The Nile River Meets the Mediterranean Sea

North East on the African continent, about 150km from the Mediterranean coast line lies the depocenter of the Nile River system, The Nile Delta. It encompasses about 22000km 2 making up most of the Egyptian arable land. It is also gas-producing area of Egypt. The Nile delta formed in the late Pliocene. It developed during Pleistocene as the Nile River actively carried sediments northwards from the African Lake Plateau, navigating its way to the North of Cairo, discharging tons of sediments to the Mediterranean Sea. Over the past 7000 years this delta has been prograding through the delta plains to offshore. However, for the past 150 years, the Nile Delta in its destruction phase, deficient of sediments. This due to sea-level changes, climatic oscillations, subsidence, and sedimentation processes. This affect the agriculture the ecosystems operating in that region.

Introduction

When a sediment laden river meets a standing body of water, be it a sea or a lake, its velocity decreases and the sediments are deposited near the entry point forming a delta (Bhattacharya,2010). A delta forms only when there is a rapid supply of sediments than they are redistributed by basinal processes such as longshore drift, wave or tidal currents. This means the morphology will differ, depending on space available at the entry point, salinity differences supplying river and the receiving water body, the amount of sediments deposited, the geometry and depth into which the river flows into, and the energy of the coastal processes that work and redistribute these sediments (Sentini, 1989).

There are number of ways to classify deltas however, the commonly used is the Tripartite Classification (figure 3) (Bhattacharya,2010), which simply groups them according to the dominant processes that control the accumulation and redistribution of the deposited sediments: river dominated, tide dominated, and wave dominated. A river-dominated delta forms in a low energy water body, with little or no wave/tidal activity (Reading,1996). Here, the river currents reach beyond the shoreline to deposit sediments and delta switching is very common because of the many distributaries. Tide-dominated deltas are formed in sea where tidal currents are strong and have an irregular plan view just like the Ganges-Brahmaputra delta, in India. Lastly, are wave-dominated deltas that form in large lakes or seas where there are large waves generated by strong prevailing winds. These show smooth fronts and arcuate to cuspate margins. (Bhattacharya, 2010) This essay is concerned with the wave-dominated Nile Delta (figure 1 and 2). It explores its different deltaic environments from the bifurcation of Nile River to the Mediterranean Sea floor, focusing on characteristics of sediments and processes that brought them about.

The Nile River flows northward from the Ethiopian Highland and the Equatorial Plateau, crossing several vast regions which include The Sudd, Central Sudan, Ethiopia, Cataract region of Sudan to Egypt (Sentini,1989). It represents the major source of sediments in the Mediterranean Sea. These sediments, Nile sands, are derived from the East African Rift System and craton but their composition changes as the Nile transverse through areas of contrasting geology and climate and finally deposited as large volumes of coarse delta sand and offshore thick turbidites forming the Nile cone in the deep ocean northeast of Africa (Garzanti et al,2006).

Progradation

As the Nile approaches the Mediterranean Sea the gradient decreases and the black volcanic sand and mud continuously settled out of suspension during episodic floods, at the mouth north of Cairo. During this time period (about 7.5 ka), the sea level was lower than at present which allowed high sediment supply and little no to erosive forces. the Nile Delta formed during late Miocene (Stanley and Warne,1998). Thus, it can be argued that the progradation of the Nile Delta was primary due a balance between sediment supply and limited coastal processes. Sentini (1989) adds that Nile Delta developed farther out towards the sea through channel extension followed by channel switching, as seen in figure 2 the main Nile channel bifurcates into Damietta and Rosetta branches leading to formation of three distinct deltaic environments: the delta plain, delta front and the prodelta. These environment correspond to the topset, foreset and bottom set series of layers of a delta, which can be clearly seen in a cross-section view (Bhatacharya,2010).

Sub Deltaic Environments

Bhattacharya (2010) defines delta plain as the subaerial, topmost environment close to the main fluvial channel with active and abandoned and active distributary channel network where river processes dominate. The fan-shaped Nile delta plain covers an area of about 22 000km2, from the apex to the approximately 225km long coast. Along the main distributary channels, Rosetta(west) and Damietta(east), lies an interdistributary bay with natural levees, crevasse splays and peats (Hamza,2009). On the delta margins lie two distinct sediment promontories, deposited by the respective branches, which continue to deliver sediments to the shoreline.

