versão impressa ISSN 1026-8774
Rev. mex. cienc. geol vol.29 no.2 México ago. 2012
Facies analysis and depositional environments of the Upper Cretaceous Sadr unit in the Nakhlak area, Central Iran
Análisis de facies y ambientes de depósito de la unidad Sadr del Cretácico Superior en el área de Nakhlak, centro de Irán
Seyed Hamid Vaziri1*, Franz T. Fürsich2, and Nader KohansalGhadimvand1
1 Department of Geology, Faculty of Basic Sciences, North Tehran Branch, Islamic Azad University, P.O. Box 19585851, Tehran, Iran. * email@example.com
2 GeoZentrum Nordbayern der Universität ErlangenNürnberg, Fachgruppe PaläoUmwelt, Loewenichstrasse 28, D91054 Erlangen, Germany.
Manuscript received: September 8, 2011.
Corrected manuscript received: January 14, 2012.
Manuscript accepted: January 31, 2012.
The up to 258 m thick, carbonatesiliciclastic Upper Cenomanian to Campanian rocks (Sadr unit), which crop out widely in the Nakhlak area of central Iran, consist of conglomerates, sandy limestones and dolostones, calcareous sandstones, sandyargillaceous limestones and reefal limestones. The lower boundary of the studied section is an angular unconformity and its upper boundary is faulted. Sedimentological and palaeontological data indicate that Upper Cretaceous Sadr unit of Nakhlak area is equivalent to shallow carbonate platform successions of Upper Cretaceous rocks in central Iran, which belong to the central Iranian Plate and were deposited in marginal marine, shallow shelf and moderately deep marine environments. This geological unit can be divided into carbonates, siliciclastics, and mixed carbonatesiliciclastics groups. The siliciclastic facies group was deposited as shorelines, tidal flats, lagoons, and barrier bars indicating shallow shelf environments. The mixed carbonatesiliciclastics facies group was formed in a coastaldelta complex and the carbonate facies group took initially place on a homoclinal ramp which later developed into a rimmed platform due to the expansion of the rudist barrier facies.
Key words: siliciclastic, carbonates, homoclinal ramp, rimmed shelf, Cretaceous, Sadr unit, Nakhlak, Central Iran.
La unidad Sadr, que alcanza 258 m de espesor de carbonatossiliciclásticos del Cenomaniano Superior al Campaniano, aflora ampliamente en el area de Nakhlak en Irán central y consiste de conglomerados, calizas arenosas y dolomitas, areniscas calcáreas, calizas arenoarcillosas y calizas arrecifales. El límite inferior de la sección de estudio es una discordancia angular y su límite superior se encuentra fallado. Datos sedimentológicos y paleontológicos indican que la unidad Sadr del Cretácico Superior en el área de Nakhlak es equivalente a sucesiones de plataforma carbonatada somera en Irán central, y pertenecen a la Plataforma de Irán central, y fueron depositados en ambientes marginales marinos, de plataforma somera y moderadamente profundos. Esta unidad geológica puede ser dividida en grupos de carbonatos, siliciclásticos y mezcla de carbonatossiliciclásticos. El grupo de facies silicilásticas fue depositado en costa, planicies de marea, lagunas y barras de margen, indicando ambientes de plataforma somera. El grupo de las facies mixtas de carbonatossiliciclásticos, fue formado en un complejo costerodeltáicoy el grupo de facies carbonatadas inició su depósito en una rampa homoclinal, que posteriormente se desarrolló en una plataforma cerrada, debido a la expansión de facies de barrera de rudistas.
Palabras clave: siliciclásticos, carbonatos, rampa homoclinal, plataforma cerrada, Cretácico, unidad Sadr, Nakhlak, Irán Central.
The Nakhlak area lies in a structural region of Central Iran and is situated north of the socalled Yazd Block. It covers an area between 53°45'N and 53°54'N and 30°30'E and 33°37'E. The area consists of preTriassic? ophiolites and a Triassic (Nakhlak Group; Alam, Baqoroq, and Ashin formations), Upper Cretaceous (Sadr unit) and Paleocene (Khaled unit) sedimentary cover. These sedimentary units exhibit considerable thickness (3250 m) and represent a range of environments (Figure 1).
