Red Sea Basins - South Valley University · Red Sea Basins TECTONO-SEDIMENTARY EVOLUTION OF THE NW PARTS OF THE RED SEA The Syn Rift by Prof. Dr. Abbas Mansour

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

2. The Syn-Rift Series• In contrast with the pre-rift sedimentary cover, the

Miocene syn-rift deposits exhibit numerous and spectacular changes in thickness and facies. This result in the multiplicity of formation names based both on offshore and outcropping sections, leading to confusion and inconsistencies. The Neogene sequence therefore is devided into four major groups, A to D, each limited by an angular discordance of regional extension which seems to reflect the major stages of rifting (Montenat et al., 1986a,b) (Fig. 7). Correlations with the series outcropping on the Saudian margin have been proposed (Purser and H?tzl, 1988).

2. The Syn-Rift Formations

2. The Syn-Rift Series• 2.a. Group A sequence: Early stage of rift

sedimentation• The presence of biostratigraphically dated

Oligocene sediments is not established. In the areas studied there is no equivalent of the marine Chattian beds described by Dullo et al. (1983) on the Saudian margin. Therefore, a long period of emersion took place between the formation of Eocene and Lower Miocene marine deposits.


sedimentation• At the base of the group A sequence there exists locally a monogenic

breccia composed of Eocene chert and limestone with a reddish sandy matrix (< 50 m), grading upwards to red sands and silts, including conglomerate lenses, paleosoils and continental carbonates (< 300 m). These red beds are widely distributed, having been reported from the Gulf of Suez, along the western Red Sea shores from Egypt to north Sudan, and the Saudi Arabian margin. The Abu Zenima Formation in Sinai and the Nakheil (Gebel Duwi) and Ranga (Abu Ghusun) formations on the Red Sea coast pertain to the same complex. This ”Red Series” has been deposited in an alluvial plain environment. The coarsening-up sequences express progradation of proximal fans on this flood-plain. Channels develop progressively, replacing the initial debris-

flows.

2. The Syn-Rift Series







sedimentation• The predominance of fine-grained detritus suggests

low-relief hinterland. Weakly developed pedogenetic horizons, frequent desiccation features and a good preservation of feldspars in sands, indicate a relatively arid climate. Sediments generally are very poorly sorted, the conglomerates often having a sandy or muddy matrix indicative of rapid sedimentation, possibly during periods of flash flooding. Soil horizons are thin and root traces rare, suggesting a moderately dry climate.


sedimentation• In some places (e.g. Safaga, Quseir), the coarser

basement material tends to become progressively more abundant upwards, reflecting the gradual exhumation of Basement Rocks. These detrital discharges covered an extensive, low-relief region with shallow depressions. In spite of these relatively modest morphologies the sequence exhibits frequent synsedimentary deformationphenomena (slump and hydroplastic deformations). Most of these structures reflect seismic activity(Roussel, 1986; Roussel et al., 1986; Thiriet, 1987; Plaziat et al., 1988).


sedimentation• Within the upper parts of Group A, the alluvial deposits are

replaced in several localities by sediments reflecting confined, permanent water conditions such as laminated anoxic lacustrinecarbonates (northern Mellaha) or marine evaporites associated with laminated fetid carhonate, diatomites or stromatolites. In the Abu Ghusun area, these evaporites (”lower evaporites”) occur in Miocene hemi-grabens while adjacent horsts and fault-scarps are draped by dolomitic algal laminites and stromatolitic domes, which are brecciated under paleo-sloped situations. These evaporites were deposited within a subaquatic environment and the degree of local restriction is closely related to the morpho-structural pattern of the area (Montenat et al., 1986c)(Fig. 8).


sedimentation• These transitional facies are represented by

the lowermost evaporite beds at RasHonkorab, Wadi el Gemal and Wadi Gasus(Rosa evaporite Mb, Fig. 9) discussed by Orszag-Sperber et al. (1993), as well as by algal laminites at Ras Honkorab, Wadi el Gemal (Philobbos et al., 1993) and at WadiMarwa (West Esh Mellaha).


sedimentation• An open marine environment developed on the eastern edge

of the Gulf of Suez during the late Group A (Late Aquitanian? – Early Burdigalian times). At Abu Zenima and south of Quseir (Wadi Sharm-el-Bahari) these continental beds include some basalt flows. At the base of the WadiNukhul Red Series, basalt pebbles are reworked from a nearby dike whose radiometric age is 26-25 Ma. At AhuZenima these red beds are capped by a basalt flow with an age of 22 Ma. In the Quseir region, at Sharm-el-Bahari, the basalt flow interbedded in the upper half of the Red Series dated as 24.9 Ma (Roussel, 1986).


