Petrografía y geoquímica del Miembro de Arenisca Ahwas de la Formación Asmari, Zagros, Irán: Implicaciones para la proveniencia y el ambiente tectónico

Mahdi Jafarzadeh and Mahboobeh Hosseini–Barzi*

Faculty of Earth Science, Geology Department, Shahid Beheshti University, Iran. * hosseini@khayam.ut.ac.ir

Manuscript received: December 3, 2007
Corrected manuscript received: March 2, 2008 ]]> Manuscript accepted: March 3, 2008

ABSTRACT

An integrated petrographic and geochemical study of the Ahwaz Sandstone Member ofOligocene–Miocene age, Asmari Formation in Zagros, southwest Iran, was carried out to infer their provenance and tectonic setting. This study is based on the analysis of core samples from three subsurface sections (wells Az–85, Az–11, andAz–89) in Ahwaz oilfield in the Dezful embayment subzone.

On the basis of the framework composition (point counting) and whole–rock geochemistry (major elements), the sandstones are classified as quartzarenite, sublitharenite, and subarkose types. Petrographic studies reveal that these sandstones contain quartz, feldspars and fragments of sedimentary and metasedimentary rocks. The modal analysis data of 50 collected (medium size and well sorted) samples, imply a recycled orogen and craton interior tectonic provenance. Moreover, petrographic point count data indicate quartz–rich sedimentary (recycled), middle to high–grade metamorphic, andplutonic parent rocks for the Ahwaz Sandstone. Tectonic setting discrimination diagrams based on major elements suggest a quarizose sedimentary provenance in a passive continental margin. As indicated by the CIW index (chemical index of weathering) of the Ahwaz Sandstone (average value of 67) their source area underwent "intense" recycling but "moderate" degree of chemical weathering. The petrography and geochemistry results are consistent with a semiarid climate and low–relief highlands.

Key words: provenance, tectonic setting, sandstone, geochemistry, Ahwaz Sandstone Member, Asmari Formation, Iran.

RESUMEN

Un estudio petrográfico y geoquímico del Miembro de Arenisca Ahwaz del Oligoceno–Mioceno, Formación Asmari en Zagros, suroeste de Iran, fue realizado con el objeto de inferir su proveniencia y el ambiente tectónico. Este estudio se basó en el análisis de muestras de núcleos de tres secciones (pozos Az–85, Az–11, y Az–89) del campo petrolero Ahwaz, en la subzona de la bahía de Dezful.

Usando la abundancia de los minerales principales (conteo de puntos) y la geoquímica de roca total (elementos mayores), las areniscas se clasificaron como cuarzoarenita, sublitoarenita y subarcosas. El estudio petrográfico revela que estas areniscas consisten de cuarzo, feldespatos y fragmentos de rocas sedimentarias y metasedimentarias. El análisis modal de 50 muestras recolectadas de grano medio y bien clasificadas, indica la proveniencia tectónica de un orógeno reciclado y del interior del cratón. Además, los datos petrográficos del conteo de puntos señalan que las rocas parentales fueron rocas sedimentarias ricas en cuarzo (recicladas), metamórficas de grado medio a alto, así como plutónicas. Los diagramas para ambientes tectónicos basados en elementos mayores sugieren una proveniencia sedimentaria rica en cuarzo en un ambiente tectónico de margen pasivo continental. Como lo indica el índice CIW' (índice químico de intemperismo) para las muestras de la Arenisca Ahwaz (con valor promedio de 67) su área fuente fue sujeta a reciclaje intenso, pero a intemperismo químico de grado moderado. Los resultados de la petrografía y la geoquímica son consistentes con un clima semiárido y con terrenos poco elevados (de bajo relieve).

INTRODUCTION

The combination of petrography and geochemistry data of sedimentary rocks can reveal the nature of source regions, the tectonic setting of sedimentary basins, and paleoclimate conditions (e.g., Dickinson and Suczek, 1979; Valloni and Mezzardi, 1984; Bhatia and Crook, 1986; McLennan et al, 1993; Armstrong–Altrin et al, 2004). Basu et al. (1975) and Tortosa et al. (1991) used the frequency of different types of quartz grains to infer the type of source rocks. Thus, simple petrographic descriptions of different quartz constituents canbe utilized for this purpose (Folk, 1974; Blatt et al, 1980; Asiedu et al, 2000). The determination of the tectonic setting of sandstones using the framework mineral composition (detrital modes) was first proposed by Crook (1974) and has since undergone considerable refinement (e.g., Dickinson and Suczek, 1979; Dickinson et al, 1983, Weltje, 2002, Basu, 2003). The main assumption behind sandstone provenance studies is that different tectonic settings contain their own rock types, which when eroded, produce sandstones with specific compositional ranges (e.g., Dickinson and Suczek, 1979; Dickinson e/ al, 1983; Dickinson, 1985).

