Subvolcanic and hydrovolcanic activities in the Bahariya depression (Egypt)

Norbert Brügge, Germany

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 Hefhuf 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 Naqab 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 Hefhuf, 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 Hefhuf 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 Maysarah 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 Cenomanian 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.


Bahariya depression:  Stratigraphic profile

Iron ore deposits/mines El Ghorabi, El Harra and El Gedida

Iron ores at Gebel Ghorabi, occurring mainly in lower Eocene limestone, 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.


What is new ?

Concentric Structures and Hydrothermal Venting in the Western Desert, Egypt
Adriano Mazzini
et al. -- Front. Earth Sci., 18 October 2019

"Here we describe for the first time the results of mapping and characterization of about 100 large concentric circular structures found in Early Cenomanian argillaceous strata of the Bahariya depression.
The mapped circular structures increase in number approaching the fault zone. These features are up to 10 m high and 625 m wide.
Halite-cemented brecciated sediments from different geological units have been sampled in the central part of the concentric circular structures implying a subsurface mechanism involved in their formation. Petrography analyses revealed also the presence of high- and low-temperature minerals (e.g., Ba-K-feldspars and ferroaluminoceladonite) suggesting former phases of hydrothermal circulation. Soil-gas flux profiles (CO2 and CH4) reveal a modest CO2 increase when crossing the central part of the circular structures inferring enhanced permeability. Field and laboratory data are consistent with a scenario envisaging a diffused and vigorous hydrothermal venting."


Sediments dug out from the central part of the CS show brecciated shale and
 sandstone clasts within a fine-grained matrix cemented by halite (white crystals)


Photo Gallery

 Jebel Hefhuf & Mandisha basalt flows

Mandisha basalt plateaus from space

The Jebel Hefhuf  is aligned along the central segment of the Mid Bahariya fault. The exposure of the mafic rocks covers an area subelliptical in shape elongated toward the NW, reaching the maximum thickness on the outer margin. The hill consists of two basaltic sheets (sills) with well-developed colonnade overlying the cross-bedded sandstones and clastics of El Heiz and Bahariya Fms. and partially concordantly overlain by the sedimentary deposits of El Hefuf Fm. (El Akkad and Issawi, 1963; Medani, 1995). Toward the east, doleritic dikes reach a thickness up to 3 m and dissect the Campanian rocks of El Hefhuf Fm. Basaltic flows are observed in the central and eastern side of the area flooding the substratum of the Cenomanian clastics (El Heiz Fm.).

The basaltic Mandisha lava flows have a thickness of 10 m and thin down to 4 m moving northwards away from the Mid Bahariya Fault. The lava flows can be subdivided into an upper fresh unit with spectacular columnar joints and a lower moderately weathered unit. The latter has intensive fracturing and a continuous sharp contact with the underlying Cenomanian rocks. Medani (1995) described a set of doleritic dikes as a feeder of the basaltic flows which are dissected normal faults distinguished with brecciated basaltic zones and rotation of the adjacent sedimentary rocks. To the east, the Jebel Agoz hill forms an isolated exposure separated from Mandisha normal fault that cuts through the basaltic flow and the underlying Bahariya Form.. The basaltic flows consist of vertical and horizontal tier of the rounded columnar joints particularly in the lower part of the basaltic flow (El Qaluabi, 1974; Khalaf et al., 2018). Further to the northeast, the Jebel Maysarah basaltic flows occur as an isolated NE-elongated sub-triangular outcrop. According to Tosson (1964), these fissure eruption basalts occurred at the floor of the Bahariya depression.

Jebel Marsus flow basalts
The Jebel Marsus has a positive and plug-like shape reaching a thickness of more than 170 m above the base of the depression. The basaltic flow has a distinct columnar structure resting on a ~2 m thick pyroclastic mound of scoria lapilli, blocks, and bombs with mainly sub-millimeter vesicles exhibiting cauliflower structure (Khalaf et al., 2018). Both pyroclastics and flow are overlying the sandstones and shales of the Bahariya Fm.

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

Hill with fantastic basalt columns


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

Here basalt cover pyroclastic deposits

Here basalt cover deposits of the Bahariya Fm


The Bahariya monogenetic volcanic field is characterized by important geomorphological features. There have been some geosites recognized such as the scoria cone, the lava flows and their surface morphological features, the pseudopillow fractures, columnar joints, peperites, tumuli, and  rootless cones. (Source: Ezz El Din Abdel Hakim Khalaf et al., 2019)


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 hydrovolcanic structures in the Gilf Kebir region, in the White Desert and Black Desert as well as the Libyan Desert Glass area with its kimberlte pipes. All structures are placed on a line, which proceeds from southwest to northeast. Can we suppose here a connection  to a Lineament ?