The Nakhlak Group is an exotic Triassic succession in central Iran. Lithologically as well as palaeontologically, the Triassic strata of Nakhlak differ completely from the shallowwater carbonate platform successions of the Lower and Middle Triassic of Iran. The only Triassic succession that litholigically and palaeontologically resembles the Nakhlak Group to some extent is the Triassic succession of the Aghdarband area in northeastern Iran (Davoudzadeh and SeyedEmami, 1972; Ruttner, 1991; Alavi et al., 1997; Vaziri, 1996, 2001, Vaziri and Fürsich, 2007; Balini et al., 2009; Zanchi et al., 2009), whereas lithological and palaeontological data indicate that the Cretaceous rocks of the Nakhlak area are equivalent to shallow carbonate platform successions of Upper Cretaceous rocks in central Iran, which belong to the central Iranian Plate (Vaziri et al., 2005).
The relationship of this geological unit to Southern Iran, i.e. coastal Fars and the Arabian Plate sequences has not been described yet but maybe it can be correlated with some parts of the Lower Sarvak (Cenomanian), Ilam (SantonianCampanian) and Tarbur (CampanianMaastrichtian) formations in Zagros region and Inner Fars in Iran, Maudud and Mishrif formations in the United Arab Emirates and Natih Formation in Oman (Razin et al., 2010; van Buchem et al., 2002). KhosrowTehrani (1977) indicated that Upper Cretaceous rocks of Nakhlak area can be correlated with those of Zagros region based on microfauna (foraminifera). This geological unit contains economically valuable minerals such as zinc and silver (Bariand, 1963; Burnol, 1968; Holzer and Ghassemipour, 1973; Cherepovsky et al., 1982).
The main objectives of this paper are to describe and interpret the facies and depositional environments of the Upper Cretaceous Sadr unit in the Nakhlak area in order to understand the geological evolution of the area during that time interval.
The Upper Cenomanian to Campanian rocks (Sadr unit) crop out across a large area of Nakhlak and reach a thickness of 258 m in the studied section (Figure. 1). This section is situated south of Nakhlak village (coordinates: N 33°32'09"; E 53°50'33") and consists of conglomerates, sandy limestones, calcareous sandstones, sandy dolostones, sandyargillaceous limestones, sandy dolomitic limestones, and reefal limestones that have been subdivided into ten subunits on the basis of their facies characteristics and described (Figure 2; Vaziri et al., 2005). The rich foraminiferal assemblage from different subunits of the studied section indicates a Late Cenomanian to Campanian age (KhosrowTehrani, 1977; Vaziri et al., 2005). Biostratigraphic studies of the Upper Cretaceous rocks of the Nakhlak (KhosrowTehrani 1977; Vaziri et al., 2005), Choopanan and Hafttoman areas (KhosrowTehrani, 1977), as well as of the Kerman area of eastcentral Iran (Faryabi, 2003) point to a hiatus during TuronianConiacian time.
The Upper Cretaceous Sadr unit overlies the Triassic rocks with an angular unconformity (Figures 3a3c) and their top is faulted and overlain by Paleocene (DanianThanetian) rocks (Figure 3a, I; Vaziri, 1996; Vaziri and Fürsich, 2007; Balini et al., 2009, Vaziri, 2011). Pebbles and fragments of Upper Cretaceous limestones in the basal red conglomerate and conglomeratic sandstone of the Paleocene sequence indicate a gap in sedimentation (disconformity) between the Upper Cretaceous and Paleocene rocks and that the fault contact is secondary.
MATERIALS AND MEHTODS
During field work in the Nakhlak area, one stratigraphic section of the Upper Cretaceous Sadr unit was studied, 77 rock samples were collected and facies were studied along the layers trend, which indicated similar characteristics. Thin sections of collected samples were studied under a polarized microscope. For classification purposes the concepts of Folk (1974) have been used for siliciclastic facies group (especially sandstones) and those of Dunham (1962) and Flügel (2004) for carbonate facies group. The petrography was subsequently studied at the GeoZentrum Nordbayern, University of ErlangenNürnberg, Germany within the framework of a DAADsponsored research stay.