Although some geochemical ratios can be altered during weathering (through oxidation) (Taylor and McLennan, 1985) and/or diagenesis (Nesbitt and Young, 1989; Milodowski and Zalasiewicz, 1991), as long as the bulk composition of a rock is not totally altered, geochemical analysis is a valuable tool in the study of sandstones (McLennan et al, 1993). Major element discrimination diagrams (e.g., Bhatia, 1983) have been used to discriminate the tectonic settings of sedimentary basins and have been commonly applied in more recent publications (e.g., Kroonenberg, 1994; Zimmermann and Bahlburg, 2003; Armstrong–Altrin et al, 2004), although,caution is required in their indiscriminate use (Armstrong–Altrin and Verma, 2005). The most important clues for the tectonic setting of the basin comes from the relative depletion of oxides like CaO and Na₂O (the most mobile elements) and enrichment of SiO₂ and TiO₂ (the most immobile elements), among others. These oxides are assumed to show enrichment or depletion in quartz, K–feldspar, micas, and plagioclase. The ratio of the most immobile elements to the most mobile ones increases toward passive margins due to the relative tectonic stability (Bhatia 1983; Kroonenberg 1994; Roser and Korsch 1986; Zimmermann and Bahlburg, 2003; Armstrong–Altrin et al, 2004) and hence prolonged weathering. This can be recorded in sediments as paleoclimate index (Nesbitt and Young, 1982; Harnois, 1988; Chittleborough, 1991) and high degree of sediment recycling.

The carbonate rocks of the Asmari Formation (the major oil reservoir in the Zagros mountain) are well studied (Lacassagne, 1963; Seyrafian, 2000; Vaziri–Moghaddam et al, 2006). However, only a few petrography and palaeo–environment studies of Ahwaz Sandstone Member of this formation (e.g., Zahedinezhad, 1987; Buck, 1991) have been reported. In this study, we present new petrography, point count and geochemical data from three subsurface sections (from three wells: Az–85, Az–11 and Az–89) of the Ahwaz Sandstone Member in the Dezful embayment zone, southwest of Zagros structural province (Figure 1). These data were utilized for reconstructing the parent rock assemblages of these sandstones, their tectonic provenance, and the physiographic conditions under which these sediments deposited. Although these kinds of results from Asmari reservoir are of considerable importance for oil industries, the consistency of different approaches (petrography, point counting and geochemistry) in provenance and tectonic setting studies of sandstones like Ahwaz Sandstone is of further significance, being the main reason why we carried out such an integrated study.

GENERAL GEOLOGY OF THE STUDY AREA

The geological evidence suggests that the Zagros region was part of a passive continental margin, which subsequently underwent rifting during the Permo–Triassic and collision during the Late Tertiary (Stocklin, 1974; Berberian and King, 1981; Beydoun et al, 1992). In fact, the Zagros fold–thrust belt lies on the northeastern margin of the Arabian plate and has been divided into NW–SE trending structural zones (imbricated and simply folded belt) parallel to the plate margin separated by major fault zones such as the High Zagros and Mountain Front Faults (Figure 1). In addition to the tectonic divisions parallel to the mountain belt, the belt has also been divided laterally to the Lurestan, Dezful embayment and Fars regions from northwest to southeast.

The Asmari Formation (one of the best–known carbonate reservoirs in the world) was deposited in the Oligocene–Miocene shallow marine environment of the Zagros foreland basin (Alavi, 2004) and it is best developed in the Dezful embayment zone. Lithologically, the Asmari Formation consists of 314 m of limestones, dolomitic limestones, and argillaceous limestones (Motiei, 1993). In the south of Dezful embayment, its lithology changes into a mixed siliciclastic–carbonate deposit consisting of carbonate beds with several intervals of sandstone, sandy limestone and shale. This facies provides the Ahwaz Sandstone Member in some oil fields such as Ahwaz, Marun and Mansuri (Motiei, 1993).

The Ahwaz Sandstone is a shoreface deposit. Their distribution along the strike of the Zagros foreland basin is restricted to the southwestern margin as a number of elongate northwest–southeast siliciclastic bars (Alavi, 2004). Alsharhan and Nairn (1997) considered the Ahwaz Sandstone to correlate with Ghar Formation in Kuwait whose clastic input is believed to have been derived from the pre–rift uplift of the Red Sea to the west (Alsharhan and Nairn, 1997). The localization of the Ghar/Ahwaz delta of southern Iraq and southwestern Iran was probably influenced by deep–seated 'Hercynian'–age (330 Ma) lineaments that extended northward from the Central Arabian Arch (Ziegler, 2001).

