Red Sea Basins - South Valley University Sea Basins TECTONO-SEDIMENTARY ... not limited to local...

Red Sea BasinsRed Sea Basins

TECTONOTECTONO--SEDIMENTARY SEDIMENTARY EVOLUTION OF THE NW PARTS OF EVOLUTION OF THE NW PARTS OF

THE RED SEA The THE RED SEA The SynSyn RiftRift

byProf. Dr. Abbas Mansour

Regional Pliocene and Quaternary Sedimentation

• Post-Miocene movement obviously is not limited to local tectonic adjustment; not only does it affect both the axial and peripheral parts of the rift, but it is important along major transform (Aqaba) trends limiting the Gulf of Suez from the N. Red Sea. The precise sedimentary expression of these major rift movements are poorly known and may be summarized as follows:


• a) Axial subsidence: Seismic profiles (Guennoc et al., 1988; Cochran and Martinez, 1988) and DSDP an other short cores Degens and Ross, 1969, Taviani, 1992), show that the axial parts of the N. Red Sea are affected by numerous scarps which cut ”reflector S”, resumed to be the top of the Late Miocene evaporites. The post-evaporite sediments discussed by Stoffers and Ross (1977) are essentially calcareous muds and silts rich in planktonic microfaunas and detrital silicate minerals. Closer study by Taviani (1992) and Purser and Philobbos (1993) indicate that these relatively homogeneous sediments, in fact, are remarkably variable.


• Although beigecoloured marls with planktonic foraminifera and pteropods are predominant, there are numerous beds of well-sorted sand. In one 16m core collected by the French vessel ”Marion Dufresne” form a depth of 1696m off Port Sudan, twelve sandy horizons were measured (Fig. 16). Each begins with a sharp base affecting underlying planktonic muds, and grades upwards from coarse sand to silt, the latter frequently laminated. The basal sandy parts may be composed either of planktonic forams and pteropods(as noted by Taviani, 1992), or of fine gravels composed of blackened or red siliceous detritus and pteropods. These units exhibit many of the typical attributes of turbidites, resedimentation of oceanic sediments probably being related to tectonic instability of the axial scarps.


• b) Peripheral uplift: The wide offshore zone situated between the axial depression and the present shoreline is also affected by numerous post-Miocene faults and diapiricintrusions (Guennoc et al., 1988). One major fault would appear to be situated fairly close to the Egyptian NW Red Sea shoreline whose linear nature suggests that these late structural movements may be relatively large- scale and linear (Bosworth, 1993).


• Onshore, basement uplift in late Pliocene and Quaternary times is expressed by important (100m) accumulations of large (1m) basement boulders which occur at altitudes of about 20-50m near the foot of the peripheral basement range. Seemingly of Quaternay age, they are typical of much of the NW Red Sea coastal area. Overlying alluvial cones and terraces reflect the combined effects of peripheral uplift and climatic evolution, discussed by Baltzer et al. (1993).


• c) Transform Aqaba Zone: This structurally active slope is crossed by a series of seismic profiles (Guennoc et al., 1988) which demonstrate the tectonic complexity of this zone. Although no cores are available, seismic profiles indicate a highly deformed sequence which has accumulated on the structural slope limiting the S end of the Gulf of Suez. Sediments, initially deposited in the relatively shallow (less than 80m) Gulf of Suez, presumably are swept southeastwards by wind-driven currents to accumulate below wave base on the structural slope.

The Geometry of Early Rift Sediments with Respect to Subsequent Rift Development

• The intimate relationships between early rift tectonics and contemporaneous continental and marine sedimentation is evident, both with respect to depression fillings and early platform evolution (Fig. 10). It is stressed that these early phases differ considerably from the subsequent sediment filling, this latter being conditioned mainly by the progressive deepenning of the rift axis.


• Firstly, the geometry of the initial, continental deposits was often oblique with respect to the rift axis, being related to the proto-rift, strike-slip fault system. Latter structural development relating to clysmic, normal faulting resulted in numerous platforms whose main axes were generally parallel to the rift. However, because there was no major axial subsidence, presumably there was no important lateral variation (at least on the platforms) between the periphery and the geographic center of this early basin.


