Sedimentology of Veresk Member of Elika Formation (Triassic) in Shahmirzad area with special reference to Karstification

Shahmirzad area is located about 25 km north of Semnan in the southern flank of Alborze
Mountains. This area is located between latitude of 36°to 35° 40' N and longitude of 53° to
30° 53' E. This study is based on two measured stratigraphic sections. The first section is
located along the road from Shahmirzad to Foladmahale which is about from located 6 km
Shahmirzad with the thickness of about 460 m. Elika Formation in the this section is divided
into three portions including vermicular Limestone, dolomite and Veresk limestone.The
second locatity is along the road from Shahmirzad to Chashm. Since karstification is very
important in Veresk unit ,it has been studied in detail at this location. Measured section at this
location is about 190 m (20 m of middle partion and 150 m of Veresk partion). Based on
petrologic studies of 215 thin sections, identify three sets of facies associations indentified
includins A1 dolomitic fenestral lime mudstone: A2 collapes breccias A3 dolomitic stromatolite
boundstone A4 intraclast pellet grainstone/packstone B B2 bioclast pellet intraclast
packston/wackestone B2 bioclast peloidal ooid intraclast packstone/grainstone) and C C1
bioclast peloid intraclast grainstone C2 Bioclast ooid peloid intraclast grainstone). These facies
associations have been deposited in a ramp type carbonate platform environments with a very
low slope in tidal flat, lagoon and shaol. Diagenetic processes affected in the upper- middle
portions of Elika Formation and micritization, cementation, compaction, neomorphic, dissolution,
dolomitization, dedolomitization, hematitization, pyritization, silicification and fracturing that
operated in the marine , meteoric, burial diagenetic environments. Carbone isotope versus oxygen
isotope diagram reperesents burial and meteoric diagenesis. The diagram of Mn versus Sr/Ca
shows a semi closed diagenetic system. Sequence stratigraphic studies indicate the presence of
three sedimentary sequencs with first and second sequence boundris that is located in a supper
sedimentary cycles called UAA3, UAA2. Karst geomorphology can be seen (Karen, karst
spring, caves) more in the Veresk portion of Elika Formation than the middle dolomite
portions how ever advanced geomorphology features such as doline, polje and ect have not
developed in the region. Diagram of rainfall and discharge versus time show the spring
discharge is largely influenced by rainfall and it could be a reason for karstification in
shallower parts of the limestone. The structural model of the fault zone and rose diagram
show faults are NW-SE trend which is the same for most joints. These fractures caused water
penetration and the findly formed karst features in the same direction. The piper diagram of
spring water show that the water is calcium bicarbonate- magnesium type which dissolution
the limestone Veresk unit and middle dolomite unit.