SAMPLING AND METHODS

Nine hundred samples were collected from three wells in the Ahwaz oil field (well no. Az–85A, Az–89 and Az–11; 300 samples from each well) to represent the entire thickness of the Asmari Formation and thin sections were prepared, which were etched and stained for calcite and dolomite as matrix. Following a detailed petrographic study of thin sections, 50 thin sections of the Ahwaz Sandstone were selected for modal analysis, among 450 sandstone samples, from the three subsurface sections (12 samples from well Az–85 A, 13 samples from well Az–89 and25 samples from well Az–11) (Figure 2). The sample selection was based on petrography to choose well–sorted and unweathered, fine– to medium–grained, sand–size samples and to cover the entire thickness of the formation. Framework mineral composition (modal analysis) was quantified using the point–counting method of Gazzi–Dickinson as described by Ingersoll et al. (1984).

In the Gazzi–Dickinson method (Ingersoll et al., 1984), minerals > 0.625 mm within lithoclasts are counted acording to the mineral type (phaneritic grains: crystal size exceeding 0.0625 mm; aphanitic grains: crystal size less than 0.0625 mm). Classification of grain types was done following the Dickinson (1985) method (Table 1). The grid spacing used in point counting exceeded the grain size so as to avoid individual grains being counted more than once (e.g., Van Der Plas and Tobi, 1965). In order to reconstruct the original detrital composition of the sandstone, the effects of diagenesis such as calcitization, zeolitization, and albitization of feldspars (Shanmugam, 1985) were taken into consideration as much as possible during counting. Framework grains were counted for 300 to 350 counts per thin section; recalculated modal analysis data from point counting of the framework grains are listed in Table 2.

Chemical analyses (major elements) of nine selected samples (finer sandstones from the three wells were selected because they are likely to provide better geochemical results than the coarser grained rocks) were performed by X–ray fluorescence (XRF) spectrometry technique on fused beads (Rollinson, 1993), at the laboratories of the Geological Society of Iran (Table 3). Analytical precision is better than 3% for the major oxides. Relative errors on major elements are usually <2% and loss on ignition (LOI) was determined by heating the dried samples to 950 °C for 2 hours. Moreover, the total iron is expressed as Fe₂O₃.

DETRITAL FRAMEWORK COMPONENTS AND CLASSIFICATION

Quartz is the dominating framework grain in the studied thin sections (Figure 3a–d; also see Table 2). Monocrystalline quartz (Qm) occurs in three variants: non–undulose, slightly undulóse (<5°), and undulóse (>5°) (Scholle, 1979; Basu, 1985; Tortosa et al., 1991). The polycrystalline quartz (Qpq) was distinguished into two groups: polycrystaline quartz with 2–3 subgrains and polycrystalline quartz with more than 3 subgrains (Figure 3b). Chert was considered as a monomineral (Folk, 1974) and was found only in a few samples. Some of the non–undulatory monocrystalline quartz contain inclusions of apatite and rutile needles, which could have been derived from plutonic source rocks or sedimentary recycling (Figure 3c–d).

All studied thin sections contain small amounts of potassium feldspar (K), including twinned microcline. Furthermore, the low potassium values (mean value for K₂O=0.94±0.40%; n=9) of the bulk rock analyses of sandstone samples, support the results of the petrography concerning the low K–feldspar content (Table 3). Plagioclase occurs in some thin sections and seems to be linked to samples with smaller grain sizes (fine–grained sand). Petrographic study of these sandstones reveals that K–feldspar is more abundant than plagioclase, which can be originated from durability of K–feldspars, non–occurrence of plagioclase–bearing source rocks, or both. The feldspars are fresh, which implies limited chemical weathering.

The thin sections contain lesser amount of lithic fragments than quartz and feldspar grains. Lithoclasts found in the studied samples include: siltstones, sandstones, carbonate fragments (rare), and metamorphic fragments. Volcanic fragments are absent in these sandstones.

Metamorphic lithoclasts are usually made up of metamorphic polycrystalline quartz fragments of high–grade metamorphic origin. The recrystallized subcrystals of a metamorphic polycrystalline quartz are lengthened and display different extinction angles under crossed polars due to the variable orientation of the C–axis (Scholle, 1979). Typical source rocks for these lithoclasts would be gneisses (Scholle, 1979).

