Introduction

The most prominent long-term warming of the Cenozoic was reported in the Early Paleogene time (66.0–23.0 mya), including a pronounced short-term warming event, i.e., Paleocene-Eocene Thermal Maximum (PETM) occurred around 55.8 million years ago (Zachos et al. 2001; Gilmour et al. 2014). Previous studies from Spain and Egypt (e.g., Orue-Etxebarria et al. 2001; Pujalte et al. 2003; Baceta et al. 2005; Rasser et al. 2005; Scheibner et al. 2005; Scheibner and Speijer 2008) correlated the larger foraminiferal evolution to the PETM environmental changes. Due to the rising temperatures (~ 5–8 °C) during the Early Paleogene, the reef-building corals in the Tethyan domain were declined, and the larger benthic foraminifera, which show greater tolerance to higher temperatures, became expanded (Hottinger 1998; Orue-Etxebarria et al. 2001; Scheibner et al. 2005; Scheibner and Speijer 2008; Martín-Martín et al. 2020).

Larger benthic foraminifera are diverse and widely-distributed in various shallow carbonate platform environments, and are considered as excellent biostratigraphic and paleoenvironmental markers (Beavington-Penney and Racey 2004; Özgen-Erdem et al. 2007; Scheibner et al. 2007; Scheibner and Speijer 2009; Boudagher-Fadel et al. 2015; Sarkar 2019; Hadi et al. 2021). According to the studies of Hottinger (1960) and Schaub (1981) on the Early Paleogene alveolinids and nummulitids, twenty shallow benthic zones (SBZs) were documented in the Neo-Tethyan realm, and were used for high-resolution biostratigraphy in shallow-marine environments (Serra-Kiel et al. 1998, 2020; Beavington-Penney and Racey 2004; Scheibner et al. 2005; Özgen-Erdem et al. 2007; Scheibner and Speijer 2009; Drobne et al. 2011; Hadi et al. 2016; Amirshahkarami and Zebarjadi 2018).

The distribution pattern of larger foraminifera is influenced by a complex set of inter-related parameters, including temperature, light, nutrients availability, substrate, depth, hydrodynamic energy, and salinity (Hottinger 1983; Pecheux 1995; Duxbury and Duxbury 1997; Geel 2000; Scott et al. 2001; Leckie and Olson 2003; Beavington-Penney and Racey 2004; Rasser et al. 2005; Scheibner et al. 2005; Boudagher-Fadel 2008; Bassi et al. 2013; Tomassetti et al. 2016; Eder et al. 2018; Torres-Silva et al. 2019; Martín-Martín et al. 2020; El-Ayyat 2022). Nummulites banks are the most prominent larger foraminiferal accumulations, with a great distribution in the Eocene carbonate deposits along the northern and southern Tethyan margins (Racey 2001; Jorry 2004; Jorry et al. 2006; Martín-Martín et al. 2020, 2021; El-Ayyat 2022). The origin of these banks has been attributed to biological (i.e. reproduction strategies of larger foraminifera) and physical (i.e. winnowing) processes (Aigner 1982; Loucks et al. 1998; Racey 2001; Hasler 2004; Beavington-Penney et al. 2005, 2006; Jorry et al. 2006).

The study area represents the northern part of the Eastern Desert of Egypt, comprising the Northern and Southern Galala Plateaus (Fig. 1). The Paleocene–Eocene rocks in the study area were mostly deposited over a low-angle homoclinal ramp with a gentle, southward dipping (El-Ayyat 2022), represented at the Galala Plateaus by the Southern Galala Formation (Abdallah et al. 1970). The Southern Galala Formation underwent several studies from stratigraphical, paleontological, and paleoenvironmental aspects (e.g., Kuss et al. 2000; Ismail et al. 2007; Morsi and Scheibner 2009; Höntzsch et al. 2011a; El-Ayyat and Obaidalla 2013). Nevertheless, the paleonenvironmental reconstruction of the study area was based only on facies analysis (Scheibner et al. 2001, 2003; Höntzsch et al. 2011b; El-Ayyat and Obaidalla 2013), overlooking the paleoenvironmental and paleoecological significance of the included larger foraminifera. The larger foraminiferal content of the Southern Galala Formation has been previously reported (Aigner 1982; Scheibner et al. 2005; Scheibner and Speijer 2009). However, these studies focused on the Southern Galala Plateau and extended southward, neglecting the Paleocene-Eocene outcrops at the Northern Galala Plateau.

Fig. 1
figure 1

Location map and geological map of the study area, modified after Abd-Elhameed et al. (2020)

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Therefore, the main objectives of the present study are; (1) to report the presence of the Thanetian–Cuisian larger benthic foraminiferal assemblages from the Southern Galala Formation at the Northern and Southern Galala Plateaus, (2) to correlate their stratigraphic distributions with the Tethyan biozonations of Hottinger (1960) and Serra-Kiel et al. (1998), (3) and to discuss the paleoenvironment and paleoecology through microfacies analysis of the studied interval, along with the included LBF communities.

Geologic setting and lithostratigraphy

Egypt was situated at the southern margin of the Tethys during the Late Cretaceous-Early Paleogene time, and a south-dipping carbonate platform developed in north Eastern Desert; Galala Plateaus (Scheibner and Speijer 2008; Mahfouz and Metwally 2020). In north Eastern Desert, on the western side of the Gulf of Suez, the Galala Plateaus include the Northern and Southern Galala Plateaus, separated by the 30 km-wide Wadi Araba (Fig. 1). These plateaus represent an isolated Late Cretaceous to Eocene carbonate platform at the southern margin of the Tethys, referred to as the unstable shelf of northern Egypt (Youssef 2003). The evolution of the platform is related to the tectonic activity of the ENE-WSW striking Wadi Araba Fault, which forms part of the Syrian Arc-Fold-Belt (Krenkel 1925; Moustafa and Khalil 1995; Hussein and Abdallah 2001). A major phase of tectonic activity occurred along the Syrian Arc-Fold-Belt during the Early Eocene (Shahar 1994). Regional uplift and subsidence triggered the formation of ENE–WSW striking basins, submarine swells and subaerially exposed plateaus on the unstable shelf (Said 1990; Schütz 1994).

