Investigation on Faults Activity in Beheshtabad Dam Site Regarding on Quaternary Deposits

Behashtabad Damsite is located in High Zagros Zone, at 6-km distance from the south of Ardal Thrust Fault and 7-km from the north of Dopolan Thrust Fault. The outcropped deposits in the studied area locate in Zagros thrust zone. Paleozoic deposits contain red shales and sands belonging to Cambrian. Mesozoic deposits mostly belong to Cretaceous. The lower part contains limestones and dolomite limestones belonging to Fahlian-Dariyan. The middle part includes limestones, bituminous marly limestones, sandy limestones and thin-bedded chert belonging to Kajhdomi. Finally, the upper part contains thick-bedded limestones, abundant with karstic vugs. Thichness of the bed is 900 m and it is related to Sarvak-Ilam Formation. Goorpy Formation includes marly, silt and shaly limestones, marlstones and siltstones. Cenozoic deposits include Jahrom Formation which is made of dolomite limestones and dolomites belonging to Eocene-Oligocene. Chert nodules and shapeless cherts are seen in some beds. Assemary Formation is made of thick-bedded limestones belonging to Oligo-Miocene. Razak Formation, on the other hand, is made of thin-bedded sandy limestones, light green to gray marlstone and conglomerates. Finally, Bakhtiary Formation includes conglomerate outcrops and red sandstone belonging to Polio- Pleistocene. Quaternary deposits mostly include lacustrine sediments and alluvial fans have covered the rocky units in dam site and reservoir. These young deposits spread in a vast area in vicinity plains.
The Dam site is located in Sheikh Mahmoud valley at southern limb of Sangvil Anticline. The study area is located at about 6 Kms south of Ardal fault and 7 Kms north of Dopolan fault with coordinates of 510 39' 10" E and 300 52' 39" N. The anticline axis is in northwestern-southeastern direction with plunge of 16° toward the southeast. This anticline is made of Jahrom-Assemary limestone and Razak marly formations. This zone is a Seismic one in Zagros Zone. Seismo tectonic activities have caused some mega fault in this zone which are mostly reverse thrust ones. The faults at the dam site are mostly secondary thrust faults, which are due to the activity of the main thrust faults. Such faults are seen in the dam site which will definitely threaten the structure. Accordingly, active faults in the dam site area were studied specifically. These local faults are covered by lacustrine deposits. So, these sediments are evidences of last events. Any movement and activity of faults, affect directly these young deposits. Investigating last movement of these faults is possible by studying lacustrine deposits belonging to quaternary period. In terms of Geomorphology, the major Morphology of study area is concluded from high mountains. They are included from limestone, dolomitic limestone and dolomite. Marlstones are located at lower altitudes. Plains and lowlands areas are covered by lake sediments.
Based on field studies, F4 fault is one of the main local faults crossed by F1 fault. Any movement in F4 fault will be transmitted to F1 fault trace. Regarding study on F1 and F4 faults, a trench on lacustrine deposits was drilled perpendicular to F1 fault trace. There was no evidence on replacement of these young deposits. So, it`s concluded deposits ‘age is showing that the latest movement of these faults is occurred before lacustrine deposits composition. In this study, the OLS method was applied to determine the age of quaternary deposits.
Luminescence Dating Techniques are the ones based on the increase of electric charge in crystal minerals (caused by radioactivity). Such method is applied by geologists and archeologists to determine the time of the last exposure of the minerals. The estimated age in this method actually indicates when the older deposits were covered in 1-700 ka timespan. Luminescence dating was proposed by David Huntley in physic department of Simon Fraser University in British Columbia in 1985. Basis of this method is the electrons being emitted by alpha, beta and gamma radiations, which is applicable for the deposits containing quartz and feldspar.
Thermo-luminescence dating of operation method (OSL) on Quaternary deposits located on faults, a good way to approve or disapprove Late Quaternary faulting at the site is by faults in the area.According to the investigations using OSL method, these deposits belong to 190,000 years ago, and no activity evidences have been seen such as cracking, joints, etc on them. Based on seismotectonic principles, regarding the age of Quaternary Deposits, all of faults around the dam site are denied to be active.