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Interpretation the Origin and Tectonic Setting of Coastal Sediments in the Northeastern of Oman Sea

Ahrari Rudi M1 and Afarin M2*

1Faculty of Marine Sciences, Chabahar Maritime University

2Chabahar Oceanography Centre (Sea of Oman and the Indian Ocean)

*Corresponding Author:
Afrin M
Chabahar Oceanography Centre
(Sea of Oman and the Indian Ocean), Iran
Tel: + 05435339198
E-mail: [email protected]

Received date: June 07, 2016; Accepted date: June 14, 2016; Published date: June 21, 2016

Citation: Rudi AM, Afarin M (2016) Interpretation the Origin and Tectonic Setting of Coastal Sediments in the Northeastern of Oman Sea. Int J Waste Resour 6:224. doi:10.4172/2252-5211.1000224

Copyright: © 2016 Rudi AM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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This study investigates geochemistary of major and trace elements of sandstones and mudstones in the Chabahar area located of northeastern Oman Sea. In this study, 52 samples were taken from 5 sections included of Tiss, Ramin, Lipar, Gorankash and Garindar estuary. Then have been done grain size and chemical analysis using XRF and ICP AES methods in geological survey of Iran. Comparison of major elements values with the average amounts of upper continental crust indicate due to sedimentation processes or the lack of source rocks contain sodium plagioclase, the amount of Na2O and Fe2O3 in continental crust is highly depleted. Plotted geochemical data of major elements from Late Miocene-Pleistocene, age 10 mudstone and sandstone samples in east coasts of Chabahar in Makran zone, on siliciclastic rocks classification diagrams, showed that the sandstones are genus wackstone, Mudstones and shale. Values range the chemical index of alteration (CIA) and plagioclase index of alteration (PIA) respectively is from 40 to 60 and 40 to 64. However, most samples have values less than 50, suggesting a low to moderate degree of alteration (weathering) and arid to semi-arid climate during deposition in the source area. Major elements geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on assotiated trace elements, such as Ti, Zr, La, Sc and Th, show that most data located in the active continental margin field and tectonic setting have developed in active continental margin (ACM). Finally, geochemical data and using from sediments silisiclastic discriminate diagrams on the major elements in these rocks shows that the quartzose sedimentary provenance.


Chabahar; Makran zone; Geochemistry; Siliciclastic rocks


The chemical and mineralogical composition of clastic sedimentary rocks is controlled by various factors such as source rock composition, chemical and physical weathering and sedimantition processes such as mechanical sorting decomposition and diagenesis [1-4]. It is on this basic, therefore, that the chemistry of fine-grained clastic sedimentary rocks has been long utilized for making inference on source rock compositions, palaeoclimatic conditions and tectonic setting [5]. The relationship between the composition of siliciclastic sediments and their tectonic setting and provenance has been examined by many workers [6-12]. The geochemistry of siliciclastic sedimentary rocks may reflect the signature of parent materials [5,13], and literature provides many examples of ways to interpret geochemical data to understand sedimentary rock provenance [9,11,14-18]. Because chemical composition is also a function of weathering, transportation, and diagenesis, much emphasis has been put on relatively immobile elements such as Cr, Co, Th, Y, Zr, Hf, Nb and Sc. The low mobility of these elements during sedimentary processes enables a better discrimination of the provenance and tectonic setting [5,10,13,19,20]. Along the Makran coastline, the formation of the coastal escapments and fluid migration has been attributed to repeated pulses of vertical movments [21-24]. Although tectonic events interrupted the formation of complete sedimentary cycles in the shelf and shoreline exposed in these escapments, the effects of these tectonic movments did not uniformly influence the sedimentary records along the coastline [24]. Geologic studies have concentrated on the Quaternary rocks exposed in southeast Iran, as well as in the Oman Sea. In contrast, little is known about the Cenozoic rocks in Chabahar. In general, the papers published on the Quaternaryrocks of Chabahar deal mainly with their sequence stratigraphy and sedimentology. No detailed studies of their geochemistry and tectonic setting have been made. The present work incorporates geochemical composition of the Quaternarysandstone and mudstone exposed southeast of Chabahar area, in an effort to disclose their provenance and tectonic setting. An attempt is also made to depict the paleo-weathering conditions of the source area during their deposition.

