Depositional environments of Danian/Selandian of Kurkur Formation, Kurkur Oasis, south Western Desert, Egypt

1 Introduction

The Paleogene succession in Egypt spans a vast expanse of land. These rocks span from the border between Sudan and Egypt in the Western Desert. The region of southern Egypt had multiple maritime transgressions throughout the Paleogene period. The region that experienced the most significant Transgression, which persisted until the Early Eocene (Luger 1988). These violations were intermittently interrupted by modest regressive episodes. The transgressions in southern Egypt were not widespread, but rather limited to the Dakhla basin, mostly because to tectonic factors. During the Maastrichtian-Paleocene period, the southern region of the Upper Nile Platform was characterized by marginal marine facies, as described by Luger in 1988.

The geological formations in southern Egypt during the upper Cretaceous-lower Eocene period show a gradual separation of the Nile Valley and the Garra El Arbain facies from the oases of Dakhla, Kharga, and Paris to the oases of Kurkur and Dungul. The Nile Valley facies is separated into four units: the Dakhla Formation, Tarawan Formation, Esna Formation, and Thebes Formation. The Garra El Arbain facies is found in the western and southern regions of Aswan, as well as in the southern region of Kharga Oasis along Darb El Arbain (Isaawi 1972). The examined area contains the Cretaceous-Paleogene succession, namely the Garra El Arbain facies. This succession is divided into four units: the Dakhala Formation, the Kurkur Formation, the Garra Formation, and the Dungul Formation (Issawi 1972). The Paleogene succession in the research area has been investigated by several authors, including Hendriks et al. (1984, 1987), Hewaidy (1994), Faris and Hewaidy (1995), and Hewaidy et al. (2014).

The Kurkur Formation has garnered significant attention from numerous authors. The paleoenvironments of the Kurkur Formation were examined by analyzing several fossil groups, as documented by Schrank (1983), Hewaidy and Soliman (1993), Strugo and Hewaidy (1993), Tantawy (1994), Ouda and Tantawy (1996), Hewaidy and Azab (2002), Youssef (2003), Aref and Youssef (2004), and Youssef and Mutterlose (2004). Previous research did not focus on using microfacies analysis to interpret the depositional conditions that existed during the sedimentary sequence deposition. Hence, the primary objective of this study is to utilize a combination of microfacies analysis and foraminiferal content to determine the depositional environments of the Kurkur Formations in the Southwestern Desert of Egypt (Fig. 1).

Close

Fig. 1

Geological map and locations of the studied sections.

Export to PPT

2 Geological setting

The Arabian craton, which formed during the late Paleozoic era, experienced the intrusion of shallow seas that originated from parts of the Tethys system, likely located north of Egypt. There is a consensus on a distinct separation between the Arabian nucleus (Arabo Nubian massive) and the shelf areas, however the boundaries between the various units of the shelf area are poorly defined (Said, 1961). The shelf area of Egypt, which surrounds the Arabo-Nubian massif, can be separated into two parts throughout the Late Cretaceous and Early Paleocene periods: the Stable Shelf and the Unstable Shelf (Said, 1962) as shown in Fig. 1. The Stable Shelf, located in southern Egypt, is characterized by indistinct boundaries and is covered by a layer of sediments measuring less than 1000 m in thickness. This encompasses the extensively spread Nubia sandstone, which is overlain, in places that are not basins, by shallow marine deposits from the significant Late Cretaceous-Early Tertiary transgression. The Stable Shelf encompasses the basement exposures of the Arabo Nubian craton in eastern Egypt. To the west, it forms a large tectonically low region (Kerdany and Cherif, 1990). This region can be divided into two primary intracratonic basins that generally extend in a north–south direction. The Dakhla Basin spans the western region of the country and has a significant opening towards the north in the Western Desert. The upper Nile Basin spans the eastern regions of the country, stretching from a longitude between Kharga and Dakhla to the west, perhaps reaching even further east beyond the current coastline of the Red Sea. The basin likely covered the area where the current basement exposures of the Red Sea range are located. These exposures were raised during the late Paleocene and subsequent periods (Bisewski, 1982). The basins are bounded on the southwest by the Calanscio-Uweinat-Gilf El Kebir High and on the south by a series of uplifts known as the Uweinat-Aswan High. These uplifts extend eastwards to the Eastern Desert basement exposures. The elevation of this landform, which is covered by a thin layer of sedimentary rock from the Mesozoic era, is believed to have been created from the late Paleozoic to early Mesozoic periods through the upward movement along significant faults that run from east to west.

