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Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran
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Journal of Asian Earth Sciences 33 (2008) 267–277 www.elsevier.com/locate/jaes
Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran Mohammad H. Adabi *, Elham Asadi Mehmandosti School of Earth Sciences, Shahid Beheshti University, Evin, Velenjak Daneshjo Blv., Tehran, Iran Received 3 November 2006; received in revised form 5 January 2008; accepted 8 January 2008
Abstract The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran. Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and d18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C. Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The d18O and d13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Microfacies; Geochemistry; Ilam Formation; Zagros; Iran
1. Introduction Aragonite is the predominant mineral, along with some high-Mg calcite, forming in modern tropical warm waters (Milliman, 1974). In modern temperate carbonates, highMg calcite predominates over low-Mg calcite, with minor amounts of aragonite (Rao, 1981a; Nelson, 1988). In subpolar cold-water carbonates, low-Mg calcite is the dominant mineral (Rao, 1981b). Experimental studies (Kinsman and Holland, 1969; Burton and Walter, 1987) indicate that low-Mg calcite forms alone in water temperatures of <5 °C from normal seawa*
Corresponding author. Tel.: +98 61362233602; fax: +98 21 22431690. E-mail addresses: [email protected], [email protected] (M.H. Adabi). 1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2008.01.002
ter, and also confirms that primary carbonate morphology and mineralogy depend primarily on water temperatures, similar to observations for modern carbonates. Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg, 1983; Wilkinson and Algeo, 1989). Variation in carbonate mineralogy has been related to the position of global sea level (Wilkinson et al., 1985), changes in rates of seafloor spreading (e.g. Mackenzie and Pigott, 1981; Hardie, 1996), PCO2 level (e.g. Sandberg, 1985; Makenzie and Morse 1992; Hallock, 1997) and Mg/Ca ratio related to spreading rate (e.g. Stanly and Hardie, 1998). However, other researchers suggested that the
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assumption of a change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitude (e.g. Nelson, 1988; Adabi, 2004). Evaluation of Ordovician Gordon Group carbonates of Tasmania (Australia), and the Upper Jurassic Mozduran limestone in the Kopet-Dagh Basin in north-east Iran, based on petrographic and geochemical criteria, indicate that aragonite, not calcite, was the dominate mineral in these warm-water, subtropical carbonates (Rao, 1991; Adabi and Rao, 1991). In the Recent, aragonite is the predominant mineral in warm, shallow-marine carbonates and calcite the dominant mineral in marine cool-water carbonates (James and Clarke, 1997). The present study uses petrographic evidence, elemental and d18O and d13C values and compares these with modern tropical (aragonitic) and temperate (calcitic) carbonates and originally aragonitic Ordovician subtropical carbonates and originally calcite Permian subpolar cold-water carbonates of Tasmania, and aragonitic shallow subtropical carbonates of the Upper Jurassic Mozduran Formation of Iran to understand the original carbonate mineralogy, depositional facies and diagenetic characteristics of the Ilam Formation. In addition, the geochemical evidence was used to determine the boundary between the Ilam and Sarvak Formations, which is difficult to place due to similar lithologies in these formations. 2. Geological setting The study area is located 18.5 km north of Izeh, Zagros area, Iran (Fig. 1). The Ilam Formation (Santonian–Campanian), which is part of the Bangestan Group consists mainly of fossiliferous limestone (Fig. 2). The type section of the Ilam Formation is situated in the Kabirkoh area, Lurestan, and is overlain by the Surgah
Formation and underlain by the Gurpi Formation. However, in the studied area this unit, conformably and without any recognizable boundary, overlies the Sarvak Formation, the Surgah Formation being locally absent. The thickness of the Ilam Formation in the study area is 95 m, and it contains macro- and microfossils of algae, echinoderms, bivalves, gastropods and benthic and pelagic foraminifera. 3. Methods of study Samples were collected near significant lithological changes, especially near the probable boundaries. One hundred and twenty uncovered thin sections were studied for petrographic and facies analysis. Thin sections were stained by potassium ferricyanide and alizarin-red S solution (Dickson, 1965). Forty-two powdered micritic samples were analysed by atomic absorption spectrometer for Ca, Mg, Sr, Na, Mn, Fe at the Geology Department of the Shahid Beheshti University, Tehran, Iran. Precision was ±0.5% for Ca and Mg and ±5 ppm for Sr, Na, Mn and Fe (Robinson, 1980). Twenty powdered samples which had previously been analysed for major and minor elements, were analysed with a Micromass, 602D for oxygen and carbon isotopes at the Central Science Laboratory, University of Tasmania, Australia. Fifteen mg of powdered samples were allowed to react with anhydrous phosphoric acid in reaction tubes under vacuum at 25 °C for 24 h. The CO2 extract from each sample was analysed for d18O and d13C by mass spectrometry. Precision of data is ±0.1‰ for both d18O and d13C and these values were reported relative to PDB. Selected samples were observed with a cathodoluminescence microscope (Nikon Cl, CCL 8200) at the Research Institute of Petroleum Industry (R.I.P.I).
Fig. 1. Location map of the study area in Tang-E Rashid, Izeh.
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Fig. 2. Stratigraphic column of the Ilam Formation in the study area.
4. Petrography The Ilam carbonates consist of a large variety of skeletal and non-skeletal grains, calcite cements, micrite and early and late diagenetic dolomites. 4.1. Skeletal grains Skeletal grains are mostly bivalves, crinoids, gastropods, algae and abundant benthic and pelagic foraminifera. Radiolitidae and Hippuritidae are the two main kinds of rudist. Red algae (Permocalculus and Hensonella) are more abundant than green algae (Dasycladacea). Miliolids, Nezzazata, Dicyclina are common and Rotalia skurnsis is the major type of benthic foraminifera in the Ilam Formation. 4.2. Non-skeletal grains Non-skeletal grains are abundant and consist mainly of ooids, intraclast and peloids. The nuclei of ooids are commonly peloids, and in some examples the nuclei consist of fossil fragments such as echinoderms, gastropod and brachiopod. The cortex thickness of ooids is inversely related
to the size of the nucleus. In some examples, subhedral to almost euhedral dolomite rhombs partially to completely obliterate the ooid structure. Different types of microstructure are recognized in the Ilam oolites: tangential, radial and composite structures. Composite (compound) ooids are the least abundant ooid type in the carbonate samples studied. Peloids are also abundant. Most pellets in the Ilam carbonates are uniform in size, but in a few samples they show considerable irregularity. Pellets in all samples range from 0.05 to 0.2 mm (average 0.125 mm). It is assumed that the small, even-size, well-sorted, uniform-shaped and dark coloured peloids in the Ilam limestone are faecal pellets. Some pellets were trapped by rudists. Intraclasts occur in some samples. They are generally polymodal in size ranging from 0.6 to 2.3 mm. Most intraclasts are internally homogenous and consist of micrite, while others contain bioclasts and peloids. 4.3. Cementation Different generations of sparry calcite cement were recognized in the Ilam limestone, ranging from marine through meteoric to some burial cements.