Regional and Local Geologic Setting

The Makran accretionary prism results from active subduction of the Arabian plate beneath the Iranian and Afghan continental blocks since the Late Cretaceous times [25-27]. The plate convergence rate has been estimated between 2.5 and 4 cm yr−1 [22,26,28,29]. The Makran subduction zone is an area of significant seismic activity [30-33], periodically affected by devastating earthquakes such as the 1945 Makran earthquake (Mw 8.1–8.3), which is the largest known in this region [21,31,34,35]. Low- to moderate-magnitude earthquakes (mainly related to thrust activity) occur more frequently (b50 y return time) in the Makran area (not restricted to the coast) [30,31]. The pattern of seismicity is distributed over a 700 km long and 200 km wide segment of plate boundary [31,32]. The Makran accretionary prism is more than 350 km wide [22]. Over 60% of the prism is presently sub-aerial, separated from the submarine part (100–150 km) by a nearly undeformed continental shelf [27]. As subduction proceeded, from Paleocene time, the oceanic depo-trough shifted episodicslly southward; with each shift a thrust slice of the sedimentary prism accreted onto the northern continental blocks. The differing rate of emergence along the coastline resulted in the formation of marine terraces of different heights [21,24]. The studied sediments are exposed east Gulf of Chabahar, along 25 km of the Makran coast of the Oman Sea (Figure 1). This coastline defines the southern boundary of the Coastal Makran Zone of [36]. The Miocene-Pleistocene sediments (Geological Survey of Iran, 1996) of the Coastal Makran Zone consists of mixed silisiclastic-carbonate deposits, which are known to be shallow marine molasses deposits and comprise mostly material recycled from Makran flyschs and order molasses exposed in the northern highlands.


Figure 1: Geologic map of the study area (Chabahar Gulf) that shows measured sections.

Materials and Methods

Samples presented here include mudstones, siltstones and fine-grained sandstones (here termed as mudrocks) collected from the Makran flyschs in view of the fact that fine-grained clastic sedimentary rocks are more useful in geochemical studies than the coarser ones [5]. Mudrocks are fine-grained siliciclastic rocks rich in clay minerals [36,37]. Clays preserve source rock chemical signstures due to the fact their mineralogy is rarely affected during diagenesis and metamorphisis [38]. Fifty-two samples was taken of clastic sedimentary rocks (from Tis, Ramin, Gurankash, Lipar and Khur-e-garrindar sections), and 22 samples were chosen for geochemical analysis. Chemical analyses were carried out at the laboratory of the Geological Survey of Iran. Major elements were analyzed by automatic X-ray fluoreseence spectrometer (XRF) using fusion glasses made from a 1:5 mixture of powdered sample and Li2B4O7 flux [39]. Trace and Rare Earth Elements were analyzed with a Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Loss on ignition was determined by heating the samples at 1000°C for two hours. The analytical reproducibility as deduced from replicate analyses is better than 3.9% for most trace elements and better than 0.9% for most major elements except P2O5 (3.9%), Na2O (1.5%) and K2O (1.3%). The accuracy of the ICP measurements as computed from replicate analysis of the USGS SY-3 rock standard is better than 2% for SiO2, Al2O3, Fe2O3, MnO, CaO, Na2O and TiO2 while the accuracy for P2O5, K2O, and MgO data are 7.4%, 3.5% and 3.7% respectively. The accuracy for most trace element analyses is within 0.54-3.9% (except for V which is 10%) whereas reproducibility of the ICP measurements is better than 7%.

Petrography and Textural Analysis

The relative proportions of the sand, silt and clay in the sandstones studied are shown in Figure 2 and plotted on the diagram of Folk (1980) (Figure 3). Most of the samples are texturally classified as silty sand, sandy mud, sandy silt and muddy sand.


