الفهرس 
GENERAL INTRODUCTION AND GEOLOGICAL LITERATUREOnly 14 pages are availabe for public view
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Abstract
Gebel Ataqa is a tilted fault block located on the northwestern side of
the Gulf of Suez. The exposed rock units in Gebel Ataqa area are of
Cenomanian to Recent age. The eastern scarps are composed of
Cretaceous strata which include the Galala Formation (Cenomanian),
Maghra El-Hadida Formation (Turonian), Adabiya Formation
(Campanian/Maastrichtian), and Maghra El-Bahari Formation (Upper
Cretaceous?, Lower Tertiarry?).
Middle Eocene rocks cap most of the study area and form the main
plateau and ridges. The central part of the study area is entirely
occupied by outcrops of the Middle Eocene nummulitic limestone with
intercalations of chalky limestone and marl. The Middle Eocene rocks
include the Suez and El-Ramiya Formations. The Upper Eocene
rocks overlie the Middle Eocene rocks and underlie the Oligocene
sediments. The Upper Eocene outcrops are composed of sandy
fossiliferous marl, marl and sandy limestone with banks of Carolia and
other oysters. They can be easily differentiated from the Middle
Eocene rocks by their ochreous color and sandy and shaly nature.
They occur as outliers at the foot of Gebel Ataqa and are faulted
down against the Middle Eocene rocks along the northern scarps of
the study area.
The Oligocene sediments in Gebel Ataqa area are scattered in the
low lands and are composed of occasionally cemented sands and
gravels with silicified wood. They occupy the western part of the study
area in Wadi Hagul and the foot of Gebel Kahailiya. The thickness of
the Oligocene sediments is evidently increasing from east to west
(from Gebel Ataqa to Gebel Kahailiya).
The Miocene strata are exposed in the southern and southwestern
parts of the study area in Wadi Hagul. At few localities, the Miocene
rocks are faulted down against the Middle Eocene rocks by NW-SE
oriented normal faults. The Lower Miocene unit is composed of marl,
shale, sandstone and marly limestone. A thick conglomerate bed has
been recorded in the calcareous sandstone. The Middle Miocene unit
is composed of white algal limestone, shale and clay. The Upper
Miocene unit is of non-marine origin and consists of sands, sandstone
cemented by calcareous matter and flint pebbles.
The depressions and flat lying areas in the northern plain, along the
coastal area in the eastern and southwestern geographically low
areas are covered by alluvial Quaternary deposits (sands and
gravels). The wadi deposits are found in the seasonal streams and
valley floors. The Quaternary talus, composed of rock fragments and
boulders, is scattered at the foot of steep scarps.
Unconformities are present between the Turonian and Campanian-
Maastrichtian, Upper Cretaceous and Middle Eocene, Upper Eocene
and Oligocene, and Oligocene and Miocene rocks.
Gebel Ataqa is a triangular tilted fault block dissected by four sets of
normal faults oriented NW-SE to NNW-SSE, E-W, NE-SW and N-S.
The dip of the strata is generally gentle (10° to 18°) to the southwest.
However, bedding dip angles are steeper near the faults reaching up
to 50° to 64°. Gebel Ataqa is bounded by steeply dipping normal
faults from the north, east and southwest. However, its southern part
is not defined by faults in the mapped area.
The Cretaceous rocks in the eastern scarps are directly overlain by
the Middle Eocene dolomitic limestone and their relationship is
angular unconformity. In the northern part of the study area, the
Upper Eocene rocks are faulted down against the Middle Eocene
rocks. In the western part of the study area, the Oligocene sediments
are exposed in Wadi Hagul at the foot of Gebel Kahailiya. In the
southern and southwestern parts of the area, the Miocene rocks are
exposed and are faulted down against the Middle Eocene rocks at
some localities.
In Gebel Homeira, the Middle Eocene rocks are capping a horst
block. In the northern scarp of Gebel Homeira, the Miocene rocks are
faulted down whereas in southern scarp the Upper Eocene rocks are
faulted down against the Middle Eocene rocks. In the southwestern
part of Gebel Ataqa, interference of the NW-SE oriented faults and
the E-W oriented faults develop a zigzag fault arrangement. The NWSE
oriented faults are not terminated by E-W oriented faults; rather
they are linked by them.
There is a great deal of similarity in the structural setting of Gebel
Ataqa to that of the Cairo-Suez area. The E-W oriented en echelon
fault belts form transfer zones between the NW-SE oriented faults.
