Paleobiodiversity, Paleobiogeography, and Paleoenvironments of the Middle–Upper Eocene Benthic Foraminifera in the Fayum Area, Western Desert, Egypt

Abstract

The middle–upper Eocene successions of northwest Fayum, Egypt, provide a crucial archive for reconstructing paleoenvironmental conditions and paleobiogeographical patterns of the southern Tethys realm. Stratigraphically, the investigated section is subdivided into three rock units: the Gehannam Formation (Bartonian-Priabonian), the Birket Qarun Formation, and the Qasr El Sagha Formation (Priabonian). A total of 101 benthic foraminiferal taxa, representing 31 genera, 23 families, 13 superfamilies, and four suborders, were identified. The middle–late Eocene age is primarily determined by the co-occurrence of index spinose planktonic foraminifera (Acarinina spp., Morozovelloides spp., and Globigerinatheka semiinvoluta) and benthic foraminiferal assemblages, further supported by the presence of the nannofossil marker Chiasmolithus oamaruensis. Four local benthic biozones are identified and correlated with coeval zones in nearby areas. Quantitative analyses of benthic foraminiferal individuals, diversity indices, ecological parameters, and the benthic foraminiferal oxygen index (BFOI) reveal distinct environmental shifts. The rock unit occupied by the late middle Eocene assemblages is diversified and dominated by calcareous infaunal taxa (e.g., Bolivina spp., Fursenkoina spp., and Nonionella spp.), indicative of low-oxygen outer neritic conditions associated with elevated organic influx. In contrast, the late Eocene Birket Qarun and Qasr El Sagha showed an increase in epifaunal forms and reduced diversity, suggesting a transition to dysoxic-oxic conditions. Paleobiogeographical analysis indicates a strong affinity with the Tethyan realm, with potential faunal exchange through the Trans-Saharan Seaway. These findings enhance our understanding of Paleogene marine connections between the Tethyan and Indo-Pacific realms, contributing to broader discussions on Eocene paleobiogeography and depositional dynamics in North Africa.

1. Introduction

The Eocene Epoch (56 to 33.9 Ma; [1]) is characterized by a significant global climatic shift and sea-level fluctuations [2]. Consequently, reconstructing paleoclimatic conditions is a pivotal approach for comprehending paleoenvironmental evolution [3,4]. The Tethyan realm, a vast tropical seaway stretching from the present-day Mediterranean to Southeast Asia, was a hotspot of marine biodiversity and played an essential role in global biogeochemical cycles during this time [5]. The sedimentary records of the Tethyan realm, particularly those preserved in shallow marine and coastal settings, provide invaluable insights into the paleoenvironmental dynamics of the Eocene time. These records are often rich in microfossils, including benthic foraminifera, which serve as powerful tools for reconstructing the depositional environments and understanding the interplay between global climate change and local sedimentary processes [6,7]. Therefore, many authors directed their attention to studying these sediments, with a particular emphasis on foraminifera (both small and large benthic foraminifera), as well as calcareous plankton and nannofossils, offering a better understanding of biostratigraphic determinations, sea-level changes, and environmental shifts that influenced sediment deposition and faunal distribution [8,9,10,11,12,13,14,15,16,17,18]. Within the Tethyan realm, the Eocene strata of the Fayum Depression in Egypt are characterized by widespread mixed carbonate-siliciclastic successions, exhibiting pronounced vertical and lateral lithological variations that reflect dynamic shifts in depositional environments and paleogeographic conditions [4,19,20,21,22,23]. The Fayum region is renowned for its exceptional preservation of both marine and terrestrial fossils, offering a unique window into the environmental and ecological conditions of the northeastern African margin during the Eocene [24]. Additionally, they are also characterized by the occurrence of planktonic foraminifera, large and small benthic foraminifera, ostracods, bryozoans, calcareous nannofossils, and corals, which are considered important tools to reconstruct the paleoenvironmental conditions [25,26,27,28,29]. The frequent occurrence of benthic foraminifera in the shallower parts of the depositional settings is indicative of the paleo-water depth and paleoenvironments [4,30]. The diversity, abundance, and morphological features of these microfauna are primarily correlated with organic flux, food supply, and oxygen levels [31]. Furthermore, both large and small benthic foraminifera provide valuable biostratigraphic age determinations in shallow water environments due to the scarcity of index planktonic foraminifera [32,33,34,35].

The Eocene exposures in the Fayum Province, situated on the western side of the Nile Valley, have been extensively studied, with a primary focus on stratigraphy and systematic paleontology [25,28,29,36,37,38,39,40,41,42,43,44,45,46,47]. However, these studies have not fully explored the potential of benthic foraminifera for paleobiogeographic and paleoenvironmental reconstructions. To address this gap, the present study integrates micropaleontological data to provide a comprehensive understanding of paleoecology and paleoenvironmental conditions during the deposition of the Eocene succession. By combining detailed lithostratigraphic analysis with refined biostratigraphic zonations and correlations to adjacent regions, this study sheds new light on the depositional history of the Fayum area. Furthermore, it emphasizes the paleobiogeographic distribution of benthic foraminiferal species within the Tethyan province, tracing their spatial and temporal occurrences to better understand their evolutionary and environmental significance during the Eocene.

2. Geologic Setting and Stratigraphy

The Fayum area is located on the western bank of the Nile Valley, Egypt, situated approximately 100 km southwest of Cairo City. It covers an area of about 1500 km², lies between latitudes of 28°60′ and 29°45′ N and longitudes of 30°00′ and 31°15′ E. The evolution of the Fayum basin was influenced by the tectonic movements between the African and Eurasian plates [48]. This area is covered by sedimentary deposits from the middle Eocene to Miocene and is unconformably overlaid by Pliocene and Pleistocene sediments [43,49,50]. The studied section is located in the northwestern part of the Qarun Lake, between a latitude of 30°26′0.8″ E and a longitude of 29°27′25.8″ N (Figure 1). The investigated section attains a thickness of about 205 m and is classified into three rock units arranged in ascending stratigraphic order as follows: the Gehannam Formation (middle–upper Eocene), the Birket Qarun, and the Qasr El Sagha formations (upper Eocene) (Figure 2).