Figure 2: Chart of texture from the Quaternary sandstones in Chabahar area, southeast Iran.


Figure 3: Plot of the Miocene-Pleistocene sediments in the ternary diagram of Folk.

Microscopic studies indicate that the Quaternary siliciclastic rocks in the study area are formed mainly of sandstone, greywacke, siltstone and mudstone (Figure 4). Quartz is the dominant detrital mineral; varying from very fine to fine sand-sized grains (Figure 4). The grains range from sub angular to sub rounded and from poorly to moderately sorted. Rocks having well-rounded and sorted grains are present in certain horizons and reflect a more mature depositional setting. Most quartz grains are monocrystalline and some of the quartz grains are partly sutured. In the Pettijohn [40] scheme, these rocks are classified as litharenite, sub-litharenite and mudstone. The slight enrichment of Na and K-feldspar over plagioclase is attributed either to prolonged weathering, transportation or recycling.


Figure 4: Photomicrograph of sub-litharenite (a, b), litharenite (c, d) and mudstones (e, f).


The results of the major and trace element analysis of the mudstones and sandstones of the coastal Makran, Chabahar area (22 samples) are listed in Tables 1 and 2, respectively.

Major (w %) B-2 B-4 C-2 C-4 D-2 D-4 E-2 E-4 E-6 E-8 A-2 A-4 Average
SiO2 58.14 57.99 59.52 59.50 59.71 54.51 54.86 55.13 55.13 56.45 53.89 53.91 56.32
Al2O3 10.52 11.42 9.77 11.31 10.79 14.43 14.36 15.18 14.38 14.48 15.17 14.95 13.11
Fe2O3 3.9 4.11 3.75 4.02 3.67 5.94 6.15 6.49 5.51 6.43 6.06 6.49 5.21
MgO 3 3.06 3.22 3.39 3.98 4.07 3.62 3.79 3.63 3.60 3.62 3.52 3.47
CaO 9.2 8.43 10.57 7.62 7.93 5.42 5.73 5.16 6.64 5.63 5.07 5.14 6.82
Na2O 2.02 2.07 1.59 1.89 1.59 1.38 1.66 1.29 1.26 1.32 1.65 1.82 1.66
K2O 1.66 1.76 1.55 1.86 1.55 2.57 2.57 2.86 2.48 2.67 2.67 2.86 2.25
TiO2 0.57 0.57 0.57 0.59 0.61 0.64 0.66 0.69 0.68 0.74 0.66 0.73 0.65
P2O5 0.15 0.14 0.20 0.16 0.19 0.18 0.16 0.18 0.19 0.18 0.18 0.18 0.18
L0I 9.9 9.44 8.62 9.02 9.72 10.18 9.50 7.98 9.50 9.44 9.82 9.56 9.48
Total 99.06 99.09 99.48 99.36 99.23 99.42 99.37 99.56 99.40 99.15 98.89 99.16 99.25
K2O/ Na2O 0.82 0.85 0.97 0.98 0.79 1.86 1.55 2.22 1.97 2.02 1.62 1.57 1.42
Fe2O3+MgO 6.9 7.17 6.97 7.41 6.65 10 9.77 10.28 9.14 10 9.68 10.52 8.73
Al2O3/ SiO2 0.18 0.20 0.16 0.19 0.18 0.26 0.26 0.27 0.26 0.27 0.28 0.28 0.23
CIA 46 48 40 50 48 60 59 61 58 60 62 60 60
PIA 45 48 40 50 48 63 61 62 64 56 65 63 62

Table 1: Chemical composition of Major elements of the Quaternary sandstones and mudstones, Chabahar area

