Introduction

The Paleozoic rocks in Egypt are becoming more important in the strategic planes of exploration for petroleum in the Western Desert of Egypt by the date. They are the least studied rocks among the subsurface successions in the Western Desert, where the Cretaceous and Jurassic intervals remained the central target for petroleum exploration. The Carboniferous surface rocks in Sinai and Eastern Desert were the first Paleozoic strata that had been discovered in the Egypt (Wanas 2011). In Sinai, the presence of the Paleozoic (Carboniferous) rocks has been recorded by Bauerman (1869). The knowledge about the subsurface Paleozoic rocks in Egypt generally comes from the scattered studies that outlined their geographical occurrences, lithostratigraphy and biostratigraphic content (e.g., Amin 1961; Said and Andrawis 1961; Said 1962; Soliman and El Fetouh 1970; Andrawis 1972; Barakat 1982; Abdel Sattar 1983; El-Dakkak 1988; Andrawis 1990; El Shamma et al. 1996; El Shamma et al. 1998; El Shamma et al. 2012; Wanas 2011). They were recorded in 11 wells in the Western Desert to the date of Hantar (1990), which are mostly located to the west of longitude 27° E. Mostly, these records of marine Carboniferous rocks used the micropaleontological criteria of Said and Andrawis 1961, who recovered a well-preserved Lower Carboniferous (Viséan) microfossils assemblages from the subsurface Western Desert in Faghur-1x (this study) and Mamura-1 wells, Fig. 1. These microfossil assemblages delineated the biozonation of Andrawis (1990) for the Carboniferous strata in northern Western Desert. These works used the Carboniferous foraminiferal scheme for the Paleotethyan provincialism, which are found to be correlatable with Russian and Turkish Platforms. Additionally, almost the same core intervals that were used in the study of Said and Andrawis (1961) were used by El Shamma et al. (1996), who identified significant Devonian–Carboniferous microflora from Faghur-1x and NWD 302-1 wells. Accordingly, Faghur-1x well is considered as one of the most important wells for the Paleozoic lithological and biological studies. Through these extensive studies in Faghur-1x (FRX-1) well (Said and Andrawis 1961; Keeley 1989, 1994; El Dakkak 1988; El Shamma et al. 1996; Wanas 2011), the Devonian-Carboniferous boundary was placed at the depth of 6840 ft.

Fig. 1
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Location map of the Faghur-1x well in the Western Desert of Egypt. The line AA’ is the base of the geological cross section in Fig. 3. It is adopted from Keeley 1994

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On the other hand, there is a prominent evidence for the active glaciations that were especially well developed in the southern hemisphere at the Gowndwana land. The continental ice caps extended from the late Viséan to Sakmarian and reached to the latitude 35° S from the South Pole (Frakes et al. 1992; Stanley and Powell 2003). An extensive mass extinction of marine fauna accompanied the development of such ice caps especially at the late Serpukhovian. Being controlled by the glacioeustacy, the basinal migration was partially allowed during the interglacial periods (sea level rise) and the taxonomical diversity was maintained. The northwestern African Sahara basins such as Tindouf, Béchar, and Reggan represented an ecological refuge for at least foraminiferal assemblages (Cózar et al. 2014). These refuges are formed as a result of the active tectonism created partially isolated basins with stable conditions for species survive longer. The north to north western oceanic currents from the opening Palaeotethyan parallel to the coast is responsible for that longer survival of species are more common in the western basins that these are the eastern (Cózar et al. 2014). The Tehenu Basin, where the Faghur-1x well is located, is near to these basins in the eastern African Sahara.

In the present work, foraminiferal assemblages from some cores of Faghur-1x are revisited and revised. The foraminiferal biostratigraphy is used to discuss the correct Devonian-Carboniferous boundary that has been published in previously mentioned studies. The studied foraminifera were identified and microphotographed to update the records of Said and Andrawis (1961). The paleoenvironmental conditions were investigated based on these foraminiferal assemblages and other associated microfossils. These associations were compared with their coeval association from the Palaeotethyan regions in biostratigraphical and paleoenvironmental aspects. This comparison is the first opportunity to evaluate the Tehenu Basin as a segment of the ecological refuge in northwestern African basin.

Geological settings

Kostandi 1959 pointed out that a fast rise of the Carboniferous Sea covered almost all of the northern Egypt and a purely marine deposition must have taken place in an area not far north of latitude 30° N in Sinai and the Eastern Desert. An expansion of the Carboniferous sea developed as a gulf along the Clysmic trend and covered the area now occupied by the Gulf of Suez and its borderland. In the north, the sea mainly deposited limestone, while in the Carboniferous gulf to the south clastics of black shales and sands were deposited, Figs. 2 and 3.