2.1. The Gehannam Formation (Bartonian-Priabonian)

Said [50] previously named this rock unit the Ravine beds at Qaret Gehannam in the western part of the Fayum area. In the present study, the Gehannam Formation is 50 m thick, occupying the lower part of the studied section. The basal part of this rock unit is composed of fissile shales, displaying a high abundance of foraminiferal assemblages. This is followed by interbedded bioturbated sandstones and sandy shales, marked by low occurrences of the microfaunal content and the complete absence of benthic foraminifers at some intervals. The middle part comprises intercalations of bioturbated sandstones and sandy siltstones with abundant macrofauna (Figure 3A), particularly pelecypods and turritellid gastropods (e.g., Turritella spp.).

The uppermost part of this formation consists of argillaceous bioturbated sandstones and gypsiferous shale with iron oxide patches representing the unconformity surface separating the Gehannam Formation from the overlying Birket Qarun Formation (Figure 2 and Figure 3B). The Gehannam Formation is assigned to the late middle-to-late Eocene (Bartonian–Priabonian) based on the presence of index spinose and small-sized planktonic foraminifera (Morozovelloides spp., Acarinina spp., Globigerinatheka semiinvoluta), further corroborated by the occurrence of the marker nannofossil Chiasmolithus oamaruensis [28,29,36,52,53].

2.2. The Birket Qarun Formation (Priabonian)

This rock unit was first recognized as the Birket Qarun Series for the sandstones and shales layers enriched in Nummulites spp. and Operculina spp. in the northern part of the Qarun Lake [49]. In this study, the Birket Qarun Formation has a total thickness of about 98 m, unconformably overlying the middle–upper Eocene Gehannam Formation and underlying the upper Eocene Qasr El Sagha Formation. The lower part of this formation comprises cross-bedded sandstones (Figure 2 and Figure 3C), followed by highly fossiliferous and bioturbated sandy coquina beds, which are densely populated with Carolia, oysters, and gastropods. These layers are followed by an interval distinguished by signs of disturbance, as indicated by the presence of highly burrowed and rhizoliths-bearing strata. The middle part of the Birket Qarun Formation has an approximate thickness of 6 m, consisting of intercalated dark gray fissile sandy shales and brown siltstones with gypsum veinlets. The topmost part of this rock unit is made up of gypsiferous shale, marked by intense bioturbation activity, as evidenced by Thalassinoides burrows (Figure 2 and Figure 3D). Furthermore, the occurrence of Kerunia cornuta, a typical brachipod specimen, was observed, displaying varieties of taphonomic features such as abrasion and bioerosion. This formation is assigned to the late Eocene (Priabonian) age based on its stratigraphic position, where it overlies the middle–late Eocene Gehannam Formation, along with the occurrence of Carolia placunoides and the previous studies carried out on the same rock unit [25,26,28,29,38,39,43,52,54,55].

2.3. The Qasr El Sagha Formation (Priabonian)

This term was proposed by Said [50], which was initially introduced as the Qasr El Sagha series to describe the Carolia-bearing strata at the temple member in the Fayum area [49]. In the present study, the Qasr El Sagha Formation is 57 m thick. The lower part attains 2 m thick and is made up of fossiliferous, fine-grained sandstone followed by repeated cycles of fissile mudstone and fossiliferous siltstones, capped by highly fossiliferous and bioturbated sandstones flooded with shell fragments of Ostrea, Carolia, and gastropods. Upwards, a dark brown siltstone bed attains about 1 m thick and is notable for its high abundance of abraded Kerunia spp. specimens followed by bioturbated ferruginous mudstone affected with Thalassinoides burrows (Figure 3E,F). The upper part of this rock unit consists of sandstones rich in oysters and Carolia banks aligned parallel to the bedding planes (Figure 2 and Figure 3G,H). This interval is also characterized by extensive bioturbation activity, as evidenced by the prominent burrows of Thalassinoides. This part is also marked by the complete absence of any benthic foraminifera and witnessed only the occurrence of macrofauna (bivalves and gastropods). The Qasr El Sagha Formation is attributed to the late Eocene (Priabonian) age according to its stratigraphic position, as well as the occurrence of Carolia placunoides and supporting evidence from previous studies that were conducted on the same formation [25,26,28,29,36,38,43,52,54,55].

3. Materials and Methods

This study is based on the analysis of 60 rock samples collected from the exposed section of the present study, with sampling intervals ranging from 50 cm to 2 m. Each sample, approximately 100 g in weight, was soaked in water for 24 h to facilitate disintegration. The samples were then washed using a 63 µm sieve under a direct flow of running water to ensure adequate separation of finer particles. The hard samples were submerged in hydrogen peroxide for two days after being carefully crushed. To make the picking up of the foraminiferal content easier, the residue from the washing process was dried and separated into several fractions using sieves with mesh sizes higher than 63, 90, 125, and 250 µm. A standard weight of 5 g from each size was used and examined using a binocular microscope (OPTIKA, Via Rigla, 30, Bergamo, Italy). Following the scheme of Loeblich and Tappan [56], the picked benthic foraminiferal species were identified and classified.

The total number of the benthic foraminifera was counted for each sample, and the percentages of benthic (B%), planktonic (P%), calcareous (C%), agglutinated (Ag%), epifaunal (Ep%), and infaunal species (In%) were calculated for each sample. Furthermore, the ternary diagram of Murray [57] was used to assess the salinity and depositional environment settings based on the percentages of the following suborders: Miliolina (M%), Textulariina (T%), and Rotaliina (R%). The oxygen content of the bottom waters was determined using the benthic foraminiferal oxygen index (BFOI). This index relies on the classification of Kaiho [58], who divided the benthic foraminifers into three groups as follows: oxic (O), suboxic (S), and dysoxic (D), where the BFOI formula is represented by [O/(O + D) × 100] (Supplementary Table S1).

To analyze the paleobiogeographic relationships, Q-mode clustering was carried out on the benthic foraminiferal taxa identified from the present study and those previously documented in the southern and northwestern Tethys areas and the North Atlantic regions as well. The retrieved dendrogram was reconstructed using PAST software, version 4.13 [59], via the paired group method and Euclidean similarity index.

The smear slide technique was followed for the nannofossil investigation, and the rock samples were scratched and soaked in 30 mL distilled water for approximately 2 h. After that, a few drops of distilled water containing the suspended sediments were put on a thin glass slide and left for about 24 h in order to make a thin film of the sediment on the slide. Subsequently, the slide was stuck on another thicker glass slide to cover and protect the slide carrying the dry suspension film by using a few drops of mounting medium (Canada Balsam). Nannofossils were examined and observed using a polarized microscope with 1000× magnification. The investigated calcareous nannofossils were identified following the standard schemes [60,61].