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Isopachous cements form fringes around grains (Fig. 3A). These cements were possibly composed of aragonite, due to the identical morphology to that of recent warm-water shallow-marine aragonitic cements (Given and Wilkinson, 1985; Adabi and Rao, 1991). Cathodoluminescence petrography shows that the acicular to fibrous cements are dull to non-luminescent (Fig. 3B), indicating a marine origin (Machel, 1985). Other petrographic evidence which supports marine origin are: (1) they are the first generation cements which predate other cements, (2) they have acicular to fibrous fabrics, (3) they commonly form isopachous fringes around the grains or cavity wall, (4) they are succeeded by bladed, equant cement and then followed by later coarsely crystalline sparry calcite cement. In the Ilam limestones, bladed calcite cements occur both interparticle, and within cavities (Fig. 3C). Individual crystals often show a bladed crystal morphology with scalenohedral terminations. The bladed cement in the Ilam limestones is commonly followed by a later generation of equant, drusy calcite. Cathodoluminescence petrography illustrates that bladed calcite crystals are mostly non-luminescent. However, in a few thin sections early bladed cements exhibit a bright luminescence. In the Ilam limestones equant cement is present (Fig. 3D). The coarse equant cements are mostly of meteoric origin. Syntaxial or epitaxial overgrowth cements grow as single crystals on echinoderm grains (Fig. 4A and B). Two kinds of syntaxial overgrowth cement are recognized in the Ilam Formation: (1) turbid (marine) syntaxial and (2) clear syntaxial overgrowth cements (meteoric origin). Poikilotopic sparry calcite cement is recognized by its very large crystals with the ability to enclose several grains (Fig. 4C). Under cathodoluminescence these cements typi-
cally have a dull luminescence (Fig. 4B), indicating a burial origin. 5. Facies analysis and depositional environment Detailed petrographic investigations have led to the recognition of four microfacies belts: tidal flat, lagoon, shoal and open marine. 5.1. Tidal flat The tidal flat sediments are composed of mudstone with fenestral fabric, and dolomicrite (Fig. 5A and B). Fenestral lime mudstones are interpreted to have been deposited in the supratidal environment. Dolomicrite facies ranging in size from 20 to 70 lm, contain wind-blown, silt-size quartz grains. Some researchers believe that dolomicrite forms during very early diagenesis in supratidal to intertidal environments (Sibley and Gregg, 1987; Gregg and Shelton, 1990). 5.2. Lagoon From the shoreline towards the seas, this facies consists of bioclast mudstone wackestone, bioclast wackestone and bioturbated mudstone wackestone (Fig. 5C–E). Restricted conditions are suggested by bioturbation, the lack of a normal-marine biota and abundant skeletal components of restricted biota (benthic foraminifera such as miliolids). 5.3. Shoal Shoal sediments are composed of bioclast peloidal packstone, ooid grainstone and ooid intraclast grainstone
Fig. 3. (A) Isopachous fibrous cement around a grain, ppl, (B) same area as in (A) under cathodoluminescence. This cement shows a dull luminescence indicating a marine origin, (C) bladed cement with scalenohedral terminations, (D) equant cement.
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Fig. 4. (A) Syntaxial cement around an echinoderm, ppl, (B) same area as in (A) under cathodoluminescence which shows bright luminescence, possibly of meteoric origin, (C) Poikilotopic cement surrounding grains, (D) same area as in (C) under cathodoluminescence, showing dark to non-luminescence, indicating burial origin .
Fig. 5. Tidal flat facies: (A) mudstone with fenestral and geopetal fabrics, (B) dolomicrite with silt-size quartz grains; lagoon facies: (C) bioclast mudstone wackestone includes echinoderm spine, (D) bioclast wackestone includes benthic foraminifera (Nezzazata), (E) bioturbated mudstone wackestone.
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(Fig. 6A–C). The shoal facies was deposited in the platform margin subenvironment, separating the open marine and restricted lagoon environments. The abundance of ooids and the mud-free nature of this facies indicate high energy conditions (Tucker et al., 1993). Bioclast peloidal packstone and ooid packstone occur in the leeward part of the lagoon. 5.4. Open marine The open marine facies includes bioclast ooid packstone and bioclast wackestone. The open marine facies belt was developed at the seaward end of the platform margin where the shoal facies graded to the open marine bioclastic wackestone (Fig. 6D). This formation is interpreted to be a carbonate ramp with a very gentle slope (Fig. 7). The lack of any marginal reef development, absence of major break of slope from shoreline into deeper water, and the presence of landward high-energy grainstone facies is consistent with the Ilam Formation having been deposited on a carbonate ramp (Wright, 1986; Tucker et al., 1993).
Isopachous fibrous marine cements in the Ilam limestone were probably aragonite in origin which is the dominant marine cement in modern warm-water carbonates (Given and Wilkinson, 1985; Fig. 3A and B). The occurrence of spalled ooids, where outer laminae are detached from inner laminae, may have resulted from the dissolution of aragonite (Adabi and Rao, 1991). 6.2. Geochemistry
6.1. Petrographic evidence of aragonite mineralogy
6.2.1. Major and minor elements 6.2.1.1. Strontium. The concentration of Sr in recent tropical carbonate sediments ranges from 8000 to 10,000 ppm (Milliman, 1974), whereas in recent temperate carbonates it ranges from 1642 to 5007 ppm (Adabi and Rao, 1991). The Sr content varies due to carbonate mineralogy. Sr increases with increasing aragonite content (Adabi and Rao, 1991) and decreases with increasing calcite content. Concentrations of Sr have also been directly related to increasing water temperature (Morse and Mackenzie, 1990). The concentration of Sr in the Ilam limestone samples ranges from 107 to 1275 ppm (Fig. 8A). Sr values of these samples indicate moderate diagenesis. This is due to the replacement of aragonite by calcite, probably during two stages of diagenetic stabilization (Al-Aasm and Veizer, 1986).