• Finally, with increasing subsidence from the Middle Miocene evaporites onwards, there developed a marked regional change in sediment type, the axial-parts of the rift, notably during the Pliocene, being enriched in fine planctonicsediments. Clearly, the early rift phases, occupied a particular part of this overall evolution.

• Subsequent rift development relating to axial subsidence appears to be expressed by a simpler tectonic style with the predominance of normal faulting. This would suggest that sediment distribution patterns would become simpler as the rift evolved. .


• Secondly, the NW parts of the Red Sea rift seem to reflect a centrifugal migration of faulting and subsidence, a similar situation existing in the Gulf of Suez. Thus, early phases of block faulting and their related sediments remain unmodified by subsequent tectonics which ”migrate”towards the axis of the rift (Fig. 10). In other words, in the peripheral parts of the basin, early close relationships between tectonics and sedimentation, although ”fossilised”, are not rejuvenated to any significant degree during later Mio-Plioceoe sedimentation. As such, the early structural platforms and their sediments, at least in the peripheral parts of the rift, tend to loose their identity as they become drowned below a blanket of younger Pliocene sediment. This is stimulated by the late, regional uplift of the shoulders of the rift and increasing terrigenous input. Subsequent Quaternary erosion has exhumed these early Miocene platforms (Fig. 10).


• Thirdly, the drowning of early Miocenc platforms and depressions by Mio-Pliocene infilling, especially in the peripheral parts of the rift, is perturbed localy by halokinetic movements of the Miocene evaporites. Post evaporite, Pliocene sediments are deformed into domal highs whose bathymetric relief has influenced Plioceoesedimentation. At two localities between Quseirand Mersa Alam, dolomitised Pliocene reefs are coincident with local Pliocene doming (Fig. 18).


• Fienlly, the thermally induced subsidence becomes very marked during the Middle and Upper Miocene when important thickness of evaporites were deposited both in the Gulf of Suez and in the Red Sea. According to Cochran and Martinez (1988) subsidence of about 3000m in the Gulf of Suez is related to the presence of many listric faults whose accumulated lateral displacement is in the order of 35km. In the northern parts of the Red Sea lateral displacement is partially coinsident with sinistral movement along the Aqaba-Levant system which also began during the Upper Miocene and has continued during the Pliocene. But in spite of this important lateral movement there is no surface oceanic crust in north Red Sea, although the continental crust has thinned to 10km (Le Pichon and Gaulier, 1988). This thinning, however, has lead to the appearence of oceanic crust south of the latitude of 24N (Bonatti, 1985).

Deformation Structure Recording NeogeneEarthquakes in the Syn-rift Sediments

• The Neogene sediments of the NW Red Sea coast exhibit a variety of deformational structures affecting both continental and marine facies deposited horizontally. These stratiform deformational phenomena do not affect adjacent beds, thus indicating their near-surface, syn-sedimentary origin. Their highly variable character depends mainly on the mechanical properties of the sediments at the time of their deformation. Soft sands have been liquified, slightly consolidated muds, sands and gravels have been affected by fissuration and plastic folding while lithified sediments have been brecciated (Purser et al., 1993). These breccias are also folded and show both distensional and compressionalstructures. They may resemble superficially other breccias resulting from karstic dissolution of evaporites or carbonates. However, they differ from these latter by the presence of compressional folding and overthrusting and by the absence of vadose diagenetic fabrics.

Deformation Structure Recording NeogeneEarthquakes in the Syn-rift Sediments

• A common origin of these deformational features is explained in terms of earthquake shocks whose varied expressions may depend on the different mechanical properties of the sediments affected. The systematic study of ”seismites” as considered to be important; their stratigraphic frequence may express specific tectonic events related to rifting while the intensity of deformation within a given horizon will depend both on its susceptibility and on proximity to the epicenter.

