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Geochemistry of sandstones and shales from the Ecca Group, Karoo Supergroup, in the Eastern Cape Province of South Africa: Implications for provenance, weathering and tectonic setting

Christopher Baiyegunhi
  • Corresponding author
  • Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa
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/ Kuiwu Liu
  • Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa
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/ Oswald Gwavava
  • Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa
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Published Online: 2017-08-16 | DOI: https://doi.org/10.1515/geo-2017-0028

Abstract

Geochemical compositions of twenty-four sandstone and shale samples from the Ecca Group were analysed to decipher their provenance, paleoweathering conditions and tectonic setting. The shales have high Fe2O3, K2O, TiO2, Ce, Cu, Ga, La, Nb, Nd, Rb, Sc, Sr, Th and Y content more than the sandstones, whereas, sandstones are higher in SiO2, Hf and Zr than the shales. The positive correlations of Al2O3 with other elements as well as the abundance of Ba, Ce, Th, Rb, Zn and Zr suggest that these elements are primarily controlled by the dominant clay minerals. Tectonic discrimination diagrams revealed that the sandstones and shales are mostly of quartzose sedimentary provenance, suggesting that they were derived from a cratonic interior or recycled orogen. The binary plots of TiO2 versus Ni, TiO2 against Zr and La/Th versus Hf as well as the ternary diagrams of V-Ni-Th*10 indicate that the shales and sandstones were derived from felsic igneous rocks. A-CN-K (Al2O3-CaO-K2O) ternary diagram and indices of weathering (CIA, CIW and PIS) suggest that the granitic source rocks underwent moderate to high degree of chemical weathering. The CIA values range between 24.41% and 83.76%, indicating low to high weathering conditions. The CIW values for the studied sandstones and shales range from 25.90 to 96.25%, suggesting moderate to high intensive chemical weathering. ICV values for the sandstones and shales vary from 0.71 to 3.6 (averaging 1.20) and 0.41 to 1.05 (averaging 0.82), respectively. The k2O/Na2O ratios for the studied samples vary from 0.71 to 8.29, which reveal moderate to high maturity. The plot of CIA against ICV shows that most of the shales are geochemically mature and were derived from both weak and intensively weathered source rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance.

Keywords: Geochemistry; provenance; weathering; tectonic setting; Ecca Group

1 Introduction

Clastic sedimentary rocks have vital information about the composition, tectonic setting and evolution of continental crust, mainly when the traditional petrographic methods are unclear. Nonetheless, their chemical and mineralogical composition can be influenced by factors like source rock characteristics, weathering, sorting processes during transportation, sedimentation and diagenetic processes to an extent [13]. Trace elements such as La, Y, Sc, Cr, Th, Zr, Hf, Nb and rare earth elements (REE) are thought to be useful indicators of provenance, geological processes and tectonic setting due to their relatively low mobility and insolubility during sedimentary processes [48]. Hence, the geochemistry of clastic sediments (i.e. sandstone and shale) reflects a combination of provenance, chemical weathering, hydraulic sorting, and abrasion [2, 4, 811].

In geochemical provenance studies, fine grained sedimentary rocks like shales are considered to be the most useful rock because of their homogeneity before deposition, post-depositional impermeability and higher abundance of trace elements [4, 10, 12, 13]. Some relatively immobile elements like Sc, Th, Zr, Hf and rare earth elements (REE) show very low concentrations in natural waters and are transported almost quantitatively throughout the sedimentary process from parent rocks to clastic sediments [4, 14]. The relative distribution or enrichment of these immobile elements in felsic and basic rocks have been used to infer the relative contribution of felsic and basic sources in shales from different tectonic environments [15]. For example, La and Th are enriched in felsic rocks, whereas Sc, Cr, and Co are more concentrated in basic rocks relative to felsic rocks. These elements are relatively immobile during weathering [6, 1618]. Hence, the ratios of La or Th to Co, Sc, or Cr are sensitive indicators of source rock compositions. Similarly, felsic igneous rocks contain negative Eu anomalies (Eu/Eu* from chondritenormalized plots of the REE), whereas basic igneous rocks have little or no Eu anomalies, and the size of the negative anomalies in the provenance seems to be preserved in finegrained sediment [18]. Furthermore, some major elements such as alkali and alkali earth elements, which are water mobile elements and very sensitive to climatic change, can be used as a proxy of paleoclimate evolution [19, 20]. Geochemical data on sandstones of unmetamorphosed sedimentary sequences deposited in epicratonic or intracratonic basins also give important clues on paleoweathering conditions, variations in provenance composition and tectonic settings [2123].