The occurrence of diverse skeletal grains, abundant ooids, peloids and some intraclasts and early diagenetic dolomites in the Ilam limestone are similar to that in modern, tropical, warm shallow-marine waters (Lees, 1975).
6.2.1.2. Sodium. Concentration of Na in recent tropical abiotic aragonite ranges from 1500 to 2700 ppm (2500 ppm; Land and Hoops, 1973; Veizer, 1983; Rao and Adabi, 1992). Na concentrations in carbonate sediments are related to salinity, biological fractionation, kinetics, miner-
6. Determination of original carbonate mineralogy
Fig. 6. Shoal facies: (A) bioclast peloidal packstone, (B) ooid grainstone, (C) ooid intraclast grainstone; marine facies: (D) bioclast wackestone.
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terparts. The low Na concentrations indicate that the Ilam Formation carbonates have recrystallized during burial marine settings which will result in the loss of Na from the carbonates. The plot of Sr–Na values shows that most limestone samples fall within or close to the warm-water subtropical aragonite fields of the Mozduran Formation (Adabi and Rao, 1991) and the Gordon Group limestone of Tasmania, Australia (Rao, 1991; Fig. 8A).
Fig. 7. Schematic diagram of a ramp platform environment of Ilam carbonates.
alogy and water depth (Land and Hoops, 1973; Morrison and Brand, 1986; Rao and Adabi, 1992). Concentration of Na in the Ilam limestone samples ranges from 105 to 372 ppm (Fig. 8A). Na values are lower than those of modern warm-water aragonitic coun-
6.2.1.3. Manganese. The concentration of Mn in the Ilam limestone samples ranges from 13 to 63 ppm. In modern warm-water aragonite, Mn and Fe concentrations are less than 20 ppm (Milliman, 1974). The Sr–Mn variations show that some samples fall in the aragonite field (Fig. 8B). The low Mn concentration may indicate an original aragonite mineralogy. The bivariate plot of Sr/Ca versus Mn shows that the limestones have been stabilized by fluids in a closed diagenetic system (Brand and Veizer, 1980; Fig. 8C).
Fig. 8. (A) Sr and Na variations in the Ilam Formation, compared with fields of subpolar cold-water Permian limestone, subtropical warm-water Ordovician aragonite (Rao, 1990, 1991), recent tropical shallow-marine aragonite (Milliman, 1974), and recent temperate bulk carbonate (Rao and Adabi, 1992; Rao and Jayawardane, 1994; Rao and Amini, 1995). Note that all data falls within the aragonitic fields due to similar mineralogy. (B) Sr and Mn variations in the Ilam limestones. Note that the Ilam carbonate samples are close to the aragonite fields of Mozduran and Gordon Limestones. Fields are similar to (A). (C) Mn and Sr/Ca variations in the Ilam carbonates. This trend shows that these carbonates were affected by marine phreatic fluids in a closed diagenetic system (Brand and Veizer, 1980). (D). Mn and Sr/Na variations in the Ilam limestones. Note that the Sr/Na ratio of all samples are >1, indicating original aragonite mineralogy. Some data also fall within the recent warm-water aragonite field. Fields are similar to (A).
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6.2.1.4. Sr/Na ratio. Modern and ancient tropical carbonates differ from their non-tropical counterparts by their Sr/Na ratio and Mn contents (Rao, 1991; Winefield et al., 1996). Modern tropical aragonitic sediments have low Mn, and high Sr/Na ratio from (3 to 5); in contrast, modern temperate bulk carbonates have high Mn, and low Sr/Na ratios (1, Fig. 8D). Subpolar Permian cold-water fossils and the Permian subpolar bulk cold-water limestones also have a Sr/Na ratio of 1 (Rao, 1991). In the Ilam limestone (Fig. 8D), Sr/Na concentrations range from 1.44 to 7.5 (mean 3); this is similar to recent warm-water aragonite (Milliman, 1974).
Fig. 9. Comparison of d18O and d13C values of the Ilam carbonates with recent polar bulk carbonates (Adabi, 1996), recent temperate bulk carbonates (Rao and Adabi, 1992), recent shallow water limestone (Milliman, 1974) and Cretaceous marine limestone (Kelth and Weber, 1964).
6.2.2. Oxygen and carbon isotopes The d18O values in the Ilam limestone range from 3.8‰ to 4.8‰ PDB (mean 4.2‰ PDB), whereas d13C values range from 0.9‰ to 3.65‰ PDB (mean 2.3‰ PDB, Fig. 9). d18O–d13C values from the Ilam Formation suggest diagenetic alteration in a marine phreatic setting. The rela-
Fig. 10. Variations of Sr (A), Sr/Na (B) and Sr/Ca (C) along the thickness of the Ilam and Sarvak carbonate sequences.
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tively higher concentrations of Sr and Na in the Ilam Formation, compared to abiotic calcite, also support this conclusion. It is important to note that most samples fall within the Cretaceous marine isotopic fields, indicating similar age, and minor diagenetic effects. Bivariate plots of trace elements versus carbon and oxygen isotope values were used to determine original carbonate mineralogy. 7. Recognition of boundary between Ilam and Sarvak Formations by geochemical analysis In view of the similar lithology, the recognition of the exact boundary between the Ilam and Sarvak Formations in the study area is difficult. Thus it was not possible to determine the boundary between these two formations based only on petrography and field observations. The elemental and isotopic compositions of Ilam and Sarvak Formations were used to determine the exact boundary. 7.1. Variation of Sr, Sr/Na and Sr/Ca In carbonates, Sr values are used to indicate an original aragonite mineralogy (e.g., Veizer, 1983; Wilkinson and Algeo, 1989; Adabi and Rao, 1991). Winefield et al. (1996) suggested that the Sr/Na ratio is useful in differentiating different carbonate facies. In this case, variations of Sr, and the ratio of Sr/Na and Sr/Ca, are useful tools for distinguishing the Ilam and Sarvak Formations (Fig. 10). Variations of Sr, Sr/Na and Sr/ Ca in the Sarvak Formation are lower than in the Ilam For-
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mation, probably as a result of a different physico-chemical condition of seawater during Ilam and Sarvak deposition.
