Red Sea Coast of Egypt. by Prof.. Dr. Abbas Mansour

Transcription

1 Red Sea Coast of Egypt by Prof.. Dr. Abbas Mansour

2

3 INTRODUCTION The Red Sea extends northwestward from the narrow strait of Bab ElMandab to the southern tip of Sinai, Egypt. The zone where land and sea meet is a variety of complex environments. The coastal zone is characterized by shallow banks and reefs, but depths of 100s ms are reached within a few kms off the shore, with commonly no effective continental shelf (Braithwaite, 1987).

4 INTRODUCTION Fringing reef is the seaward extension of the coastal plain in which depressions, mostly valley mouths, locally called "sharms" or "marsas" and some lagoons were formed by the drainage systems during glacial lowstands of the sea level. Human impacts and activities have great effect on the coastal zone and its coral reefs. For its economic and applied aspects, to utilize the coastal zone to its capacity, and yet not plunder its resources, an extensive knowledge of its complex environments is needed.

5 Red Sea and Medicine Other than the recreation and industrial activities, recently, patients suffering from certain diseases such as tsoriasis and rheumatism are treated in the coastal area of Safaga. Sediments are probably one of the main factors for this treatment as they reflect interaction with the hydrosphere, atmosphere, and biosphere. Therefore, good knowledge of the sediment, its composition, minerals and origin helps the evaluation of the medicinal point.

6 Topography The Red Sea extends from Suez to Bab ElMandeb and lies in a fault depression in the African-Arabian Shield. At its northern end it bifurcates into the Gulfs of Suez and Aqaba; the floor of the former is smooth and shallow (~60-70m), whereas, by contrast, the Gulf of Aqaba reaches a max. depth of 1720m. South of the Gulfs, the opposite coasts of the Red Sea run remarkably straight as far as Lat. 25 N. The width of this northern portion of the sea is approximately 180km, and the sea floor consists of a single main trough ab. 1300m deep.

7 Topography South of Lat. 24 a deep incision, referred to as the axial trough, appears in the main basin. This can be followed along the length of the Red Sea as far as 16 N. North of Lat. 22 N the coastal plains tend to be narrower, with terraces well above the present shoreline emergence to the N and of submergence to the S. The narrow coastal plains here climb directly into fault-bounded blocks of Basement Rocks less than 5 to 10 km from the shoreline. Landward from the littoral zone and coastal plain, the topography varies considerably. The plains in southern Egypt are bordered by lower slopes merging seaward as interdigitating fans and pediments broken by outliers of older rocks.

8 Coastal Structure and Formations The Red Sea is the place with the earth s largest geologic features; the mid-ocean rift system strikes a continental platform, and splits it. It is now fairly well established that the Red Sea rift has been separating Africa from Arabia for ab. 70 million years. Rifting has not been continuous, and pauses in the first half of the Tertiary saw important episodes of basaltic volcanic activity manifested by volcanic islands and other southern coastal reef foundations, and lava flows.

9 Gulf of Aqaba The Gulf of Aqaba seems not to be a spreading center at present, but instead the Arabian side is sliding northwards relative to Sinai. This center is believed to change to strike-slip faulting in the southern third of the Gulf as the Arabian plate slides more in parallel with the Sinai (Friedman, 1985), and this has led to three deep basins in the Gulf of Aqaba. The Gulf with its deeper water, contains more than 2,000 to 3,000m of sediment infill, 5,000m of sediment infill in its northern and southern ends and somewhat less in the central area (Avraham, a.o., 1979).

10 Gulf of Suez The Gulf of Suez occupies a wide valley bordered by wide plains of low relief. It appears to be spreading and exhibits normal faulting. It has no sill at its connection with the Red Sea. It has been thought that the Gulf of Suez was developed in Miocene with the first stage of formation of the Red Sea, and that only the Gulf of Aqaba marginal faults were formed at that time. The faults that actually bound the modern Gulf and that lie inside the marginal faults (separated from them by the marginal blocks) are considered to have been developed with the second stage of tectonism only 4 to 5 million years ago. The depth of sediment infill of the Gulf of Suez has been known to reach 5000m and most likely to include Miocene-age sediments (Said, 1990).

11 Important Events for Present Day Ecology The important events for present day ecology derive partly from the eventual but not continuous connection with the Indian Ocean which began 2-5 MaBP, and partly from the intense erosion during the Pliocene and Pleistocene.

12 Important Events for Present Day Ecology The former period brought the modern Indian Ocean fauna and flora, including reef-forming and reef-dwelling organisms, into the Red Sea. The erosion later was caused by heavy rain probably in two phases, the last phase between 35 and 17 thousand years ago (KaBP)(Jado and Zoetl, 1984), i.e. during the lowest sea water stand. During both phases of erosion, sheet flow of alluvial material formed alluvial fans and outwash systems. The material of the fans was derived from the older rocks of the adjacent Red Sea Mountains.

13 Aspects of Erosion Two aspects of this erosion are important. First, it cut through some of the early, developing fringing reefs repeatedly, commonly down to around the contemporary sea level or possibly slightly deeper. This appears to be the case especially in the early Pleistocene rainy period when sea level was 30 to 60 m below present. At the same time more alluvial material covered extensive areas of fringing reef. Solution by rain also might have added minor karst relief directly, in comparison to the erosion from sediments.

14 Aspects of Erosion Secondly, large amounts of terrestrial material ended up offshore in shallow water forming important new substrates colonized in many cases by new coral reef growth, and developed extensive areas of sheltered habitat retaining a soft substrate character. The extent of erosion influence can be seen by the major wadi systems (called sharms, small gulfs or marsas) formed during the Pleistocene, and which were essentially cut by abrasion into the existing substrate. In some areas by contrast, the overall low lying terrain precludes wadi formation, though terrigenous sediments grade seaward for a considerable distance.