Fig. 2
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The paleogeographic map of Egypt during Carboniferous–Permian. It is adopted from Keeley 1994

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Fig. 3
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Geological cross section through the AA’ line in Fig. 1 in the northern part of the Western Desert. It is adopted from Keeley 1994

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In the Western Desert, the Carboniferous sediments were recorded from 11 wells west of the longitude 27° E, Figs. 2 and 3. The presence of marine Carboniferous carbonate deposits in some of the wells like Faghur-1x suggested that an open link occurred between the Carboniferous gulf and the shallow Carboniferous sea of the Libyan Desert. The carboniferous sediments exhibit lateral variation of facies from north to south, where fully marine and carbonate sediments are thicker and more common to the north and west (Hantar 1990; Klitzsch 1990). In these western areas, the strata are more or less of fully marine origin; consisting of sandstone, siltstone and shale with thin limestone intercalations, mainly at the top. This description is identical with and typical for the lithological content of Lower Carboniferous rocks of Faghur-1 well, which extend from the depth 6030 to 7780 ft (1750 ft), Fig. 4. This lithology is distinctive for the Desouqy and Dhiffah formations, which are assembled with the lower Zeiton and the upper Safi formations in the “Faghur Group” to describe the Devonian through Permo-Carboniferous subsurface rock successions in the Western Desert by Paleoservice 1986. This classification is widely applicable in the Paleozoic studies (e.g., Keeley 1989; Keeley 1994; El Shamma et al. 1996; Wanas 2011). The Desouqy Formation is laying on the Zeitoun Formation, which is only completely preserved in the Faghur-1x well. Towards the south-east, progressively earlier stages of first the Late, and then the entire Devonian are missing under Lower Carboniferous strata (Keeley 1989), Figs. 2 and 3. The lithological composition of this Desouqy Formation in Faghur-1x incorporates alluvial shoreface and delta-top sandstones thick beds of sandstone and thin layers of shales. Dhiffah Formation is overlying the Desouqy Formation conformably. In the Dhiffah Formation, there are four beds of limestone. The lower group consists of one bed and this is where the studied core materials (Core#10) were collected. The upper group consists of three beds. They are separated by few shoreface sandstones and open marine shale intercalations, Fig. 4. These two groups referred to the “Lower Limestone and Upper Limestone” by Keeley 1989, 1994. The Permo-Carboniferous Safi Formation is overlying the Dhiffah Formation.

Fig. 4
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The lithological and lithostratigraphical section of Faghur-1x well. The microfossil content is after the composite log of Pan-American (1968). 1* is the Devonian–Carboniferous boundary that used in El Dakkak (1998), El Shamma et al. (1996) and Wanas (2011). 2* is the Devonian–Carboniferous boundary that is used in Keeley 1989

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The deposition of Faghur Group comprised a part of sedimentation phase II that extended through Early Tournaisian–Middle Permian (Keeley 1994). The deposition was affected primarily by eustatic changes and dominantly by the tectonics of the Calanshiyu ‘Awaynat Arch. This tectonic arc shaped the Tehenu Basin almost parallel to the Libyan-Egyptian Border (Klitzsch 1971), Figs. 2 and 3.

Material and methods

The collected materials are from the top parts of the core #10 in the Faghur-1x well, Fig. 4. Two versions of composite logs were available for Faghur-1x well (Pan-American 1959, unpublished data, 1968). The core was cut at depth interval of 6887–6928 ft and studied materials were prepared from the depths of 6887 and 6890 ft. The material were cut and polished by Carborandum and a couple of slides were prepared from each depth. The slides were then examined and microphotographed by polarized light Olympus Microscopic System. The identified taxa were arranged taxonomically according to Boudagher-Fadel (2008) in Plates 1 and 2.