FACIES ANALYSIS AND DEPOSITIONAL ENVIRONMENTS
The lower part of Upper Cretaceous Sadr unit in the Nakhlak area is transgressive and consists of basal polymictic conglomerates occurring in pockets (Figure 3d), calcareous sandstones, and sandy limestones and dolostones, which are followed by dolostones, calcareous sandstones and shallowwater rudistbearing limestones (Figures 3e3h). The Sadr unit can be divided into carbonates, siliciclastics, and mixed carbonatesiliciclastics groups as follows, and there is no evidence of continental sedimentation.
Mixed carbonatesiliciclastic facies group
Mixed carbonatesiliciclastic facies group have considerable extension and are observable in nearly all subunits of the studied section. Sand and gravelsized detrital grains with fragmented and reworked allochems and a micritic to sparitic matrix indicate that the sedimentary basin frequently received detrital sediments from continental sources, brought in by rivers, and distributed and mixed by currents and storms. Sediments of the subunits 13 of the Upper Cretaceous Sadr unit are mainly composed of these facies. The most important microfacies types are briefly described below:
Pebbly to sandy bioclasticpackstone to fossiliferous pebbly sandstone
Siliciclastic grains account for 3040% and range in size from coarse sand to pebbles made up of monocrystalline quartz and metaquartzite. Carbonate fragments account also for 3040%, and consist especially of bioclasts such as brachiopods, bivalves (especially rudists), echinoids and gastropods fragments. The matrix consists of micrite and microsparite or sparite/sparry dolomite. Most carbonate grains are reworked and most likely are from within the basin. These facies can be observed in subunits 3, 6, 7 and at the base of subunit 5 (Figures 4, 5d, 5e).
Bioclasticwackestone/sandy packstone to fossiliferous sandstone
These mixed facies are finergrained than the previous ones and the percentage of pebbles and carbonate grains is lower, and lithoclasts consist mostly of microfossils. The matrix is micrite and microspar, which indicates a lower energy level than in the previous facies (Figures 5f, 5g). As observed in the field, these mixed facies often occur as alternations of sandstone and carbonates. In subunits 14 (Figure 4), mixed facies are very common and show thick coarseningupward sedimentary cycles. Within these cycles thinner coarseningupward cycles are also seen and the sediments are strongly bioturbated (Figure 6d). These mixed carbonatesiliciclastic facies mainly formed close to the shoreline and possibly in the vicinity of deltas.
Medium to coarse sandy dolostone to dolomitic sandstone
Dolomite rhombs and detrital sand grains (monocrystalline quartz with undulose extinction) and a few cherts grains are observed. Commonly, the dolomite rhombs consist of ferric dolomite and have zonal structure (Figure 5h). With increasing percentage of quartz and decreasing percentage of dolomite crystals, this facies changes to dolomitic sandstone (sandstone of the subchert arenite group). In this facies, only shadows of allochems (crinoids and bivalves) can be seen, because primary texture has been affected by dolomitization. This facies is arranged in small finingupward sedimentary cycles and reflect a gradual sealevel rise with increasing water depth. This facies can be observed in subunits 3, 4, 6 and 9.
Mixed carbonatesiliciclastic facies group consists mainly of pebbly to sandy limestones/or dolostones, calcareous sandstones and pebbly dolomitic sandstones. Detrital grains include metaquartzites and chert granules. Carbonate components consist mainly of skeletal fragments of brachiopods, bivalves (e.g., rudists), echinoderms, algae, bryozoans, and foraminifera, in addition to some intraclasts, extraclasts, and peloids (Figures 5d5g). Field and petrographic studies of the mixed carbonatesiliciclastic facies group, for example coarseningupward cycles, largescaled crossbeddings and greycoloured sediments, show that these facies were formed in a coastaldelta complex. The coast is a spread area that includes deltas, beaches, barrier islands, cheniers and also coastal plains adjacent to the shoreline (Summerfield, 1991). Sedimentary shorelines can be divided into lobated and linear shorelines (e.g., Selley, 1996). The first set includes deltas and the second set mainly includes beach/barrierlagoon complexes. The mixed carbonatesiliciclastic facies group of the Upper Cretaceous Sadr unit of the Nakhlak is probably belongs to linear shorelines (e.g., Selley, 1996).