4. Results

4.1. Biostratigraphy

The investigation of the collected rock samples revealed 101 species and subspecies, which belong to 4 suborders, 13 superfamilies, 23 families, and 31 genera. Some representatives of the recorded benthic foraminifers and the marker planktonic foraminifera and nannofossils are shown in Figure 4, Figure 5 and Figure 6. The vertical distribution of the identified benthic foraminiferal species enables us to construct four local benthic biozones of middle–late Eocene age (Figure 2, Figure 7, Figure 8 and Figure 9). The recorded biozones are discussed in detail and correlated with their equivalents in nearby areas in Egypt (Figure 10). These biozones are arranged from older to younger as follows:

4.1.1. Bolivina carinata Zone

• Category: Lowest Occurrence Zone.
• Age: late middle Eocene (Bartonian).
• Definition: Interval from the lowest occurrence (LO) of Bulimina jacksonensis to the lowest occurrence of Lenticulina costata.
• Occurrence: The lower part of this zone is unexposed, occupying the lower and middle parts of the Gehannam Formation (samples 1–17), with a thickness of about 35 m (Figure 2 and Figure 7).
• Assemblages: Fifty-five species and subspecies are recorded from this zone. The lower part of this zone is characterized by the occurrence of planktonic foraminifera and high dominance and diversity of the recorded benthic foraminifera (49 species), whereas the middle and upper parts are marked by low representation compared to the lower part (23 species). The most common species are Bolivina anglica, B. jacksonensis, B. jacksonensis striatella, Bulimina jacksonensis, Cancris auriculus primitivus, C. danvillensis, Cibicidoides yankaulensis, Fursenkoina dibolensis, F. squamosa, Lenticulina cultrata, L. insula, Nonionella insecta, N. spissa, Uvigerina peregrina, U. subperegrina, and U. seriata.
• Correlation: The present zone matches with the standard middle Eocene planktonic foraminifera E13 Morozovella crassatus Zone and the lower part of the late Eocene E14 Globigerinatheka semiinvoluta Zone [62]. Globally, this zone corresponds to the top part of the following shallow benthic zones (SBZs): Biozone 51 (Nummulites-Alveolina assemblage subzone) of Wynd [63] from south Iran, SBZ 17–18 of Serra-Kiel et al. [35], Assemblage Zone B of Babazadeh and Cluzel [64], and Assemblage Zone of Changaei et al. [65]. It is completely equivalent to the Brizalina cookie/Nonionella insecta Zone and the Nonion scaphum Zone that were previously recorded from north Eastern Desert and Helwan area [4,32]. It is also matched with the top part of the Palmula ansaryi Zone from the middle Eocene of north Eastern Desert [30,66] and the Nile Valley [39], as shown in Figure 10. In addition, it correlates with the Bolivina jacksonensis/Pararotalia audouini Zone and the Uvigerina mediterranea Subzone from the north Eastern Desert [67,68]. It is equated to the top part of the following zones: the Uvigerina rippensis/Uvigerina churchi Zone from Western Sinai [69], the Nonion scaphum/Pararotalia audouini Zone from the Nile Valley [70], and Lenticulina alatolimbata [71], Bulimina jacksonensis/Uvigerina jacksonensis [36], and the Lenticulina costata [42] zones that were defined from the Fayum area (Figure 10). Moreover, it is coinciding with the large benthic foraminifera Nummulites beaumonti Zone recorded from the middle Eocene of the Nile Valley [72].
• Age: Based on the occurrence of the spinose planktonic species Acarinina rohri, Morozovelloides spp., and Globigerinatheka semiinvoluta, this zone corresponds to the planktonic Morozovelloides crassatus Zone (E13) and the lower part of the Globigerinatheka semiinvoluta Zone (E14), assigning a late middle Eocene (Bartonian) age [32,62,73]. Previously in Egypt, the Bartonian/Priabonian boundary was placed at the first appearance of Globigerinatheka semiinvoluta [28,29]. However, Strougo [74] stated that the first appearance of the Globigerinatheka semiinvoluta, in conjunction with the disappearance of large Acarininids, Truncorotaloides, and Morozovelloides, marks the middle/upper Eocene boundary. Additionally, some authors located the boundary at the top of a new local biozone named the Turborotalia pseudoampliapertura Zone [51,54]. In the Mediterranean and tropical areas, the Bartonian/Priabonian boundary is placed at the last occurrence of the middle Eocene planktonic spinose forms [52,75,76], either at the P14/P15 zonal boundary or in the lower part of zone P15 [76,77]. However, several studies mentioned that Globigerinatheka semiinvoluta appeared below the last appearance of the spinose planktonic forms [62,73,78]. Therefore, the middle/upper Eocene (Bartonian/Priabonian) boundary in the present study is placed within the Globigerinatheka semiinvoluta Zone, at the top of the Bolivina carinata Zone (at the base of sample 18), according to Wade et al. [62]. Furthermore, this boundary is marked at the NP17/NP18 boundary by the lowest occurrence of Chiasmolithus oamaruensis [61], showing a good correlation with Subchron C17n.1n and Zone SBZ18A documented by Özcan et al. [79].

4.1.2. Cancris auriculus primitivus Zone

• Category: Concurrent-Range Zone.
• Age: late Eocene (Priabonian).
• Definition: Interval from the lowest occurrence (LO) of Lenticulina costata to the highest occurrence (HO) of Cancris auriculus primitivus.
• Occurrence: It represents the topmost part of the Gehannam Formation (samples 18–21), attaining a thickness of about 15 m (Figure 2 and Figure 7).
• Assemblages: Fifty-nine species and subspecies are identified from this zone. This zone is characterized by the highest dominance and diversity of the recorded benthic foraminifera. The most abundant species are Bolivina anglica, B. jacksonensis, B. j. striatella, B. kuriani, Bulimina jacksonensis, Cancris auriculus primitivus, C. danvillensis, C. turgidus, Fursenkoina dibolensis, F. squamosa, Hofkeruva mioza, Lenticulina cultrata, Nonionella africana, N. insecta, N. spissa, Uvigerina peregrina, U. subperegrina, U. rippensis, U. seriata, and U. yazoensis.
• Correlation: The present zone matches with the standard late Eocene planktonic foraminifera Globigerinatheka semiinvoluta Zone [62]. It correlates with the Bolivina carinata Zone from the Helwan area [32], the Bulimina jacksonensis/Uvigerina mediterranea Zone, and the lower part of the Bulimina jacksonensis Zone recorded from the north Eastern Desert [30,66]. It is equated to the Bulimina jacksonensis/Uvigerina jacksonensis Zone [72] and the Bulimina jacksonensis/Uvigerina mediterranea Zone [39] in the Nile Valley (Figure 10). It is also correlated with the lower part of the Uvigerina continusa/Bulimina jacksonensis Zone from Western Sinai [69]. Moreover, this zone is coeval with the topmost part of the Bulimina jacksonensis/Uvigerina jacksonensis Zone [36] and the lower part of the Nonionella longicamerata Zone [42] from the Fayum area (Figure 10).
• Age: This zone is attributed to the late Eocene (Priabonian) age and correlated to the planktonic Globigerinatheka semiinvoluta Zone (E14), based on the occurrence of the index planktonic foraminifera Globigerinatheka semiinvoluta [28,62,80,81], along with the rare base occurrence of the marker nannofossil Chiasmolithus oamaruensis, which defines the base of Zone NP18 [82] and the base of CNE17 [83]. In addition, the recorded benthic assemblages within the present zone show strong similarity with those identified in Fayum, the Nile Valley, and the north Eastern Desert [30,32,36,39,42,66,72].