Individual faults within the E-W en echelon fault belt overlap each
other. Their overlapping areas are marked by relay ramps. The dip
angles of the E-W oriented faults in the en echelon fault belt are
generally steeper than the NW-SE oriented faults. The dip angles of
E-W en echelon faults vary from 70° to 88° whereas the NW-SE
oriented faults have dip angles varying from 65° to 75°.
The NW-SE and E-W oriented faults were formed as a result of the
same tectonic episode contemporaneous with the Oligocene-Miocene
rifting of the Suez rift. The NW-SE oriented faults were formed by NESW
trending tensional stresses originated by the Suez rifting.
However, the E-W oriented faults were formed by rejuvenation of the
old E-W oriented deep-seated faults.
The southwest tilt of the Ataqa block can be attributed to the rotation
of the downthrown side of the Hagul Fault which is, most probably,
listric and is a part of rift-bounding faults of the Suez rift, therefore the
Ataqa block is one of the rift blocks.
Fractures are measured in the field in the Middle Eocene dolomitic
limestone. Fractures measured in similar lithologies support the
correlation of different outcrops scattered over the study area.
Fracture data show that there are four sets of steeply dipping
fractures striking NW-SE, N-S, and WNW-ESE and NE-SW or ENEWSW.
There is significant similarity in fracture orientations at all
outcrops having the same range of orientations for dominant, subdominant
and least occurring fracture sets. Moreover the dips of the
fractures are also within a similar range.
The vast majority of fractures are steeply dipping ranging from 70° to
90°. Most of the fractures are 20-30 centimeters apart. However,
some of them have spacing between 10-20 and 30-40 centimeters.
The average fracture density (FD) calculated for data sets from ten
random outcrops is 4.5 fractures per meter. It can be considered as
average FD over the study area (excluding fault damage zones).
The similarity and consistency in fracture orientation in the northern
part of Gebel Ataqa which is characterized by E-W oriented normal
faults and the western part of Gebel Ataqa which is characterized by
NW-SE oriented normal faults reveal that the origin of fractures
(excluding fractures in the fault damage zones) is attributed to the
tensile stresses related to the Suez rift rather to the occurrence of
nearby faults.
The N-S fracture set is regarded as shear fractures due to its
orientation at an acute angle to the maximum horizontal stress axis.
The origin of the other two fracture sets (WNW-ESE and NE-SW
oriented) can be attributed to the local stress directions.
The fault damage zone study is based on fracture density, orientation
and spacing measured along scanlines perpendicular to the faults.
There are four sets of fractures in the damage zones of the E-W en
echelon normal faults and three sets of fractures in the damage zones
of the NW-SE oriented normal faults. The dominant fracture sets in
the damage zones of both fault populations strike parallel or subparallel
to their respective faults indicating the direction of the local
stress involved in their genesis. The fractures in the damage zones
are mainly formed by the local stress rather than the regional stress.
Otherwise the NW-SE fractures would be dominant in the damage
zones of the E-W faults. Fracture orientations in the damage zones of
the NW-SE oriented faults clearly show that their origin can be
attributed to the local stresses associated with the fault and its
displacement.
The majority of the fractures of the E-W oriented faults are parallel or
sub-parallel to the fault. It is also supported from the fact that the NESW
oriented fractures in the damage zones of the E-W oriented en
echelon normal faults were formed in relation to the relay ramps
between the overlapping areas of these faults (formed perpendicular
to the dip direction of the relay ramps and mostly dip opposite to dip
direction of the relay ramps).
The ten measured fault damage zones of the E-W en echelon normal
faults are related to four faults generated from the same tectonic
event affecting the same lithology with similar burial histories. The
conditions and the mechanisms of deformation are thus similar.
However, one cannot ignore the potential effects of varying
rheological properties across the faults.
Fracture orientations show great similarity in all the fault damage
zones of a single fault population having the same range of
orientation for dominant, sub-dominant and least occurring fracture
sets. The vast majority of fractures are steeply dipping ranging from
70° to 90°. Fracture spacing increases significantly as we move away
from the fault within the damage zone for all the studied faults. The
highest FD values have been measured in the proximity of the fault
surfaces. FD values decrease gradually as we move away from the
fault.
The average half width of the damage zones of the E-W oriented
faults is 19.5 meters. On the other hand, the average half width of the
damage zones of NW-SE oriented faults (for SWD) is 12 meters and
average width of damage zones (for DWD) is 30 meters. The widths
of the damage zones in the middle part of the fault are significantly
greater than those towards the fault ends.
On the basis of field studies in Gebel Ataqa, it can be assertively
concluded that the present-day structural setting of Ataqa block is
controlled and evolved by the Oligocene-Miocene and post-Miocene
deformations. It is a unique example of a rift-related normal fault
system.