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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. |