4.1.3. Cibicides westi Zone

• Category: Concurrent-Range Zone.
• Age: late Eocene (Priabonian).
• Definition: Interval from the lowest occurrence (LO) of Cibicides mabahethi to the highest occurrence (HO) of Cibicides westi.
• Occurrence: This zone corresponds to the whole thickness of the Birket Qarun Formation (samples 22–35), with a thickness of about 97 m (Figure 2 and Figure 8).
• Assemblages: Twelve species and subspecies are recorded from this zone. This zone is marked by the lowest dominance and diversity compared with the other zones. The identified species are represented by Bolivina anglica, B. carinata, B. j. striatella, Cibicides mabahethi, C. westi, Cibicidoides laurisae, C. yankaulensis, Lobatula lobatula, Lenticulina alabamensis, L. cultrata, L. insula, and Uvigerina isidroensis.
• Correlation: In the present study, this zone is well correlated with the Cancris auriculus Zone [30] and the middle part of the Bulimina jacksonensis Zone [66] from the late Eocene of north Eastern Desert (Figure 10). It coincides with the lower and middle parts of the Halkyardia minima Zone from the upper Eocene succession in the Helwan area [32]. Also, it completely matches the Cancris cocoaensis/Massilina decoratus Zone [72] and is coeval to the lower and middle parts of the Nonion scaphum/Cibicides pygmous Zone [39] in the Nile Valley (Figure 10). It is correlated to the middle part of the Uvigerina continusa/Bulimina jacksonensis Zone recorded from Western Sinai [69]. Additionally, this zone correlates with the following zones in the Fayum area: the lower part of the late Eocene Nonion scaphum/Cancris subconicus Zone [36] and the middle and top parts of the Nonionella longicamerata Zone [42]; see Figure 10.
• Age: This zone witnessed a complete absence of planktonic foraminifera and nannofossil assemblages. Based on the abovementioned data, stratigraphic position, as well as the occurrence of the index Carolia placunoides, and previous studies that have been performed on the studied section, this zone is assigned to a late Eocene (Priabonian) age [26,38,84].

4.1.4. Nonionella spissa Zone

• Category: Concurrent-Range Zone.
• Age: late Eocene (Priabonian).
• Definition: Interval from the lowest occurrence (LO) of Lenticulina yaguatensis to the highest occurrence (HO) of Lenticulina ellisorae.
• Occurrence: This zone represents the lower and middle parts of the Qasr El Sagha Formation (samples 36–51), measuring a thickness of about 40 m (Figure 2 and Figure 9).
• Assemblages: Fifty-five species and subspecies are recorded from this zone, which is characterized by high diversity and moderate dominance. The most common species are Bolivina carinata, Cancris auriculus primitivus, C. danvillensis, Lenticulina alabamensis, L. alatolimbata, L. cuvillieri, L. limbata, L. yaguatensis, Nonionella insecta, N. spissa, and Spiroloculina depressa.
• Correlation: In the present study, this zone is equivalent to the late Eocene Pararotalia audouini Zone [30] and the top part of the Bulimina jacksonensis Zone [66] from north Eastern Desert (Figure 10). It is matched with the top part of the Halkyardia minima Zone from the Helwan area [32] and with the Nonion scaphum/Cibicides pygmous Zone in the Nile Valley [39]. It also correlates with the middle and upper parts of the Uvigerina continusa/Bulimina jacksonensis Zone at Western Sinai [69]. Moreover, it fits with the lower part of the Quinqueloculina seminula/Q. triangularis Zone [42] and with the middle and top parts of the Nonion scaphum/Cancris subconicus Zone [36] in the Fayum area. Regarding the large benthic foraminiferal zonation, this zone is well correlated to the Operculina pyramidium Zone, which was recognized from the Nile Valley [72], as shown in Figure 10.
• Age: Based on the stratigraphic position of this zone, besides the occurrence of the index macrofauna (Carolia spp.), and the previous studies that have been carried out on the studied section, a late Eocene (Priabonian) age is attributed to this zone.

Figure 10. Biostratigraphic correlation of the benthic foraminiferal zones in the present study with their counterparts [4,30,32,36,39,42,66,67,68,69,70,71,72].

Figure 10. Biostratigraphic correlation of the benthic foraminiferal zones in the present study with their counterparts [4,30,32,36,39,42,66,67,68,69,70,71,72].

4.2. Ecological Parameters and Diversity Indices

The studied samples display significant variations in the recorded benthic foraminiferal assemblages. For each sample, the total number of the recorded benthic species, the infaunal/epifaunal ratio, and the BFOI were calculated (Table S1). The lower (sample 1) and topmost parts (samples 18–20) of the Gehannam Formation are characterized by high occurrences of the recorded benthic foraminifers compared to the middle parts of the same rock unit. These intervals are marked by high species richness, with 49 species in sample 1 and from 24 to 32 species in samples 18–21, where most of the recorded taxa are infaunal species (Bolivina ssp., Uvigerina spp., and Fursenkoina spp.), which is reflected in the elevated values of infaunal/epifaunal ratios (ranging from 51 to 92%) and low BFOI values (from 8 to 49). In addition, these parts exhibit the highest Shannon diversity index (ranging from 2.36 to 3.23) and the lowest dominance (ranging from 0.05 to 0.15; see

Table 1. Benthic foraminiferal species richness, individuals, and diversity indices of the investigated samples.