Several researchers such as [24] and [25] have proposed K2O/Na2O versus SiO2 tectonic setting discrimination diagrams for sedimentary rocks in order to identify tectonic setting of unknown basins. These diagrams are still commonly used to deduce the tectonic setting of ancient basins. However, more detailed results can be obtained using the calc-alkaline oxide ternary diagram (CaO-Na2O-K2O) of [26] and modified by [27]. [19, 28] documented that the index of compositional variability (ICV), K2O/Al2O3 ratio, chemical index of alteration (CIA) and Al2O3-(CaO+Na2O)-k2O (A-CN-K) ternary plots are useful geochemical parameters for the study of provenance and maturity of the rocks. In addition, [25, 29, 30] used alkali metal oxides to reveal information about the provenance of clastic sediments. Recent geochemical investigations on sandstones and shales have focused on deciphering the provenance and tectonic evolution of sedimentary basins [22, 31, 32]. However, little attention has been paid to the study of provenance in this region, despite the fact that the targeted carbonaceous shales for shale gas exploration in the region is hosted in the Ecca Group. Furthermore, it is also important in understanding the tectonic evolution of the southeastern Karoo basin. To date, the geochemistry of fine-grained rocks of the Ecca Group in the Eastern Cape Province of South Africa has not been studied in detail to determine their source rock characteristics, provenance and tectonic setting. This study was therefore aimed at evaluating the geochemistry of the Ecca shales and sandstones in the study area in order to provide information on the source rock characteristics, provenance, paleoweathering and tectonic setting using their major, trace and rare earth elements geochemistry.

2 General geology and stratigraphy

The word “Karoo” was derived from the Main Karoo Basin of South Africa to describe sedimentary fill of all basins of akin age across Gondwana. The Main Karoo Basin of South Africa is bordered in the southern part by a fold belt (Cape Fold Belt), while the northern part is held by Achaean Kaapvaal Craton [33]. It is a unique type of basin of all the Karoo basins in southern Africa because it contains the thickest and stratigraphically most complete mega-sequence of several depositories of the Permo- Carboniferous to Jurassic age sediments in southwestern Gondwana continent [33]. In addition, it serves as a datum for classifying Karoo basins in central and southern Africa. The bulk of the Karoo strata occur in the Main Karoo Basin, with maximum preserved thickness adjacent to the Cape Fold Belt in excess of 6 km [34]. The basin covers up to 700, 000 km2 and represents about 110 Ma of sedimentation spanning from 290 Ma to 180 Ma, and has its rocks covering almost half of the area of South Africa [35]. The sedimentary succession reflects changing environments from glacial to marine, deltaic, fluvial and finally aeolian [36].

The Karoo Supergroup is believed to have originated from the Gondwana Supercontinent [33]. The several lithospheric plates that separated to form the current Southern Hemisphere continents and India was once together as the Gondwana supercontinent. The southern African remnant of these continents contains the Karoo basins that include the Main Karoo Basin and Great Kalahari Basin (Kalahari Karoo, Aranos, and Mid-Zambezi Basins), as well as other smaller basins in South Africa, Namibia, Zimbabwe and Mozambique. The Karoo Supergroup has evolved from two distinct tectonic regimes sourced from the southern and the northern margin of Gondwana [37]. The southern tectonic regimes are believed to be related to processes of subduction and orogenesis along the Panthalassan (paleo- Pacific) margin of Gondwana, this resulted in the formation of a retro-arc foreland system known as the “Main Karoo Basin” in association with the primary subsidence mechanism represented by flexural and dynamic loading. The northern event was associated with extensional stresses that propagated southwards into the supercontinent from the divergent Tethyan margin of Gondwana. The Karoo Supergroup is a 12 km thick sequence of sedimentary rocks that was deposited in a large intracratonic retro-arc foreland basin in south-western Gondwana [38]. Generally, the Karoo Supergroup is subdivided into five main groups, namely; the Dwyka (Late Carboniferous), Ecca (Late Carboniferous-Early Permian), Beaufort (Late Permian-Middle Triassic), Stormberg (Late Triassic-Early Jurassic) and Drakensberg Groups (Middle Jurassic) [39]; Figure 1.The Drakensberg lavas are believed to have terminated sedimentation in the basin in the Middle Jurassic [36].

Areal distributions of lithostratigraphic units in the Main Karoo Basin (after [33, 40]).
Figure 1

Areal distributions of lithostratigraphic units in the Main Karoo Basin (after [33, 40]).

The areal distributions of lithostratigraphic units in the Main Karoo Basin is depicted in Figure 1 and Table 1. The term “Ecca” for argillaceous sedimentary strata exposed in the Ecca Pass, near Grahamstown in the Eastern Cape Province, South Africa [37]. Stratigraphically, the Ecca Group in the study area can be subdivided into five formations, namely, the Prince Albert, Whitehill, Collingham, Ripon, and Fort Brown Formations [4043]; Figure ??]. It is estimated that the group attained a thickness of about 3000 m in the southern part of the basin, while it is considerably thinner elsewhere in the northern part of the basin [44]. More details on the geology, stratigraphy and tectonic setting of the Karoo Basin can be found in [4554].