The Different Structural Stages during Rift Development

• a. The pre-rift situation • In the regions where the Cretaceous – Eocene cover occurs

(Gulf of Suez and northwestern parts of the Red Sea), it is clear that there have been only subtle and local deformations prior to deposition of the lowest units of Group A. These latter lie on the Eocene with an erosional unconformity, or with a low angular discordance. The pre-Miocene structuration began by Upper Cretaceous and Eocene synsedimentary deformations (frequent collapses and brecciation in the upper part of the Eocene Thebes Formation). These phenomena are mainly localized along major trends of faults (N110, Gebel Duwi; N060, Gebel Dara). They may be considered as an attenuated effect of the Syrian arc tectonics, developed in the northernmost region.


• b. The initial phase of rifting: strike-slip movements

• Early structuration at the beginning of Group A is characterized by a compressional reactivation of the inherited fault-pattern. The direction of regional shortening is close to N140 (Jarrige et al., 1986a), i.e. the same as for the previous folding episode of the Syrian arc. It is expressed by sinistral strike-slip faulting along N00 to N020 ”Aqaba” faults and dextral movements on N100 – 120 ”Duwi” faults. The associated structures include drag- folds with inclined axis and kilometer-sized folds located most frequently at the tips of sinistral strike-slip faults (ex. Abu Rudeis area).


• The normal N140 ”clysmic” faults also began to form during the early phases of deformation. They are associated with the submeridian sinistral or dextral faults, in a ”zig-zag” fault pattern, resulting in the creation of large rhomboid-shaped panels (ex., the Zeit-Mellaha area)(Fig. 13).

• Synsedimentary deformation of lowest Group A sediments is often intense, especially where located along lines of submeridian-trending strike-slip faults (West Mellaha; Safaga). Compressive structures related to this early phase of the rifting are more subtle south of Safaga; from Quseirto Ras Benas submeridian faults are weakly expressed. However, strike-slip movements occur mainly along dextral N100-120 trending faults. In the Duwi area (Jarrige et al., 1986b), they affect the lowest Group A; in Abu Ghusunthese dextral strike-slips acted prior to, and after deposition of the Red Series (Lower Group A).


• c. Lack of evidence of initial doming • In the Gulf of Suez and the northern part of the Red Sea, the

Cretaceous – Eocene series are preserved within the axial parts of the rift where they are covered by Miocene deposits. Furthermore, there are no obvious lithologicalchanges within the pre-rift open marine Thebes carbonates as one approaches their southern limit indicating that it is an erosional one. This is an indication that the initiation of rifting was not associated with doming. As already mentioned, the sedimentary properties of Group A sedimentation (Red Series) suggest low relief conditions: initial structuration has not induced contrasting morphologies. These reliefs appear progressively, due to the increasing importance of clysmic faults. The progressive but discontinuous uplift of the rift shoulders is expressed by coarse clastic input during the later stages of the rifting.


• d. The tilted block pattern • During sedimentation of the upper parts of Group

A, the lateral fault displacements decreased in importance and tended to be replaced by normal fault movements. Nevertheless the persistence of late strike-slip movements has been noted locally in the Gulf of Suez (Gigot et al, 1985). Normal faulting oriented in various directions gradually predominated. This resulted in the formation of large antithetically tilted blocks generally limited by clysmically-oriented faults (Zeit and Mellahablocks – Gauthier, 1986; Prat et al., 1986), although in some areas they may have submeridian (Safaga) or N100 – 120 (Abu Ghusun) orentations (Fig. 13).


• Groups of tilted clysmically-oriented blocks with opposing dip orientations are separated from one another by ”Aqaba”or ”Duwi” trending normal faults that previously acted as strike-slip faults (ex. Duwi area – Jarrige et al., 1986b).

• This tectonic framework generated clearly delimited structural depressions within which were deposited restricted marine (”lower evaporites”) or lacustrinesediments of upper Group A (Fig. 8). On the eastern side of the Gulf of Suez and in off-shore areas of this latter region, (Brown, 1980), these highly subsident depressions are filled with mixed pelagic and gravity flows deposits.

• The activation of normal faults favored erosion and important discharges of coarse detritus, forming the upper part of Group A. In many places (Quseir, Safaga), the first appearance of basement material within these clastics is recorded.

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