"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.
|
|
Gebel Ghorabi and iron ore mines
El Harra, El Gedida
|
|
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. 
Old Iron ore mine nearby Aswan
|
|
Bahariya oasis with a part of the basalt
area from space
|
Mandisha basalt area on map
|
|
Location 2
|
Northern large basalt plateau (1)
|
|
"English Mountain"
|
Location 1 (?)
|
|
Field with fantastic basalt columns
|
"English Mountain"
|
|
Basalt columns
|
"English Mountain"
|
|
Known Black Desert flood basalt remains
3 -- 28°15'55''N/28°45'10"E
4 -- 28°12'43"N/28°45'13"E
|
Little basalt hat over ferruginous layers
|
|
Location 3
|
Location 3
|
|
Giant basaltic columns (3)
|
|
|
Hill with basalt on top (4)
|
Location 4
|
|
Basalt cover ferruginous layers (4)
|
Location 4
|
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 and Black Desert as well as the
Libyan Desert Glass. All structures
are placed on a line, which proceeds from southwest to northeast. Can we suppose
here a connection between continental drifting and a Hot Spot? |
|
|
|
|
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.
|