Table 1

Lithostratigraphy of the Karoo Supergroup in the study area [35].

3 Methodology

Twenty-four rock samples of the shale and sandstone representing various formations of the Ecca Group were analysed for the major and trace element (including rare earth element) concentrations. X-ray Fluorescence (XRF) analysis was performed at the Council for Geoscience in Pretoria, South Africa. XRF analysis of the major and trace elements geochemistry of the samples was performed using MagiX Fast, XRF spectrometer. Bulk sample preparation consists of drying where necessary, crushing to 10 mm, splitting and milling in a tungsten carbide milling pot to less than 75 µm. Major element analyses were analysed on fused beads, while trace elements were executed on pressed powder pellets. Major elements were analysed following the procedure documented by [55], while the concentration of trace and rare earth elements were determined by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) following the procedure described by [56]. Accuracy and precision was estimated and monitored from the control samples and duplicates. The analytical precision is better than 5% for major and trace elements. Samples were dried at 100°C (weightA) and heated at 1000°C (weight B), to determine the percentage loss on ignition (LOI) using the following equation:

%LOI=WeightAWeightBWeightAWeightcrucible×100

Several discriminatory plots of the major and trace elements were used to determine the provenance and tectonic setting. The ternary plot of Al2O3-(CaO+Na2O)-k2O (represented as A-CN-K) as well as the formulas for chemical index of alteration (CIA), chemical index of weathering (CIW) and plagioclase index of alteration (PIA) were used to quantify the degree of weathering. In the formulas, CaO* is the amount of CaO incorporated in the silicate fraction of the rock. In this study, correction for CaO from carbonate contribution was not performed due to the absence of CO2 value. Thus, to compute for CaO* from the silicate fraction, the assumption proposed by [57] was employed. Based on this, CaO values were accepted only if CaO<Na2O. Thus, when CaO>Na2O, it was presumed that the concentration of CaO is the same with that of Na2O. This procedure provides measure for the ratio of the secondary aluminous mineral to feldspar, and forms a basis for the measure of intensity of weathering. Index compositional variation (ICV) proposed by [58] was used to determine maturity of the sediments.

4 Results

4.1 Major elements

The major element compositions are quite variable but still comparable with the average compositions documented by [5961] (Table 2). All the samples show high concentration of SiO2, ranging from 55.94 to 87.99%. The Al2O3, CaO and Fe2O3 contents are moderately high, ranging from 5.91 to 16.10%, 0.06 to 16.67% and 0.47 to 6.88%, respectively. The concentrations of TiO2, MnO, MgO, Na2O, K2O and P2O5 are generally low, ranging from 0.13 to 0.80%, 0.004 to 0.197%, 0.25 to 1.58%, 0.17 to 3.27%, 0.95 to 4.42% and 0.053 to 0.219%, respectively. The sandstones are higher in SiO2 content more than the shales. On the other hand, shales are higher in Fe2O3, K2O and TiO2 contents more than the sandstones, which reflect their association with clay-sized phases [62].

Table 2

Comparing average chemical composition of the sandstones and shales from the Ecca Group with published average shales.

The abundance of Al2O3 was used as a normalization factor to make comparisons among the different lithologies because of their immobile nature during weathering, diagenesis and metamorphism [63]. Major oxides of the studied shales and sandstones were plotted against Al2O3 as depicted in Figures 2 and 3. In addition, average UCC (Upper Continental Crust) and PAAS (Post-Archaean Australian Shale) values were extracted from [64] and [4], respectively and included in the plots for comparison purposes. In the shale samples, major elements like TiO2, Fe2O3, MgO and K2O shows positive correlation with Al2O3, whereas MnO, CaO, Na2O and P2O5 shows no particular trend (Figures 2). Similarly, in the sandstone samples, TiO2, Fe2O3, MgO, K2O and P2O5 shows positive correlation with Al2O3, while TiO2, MnO, CaO and Na2O shows no particular trend (Figure 3). The strong positive correlation of these major oxides with Al2O3 indicates that they are associated with micaceous/clay minerals.

Major elements versus Al2O3 graph showing the distribution of shale samples from the Ecca Group. Average data of UCC and PAAS from [64] and [4], respectively are also plotted for comparison.
Figure 2

Major elements versus Al2O3 graph showing the distribution of shale samples from the Ecca Group. Average data of UCC and PAAS from [64] and [4], respectively are also plotted for comparison.

