Journal of Earth Science & Climatic Change
Open Access

Khartoum Basin; Omdurman Formation; Sedimentology; Facies; Depositional environments

Introduction

Background information: Most of the Sudanese sedimentary basins were created by rifts that are related to the East Africa rifts. These rift basins occurred as the result of the formation of the West and Central African Shear Zone (WCSAZ), due to the separation of African and South American continents from the Gondwana supercontinent during the late Jurassic - early Cretaceous. It has been found that the rifts in central and southern Sudan end at the Blue Nile, the White Nile, Atbara, and the Nile rifts, the structures of these rifts are parallel to the Red Sea. It may be found that the process of separation of the Arabian Plate from the continent of Africa is responsible for the appearance of these structures, and the development of these grooves arose from the processes of doming and volcanism of the Middle Ages. These dome canyons are found in southern Sudan, and they are the bottoms of Muglad, Melute, Abu Gabra, and the Blue Nile, which were studied by the Chevron Petroleum Company. This shear zone propagates into Africa from Cameroon, Nigeria through Chad to Central Africa Republic through the Republic of South Sudan and Sudan. Several rift basins, including those of Sudan, were formed as part of the extensional rift basins along the West and Central African Shear Zone [1].

Sudan rift basin structures, that appear in an NW-SW direction, have been investigated by using potential data from the field, well loggings, and seismic reflection data. These rift Basins are confined by listric normal faults, and each of them varies in how much they extend.

At the bottom of Khartoum basin, the early volcanic activity that occurred in Al-Sabaloqa and the appearance of igneous rocks that lie on top of the basic rocks of the Pre-Cambrian and the period between the Ordovician and the Devonian consists of basalt, and therefore its age falls within the range of igneous rocks of the Al-Sabloqa complex, and thus the origin of the basalt is for the Silurian. Also, the bottom of the Khartoum basin may be due to volcanic events that led to the formation of the igneous rocks of the Sabloqa. Volcanic rocks are spread in various locations in western and northern Sudan. There are sequences of faults in the E-W direction west of Jebel Markhiyat and they affect the upper Omdurman sediments with the presence of a few cracks in Jebel Awliya, and in the far northwest of Omdurman, there are a group of deep cracks through which basalt rocks appear. Wycishetal (1990) indicated that the development of the structural rifts occurred to the north of Khartoum during the early Cretaceous, and the development of major canyons began during the Upper Jurassic and probably continued until the Albian.

The study area is located as part of the Khartoum Sedimentary Basin, in Khartoum state, Sudan. The area bounded between longitude 32°18'24.5" - 32°44'41.2" E and latitude 16°20'19.6"- 15°11'36.5” N.

This paper discusses the following objectives:

1. Facies analysis of Omdurman Formation.

2. Inferring paleogeography & paleoenvironment in the area of study.

3. To determine the environment of deposition of the Basin.

Sudanese basins are connected to the extension through the Afro- Arabian area in association with the origination of the African and South American continents of Gondowana as well as the seafloor spreading of the proto-Indian Ocean in the Jurassic. Followed by the second stage of the development that happened in the Cretaceous, in response to faulting along the Central African Shear Zone and the opening of the South Atlantic. The Khartoum Basin is a portion of the Blue Nile rift basin system, which extends southeast across the Blue Nile and is located between the White and Blue Niles to the north [2]. The basin may be a ‘sag’ basin because its boundaries do not appear to be governed by large bounding faults, and it may be rather shallow, with a maximum sediment thickness of not more than a few km. The Mozambique Belt's Neoproteozoic Pan-African metamorphic and igneous rocks, which are exposed in the Sabaloka region north of Khartoum, represent the unconformably placed base of the basin sediments . According to, juvenile arc assemblages were accreted onto an ancient continental edge that generated the gneisses found there. Much of the research region is covered by the Albian age Omdurman Formation, which is made up of the Umm Badda and Merkhiyat members. The Nubian sandstone, which is made up of sandstones, pebbly sandstones, and conglomerates and is frequently covered in ferruginous layers, is the predominant lithology found in these members. Between the White and Blue Niles as well as on the east bank of the Blue Nile is where the late Tertiary and Quaternary Gezeria formation is observed. Sandstones, silts, and clays make up the formation. The Nile Silt and Wadi deposits are two examples of the region's quaternary deposits.

