Stratigraphic and Paleoenvironmental Studies on the Lower Eocene Succession at El-Guss Abu Said Plateau, Farafra Oasis, Western Desert, Egypt

Boukhary et al. (1995) considered the transitional rocks, which occur between the shale of the Esna Formation at the base and the limestone of the Farafra Formation at the top, as an independent unit and named the Nusf Formation to describe this section. The Nusf Formation (Fig. 3C) is readily distinguishable from the underlying Esna shale and overlying Farafra limestone (Fig. 3D). The lithology includes ∼67.5 m thickness of well-bedded, varve-like intercalations of marl and limestone and is marked by two prominent nodular limestone ledges. It is generally nummulitic at several horizons. The overlying Farafra Formation is characterized by ∼30 m thickness of alveolinid-rich, slightly argillaceous, moderately hard limestone beds that become harder and more dolomitic upward. The Farafra Formation is separated from the underlying Nusf Formation by an undulating, erosional surface (Fig. 3A). The presence of such surfaces was postulated to indicate tectonic activity by Said & Kerdany (1961). Said (1962) reported a similar feature between the Maastrichtian and late Paleocene sediments, which he attributed to tectonic uplift that occurred at that time. Later, this idea attained general acceptance, as mentioned by Youssef & Abdel Aziz (1971) who reported many unconformities at Farafra Oasis and by Mahfouz et al. (2021) who attributed the shallowing of sea level at some intervals in the Maastrichtian deposits of the Aqaabat section at Farafra to syn-sedimentary tectonic events. Additional hiatuses were recorded elsewhere in Farafra. For example, at the Aqaabat section, Maastrichtian–Selandian and Paleocene–Eocene hiatuses were reported by Obaidalla et al. (2006).

Several previous studies examined foraminiferal assemblages at Farafra Oasis. Said & Kerdany (1961) assigned the age of the Esna Formation at Farafra Oasis to the late Paleocene, based upon the occurrences of primitive Nummulites and alveolinids species. Abdel-Kireem & Samir (1995) studied the planktonic foraminifera of the Northern Gunna section, assigning most of the Esna Formation to the early Eocene (Morozovella subbotina–Acarinina pentacamerata zones). Haggag et al. (2010) also assigned the Esna Formation to late Paleocene–early Eocene in age, considering the Esna and Nusf Formation as one stratigraphic unit regardless the lithostratigraphic and paleoenvironmental considerations that differentiate these units. In our present study, the assemblages of Esna Formation of El-Guss Abu Said plateau indicate its deposition during early Eocene time (Morozovella marginodentata E3 Zone and Morozovella formosa formosa (E4) Zone) while the lowermost part of Esna Formation seems to be unexposed in the study area. Previous detailed taxonomic study of Nusf Formation nummulites assigned it to be Ypresian in age (Shreif et al., 2019). The Farafra Formation has only alveolinids and needs more detailed further studies to determine its age; the authors encourage such future research. Said & Kerdany (1961) reported an unconformity between the Maastrichtian and late Paleocene deposits in sections at the Farafra Oasis, consistent with the reported tectonic uplift that occurred during the early Eocene of the study area. Subsequently, Youssef & Abdel-Aziz (1971) reported many other local unconformities at Farafra Oasis that are associated with tectonics. These previous reports inspired us to investigate the benthic foraminifera and depositional environment of El-Guss Abu Said strata to elucidate the stratigraphy and nature of sedimentation there.

Seventy-three benthic foraminiferal species from 86 samples from El-Guss Abu Said Plateau were identified to provide evidence to interpret the paleoenvironments of the rock units that represent the Esna, Nusf, and Farafra formations. Identifications were made using Le Roy (1953), Loeblich & Tappan (1987), and Bolli et al. (1994). The planktonic/benthic (P/B) ratios for all samples were also utilized in interpreting the different marine habitats. A benthic foraminiferal distribution chart was created (Fig. 4), and the P/B ratios for all samples were calculated (Fig. 5). These compilations revealed that samples 1–43 from the lowermost part of the El-Guss Abu Said succession showed a dominance of planktonics in the lower part of Esna Formation, while the benthic taxa found comprised benthic Biofacies A. In the upper part of the Esna Formation (samples 41–50), the P/B ratios declined, and benthic foraminiferal Biofacies B predominated. The P/B ratio remained <0.5 in the lower part of the Nusf Formation (samples 51–60), above which planktonics were seldom found. Up section through the Nusf and into the Farafra formations (samples 61–82), the foraminiferal assemblage was represented by two previously identified species of Nummulites (Nummulites solitarius and Nummulites fraasi). These Nummulites were accompanied by other shallow-water benthic foraminifera and alveolinids (Biofacies C). The presence of alveolinids, which tended to be open-platform dwellers, provides evidence for further shallowing. These shallow benthic forms also were found in samples 83–86 from the Farafra Formation. Biofacies based on the benthic foraminiferal associations and P/B ratios, and the paleoenvironments they represent, are interpreted as follows:

