Diagenetic history of Mozduran Formation and the relation between dolomditization and depositional sequences in Dasht-e-Gonbad area, west of the Kopet-Dagh basin

  In this study, the Upper Jurassic carbonate strata in the Kopet-Dagh sedimentary basin (Mozduran Formation) has been investigated in the Gonbad plain in the west of Golestan Forest National Park. Mozduran Formation in the studied section is composed of muddy limestone containing chert nodules with interbeds of marl and shale that is conformably covered the shale beds of Kashaf-Rud Formation with a sharp boundary. The aim of this study is to interpret the diagenetic history of the Mozduran Formation and to interpret the relation between dolomitization and sequence stratigraphy. One of the most important applications of geochemical studies in carbonate rocks is to determine the composition of primary mineralogy, interpret sedimentary environment and depositional conditions, the palaeo temperature, alteration, differentiation of different diagenetic environments and determination of diagenetic trends (Adabi & Asadi Mehmandosti, 2008). Material and In this study, samples collected from surface section in south of Cheshmeh-Khan village and the data of Qezel-Tapeh well #2 have been used. 130 samples of limestone and dolomite and 8 samples of sandstone from Cheshmeh-Khan section (with thickness of 236 meters) and 103 samples from Qezel-Tapeh well #2 were analyzed and studied. Thin-sections stained with Alizarin Red-S, using Dickson (1965) method to differentiate calcite from dolomite. Global sea level curves presented by Haq et al. (1987) were used and compared with interpreted sea level curve for the sequence stratigraphic analysis. 4 samples of dolomite and 2 samples of pyrite were studied by scanning electron microscopy (SEM) and also analyzed by EDX. In order to study the geochemistry of Mozduran Formation in the Cheshmeh-Khan section, 20 samples (10 samples of micrite and 10 samples of dolomite) were selected for determination of the major elements (Mg and Ca) in terms of percentage and minor elements (Fe, Mn, Sr) in terms of ppm, samples were analyzed by atomic absorption (AAS). Results and Diagenetic studies indicate that the most important diagenetic processes affected these sediments are dolomitization, cementation, micritization, dissolution, physical and chemical compaction and neomorphism. These processes have affected sediments in three meteoric, marine and burial environments. In general, early diagenetic peocesses in marine environment include micritization, formation of isopachous cement and primary dolomite. Formation of isopachous, syntaxial and drusy cements, dissolution (formation of vugy porosity) and neomorphism have occurred at the stage of meteoric diagenesis. Last diagenetic processes (in shallow and deep burial stages) include dolomitization, mechanical compaction, stylolization and formation of fractures.
Neomorphism has seen to operate in three different processes: conversion of calcareous mud to coarse calcite crystals in mud-supported facies, replacement of calcite in aragonite grains, and converting fine-crystalline to coarse-crystalline dolomites. Based on the size and boundaries of crystals (Warren, 2000), four types of dolomite have been identified in the Mozduran Formation, including very fine-crystalline dolomite, mosaic microcrystalline dolomite, medium-crystalline dolomite and coarse- crystalline dolomite. Dolomites formed as primary and replacement in matrix and allochems. The presence of dolomite along dissolution veins and stylolite as well as coarse crystal dolomites indicate that shallow burial processes probably played an important role in the formation of these dolomites. Microcrystalline dolomite (D1) is perfectly indicative of surface conditions, low temperature and probably supratidal environment, due to very small size of crystals, the absence of fossils and the preservation of the primary sedimentary texture (Adabi, 2009). Super saturated alkaline condition with high pH are also suitable for the formation of these primary dolomite (Deng, et al., 2010).
Sibley and Gregg (1987) are believed that semi-shaped straight fabric of coarse-crystalline dolomite (D4) are the result of the slow growth of crystals at low temperatures during shallow burial and formed from recrystallization of finer crystals. Elemental analysis of the studied rocks shows that the effect of diagenic processes, specially diagenetic alteration on a semi-closed system with high water-to-rock exchange on Mozduran Formation was high and the low Sr/Mn ratio shows a high dissolution rate in the formation of dolomites (Rao & Amini, 1995). Very fine crystals of dolomite (the first type) formed in a supratidal environment (Blendinger, 2004), therefore they can form at the end of some shallowing upward sequences (third, fourth, and fifth sequences). The dispersion and abundance of fine crystals of dolomites in the upper parts of sequences represent sea level fluctuation during sedimentation (Khalifa, 2005)