The Early Eocene Galala Plateaus represent one of the southernmost plateaus of the unstable shelf. According to regional tectono-sedimentary constraints, three major depositional units can be distinguished; the Northern Galala/Wadi Araba High (NGWA), a transitional slope zone, and the Southern Galala Subbasin (SGS; Höntzsch et al. 2011b). The NGWA represents subaerially exposed to shallow-marine inner platform environments. Due to the synsedimentary monoclinal uplift and following erosion along the Wadi Araba Fault since the Late Cretaceous, major inner-ramp deposits were eroded or intensively altered (Moustafa and Khalil 1995). The connection between the NGWA and the SGS is represented by a transitional slope zone (mid ramp to outer ramp).

The study area stretches from the southern rim of the Northern Galala Plateau (Askhar El-Bahari section) to the northern rim of the Southern Galala Plateau (Umm Khayshar section), where the upper Paleocene–lower Eocene carbonate platform were subdivided into three biotic stages (I–III) based on the paleoclimatic changes associated with larger foraminiferal evolution (Scheibner et al. 2005).

The biotic stage I is characterized by the occurrence of coralgal reefs during the progradational phase of the carbonate platform, while the biotic stage II shows a change from coralgal reefs to larger foraminiferal shoals at the platform margin during the retrogradational phase of the carbonate platform, as a result of the increase in water temperature. The short-term effects of the Paleocene–Eocene Thermal Maximum (PETM) during the Early Eocene progradational phase (i.e., temperature increase, eutrophication) caused a change from the dominance of miscellanids and ranikothalids to alveolinids and nummulitids, characterizing the biotic stage III (Scheibner et al. 2005).

The upper Paleocene–lower Eocene deposits at the Galala Plateaus are represented by the Southern Galala Formation. It was first introduced by Abdallah et al. (1970) at Gebel Thelmet, on the western side of the Gulf of Suez to describe the Paleocene–Eocene rocks which has been affected by many tectonic activities related to the Syrian arc system. The Southern Galala Formation unconformably overlies the Campanian–Maastrichtian Sudr Formation (Fig. 2; Abdallah et al. 1970; Ismail et al. 2007). The base of the Southern Galala Formation is represented by a 3 m-thick white-yellow sandstone bed at Umm Khayshar area (Fig. 2), and a 3.5 m-thick conglomeratic bed at Askhar El-Bahari area (Fig. 2). Upwards at Umm Khayshar area, the sandstone bed is followed by about 65 m-thick yellowish-white to greyish-white fossiliferous limestones. These limestones are represented by bioclastic floatstone facies, followed by Nummulites grainstone and Nummulites packstone facies (Fig. 2). On the other hand, at Askhar El-Bahari area, the conglomeratic bed is overlain by about 60 m-thick white to greyish-white, fossiliferous limestones, represented by bioclastic rudstone facies, followed by Nummulites-Alveolina wacke-packstone and Nummulites grainstone facies. A 4 m-thick pinkish carbonate breccia (brecciated grainstones) bed caps the succession (Fig. 2).

Fig. 2
figure 2

Lithostratigraphic section of the Paleocene–Eocene Southern Galala Formation at Umm Khayshar and Askhar El-Bahari areas, with the distribution of the recognized microfacies types

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Materials and methods

The upper Paleocene–lower Eocene successions are investigated from two sections; Umm Khayshar section at the northern scarp of the Southern Galala Plateau (Lat. 28° 55ʹ 34″ and Long. 32° 10ʹ 25″) and Askhar El-Bahari section at the southern scarp of the Northern Galala Plateau (Lat. 29° 03ʹ 04″ and Long. 32° 01ʹ 09″; Fig. 1). These sections are measured, described, and sampled for microfacies and benthic larger foraminifera analyses.

Fifty-four thin sections of standard size are prepared, analyzed, and photographed using Polarizing Microscope with attached digital camera from Beni-Suef University. Thin-section studies yield different microfacies types, as well as diverse benthic larger foraminiferal content. The recognized microfacies types are described and classified following the scheme of Dunham (1962), with modifications of Embry and Klovan (1972) and Scholle (2003).

Furthermore, twenty-eight larger benthic foraminiferal species belonging to thirteen genera are identified following the taxonomic descriptions of Schwager (1883), Leymerie (1846), Heim (1908), Reichel (1937), Davies (1937), de Cizancourt (1945), Henson (1948), Hottinger (1960, 2014), Drobne (1975), Kuss and Leppig (1989), Wan (1990), and Tosquella and Serra-Kiel (1998). All the materials are housed in the Museum of the Geology Department, Faculty of Science, Helwan University, Egypt.

Results

Biostratigraphy

Twenty-eight larger foraminiferal species are recorded from the studied upper Paleocene (Thanetian)-lower Eocene (Ilerdian-Cuisian) successions, and their stratigraphic distributions are correlated with the Tethyan biozonations of Hottinger (1960) and Serra-Kiel et al. (1998) (Figs. 3 and 4).

Fig. 3
figure 3

The distribution of the identified larger benthic foraminifera from the Paleocene-Eocene Southern Galala Formation at Umm Khayshar area

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Fig. 4
figure 4

The distribution of the identified larger benthic foraminifera from the Paleocene-Eocene Southern Galala Formation at Askhar El-Bahari area

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The lower part of the Thanetian has been described as the Glomalveolina primaeva zone by Hottinger (1960) or as SBZ3 by Serra-Kiel et al. (1998). This interval is represented by the total range of Glomalveolina primaeva in the two studied sections (Figs. 3 and 4). In addition, Elazigina lenticula and Discocyclina sp. in Umm Khayshar section and G. dachelensis, Periloculina slovenica, and Idalina sp. in Askhar El-Bahari section are associated with G. primaeva in this interval (Fig. 5).