Elements B-1 B-3 C-1 C-3 D-1 D-3 E-1 E-3 A-1 A-3
Ag 0.1 0.1 0.1 0.3 0.0 <0.1 0.2 0.1 0.2 0.2
As 3.6 8.5 7.7 8.0 7.9 7.3 5.8 4.8 9.5 8.4
Ba 183.4 255.7 164.4 174.4 158.2 248.8 227.6 193.3 232.0 230
Be 1.9 2.0 1.3 1.0 1.1 2.3 2.1 1.7 2.3 1.8
Bi 0.8 0.8 0.7 0.5 0.6 0.8 0.7 0.7 0.7 0.7
Cd 0.2 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2
Ce 48.8 51.2 39.6 41.7 39.1 64.4 53.2 55.4 57.9 58.5
Co 32.6 34.2 20.6 17.2 19.9 32.5 28.2 23.1 33.3 33.4
Cr 123.2 147.9 86.4 144.1 91.6 158.7 142.2 125.3 162.7 160.8
Cu 40.6 37.4 24.0 15.4 17.5 32.7 31.3 14.5 22.9 23
Dy 6.6 5.9 3.7 3.6 3.0 7.8 6.6 5.8 7.2 6.8
Er 2.5 2.4 2.3 2.8 2.8 2.4 2.7 2.5 2.5 2.5
Eu 2.5 2.5 1.5 2.1 1.5 1.9 1.8 2.0 1.9 2
Ga 11.6 6.6 5.8 8.2 5.8 13.0 10.4 6.3 11.1 10
Gd 9.3 9.0 6.9 6.0 5.5 8.7 8.0 6.9 8.6 8.5
Ge 2.3 1.6 3.4 2.5 3.2 3.3 2.4 2.1 2.9 3
Hf 2.7 3.9 0.8 4.4 3.4 2.1 3.5 4.3 2.8 2.6
Hg 0.07 0.06 0.05 0.07 0.05 0.05 0.07 0.05 0.06 0.05
Ho 1.1 1.1 1.0 0.9 0.8 1.1 1.0 0.9 1.0 1.0
La 24.4 25.6 21.4 24.3 19.8 32.1 27.9 29.5 29.3 30
Li 44.3 46.4 31.0 23.9 28.1 49.8 42.8 39.2 48.6 45.2
Lu 0.6 0.7 0.4 0.3 0.3 0.6 0.5 0.4 0.6 0.5
Mn 503.7 502.4 586.5 717.1 581.1 805.5 543.0 829.6 787.8 802.5
Mo 0.9 0.8 0.9 0.9 0.9 1.1 1.0 1.0 1.2 1.4
Nb 17.2 17.9 14.4 12.4 13.1 21.9 18.9 20.2 20.8 20.5
Nd 24.2 27.5 26.7 25.0 24.4 28.7 29.7 30.1 25.6 24.3
Ni 159.5 161.5 105.4 71.5 94.5 157.0 151.6 109.4 161.6 158
P 873.6 881.2 794.1 687.3 656.8 866.3 884.6 1024.0 912.0 932
Pb 15.3 9.9 13.0 11.4 9.4 23.2 21.2 7.8 11.3 12
Pr 14.5 14.0 9.4 7.7 8.0 13.9 12.7 9.1 14.0 13
Rb 83.4 93.4 78.3 76.2 88.4 90.1 98.7 82.7 91.6 90.2
S 1446.9 1029.9 979.8 141.0 304.2 216.1 2883.3 211.8 126.9 355
Sb 0.4 0.4 0.3 0.4 0.5 0.4 0.3 0.4 0.3 0.4
Sc 13.6 13.9 9.9 8.3 8.7 17.7 14.7 13.4 16.1 15
Sm 5.3 4.8 4.9 4.5 4.0 6.6 5.6 7.0 5.3 6
Sn 5.3 5.2 3.4 2.8 2.8 5.0 4.6 3.4 5.3 4
Sr 149.5 137.8 514.1 303.1 197.1 211.3 217.1 221.5 172.9 210
Ta 1.1 1.8 1.4 1.0 1.2 1.0 1.0 1.9 1.9 1.8
Th 12.2 12.1 9.3 6.3 6.6 14.7 13.3 10.5 13.0 12.3
Ti 5566.9 5821.3 4774.7 4075.5 4283.3 7052.2 6276.5 6523.3 6603.6 594.9
Tl 1.9 1.0 2.5 1.4 0.7 2.1 2.0 1.1 1.2 1.8
U 6.6 6.6 4.7 3.4 3.6 6.2 5.9 4.8 6.2 5.8
V 113.8 116.8 92.6 79.1 77.0 147.3 127.8 113.0 141.8 133
Y 23.8 25.1 24.8 25.8 21.5 31.8 27.8 32.4 29.4 30
Yb 3.2 2.9 2.3 2.2 2.1 3.5 3.1 3.2 3.3 3.5
Zn 95.9 93.5 62.0 48.4 56.1 107.0 91.3 78.6 104.0 89
Zr 167.3 186.6 164.4 156.7 141.4 225.5 203.9 218.8 211.7 2.15
Th/U 2.2 2.4 2 2.2 1.8 1.8 2 1.8 1.8 2.4
La/Sc 1.9 1.8 1.8 2.2 2.3 2.9 2.2 1.8 1.8 2.9
Sc/Th 1.1 1.2 1.2 1.3 1.3 1.3 1 1.1 1 1.3
La/Th 2.1 2.2 2.2 2.8 3 3.8 2.3 2 2 3.8
Co/Th 2.1 2.2 2.6 2.2 3 2.7 2.2 2.7 2.1 2.8
Cr/Th 10.7 10.8 12.5 12 13.9 22.9 9.3 10 9.3 22.9

Table 2: Chemical composition of Trace elements of the Quaternary sandstones and mudstones, Chabahar area.

Major elements

The Major element distributions reflect the mineralogy of the studied samples. The SiO2 content is low and varies from 53.9% to59.7%, with an average of 56.4%. Silica is below the mean value for the upper continental crust (UCC) in most studied samples [5]. Sandstones are higher in SiO2 content than mudstones (Figure 5). Similarly, mudstones are higher in K2O, Fe2O3 and TiO2 contents than sandstones, which reflect their association with clay-sized phases. Al2O3 abundances were used as a normalization factor to make comparisons among the different lithologies, because of its immobile nature during weathering, diagenesis, and metamorphism. In Figure 5, major oxides are plotted against Al2O3. Average UCC values [5] are also included for comparison. Among other major elements, Fe2O3, MgO, K2O, TiO2 and P2O5 are consequently showing strong positive correlations with Al2O3, whereas CaO and Na2O do not exhibit a trend (Figure 5). The, strong positive correlations of the major oxides with Al2O3 indicate that they are associated with micaceous/clay minerals. In comparison with UCC, the studied mudstones are low in MgO, CaO, Na2O and high in Al2O3 and TiO2. Al and Ti are easily absorbed by clays and concentrate in the finer, more weathered materials. In addition, XRD analysis of the studied mudstones and sandstones reveals that they are associated with the mineral chlorite (Al2O3) [41]. On average, the studied mudstones have lower SiO2 abundances relative to UCC therefore the observed variations are probably due to quartz dilution effect.


Figure 5: Co-variation of major elements versus Al2O3 for the study samples. Fe2O3, MgO and K2O show positive correlation of Al2O3 with the major elements, SiO2, CaO and Na2O show negative correlation.

Generally, most elements increase with rise in Al2O3 from silty sandstone to sandy mudstone. Positive correlations with Al2O3 are shown by most of the major elements (Figure 5). Marked negative correlation is shown by SiO2 confirming that much of SiO2 is present as quartz grains. These trends may be controlled by the silt fraction [42]. The trend of decreasing K2O and Na2O with increasing SiO2 is sensitive to grain size, probably due to the decrease of clay content in the coarser sediments. The low K2O/Na2O ratios of due to sedimentation processes or the lack of source rocks contain sodium plagioclase [43-45]. A positive correlation between K2O and Al2O3 implies that the concentrations of the K-bearing minerals have significant influence on Al distribution and suggests that the abundance of these elements is primarily controlled by the content of clay minerals [43,46].