7.2. Carbon and oxygen isotopes Mean d18O values for the Sarvak Formation (5.37‰ PDB) are lighter than mean d18O values of the Ilam Formation (4.40‰ PDB). Thus, this difference can be used to recognize the boundary between the Ilam and Sarvak Formations within the stratigraphic succession (Fig. 11A). In a similar way to d18O, the d13C values in the Ilam Formation are useful for the recognition of the stratigraphic boundary between the two formations. All carbon isotope values are positive in the Ilam Formation, in contrast to negative d13C values in the Sarvak Formation (Fig. 11B). The C–O isotope data, along with elemental compositions indicate clearly that this is a subaerial exposure surface, below which meteoric diagenesis affected (at least) the upper few metres of the Sarvak Formation (Algeo et al., 1992; Algeo, 1996). Variations of Sr, Sr/Na, Sr/Ca and d18O and d13C values indicate that the boundary between the Ilam and Sarvak Formations is located around 16 m from the base of the section. The least-altered carbonate sample, with a d18O value of 3.8‰ PDB, was used to calculate a temperature during the relatively shallow burial, using the equation of Anderson and Arthur, 1983:
T ð CÞ ¼ 16 4:14ðdC dW Þ þ 0:13ðdC dW Þ
2
Fig. 11. (A) Variation of d18O along the stratigraphic column of Ilam and Sarvak Formations. (B) Variation of d13C along the stratigraphic column of the Ilam and Sarvak Formations. Note that the oxygen and carbon isotope data indicate that Ilam carbonates have stabilized in the marine phreatic environment, while the negative d13C values of Sarvak Formation indicate a subaerial exposure surface, below which meteoric diageneis influenced the upper few metres of the Sarvak Formation.
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where T is temperature (in °C), dC is the heaviest oxygen isotope value in studied samples and dW is the oxygen isotope value of marine water in the Cretaceous (in SMOW), i.e. 1‰ SMOW (Barron, 1983). This calculation gives an early shallow burial fluid temperature of about 28 °C. 8. Conclusions The present study indicates that the Ilam Formation was deposited on a carbonate ramp with four microfacies: tidal flat, lagoon, shoal and open marine. Bivariate plots of minor and major elements, oxygen and carbon isotope values, along with petrographic studies indicate that the original carbonate mineralogy was dominantly aragonite in the Ilam Formation. Major and minor element variations (such as Sr, Sr/Na and Sr/Ca) and oxygen and carbon isotope values can be used to distinguish the boundary between the Ilam and Sarvak Formations in the Izeh zone. Geochemical results clearly indicate that this is subaerial exposure surface, and that at least the upper few metres of the Sarvak Formation were affected by meteoric diagenesis. A temperature calculation based on the heaviest oxygen isotope value indicates that the very early, shallow burial fluid temperature was around 28 °C during Ilam carbonate deposition. Variations of d18O values versus Mn and Sr/Ca versus Mn suggest that the Ilam carbonates were stabilized in the marine phreatic environment. Acknowledgments The authors thank the School of Earth Sciences, Shahid Beheshti University, Iran, for elemental analysis, the Central Science Lab, University of Tasmania, Australia for isotope analysis, National Iranian South Oil Company (NISOC) for logistic support and Research Institute of Iranian Oil Company for CL Photomicrographs. We appreciate the helpful comments and careful review of this manuscript by Dr. Zahra Amini and Dr. Paul Davidson. References Adabi, M.H., 1996. Sedimentology and geochemistry of Upper Jurassic (Iran) and Precambrian (Tasmania) carbonates. Unpublished Ph.D. Thesis, University of Tasmania, Australia, 407 p. Adabi, M.H., 2004. A re-evaluation of aragonite versus calcite seas. Carbonates and Evaporites 19, 133–141. Adabi, M.H., Rao, C.P., 1991. Petrographic and geochemical evidence for original aragonitic mineralogy of Upper Jurassic carbonates (Mozduran Formation), Sarakhs area, Iran. Sedimentary Geology 72, 253–267. Al-Aasm, I.S., Veizer, J., 1986. Diagenetic stabilization of aragonite and low-Mg calcite, I. Trace element in rudists. Journal of Sedimentary Petrology 56, 138–152. Algeo, T.J., 1996. Meteoric water/rock ratios and the significance of sequence and parasequence boundaries in the Gobbler Formation (Middle Pennsylvanian) of south-central New Mexico. In: Witzke, B.J., Ludvigson, G.A., Day, J. (Eds.), Paleozoic Sequence Stratigra-
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M.H. Adabi, E.A. Mehmandosti / Journal of Asian Earth Sciences 33 (2008) 267–277 Rao, C.P., 1990. Petrography, trace elements and oxygen and carbon isotopes of Gordon Group carbonate (Ordovician), Florentine Valley, Tasmania, Australia. Sedimentary Geology 66, 83–97. Rao, C.P., 1991. Geochemical differences between subtropical (Ordovician), temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, Tasmania, Australia. Carbonates and Evaporites 6, 83–106. Rao, C.P., Adabi, M.H., 1992. Carbonate minerals, major and minor elements and oxygen and carbon isotopes and their variation with water depth in cool, temperate carbonates, western Tasmania, Australia. Marine Geology 103, 249–272. Rao, C.P., Amini, Z.Z., 1995. Faunal relationship to grain-size, mineralogy and geochemistry in recent temperate shelf carbonates, western Tasmania, Australia. Carbonates and Evaporites 10, 114–123. Rao, C.P., Jayawardane, M.P.J., 1994. Major minerals, elemental and isotopic composition in modern temperate shelf carbonates, eastern Tasmania, Australia: implications for the occurrence of extensive ancient non-tropical carbonates. Palaeogeography, Palaeoclimatology, Palaeoecology 107, 49–63. Robinson, P., 1980. Determination of calcium, magnesium, manganese, strontium and iron in the carbonate fraction of limestones and dolomites. Chemical Geology 28, 135–146. Sandberg, P.A., 1983. An oscillating trend in Phanerozoic nonskeletal carbonate mineralogy. Nature 305, 19–22. Sandberg, P.A., 1985. Aragonite cements and their occurrence in ancient limestones. In: Schneidermann, N., Harris, P.M. (Eds.), Carbonate Cements, vol. 36. Society of Economic Paleontology and Mineralogy, Special Publication, pp. 33–57.