The existence of pebble and sandsized (quartz, feldspar and chert) siliciclastic grains as well as the presence of various allochems, especially bioclasts, and the carbonate matrix indicates that these facies were affected by marine and continental processes. Variable textural maturity of the siliciclastic sediment samples, plenty of allochems and lime mud in other samples, coarsening upwards sedimentary cycles and plenty of biogenic structures show that these facies were deposited in various subenvironments of a shorebeach complex.
Siliciclastic facies group
This facies group consists mainly of siliciclastic and other detrial grains derived from Triassic rocks of the Nakhlak area. The grains consist of quartz and reworked carbonate grains such as carbonate lithoclasts and eroded allochems. This facies group is composed of a variety of gravel, sand, and siltsized grains and exhibit sedimentary structures such as horizontal lamination, lenticular bedding, and flaser bedding, normal and reverses graded bedding, mud cracks, casts of evaporite minerals, plant roots and remains of paleosoil (caliche). Sandstone and conglomerate beds alternate and exhibit numerous sedimentary structures such as imbrications and trough cross bedding. Siliciclastic mudstones are seen between the sandstone beds. Chert arenite and quartzarenite facies have good textural maturity and mineralogy and may display parallel lamination and crossbedding. The most important facies types are as follows:
Brown, finegrained oligomictic conglomerate
This gravel facies is intercalated between sandstones in subunit 5 (Figure 4) and occurs partly as orthoconglomerate and partly as paraconglomerate. Gravel grains consist of metaquartzite, monocrystalline quartz and chert, and are lightcoloured. The sorting of grains is good but their rounding usually is poor to moderate. The conglomerate is variably sandy and occasionally grades into gravely sandstone. Its brown colour is caused by the dolomitized carbonate cement with hematite impregnation (Figure 6a). The conglomerate has a wide lateral extension and, according to its lithological and petrographic features such as textural and compositional maturity, flat bedding, finingupward graded bedding and marine fauna, is interpreted as shoreline facies.
Fossiliferous, mature subchert arenite with dolomiticcalcareous cement
In this facies, quartz and chert grains are cemented by carbonate. Commonly, grains are medium in size, and their rounding and sorting are not good. The cement is impregnated with hematite. Quartz and chert pebbles and macroand microfossil fragments are characteristic elements of this facies (Figures 5a, 5b). The general colour of this facies is brownish and small detrital grains can be seen (Figure 6b). The conglomeratic facies has been deposited at relatively highenergy shorelines. Textural maturity and fragments of echinoderms, bivalves, foraminifera, and algae support this interpretation. This facies mostly occurs in subunits 5, 6, 7, and 8 (Figure 4).
Crossbedded chertarenite to mature, coarsegrained quartzarenite with dolomiticcalcareous cement
This siliciclastic facies includes coarseto very coarsegrained sandstones and some conglomerates. The percentage of chert grains varies from very low to 30%. Other grains are composed of monocrystalline quartz and metaquartzite. Most quartz grains have undulose extinction and metamorphic or plutonic sources. The components are embedded in rhombohedral ferric dolomitic or granular, blocky and poikilotopic calcite cements (Figure 5c).The sandstone beds show lightgray to white, medium and thick beds with large scale cross stratifications. Both field and petrographic evidence suggest that this facies represents sand waves organized in barrier bars. This facies attains considerable thickness in subunit 5 (32 m) (Figure 4).
Finegrained siliciclastic facies include mudstone and shale. Some of the mudstones are brownish or yellowish and exhibit mud cracks (Figure 6c), burrows and lamination. These deposits represent tidal flats, in particular supratidal areas. Some other mudstones are black and are composed of thin finingupward cycles overlying sandy deposits. Very probably, these mudstones were formed during transgressive events, because of finingupward cycles and black coloured representing deposition under reduce conditions.
Shaly facies alongside with creamy to lightgreen carbonates represent open marine conditions deposited below the storm wavebase.
The siliciclastic facies group of the Upper Cretaceous Sadr unit of the Nakhlak area were deposited in various subenvironments of siliciclastic shorelines, especially tidal flats, lagoonal, sandy barriers, and open marine areas. These facies were formed on a shallow shelf during a relative sealevel highstand. During that time, the carbonates were highly diluted by abundant input of landderived siliciclastic material. Sedimentation took place along linear shorelines in a warm and dry climate which is supported by the presence of mud cracks and casts of evaporate minerals. In this setting, deltas are common, but barrier islands situated parallel to the shores are widespread. These sandy islands may have separated lagoons from the open sea and may have been cut by a few tidal dissected channels. This complicated system may have formed whenever carbonate regime changed to a siliciclastic regime in connection with a relative fall in sea level or evolution of continental environments to coastal environments during a transgressive phase and relative sealevel rise. Shales are highly bioturbated and intercalated between sandstones of the shore face subenvironment. They are interpreted to represent offshore and open marine environments.