Fig. 5
figure 5

a Glomalveolina primaeva (Reichel 1937), PEF-1. b Glomalveolina levis Hottinger 1960, PEF-2. c Glomalveolina dachelensis (Schwager 1883), PEF-3. d Idalina sp., PEF-5. e Periloculina slovenica Drobne, 1974, PEF-6. f Miscellanea rhomboidea Kuss and Leppig 1989, PEF-7. g Hottingerina lukasi Drobne 1975, PEF-13. h Nummulites catari Tosquella and Serra-Kiel 1998, PEF-16. i Elazigina lenticula (Hottinger 2014), PEF-17. j Discocyclina sp., PEF-18. k Nummulites atacicus (Leymerie 1846), PEF-23. l Nummulites globulus Leymerie 1846, PEF-36. m Orbitolites minimus (Henson, 1950), PEF-55. n Fabularia zitteli Schwager 1883, PEF-56

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The upper part of the Thanetian has been described as the Glomalveolina levis zone by Hottinger (1960) or as SBZ4 by Serra-Kiel et al. (1998). It is defined by the total range of Glomalveolina levis in Askhar El-Bahari section and that of Nummulites catari in Umm Khayshar section (Figs. 3 and 4). Furthermore, Elazigina lenticula, Miscellanea rhomboidea, and Discocyclina sp. in Umm Khayshar section and G. dachelensis, Miscellanea rhomboidea, and Hottingerina lukasi in Askhar El-Bahari section are also recognized in this interval (Fig. 5).

The Early Ilerdian has been established as the Alveolina cucumiformis (junior synonym of A. vredenburgi; Hottinger et al. 1998) and A. ellipsoidalis zones by Hottinger (1960) or as SBZ5 and SBZ6, respectively, by Serra-Kiel et al. (1998). This interval is represented by the total range of Alveolina vredenburgi in Umm Khayshar section and that of A. ellipsoidalis in Askhar El-Bahari section (Figs. 3 and 4). Other Alveolina spp., e.g., A. minervensis, A. pasticillata, A. piper, A. rutimeyeri, A. subovata, and Fabularia zitteli are also included in Askhar El-Bahari section. Since the index species of SBZ5 and SBZ6 co-occur in the present study, the boundary between both zones cannot be traced (Figs. 5 and 6).

Fig. 6
figure 6

a Assilina subspinoza Davies, 1937, PEF-57. b Alveolina vredenburgi Davies, 1937, PEF-58. c Alveolina aragonensis Hottinger 1960, PEF-60. d Alveolina ellipsoidalis Schwager 1883, PEF-61. e Alveolina minervensis Hottinger 1960, PEF-62. f Alveolina pasticillata Schwager 1883, PEF-63. g Alveolina piper Hottinger 1960, PEF-66. h Alveolina rutimeyeri Hottinger, 1962, PEF-68. i Alveolina subovata Wan 1990, PEF-69. j Nummulites fossulatus de Cizancourt, 1946, PEF-70. k Nummulites sp., PEF-78. l Assilina major Heim 1908, PEF-107. m Assilina sp., PEF-108. n Lockhartia sp., PEF-113

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The Middle Ilerdian has been described as the Alveolina corbarica zone by Hottinger (1960) or as SBZ8 by Serra-Kiel et al. (1998). However, A. corbarica is not recorded in the studied sections, and instead, the total range of Nummulites atacicus defines this interval (Figs. 3 and 4). In addition, Nummulites globulus in Umm Khayshar section and Orbitolites minimus and Nummulites globulus in Askhar El-Bahari section are also recognized in this interval (Figs. 5 and 6).

The Late Ilerdian has been established as the Alveolina trempina zone by Hottinger (1960) or as SBZ9 by Serra-Kiel et al. (1998). Nevertheless, A. trempina is not recognized in the present study. Therefore, the lower boundary of this interval is defined by the last occurrence of Nummulites atacicus and the upper boundary is defined by the last occurrence of Nummulites globulus (Figs. 3 and 4). Alveolina aragonensis is also associated with Nummulites globulus in Umm Khayshar section (Figs. 5 and 6).

The Early Cuisian has been described as the Alveolina oblonga zone by Hottinger (1960) or as SBZ10 by Serra-Kiel et al. (1998). A. oblonga is not reported in the studied sections, and therefore the total range of Nummulites fossulatus defines this interval (Figs. 3 and 4). Assilina subspinoza is also recorded, along with Nummulites fossulatus in Umm Khayshar section (Fig. 6).

The Middle Cuisian has been established as the Alveolina dainellii zone by Hottinger (1960) or as SBZ11 by Serra-Kiel et al. (1998). However, A. dainellii is not recognized in the present study. Therefore, the base of this interval is defined by the last occurrence of Nummulites fossulatus and the top is defined by the first occurrence of Assilina major (Figs. 3 and 4). Only Nummulites sp. is recorded from this interval in the studied sections (Fig. 6).

The Late Cuisian has been described as the Alveolina violae zone by Hottinger (1960) or as SBZ12 by Serra-Kiel et al. (1998). A. violae is not recorded in the studied sections and, therefore, the total range of Assilina major defines this interval (Figs. 3 and 4). Furthermore, Nummulites sp. and Lockhartia sp. are also included in this interval in the two studied sections (Fig. 6).

Microfacies analysis, paleoenvironmental and paleoecological interpretation

The microfacies analysis of the studied upper Paleocene–lower Eocene successions reveals seven microfacies types (MFT 1–7), reflecting various depositional environments (Fig. 9).

Calcareous sandstone (MFT-1)

This microfacies type is recorded from the lowermost part of the Southern Galala Formation in Umm Khayshar section (Fig. 2). It is made up of sub-angular to sub-rounded, fine to medium, sand-sized quartz grains. In addition, few sparry calcite crystals, ferruginous material, iron oxides, glauconite grains, and euhedral, zoned dolomite crystals are also included in this microfacies (Fig. 7a).

Fig. 7
figure 7

a Calcareous sandstone (MFT-1). b, c Sandy bioclastic rudstone (MFT-2), with larger benthic foraminifera (LBF), echinoids (Ec), and bivalves (Bv). d–f Brecciated grainstone (MFT-3), with echinoids, algal plates, and bivalves. f a rock sample of the brecciated grainstone facies. g, h Bioclastic floatstone (MFT-4), showing the dominant bivalve oyster shells and larger foraminifera

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The complete absence of faunal components shown in the calcareous sandstone microfacies (MFT-1) and the presence of some glauconite grains reflect a near-shore environment, with restricted circulation conditions (Saber 2012). Based on the dominance of quartz grains and the presence of sparite and dolomite crystals, a deposition in a restricted tidal flat environment, with high water energy, is proposed for this microfacies (Fig. 9).