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Sibley, D.F., Gregg, J.M., 1987. Classificatin of dolomite rock texture. Journal of Sedimentary Petrology 57, 967–975. Stanly, M.S., Hardie, L.A., 1998. Secular oscillations in the carbonate mineralogy of reef-building and sediment- producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeography, Palaeoclimatology, Palaeoecology 144, 3–19. Tucker, M.E., Calvet, F., Hunt, D., 1993. Sequence stratigraphy of carbonate ramps: systems tracts, models and application to the Muschelkalk carbonate platform of eastern Spain. In: Posamentier, H.W., Summerhayes, C.P., Haq, B.U., Allen, G.P. (Eds.), Sequence Stratigraphy and Facies Associations, vol. 18. International Association of Sedimentology, Special Publication, pp. 397–415. Veizer, J., 1983. Trace elements and isotopes in sedimentary carbonates. Reviews in Mineralogy 11, 265–300. Wilkinson, B.H., Algeo, T.J., 1989. Sedimentary carbonate record of calcium-magnesium cycling. American Journal of Science 289, 1158– 1194. Wilkinson, B.H., Owen, R.M., Carroll, A.R., 1985. Submarine hydrothermal weathering, global eustasy, and carbonate polymorphism in Phanerozoic marine oolites. Journal of Sedimentary Petrology 55, 171– 183. Winefield, P.R., Nelson, C.S., Hodder, A.P.W., 1996. Discriminating temperate carbonates and their diagenetic environments using bulk elemental geochemistry: a reconnaissance study based on New Zealand Cenozoic limestones. Carbonates and Evaporites 11, 19–31. Wright, V.P., 1986. Facies sequences on a carbonate ramp: the Carboniferous Limestone of South Wales. Sedimentology 33, 221–241.
Journal of Asian Earth Sciences 33 (2008) 267–277 www.elsevier.com/locate/jaes
Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran Mohammad H. Adabi *, Elham Asadi Mehmandosti School of Earth Sciences, Shahid Beheshti University, Evin, Velenjak Daneshjo Blv., Tehran, Iran Received 3 November 2006; received in revised form 5 January 2008; accepted 8 January 2008
Abstract The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran. Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and d18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C. Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The d18O and d13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Microfacies; Geochemistry; Ilam Formation; Zagros; Iran
1. Introduction Aragonite is the predominant mineral, along with some high-Mg calcite, forming in modern tropical warm waters (Milliman, 1974). In modern temperate carbonates, highMg calcite predominates over low-Mg calcite, with minor amounts of aragonite (Rao, 1981a; Nelson, 1988). In subpolar cold-water carbonates, low-Mg calcite is the dominant mineral (Rao, 1981b). Experimental studies (Kinsman and Holland, 1969; Burton and Walter, 1987) indicate that low-Mg calcite forms alone in water temperatures of <5 °C from normal seawa*
Corresponding author. Tel.: +98 61362233602; fax: +98 21 22431690. E-mail addresses: [email protected], [email protected] (M.H. Adabi). 1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2008.01.002
ter, and also confirms that primary carbonate morphology and mineralogy depend primarily on water temperatures, similar to observations for modern carbonates. Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg, 1983; Wilkinson and Algeo, 1989). Variation in carbonate mineralogy has been related to the position of global sea level (Wilkinson et al., 1985), changes in rates of seafloor spreading (e.g. Mackenzie and Pigott, 1981; Hardie, 1996), PCO2 level (e.g. Sandberg, 1985; Makenzie and Morse 1992; Hallock, 1997) and Mg/Ca ratio related to spreading rate (e.g. Stanly and Hardie, 1998). However, other researchers suggested that the
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assumption of a change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitude (e.g. Nelson, 1988; Adabi, 2004). Evaluation of Ordovician Gordon Group carbonates of Tasmania (Australia), and the Upper Jurassic Mozduran limestone in the Kopet-Dagh Basin in north-east Iran, based on petrographic and geochemical criteria, indicate that aragonite, not calcite, was the dominate mineral in these warm-water, subtropical carbonates (Rao, 1991; Adabi and Rao, 1991). In the Recent, aragonite is the predominant mineral in warm, shallow-marine carbonates and calcite the dominant mineral in marine cool-water carbonates (James and Clarke, 1997). The present study uses petrographic evidence, elemental and d18O and d13C values and compares these with modern tropical (aragonitic) and temperate (calcitic) carbonates and originally aragonitic Ordovician subtropical carbonates and originally calcite Permian subpolar cold-water carbonates of Tasmania, and aragonitic shallow subtropical carbonates of the Upper Jurassic Mozduran Formation of Iran to understand the original carbonate mineralogy, depositional facies and diagenetic characteristics of the Ilam Formation. In addition, the geochemical evidence was used to determine the boundary between the Ilam and Sarvak Formations, which is difficult to place due to similar lithologies in these formations. 2. Geological setting The study area is located 18.5 km north of Izeh, Zagros area, Iran (Fig. 1). The Ilam Formation (Santonian–Campanian), which is part of the Bangestan Group consists mainly of fossiliferous limestone (Fig. 2). The type section of the Ilam Formation is situated in the Kabirkoh area, Lurestan, and is overlain by the Surgah
Formation and underlain by the Gurpi Formation. However, in the studied area this unit, conformably and without any recognizable boundary, overlies the Sarvak Formation, the Surgah Formation being locally absent. The thickness of the Ilam Formation in the study area is 95 m, and it contains macro- and microfossils of algae, echinoderms, bivalves, gastropods and benthic and pelagic foraminifera. 3. Methods of study Samples were collected near significant lithological changes, especially near the probable boundaries. One hundred and twenty uncovered thin sections were studied for petrographic and facies analysis. Thin sections were stained by potassium ferricyanide and alizarin-red S solution (Dickson, 1965). Forty-two powdered micritic samples were analysed by atomic absorption spectrometer for Ca, Mg, Sr, Na, Mn, Fe at the Geology Department of the Shahid Beheshti University, Tehran, Iran. Precision was ±0.5% for Ca and Mg and ±5 ppm for Sr, Na, Mn and Fe (Robinson, 1980). Twenty powdered samples which had previously been analysed for major and minor elements, were analysed with a Micromass, 602D for oxygen and carbon isotopes at the Central Science Laboratory, University of Tasmania, Australia. Fifteen mg of powdered samples were allowed to react with anhydrous phosphoric acid in reaction tubes under vacuum at 25 °C for 24 h. The CO2 extract from each sample was analysed for d18O and d13C by mass spectrometry. Precision of data is ±0.1‰ for both d18O and d13C and these values were reported relative to PDB. Selected samples were observed with a cathodoluminescence microscope (Nikon Cl, CCL 8200) at the Research Institute of Petroleum Industry (R.I.P.I).