The initial extensive marine advance of the Mesozoic period over the upper Nile basin took place during the Cenomanian epoch and extended to at least the latitude of Aswan. The Turonian regression had an impact on the upper Nile basin. The Coniacian period signifies a transgression event (Said, 1990) that resulted in the sea advancing inland, reaching as far as Nubia and beyond, and submerging the entire Nile basin. The Santonian period was characterized by a regression, during which the sea was limited to the tectonic basins of northern Egypt. A significant violation occurred in the Campanian period, during which a shallow sea engulfed the majority of Egypt. Following a brief period of regression in the early Maastrichtian, the sea progressed and engulfed more extensive regions of Egypt than at any other period. The depth of the sea increased significantly. In the northern regions, there was a buildup of pristine chalk deposits in the Sudr area (Sinai and Gulf of Suez) and the Khoman area (north Western Desert). The southern region experienced the deposition of shales, specifically in the lower part of the Dakhla formation. During the Paleocene, a significant portion of the Stable Shelf experienced flooding from the sea due to the greatest transgression. The identification of a maritime Paleocene outcrop in Gebel Abyad, located in northern Sudan (Barazi and Kuss, 1987), indicates that the shoreline during that time was approximately 400 km further south. The Gebel Abyad facies bears resemblance to the Arbain facies found in the Paleocene of southern Egypt, suggesting the possibility that both were deposited in a single basin. In the Early Eocene, the shoreline moved back to southern Egypt, resulting in the coverage of a large embayment with relatively thin and consistent sediments, measuring less than 500 m in thickness. Sediments thicker than 500 m are only found in a few number of basins.

3 Lithostratigraphy

The term “Kurkur Formation” was introduced by Issawi in 1968 to characterize the brownish sediments that consist of at least two carbonate beds separated by shale or sandstone strata, together with sandy clay intercalations. The word “Kurkurstufe,” initially coined by (Blankenhorn 1900) to refer to the fossil-rich layers at Kurkur Oasis, has been supplanted. The Kurkur Formation in the study region is distinguished by its brownish color, which stands out in contrast to the greenish or grayish shale of the Kiseiba Formation below and the white, chalky limestone beds of the Garra Formation above (Table 1: supplementary). The Kurkur Formation consists of brown to dark brown, hard to semi-hard, argillaceous limestone with rich fossil content, phosphatic deposits, and oolitic limestone. It also contains layers of silty shale and marl. The Kurkur Formation is observed on the steep slope of Sinn El Kaddab and forms a noticeable shelf that sits above the Kiseiba Formation in the southern region of Gebel Garra. The Kurkur Formation is located in close proximity to the wadi bottom at Kurkur Oasis due to its specific structural features, as noted by Issawi 1968. The thickness of the rock layers in the Gebel Garra, Wadi Abu-Sayal, Kurkur Oasis, and Sinn El Kaddab portions are 6 m, 6 m, 10 m, and 20 m, respectively. The Kurkur Formation has substantial differences in both thickness and lithology while moving from the southern to the northern regions. The succession has a significant amount of fossils, specifically bivalves, gastropods, and echinoids. The shale intercalations include a large number of microfossils, but in other areas, there is a complete absence of fauna. The comprehensive lithostratigraphic sequence of the Kurkur Formation in the examined sections is outlined in Figs. 2–5 a.

Close

Fig. 2

a) Composite lithostratigraphic section of the Kurkur Formation, foraminiferal content, microfacies and depositional environments at Gebel Garra section; b) range chart of planktonic foraminifera and the sea level change of the studied succession at Gebel Garra section.

Export to PPT

Close

Fig. 3

a) Composite lithostratigraphic section of the Kurkur Formation, foraminiferal content, microfacies and depositional environments at Wadi Abu-Sayal section; b) range chart of planktonic foraminifera and the sea level change of the studied succession at Wadi Abu-Sayal section.

Export to PPT

Close

Fig. 4

a) Composite lithostratigraphic section of the Kurkur Formation, foraminiferal content, microfacies and depositional environments at Kurkur Oasis section; b) range chart of planktonic foraminifera and the sea level change of the studied succession at Kurkur Oasis section.

Export to PPT

Close

Fig. 5

a) Composite lithostratigraphic section of the Kurkur Formation, foraminiferal content, microfacies and depositional environments at Sin El-Kaddab section; b) range chart of planktonic foraminifera and the sea level change of the studied succession at Sin El-Kaddab section.