Major elements versus Al2O3 graph showing the distribution of sandstone samples from the Ecca Group. Average data of UCC and PAAS from [64] and [4], respectively are also plotted for comparison.
Figure 3

Major elements versus Al2O3 graph showing the distribution of sandstone samples from the Ecca Group. Average data of UCC and PAAS from [64] and [4], respectively are also plotted for comparison.

The shale and sandstone samples were normalized to UCC [64] and PAAS [4] as depicted in Figures 4. Relative to UCC, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, CaO, K2O and P2O5 in the sandstones are 1.09, 0.79, 0.73, 0.71, 0.65, 0.59, 0.83 and 1.07, respectively, which are generally comparable with the UCC. On the other hand, the average concentration of Na2O and MgO relative to UCC are 0.36 and 0.37, respectively, which are generally low as compared to UCC. The depletion of Na2O (< 1%) in the Ecca rocks can be attributed to a relatively smaller amount of Na-rich plagioclase in them. K2O and Na2O contents and their ratios (k2O/Na2O > 1) revealed that K-feldspar dominates over plagioclase (albite) feldspar. K2O enrichment relates to the presence of illite as common clay mineral in the shales and sandstones. In addition, the enrichment of CaO (averaging 0.59 relative to UCC) can be attributed to the presence of diagenetic calcite cement. Relative to UCC, the shales are low in MgO, CaO, Na2O, MnO and high in SiO2, TiO2 and Al2O3 .Al and Ti are easily absorbed on clays and concentrate in the finer, more weathered materials [65]. In support of this, XRD analysis of the Ecca shales revealed that they are dominated by kaolinite (Al2Si2O5(OH)4). In comparison with PAAS, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, CaO, Na2O, K2O and P2O5 are 1.16, 0.52, 0.62, 0.55, 0.59, 1.64, 1.27, 0.63 and 0.80, respectively, which are generally comparable with PAAS. Conversely, the average concentration of MgO relative to PAAS is low, averaging 0.43.

Spider plot of major elements showing (a) Ecca shales normalized against UCC, (b) Ecca sandstones normalized against UCC, (c) Ecca shales normalized against PAAS, (d) Ecca sandstones normalized against PAAS (after [64] and [4]).
Figure 4

Spider plot of major elements showing (a) Ecca shales normalized against UCC, (b) Ecca sandstones normalized against UCC, (c) Ecca shales normalized against PAAS, (d) Ecca sandstones normalized against PAAS (after [64] and [4]).

4.2 Trace Elements

The trace element compositions are quite variable but still comparable with the average compositions documented by [60] and [66]. In the sandstones and shales, the contents of large ion lithophile elements (LILE) like Rb, Ba, Sr and Th vary from 39 to 197 ppm, 208 to 909 ppm, 23 to 340 ppm and 2.8 to 23 ppm, respectively. The content of high field strength elements (HFSE) like Zr, Y and Nb range from 108 to 652 ppm, 9.1 to 35 ppm and 5 to 17 ppm, respectively. Similarly, transition trace elements (TTE) like Sc, V, Cr, Ni and Zn range from 3 to 16 ppm, 6.4 to 96 ppm, 4.8 to 85 ppm, 2.1 to 31 ppm and 1.5 to 121 ppm, respectively. Generally, shales have high Ce, Cu, Ga, La, Nb, Nd, Rb, Sc, Sr, Th and Y content than the sandstones. On the other hand, sandstones are higher in Hf and Zr than the shales. The high Sr content in shales indicate that Sr may be associated with calcite minerals. Based on the LILE average values, except for Sr and Ba, almost all the shale samples exhibit similar LILE abundances relative to UCC and PAAS (Figures 5a and 5b).

Spider plot of trace elements showing (a) Ecca shales normalized against UCC, (b) Ecca shales normalized against PAAS, (c) Ecca sandstones normalized against UCC, (d) Ecca sandstones normalized against PAAS (after [4, 64]).
Figure 5

Spider plot of trace elements showing (a) Ecca shales normalized against UCC, (b) Ecca shales normalized against PAAS, (c) Ecca sandstones normalized against UCC, (d) Ecca sandstones normalized against PAAS (after [4, 64]).

In contrast, the sandstones exhibit similar Th and U contents relative to UCC and PAAS but are depleted in Rb, Ba and Sr (Figure 5c and 5d). Th has very strong positive correlations with Nb in the shales and sandstones. This possibly implies that it may have been controlled by clays and/or other phases associated with clay minerals. Rb and Ba are positively correlated in the sandstone and shales, perhaps indicating a similar geochemical behaviour. These correlations indicate that their distributions are mainly controlled by illites and other minor clays. HFSE elements are enriched in felsic rocks rather than mafic rocks [67]. The concentrations of Zr, Hf and Y in the shales and sandstones are comparable with the UCC contents (Figure 5a), whereas Nb is depleted. