versión impresa ISSN 1026-8774
Rev. mex. cienc. geol v.25 n.2 México ago. 2008
Petrography and geochemistry of Ahwaz Sandstone Member of Asmari Formation, Zagros, Iran: implications on provenance and tectonic setting
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 HosseiniBarzi*
Faculty of Earth Science, Geology Department, Shahid Beheshti University, Iran. * email@example.com
Manuscript received: December 3, 2007
Corrected manuscript received: March 2, 2008
Manuscript accepted: March 3, 2008
An integrated petrographic and geochemical study of the Ahwaz Sandstone Member ofOligoceneMiocene 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 Az85, Az11, andAz89) in Ahwaz oilfield in the Dezful embayment subzone.
On the basis of the framework composition (point counting) and wholerock 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 quartzrich sedimentary (recycled), middle to highgrade 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 lowrelief highlands.
Key words: provenance, tectonic setting, sandstone, geochemistry, Ahwaz Sandstone Member, Asmari Formation, Iran.
Un estudio petrográfico y geoquímico del Miembro de Arenisca Ahwaz del OligocenoMioceno, 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 Az85, Az11, y Az89) 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).
Palabras clave: proveniencia, ambiente tectónico, arenisca, geoquímica, Miembro de Arenisca Ahwaz, Formación Asmari, Irán.
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; ArmstrongAltrin 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; ArmstrongAltrin et al, 2004), although,caution is required in their indiscriminate use (ArmstrongAltrin 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, Kfeldspar, 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; ArmstrongAltrin 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; VaziriMoghaddam et al, 2006). However, only a few petrography and palaeoenvironment 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: Az85, Az11 and Az89) 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 PermoTriassic and collision during the Late Tertiary (Stocklin, 1974; Berberian and King, 1981; Beydoun et al, 1992). In fact, the Zagros foldthrust belt lies on the northeastern margin of the Arabian plate and has been divided into NWSE 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 bestknown carbonate reservoirs in the world) was deposited in the OligoceneMiocene 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 siliciclasticcarbonate 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 northwestsoutheast 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 prerift 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 deepseated '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. Az85A, Az89 and Az11; 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 Az85 A, 13 samples from well Az89 and25 samples from well Az11) (Figure 2). The sample selection was based on petrography to choose wellsorted and unweathered, fine to mediumgrained, sandsize samples and to cover the entire thickness of the formation. Framework mineral composition (modal analysis) was quantified using the pointcounting method of GazziDickinson as described by Ingersoll et al. (1984).
In the GazziDickinson 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 Xray 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
The detrital framework grains of the Ahwaz Sandstone Member include quartz, feldspars, rock fragments and accessory minerals such as zircon and sphene. By studying the texture of these sandstone we observed that fine grains are angular, and coarser ones are rounded to broken rounded. This occurs because in a highenergy environment or through longdistance transport, the wellsorted, coarsegrained sands not only round completely, but also break to finegrained, angular sands, which results in a decrease of the sediment sorting. Although chemical mechanisms can round the grains by corroding them, these mechanisms round the finegrained sands more than the coarser ones (more exposed surface in finegrained particles), which is not the case in this study. Therefore, the heterogeneous roundness of grains for different grain sizes in the Ahwaz Sandstone reflect the importance of mechanical factors for the final grain shapes.
Quartz is the dominating framework grain in the studied thin sections (Figure 3ad; also see Table 2). Monocrystalline quartz (Qm) occurs in three variants: nonundulose, 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 23 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 nonundulatory monocrystalline quartz contain inclusions of apatite and rutile needles, which could have been derived from plutonic source rocks or sedimentary recycling (Figure 3cd).
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 Kfeldspar content (Table 3). Plagioclase occurs in some thin sections and seems to be linked to samples with smaller grain sizes (finegrained sand). Petrographic study of these sandstones reveals that Kfeldspar is more abundant than plagioclase, which can be originated from durability of Kfeldspars, nonoccurrence of plagioclasebearing 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 highgrade 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 Caxis (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 wellrounded grains (Figure 3e). Other heavy minerals observed in thinsection include tourmaline and rutile. Also, resedimented quartz grains with thin, rounded, syntaxial relictic rims of quartz cement (Figure 3f) are common. Calcite and dolomite occur in the wellcemented sandstone samples.
Using Folk (1974) classification (QFL ternary plot), the Ahwaz Sandstone samples were classified as quartzarenite, 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.
PARENT ROCK LITHOLOGY
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 cryptopolycrystalline quartz grains, both suggestive of metamorphic source rocks. Nevertheless, the presence of strainfree 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. nonundulatory and undulatory monocrystalline quartz in a doubletriangular 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 arrowshaped 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 granulitefacies rocks.
Accordingly, the monocrystalline (Basu et al., 1975; Tortosa et al. 1991) and polycrystaline quartz grain characteristics (Folk, 1974; Blatt et al., 1980; Asiedu et al., 2000) as well as the logarithmic ratios of quartz to feldspars and rock fragments (diagram introduced by Weltje et al., 1998) (Figure 7a) and quartz grain inclusions imply a mixed origin from plutonic and medium to highgrade metamorphic rocks for the Ahwaz Sandstone. Furthermore, the evidence of recycling (rounded zircon and rounded overgrowths in some quartz grains) (Figure 3ef) in the studied samples, indicate that quartzrich sedimentary rocks should be considered as one of the major source rocks.
In order to use major elements for provenance interpretations we considered the discriminant functions of Roser and Korsch (1988), which use A12O3, TiO2, Fe2O3t, 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).
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 lowlying 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 ArmstrongAltrin and Verma (2005). The highMgO 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
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. Coarsegrained 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 quartzrich sandstones. Therefore, the petrographic evidence suggests that the compositional maturity of these sandstones may be due to recycling and long transport on lowrelief 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 lowrelief and tropical, humid climatic conditions. The samples plotting in the field number 2 were either deposited on a lowrelief with a temperate and subhumid 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 firstcycle 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 (ArmstrongAltrin 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 AndelMaguid, 2006).
Petrographic evidence from the Ahwaz Sandstone (heterogeneous roundness for different grain sizes; fresh coarsegrained feldspars) is consistent with a more important role for mechanical weathering than for chemical weathering. Similarly, petrographic evidence of rounded quartz overgrowths can be related to an intense recycling of their source rock. The geochemical (Figure 9) and point count (Figure 10) data as well as CIW values are consistent with an active recycling in an arid/semiarid climate for Ahwaz Sandstone. However, more accurate interpretation of climate conditions of these deposits would require further studies such as clay mineral investigations.
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 highgrade 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 OligoceneMiocene Zagros foreland basin.
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. ArmstrongAltrin 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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