However, according to the geochemical classification of [47], most of the studied mudstones and sandstones samples plot as tight clusters in the wacky field (Figure 6). Only a few samples are enriched in Fe2O3 and fall in the shale and litharenite field. In terms of major element compositions (Table 1), studied sandstones are relatively enriched in MgO, Fe2O3 and Al2O3 and depleted in Na2O and K2O. Mudstones have a higher proportion of Al2O3 and are depleted in Na2O, which reflects the greater proporition of clay minerals (especially chlorite and sericite) in the finer-grained deposits.


Figure 6: Log diagram Fe2O3/K2O with SiO2/Al2O3 and geochemical classification of the mudstone and sandstones in the study area.

Trace elements

The major and trace element compositions for the studied samples (sandstones and mudstones) are highly comparable and are suggestive of similar protolith (Table 1). However, the depletion in TiO2, Al2O3 and the transition trace elements particularly Co, Ni, Sc and V, which are normally enriched in mafic rocks [48]. These elements are mainly concentrated in the clays or metal oxides [49]. Vanadium is positively correlated with TiO2 (r = 0.71). It is possibly adsorbed on kaolinite and associated with iron oxide minerals [50,51].

The negative correlation between SiO2 and the trace elements suggests most of the trace elements are concentrated in the clay fraction. Ba is positively correlated with K (r = 0.54), suggesting that the K bearing clay minerals control its abundance [43,52,53]. Positive correlation of Co, Ni, Cu, Zn, Li and V with both Al2O3 (r = 0.91, 0.48, 0.23, 0.36, 0.22 and 0.41, respectively, Figure 7) and Fe2O3 (r = 0.90, 0.35, 0.18, 0.25, 0.28 and 0.13 respectively, Figure 8). Cr is positively correlated with SiO2 (r = 0.40) and negatively correlated with most major and trace elements, suggesting its possible association with calcite.


Figure 7: Plots of Ba, Cr, Ni, Rb, Sr, Y, Zr, La and Ce versus Al2O3 for the study samples.


Figure 8: Plots of Ba, Cr, Ni, Rb, Sr, Y, Zr, La and Ce versus Fe2O3 for the study samples.


Geochemistry and tectonic setting

Major elements geochemistry has been employed largely to decipher ancient tectonic setting(Maynard et al., [54]; Bhatia [9]; Roser and Korsch, 1985, 1986, 1988; Bhatia and Crook [10]; Merodio and Spalletti, 1990; Spalletti et al., 1991, 1992).. On SiO2-K2O/Na2O, Al2O3/SiO2-Fe2O3+MgO, TiO2-Fe2O3+MgO, and SiO2/Al2O3-K2O/ Na2O diagrams, the studied samples show a considerable range of compositions (Figure 9). As a result, interpretations of the tectonic setting using these compositions are not straightforward [9,11,54]. Na and K are among the most mobile major elements, and the use of Na2O and K2O to discriminate tectonic settings may be regarded cautioualy. As discussed previously, it should be noted that Fe2O3 likely was enriched during the sedimentary processes and early diagenesis.


Figure 9: Tectonic setting discrimination diagrams based on major elements compositions of the Quaternary mudstones and sandstones of the coastal makran. (a) K2O/Na2O vs. SiO2 (Roser and Korsch, 1986); (b) TiO2 vs. Fe2O3+MgO (Bhatia, 1983); (c) Al2O3/SiO2 vs. Fe2O3+MgO (Bhatia, 1983); (d) SiO2/Al2O3 vs. K2O/Na2O (Maynard et al., 1982; Roser and Korsch, 1986; Gu et al., 2002). ACM, active continental margin; PM, passive margin; CIA. continental island arc; OIA, oceanic island arc; IA, island arc; A1, arc setting, basaltic and andesitic detritus; A2, evolved arc setting, felsitic-plutonic detritus

Several trace elements, such as Cr, Co, Th, Sc, La and Zr, should be immobile under depositional conditions and diagnose specific source rocks [5,10,55]. Hence, thay are more useful in discriminating tectonic environments than the major elements. In the ternary, Th-Sc-Zr/10 and La-Th-Sc plots proposed by Bhatia and Crook [10] data for the study samples are scattered, but most fall within the active continental margin (ACM) fields (Figure 10).