Plate 1
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1–8 Omphalotis omphalota; 1 diameter 211 μm; 2 diameter 201 μm; 3 diameter 184 μm; 4 diameter 203 μm; 5 diameter 195 μm; 6 diameter 100 μm; 7 diameter 95 μm; 8 diameter 187 μm; 9 diameter 175 μm;10 diameter 181 μm; 11, 12 Omphalotis sp. 2; 11 diameter 340 μm; 12 diameter 260 μm; 13–15 Omphalotis sp. 3; 13–17, 13 diameter 213 μm; 14 diameter 191 μm; 15 diameter 183 μm; 16 diameter 221 μm; 17 diameter 172 μm; 18 Paraarchaediscus stilus, diameter 112 μm; 19–22 Araarchaediscus koktjubensis; 19 diameter 204 μm; 20 diameter 213 μm; 21 diameter 204 μm; 22 diameter 221 μm; 23–25 Archaediscus krestovnikovi; 23 diameter 196 μm; 24 diameter 187 μm; 25 diameter 175 μm; 26 Archaediscus complanatus, diameter 217 μm; 27 Archaediscus inflatus, diameter 215 μm; 28, 29 Archaediscus karreri; 28 diameter 184 μm; 29 diameter 177 μm; 30, 31, Eotubertina sp.; 30 diameter 125 μm; 31 diameter 155 μm

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Plate 2
figure 6

1 Tetrataxis conica, diameter 185 μm; 2–6 Cribrostomum lecomptei; 2 diameter 487 μm; 3 diameter 387 μm; 4 diameter 251 μm; 5 diameter 224 μm; 6 diameter 186 μm; 7 Palaeotextularia angulata, diameter 176 μm; 8–10 Palaeotextularia longiseptata; 8 diameter 367 μm; 9 diameter 586 μm; 10 diameter 240 μm; 11 Auroria, diameter 142 μm; 12,13 Draffania biloba; 12 diameter 168 μm; 13 diameter 174 μm; 14 Corals, scale bar 500 μm; 15 Alga incerta, scale bar 500 μm; 16 Brayozoa, scale bar 500 μm; 17 A, ostracod; B, C Koninckopora sp., scale bar 1000 μm

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Results and discussions

Biostratigraphy

Despite of their small size, the studied materials are unique because of two reasons. Firstly, most of the Carboniferous calcareous sediments exist only in the subsurface which is being reported by unpublished data in petroleum companies. In addition, only a little information can be obtained from surface the mainly clastic carboniferous exposures. Secondly, the samples are cores, which allowed the examination polished thin section from precise depths that cannot be obtained from the cutting samples. The microscopic examination of core materials revealed a low diversity and medium abundant foraminiferal assemblage. The individuals are well preserved, which allowed fine identification of some significant Carboniferous foraminifera. Twenty species were identified from 15 genera. However, many other forms appeared in the assemblage in inadequate positions that did not allow fine identification. The identified assemblage consists of Omphalotis omphalota, Omphalotis sp. 2, Omphalotis sp. 3, Paraarchaediscus stilus, Paraarchaediscus koktjubensis, Archaediscus krestovnikovi, Archaediscus complanatus, Archaediscus inflatus, Archaediscus karreri, Diplosphearina inequalis, Eotubertina sp., Tetrataxis conica, Cribrostomum lecomptei, Palaeotextularia angulata, Palaeotextularia longiseptata and the microproblematicum Draffania biloba, Plates 1 and 2.

The Carboniferous foraminiferal biozonation were discussed previously in Said and Andrawis 1961; Andrawis 1990. This biozonation consisted mainly of seven biozones, which are Tetrataxis conica, Endothyramopsis crassa and Archaediscus krestovnikovi for Viséan, Eostaffella postmosquensis, Ozawainella umbonata and Hemigordius simplex for Namurian and Hyperammina earlandia assemblage for Westphalian. This biozonation is correlatable with biozonation scheme from the Western Europe and Russian Platforms. The Western Europe Namurian Stage is replaced with the Russian Platform Serpukhovian in the new Geological Time Scale (Davydov et al. 2012).

The species of the genus Omphalotis (O. Omphalota, O. sp. 1 and O. sp. 2) are common in the Palaeotethyan regions of the Serpukhovian, especially in the northern basins. They are well recognized through late Viséan (Asbian-Brigantian) in Ireland, North England, Spain, Iran and Russia (Herbig and Mamet 1985; Rukina 1996; Gallagher 1998; Kalvoda 2002; Cózar and Somerville 2004; Somerville and Cózar 2005; Cózar et al. 2011, Zandkarimi et al. 2016). In the Western Africa, they remarkably flourish in the southern Palaeotethys basins like Tindouf, Béchar and Reggan through the late Viséan to Brigantian (Cózar et al. 2014). The present study is an extension of the Omphalotis occurrences in such basins to the Eastern Africa basins that were completely separated from them by African Shield in northern Saharan regions.