Export to PPT

4 Materials and methods

Four stratigraphic outcrops were surveyed, and 25 rock samples including macrofossils were obtained to describe the microfacies analysis and depositional settings of the middle to early late Paleocene Kurkur carbonates in the Southwestern Desert of Egypt. The studied sections, listed in order from north to south, consist of Gebel Garra (32° 30′E, 24° 09′N), Wadi Abu-Sayal (32° 24′E, 23° 58′N), Kurkur Oasis (32° 8′E, 23° 45′N), and Sinn El-Kaddab (32° 19′E, 23° 40′N). A total of twenty-five thin sections were meticulously constructed for the purpose of conducting microfacies study. The microfacies description and interpretation followed the classification of Dunham (1962). The shale intercalations were collected for the purpose of extracting foraminifera. The samples were washed and prepared using the conventional approach (Rückheim et al. 2006) to separate the foraminifera. A twofold treatment was applied to ensure complete disaggregation.

5 Biostratigraphy

The biostratigraphic scheme for the planktonic foraminifera of the Paleocene-Early Eocene is established using the Tethyan zonal concepts proposed by Berggren et al., 1995. Based on the vertical distribution of planktonic foraminifera in the examined sequence, the Kurkur Formation can be divided into two distinct biozones: Praemurica trinidadensis (P1d) and Morozovella angulate (P3a). The age of the succession from the early to late Paleocene period has been determined by studying the planktonic foraminifera found in the Kurkur formation. The presence of planktonic foraminifera in the sediments of the Kurkur Formation was infrequent. Hewaidy and Soliman (1993) determined that the Kurkur Formation (M. uncinata Zone) is of middle Paleocene age. The Early-Late Paleocene is attributed to the Kurkur Formation, as stated by Luger 1985. The Kurkur Formation, specifically the Early-Late Paleocene period, has been identified based on the presence of planktonic foraminifera and calcareous nannofossils (Youssef and Muterlose 2004). The Kurkur Formation in the examined area contains a hiatus that includes Zone P2 (Praemurica uncinata Zone). For the range chart and water depth please (see Figs. 6-9b).

Close

Fig. 6

Examples of the microfacies analyses of the Gebel Garra section. A: Foraminifera bioclastic packstone/grainstone with benthic foraminifera and shell fragments, lower part of the section. B: Bioclastic oolitic packstone/grainstone with concentric oolites, echinoids, and shell fragments, lower part of the section. C: Bryozoan bioclastic packstone/grainstone with phosphatic pellets, bryozoans, and shell fragments, lower part of the section. D: Coralline oolitic bioclastic packstone/grainstone with corals, gastropods, and shell fragments, lower part of the section. E: Oolitic grainstone with concentric oolites, upper part of the section. F: Ostracodal oolitic bioclastic grainstone with recrystallized ostracods, concentric oolites, and shell fragments, upper part of the section.

Export to PPT

Close

Fig. 7

Examples of the microfacies analyses of Wadi Abu-Sayalsection. A: Oolitic grainstone with fine to medium concentric oolites, upper part of the section. B: Ostracodal oolitic packstone/grainstone with ostracods, benthic foraminifera, and oolites, lower part of the section. C: Pisolitic–oolitic packstone/floatstone, lower part of the section. D: Foraminifera wackestone/packstone with planktonic foraminifera and oolites, lower part of the section. E: Oolitic packstone/grainstone with gastropods and concentric oolites, upper part of the section. F: Ostracodal oolitic packstone/grainstone, lower part of the section.

Export to PPT

Close

Fig. 8

Examples of the microfacies analyses of Kurkur Oasis section. A: Gastropod oolitic bioclastic packstone, upper part of the section. B: Gastropod oolitic bioclastic packstone, upper part of the section. C: Bioclastic packstone, upper part of the section. D: Echinoidal oolitic bioclastic packstone, upper part of the section. E: Oolitic packstone with concentric fine to medium oolites, lower part of the section. F: Bioclastic packstone with shell fragments, upper part of the section.

Export to PPT

Close

Fig. 9

Examples of the microfacies analyses of the Sinn El-Kaddab section. A: Bioclastic wackestone/packstone with planktonic foraminifera and shell fragments, lower part of the section. B: Echinoidal bioclastic packstone/grainstone with echinoid plates and spines, upper part of the section. C: Bioclastic floatstone with pesoids, lower part of the section. D: Bioclastic grainstone, lower part of the section. E: Bioclastic wackestone with planktonic foraminifera, lower part of the section. F: Bioclastic wackestone/packstone with mostly recrystallized shell fragments, upper part of the section.