There are several studies conducted in the Khartoum Sedimentary Basin, Kheiralla, (1966) studied Nubian sandstone in the Wadi Alneel area, he could have classified the Nubian sandstone into five units. In addition to the re-nomination of Basal Conglomerate to the Omdurman Formation. Omer, (1975) found out that most of the Constant imprecation in Nubian sandstone around Khartoum ensures sediments of a fluvial environment and semi-arid climate. Berir, (1993) was considered the main reference of the sedimentological studies in the Khartoum sedimentary Basin, he classified the Omdurman Formation as upper and lower. Moreover, pollens and spores were described for the first time was the first person to study the paleontology of the Khartoum basin and proposed the Cenonian age for the basin. Eissaw, (1999), concluded that the depositional environment of the Ummbada member is Meandering rives, flood plain, and lakes environment [3], while the Merkhyat member was precipitated by Braided rivers, and corrected the age of Ummbada member to Late Albian Early Tournian. In addition to some unpublished reports.

Materials and Methods of Investigation

Fieldwork method: Geological fieldwork was conducted by visiting targeted areas distributed in eight outcrops in the study area and the following data were collected:

• Vertical sedimentary profiles were drawn based on the observations.

• Data collection and inferring sedimentary facies and their vertical and horizontal relationships.

• Samples collection which represented sedimentary facies for the vertical and horizontal profiles.

• Photographic imaging.

There are some other tools were used in this study for instance: A hand Lens to compare sedimentary grain sizes, GPS for coordination, tape measure for length measurements, in addition to compass device for the slopes and inclinations measurements, as well as Hummer for breaking and sampling the rocks, moreover, Digital Camera for photographing.

The Result of the Study

Lithofacies analysis: In this part, lithofacies are described to determine the types of facies, depositional environment, paleogeography, and paleocurrent direction. Facies analysis depends on two types of sections: Vertical and lateral sections, based on their grain size, thickness, sedimentary structures, fossil content, and colour, these facies were described.

Matrix-supported conglomerate facies (Gm): It constitutes 13.9% of the total facies. It consists mainly of quartz pebbles with other components such as clay, and sometimes it is supported by pebbles, as in Jabal Al-Ruyan, when its grains range in diameter and their average size is 3 cm. These facies represent basal lag sediments reaching a thickness of (4m) in Jabal Al-Ruyan, and in this case, they represent longitudinal barriers sediments in the braided rivers [4].

Trough cross bedded sandstone facies (St): It represents about 74.9% of the total facies in the study area and its colors range from (red, yellow, grey, and brown to pink) and its range of grain size is between medium to coarse and sometimes fine. This facies represents the sediments of the channel under the sand flow, and it represents the migration of dunes. There are two types of them: a small concave layer and a large one. The latter is often at the bottom of the channel, and the small one is at the cap of the channel. It signifies a shift in the level of intensity of the current and also indicates depth.

Planner cross bedded sandstone facies (Sp): It represents 1.8% of the total facies in the region. It ranges in color from light red to light yellow and its grain size is medium to coarse. It represents longitudinal bar sediments in a high-flow regime.

Fine lamination siltstone facies (FI): It constitutes 6.3% of the total facies in the study area. Its colors range from gray to light gray. It consists of kaolin clay and appears clearly in the Omdurman Formation (Umbada Member) west of Al-Markhiyat. It represents the upper part of the channel and is classified as an over-bank deposit, especially in the Jebel Awlia area. It represents the river fissure and the end of a depositional cycle [5].

Massive mudstone facies (Fm): It constitutes 1.9% of the total facies and is in the form of a thin layer rich in iron, which indicates that it was deposited in a calm environment and represents the end of a depositional cycle or nearby floodplain deposits.

Ripple marks (Sr): It represents about 1% of the total facies present in the study area. This facies is found only in the Jabal Awliya area and indicates the weakness of the current and the depth of the water).

