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Facies Analysis and Depositional Environment of the Permian Siliciclastic-Carbonate Transition, Central Alborz, Iran

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The Permian rocks in Gaduk Area (east of Firuzkuh, Central Alborz) occur as a mixed siliciclastic-carbonate succession, dominated by siliciclastic facies (Doroud Formation) in the lower and carbonate facies (Ruteh Formation) in the upper half. This
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    Facies Analysis and Depositional Environment of the PermianSiliciclastic-Carbonate Transition, Central Alborz, Iran Mohammad Lankarani a , Abdolhossein Amini *   a ,   and Hossein Mosaddegh b    a School of Geology, University College of Science, University of Tehran, Tehran, Iran  b School of Earth Sciences, Damghan University of Basic Sciences, Damghan, Iran Received: 13 May 2007; Received in Revised form: 13 August 20070;Accepted: 15 October 2007; Available on Line: 8 July 2008.   Abstract: The Permian rocks in Gaduk Area (east of Firuzkuh, Central Alborz) occur as a mixedsiliciclastic-carbonate succession, dominated by siliciclastic facies (Doroud Formation) in thelower and carbonate facies (Ruteh Formation) in the upper half. This succession overlies theLower Carboniferous Mobarak Formation and is underlain by Lower Triassic carbonates of Elikah Formation unconformably. Field studies and Laboratory measurements resulted inrecognition of 4 terrigenous and 13 carbonate facies in the studied section. Siliceous cementedoligomictic orthoconglomerate and quartzarenite, calcite cemented quartzarenite (Lc–bearing inplaces), and red siltstone are common terrigenous facies. Fair laminated homogenous dolomiteto lime mudstone, peloid grainstone/packstone, bioclast oncoid grainstone/packstone,bioturbiated bioclast wackestone/packstone, sandy ooid grainstone, green algal bioclastgrainstone, sandy bioclast grainstone/rudstone, oriented sandy bioclast packstone, sandy bioclastfloatstone, cephalopod bioclast wackestone and sponge spicule bioclast wackestone are commoncarbonate facies. A homoclinal carbonate ramp, with scattered patch reefs and a siliciclasticshallow marine system (siliciclastic shelf) were interpreted as depositional environment of thecarbonate and terrigenous facies respectively. Results from this study show that sedimentation inthe area commenced with siliciclastic facies, after the early Permian eustatic sea-level rise. Thisshelf was gradually changed into a carbonate ramp setting mostly due to relative sea level rise,result of which occurred as development of Middle Permian deposits. Lack of Upper Permiandeposits is related to a regional sea level fall, which resulted in development of a laterite horizonin the area. Keywords:   Permian, Firuzkuh, siliciclastic-carbonate transition, paleotethys, Central Alborz, Iran. * Corresponding author.Email address: ahamini@khayam.ut.ac.irFax: +98-21-66491623 Introduction The Lower Permian Doroud, Middle PermianRuteh, and Upper Permian Nesen formations havea considerable outcrop throughout the AlborzMountains. The Lower Permian DoroudFormation was first investigated by Asseretto(1963) in Jajroud valley (Central Alborz), whereis comprised of conglomerates, sandstones, andred shales with minor beds of Fusulinid bearinglimestone. The Middle Permian Ruteh Formationis well described in its type section (Ruteh valley),where consists of grey to dark grey limestoneswith intercalations of marls (Asseretto, 1963). The The Journal of Damghan University of Basic Sciences, Volume 2 , Number 1 , 200 9 , 25-36  Available online at: http://www.dubs.ac.ir/Scientific_journal.htm    2 6 Lankarani et al. JDUBS, Vol 2 , No 1 , 200 9   Upper Permian Nesen Formation was firstdescribed by Glaus (1964) in Noor valley (typelocality). This formation is dominated bysandstones, limestones, black shales, dark marlsand nodular limestones. In general, the UpperPermian deposits are well exposed in the northernflank, whereas the Middle and Lower Permiandeposits crop out in the southern flank of AlborzMountains.Despite numerous studies on the formations (e.g.Bozorgnia, 1973; Okhravi and Alizadeh Ketek Lahijani, 1994; Lasemi and Mokhtarpour, 1994;Vaziri, 1994; Mokhtarpour, 1998; Ghasemi-Nejad, 2002), their sedimentary petrology anddepositional environment in Central Alborz arenot well understood yet. This study, as part of awider project which investigates sedimentarypetrology and depositional environment of theformations in Alborz Region, aims to discuss themajor characteristics of constituted rocks and theirdepositional environment in Gaduk Area. Thestudy area is located 10 Km east of Firuzkuh cityat longitude of 52° 53' 05" east and latitude of 35°49' 25" north (Fig. 1). From tectonic point of view, the area is part of Central Alborz Zone(Berberian and King, 1981; Alavi, 1991) (Fig. 2).In this region the terrigenous and carbonate rocksof Lower to Middle Permian (Doroud and Rutehformations) are well exposed but no outcrops of the Upper Permian Nesen Formation is observed.These Formations are exposed in southern flank of Gaduk syncline, in which the Paleozoic andMesozoic successions crop out in some extent(Fig. 3). The Lower Permian Doroud Formationdisconformably overlies the Lower CarboniferousMobarak Formation and transitionally passes tothe Middle Permian carbonates (RutehFormation), which are in turn overlain by Basaltto Andesitic rocks. Discrimination of Doroud andRuteh formations is only possible bypaleontological studies (Bozorgnia, 1973).Absence of the Nesen Formation is related toregional exposure in the area during UpperPermian to Triassic period, result of which isrecorded by distinct paleosol horizons. This isprobably due to the global (eustatic) sea level fallat the end of Guadalupian (Ross and Ross, 1987).Some basaltic to andesitic rocks along withlaterite horizons characterize Permian-Triassicboundary in the area.Results from sedimentary petrology anddepositional environment of the formations inGaduk Area along with similar researches fromother parts of the Alborz are expected to provide abasis for reconstruction of PermianPaleogeography in the Alborz region. Figure 1. Location map of the studied area.      