Sandy bioclastic rudstone (MFT-2)

This microfacies type is recorded from the lower part of the Southern Galala Formation, representing the Late Paleocene interval in Askhar El-Bahari section (Fig. 2). It is characterized by larger benthic foraminifera (e.g. Glomalveolina and Hottingerina), embedded in a groundmass of sparry calcite, with sub-angular-sub-rounded, fine- to medium-grained quartz. Moreover, bivalve shell fragments, gastropod shells, echinoid fragments, crinoid columnals, and halimedacean and dasycladacean algal plates are recorded in this microfacies (Fig. 7b, c).

The sandy bioclastic rudstone microfacies (MFT-2) are characterized by porcelaneous larger foraminifera, e.g. Glomalveolina, Hottingerina, Idalina, and Periloculina. Hottingerina lukasi has been reported from the shallow platform interior (Scheibner and Speijer 2009). Away from oceanic waves, alveolinids inhibit sheltered habitats, i.e., restricted lagoonal settings (Hottinger 1997; Murray 2006; Babazadeh and Alavi 2013; Coletti et al. 2021). The imperforate miliolid foraminifera (e.g. Idalina and Periloculina) also suggest an inner restricted lagoon environment (El-Ayyat 2022). The alveolinids and miliolids have been known as epiphytes, living within vegetated substrates (Brasier 1975; Beavington-Penny et al. 2006; Tomassetti et al. 2016; Martín-Martín et al. 2020; Coletti et al. 2021). They are also reflective of euphotic and moderately oligotrophic conditions (Hottinger 1997; Samankassou 2002; Barattolo et al. 2007; Sarkar 2019; Martín-Martín et al. 2020; El-Ayyat 2022). In addition, the abundance of miliolids suggests normal to hyper salinity conditions lower than 50 psu (Brasier 1975; Murray 2006; Mossadegh et al. 2009; Chan et al. 2017; Sarkar 2019). The presence of green algae characterizes euphotic, oligotrophic shallow-marine settings (Wray 1977; Brandano et al. 2015; Sarkar 2019; Mattern et al. 2021). Moreover, the presence of fine- to medium-grained quartz and sparitic matrix suggests a position close to a source area, with high water energy related to tidal currents or waves (Hadi et al. 2015). Therefore, this microfacies can be attributed to a restricted, high-energy inner ramp lagoon, with euphotic, oligotrophic, and normal to hypersaline waters (Fig. 9). Similar microfacies have been reported from Spain (Martín-Martín et al. 2020) and India (Sarkar 2019), and interpreted a restricted inner ramp lagoon. However, Martín–Martín et al. (2020) also recorded dasyclad and codial green algae, indicating a low-energy marine environment.

Brecciated grainstone (MFT-3)

It is recorded from the uppermost part of the Southern Galala Formation in Askhar El-Bahari section (Fig. 2). The ferruginous calcareous sandstone groundmass of this microfacies consists mainly of fine- to medium-grained, sub-angular to sub-rounded quartz, with few ferruginous material and sparite crystals. Sub-angular clasts of sandy bioclastic grainstone, with fragments of bivalves, echinoids, crinoids, and green algal plates, are embedded in the groundmass (Fig. 7d–f).

The allochthonous components of the brecciated grainstone facies (MFT-3) are represented by skeletal debris, e.g., fragments of bivalves, echinoids, and crinoids, in a sparitic matrix. Such skeletal debris and the sub-angular shape of the clasts suggest high water energy at the flanks of a reef/shoal (Schlagintweit and Gawlick 2008; Pleş et al. 2019). The low diversity of the recorded fauna reveals restricted circulation conditions. Hence, a deposition in a high-energy, restricted, inner ramp (back-shoal) environment is proposed (Fig. 9). Similar microfacies has been reported from Slovenia (Zamagni et al. 2008) and Iran (Bagherpour and Vaziri 2011; Amirshahkarami and Zebarjadi 2018), and interpreted as restricted, back-reef deposits.

Bioclastic floatstone (MFT-4)

It is recorded from the lower part of the Southern Galala Formation, representing the Late Paleocene interval in Umm Khayshar section (Fig. 2). This microfacies consists mainly of well-sorted bivalve oyster shells (Fig. 7g), with foliated structure, along with larger benthic foraminifera, e.g., Nummulites and Discocyclina (Fig. 7h). These skeletal components are embedded in a sparitic groundmass, with few fine- to medium-grained quartz and some concentric ooids.

The bioclastic floatstone microfacies (MFT-4) is characterized by abundant oyster fragments, larger foraminiferal tests, fine- to medium-grained quartz, and concentric ooids, indicating high-energy conditions above fair-weather wave base (FWWB), likely related to a shoal environment at a shelf-edge or bank-margin facies (Chablais et al. 2010; Sarkar 2019; Hadi et al. 2021). The co-occurrence of alveolinids (e.g. Glomalveolina) and nummulitids (e.g., Nummulites) suggests euphotic to mesophotic, meso-oligotrophic, open-marine, proximal to distal inner ramp environment, with normal salinity (32–38 psu) and high water energy, close to the FWWB (Geel 2000; Mossadegh et al. 2009; Hadi et al. 2016, 2021; Sarkar 2019). The orthophragminids (e.g., Discocyclina) have been reported from a wide array of environments at the lower limit of the photic zone, from inner ramp back- and fore-reef/shoal environments (Ghose 1977; Babazadeh and Alavi 2013; Hadi et al. 2015; Coletti et al. 2021) to deeper, middle-outer ramp environments (Eichenseer and Luterbacher 1992; Gilham and Bristow 1998; Beavington-Penney and Racey 2004; Babazadeh and Alavi 2013), with meso-oligotrophic and normal salinity conditions (Anketell and Mriheel 2000; Hadi et al. 2019; Martín–Martín et al. 2020). The orthophragminids recorded in this microfacies are represented by ovate forms, characterizing the inner ramp settings (Loucks et al. 1998; Zamagani et al. 2008). Discocyclina, Nummulites, and rotaliids (e.g., Elazigina) show preference for soft sandy and muddy substrates (Scheibner et al. 2005, 2007; Zamagani et al. 2008). Consequently, MFT-4 is attributed to a high-energy, distal inner ramp shoal environment, in agreement with their stratigraphic association with core- and back-bank deposits, with euphotic to mesophotic, meso-oligotrophic, and normal salinity conditions (Fig. 9). Although this microfacies show some features similar to that reported from Spain, France, and Slovenia (Rasser et al. 2005; Scheibner et al. 2007; Zamagani et al. 2008) with the predominance of larger benthic foraminifera, the corals and red algae are completely absent.