Cribrostomum lecomptei is most remarkable for the late Viséan–early Serpukhovian in many of the Paleotethyan basins (Pille 2008). It extends geographically from the northern regions in Ireland and Spain (Asbian-Brigantian; Gallagher 1998; Cózar 2000; Cózar and Somerville 2004; Cózar and Somerville 2005). In Western African Sahara at Tindouf Basin, it is recorded in the late Serpukhovian (Cózar et al. 2014).

Tetrataxis conica occurs for the first time in the latest Tournaisian. It is abundant in the Carboniferous Paleotethyan basins (Viséan of Ireland, Gallagher et al. 2006; Viséan-Serpukhovian of Spain, Cózar 2000; Herbig and Mamet 1985; Serpukhovian-Bashkirian of Iran, Leven et al. 2006; Zandkarimi et al. 2016; Western Arica Sahara-Cózar et al. 2014). Level of extinction undermined, probably Permian (Armstrong and Mamet 1977). Palaeotextularia longiseptata is recorded in the Viséan of the Guadiato Area in Spain (Cózar 2000).

Archaediscus krestovnikovi is essential marker for Viséan and originally described from the Late Viséan of the Russian Platform (Armstrong and Mamet 1977; Davydov et al. 2012). Nevertheless, it is recorded from Serpukhovian Palaeotethyan basins (Serpukhovian-Bashkirian of Iran, Leven et al. 2006; Serpukhovian of Turkey, Atakul-Özdemir et al. 2011).

The last occurrence Archaediscus karreri marked the Viséan/Serpukhovian boundary in the Palaeotethys basins and in the Sahara Platform (Mamet et al. 1966; Mamet 1972, Cózar and Somerville 2004). However, this species is recorded in the Tindouf Basin in Western Africa through the early Serpukhovian (Cózar et al. 2014). In addition, it is documented in the early Serpukhovian of the Rhadames Basin in western Libya (Massa and Vachard 1979). It is also traced through the late Serpukhovian and Bashkirian in the Béchar Basin (Sebbar and Lys 1989; Cózar et al. 2014).

In addition to the identified foraminifera, the microscopic examination revealed a diverse and abundant microfossil assemblage. This assemblage included a well preserved gastropods, brachiopods, ostracods, crinoidal ossicles, bryozoans frond-like fenestrate types and stick-like colonies, echinoderms, algal Calcisphaera and the dasyclad Koninckopora and the microproblematicum Draffania biloba. These contents are distinctive for the Paleozoic in general and the Carboniferous in particular. The bryozoan frond-like fenestrate types and stick-like colonies are common at the Carboniferous of Alaska (Armstrong and Mamet 1977). Calcisphaera and the dasyclad Koninckopora are markers for Viséan in Eurasia, North America, Australia, and North Africa. They are more abundant in the Paleotethyan realm (Armstrong and Mamet 1977; Cózar et al. 2008; Gallagher and Somerville 2003; Cózar et al. 2014).

The microproblematicum Draffania biloba is among recorded in the assemblages from late Viséan to early Serpukhovian in many of the Palaeotethyan region in UK and Western African Sahara in Tindouf Basin (Hallett 1971; Cózar and Somerville 2004; Cózar et al. 2014).

In the present contribution, the coexistence of the, Omphalotis spp., Cribrostomum lecomptei, Tetrataxis conica, Archaediscus karreri, Archaediscus krestovnikovi and Draffania biloba suggests a late Viséan-early Serpukhovian age for the studied core #10 in Faghur-1x well.

Paleoenvironment of deposition

The composition of the studied limestone in Core#10 in Faghur-1x well is medium-sorted, medium-grained, essentially bryozoan-crinoid-echinodermal-foraminifer packstone-grainstone with mostly sparry calcite cement. Some fragments show signs of reworking and rounding and various degrees of coating. Others show in situ accumulation. The high abundance of algal mats, calcispheres and the well-segregated bedding of clastics and carbonate sediments point to deposition in the restricted platform to open platform in a lagoonal framework environment (Armstrong and Mamet 1977). The bryozoan thickets indicate deposition in deep waters (Gallagher 1998). Tetrataxis are interpreted to have thrived at water depths below 20 m in algal-poor facies (Gallagher 1998). In addition, Draffania is a microproblematicum that characterize the low energy, deeper water, subtidal, bryozoan-rich facies that mostly lacks or with few calcareous algae (Gallagher and Somerville 2003).

