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Hamed Hooshmand, Geological Survey of Iran, (Islamic Republic of Iran)
Mohammad hossein Adabi, Shahid Beheshti University (Islamic Republic of Iran)
Abbas Sadeghi, Shahid Beheshti University (Islamic Republic of Iran)
Hassan Amiri Bakhtiyar, NISOC (Islamic Republic of Iran)
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The Maastrichtian Tarbur rudist-bearing Formation with abundance of large foraminifera was deposited in Zagros basin in South of Iran near the Persian Gulf. This Formation gradually and conformable overlies the Gurpi Formation and is overlain by the Sachoon evaporate Formation with conformably and sharp boundary. The Khan-e-Nahr surface section (northeast of Jahrom) with 256 mete thickness has been chosen for this study. This section has two dolomite intervals that one of them is near the basement with 6 meter thickness and the other one with 60 meter thickness is in the uppermost part of the section and is associated with marl layers.
based on field observations and petrographic studies four different dolomite types have been recognized that resulted from multistage dolomitization. Dolomite type one consist of very fine to finely-crystalline dolomite. Dolomite crystal size range from 10 to 40 microns with an average size of approximately 30 micron. These dolomites characterized by dark color, nonporous, unfossiliferou and preserving trace of depositional texture forming penecontemporaneously or by early replacement of carbonate mud soon after deposition with Mg possibly derived from seawater.
Dolomite type two consist of fine to medium-crystalline euhedral rhombs, with size ranging from 40 to 120 microns with an average size of approximately 70 micron with in inter crystalline area filled by sparry cement. This dolomite type results from replacement of precursor calcium carbonate mainly during early shallow burial and Mg was probably derived from dissolution of high-Mg calcite.
Dolomite type three is comprised of unimodal mosaics of subhedral to anhedral planer-s crystals. The crystal size of these hypidiotopic dolomites ranging from 120 to 180 microns with an average of 140 micron. Most of dolomite rhombs have a cloudy surface contain inclusion of micrite. Many cloudy rhombs are overgrown with a single clear euhedral rim. This type of dolomite is considered to represent a diagenetic replacement of preexisting limestone and/or recrystallization of an early formed dolomite, possibly after some burial.
Dolomite type four comprise of clear, coarsely crystalline planer-C (cement), often euhedral dolomite crystals, the crystal size ranging from 180 to 350 micron and lining voids, vugs and fractures then occurring major void-filling dolomitization. Paragenetic relationships indicate that euhedral and planer-C dolomite cement are the last major dolomite generation as they postdate all over dolomite types.
Compaction of marls and clay diagenesis, combined with burial brines could produce enough magnesium for shallow to somedeal deep burial dolomitization (dolomite types three) and dolomite cement (type four).
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