Nummulites-Alveolina wacke-packstone (MFT-5)

This microfacies type is recorded from the middle part of the Southern Galala Formation in Askhar El-Bahari section (Fig. 2). It is made up of micritic groundmass, encompassing various skeletal components. These components are dominated by larger benthic foraminifera, e.g., A-form Nummulites, Alveolina, and Orbitolites, with some bivalve shells and green algal plates (Fig. 8a, b).

Fig. 8
figure 8

a, b Nummulites-Alveolina wacke-packstone (MFT-5), with larger benthic foraminifera and algae. c, d Nummulites grainstone (MFT-6), showing the larger benthic foraminifera and echinoids. e, f Nummulites packstone (MFT-7), showing the larger benthic and planktonic foraminifera

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The Nummulites-Alveolina wacke-packstone microfacies (MFT-5) are dominated by nummulitids, alveolinids, and orbitolitids. The alveolinids (e.g., Alveolina and Fabularia spp.) show a great tolerance to salinity and temperature fluctuations, and therefore, could inhibit any part within shallow-marine carbonate platforms (Drobne et al. 2011; Hadi et al. 2015, 2018; Sarkar 2019). They are also indicative of euphotic, meso-oligotrophic conditions (Tomassetti et al. 2016; Coletti et al. 2021). The wide distribution of alveolinids, orbitolitids (e.g., Orbitolites), and green algae, in a wacke-packstone matrix indicates euphotic, oligotrophic inner ramp environment, with moderate-high water energy (Wray 1977; Hottinger 1997, 1983; Scheibner et al. 2007; Babazadeh and Alavi 2013; Hadi et al. 2015; Coletti et al. 2021). Imperforate porcelaneous foraminifera reflect higher salinity conditions (< 50 psu; Mossadegh et al. 2009; El-Ayyat 2022). Hottinger (1977) and Hadi et al. (2016) interpreted the alveolinid-dominated carbonate successions as of back-bank origin. The co-occurrence of alveolinids and A-form Nummulites suggests euphotic to mesophotic, meso-oligotrophic, proximal to distal inner ramp settings, with high-energy conditions (Geel 2000; Beavington-Penney et al. 2006; Zamagani et al. 2008; Sarkar 2019; Hadi et al. 2021). It can be concluded that this microfacies has been deposited in a distal inner ramp (back-bank) environment, with moderate-high water energy, meso-euphotic, oligotrophic, and normal to hyper salinity conditions (Fig. 9). This microfacies type shows great similarity to that of Hadi et al. (2016), with high abundance of alveolinids, A-forms, and other porcelaneous foraminifera, that was indicator of the inner part of ramp with high-energy conditions.

Fig. 9
figure 9

Paleoenvironmental model for the Paleocene–Lower Eocene successions in the study area, with the distribution of the recognized microfacies types and the recorded larger benthic foraminiferal groups. FWWB Fair weather wave base

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Nummulites grainstone (MFT-6)

This microfacies type is recorded from the upper part of the Southern Galala Formation in the two studied sections, representing the Early Eocene (Cuisian) interval (Fig. 2). It encompasses highly diversified biota in a sparitic groundmass, with some quartz grains and calcareous ooids. Larger benthic foraminifera, mainly Nummulites and Assilina spp., bivalve shells, gastropod shells, and echinoid fragments are the main skeletal components of this microfacies (Fig. 8c, d).

The Nummulites grainstone microfacies (MFT-6) contains abundant larger foraminifera (e.g. Nummulites and Assilina spp.) and echinoids, reflecting open-marine conditions (Racey 2001; Saber 2012; El-Ayyat 2022). The appearance of Assilina and Nummulites is indicative of an increase in water depth (Hadi et al. 2015). The presence of A-form and rare B-form nummulitids, along with the disappearance of the porcelaneous foraminifera, e.g., Alveolina, in a grainstone matrix reflects oligotrophic proximal to distal inner ramp palaeohighs, influenced by winnowing and current processes (Langer and Hottinger 2000; Zamagani et al. 2008; Hadi et al. 2021). The presence of calcareous ooids and fine- to medium-grained quartz suggests a higher water energy event, possibly related to tidal currents or waves (Hadi et al. 2015). The presence of nummulitids (e.g., Nummulites and Assilina) indicates preference for a soft substrate, and reflects meso-oligophotic and normal salinity conditions (32–38 psu; Hottinger 1997; Scheibner et al. 2005, 2007; Martín–Martín et al. 2020). The minor association of Nummulites with micro- and macrofauna confirms the deposition in nutrient-poor waters (Hallock 1985). Therefore, this microfacies reflect the development of Nummulites accumulations (banks) in a distal inner ramp environment, which were then transported into the proximal middle ramp environment, with considerable wave and current actions, meso-oligophotic, oligotrophic, and normal salinity conditions (Fig. 9). Similar microfacies type has been reported from Iran (Hadi et al. 2016), where the accumulations of A-form Nummulites-dominated with robust and ovoid tests are interpreted as palaeohighs developed in shallow inner-ramp setting.

Nummulites packstone (MFT-7)

It is recorded from the middle part of the Southern Galala Formation in Umm Khayshar section (Fig. 2). This microfacies consist of highly diversified faunal content, embedded in a groundmass of microcrystalline calcite. The faunal content is dominated by larger benthic foraminifera, e.g., Nummulites and Alveolina, planktonic foraminifera, bivalve shells, echinoid and crinoid fragments (Fig. 8e, f).