sedimentation• From these data it appears that the Group A Red Series is

probably diachronnus (Montenat et al., 1986b; Ott d’Estevou et al., 1986b). An Early Miocene age (Aquitanian) between 26 and 22 Ma is proposed for these levels on the edges of the Gulf of Suez. At Sharm-el-Bahari, where an important sequence of red sands occurs below the 24.9 Ma basalt, it is probably earlier. Therefore, the series probably begins in Oligocene. Farther to the south, at Abu Ghusun, the red sands are followed by an evaporiticsequence, the top of which has yielded an Early Miocene (Aquitanian) microfauna; the red beds are clearly older. In Northern Sudan (Marsa Shinab- Montinat, 1988) the same series including basalt flows, could also be considered as pre-Miocene. Nevertheless, sufficient radiometric data are still lacking.


sedimentation• Diachronism of the earlier syn-rift deposits (i.e. older

southwards) seems likely but is not yet confirmed; it is of considerable importance to the understanding of rifting processes.

• As previously noted, the evaporites located in the late Group A are dated as Aquitanian at their top in the Abu Ghusun area but they could be somewhat more recent (Early Burdigalian) in Safaga (Thiriet, 1987). On the eastern side of the Gulf of Suez open marine deposits equivalent to the upper part of Group A arerepresented by the Lower Gharandal series of Early Miocene age (Late Aquitanian – Early Burdigalian; Garfunkel and Bartov, 1977; Ott d’Estevou et al., 1986b; Fig. 8).


sedimentation• Distribution of Group A sediments: Group A

sediments outcrop only along the periphery of the rift where they are best developed within certain grabens (Fig. 10). Their localization within structural depressions formed prior to Group A sedimentation, is similar to that on the northeastern margin of the rift (Saudi Arabia). These morpho-structural depressions have acted as conduits along which the continental deposits have progradedtowards the axis of the rift.


sedimentation• Distribution of Group A sediments: Group A

sediments also form structural highs (Fig. 10) but these result mainly: from post-Group A deformation. In general, positive blocks of basement rock are covered directly by Group B or C sediments. In many parts of the regions studied, as in Saudi Arabia, Group A continental detritus is localized within grabens formed during early rifting.


2. The Syn-Rift Series• 2.b. An important, early syn-rift deformational

phase: the post Group A unconformity• The vertical and littoral variations within these

continental sediments suggest progressive structural development which culminated towards the end of Group A sedimentation with important vertical movement which occurred both along the preexisting strike-slip faults and, especially, along a series of somewhat younger normal faults oriented parallel to the main axis of the rift. .

2.b. An important, early syn-rift deformational phase: the post Group A unconformity

• Termed ”clysmic trend” by Hume (l921), these elongated horsts and half-grabens were tilted either towards the W-periphery, notably in the W parts of the Red Sea (Gebels Zeit, Mellaha, Abu Shaar, etc), or towards the main axis of the rift in the region of Quseir (Roussel et al., l986). Vertical movements, probably in the order of l00 m, resulted in an important erosion of the earlier Group A series. A marked angular unconformity thus separates the initial (Group A) continental series from the subsequent Group B marine formations. .

2.b. An important, early syn-rift deformational phase: the post Group A unconformity

• This unconformity, visible in many localities between Safaga and Ras Banas (Wadi Gasus, Sharm el Bahari, Sharm el Qibli, – Orszag-Sperber and Plaztat, 1990) and at Ras Honkorab where it separates Group A evaporites and marls containing Aquitanian microfaunns (Montenat et al., l986), from Group B marine carbonates of approximately Burdigalian age. An important unconformity expressing a ”mid clysmic tectonic event” (Garfunkel and Bartov, l977) is wel1 known in the Gulf of Suez region where it occurs within the thick Gharandal marine shales of early Miocene age. Although this important discordance is not dated with precision either in the NW Red Sea or in the Gulf of Suez, it may well record a common structural event..

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Red Sea Basins - South Valley University · Red Sea Basins TECTONO-SEDIMENTARY EVOLUTION OF THE NW PARTS OF THE RED SEA The Syn Rift by Prof. Dr. Abbas Mansour