15 Geomorphology and Shoreline terraces The high mountains extending along the western Red Sea coast characterize the inland geomorphology. Many wadis fan-out of these hills, being dry most of the time with sporadic and abrupt fluvial activity.

16 Shoreline terraces During the time of considerable alluvial activity in the last 140,000 years, reefs were developed along the coast. Their growth was irregular and discontinuous for two main reasons: First changing levels of sea relative to shoreline developing terraces, and secondly because of alluvial activity with sea level 20 to 60 m below present, fringing reef was developed on the contemporary shoreline, and patch reefs was developed on any older limestone (or other) platforms (Sheppard, a.o., 1992).

17 Shoreline terraces The upper surface of these reefs was just covered at low water with fall in level, erosion of the previous reef growth would occur. At the same time, sheets of alluvial material spread into the coastal zone and sea, filling eroded channels and smothering sections of growing reef. This alluvial activity was marked in the Gulf of Aqaba and Red Sea, partly because of the steep mountains very close to shore.

18 Shoreline terraces Where these alluvial fans ended up in water, they become consolidated fairly rapidly to the point forming substrate for new reef growth. Present reefs are a blanket of recent growth covering the Pleistocene highs. The high load of terrigenous deposits transported to the shore by wadis, and hence the high turbidity of sea water, prevented the formation of coral reefs and this gave rise to the formation of embayments (Sheppard, a.o., 1992).

19 Shoreline terraces Series of raised fossil coral reefs higher than current sea level are one of the most striking features of the Egyptian Red Sea coastline. Quaternary reef terraces are 2-6 terraces (Fig. 2), three of them are distinguished in several areas. Each exhibits a short distance lateral facies development, which begins at the shore with beach, mainly siliciclastics, and ends at the reef zone by carbonate sediments (Mansour, 2000).

20 Shoreline terraces Strong similarity can be noticed between sedimentary facies (climatic conditions) of ancient Pleistocene sediments and those now present in modern fringing reefs. Reefs with their siliciclastic associations occur in the form of repeated cycles reflecting the tectonic effect and/or sealevel changes (Mansour, 2000).

21 Shoreline terraces ElAsmar and Attia (1996) noted 4 terraces in the Sinai, and Guilcher (1988) refers to series of 11 uplifted reefs on the Tiran Islands at the entrance of Gulf of Aqaba extending vertically to 320m. Elevation of the lower terrace is different from place to place and generally exceeds southward along the coast (Mansour, 1993).

22 Shoreline terraces These terraces are occasionally covered with a thin veneer of alluvial fan deposits from the Basement Complex in the W and with raised beach deposits eastward. Some evaporite plains, a few cm thick, usually exist on the upper part of raised beach close to the sea.

23 Shoreline terraces Tectonism controls the distribution of the depositional systems and influences the number, thickness, extension, and elevations of the reef sequences. One or more of the depositional sequences may occur within one terrace. In some places (ex. Dusht ElDabaa, 25km S Hurghada and at Marsa Kalawi, ab. 28 km S Safaga), 2 reef cycles are observed within the younger terrace, separated by an unconformity or bodies of terrigenous sediments (Fig. 3). This can be explained by the effect of eustatic sea level rise during the last interglacial stage. The occurrence of evaporites on the surface of the lower terrace supports the effect of sea level change.

24 Coastal environments Alluvial fans Impressive evidence of stream erosion and deposition exists in the areas of the Egyptian Red Sea and its Gulfs of Suez and Aqaba. Infrequent rain does fall catchment areas of rocky desert highlands funnel flashfloods building alluvium fans several sq. km in extent (Fig. 4).

25 Coastal environments Alluvial fans Below the water surface submarine alluvium cones are formed, especially on the gentle slopes. On the extremely steep slope areas along the coast, no subaerial fans are formed. Larger fans alone or anastomosing at the mouth of more than one wadi (ex. W. Kid, W. Um Ghieg) have gentle slopes and finer particles; sand grains and sand layers are more common than the gravels.

26 Coastal environments Alluvial fans and Erosion Submarine canyons of the Gulf of Aqaba cut down 130m into alluvial cones offshore from the fans. These fans themselves have been partially eroded, as shown by the presence of terraces. These are the effects of unusual conditions that prevailed during the late Wuerm glaciation ab. 11,000 years ago (Friedman, 1985).

27 Coastal environments Alluvial fans and Tectonics Recent tectonic events have also affected the fans through elevation resulting in further erosion and terrace formation. Locally, modern fault movements can be seen to have cut across and displaced the fan deposits as well.

28 Coastal environments Alluvial fans, Sediments Detrital sediments are brought into the sea from the slopes. The coarsest material remains on the beaches and progressively finer particles reach greater distance and depth, where they are strongly diluted by biogenic material. Unconsolidated boulders of Basement Rocks cover steep slopes at depth between 10 and 40m (ex. at the mouth of W. Dahab, Gulf of Aqaba).

29 Coastal environments Alluvial fans, Uses The Quaternary alluvial fan gravels are one of the preferred aggregates to be used for constructions, buildings, highways, airport runways, and all other structures. In different localities, most of them can be used as quarry sites due to their surface occurrence and very close to roads.

30 Coastal environments Alluvial fans, Uses Deposits close to granitic rocks are very rich with feldspars, which can be used in ceramics. Alluvial fans can be good sources of ground water along the coast. The abundance of these coastal sediments can be useful in the development of the Red Sea area.