Sample No.	FM	Taxa_S	Individuals	Dominance_D	Shannon_H
1	Gehannam Fm	49	406	0.06309	3.227
2		5	12	0.2361	1.517
4		12	76	0.1908	2.025
5		2	3	0.5556	0.6365
6		11	45	0.1625	2.04
7		3	3	0.3333	1.099
9		1	1	1	0
10		10	40	0.16	2.008
11		5	6	0.2222	1.561
13		4	6	0.2778	1.33
14		1	2	1	0
15		2	3	0.5556	0.6365
16		2	3	0.5556	0.6365
18		32	188	0.05794	3.137
19		33	190	0.05518	3.159
20		24	339	0.1542	2.364
21		31	577	0.1188	2.672
22	Birket Qarun Fm	4	9	0.2593	1.369
25		8	20	0.14	2.016
27		8	21	0.1429	2.004
29		8	14	0.1429	2.008
33		9	19	0.1302	2.114
34		9	21	0.1202	2.153
35		4	8	0.2813	1.321
37	Qasr El Sagha Fm	14	62	0.1733	2.177
40		12	39	0.1006	2.373
46		27	66	0.06336	3.035
47		23	88	0.1059	2.631
48		16	158	0.378	1.656
50		1	2	1	0
51		2	3	0.5556	0.6365
52–60	Barren

). In contrast, the middle part (samples 2–17) is characterized by a lower abundance of benthic foraminifera, exhibiting reduced species richness (ranging from 1 to 12 species), lower Shannon diversity (ranging from 0.63 to 2.04), and higher dominance (ranging from 0.16 to 0.55).

These samples are also marked by the occurrence of macrofauna (gastropods and bivalves), trace fossils (Thalassinoides sp.), and plant remains (rhizoliths). The recorded species in the Gehannam Formation mostly belong to suborder Rotaliina (83 to 100%), followed by suborder Miliolina (reaches up to 17% in sample 11), whereas suborder Textulariina is almost absent and displays low percentages (from 1 to 17% in samples 1, 3, and 19; see Table S1 and Figure 11).

Moreover, the Birket Qarun Formation exhibits a significant decrease in the recorded benthic foraminiferal assemblages, indicated by the lowest richness (ranging from 4 to 8 species), Shannon diversity (ranging from 1.32 to 2.14), and the highest dominance (ranging from 0.13 to 0.28; see Table 1). Moreover, it is marked by a high dominance of macrofauna (gastropods and bivalves), trace fossils (Thalassinoides spp.), and plant remains. All benthic foraminifera species recognized in the Birket Qarun Formation belong to the suborder Rotaliina (100%), while Miliolina and Textulariina suborders are absent (Table S1 and Figure 11).

Upwards, the Qasr El Sagha Formation showed a remarkable increase in epifaunal species, where infaunal/epifaunal species decreased (with an average of 40%) and the BFOI increased (mean of 60%, Table S1). Furthermore, this rock unit is characterized by moderate to high species richness (up to 27 species in sample 46), high Shannon diversity index (up to 3.03), and low dominance (from 0.06 to 0.37; see Table 1). However, the topmost part of the Qasr El Sagha Formation experienced a complete absence of benthic foraminiferal assemblages. This rock unit is also distinguished by a high abundance of molluscan shells, such as gastropods and bivalves (Carolia spp.), coupled with the high activity of bioturbation (Thalassinoides sp.). Most of the benthic taxa identified from the Qasr El Sagha Formation belong to the suborder Rotaliina (ranging from 83 to 100%), followed by the suborder Miliolina (17% in sample 47), while suborder Textulariina is absent (Table S1 and Figure 11).

The nannofossil assemblages display disparate distribution patterns within the studied rock units. The Gehannam Formation exhibits sporadic occurrences of nannofossils (samples 1, 10, and 18–21), whereas the complete absence of nannofossil taxa characterizes both the Birket Qarun and the Qasr El Sagha formations.

5. Discussion

5.1. Depositional Environments

The findings of the present study indicate significant variations in biotic traits, including diversity and dominance, which were mainly influenced by changes in environmental factors such as oxygen levels, water depth, and type of substrate. The depositional environments of the Gehannam Formation have been subjected to various interpretations, ranging from the middle to the outer shelf of shallow marine settings [85], deep marine environments [24], and the inner to outer shelf [43], reflecting the challenges and complexity of reconstructing the depositional settings of this rock unit. The depositional environment of the Gehannam Formation is established through a multiproxy approach that integrates sedimentological, micropaleontological, macrofaunal, and geochemical data. These lines of evidence collectively provide a robust and contemporary framework for reconstructing its depositional settings. Sedimentologically, the lower and upper parts of the formation are characterized by well-bedded shales and fine-grained mudstones, indicative of deposition in a low-energy marine setting, consistent with the findings of Hadi et al. [86] and Wilson [87]. Notably, these intervals exhibit high abundances of planktonic foraminifera and infaunal benthic species (e.g., Bolivina spp., Uvigerina spp., and Fursenkoina spp.), suggesting deposition in quieter, deeper marine environments with reduced oxygen levels [4,30,88]. The co-occurrences of these infaunal benthic taxa were assigned as bioindicators for the shelf and upper bathyal settings [89,90] (see Figure 12). Bulimina and Uvigerina genera are also distinguishing the oxygen-minimum and the organic-rich sediments [91]. Additionally, the low values of the BFOI (27 in sample 1 and ranging from 8 to 49 in samples 18–21), coupled with high P/B% (reaching 43% in sample 1 and varying from 13 to 27% in samples 18–21, see Table S1), in these intervals suggest middle to outer shelf environments [41,70]. Additionally, the low values of the arenaceous/calcareous ratio (1% in sample 1, 2% in sample 19) characterize deposition in environments with high temperature, normal salinity, and high calcium carbonate above the Carbonate Compensation Depth (CCCD) [92,93]. In contrast, the middle portion of the Gehannam Formation comprises intercalations of bioturbated calcareous sandstones and sandy siltstones, rich in macrofauna, particularly pelecypods and turritellid gastropods (e.g., Turritella spp.). The presence of bioturbation and a diverse macrofaunal assemblage indicates well-oxygenated and warm conditions, characteristic of inner shelf depositional environments [4,21,94,95]. In addition, this part displays a remarkable increase in the epifaunal taxa with shallow water ecological preferences, which is shown in the elevated values of BFOI (reaching 50, Table S1), indicating that the deposition occurred in a shallow marine environment with high oxygen levels. Furthermore, the occurrence of Lenticulina spp., Lobatula lobatula, Eponides spp., and Cancris spp. indicate inner to middle shelf environments with high oxygen conditions [4,30] (Figure 12). The observed faunal shift, accompanied by a progressive decrease in mean grain size from fissile mudstones and shales to silt and argillaceous sandstones, along with the presence of bioturbated muddy sediments, signifies a transition to a lower-energy depositional environment [96]. Such fining-upward sequences are typically associated with a decline in hydrodynamic energy, facilitating the settling of finer sediments. Furthermore, the prevalence of bioturbation within these muddy sediments suggests increased benthic activity, indicative of stable, low-energy conditions. Geochemical analyses on the same samples further support these interpretations. Sayed et al. [84] reported that paleowater depth and redox-sensitive elemental ratios, including Fe/Mn, Ni/Co, and V/(V+Ni), confirm deposition under a well-oxygenated shallow marine environment.