Fig. 1. Location map of the study area in Tang-E Rashid, Izeh.
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Fig. 2. Stratigraphic column of the Ilam Formation in the study area.
4. Petrography The Ilam carbonates consist of a large variety of skeletal and non-skeletal grains, calcite cements, micrite and early and late diagenetic dolomites. 4.1. Skeletal grains Skeletal grains are mostly bivalves, crinoids, gastropods, algae and abundant benthic and pelagic foraminifera. Radiolitidae and Hippuritidae are the two main kinds of rudist. Red algae (Permocalculus and Hensonella) are more abundant than green algae (Dasycladacea). Miliolids, Nezzazata, Dicyclina are common and Rotalia skurnsis is the major type of benthic foraminifera in the Ilam Formation. 4.2. Non-skeletal grains Non-skeletal grains are abundant and consist mainly of ooids, intraclast and peloids. The nuclei of ooids are commonly peloids, and in some examples the nuclei consist of fossil fragments such as echinoderms, gastropod and brachiopod. The cortex thickness of ooids is inversely related
to the size of the nucleus. In some examples, subhedral to almost euhedral dolomite rhombs partially to completely obliterate the ooid structure. Different types of microstructure are recognized in the Ilam oolites: tangential, radial and composite structures. Composite (compound) ooids are the least abundant ooid type in the carbonate samples studied. Peloids are also abundant. Most pellets in the Ilam carbonates are uniform in size, but in a few samples they show considerable irregularity. Pellets in all samples range from 0.05 to 0.2 mm (average 0.125 mm). It is assumed that the small, even-size, well-sorted, uniform-shaped and dark coloured peloids in the Ilam limestone are faecal pellets. Some pellets were trapped by rudists. Intraclasts occur in some samples. They are generally polymodal in size ranging from 0.6 to 2.3 mm. Most intraclasts are internally homogenous and consist of micrite, while others contain bioclasts and peloids. 4.3. Cementation Different generations of sparry calcite cement were recognized in the Ilam limestone, ranging from marine through meteoric to some burial cements.
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Isopachous cements form fringes around grains (Fig. 3A). These cements were possibly composed of aragonite, due to the identical morphology to that of recent warm-water shallow-marine aragonitic cements (Given and Wilkinson, 1985; Adabi and Rao, 1991). Cathodoluminescence petrography shows that the acicular to fibrous cements are dull to non-luminescent (Fig. 3B), indicating a marine origin (Machel, 1985). Other petrographic evidence which supports marine origin are: (1) they are the first generation cements which predate other cements, (2) they have acicular to fibrous fabrics, (3) they commonly form isopachous fringes around the grains or cavity wall, (4) they are succeeded by bladed, equant cement and then followed by later coarsely crystalline sparry calcite cement. In the Ilam limestones, bladed calcite cements occur both interparticle, and within cavities (Fig. 3C). Individual crystals often show a bladed crystal morphology with scalenohedral terminations. The bladed cement in the Ilam limestones is commonly followed by a later generation of equant, drusy calcite. Cathodoluminescence petrography illustrates that bladed calcite crystals are mostly non-luminescent. However, in a few thin sections early bladed cements exhibit a bright luminescence. In the Ilam limestones equant cement is present (Fig. 3D). The coarse equant cements are mostly of meteoric origin. Syntaxial or epitaxial overgrowth cements grow as single crystals on echinoderm grains (Fig. 4A and B). Two kinds of syntaxial overgrowth cement are recognized in the Ilam Formation: (1) turbid (marine) syntaxial and (2) clear syntaxial overgrowth cements (meteoric origin). Poikilotopic sparry calcite cement is recognized by its very large crystals with the ability to enclose several grains (Fig. 4C). Under cathodoluminescence these cements typi-
cally have a dull luminescence (Fig. 4B), indicating a burial origin. 5. Facies analysis and depositional environment Detailed petrographic investigations have led to the recognition of four microfacies belts: tidal flat, lagoon, shoal and open marine. 5.1. Tidal flat The tidal flat sediments are composed of mudstone with fenestral fabric, and dolomicrite (Fig. 5A and B). Fenestral lime mudstones are interpreted to have been deposited in the supratidal environment. Dolomicrite facies ranging in size from 20 to 70 lm, contain wind-blown, silt-size quartz grains. Some researchers believe that dolomicrite forms during very early diagenesis in supratidal to intertidal environments (Sibley and Gregg, 1987; Gregg and Shelton, 1990). 5.2. Lagoon From the shoreline towards the seas, this facies consists of bioclast mudstone wackestone, bioclast wackestone and bioturbated mudstone wackestone (Fig. 5C–E). Restricted conditions are suggested by bioturbation, the lack of a normal-marine biota and abundant skeletal components of restricted biota (benthic foraminifera such as miliolids). 5.3. Shoal Shoal sediments are composed of bioclast peloidal packstone, ooid grainstone and ooid intraclast grainstone
Fig. 3. (A) Isopachous fibrous cement around a grain, ppl, (B) same area as in (A) under cathodoluminescence. This cement shows a dull luminescence indicating a marine origin, (C) bladed cement with scalenohedral terminations, (D) equant cement.
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Fig. 4. (A) Syntaxial cement around an echinoderm, ppl, (B) same area as in (A) under cathodoluminescence which shows bright luminescence, possibly of meteoric origin, (C) Poikilotopic cement surrounding grains, (D) same area as in (C) under cathodoluminescence, showing dark to non-luminescence, indicating burial origin .
Fig. 5. Tidal flat facies: (A) mudstone with fenestral and geopetal fabrics, (B) dolomicrite with silt-size quartz grains; lagoon facies: (C) bioclast mudstone wackestone includes echinoderm spine, (D) bioclast wackestone includes benthic foraminifera (Nezzazata), (E) bioturbated mudstone wackestone.