The Nummulites packstone (MFT-7) is dominated by Nummulites, co-occurring with rare larger porcelaneous foraminifera, e.g., Alveolina, suggesting downslope transport and deposition in distal inner ramp to proximal middle ramp settings, below the fair-weather wave base (Hadi et al. 2015, 2021). The B-form Nummulites of this microfacies can be indicative of deeper waters (Ćosović et al. 2004; Machaniec et al. 2011; Sarkar 2019) in a fore-bank environment (El-Ayyat 2022). These nummulitids indicate soft substrate, normal water salinity (32–38 psu), oligotrophic, and meso-oligophotic conditions (Scheibner et al. 2005; Mossadegh et al. 2009; Martín–Martín et al. 2020). Furthermore, the wide distribution of planktonic foraminifera reflects open-marine, middle to outer ramp settings, with oligotrophic settings and salinity between 32 and 40 psu (Beavington-Penney et al. 2004; Murray 2006). It can be concluded that this microfacies originated in a middle ramp (fore-bank) environment, with oligophotic, oligotrophic, and normal salinity conditions (Fig. 9). Martín–Martín et al. (2020) reported similar microfacies type from Spain with abundant lenticular nummulitids in packstone–wackestone beds. Nevertheless, the mixing of flattened tests of Assilina and operculiniform Assilina reflected an intermediate depth into the middle ramp environment.

Discussion

Paleocene–Eocene boundary

The Paleocene–Eocene boundary was delineated officially by the onset of the Carbon Isotopic Excursion (CIE; Aubry and Ouda 2003), corresponding to the base of P5b and NP9b zones (Dupuis et al. 2003; Scheibner and Speijer 2009; Zhang et al. 2013). On the other hand, there are two contrasting opinions concerning the position of P-E boundary with respect to the shallow benthic zones. Serra-Kiel et al. (1998) and Hottinger (1998, 2001) suggested that the P–E boundary spanned the time intervals from the lower part of SBZ6 to the middle part of SBZ7.

On the contrary, Scheibner et al. (205) and Scheibner and Speijer (2009) placed the P–E boundary at the transition between SBZ4 and SBZ5 in Egypt, coinciding with the base of the Ilerdian, with the index species for SBZ4 below the onset of the CIE and SBZ5 index species above it. This proposition is also consistent with data from the Pyrenean basin (Orue-Etxebarria et al. 2001; Pujalte et al. 2009a, b a, b; Serra-Kiel et al. 2020).

In the studied sections, the Paleocene–Eocene boundary is marked by the last occurrence of the index species for SBZ4, e.g., Glomalveolina levis and Hottingerina lukasi, and the first occurrence of the index species for SBZ5/6, e.g. Alveolina vredenburgi and Alveolina ellipsoidalis (Figs. 3 and 4).

Factors controlling the LBF distribution

Larger benthic foraminifera are the most prevailing faunal components of the recognized microfacies types. According to Scott et al. (2001), LBF are highly influenced by even subtle variations in environmental conditions. Therefore, they are considered as excellent markers for paleoenvironmental parameters, including temperature, light, nutrients availability, substrate, depth, hydrodynamic energy, and salinity (Hottinger 1977, 1983; Pecheux 1995; Duxbury and Duxbury 1997; Geel 2000; Scott et al. 2001; Leckie and Olson 2003; Beavington-Penney and Racey 2004; Rasser et al. 2005; Boudagher-Fadel 2008; Drobne et al. 2011; Tomassetti et al. 2016; Eder et al. 2018; Torres-Silva et al. 2019; El-Ayyat 2022). The most influential parameters, in shallow shelves, are nutrients sources and light intensity, where primary production is most pronounced (Duxbury and Duxbury 1997).

Light intensity

The light intensity relies on water depth and water transparency, i.e., light intensity reduces and water transparency increases as water depth increases (Hallock, 1987). According to light dependence, there are three groups of organisms; light-dependent organisms (e.g., corals, algae, and porcelaneous foraminifers), low light-dependent organisms (e.g., algae, Operculines, and Lepidocyclines), and light-independent organisms (e.g., bryozoa, mollusca, crinoids, and sponges). Different larger benthic foraminiferal taxa are good indicators to reconstruct the paleo-depth conditions of seawater. Porcelaneous forms, for example, are light dependent and abundant in shallow-marine settings (inner ramp environment; Fig. 9), while they are rare in deeper settings (Martín-Martín et al. 2020). On the other hand, the quantity of hyaline foraminifera (e.g. nummulitids) is mostly enhanced with increasing the water depth.

Lenticular shells of Nummulites are diverse in shallow-marine settings, inhabiting the basal portion of the top of the photic zone (e.g., Nummulites bank), but thin-shelled tests considerably occur in deep waters (fore-bank environment; Beavington-Penney et al. 2004). Such variation in shell shapes is indicative of decreasing light grades at deeper settings (BouDagher-Fadel 2008). The green calcareous algae recorded in the shallow-marine sediments within the Southern Galala Formation (Figs. 7 and 8) are typical of euphotic zone, with ultimate depth of prolific growth commonly about 20–30 m depth. Furthermore, planktonic foraminifera are recorded in a middle ramp environment in open-marine settings (Fig. 9), reflecting oligophotic conditions (BouDagher-Fadel 2008). It can be concluded that the decrease in light intensity affected the abundance and dominance of porcelaneous forms (e.g., alveolinids) and replaced them with hyaline forms (e.g., nummulitids).

Temperature

The impact of temperature on benthic fauna is illustrated through affecting the metabolic performance and dissolution of CO2, i.e., a decrease in temperature leads to a decrease in the metabolic performance and therefore the nutrient requirements of the organism (Geel 2000). In general, most of the LBF inhibit subtropical to tropical environments, at temperatures of about 18–23 °C (Lee 1990; Ungaro 1994; Hottinger 1997; Rasser et al. 2005; Wilson and Vecsei 2005; Brandano et al. 2009; El-Ayyat 2022), while their absence points to cooler or more nutrient-rich environments (El-Ayyat 2022). However, some extant larger foraminifera can tolerate water temperatures as low as 10–11 °C, e.g., Amphistegines, Sorites, Amphisorus, and Amphistegines (Li et al. 1999). The co-occurrence of larger benthic foraminifera and green calcareous algae, to some extent, would reside in warm waters having temperatures of about 18–20 (Brandano et al. 2009), proposing a tropical–subtropical climate for the studied succession.