Figure 10: Tectonic setting discrimination diagrams based on trace elements compositions of studied samples (after Bhatia and Crook, 1986; Schandl and Gorton, 2002). (a,b) Ternary diagrams La-Th-Sc and Th-Sc-Zr/10. (c,d) Plots of Th/Ta vs. Yb and Th/Hf vs. Ta/Hf. (A) Oceanic island arc, (B) Continental island arc, (C) Active continental margin and (D) Passive margin.

Geochemistry and provenance

Several ratios and plots may be used to define the source rock of siliciclastic deposites. Presence of greater than 70% SiO2 implies the sandstones are rich in quartz from quartz-rich crystalline provenance. K2O/Na2O ratio can be considered as a simplified chemical provenance indicator [56]. High values of this ratio reflect derivation from granites rather than from basic rocks. This is also confirmed by the clay mineral content, as illite and kaolinite were considered to inherit from weathering horizons and soils developed on silicic rocks. In addition, the absence of chlorite and smectite clay minerals also precludes mafic source rocks. Felsic source rocks usually contain markedly lower concentrations of Cr, Co, Ni and V elements and higher concentrations of Ba and Sr then mafic source rocks. This is reflected in the sediments, indicating the original protolith chemistry [5,43,57]. The studied samples examined here are enriched in Sr and Ba and depleted in transitional elements such as Cr, Co, Ni and V, reflecting the negligible role of mafic provenance [46]. Also, these samples have elevated Ba contents but are depleted in Co. The higher Ba/Co ratio (Table 1) is suggestive of sediments derived from weathered quartzose source.

In the discrimination diagram for sedimentary provenance after [12], all samples of the studied sandstones and mudstones plot in the quartzose sedimentary field (Figure 11).


Figure 11: Provenance discriminant function diagram using major elements.

However, the Th/U ratios are very useful in determining the source characteristics of clastic sedimentary rocks [58]. The present average crust the Th/U ratios of 4.25-4.30 whereas the values for upper and lower mantle are 2.6 and 3.8, respectively [59]. Although, sometimes higher Th/U ratios have been related to oxidative weathering and removal of U, yet, clastic sedimentary rocks derived from the upper crust are characterized by ratios equal to or greater than 4 whereas ratios lower than 4 have been related to a mantle contribution [58]. The sandstone and mudstone of Chabahar area, mean Th/U ratios of 1.8-2.4, respectively, which characteristics are suggestive of mantle parentage. The elevated Th/U ratios in the studied samples (sandstone and mudstone) could be attributed to either increased weathering intensity or variation in oxidation state during deposition which would permit U mobility [58]. On the Th/U versus Th diagram, all sandstone and mudstone samples from the Chabahar area, derived from the mantle (Figure 12).


Figure 12: Plot of Th/U versus the Th for the sandstone and mudstone of Chabahar area, southeast Iran. The grey box shows the typical range of upper crustal protolithes.

The geochemical variations between elements such as Th and La (indicative of a felsic source) and Sc (indicative of a mafic source) have been used to distinguish between felsic and mafic provenances by various outhors [5]. Th/Sc ratios are useful indicators of source rock processes and are unaffected by sedimentary processes [5]. The Th versus Sc plot (Figure 13), adopted from McLennan [60], reveals two dominant source areas, a continental source with Th/Sc ratios near 1 and intermediate source with Th/Sc ratios 1-0.6 and mafic source with Th/Sc ratios lower 0.6 (see; Figure 14). This observation further suggests that the sandstones and mudstones were formed by more ultrabasic detritus (intermediate source).