Carbonate facies group
Thickbedded limestones account for a considerable part of subunit 10 (Figure 4). In other subunits, carbonate beds (calcareous or dolomitic) generally occur alternating or interbedded with siliciclastic rocks or mixed carbonatesiliciclastic facies group. The most important facies types are as follows:
Dolomiticlime mudstone to laminar dolomudstone with casts of evaporite minerals
These facies include all samples that are composed of finely crystalline dolostone (Figure 7j); do not have allochems and only rare peloids and intraclasts. Calcium carbonate mud was changed during diagenesis to microspar. The most important structures and fabrics of these sediments include lamination, mud cracks, casts of evaporate minerals and fenestral fabrics (Figure 7j). These fenestral fabrics can be observed as birds eyes and usually form fenestral porosities while the others have been filled with sparry calcite. These thinto mediumbedded sediments can be attributed to interand supratidal environments (Gebelin, 1977; James, 1979; Shinn, 1968, 1983; Sellwood, 1991) (Figure 5e). Erosional sedimentary structures such as scour and fill structures occurring in some beds are common on tidal flats (Reineck and Singh, 1980; Elliott, 1991; Reading and Collinson, 1996). Similar sediments form at present day on supratidal flats of Florida and the Bahama and on sabkha flats of the Persian Gulf (Shinn, 1986).
Lime mudstone to bioclasticwackestone with casts of evaporite minerals
This facies contains bioclasts (115 %), some quartz grains, and a few peloids. Bioclasts include foraminifera and rarely ostracods. The pore spaces were filled with sparite. In some samples, a geopetal fabric formed in these cavities (Figures 7a, 7b).
Peloidal bioclastic wackestone
In this facies, peloids account for 10% and finegrained bioclasts including benthic foraminifera and ostracods for approximately 510% of the total rock volume. Locally, the micritic matrix has been converted to microspar and pseudospar (Figure 7c). All evidence such as plenty of lime mud, fragments of foraminifera from offshore environments and peloids indicate a quiet environment.
Peloidal bioclastic packstone to grainstone
This facies differs from the former facies by its higher amount of allochems and type of matrix. The facies contains 2025% bioclasts (foraminifera, ostracods and bivalves) and 2025% peloids. The size of the bioclasts is fine to mediumgrained sand and the fabric is grainsupported (Figure 7d). This indicates that the sediment has been deposited under elevated energy conditions.
Field and petrographic evidence suggests that the three above facies represent restricted shallowwater inner platform areas resembling recent inner platforms such as the Persian Gulf, Florida, and the Bahamas (Bathurst, 1975; Shinn, 1986; Purser and Evans, 1973). Abundant lime mud is evidence of sedimentation in a lowenergy environment. In some of the facies, a bioturbation fabric (mottling) and micritization of skeleted grains occur, which resulted from the activity of endolithic organisms in quiet environments such as lagoons (Flügel, 2004). Plenty of peloids and tests of benthic foraminifera such as miliolids and algae also support sedimentation of these facies in lagoonal environments (Tucker and Wright, 1990; Wright and Burchette, 1996).
Intraclastic to bioclastic grainstone
The allochems are coarse sand to gravelsized and set in a sparry calcite cement. The most important bioclasts are foraminifera, echinoderm, brachiopod, bryozoan and bivalve fragments (Figures 7e, 7f). Granular cement between allochems and syntaxial rim cement around echinoderm fragments are the most important cement fabrics. This facies is not widely distributed. Skeletal grains and lack of lime mud indicate turbulent environments such as shoals. Intraclasts were formed in tidal flat channels or estuary channels.
