). In addition, the continental crust has developed, and the abundance of Cr on Earth’s surface has decreased. The lowest abundances of concentrations of compatible trace elements, such as Cr and Ni. are present in the alkali basalts, in which Cr is from 55 to 403 ppm and Ni varies from 39 to 238 ppm (Johnson et al. 2005).

Interpretation of thermal behaviors

Through the present study can conclude that the different rock types (Egyptian alabaster, recrystallized limestone, and basalt) when subjected to thermal treatment, showed high curve variations due to many factors such as their mineral and chemical compositions. Mafic rocks such as, basalt is suitable for conserving high-temperature thermal energy, because do not exhibit any thermal events when exposed to thermal influences by DSC between 30 and 1000 °C. Unlike carbonates such as, (Egyptian alabaster and recrystallized limestone) which are not suitable for maintaining high temperatures between 695 and 965 °C and decompose into CaO and CO2 due to the decarbonization of the calcite mineral.

The genesis of the Egyptian alabaster

The genesis of the formation of the Egyptian alabaster depended on dissolving the Eocene limestone along fractures by meteoric water and thermal groundwater, causing vugs or caves filled with calcium carbonate saturated solutions (the dissolved limestone) to solidify with time forming the Egyptian alabaster. These processes took place in a comparatively high-temperature range of 100–170 °C within open veins at shallower levels or the karst systems (Klemm and Klemm 1991). It is noticed that the Egyptian alabaster in the present area either in contact with basaltic plugs or in the southern W. Mawathil area is controlled by the NW fault trend. According to Amin et al. (2022), the Egyptian alabaster in Egypt was formed by the dissolving of limestone by thermal waters in the presence of meteoric water during the rainy periods (Pleistocene age), followed by, the re-precipitation of the karst products such as, “Egyptian alabaster” from calcium carbonate saturated water.

The genesis of the basaltic plugs

The petrographical and geochemical results revealed that the volcanic rocks highlighted in the study area are basalt of alkali affinity that could be related to the Red Sea rift process. These basalts, which comprise only anhydrous minerals such as plagioclase, orthopyroxene, and olivine are enriched in Ba and Sr which illustrate the early fractionation of plagioclase during the magma cooling.

Conclusion

The following conclusions arise:

  • The lithostratigraphical study shows that the study area includes three rock units arranged from base to top: El Fashn Formation, Sannur Formation, and Maadi Formation of the late Middle to Late Eocene age, underlies the Oligocene basaltic plugs which recorded east of Beni-Suef City as the new occurrence of igneous rocks.

  • The rock units of the area are controlled by numerous faults mainly of normal types, and most of the wadis crossing the area are running along major fault zones. The basaltic plugs in the study area are extruded along a major fault striking in the NW–SE direction. This fault is parallel to the fault trends controlling the alabaster quarries of the W. Mawathil area. This trend is parallel to the Red Sea graben indicating the relation between the Red Sea tectonism and Oligocene basalt, and with alabaster formation.

  • Petrographically, the Eocene deposits show ten microfacies indicating its deposition in a subtidal shallow marine environment except for microfacies of the Egyptian alabaster that show that its formation is due to the hydrothermal waters associated basaltic magma within open cavities or karstic systems in the Eocene carbonate.

  • The petrographical and geochemical results revealed that the volcanic rocks highlighted in the study area are basalt of alkali affinity that could be related to the Red Sea rift process. These basalts, which comprise only anhydrous minerals such as plagioclase, orthopyroxene, and olivine are enriched in Ba and Sr which illustrate the early fractionation of plagioclase during the magma cooling.

  • TGA analysis showed that the carbonate rocks are not suitable for maintaining high temperatures between 695 and 965 °C, unlike the basaltic rocks which conserve high-temperature thermal energy because do not exhibit any thermal events when exposed to thermal influences by DSC between 30 and 1000 °C.