In the present study, we distinguish different biofacies based on the predominant genera rather than species according to benthic foraminiferal depth zonation of Culver (1988). Samples 1–43 with foraminiferal assemblages that represent Biofacies A were from the lowermost part of the Esna Formation. Benthic species that are known as representatives for upper bathyal environments (200–600 m water depth) are restricted to these samples from the lower Esna. These taxa are common here including Valvulinaria scrobiculata, Valvulinaria aegyptiaca, Gyroidinoides giradanus, and Bulimina farafraensis. Other abundant species include Protoglobobulimina pupoides, Laevidentalina colei, Dentalina frontierensis, and Spiroplectammina esnaensis, while Ramulina tubensis and Ramulina elongata were rare and restricted to this biofacies (Figs. 6, 7). Middle to upper bathyal genera like Lenticulina are also abundant (Lenticulina alabamensis). The abundance of certain genera like Bulimina, Gyroidinoides, and Valvulinaria (which are found only in the upper bathyal environment; Culver, 1988) and the complete absence of shallow benthic taxa (which is consistent with relatively high P/B ratios in these samples) characterizes the environment as outer-shelf, upper bathyal in depth.

Samples 44–50 with foraminiferal assemblages that represent Biofacies B were from the uppermost part of the Esna Formation. Although this biofacies has many species that are extended from Biofacies A (e.g., Cancris auricula, Eponides lotus, and Paralabamina lunata), the restricted appearance of certain genera like Cibicides and Cibicidoides in addition to the abundant occurrence of Nonionella (Nonionella insecta), which cannot be found deeper in outer ne-ritic waters (Culver, 1988), indicates the paleoenvironment of Biofacies B was middle shelf, 50–100 m water depth. Cibicidoides farafraensis was recorded for the first time in this biofacies, and Cibicides decoratus was common and restricted only to this biofacies (Figs. 7, 8). Species such as Lenticulina budensis, Lenticulina midwayensis, and Sigmoilina proboscidae, which are known to extend from middle neritic to upper bathyal depths according to the depth zonation of Culver (1988), are rare in Biofacies B.

Samples 51–86, with foraminiferal assemblages that represent Biofacies C, were found throughout the Nusf and Farafra formations. The predominance of nummulites and alveolinids and scarcity of smaller benthic foraminifera reveal the shallowing from a shale-lithology middle-outer shelf environment of the Esna Formation to a progressive increase in carbonates, indicating the transition to a carbonate ramp or platform in the Nusf Formation. Two species of Nummulites are abundant in this biofacies: Nummulites solitarus de la Harpe 1883 and N. fraasi de la Harpe 1883. These two species were identified previously by Shreif et al. (2019), who dated the Nusf Formation as early Eocene (Ypresian) in age (SBZ5–SBZ6) according to the standard shallow-benthic zonal scheme of Serra-Kiel et al. (1998) and Papazzoni et al. (2017). The Farafra Formation is also rich in alveolinids only, indicating the continuity of the platform environment during the deposition of the Farafra Formation, in spite of the presence of an irregular erosional surface with some iron oxides separating the Nusf Formation from the Farafra Formation and a limestone ledge separating the Esna Formation from the Nusf Formation. These probable erosional surfaces indicate a cessation in deposition for some time during the early Eocene. The complete absence of planktonic foraminifera in the Nusf and Farafra Formations stands as an obstacle for determining the span of this hiatus accurately.

The lithological evidence, combined with the biofacies assessments, indicate shoaling episodes through the succession. Moreover, the presence of an irregular erosional surface between the Nusf and the Farafra formations and a ledge of limestone between the Esna and Nusf Formations indicate that this platform was likely subaerially exposed between the depositional episodes that produced the two formations. The evidence for shoaling includes the progressive increase in mid-shelf benthic taxa and decreasing P/B ratios up-section. The abrupt decrease in the P/B ratio at the top of the Esna Formation and the erosional surface at the top of the Esna Formation indicate progressive or stepwise shallowing of the shelf, ultimately resulting in subaerial exposure at the top of the Nusf Formation, followed by the return of shallow-water conditions that produced the overlying Farafra Formation. The transition from shale to marl to limestones indicates a reduction in terrigenous sediments input from land to the marine shelf. That reduction in runoff can indicate that the environment is getting shallower. Although it can also result from a reduction in rainfall thereby reducing terrestrial runoff, which means a change in local climate (Hallock & Schlager, 1986). However, the P/B ratios and occurrence of outer shelf/upper slope benthic species, according to the depth zonation of Culver (1988), indicate that the original habitat of the Esna Formation was indeed deeper. Moreover, tectonic activity has been noted in the study area where the Farafra Formation of El-Guss Abu Said is affected by faulting. Regional uplifting processes in the study area are also well known and recorded by many previous authors who referred it to as tectonic in origin (Obaidalla et al., 2006; Faris et al., 2013; Ismail & Daghriri, 2015). So, the potential influence of rainfall, runoff, and climate seems to be of lesser influence than the probable tectonics that may have occurred during the early Eocene.