A limited range of heavy minerals was observed in thin section. The most common is zircon, which occurs as well–rounded grains (Figure 3e). Other heavy minerals observed in thin–section include tourmaline and rutile. Also, re–sedimented quartz grains with thin, rounded, syntaxial relictic rims of quartz cement (Figure 3f) are common. Calcite and dolomite occur in the well–cemented sandstone samples.

Using Folk (1974) classification (QFL ternary plot), the Ahwaz Sandstone samples were classified as quartza–renite, sublitharenite and subarkose, thus reflecting their slightly mineralogical matured character (Figure 4). The geochemical classification diagrams of Pettijohn et al. (1972) and Herrón (1988) for these sandstones (Figure 5) provide the same results.

Because of the low abundance of feldspars, rock fragments and heavy minerals (Table 2), recognition of the source rock lithology of the Ahwaz Sandstone was established mainly by the study of quartz types.

Petrographic analysis of the thin sections shows small percentages of polycrystalline quartz grains (Table 2), which are of two types: the first type correspond to polycrystalline grains composed of five or more crystals with straight to slightly curved intercrystalline boundaries. The second type consists of polycrystalline quartz grains composed of more than five elongated crystals, exhibiting irregular to crenulated intercry stal boundaries (Figure 3b). The first type suggests an origin for the Ahwaz Sandstone from plutonic igneous rocks (Folk, 1974; Blatt et al, 1980), while the second type indicates an origin from metamorphic source rocks (Blatt et al. ,1980; Asiedu et al., 2000). Moreover, the Ahwaz Sandstone consists of monocrystalline quartz grains showing strong undulatory extinction and crypto–polycrystalline quartz grains, both suggestive of metamorphic source rocks. Nevertheless, the presence of strain–free quartz grains suggests plutonic source rock (e.g., Basu, 1985).

To evaluate the relative importance of plutonic and metamorphic rocks as quartz sources, we plotted polycrystalline quartz vs. non–undulatory and undulatory monocrystalline quartz in a double–triangular diagram following the technique of Basu et al. (1975) and Tortosa et al. (1991) (Figure 6). These approaches give dramatically different results; a medium to low metamorphic origin and a granitic source rocks. These contrasting results can be partially explained by considering that the Ahwaz Sandstone samples plot in a region of the diagram where only some of the granite sourced modern sands analyzed by Tortosa et al. (1991) plot. In contrast, the North American sands studied by Basu et al. (1975) do not contain comparable quartz populations. Furthermore, Weltje et al. (1998) diagram shows that the point count data from the Ahwaz Sandstone plot in the arrow–shaped fields (Figure 7a) at the middle of the diagram, which represents a mixture of metamorphic and plutonic source rocks.

The effect of source rock on the composition of the Ahwaz Sandstone could be distinguished by plotting the point count data on Suttner et al. (1981) diagram (Figure 8). This approach also point to a metamorphic source rock for these sandstones.

In the Ahwaz Sandstone samples, most quartz grains are colorless and may include crystals of zircon or apatite (plutonic origin) (Figure 3 c). However, a few quartz grains are dusty because of a profusion of rutile needle inclusions, and generally do not contain any other mineral inclusions (Figure 3d), which is typical of granulite–facies rocks.

In order to use major elements for provenance interpretations we considered the discriminant functions of Roser and Korsch (1988), which use A1₂O₃, TiO₂, Fe₂O₃^t, MgO, CaO, Na₂O, and K₂O contents as variables. In this diagram (Figure 9), the majority of the Ahwaz Sandstone samples plot on the quartzose sedimentary provenance field. This is equivalent to a passive margin tectonic setting (Bathia, 1983).

TECTONIC PROVENANCE

In the QtFL and QmFLt ternary diagrams of Dickinson et al. (1983), the point counting data plot in the craton interior and recycled orogen (quartzose recycled) field (Figure 10). As pointed out by Dickinson et al. (1983), sandstones plotting in the craton field are mature sandstones derived from relatively low–lying granitoid and gneissic sources, supplemented by recycled sands from associated platform or passive margin basins.

The major element geochemistry of the Ahwaz Sandstone samples (Table 3) is discussed in terms of ternary plots and discrimination diagrams to characterize the tectonic setting as proposed by Bhatia (1983) and Kroonenberg (1994). These diagrams show that the Ahwaz Sandstone was deposited in a passive continental margin (Figures 11c and 12). However, in the diagrams of Bhatia (1983; Figures 11a, 11b), the Ahwaz Sandstone samples are shifted to the right of the passive margin field because of secondary MgO enrichment related to dolomite cement. Other reason for this discrepancy may be the deficient functioning of these diagrams as documented by Armstrong–Altrin and Verma (2005). The high–MgO contents of the studied samples (average 3.4%) appear to be largely derived from dolomite (Von Eynatten, 2003), although the expected positive correlation between MgO and CaO, and between MgO and LOI was not statistically valid at 99% (or even at 95%) confidence level (using the computer program OYNYL by Verma et al, 2006a).