Boundstones occur mainly in subunit 10 (Figure 4). The lightgrey, thickbedded deposits contain rudists in growth position (Figures 3h, 5f). Apart from rudist fragments, some echinoderm, brachiopod, and foraminiferal fragments occur. These deposits change in thickness laterally and alternate with lagoonal and open marine facies (Figure 6g). The intraclastic bioclastic grainstone and rudist boundstone are here interpreted as bioclastic barriers and rudist biostromes/reef, respectively on platform margins or highenergy areas of the inner platform. Texture of the grainstone facies (lack of matrix) indicates that the sediment formed in highenergy areas. Similarly, the coarse sizes of calcareous grains support deposition in a turbulent environment (Irwin, 1965). At present day, bioclastic barriers can be observed in the Persian Gulf (Purser and Evans, 1973) and on the Bahamas Platform (Harris et al., 1985). Rudists at the margins of carbonate platforms commonly formed biostromes/reefal barriers (Ross and Skelton, 1993).
Rudist floatstone to rudstone
This facies is composed of matrix made of lime mud, and more than 50% of fine and coarse shell fragments. Bioclasts consist of rudists and some foraminifera, echinoderms, and brachiopods. Such floatstones and rudstones commonly form on reefal flanks as reefal debris (Figures 7f, 7g).
Lime mudstone to bioclasticwackestone
This facies represents the deepest carbonate facies of the Upper Cretaceous rocks of the Nakhlak area and alternates with other carbonate facies or with open marine shales. The main allochems are pelagic foraminifera such as Hedbergella delrioensis, H. planispira, H. simplex and Globotrancana sp. (approximately 15%). Other allochems include fragments of echinoderms, small brachiopods, and thin and fragile bivalves (Figure 7h). Rarely, the rocks exhibit a nodular texture. Plenty of lime mud, fossils, and the thinbedding suggest deposition below storm wavebase in an open marine environment.
The rudist float to rudstones and the mud to bioclasticwackestones probably formed in moderate energy, shallow to deepwater, quiet open platforms. Such conditions usually exist on mid to outer ramps and on open shelves and basinal areas. Rudist fragments resulted from demolition of marginal platform reefs by waves and storms. The sediment accumulated as biostromes/reef talus in reefal barriers and on slope areas of the platform margin. In summary, field and petrographic studies indicate that the carbonates facies formed in tidal flats to open marine areas (Figure 8). Some beds exhibiting a detrital base and graded bedding probably represent storm deposits (Figure 6h).
The detailed study of the Upper Cretaceous Sadr unit of the Nakhlak area indicates a range of siliciclastic and carbonate environments, which frequently changed through time. Siliciclastic facies were deposited in marginal marine environments during a relative sealevel highstand such as tidal flats, lagoons and barrier islands. Some finergrained facies including mudstones and shales belong to somewhat deeper shelf areas. The carbonate facies formed on a homoclinal ramp. At times, these platforms were rimmed by barrier bars. The facies show various sedimentary structures and textural characteristics such as lamination, evaporate pseudomorphs, fenestral fabrics, and mud cracks indicative of intertidal to supratidal conditions. Some facies contain macrofossils such as rudists which formed patch reefs, alternating with some shales and marls deposited below storm wavebase. The mixed facies types are related to mixed carbonatesiliciclastic shelf environments where abundant intraclasts of carbonate rocks have been produced and a great number of quartz and chert grains, and pebbles have been delivered from a terrestrial source. Field and petrographic studies indicate that these facies were formed in a coastaldelta complex and are probably belong to linear shorelines.
The petrographic study has been carried out at the GeoZentrum Nordbayern, University of ErlangenNürnberg, Germany with financial support to S.H. Vaziri by the German Academic Exchange Service (DAAD) in summer 2009, which is gratefully acknowledged. The first author would like to deeply thank the GeoZentrum Nordbayern (GZN), University of ErlangenNürnberg for administrative support of the research, Dr. M. Heinze and Mrs. G. Schönberger (GZN) for their kindness during his stay in Erlangen and also to Dr. J. Taheri of the Geological Survey of Iran (GSI) for useful discussions on the sedimentary log. We wish to thank Dr. J. Madhavaraju of the ERNO Institute of Geology, UNAM, Mexico and Dr. D. Jahani of the Islamic Azad University, North Tehran Branch, Iran whose comments led to improvement of the manuscript. The photographic work was done with kind help of Prof. Dr. R. Koch at the GeoZentrum Nordbayern, University of ErlangenNürnberg.
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