Salinity

Salinity has a direct influence on the evolution of larger benthic foraminifera. The existence of porcelaneous foraminifera (e.g. alveolinids and miliolids) and the lack of hyaline foraminifera, red algae, and corals mark the dominance of higher levels of hypersaline water (higher than 50 psu) in enclosed lagoon setting (Mossadegh et al. 2009). The occurrence of larger benthic foraminifera with perforated wall (such as Nummulites) refers to normal water salinity conditions, with salinity ranges from 32 to 38 psu (Mossadegh et al. 2009). Planktonic foraminifera live in open oceans and also in inland seas (e.g., the Mediterranean). Their tolerance to variations in water salinity is very limited; indeed, most extant members are stenohaline. No extant planktonic foraminifera have been recorded from waters with salinity higher than 40 psu or lower than 32 psu (Murray 2006). It can be concluded that that the upper parts of the investigated successions accumulated in normal marine salinity circumstances as indicated by the high abundance of hyaline forms (e.g., nummulitids), while the lower parts witnessed interrupted normal and higher water salinity conditions according to the dominance of porcelaneous forms (e.g., alveolinids).

Nutrients

Larger benthic foraminifera live in stabilized, nutrient deficient, oligotrophic circumstances. On the other hand, LBF cannot react competitively in case of plenty nourishing supplies (Hallock 1985). The imperforate foraminifera (e.g., miliolids), which inhibit inner confined lagoon (Fig. 9), reflect low grades of nutrients (Samankassou 2002). In addition, the presence of green algae confirms the oligotrophic shallow-marine settings (Wray 1977). The presence of nummulitids (e.g., Nummulites and Assilina) indicates oligophotic conditions (Martín–Martín et al. 2020), while their association with alveolinids suggests meso-oligotrophic circumstances (Hadi et al. 2021). Concerning the recorded Nummulites bank, no associated micro- or macrofauna has been reported, reflecting a deposition in a nutrient-poor (oligotrophic) environment and/or in a setting with significant hydrodynamic sorting. In general, sediments rich in larger benthic foraminifera are often inferred to represent a deposition in oligotrophic settings, because such sediments are overwhelmingly accompanied with organisms tolerant of nutrient-poor conditions (e.g., green calcareous algae). The existence of open-marine planktonic foraminifera indicates surface trophic to intermediate mesotrophic water conditions (BouDagher-Fadel 2008).

Substrate

The different shapes of Nummulites resulted from the reproductive strategies and environmental conditions, such as substrate and water hydrodynamic energies. These factors affect the thickness, size and shape of these shells (Ćosović et al. 2004). Brasier (1975) mentioned that thick-shelled and conical forms of larger benthic foraminifera prefer housing coarse-grained sediments, but flattened shell forms prefer soft substrates. The alveolinids and miliolids have been known as epiphytes, living within vegetated substrates (Beavington-Penny et al. 2006; Tomassetti et al. 2016; Martín-Martín et al. 2020; Coletti et al. 2021). On the other hand, nummulitids (e.g., Nummulites and Assilina), rotaliids (e.g., Elazigina), and Discocyclina show preference for soft sandy and muddy substrates (Scheibner et al. 2005, 2007; Zamagani et al. 2008). Therefore, the substrate is an important factor affecting the abundance and dominance of different larger foraminiferal groups.

Water depth and hydrodynamic behavior

Changes in water depth could affect environmental changes such as sedimentation rate, turbulence, light intensity, organic change, water hydrodynamic energy, and dissolved oxygen (Leckie and Olson 2003). Due to their sensitivity to water depth, LBF were used in the early times to estimate water depth (El-Ayyat 2022). Alveolinids avoid very shallow water and live in sheltered habitats, away from oceanic wave energies, and thus are particularly common in deeper parts of restricted lagoonal settings (Murray 2006). Some LBF such as Nummulites and Discocyclina become flatter, with thinner outer walls, with increasing water depth (middle ramp Nummulites bank; Fig. 9) and decreasing light intensity. This reflects the dependence of morphology on the gradient of depth (Pecheux 1995).

Perforate foraminifera (e.g., Nummulites and Assilina) are abundant in subtropical to tropical environments, reaching a depth down to 40–70 m (Hottinger 1997). At the lower part of the studied successions, miliolids are plentiful and mostly found in a shallow and restricted water depth. In addition, green calcareous algae show subtropical to tropical environments, with a depth of 20–80 m (Flügel 2010). On the contrary, within the middle and upper parts, Nummulites, and Assilina (as perforate foraminifera) are common and reflect open marine conditions (Fig. 9). The occurrence of planktonic foraminifera and the deficiency of benthic fauna at the upper part of the successions suggest a deposition in tranquil deeper water (Murray 2006). Accordingly, increase in water depth affected the abundance of porcelaneous forms (e.g., alveolinids and miliolids), and led to the dominance of hyaline forms (e.g., nummulitids).

The hydrodynamic behavior of LBF, which depends on density, size and their shapes, is an essential factor controlling their transport (BouDagher-Fadel 2008). Some foraminifera develop thick lamellar crust shells and others tend to develop a hard, conical shell shape with abundant secondary cells, depending on hydrodynamic water energy (Rasser et al. 2005). The wide distribution of alveolinids and orbitolitids (e.g., Orbitolites) in the inner ramp back-bank setting indicates moderate water energy conditions (Coletti et al. 2021). On the other hand, the co-occurrence of alveolinids and nummulitids in the shallow-marine inner ramp settings suggests high water energy conditions, close to the FWWB (Sarkar 2019; Hadi et al. 2021).

Upon comparison of the Pyrenean strata from middle latitudes at the northern Tethys (Scheibner et al. 2007) with the Egyptian succession from the low latitudes at the southern Tethys (this study), the overall trend within the three platform stages proposed by Scheibner et al. (2005) is the same, while latitudinal differences exist, especially in platform stages II and III. The studied successions at the Galala Plateaus are devoid of corals in both platform stages II and III. This overall trend of a less frequent coral occurrence is also apparent in the Pyrenean succession, except for the small coral patches in the late Paleocene platform stage II and the early Eocene platform stage III (Baceta et al. 2005).