Discussion and Interpretation

Depositional environment: In the study area, there are various Basal lag sediments, represented by the conglomerate facies (Gm), as well as channel sediments, represented by the trough cross-bedded sandstone facies (St), and longitudinal barrier sediments, represented by the conglomerate of large thickness, as in Al-Ruyan and Abu Walidat. The massive sand facies (Sm) and lingual barrier sediments, which are represented by the planar cross-bedded sandstone facies (Sp), as these sediments indicate deposition within the channel at the beginning of deposition. There are also silt and clay facies represented by the (Fl) and (Fm) facies, which indicate sediments above The bank.

we notice that the percentage of sand is high compared to the clay, which indicates high speed in the current, inconsistent banks, severe erosion, and a decrease in the current after a long period. This is reflected in the deposition of sandy layers with fine grains of silt and clay, and indicates that they are sediments of braided rivers.

Depositional model: Due to the lack of fossils that indicate the depositional environment, the study relied on the rock characteristics in the different layers for analysis and comparison [6].

The depositional environments are lithofacies groups, and the facies group is several facies that exist together and are believed to have a relationship with the depositional environment, due to the presence of flake clay facies (Fl) and massive clay (Fm) in the study area in Jabal Awlia, as well as in Jabal Al-Markhiyat, which Reflect a fining upward sequence. Also, the presence of (Gm) and (St) facies, which are the most common in the study area, and the lack of clay in this area indicates the lack of cohesion of the banks and lateral migration, and these facies indicate a fining upward sequence, which indicates that these sediments were deposited by braided rivers.

Paleoclimate and paleogeography: The study indicated that the prevailing climate was humid tropical during the deposition of sediments of the Upper Omdurman Formation, and the presence of lateritic soil, ironstone, and the dominance of kaolin in the region. As well as siliceous plant fossils and sand-rich river sediments. In northern Sudan, tectonic activities occurred during the Albian-Cenomanin period, which resulted in the formation of sedimentary basins consisting of a group of smaller basins, with Khartoum Basin forming one of these basins. Deposition in these basins occurred via braided rivers whose direction is northwest.

The direction of the paleo-current indicates that the source rocks are located in the southeast direction, and the presence of a conglomerate consisting of pieces of silica, as in Abu Walidat Mount, indicates that the source rocks are rich in quartz and are igneous or metamorphic [7].

Conclusion

The study area is located in Khartoum State, central Sudan, between longitudes (32°18'24.5" –32°44'41.2 ") E and latitudes (16°20'19.6" –15°11'36.5") N.

The research dealt with studying the ancient depositional environments, architecture, facies types, and sediments paleogeography of the Omdurman Formation (Upper-Cretaceous) of the Khartoum Basin's northern region. The study included an analysis of surface rock facies based on vertical and lateral profiles. Lithofacies analysis indicated that the Omdurman Formation contains matrix-supported conglomerate (Gm), trough cross-bedded sandstone facies (St), plane cross-bedded sandstone facies (Sp), laminated silt facies (Fl), and massive mudstone facies. (Fm).

The Omdurman Formation consists of two members, the first is the highest and is called the Al-Markhayyat Member (fining upward sequences) representing the deposits of braided rivers characterized by cohesive banks. In this member coarse facies are common and some fine facies appear such as the alluvial facies (Fl) and the massive mudstone facies (Fm). This facies was deposited by braided, slightly meandering rivers characterized by shallow channels. The second, which is the bottom, is called the Umbada Member, as this member shows fining upward facies sequences of vertically accumulated sediments of channels and islands.

The depositional model of the Omdurman Formation can be represented by a coarse-upward depositional cycle that appears to be controlled by internal and external factors. The change in the base level is closely linked to the gradual decrease and tectonic activity that finally led to the spread of an open or external drainage system during the period of deposition of the Markhiyat Member (lower part). The predominance of kaolinite and ironstones indicates that the Upper Cretaceous climate in the period was tropical with short terms of drought, supported by the prevalence of siliceous solidified wood, iron oxides, and a high percentage of clastic sediments. The sandy (CHs) and gravel (CHg) channel architecture was distinguished in the studied profiles.

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