SOURCE AREA WEATHERING

Climate indexes

Petrographic evidence such as heterogeneous roundness for different grains (coarser ones are rounded and finer ones are angular) implies the importance of mechanical effects for grain shape configuration. Coarse–grained feldspars are related to a low degree of chemical weathering. Moreover, the rounded quartz overgrowths indicate recycling, which, in turn, can modify the compositional data towards the quartz–rich sandstones. Therefore, the petrographic evidence suggests that the compositional maturity of these sandstones may be due to recycling and long transport on low–relief Arabian craton as has already been indicated by Ziegler (2001) and may not be related to a humid climatic condition.

The point count data for most Ahwaz Sandstone samples on Weltje et al. (1998) diagram plot in the field number 4 (Figure 7b), which points to the sedimentation in a low–relief and tropical, humid climatic conditions. The samples plotting in the field number 2 were either deposited on a low–relief with a temperate and sub–humid climate or on tropical, humid conditions within an area with a moderate relief. The diagram of Suttner et al. (1981) (Figure 8), however, indicate a metamorphic source rock in a humid climate. However, this particular diagram can discriminate only sources of metamorphic and plutonic rocks (humid or arid conditions) and does not discriminate between different tectonic settings. The diagrams (Figures 7b and 8) are defined for first–cycle sediments and the effect of recycling and long distance transportation can shift the data on these diagrams toward the humid conditions. These considerations probably imply that new diagrams should be proposed such as those suggested in the field of sedimentary rock geochemistry (Armstrong–Altrin and Verma, 2005) and already proposed in the area of igneous rock geochemistry (e.g., Agrawal et al, 2004; Verma et al, 2006b).

Geochemistry and source area weathering

Alteration of rocks during weathering results in depletion of alkalis and alkaline earth elements and preferential enrichment of A1₂O₃ (e.g., Cingolani et al., 2003). Therefore, weathering effects canbe evaluated in terms of the molecular percentage of the oxide components, using the formulae of chemical index of weathering (CIW = [A1₂O₃/(A1₂O₃ + CaO* + Na₂O] x 100; Harnois, 1988) and chemical index of alteration (CIA= [A1₂O₃/(A1₂O₃ + CaO* + Na₂O +K₂O] x 100); Nesbitt and Young, 1982). However, samples having highly variable CaO contents due to variation in calcite or dolomite abundance (such as those included in this study), may produce misleading conclusions if the CIW and CIA are used to infer the degree of weathering (Cullers, 2000). Therefore, in this study we used a modified chemical index of weathering (CIW = molecular [A1₂O₃/ (A1₂O₃ + Na₂O] x 100, in which CaO is left out of the CIW; Cullers, 2000). The CIW' value of the Ahwaz Sandstone samples vary from 32 to 79 (n=9, mean 67, s=15; median 70), which indicate a moderate weathering (recycling) for these sandstones. These CIW' values, in general, can be due to either absence of intense recycling in a humid climate or intense recycling in an arid/semiarid climate (Osae et al, 2006; Wanas and Andel–Maguid, 2006).

CONCLUSIONS

Quartz arenites, sublitharenite and subarkose of the Ahwaz Sandstone (sandy member of Asmari Formation, the major oil reservoir in Zagros mountain) have petrographic (texture, framework mineralogy, quartz types and inclusions in quartz) and geochemical characteristics that suggest quartzose recycled sedimentary rocks as the main source rocks, in addition to high–grade metamorphic and plutonic igneous rocks as minor parent rocks. Moreover, the obtained data are consistent with a long distance transport, in an arid/semiarid climate, over the Arabian shield, which supplied these sands to their depositional basin along the passive marginal coast of the Oligocene–Miocene Zagros foreland basin.

ACKNOWLEDGMENTS

We thank the National Iranian Oil Company, especially H. Ghalavand for providing us with the core samples and thin sections, and the Shahid Beheshti University of Tehran for providing laboratory equipments. In addition, we are grateful to M.H. Adabi for his considerable help throughout this study. We would like to thank IS. Armstrong–Altrin and an anonymous reviewer for helpful and constructive reviews. The manuscript has largely been improved by constructive comments and corrections from the RMCG editor, Dr S.P. Verma.

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