The observed transgressive interval in the lower part of the Gehannam Formation (sample 1) aligns with the short-term eustatic sea level rise of the PaBart1 cycle, and the topmost part (samples 18–21) corresponds to the PaPr1 sequence boundary [97]. Following upward through the middle part of the Gehannam Formation, there were remarkable lithologic changes to bioturbated sandstones (Thalassinoides spp.), coarse-grained siltstones, and sandy claystones. According to Buatois and Mángano [98], the Thalassinoides is a trace fossil characterized by complex networks of horizontal to sub-horizontal burrows with T- and Y-shaped bifurcations. They emphasize its significance in paleoenvironmental reconstructions, noting that Thalassinoides is typically associated with shallow-marine environments [99], particularly within the Cruziana ichnofacies, indicative of moderate to high-energy conditions. Noteworthy is that these sandstone beds are dominated by macrofauna such as Turitella spp., Carolia spp., and Ostrea spp. To the best of our knowledge, the Turritella spp. are suspension-feeding gastropods typically associated with normal marine salinity and moderate- to low-energy environments from the low intertidal to approximately 100 m water depth [100]. Their high abundance is often linked to nutrient-rich conditions, suggesting a productive marine setting, potentially influenced by upwelling or organic enrichment [101]. Moreover, Carolia spp. are known from muddy, soft-substrate environments, often associated with low-energy, offshore, or restricted settings [102]. Their presence suggests relatively stable conditions with periodic sedimentation events. In addition, the Ostrea spp. are sessile, suspension-feeding bivalves that attach to hard, rigid substrates or conspecific shells. They commonly indicate shallow-marine settings with moderate energy, where stable substrate availability and nutrient influx support their growth [103].

The taphonomic criteria observed in the present study were incorporated to strengthen the environmental interpretation. The shell fragmentation and abrasion was as follows: the moderate degree of fragmentation observed in Turritella and Ostrea suggests intermediate energy conditions, likely within a sublittoral to inner shelf setting, where occasional wave or current action influenced preservation [104]. Encrustation and bioerosion were as follows: the presence of bioerosion on Ostrea shells by bryozoans, along with encrusting epibionts, suggests prolonged exposure on the seafloor prior to burial, consistent with low sedimentation rates in a siliciclastic-dominated system [105]. Disarticulation of bivalves was as follows: the moderate disarticulation in Carolia and Ostrea specimens supports episodic burial events, possibly linked to storm-induced deposition or shifting sediment conditions [106].

Figure 12. Upper depth limit of dominated benthic foraminifera indicators, after Van Morkhoven et al. [107], Culver [108], Van der Zwann et al. [109], Murray [90], and Holbourn et al. [89].

Figure 12. Upper depth limit of dominated benthic foraminifera indicators, after Van Morkhoven et al. [107], Culver [108], Van der Zwann et al. [109], Murray [90], and Holbourn et al. [89].

The lithological variations, besides the occurrence of macrofauna and trace fossils, indicate shallower proximal settings compared to the lower and topmost parts of the same rock unit (Figure 13). This shallowing upward interval could probably be linked to a drop in relative sea level, coinciding with the global late Bartonian sea level fall. Additionally, highly bioturbated and fossiliferous beds reflect shallow marine environments with sufficient nutrients and well-oxygenated conditions.

Moreover, the overlying Birket Qarun Formation is interpreted as an overall shallow marine environment with high-energy conditions, corroborating with the earlier suggestions [24,71]. This interpretation is based on the observed remarkable paleontological and sedimentological features within this rock unit, such as cross-bedded sandstones indicating mainly near-shore areas, where high-energy conditions such as strong currents or wave action are the main controlling factors for these primary structures [110]. Additionally, the fossiliferous sandstone with non-oriented and fragmented oysters, Carolia spp., and gastropod shells reflects agitated environments [25]. The presence of rhizoliths-bearing strata also supports these shallower settings. The middle part of this unit exhibits characteristics of a low-energy, restricted marine to brackish depositional setting, evidenced by the occurrence of glauconitic sandstones, gypsiferous shales, and siltstone beds. Glauconite suggests slow sedimentation in a mildly reducing environment [111], while gypsum and evaporitic facies indicate episodic salinity fluctuations [112]. The lack of wave-generated structures, combined with low bioturbation and sparse trace fossil activity, suggests suboptimal benthic conditions due to periodic physicochemical stress, consistent with previous interpretations [24,113]. The increased sand fraction upward in the middle part, higher bioturbation intensity, and occasional thin-bedded rippled sandstones indicate a gradual shift toward a more open, shallow marine setting under waning energy conditions. The coarsening-upward pattern, coupled with the reappearance of wave-modified bedding, suggests a progradational trend potentially linked to a regressive phase. This interpretation aligns with previous facies models of the Eocene shallow marine deposits in North Africa, where fluctuating salinity and periodic marine incursions played a key role in shaping the depositional environments [114].

