Subvolcanic and hydrovolcanic activities in the Bahariya oasis depression (Egypt)

Norbert Brügge, Germany
Dipl.-Geol.

The Bahariya Oasis is an oval shaped depression trending in a NE-SW direction. Within the depression, the Cretaceous rocks outcrop at its base as well as at the base of the conical hills and the scarp. The Cretaceous rock succession (Sabaya, Bahariya, El Heiz and El Hafhuf Formations) comprises fluvialite to fluviomarine clastics of sandstone, claystone and shale. The Sabaya Formation of Lower Cenomanian age is coverd by Bahariya Formation which also belongs to the Lower Cenomanian. The Bahariya Formation is unconformably overlain by the Upper Cenomanian fluviomarine marly shale, sandy dolomitic limestone and calcareous sandstone of El Heiz Formation as well as by the Campanian cherty cavernous dolostone, crossbedded sandstone and phosphatic limestone of El Hafhuf Formation.
The succeeding Maeastrichtian Khoman Formation is mainly represented by chalk and chalkly limestone. It overlies conformably El Hafhuf Formation on both sides of the depression and extends southward with increasing thickness. The Eocene rocks are represented by the Naqb Formation which belongs to the Middle Eocene and comprises gray and pink limestone and by the Qalamoun Formation which belongs to the Lower Eocene. The Qalamoun Formation is deposited on the Bahariya Formation and El Hafhuf Formation to the North of Bahariya depression. The succeeding Oligocene Qatrani Formation covers Bahariya Formation at the top of the conical hills besides occurring as small outcrops within the depression. This formation consists of quartizitic sandstone, quarzite, shale and silt.

At the northern part of Gebel El Hafhuf, Oligo-Miocene basaltic and doleritic extrusions are recorded.
The types of volcanic rocks in this region can be divided into three main varieties, mainly an amygdaloidal basalt which is the oldest extruded lava followed by the intruded dolerite. Later another period of volcanicity took place and the olivine basalt porphyry was extruded covering the amygdaloidal basalt in Gebel El Heluf and the small basalt hill nearby, and helped to cover and preserve in these two hills the layer of amygdaloidal basalt below it, while in Gebel Mayesra and Mandisha where this basalt porphyry was not represented caused the lower layer of amygdaloidal basalt to be also absent and probably leached.
As to the age of these volcanic rocks there is no doubt that it is post-cretaceous. The Eocene being not represented in this part of the oasis leads the writer to conclude that the volcanic rocks are probably of Oligocene age specially that this period of Tertiary volcanicity was recorded in Egypt. The eruption was at several steps and took a long period. The variation in types of rock and alteration indicate that the formation of the volcanic rocks in this area took place at several times and more or less during long periods of volcanicity.

Columnar Oligocene flood basalt sheets cover the Eocene Bahariya Formation at Gebel Mandisha area in the Bahariya oasis depression. The Mandisha basalts are located with the position of 28° 54' E and 28°22' N, nearby the iron ore mines in the eastern direction. The basaltic intrusions took place during Oligocene, when the Gulf of Suez rift began to open. As noted earlier, hydrovolcanic solutions associated with this subvolcanic activity caused intensive mineralization and iron precipitation in parts of the depression. Iron forms as a replacement to Eocene limestone where open cast quarries are located in several places. Mostly known are the mines of El Harra area and El Gedida area at the northern edge of the depression.
 

Iron ores occurring mainly in lower Eocene limestone at Gebel Ghorabi include an oolitic type consisting essentially of goethite, a vesicular type containing hematite with pyrolusite and psilomelane, a hard, dark brown type consisting of goethite and hematite with some pyrolusite, and a low-grade limonitic type. Study of the mineral assemblages and ore textures shows that processes of metasomatic replacement, colloidal precipitation, cavity filling, and impregnation have been involved in formation of the deposits.


Gebel Ghorabi and  iron ore mines El Harra, El Gedida


Bahariya depression: Geological map



Oasis with the basalt area from space



Mandisha basalt outcrops











 



 

 



Black Desert flood basalt remains: Jebel Marsus (28°15'55''N / 28°45'10"E)



Hill with fantastic basalt columns









Basalt cover ferruginous layers



 

 

Photos (4) by C. L. Richardson, 2010        

Further areas of flood basalts in north- and middle-Egypt are distributed. They are concentrated in the region of Cairo, are found however also in the Bahariya depression and western to southwestern of it.
Remarkable is the dating of the basalt intrusions in the Oligocene age. It agrees with the dating of other subvolcanic and orthomagmatic-hydrovolcanic structures in the Gilf Kebir, in the White Desert and Black Desert as well as the Libyan Desert Glass area. All structures are placed on a line, which proceeds from southwest to northeast. Can we suppose here a connection between continental drifting,  a Hot Spot and the old Trans African Lineament ?



Scientific workers believe that the Bahariya depression was really a anticline, which developed as a result of movements along a pre-existing NE - SW fault trend. Interpretation of a seismic line from a typical profile in the Western Desert provide an idea of a tectonic event.