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(Fig. 6A–C). The shoal facies was deposited in the platform margin subenvironment, separating the open marine and restricted lagoon environments. The abundance of ooids and the mud-free nature of this facies indicate high energy conditions (Tucker et al., 1993). Bioclast peloidal packstone and ooid packstone occur in the leeward part of the lagoon. 5.4. Open marine The open marine facies includes bioclast ooid packstone and bioclast wackestone. The open marine facies belt was developed at the seaward end of the platform margin where the shoal facies graded to the open marine bioclastic wackestone (Fig. 6D). This formation is interpreted to be a carbonate ramp with a very gentle slope (Fig. 7). The lack of any marginal reef development, absence of major break of slope from shoreline into deeper water, and the presence of landward high-energy grainstone facies is consistent with the Ilam Formation having been deposited on a carbonate ramp (Wright, 1986; Tucker et al., 1993).
Isopachous fibrous marine cements in the Ilam limestone were probably aragonite in origin which is the dominant marine cement in modern warm-water carbonates (Given and Wilkinson, 1985; Fig. 3A and B). The occurrence of spalled ooids, where outer laminae are detached from inner laminae, may have resulted from the dissolution of aragonite (Adabi and Rao, 1991). 6.2. Geochemistry
6.1. Petrographic evidence of aragonite mineralogy
6.2.1. Major and minor elements 6.2.1.1. Strontium. The concentration of Sr in recent tropical carbonate sediments ranges from 8000 to 10,000 ppm (Milliman, 1974), whereas in recent temperate carbonates it ranges from 1642 to 5007 ppm (Adabi and Rao, 1991). The Sr content varies due to carbonate mineralogy. Sr increases with increasing aragonite content (Adabi and Rao, 1991) and decreases with increasing calcite content. Concentrations of Sr have also been directly related to increasing water temperature (Morse and Mackenzie, 1990). The concentration of Sr in the Ilam limestone samples ranges from 107 to 1275 ppm (Fig. 8A). Sr values of these samples indicate moderate diagenesis. This is due to the replacement of aragonite by calcite, probably during two stages of diagenetic stabilization (Al-Aasm and Veizer, 1986).
The occurrence of diverse skeletal grains, abundant ooids, peloids and some intraclasts and early diagenetic dolomites in the Ilam limestone are similar to that in modern, tropical, warm shallow-marine waters (Lees, 1975).
6.2.1.2. Sodium. Concentration of Na in recent tropical abiotic aragonite ranges from 1500 to 2700 ppm (2500 ppm; Land and Hoops, 1973; Veizer, 1983; Rao and Adabi, 1992). Na concentrations in carbonate sediments are related to salinity, biological fractionation, kinetics, miner-
6. Determination of original carbonate mineralogy
Fig. 6. Shoal facies: (A) bioclast peloidal packstone, (B) ooid grainstone, (C) ooid intraclast grainstone; marine facies: (D) bioclast wackestone.
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terparts. The low Na concentrations indicate that the Ilam Formation carbonates have recrystallized during burial marine settings which will result in the loss of Na from the carbonates. The plot of Sr–Na values shows that most limestone samples fall within or close to the warm-water subtropical aragonite fields of the Mozduran Formation (Adabi and Rao, 1991) and the Gordon Group limestone of Tasmania, Australia (Rao, 1991; Fig. 8A).
Fig. 7. Schematic diagram of a ramp platform environment of Ilam carbonates.
alogy and water depth (Land and Hoops, 1973; Morrison and Brand, 1986; Rao and Adabi, 1992). Concentration of Na in the Ilam limestone samples ranges from 105 to 372 ppm (Fig. 8A). Na values are lower than those of modern warm-water aragonitic coun-
6.2.1.3. Manganese. The concentration of Mn in the Ilam limestone samples ranges from 13 to 63 ppm. In modern warm-water aragonite, Mn and Fe concentrations are less than 20 ppm (Milliman, 1974). The Sr–Mn variations show that some samples fall in the aragonite field (Fig. 8B). The low Mn concentration may indicate an original aragonite mineralogy. The bivariate plot of Sr/Ca versus Mn shows that the limestones have been stabilized by fluids in a closed diagenetic system (Brand and Veizer, 1980; Fig. 8C).
Fig. 8. (A) Sr and Na variations in the Ilam Formation, compared with fields of subpolar cold-water Permian limestone, subtropical warm-water Ordovician aragonite (Rao, 1990, 1991), recent tropical shallow-marine aragonite (Milliman, 1974), and recent temperate bulk carbonate (Rao and Adabi, 1992; Rao and Jayawardane, 1994; Rao and Amini, 1995). Note that all data falls within the aragonitic fields due to similar mineralogy. (B) Sr and Mn variations in the Ilam limestones. Note that the Ilam carbonate samples are close to the aragonite fields of Mozduran and Gordon Limestones. Fields are similar to (A). (C) Mn and Sr/Ca variations in the Ilam carbonates. This trend shows that these carbonates were affected by marine phreatic fluids in a closed diagenetic system (Brand and Veizer, 1980). (D). Mn and Sr/Na variations in the Ilam limestones. Note that the Sr/Na ratio of all samples are >1, indicating original aragonite mineralogy. Some data also fall within the recent warm-water aragonite field. Fields are similar to (A).
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6.2.1.4. Sr/Na ratio. Modern and ancient tropical carbonates differ from their non-tropical counterparts by their Sr/Na ratio and Mn contents (Rao, 1991; Winefield et al., 1996). Modern tropical aragonitic sediments have low Mn, and high Sr/Na ratio from (3 to 5); in contrast, modern temperate bulk carbonates have high Mn, and low Sr/Na ratios (1, Fig. 8D). Subpolar Permian cold-water fossils and the Permian subpolar bulk cold-water limestones also have a Sr/Na ratio of 1 (Rao, 1991). In the Ilam limestone (Fig. 8D), Sr/Na concentrations range from 1.44 to 7.5 (mean 3); this is similar to recent warm-water aragonite (Milliman, 1974).
Fig. 9. Comparison of d18O and d13C values of the Ilam carbonates with recent polar bulk carbonates (Adabi, 1996), recent temperate bulk carbonates (Rao and Adabi, 1992), recent shallow water limestone (Milliman, 1974) and Cretaceous marine limestone (Kelth and Weber, 1964).