The reasons for these platform evolution stages were probably the same in both regions. However, the consequences were less severe in the middle latitudes of the Pyrenees, representing a latitudinal effect. In the low latitudes (this study), temperatures were too warm to sustain the growth of coral reefs and enabled the larger foraminifera to occupy this niche and flourish during the early Paleogene. On the other hand, in the middle latitudes (Pyrenees), the temperatures were cooler than in the tropics, so that coral reefs still could grow, but played only a minor role comparing with the dominant larger foraminifera (Scheibner et al. 2007).

Late Paleocene–Early Eocene paleoenvironmental evolution

The Early Palaeogene evolution of the Galala carbonate platform is strongly influenced and controlled by syndepositional tectonic activity along the Wadi Araba Fault (Höntzsch et al. 2011a). From the PETM to the latest Early Eocene, three tectono-sedimentary platform stages were classified and discussed by Scheibner et al. (2003) and Höntzsch et al. (2011b). The termination of the tectonic activity of the Wadi Araba Fault, related to the Syrian Arc System, is reflected by the absence of siliciclastics (Fig. 2). The microfacies analysis along the Late Paleocene–Early Eocene inner to middle ramp transect yields detailed information regarding environmental and biotic conditions of the platform system.

The shallow-marine inner ramp environment is dominated by photo-autotrophic larger benthic foraminiferal groups (e.g., alveolinids and nummulitids), smaller benthic foraminifera (e.g., miliolids), and halimedacean and dasycladacean green algae (Figs. 7–9). Typical low-latitude nutrient-depleted assemblages, such as corals, are absent. Other faunal components include bivalves and gastropods. The environmental impact of the PETM caused a major shift in the prevailing larger benthic foraminiferal taxa, where new taxa with different ecological strategies evolved (Orue-Etxebarria et al. 2001; Scheibner et al. 2005). Paleocene orthophragminids and miliolids were replaced by the first Eocene nummulitids. Porcelaneous alveolinids evolved from planspiral glomalveolinids to fusiform Eocene alveolinids (Hottinger 2001). Such major shift in the larger benthic foraminiferal assemblages is recorded in the middle ramp environment (Figs. 7–9). The diversification of LBF was linked to an overall increase in test size. The coeval evolving adult dimorphism of LBF is interpreted as an adaptation to the seasonality in warm and oligotrophic habitats (Hottinger 1998, 2001). The progressive increase in shell size and development of adult dimorphism are typical features of long-term maturation cycles within larger benthic foraminifera communities (Hottinger 1998; Zamagani et al. 2012). This Larger Foraminifera Turnover (LFT) was directly linked to the CIE of the PETM (Scheibner et al. 2005; Höntzsch et al. 2011a). The present study is in accordance with the studies of Scheibner et al. (2005), Scheibner and Speijer (2009), and Höntzsch et al. (2011b), except that the larger benthic foraminiferal assemblages shifted from the Paleocene ranikothalids, miscellanids, and glomalveolinids to the Eocene nummulitids and alveolinids.

Early Eocene Nummulites accumulations

Eocene Nummulites accumulations (banks) have been widely distributed along both margins of the Tethys, the northern margin (from Pyrenees to West Pacific) and the southern margin (from North Africa to Oman) (Racey 2001; Jorry 2004; Jorry et al. 2006; Martín-Martín et al. 2020, 2021; El-Ayyat 2022).

Concerning the origin of these accumulations, different processes have been proposed; biological (i.e., larger foraminiferal reproduction strategies) and physical (i.e., winnowing and sorting) processes (Aigner 1982; Loucks et al. 1998; Racey 2001; Hasler 2004; Beavington-Penney et al. 2005, 2006; Jorry et al. 2006).

The Early Eocene carbonate ramp of the Southern Galala Formation in the study area is dominated by larger benthic foraminifera (e.g., nummulitids). The Nummulites accumulations recorded in the present study have been developed on paleohighs in a distal inner ramp environment, and then transported into the surrounding proximal middle ramp environment with significant wave and current actions. This is comparable with the Eocene El-Garia Formation in Tunisia and the Ziarat Formation in Iran which were interpreted as nummulitic paleohighs in shallow-marine environments and transported into the surrounding deeper settings (Beavington-Penney et al. 2005; Hadi et al. 2021).

Conclusions

The upper Paleocene–lower Eocene Southern Galala Formation is exposed at the Northern and Southern Galala Plateaus, north Eastern Desert, Egypt, consisting mainly of fossiliferous limestones, with abundant larger benthic foraminiferal fauna. Twenty-eight larger foraminiferal species of thirteen genera are reported in the present study. In addition, their stratigraphic ranges are correlated with the Tethyan zonations of Hottinger (1960) and Serra-Kiel et al. (1998), designating nine shallow benthic zones (SBZ3-6 and SBZ8-12), involving index zonal markers, e.g., Glomalveolina primaeva, G. levis, Nummulites atacicus, Alveolina vredenburgi, and A. ellipsoidalis.

Based on the microfacies analysis of the studied interval and their fossil components, seven microfacies types are recognized, including calcareous sandstone, sandy bioclastic rudstone, brecciated grainstone, bioclastic floatstone, Nummulites-Alveolina wacke-packstone, Nummulites grainstone, and Nummulites packstone, mostly dominated by larger benthic foraminifera. The distribution of the recorded microfacies types and the paleoenvironmental and paleoecological interpretation of the dominant larger benthic foraminifera reflect depositional environments ranging from restricted inner ramp to open-marine middle ramp environments.

The larger foraminifera in the inner ramp environments are represented by alveolinids, orbitolitids, and in part by miliolids and orthophragminids, suggesting euphotic–mesophotic, meso-oligotrophic, and normal to hyper salinity conditions. Meanwhile, the middle ramp environment is dominated by nummulitids, reflecting meso-oligophotic, oligotrophic, and normal salinity conditions.

The Early Eocene (Cuisian) Nummulites accumulations that have been widely distributed along the northern and southern margins of the Tethys, reflect a development on paleohighs in a distal inner ramp environment and subsequent transportation by wave and current actions into the proximal middle ramp environment.