Figure 13: The versus Sc diagram indicating felsic and mafic provenance for the mudstones and sandstones of Chabahar area.


Figure 14: Scatter plot of Al/Na ratios versus Chemical Index of Alteration (CIA) for the studied samples. Fields are from Servaraj and Arthur (2006).

Geochemistry and source area weathering

Major elements abundances may reflect the composition of source rocks. However, in some cases, their proportions may be modified by weathering processes that affect the source rocks and by post-depositional changes that affect the sediments [61-63]. Chemical weathering affects, to a great extent, the composition of siliciclastic sediments. Through these processes the large cations (Ba and Al) remain preserved in the weathering residue in contrast to the smaller cations (Na, Ca, Sr) that can be selectively removed [44,64]. The SiO2/ Al2O3 ratios are low (3.55-6.09%; averaging 4.41%) and indicate a low degree of maturation of the mudstones and sandstones Chabahar area, southeast Iran (Table 1). The degree of weathering may assessed by the chemical index of alteration (CIA = [Al2O3 / (Al2O3 + CaO* + Na2O + K2O) × 100]; [65]) and the plagioclase index of alteration (PIA = [ Al2O3 - K2O / (Al2O3 + CaO* + Na2O + K2O) × 100]; [66]), in molecular proportions. In the equations, CaO* represents the value of Ca in the silicate fraction only or other term CaO* is the amount of CaO incorporated in the silicate fraction of the rock.

CIA and PIA values of about 45-55 imply weak weathering [60,66] whereas values of near 100 indicate intense weathering, with complete removal of the alkali and alkaline earth elements and increasing Al2O3 content. Because feldspar and volcanic glass make more than 75% of the labile material in the upper crust, these indices measure the degree of alteration of feldspar to clay minerals [5,44,66,67]. So, the CIA values represent the alteration of primary minerals (e.g., feldspars, volcanic glass, heavy minerals) to clay minerals. Low CIA values indicate a low to moderate degree of alteration of the source rocks. The calculated CIA and PIA values (Table 1) range from 40 to 60, with an average 54.5, and from 40 to 64 with an average 55.2, respectively. However, most samples have values less than 50, suggesting the low to moderate degree of alteration and weathering either of the original source rocks. In a plot of Al/Na ratios versus CIA of all sandstones and mudstones plot in the intermediate weathering field except for a few samples which plot in the low weathering field (Figure 14).

Sandstones and mudstones composition of Chabahar area expressed as a function of SiO2 percentage against total percent of Al2O3, K2O and Na2O is used the discriminate the paleoclimatice signature and to give an idea about the chemical maturity of the studied deposites (Figure 15). The majority of the studied sandstones and mudstones plot in arid field with low maturity.


Figure 15: Chemical maturity of the studied sandstone and mudstone expressed by bivariant plot SiO2-(Al2O3+K2O+Na2O).


The geochemistry of Quaternarys andstones and mudstones from Chabahar area, southeast Iran, were studied to determine their provenance, source area weathering and the tectonic setting in which they were deposited. Our attempt to distinguish tectonic setting from major element geochemistry is not successful. In this case, mobile elements such as Na and K seem controlled not only by provenance but also by mineralogical transformation and authigenesis, especially during diagenesis. Some trace elements, such as (Cr, Co, Th, Sc, La and Zr) better preserve the characteristics of source rocks because of their low mobility on the earths surface. Source rock weadering and provenance of the sandstones and mudstones sequences in Chabahar area have been assessed using geochemical studies. Major elements compositions suggest that these rocks were derived from low to moderately weathered protoliths. The low CIA and PIA values of these rocks a low to moderate degree of alteration of the source rocks. Discrimination plots by major and trace elements shows that most data lie in field, typical of active continental margin. The Th/Sc, Co/Th, La/ Sc and Cr/Th values point to a significant input of detrital material of intermediate composition.


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