Following upwards, the upper part of this rock unit is deposited in shallow marine environments under waning energy and physicochemically stressed conditions, mostly with a high to moderate sedimentation rate and salinity shifts, as evidenced by the remarkable presence of fissile mudstone layers, the scarcity of molluscan shells (bivalves and gastropods), along with the occurrence of trace fossils. Sedimentologically, the observed fining-upward sequence, characterized by a progressive decrease in grain size from intercalated argillaceous sandstone and sandstone beds to piles of shale beds, is indicative of a reduction in hydrodynamic energy, a trend commonly associated with waning depositional conditions [115]. Furthermore, well-bedded, fine-grained mudstones and bioturbated shales suggest deposition in a low-energy marine environment, consistent with previous studies on similar settings [86]. The associated decrease in macrofaunal diversity, particularly the scarcity of epifaunal suspension feeders, further supports a stressed paleoenvironment [58]. Regarding the foraminiferal assemblages, the Birket Qarun Formation is characterized by the lowest diversity (from 1.3 to 2.1), low species richness (8 species), and high dominance (from 0.21 to 0.28), which could be related to the predominant stressed conditions in an extremely shallow setting, low primary productivity, and/or a highly dynamic setting with a high sedimentation rate, which limit the biological demands of the benthic communities. In addition, most of the recorded species within this part are epifaunal taxa with shallow water ecological preferences (infaunal/epifaunal ratios range from 32 to 45%), which is shown on the elevated values of BFOI (ranges from 55 to 68), indicating that the deposition occurred in a shallow marine environment with high oxygen levels (Figure 13). Geochemically, Sayed et al. [84] confirmed a shallow marine setting with predominantly oxic conditions. The Zr/Rb and Fe/Mn ratios, a proxy for grain-size variation, and paleohydrodynamic conditions further support the interpretation of a gradual decline in energy, aligning with our sedimentological observations [3,84].

The Qasr El Sagha Formation was previously interpreted as coastal and marginal-marine environments characterized by tropical and subtropical climate conditions [49,50]. In the present study, the Qasr El Sagha Formation is interpreted as deposits of shallow marine environments with high energy and oxygen levels (Figure 13). This suggestion is supported by the high occurrence of fragmented, non-oriented oyster shells, Carolia spp., Kerunia spp., gastropods, echinoids, and corals, coupled with the occurrence of the remarkable bioturbation activity (Thalassinoides sp.), which prefers shallow settings where clear water, high oxygen levels, and warm conditions are predominant, providing favorable conditions for their biological demands [4]. Notably, Thalassinoides is frequently found in shallow-marine carbonate platforms, inner to middle shelf settings, and deltaic environments, where it is produced by burrowing crustaceans, typically decapods [116]. It is commonly associated with moderate-energy environments, such as subtidal sandbars, storm-influenced shelf settings, and lagoonal deposits, indicating oxygenated, nutrient-rich waters [98,117]. Furthermore, Thalassinoides sp. indicates a versatile range of environments, from shallow [118,119], which indicates colonization during periods of reduced sedimentation and relative substrate stability. Moreover, Thalassinoides also occur in deep-sea turbiditic deposits, slope settings, and deep-marine contourites, where they reflect oxygenated bottom conditions and low sedimentation rates [120]. It is also common in storm-influenced and tide-dominated environments, where periodic sediment influx is followed by bioturbation during quiescent phases, reflecting intermittent sedimentation dynamics [121,122]. Consequently, Thalassinoides occupy either the shallow or deep marine settings, forming Cruziana Ichnofacies, which dominate the area between the fairweather wave base to the upper part of the storm wave base [26,123]. Furthermore, the Kerunia specimens exhibited abrasion and bioersion as taphonomic criteria could reinforce our adopted interpretation of high-energy shallow-marine environments, including intertidal to subtidal settings, reefs, and rocky shorelines for the Qasr El Sagha Formation [124]. These specimens were collected previously from the same section and have influenced low levels of Gastrochaenolites borings, assigning Trypanites ichnofacies, reflecting shallow marine agitated environments [26]. Noteworthy, the Trypanites ichnofacies is an important bioerosional trace fossil assemblage that is typically associated with hard substrates, including firm grounds, lithified sediments, and rock grounds [105]. Similarly, the occurrence of fragmented and non-oriented shells of Carolia spp. indicates a shallow marine setting with high-energy conditions (Figure 13). In contrast, shells with preferred orientation in the uppermost part of this rock unit are observed, indicating rapid in-situ burial, often driven by high-energy depositional events [38]. The geochemical analysis of the Qasr El Sagha Formation [84] revealed upward low values of Zr/Rb, coupled with high Fe/Mn reflecting high energetic settings and shallow water depth [3]. Additionally, the geochemical paleo-redox elemental ratios (Ni/Co and V/(V+Ni)) for this rock unit indicate high paleo-oxygenation levels, suggesting deposition under oxic conditions [84].

The diversity index calculations of the recorded benthic foraminiferal assemblages within the Qasr El Sagha Formation exhibit high numbers of foraminiferal individuals, in conjunction with high Shannon diversity (reaching 3.035) and lower dominance (from 0.05 to 0.55, except sample 50; see Table 1), high ratios of epifaunal species, and a high BFOI, indicating that the deposition took place in shallow marine settings under well-oxygenated conditions and low to moderate organic flux (Figure 13 and Table S1). Some peaks of infaunal assemblages can be noticed (reaching 83% infaunal/epifaunal ratio in sample 48; see Table S1), which could be attributed to high sediment input to the ocean floor. This shallowing setting experienced within the Qasr El Sagha Formation was previously reported in the Fayum area [27,43,51] and is equivalent to depositional sequence 4 (DS 4) in the northeastern desert [81], which was globally correlated to the eustatic sea-level fall (TA4.3 and TA4.4) [125] and Pr1, Pr2, and Pr3 (ca. 37.2–33.9 ma) [126].

This sea-level fall coincided with a regional regression in the southern Tethys margin, which is related to regional uplift in northern Egypt as a result of the inversion of the Syrian arc system during the middle–late Eocene, representing the final stage of the collision between Africa/Arabia and Eurasia [71], resulting in the closure of the Neo-Tethys and uplift along the southern Tethys realm [72]. Similar regression patterns have been documented in the Gulf of Suez region, driven by both eustatic fall and regional tectonic uplift [127] and the Levant margin along with the eastern Mediterranean, where sedimentary sequences exhibit shallowing-upward facies successions indicative of a coeval sea-level drop [128]. Accordingly, this shallowing trend supports the hypothesis that this regression was not only a local phenomenon but part of a broader southern Tethyan margin response to global sea-level fluctuations [129]. This trend aligns with the global middle-to-late Eocene eustatic sea-level fall, which has been attributed to cooling events and early ice-sheet development in Antarctica [130]. Regionally, this regression coincides with a widespread shallowing trend recorded along the southern Tethyan margin, including the Egyptian Western Desert and adjacent basins [128,131]. The progressive reduction in accommodation space, coupled with changes in sedimentary facies and foraminiferal assemblages, supports the interpretation of a regressive phase in the studied succession, validating the link between global eustatic changes and regional tectonic influences during the late Eocene.