Export to PPT

6 Results

6.1

6.1 Microfacies analysis

The Kurkur Formation thin sections from the Garra section north of the study region contained bioclastic oolitic packstones and packstones/grainstones (Figs. 2a and 6). The predominant skeletal grains include primarily of bivalves, echinoids, gastropods, bryozoans, foraminifera, corals, and ostracods. The non-skeletal grains in the top section are characterized by phosphatic and glaucanitic patches, as well as a small number of ooids. All grains are encased within micrite and microsparite in certain areas. The cavities of corals, gastropod whorls, and foraminifera chambers contain micrite and internal sediments. The walls of the ostracod and bryozoan have undergone recrystallization (Fig. 10). The primary microfacies types found in the Wadi Abu Sayalcarbonates are bioclastic oolitic packstones/grainstones, floatstones, and grainstones (Figs. 3a and 7). The skeletal grains consist of bioclasts and biomorphs derived from ostracods, bivalves, gastropods, echinoids, and foraminifera. Non-skeletal grains primarily consist of carbonate nuclei in the form of ooids and pisoids. The primary diagenetic changes that occur in these microfacies types include micritization and silicification of certain ooids, as well as recrystallization of other skeletal grains.

Close

Fig. 10

Depositional model with the range of MF types and distribution of biogenic components on a carbonate platform for the mixed siliciclastic/carbonate facies of Kurkur Formation.

Export to PPT

The carbonate samples collected from the Kurkur Oasis area contained bioclastic oolitic wackestones, wackestones/packstones, packstones, and floatstones (Figs. 4a and 8). The skeletal grains consist of echinoid plates and spines, gastropods, bivalves, brachiopods, and corals. Certain grains exhibit diagenetic characteristics such as recrystallization, micritization, and stylolites, while others have maintained their initial structures. The non-skeletal grains consist of oolitic grains, phosphatic pellets, and tiny, subrounded quartz grains. The oolitic grains are of small to medium size and have a rounded to subrounded shape. They also include carbonate nuclei. The interstitial material between the grains consists of micrite, which transitions to microsparite in certain areas. The examination of carbonate samples from the Sin El-Kaddab section located south of the study region identified many types of sedimentary rock compositions. These include bioclastic packstones, packstones/grainstones, wackestones, wackestones/packstones, and floatstones (Figs. 5a and 9). The bioclastic grains consist of echinoid plates and spines, as well as fragments of bivalves, gastropods, foraminifera, ostracods, and scleractinian corals. The skeletal grains are primarily enclosed within the micritic matrix and serve as vital components in the formation of the rock. The majority of the skeletal grains underwent recrystallization and micritization, whereas a small number were silicified. The echinoid fragments exhibit syntaxial overgrowth. The inside chambers of gastropods, foraminifera, and ostracods are filled with sediments, micrite, phosphate pellets, and other minute skeletal grains. The non-skeletal grains are found as phosphatic pellets made of apatite, making up 5–10 % of the total grain volume.

7 Discussion

8 Conclusion

The analysis of carbonate rocks from the lower portion of the middle to early late Paleocene Kurkur Formation at the Gebel Garra, Abu-Sayal, Kurkur Oasis, and Sinn El-Kaddab sections in the Southwestern Desert showed a significant presence of wackestones, floatstones, and packstones, with a few packstones/grainstones. In contrast, the upper portion contained a large amount of packstones, packstones/grainstones, and grainstones. The lower section of the Kurkur Formation has carbonate deposits that contain a variety of fossils, including benthic and planktonic foraminifera, ostracods, and a small number of macrofossils. The upper section contains a variety of fossils, including dispersed oyster shells, gastropods, echinoids, brachiopods, benthic foraminifera, ostracods, and corals. Based on the analysis of microfacies and fossil material, it can be determined that the Kurkur Formation in the study region was deposited in intermediate neritic habitats. In the lower section, it was below the wave base, but as it moved upwards, it transitioned to shallow shelf and supratidal environments that were at or above the wave base.

CRediT authorship contribution statement

Mohamed Youssef: Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. Abdelbaset El-Sorogy: Writing – review & editing, Writing – original draft, Methodology, Funding acquisition, Data curation. Bassam A. Abuamarah: Project administration, Investigation. Mahmoud Hefny: Writing – review & editing, Software, Methodology. Saida A. Taha: Software, Methodology, Data curation.