6.2.2. Oxygen and carbon isotopes The d18O values in the Ilam limestone range from 3.8‰ to 4.8‰ PDB (mean 4.2‰ PDB), whereas d13C values range from 0.9‰ to 3.65‰ PDB (mean 2.3‰ PDB, Fig. 9). d18O–d13C values from the Ilam Formation suggest diagenetic alteration in a marine phreatic setting. The rela-
Fig. 10. Variations of Sr (A), Sr/Na (B) and Sr/Ca (C) along the thickness of the Ilam and Sarvak carbonate sequences.
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tively higher concentrations of Sr and Na in the Ilam Formation, compared to abiotic calcite, also support this conclusion. It is important to note that most samples fall within the Cretaceous marine isotopic fields, indicating similar age, and minor diagenetic effects. Bivariate plots of trace elements versus carbon and oxygen isotope values were used to determine original carbonate mineralogy. 7. Recognition of boundary between Ilam and Sarvak Formations by geochemical analysis In view of the similar lithology, the recognition of the exact boundary between the Ilam and Sarvak Formations in the study area is difficult. Thus it was not possible to determine the boundary between these two formations based only on petrography and field observations. The elemental and isotopic compositions of Ilam and Sarvak Formations were used to determine the exact boundary. 7.1. Variation of Sr, Sr/Na and Sr/Ca In carbonates, Sr values are used to indicate an original aragonite mineralogy (e.g., Veizer, 1983; Wilkinson and Algeo, 1989; Adabi and Rao, 1991). Winefield et al. (1996) suggested that the Sr/Na ratio is useful in differentiating different carbonate facies. In this case, variations of Sr, and the ratio of Sr/Na and Sr/Ca, are useful tools for distinguishing the Ilam and Sarvak Formations (Fig. 10). Variations of Sr, Sr/Na and Sr/ Ca in the Sarvak Formation are lower than in the Ilam For-
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mation, probably as a result of a different physico-chemical condition of seawater during Ilam and Sarvak deposition.
7.2. Carbon and oxygen isotopes Mean d18O values for the Sarvak Formation (5.37‰ PDB) are lighter than mean d18O values of the Ilam Formation (4.40‰ PDB). Thus, this difference can be used to recognize the boundary between the Ilam and Sarvak Formations within the stratigraphic succession (Fig. 11A). In a similar way to d18O, the d13C values in the Ilam Formation are useful for the recognition of the stratigraphic boundary between the two formations. All carbon isotope values are positive in the Ilam Formation, in contrast to negative d13C values in the Sarvak Formation (Fig. 11B). The C–O isotope data, along with elemental compositions indicate clearly that this is a subaerial exposure surface, below which meteoric diagenesis affected (at least) the upper few metres of the Sarvak Formation (Algeo et al., 1992; Algeo, 1996). Variations of Sr, Sr/Na, Sr/Ca and d18O and d13C values indicate that the boundary between the Ilam and Sarvak Formations is located around 16 m from the base of the section. The least-altered carbonate sample, with a d18O value of 3.8‰ PDB, was used to calculate a temperature during the relatively shallow burial, using the equation of Anderson and Arthur, 1983:
T ð CÞ ¼ 16 4:14ðdC dW Þ þ 0:13ðdC dW Þ
2
Fig. 11. (A) Variation of d18O along the stratigraphic column of Ilam and Sarvak Formations. (B) Variation of d13C along the stratigraphic column of the Ilam and Sarvak Formations. Note that the oxygen and carbon isotope data indicate that Ilam carbonates have stabilized in the marine phreatic environment, while the negative d13C values of Sarvak Formation indicate a subaerial exposure surface, below which meteoric diageneis influenced the upper few metres of the Sarvak Formation.
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where T is temperature (in °C), dC is the heaviest oxygen isotope value in studied samples and dW is the oxygen isotope value of marine water in the Cretaceous (in SMOW), i.e. 1‰ SMOW (Barron, 1983). This calculation gives an early shallow burial fluid temperature of about 28 °C. 8. Conclusions The present study indicates that the Ilam Formation was deposited on a carbonate ramp with four microfacies: tidal flat, lagoon, shoal and open marine. Bivariate plots of minor and major elements, oxygen and carbon isotope values, along with petrographic studies indicate that the original carbonate mineralogy was dominantly aragonite in the Ilam Formation. Major and minor element variations (such as Sr, Sr/Na and Sr/Ca) and oxygen and carbon isotope values can be used to distinguish the boundary between the Ilam and Sarvak Formations in the Izeh zone. Geochemical results clearly indicate that this is subaerial exposure surface, and that at least the upper few metres of the Sarvak Formation were affected by meteoric diagenesis. A temperature calculation based on the heaviest oxygen isotope value indicates that the very early, shallow burial fluid temperature was around 28 °C during Ilam carbonate deposition. Variations of d18O values versus Mn and Sr/Ca versus Mn suggest that the Ilam carbonates were stabilized in the marine phreatic environment. Acknowledgments The authors thank the School of Earth Sciences, Shahid Beheshti University, Iran, for elemental analysis, the Central Science Lab, University of Tasmania, Australia for isotope analysis, National Iranian South Oil Company (NISOC) for logistic support and Research Institute of Iranian Oil Company for CL Photomicrographs. We appreciate the helpful comments and careful review of this manuscript by Dr. Zahra Amini and Dr. Paul Davidson. References Adabi, M.H., 1996. Sedimentology and geochemistry of Upper Jurassic (Iran) and Precambrian (Tasmania) carbonates. Unpublished Ph.D. Thesis, University of Tasmania, Australia, 407 p. Adabi, M.H., 2004. A re-evaluation of aragonite versus calcite seas. Carbonates and Evaporites 19, 133–141. Adabi, M.H., Rao, C.P., 1991. Petrographic and geochemical evidence for original aragonitic mineralogy of Upper Jurassic carbonates (Mozduran Formation), Sarakhs area, Iran. Sedimentary Geology 72, 253–267. Al-Aasm, I.S., Veizer, J., 1986. Diagenetic stabilization of aragonite and low-Mg calcite, I. Trace element in rudists. Journal of Sedimentary Petrology 56, 138–152. Algeo, T.J., 1996. Meteoric water/rock ratios and the significance of sequence and parasequence boundaries in the Gobbler Formation (Middle Pennsylvanian) of south-central New Mexico. In: Witzke, B.J., Ludvigson, G.A., Day, J. (Eds.), Paleozoic Sequence Stratigra-
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