Depositional Environment and Diagenetic History of the Bisotoun Carbonate Succession (Lower Cretaceous), Kermanshah Area, West Iran

Introduction The Zagros orogeny belt as a part of the Alpine Himalayas orogeny belt extended about 2000km in a northwest-southeast trend from the Anatolian fault in the southeast of Turkey to the Makran zone in southern Iran. Bisotun limestone was deposited on the southeastern border of the Neotetis Sea, where it was separated from the Arabic platform by the narrow and radiolarite basin of Kermanshah. A 2500 m thick succession Bisotoun carbonate rocks of the Late Triassic to Late Cretaceous in age were exposed at the east of Kermanshah as a part of the High Zagros zone. The studied section consists of dark gray color, medium to thick bed and massive limestone. Little studies have been done on the microfacies analysis, sedimentary environments and diagenesis history of the Bisotoun limestones. Since these limestones are deposited along with the radiolarite deposits, sedimentary studies and facies analysis help to reconstruct the paleogeography, their relation with the Kermanshah radiolarite basin, as well as investigate the diagenic history of the Bisotoun carbonate successions.

Study method 105 thin sections were prepared for petrography. To recognize the calcite from dolomite under microscope, all thin sections were stained with Alizarin red solution (by Dickson 1965 method). The Cathodoluminescence (CL) microscopy and scanning electron microscope (SEM) have been conducted to study the dolomites and cements. Classification of dolomites was based on Sibley & Gregg (1987). The blue epoxy has been used to estimate the porosity in studied samples.

Results and discussion Petrographic study of 105 thin sections from this succession led to identification of 8 carbonate facies that are deposited in four depositional environments including open marine, shoal, lagoon and tidal flat. Due to the absence of turbidity facies, cortoid, ankoid, pizoid and aggregate allochem, which indicate carbonate shelf and rarely found in a carbonate ramp, and also because of gradual changes in vertical sequence, the depositional environment of Bisitoun carbonate succession is a homoclinal ramp (consisting of middle and inner ramp). Facies are classified according to Dunham (1962) and Embry & Klovan (1971) classifications. The facies analysis is based on Schlager (2002) and Flugel (2010). The main diagenetic processes that affected the studied succession are micritization, bioturbation, replacement (dolomitization, silicification, pyritization and dedolomitization), compaction, stylolitization, cementation (blocky, syntaxial overgrowth, poikilotopic, isopachous and drusy equant), fracturing and vein filling, neomorphism, dissolution and hematitizationt. These processes were conducted in marine, burial (shallow and deep burial) and uplifted (meteoric) stages of diagenetic environments. The marine diagenetic process include micritization (that destroyed the internal components of the allochems), syntaxial overgrowth calcite cement, bioturbation (that is mainly found in rudist containing facies). The burial (shallow and deep burial) diagenetic process include physical (compaction in mudsupported facies) and chemical (stylolitization) phenomena. These process affected on most carbonate facies and played important role in the studied succession. Most of the allochems are cut off by stylolitization in which the presence of dolomite and iron oxide is common. Channel porosity and stylolitization porosity are significant in studied carbonate rocks, which some of them are filled by spary calcite cement. Dolomite dissolution, dedolomitization, channel and moldic porosity were created in uplift and meteoric diagenetic environments

Conclusion Based on field observation and petrography studies it is concluded that the Bisotoun carbonate rocks have been deposited in middle and inner parts of a homoclinal ramp. The most important diagenetic processes observed in Bisotoun carbonate rocks are micritization, bioturbation, replacement, compaction, stylolitization, cementation, fracturing and vein filling, neomorphism, dissolution and hematitizationt, which are mainly occurred in marine, burial and uplift stages of diagenetic environments. Channel porosity and stylolitization porosity are significant in studied carbonate rocks