5.2. Paleobiogeography

Q-mode clustering was carried out to outline the paleogeographic distribution of the recorded benthic foraminiferal assemblages. This analysis is applied to a data matrix consisting of the recorded benthic species in this study and their paleogeographic distribution in the other five Tethys countries, including Libya, Italy, Spain, England, and France, as shown in

Table 2. Geographic distribution of the identified species (1/0 indicates present/absent).

Species	Egypt	Libya	Italy	France	Spain	England
Cancris a. primitivus	1	0	1	1	0	0
Cancris danvillensis	1	0	0	0	0	0
Cancris amplus	1	1	0	0	0	0
Cancris subconicus	1	0	1	1	0	0
Spiroloculina dorsata	1	0	0	1	0	1
Spiroloculina esnaensis	1	0	0	0	0	0
Quinqueloculina seminula	1	0	1	1	0	1
Cibicides mabahethi	1	1	0	0	0	0
Cibicidoides laurisae	1	1	0	0	0	0
Cibicides libycus	1	1	0	0	0	0
Planulina cocoaensis	1	1	0	0	0	0
Lenticulina alabamensis	1	0	0	0	0	0
Lenticulina alatolimbata	1	1	0	0	0	0
Lenticulina clerici	1	1	0	0	1	0
Lenticulina cultrata	1	1	0	0	1	0
Lenticulina cuvillieri	1	1	0	0	0	0
Lenticulina cf. ellisori	1	1	0	0	0	0
Lenticulina isidis	1	1	0	0	0	0
Lenticulina limbata	1	1	1	0	0	0
Lenticulina turbinata	1	1	0	0	0	0
Lenticulina yaguatensis	1	1	0	0	1	0
Stillostomella jacksonensis	1	1	0	0	0	0
Bolivina anglica	1	1	0	1	0	0
Bolivina brabantica	1	0	0	0	0	0
Bolivina carinata	1	1	0	1	1	0
Bolivina jacksonensis	1	1	0	0	0	0
Bolivina j. striatella	1	0	0	0	0	0
Bolivina kuriani	1	0	0	0	0	0
Brizalina cookei	1	0	0	0	0	0
Nonionella spissa	1	0	0	0	0	0
Lagena striata	1	0	1	1	0	0
Fursenkoina dibolensis	1	0	0	1	1	0
Fursenkoina squamosa	1	0	0	0	0	0
Uvigerina cocoaensis	1	1	0	1	0	0
Uvigerina hispidocostata	1	1	0	0	1	0
Uvigerina mexicana	1	1	1	0	0	0
Uvigerina rippensis	1	1	0	0	1	0
Uvigerina seriata	1	0	0	0	0	0
Bulimina jacksonensis	1	1	0	0	0	0
Textularia adalta	1	0	1	0	0	0

. Two clusters resulted from the Q-mode analysis (Figure 14), one of them representing southern Tethys countries (Egypt and Libya) and the other one referring to the northwestern Tethyan and the North Atlantic regions (Italy, Spain, England, and France). In the present study, the common species between Egypt and Libya are represented by 23 species, including Cancris amplus, Cibicides mabahethi, Cibicidoides laurisae, Planulina cocoaensis, Neoeponides schrebersi, Lenticulina alatolimbata, L. clerici, L. cultrata, L. cuvillieri, L. cf. ellisorae, L. isidis, L. limbata, L. turbinata, L. yaguatensis, Bolivina anglica, B. carinata, B. jacksonensis, Stillostomella jacksonensis, Uvigerina cocoaensis, U. hispidocostata, U. mexicana, U. rippensis, and Bulimina jacksonensis. This reflects strong similarities between the recorded species and those found in the southern Tethys province, suggesting potential migration through the Trans-Sahara Seaway, consistent with the findings of Sayed et al. [4] and El Baz [27]. This migration was further supported by means of ostracods in the southern Tethyan realm (Egypt, Tunisia, Libya) [23,132,133,134,135,136,137]. Moreover, the second cluster represents the western Tethys and north Atlantic areas, exhibiting significant minor similarities between the recorded species and those reported from France, Italy, Spain, and England, where the common species reach 12, 9, 8, and 2, respectively; see Table 2. The common species are represented by Cancris auriculus primitivus, C. subconicus, Spiroloculina bicarinata, S. dorsata, Quinqueloculina seminula, Lobatula lobatula, Lenticulina cultrata, L. limbata, L. yaguatensis, Bolivina anglica, B. carinata, Lagena striata, Fursenkoina dibolensis, Uvigerina cocoaensis, U. mexicana, U. rippensis, Textularia adalta, and Spiroplectammina adamsi. This low similarity could be linked to the benthic nature of the identified species, constraining them from moving over a long distance. Also, it can be attributed to changes in environmental and climate conditions in northern latitudes, such as cooler climates, which did not align with the requirements for their biological demands. This suggestion shows a good accordance with the interpretations of Sayed et al. [4] and Sayed et al. [23].

6. Conclusions

The studied middle–upper Eocene section exposed in the Fayum area is classified into three lithostratigraphic units as follows: The Gehannam (Bartonian-Priabonian), the Birket Qarun, and the Qasr El Sagha formations (Priabonian). The investigated samples showed variations in the density of the recorded benthic foraminiferal assemblages, which were linked to changes in the depositional conditions. The vertical distribution of the identified species allowed us to recognize four benthic biozones, namely, the Bolivina carinata Lowest Occurrence Zone (Bartonian), Cancris auriculus primitivus Concurrent-Range Zone, Cibicides westi Concurrent-Range Zone, and Nonionella spissa Concurrent-Range zones (Priabonian). The middle/upper Eocene boundary is tentatively placed based on the last and first appearance of the marker planktonic species and located herein within the Globigerinatheka semiinvoluta Zone. The paleontological data and sedimentological features have been used to evaluate the depositional settings of the middle–upper Eocene sediments in north Egypt, suggesting that the deposition took place in a shallow marine environment with high-energy conditions and oxygen levels. Meanwhile, the lower and topmost parts of the Gehannam Formation are deposited in middle to outer shelf settings with low to moderate oxygen conditions. The observed low values of the recorded benthic foraminifera in some intervals can be linked to low primary paleoproductivity due to the strong paleohydrodynamic regime and substantial sediment influx to the ocean floor, providing unfavorable conditions for the faunal proliferation. The foraminiferal assemblages in the present study are similar to those found in southern Tethys countries, reflecting a possible migration route through the Trans-Sahara Seaway. However, the slight resemblance with the North Atlantic and the northwestern Tethys province is related to their benthic nature, which constrains them to spread over a long distance, sea level fall, and/or environmental and cooler climate conditions.