These paleoenvironmental conditions in addition to the occurrence of these shallow marine beds on two levels in Dhiffah Formation above the mainly clastic Desouqy Formation indicate a full marine transgression by the end of Viséan and beginning of Serpukhovian. This marine transgression is connected to the tectonic activity in Calanshiyu’Awaynat Arch, which resulted in cyclic sea level changes (Keeley 1989; Keeley 1994) or high glacioeustacy (sea level rise) during the interglacial periods.

Tehenu Basin as segment of the western African Sahara basins

There are prominent similarities between the identified microfossil association in general and foraminiferal assemblages in particular in Faghur-1x well at the Tehenu Basin and assemblages that are identified in the western African Sahara basins (e.g., basins of Tindouf, Béchar and Reggan; Cózar et al. 2014). These similarities suggest a possible mixing of waters between these basins during the deposition. This mixing allowed better distribution of the some genera of foraminifera through the migration and permitted better surviving conditions for them to pass the mass extinction events during the extensive glaciations through the northern African Sahara basins in general. It is not possible to determine whether the foraminiferal assemblages that thrived locally in Tehenu Basin managed to pass the extinction events because of the limited sampling and difficulty to follow up the stratigraphic ranges of the species.

The diversity of the foraminiferal assemblage in the Tehenu Basin (n genera = 8) is lower than diversity of the assemblages from the western African Sahara (n genera = 10–57). This may be interpreted on the basis of that the Palaeotethyan north-northwestern warm currents from the palaeoequator did not reach effectively to the eastern basins like Tehenu Basin. Accordingly, the foraminiferal association affected more readily with glaciations and mass extinction. This assumption requires more investigations in the future on a larger sample collection.

Devonian-Carboniferous boundary in Faghour-1x well

There are two editions of the composite logs for the stratigraphy of Faghur-1x well. The older edition printed in 1958–1959 and recorded only the lithological description of the retrieved ditch cuttings and cores and the rate of drilling. In this edition, the Devonian–Lower Carboniferous boundary was placed above core# 10 at depth of 6840 ft. This edition was adopted by El Dakkak 1998; El Shamma et al. 1996; Wanas 2011. This boundary was readjusted by Mustafa 1997 and lowered to be placed above core# 11 at depth 7320 ft, depending on the palynological investigation.

The younger edition was revised by stratigraphic and biostratigraphic (using mainly foraminifera and palynology) information and was printed by Pan American United Arab Oil Company under the supervision of J.P Beckmann in November 1968. In this version, the Devonian–Lower Carboniferous boundary was placed above core #15 at depth of 7780 ft. This edition was accepted in Keeley 1989.

The findings of Carboniferous foraminifera in core #10 in the present contributions keep up the placing of the Devonian-Carboniferous beneath this core. Nevertheless, they do not support any of the reports and studies mentioned above; though, it coincides with results of Mustafa (1997).

Conclusions

The study of subsurface sample materials from the limestone beds of Core#10 in Faghur-1x well lead to identification of a well preserved microfossil assemblage. The foraminifera association is composed of Omphalotis omphalota, Omphalotis sp. 2, Omphalotis sp. 3, Paraarchaediscus stilus, Paraarchaediscus koktjubensis, Archaediscus krestovnikovi, Archaediscus complanatus, Archaediscus inflatus, Archaediscus karreri, Diplosphearina inequalis, Eotubertina sp., Tetrataxis conica, Cribrostomum sp., Palaeotextularia angulata, Palaeotextularia longiseptata and microproblematicum Draffania biloba. This assemblage is of late Viséan-early Serpukhovian age. Accordingly, this division of the Faghur-1x well sediments should be Carboniferous. The other microfossil assemblage consisted mainly of gastropods, brachiopods, ostracods, crinoidal ossicles, bryozoans frond-like fenestrate types and stick-like colonies, echinoderms and algal Calcisphaera and the dasyclad Koninckopora. This microfossil assemblage indicates deposition in the deep (> 20 m) restricted to open platform in a lagoonal framework environment.

The foraminiferal associations the Tehenu Basin indicate that it is a segment of the northern African Sahara Palaeotethyan basins like Tindouf, Béchar and Reggan. Although it has a lower foraminiferal diversity in comparison of the western basin that indicates that the warm north-northwestern Palaeotethyan currents were not effective in the Tehenu Basin.

The Devonian-Carboniferous boundary is placed beneath the core #10 according to the present investigations. This is coinciding with results from the microfloral biozonation of Mustafa (1997).