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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Microfacies Analysis and Paleoenvironment Interpretation of Upper Jurassic Carbonates of Jajarm Area, N-NE Iran 304 A. Mahboubi, R. Moussavi-Harami, A. Aghaee Department of Geology, Faculty of Sciences, Ferdowsi University of Mashhad, Iran (E-mail:amahboobi2001@yahoo.com) This research is focused on the Upper Jurassic (Oxfordian-Kimmeridgian) carbonates in Jajarm area, north-northeast Iran. These deposits have very good and wide exposure in this area. Five stratigraphic sections that are studied in Jajarm area is shown in Fig.1. Upper Jurassic sediments have transitionally over- lain by Middle Jurassic deep water carbonates and unconformable underlain with fluvial siliciclastic deposits of Early Cretaceous. Thickness increases to the west part of study area and ranges from about 300 to 700 m. These deposits mostly consist of gray to light gray and pink to red calicilutite to calcarenite and calcirudite with big ammonites, pelecypods, gastropods and brachiopods in coarser rocks. Interbedded fine and coarse crystalline dolomite are also present. Since Upper Jurassic carbonate rocks (Mozduran Formation) in Kopet-Dagh Basin (NE Iran) are a major gas reservoir, the objective of this study is comparison of lithofacies and depositional history of these formations. Based on five measured stratigraphic sections, field ob- servations, detailed petrographic considerations on more than 500 thin sections, microfacies are analyzed and depositional environment are interpreted. Microfacies petrographically are mainly consist of skeletal grains such as bivalve, foraminifer, gas- tropod, brachiopod, echinoderm, bryozoan, coral and non-skeletal carbonate grains such as ooid, intraclast and peloid. Mudstone, dolomudstone, pelloid wackestone/ packstone, pelloid ooid grainstone and bioclast wackestone/packstone are the most important microfacies that mainly deposited in intertidal-subtidal subenvironment in a carbonate platform (Flugel, 2004; Hips and Haas, 2009). The lacking reef constructers in the shallow water microfacies of study area and decreasing carbonate production led to ramp development. Transi- tional relation between microfacies in study area is sup- ported carbonate sedimentation in a carbonate ramp (e.g. Brachert et al., 2001). Present of various bioclasts in these sediments also show all subenvironments are connected together. Foraminiferal and pelloid wackestone/packstones are deposited in lagoon environment, ooid grainstone are formed in high energy conditions (Cadjenovic et al., 2008; Hofmann and Keller, 2006), bioclast (echinoderm, brachiopod) grainstone/packstone/wackestones deposited in open marine and finally radiolaria and pelagic pelecypods wackestone are deposited in deeper parts of depositional environment. Type of skeletal carbonates and increasing of non-skeletal carbonate grains especially ooids revealed that Upper Jurassic carbonate may have been precipitated in warm, tropical and supersaturated waters (e.g. Pomar et al., 2004; Hips and Haas, 2009). Based on size and shape, dolomites of study area in general divided into two groups including fine to very fine and coarse crystalline. Fine dolomites are mainly associated with silt to very fine sand quarts as well as interbeded with non fossiliferous mudstone and show these rocks may have been deposited in tidal flat condition, while coarse crystals are diagenetic dolomite and have formed in shallow to deep burial diagenetic environments. Microfacies comparison of studied sediments with Upper Jurassic deposits in Kopet-Dagh basin as well as paleogeography construction show a relative similarity in some aspects between two location, but lack of siliciclastic and evaporate deposits indicate that the carbonates of Jajarm area are deposited in a relatively deeper water condition. Key words: Jurassic; Carbonate rocks; Micro- facies analysis; Iran
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 References: Brachert T.C., Hultzsch N., Knoerich A.C., et al. Cli- matic signature in shallow water carbonate: High resolution stratigraphic markers in structurally controlled carbonate buildups (Late Miocene, Southern Spain): Palaeogeography, Palaeocli- matology, Palaeoecology, 2001, 175: 211-237. Cadjenovic D., Kilibarda Z., Radulloic N. Late Triassic to Late Jurassic evolution of the Adriatic carbonate platform and Budva Basin, southern Mon- tenegro. Sedimentary Geology, 2008, 204: 1-17. Flügel E. Microfacies of Carbonate Rocks: Analysis, Interpretation and Application. Berlin: Springer, 2004: 976. Fig.1 Location map of study area Hips K. Haas. Facies and diagenetic evaluation of the Permian–Triassic boundary interval and basal Triassic carbonates: Shallow and deep ramp sections, Hungary. Facies, 2009, 55: 421-442. Hofmann H.M., Keller M. Sequence stratigraphy and carbonate platform organization of the Devonian Santa Lucia Formation, Cantabrian Mountains, NW-Spain. Facies, 2006, 52: 149-167. Pomar L., Brandano M., Westphal H. Environmental factors influencing skeletal grain sediment as- sociations: A critical review of Miocene examples from the western Mediterranean. Sedimentology, 2004, 51: 627-651. 305
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