Thus, the paleoecology and sedimentation history can be summarized as follows. During the late Paleocene–early Eocene, siliciclastic deposition of shale in the lowermost part of the Esna Formation occurred in an outer shelf–upper bathyal environment (200–600 m water depth). Evidence includes the high P/B ratios, the abundance of bathyal genera of benthic foraminifera, and a complete absence of shallow-water benthic taxa. In the upper part of the Esna Formation, the abrupt drop in P/B ratios with continued abundance of many species encountered in the lower part of the Esna Formation, combined with the first occurrence of species known as middle neritic forms, indicates that the environment had shoaled somewhat to middle neritic depths (50–100 m water depth). Further shallowing and increasing prevalence of carbonates is evident through the Nusf Formation, culminating in the erosional surface between the Nusf and the Farafra formations. Reflooding of the platform, as indicated by limestone and dolostone lithologies in the Farafra Formation, combined with the occurrence of shallow-dwelling taxa like Nummulites and alveolinids and the rarity of smaller foraminiferal taxa, indicates the persistence of shallow-water carbonate-depositional conditions in the early Eocene. This interpretation is consistent with field observations where the lithology transitioned from being mostly shale in nature (Esna Formation) to varved-like structures of alternating thin beds of limestone and marl upward through the Nusf Formation. Immediately above an erosional surface, the lithology becomes the increasingly lithified carbonates of the Farafra Formation.

Many previous studies have considered the El-Gus Abu Said succession to represent only two lithostratigraphic units, the Esna Formation in the lower part and the Farafra Formation in the upper part. However, based on the results presented here, we completely agree with Boukhary et al. (1995) that the transitional section between the shale of the Esna Formation and the limestone of the Farafra Formation should be considered as a separate unit called the Nusf Formation. Studying the nature of sedimentation and the paleoenvironment indicated by the benthic foraminiferal assemblages has revealed an upward transition in the environment through the succession, which may be due to a tectonic uplift during the early Eocene. We investigated the benthic foraminiferal assemblages to determine the paleoenvironment and the nature of deposition of these rock units during the Eocene. This was achieved with the aid of a detailed field and lithostratigraphic study of the different rock units, which delineated three benthic biofacies: Biofacies A, which comprises the lowermost part of the Esna Formation, suggests a deep outer shelf-upper bathyal environment with the complete absence of the shallow benthic forms and a relatively high P/B ratio; Biofacies B, comprising the upper part of Esna Formation, suggests a middle shelf environment in accordance with the decreasing P/B ratio; and Biofacies C representing the Nusf Formation and a completely different carbonate platform facies. This facies is characterized by a rich occurrence of Nummulites and Alveolinids and the rarity of small foraminifera, with only very few smaller benthic foraminifera, which explains the sudden decrease in the P/B ratio at the end of the Nusf Formation and the overlying Farafra Formation.

The presence of alveolinids in the Nusf Formation extends also to the Farafra Formation, which suggests the continuity of the platform environment upward during the deposition of the Farafra Formation also even after the hiatus. This cessation of sedimentation was inferred through the lithological field observations by an irregular erosional surface separating the Nusf and Farafra formations. The scarcity of small foraminifera and the rich occurrence of alveolinids has extended also to top of the Farafra Formation, which may also point to the same environment of sedimentation extending after the hiatus between Nusf and Farafra Formations. This sudden shift from relatively deep outer shelf environment of the Esna Formation, rich in planktonic foraminifera, to a completely different carbonate platform facies of the Nusf Formation, where Nummulites and alveolinids with only small benthic foraminifera were recorded, could point to a tectonic uplift that occurred during the early Eocene.

The authors are deeply grateful to Prof. Pamela Hallock Muller, College of Marine Science, University of South Florida, and Prof. Mohammed Boukhary, Faculty of Science, Ain Shams University, for their careful review of the manuscript, helpful comments and constructive suggestions. The Appendix can be found linked to the online version of this article.