Red Sea Basins TECTONO-SEDIMENTARY EVOLUTION OF THE NW PARTS OF THE RED SEA The pre-rift rift by Prof. Dr. Abbas Mansour
1.a. The Precambrian basement and the inherited structural pattern of the rift Basement Rocks both along the marginal reliefs of the basin and within several major tilted fault blocks of the margin (ex., Gebel Abu Durba, Gebel zeit, Gebel Mellaha, etc.) are very heterogenous and polygenetic (crystalline, metamorphic and sedimentary rocks). They constitute the peripheral mountains locally exceeding 1500m in altitude. This relief has been cited as proof of early doming. Basement rocks are related to several Precambrian orogenic cycles. The Pan-African tectono-magnetic event (about 500 Ma) appears to be of major importance to the structuration of this basement. A Lower to Middle Paleozoic age has been quoted for some granitic batholiths of the Gulf of Suez (Gharib, Zeit-Burolet et al., 1982). In addition, many annular plutonic intrusions of Cretaceous age have been recorded (Steen, 1982).
1.a. The Precambrian basement and the inherited structural pattern of the rift Basement rocks exhibit a complicated structural pattern which include multidirectional fracture systems and other mechanical weaknesses including dyke swarms which are oriented N140-150. Richert et al. (1984) and Ott d Estevou et al. (1986) have indicated that the N 140 orientation of the Red Sea may be related to rejuvenated faults inherited from these basement structures. These relationships are particularly clear in the Ras Honkorab area where faults limiting a series of structural blocks are oriented parallel to the foliation within the adjacent metamorphic basement (Montenat et al., 1986).
1.a. The Precambrian basement and the inherited structural pattern of the rift.
1.a. The Precambrian basement and the inherited structural pattern of the rift The lithological and structural properties of this basement play various roles in Neogene rifting: Lithological compositions influence the quality and volume of clastic material delevered to the Neogene basin. The structural pattern of the Neogene rift is clearly inherited from the basement tectonics (Jarrige et al., 1986a).
The structural pattern of the Neogene rift is clearly inherited from the basement tectonics (Jarrige et al., 1986a). (a). Major fracturation in the Gulf of Suez and NW Red Sea often oriented N140 (clysmic direction) is inherited. It is parallel to the principal orientation of Paleozoic plutonic intrusions (Gebels Gharib and Zeit in the southern part of the Gulf). Large syngenetic dikes (about 344 Ma) which cut the former granitic (Gharib) are also oriented N140 (Ott d Estevou et al., 1986a).
The structural pattern of the Neogene rift is clearly inherited from the basement tectonics (Jarrige et al., 1986a). (b). Submeridian (N00-N020; Aqaba direction) and N100-120 (Duwi direction) Miocene faults are also inherited from basement structural discontinuities. Combined with the N140 clysmic faults, they play an important role in the rifting pattern (Fig. 7).
The structural pattern of the Neogene rift is clearly inherited from the basement tectonics (Jarrige et al., 1986a). (c). Thus, various discontinuities within the basement have influenced the rifting. It is particularly evident in the Abu Ghusun area (Montenat et al., 1986a) where the limits of Miocene faulted blocks follow the foliation direction of the metamorphic basement. Faultstrikes vary from N70 to N120 as a function of the latter. In the same zone, it should be noted that the pattern of the tilted blocks differs according to whether they developed in the easily cleavable metamorphic rocks or in the massive plutonic material.
The structural pattern of the Neogene rift is clearly inherited from the basement tectonics (Jarrige et al., 1986a). (d). However, the N050-060 fault or foliation direction (cross direction), well expressed in the basement, is rejuvenated less frequently (Dara Block-Ott d Estevou et al., 1986a) and with considerably less amplitude. Geochemical peculiarities of alkaline dikes and plutonic intrusions (Schuermann, 1942, 1971) and their emplacement along clysmically oriented discontinuities can be related to a crustal distensional episode of Paleozoic age. Then the Neogene rift could be superposed on an older zone of crustal thinning, displaying the same general NW-SE orientation.
1.b. The pre-rift sedimentary cover This sedimentary sequence (Fig. 7), lying unconformably on the basement, begins with continental Nubian sandstone whose age is supposedly Cretaceous on the western margin but includes older units of the Sinai edge. Lithologically it is comparable to those cropping out in Wadi Qena (east, Nile Valley) and in Saudi Arabia (Purser and H?tzl, 1988). These fluvio-aeolian sandstones are overlain by shaley and carbonate marine deposits of Late Cretaceous to Eocene age (Said, 1962). The marine transgression has developed southward, and the open-marine sedimentation thus begins earlier in the north (Cenomanian of Gharamul) than in the south (Campanian of Safaga or Quseir). The recorded sequence apparently ends with Lower Eocene (Ypresian) cherty limestone on the western margin; it includes Middle and Late Eocene beds on the eastern edge of the Gulf (Garfunkel and Bartov, 1977; Ott d Estevou et al., 1986b).
Dawi Formation, Basement/Nubia Sandstone
Dawi Formation
Dawi Formation, Nubia Sandstone
Dawi Formation, Nubia Sandstone- Ripple Marks
Dawi Formation, Nubia Sandstone- Bioturbation
Dawi Formation, Shale & Carbonates
This pre-continental sedimentation was not entirely uniform; notable variations in thickness, facies and sedimentary deformations reflect fault movements during Cretaceous and Eocene times. Paleogeographic features are basically E- W to NE-SW trending. They do not have a clysmic configuration and cannot be used to imply the existence of a proto-rift (Selwood and Netherwood, 1984).
Dawi Formation, Shale & Carbonates/Faults
Dawi Formation, Carbonates/Phosphates
Dawi Formation, Shale/Carbonates
1.b. The pre-rift sedimentary cover A significant example of these perturbations throughout the sedimentary cover is exposed in the Dara ridge (south of Ras Gharib; Ott d Estevou et al., 1986a). Marked thinning of the entire sequence (<100m) with local diastems, intraformational erosion and reworking (upper Campanian), synsedimentary slides and olistoliths (Eocene) occur along the NE-SW trending Dara ridge. They are related to a contemporanous tectonic phase that is strongly expressed farther to the north, in the Syrian arc domain, where the structures exhibit the same major orientation. A general sedimentary gap within basinal facies at the Maastrichtian-Paleocene boundary (Faris, 1984; Thiriet et al., 1986) may be related to an echo of these Syrian movements in the southernmost areas (Safaga, Duwi, Quseir).
Dawi Formation, Carbonates/ Monogenetic Conglomerates
Dawi Formation, Carbonates/ Monogenetic Conglomerates
Dawi Formation, Carbonates/ Chert Nodules
Dawi Formation, Carbonates/ Flute Casts
Dawi Formation, Carbonates/ Erosion
1.c. The contact between pre- and syn-rift series The early to middle Eocene (Strougo and Abul-Nasr, 1981), open marine carbonate mudstones of the Thebes Formation do not show obvious traces of upwards shallowing; at least in the NW parts of the Red Sea. They are generally capped by a relatively planar erosion surface overlain by a monogenetic conglomerate composed of residual chert nodules derived from the underlying Thebes Formation. The basal, post-eocene unconformity is generally a flat surface whose tilt varies from a few degrees (Wadi Siatin) to about 20.