A bay lie divides the delta plain into upper and lower delta plan. The upper delta plain is dominated by silt plains, small lakes, swampy areas and stream with little or no water. Fine sands and silt are covered by permanent vegetation (Sentini,1989). The lower delta plain lies below the bay lie, seaward, and occasionally receives sea water. It is dominated by brackish water environment which form a lagoon belt, dunes and sandy, shelly beaches. Here the sediments are silty clays with high organic content. Landward of the lagoon belt (i.e Idku, Burullus and Manzala Lagoon) (Frihy,2003), stiff silty and sandy are also found. Overall, the delta plain has silt and sand sized sediments making up the topsets with upward fining and seaward dipping bands in the Nile distributary branches while in the interdistributary bay there is sand and mud couplets that settled out of suspension after flooding with peats due to the high biological activity. (Frihy et al,1991).

The prograding Nile Delta front has dipping sea beds which includes both the subaqueous topset and foreset beds as the slope leads to deep water. It marks the transition from non-marine to marine environment thus both river and basinal processes operate. This forms part of very active deposition (Bhatacharya,2010), where the mouth of the Rosetta and Damietta distributary discharges, and consequently mouth bars form. These mouth bars form parallel to the coastline due to the continuous reworking of the waves which can be preserved as symmetric ripple cross lamination. Well sorted, very fine to fine sand dunes also form through deposition by the wind action from the strand plain. Beach sands, bioturbated shelly mud and thinly interbedded silt and sand form to the landward of coastal dunes and dune peat while muddy blankets form on the seaward side, both with distinct upward coarsening. Along the margins, dark grey to black mud with high organic content (Stanley and Warne,1993). Cross bedding at the mouth of distributary channel forms with lamination from along the muddy seaward slope. However, these sedimentary structures have low chances of preservation due to high sedimentation and bioturbation.

After the shelf break the bottom set beds are deposited from by settling out of suspension which is occasionally disturbed by mass flow deposition under the influence of gravity called hemipelagites (Reading,1996). Deposition by gravity creates a better chance of soft sediment deformation structures. The bottomset beds are dominated by basinal processes that lead to formation of neritic shale. From the quite settling of sedimentary load and low gradient, horizontal lamination forms. These laminations usually preserve better in anoxic waters where they will not be disturbed by bioturbation (Bhattacharya,2010). This explains the rhythmic lamination of the Nile cone. The prodelta contrasts with the coast in shape with it being asymmetric with a wider eastern end. Bioclastic debris high in CaCO2 a large proportion of this area at the same range (Sentini,1989)

Overall the Nile Delta is overlain by a thin 10-40m interbedded medium to fine grain sand, silt, evaporites and peats layer. A mosaic of environments are made from deltaic to marine with transitional beach and dune sands. These sediments unconformably overlay coarse-grained late Pleistocene clastic sands which lie underneath as sheets that formed during times that continuously cycled from a high to a low sediment flux (Sentini,1989). In some parts of the delta plain they form hills called “Gzira” by the Egyptians which are also known as turtlebacks because of their shape. Also, offshore west of Abuqir they are exposed.

The deposits of the Nile delta brought fertile, most arable soils to Egypt. Now they make makes up 2/3 of the total Egypt’s agriculture. With the growing population and to control floods, Aswan Dam was constructed. The Nile water mostly became canalised to support irrigation and livelihood of the people. The construction of the dam decreased the sediment supply to the Mediterranean coast. Along side the dam construction, natural processes like subsidence, climate change and sea level changes led to the decrease of sediment influx (Stanley and Warne,1993). These fluctuations changed regression to its transgression phase(Stanley and Warne, 1998). The delta plains are continuously being worked by basinal processes with little sediment supply (Bhatacharya, 2O10) “Nile is no longer an active delta but, rather, a completely wave-dominated coastal plain along the Mediterranean coast. The River Nile system has been so modified”, (Stanley and Warne, 1998)

Conclusion

The Nile Delta formed in the late Pliocene, continued to grow in the last 7000 years is now in its destruction phase. This delta successfully progressed from Cairo, where the Nile River deposited most of its sediments during episodic floods, to the Mediterranean Sea coast where it formed thick turbidites in the offshore. The delta geometry changed from the delta plain with topset silt beds with distributary streams, lagoons and levees to a delta front lined with beach sands with mouth bars of the Rosetta and Damietta distributary channel parallel to the coast line to the bottomset beds formed from settling of sediments out of suspension. Different processes govern each stage to bring about the gentle sloping clinofrom.