Description of Different Kinds of Dolomites in Shotori Formation (Kouhbanan area) based on Petrographic and Geochemical Studies with a Reference to the Role of Shales in the Sorkh Shale Formation as a Major Source of Mg

Carbonate and siliciclastic sedimentary sequences of Sorkh Shale and Shotori Formations (Lower and Middle Triassic) with a total thickness of 870 m, have a gradational and conformable contacts, at Behabad region of Bafgh in Central Iran. At the beginning of this sequence, shale, siltstone and sandstone gradually change into dolomitic carbonates of tidal flat sub-environment. Thick layered or massive sequence of mainly dolomitic rocks of Shotori Formation, are transitionally overlain by a limestone member namely Espahak limestone. Based on petrographic studies, four different types of dolomites (based on shape, crystal size and crystal boundaries), have been recognized. Dolomite type 1 (dolomicrite) formed during the first stage of sedimentation and under surface temperature, this is very early diagenetic dolomite. The other types, having coarse crystal size, formed during shallow to deep burial environments. Change of composition and temperature of dolomitizing fluids at several stages of diagenesis (early to late) led to formation of different types of Shotori dolomites. On the basis of geochemical studies (elemental analysis such as Ca, Mg, Sr, Na and stable isotopes such as δ13C, δ18C formation of these dolomites occurred in a reducing environment and increasing temperature. The source of Mg is sea water for dolomite type 1, but for the other types of dolomites, Mg provided was by clay minerals diagenesis and basinal brine. The results of XRD and XRF on chlorite and montmorillonite clay minerals of Sorhk Shale Formation show a decrease in MgO content from 36% and 84% respectively compared to standard clay minerals. This reduction is due to diagenesis of clay minerals that exist in shales and Mg was originated from overlain layers. Measurement of organic carbon contents of dolomites and limestones showed that dolomites have several times more organic carbon than limestones. This information confirm that limestones rich in organic matter, are more susceptible to dolomitization. Microbial structures such as stromatolites, in the Shotori Formation have major role in trapping of organic matters. The calculated paleotemperature for the formation of early dolomites is about and for late diagenetic dolomite is.