JDUBS, Vol 2 , No 1 , 200 9 Facies Analysis and Depositional Environment   … 27   Figure 2. Generalized tectonic setting of Iran. The location of studied area (in Central Alborz zone) has been shown by star(compiled from Berberian and king, 1981 and Alavi, 1991). Figure 3. Upper Paleozoic (Mobarak, Doroud and Ruteh Formations) and Lower Mesozoic (Elikah Formation) rocks in the southernlimb of Gaduk syncline. The contacts between the formations are indicated by dashed lines.    28 Lankarani et al. JDUBS, Vol 2 , No 1 , 200 9   Results and Discussion After reconnaissance study of nearly all exposuresof the formations in the East Firuzkuh, the Gaduk section was selected for detail description of rocksand sampling. This was mainly due to minimumstructural deformation and accessibility of theformations. A number of 100 oriented sampleswere collected based on lithological variations.Standard thin sections were prepared fromsamples for petrographic studies. Fordiscrimination of various carbonates, all sampleswere stained with Alizarine Red-S and PotassiumFerricynide using Dickson (1966) method. Thefrequency of required constituents (allochems,cement, matrix, detrital grains and porosity) wasdetermined using comparison charts (Bacelle andBosellini, 1965). Clasticity index was determinedby measuring of maximum diameter of theconstituent allochems and detrital componentsusing Carozzi (1989) method. Identification of terrigenous and carbonate facies was based onfield descriptions and laboratory measurements.Lithology, sedimentary structures, fossil content,geometry, matrix proportion, cement type andabundance were used for this purpose.Terrigenous and carbonate rocks were classifiedon the basis of Pettijohn et al. (1987) and Dunham(1962) respectively. Detailed description of reef facies is carried out using Embry and Klovan(1971) classification. Identified facies werecompared with standard facies from well knownenvironments (e.g. Wilson, 1975; Read, 1985;Buxton and Pedley, 1989; Flugel, 2004).Depositional environments of the carbonate facieswere discussed mostly based on their petrographiccharacteristics (Flugel, 2004). Depositionalenvironment of terrigenous facies weredetermined mainly based on theirsedimentological characteristics (textural andmineralogical maturity, sedimentary structures,composition), while comparing with standardfacies from known environments (e.g. Miall,1996, 2002; Reading, 1996; Selley, 1996).Clasticity variations of the facies along with theirsedimentological characteristics throughout thesedimentological log (Fig. 4) were used fordepositional environment analysis through time.Major characteristics of the rocks including,lithology, geometry, sedimentary structures, fossilcontents and stratal surfaces were used for theirdetailed description in the field (lithofaciesanalysis). Combination of results from fieldobservations (lithofacies analysis) and laboratorystudies (petrofacies and microfacies analysis) ledto recognition of 4 terrigenous and 13 carbonatefacies which are briefly discussed below.  Facies G: It is a pebble to granule size, siliceouscemented, well sorted, texturally andmineralogically mature oligomicticorthoconglomerate. The conglomerate has alenticular geometry and fining upward trend,marked by erosional scoured base. The faciesgrades into a low angle cross stratified sandstone(facies S 1 ). This facies is related to the bedformsof distributary channels on a shorefaceenvironment (Reading, 1996), which wasperiodically receiving sediments from the land.  Facies S 1 : This is a horizontal to low angle crossstratified, texturally and mineralogically mature,siliceous and carbonate (dominantly poikilotopicdolomite) cemented, medium to very coarsequartzarenite. This facies is marked by heavymineral horizons and sheet like geometry (Fig. 5Aand B). This facies is related to the beach(foreshore) sub-environment of a siliciclastic shelf (Reading, 1996; Selley, 1996).  Facies S  2 : The facies is a cross stratified,dolomite cemented quartzarenite with minor(<5%) carbonate lithic content. The carbonatelithics are mostly dolomitized green algae(Intraclast in nature). The carbonate lithics arerelated to carbonate deposits which developedabove fair weather wave base during sea-level riseand low sediment influx from the land. They aremost likely reworked as intraclasts during sea-level fall and high sediment supply from the land.This phenomenon is responsible for hybridappearance of the facies in places.  Facies M  : It is a fair laminated, very well sorted,dolomite cemented, heavy mineral bearing redsiltstone, composed of angular silt sized quartzgrains. This facies is related to beach berm sub-environment (subareal and oxidation conditions).Occurrence of this facies in the studied sectionindicates that the environment was subject toexposure and erosion periodically.  Facies C  1 : Facies C 1 is a parallel laminatedhomogeneous dolomite to lime mudstone.Neomorphism is observed in places. This facies issimilar to standard microfacies number 22(RMF.22) described by Flugel (2004) from rampenvironments. This facies is related to supratidalsub-environment of a ramp system.    JDUBS, Vol 2 , No 1 , 200 9 Facies Analysis and Depositional Environment   … 29   Figure 4. Representative sedimentological log of the studied section illustrating the constituent facies and their interpretedenvironments.
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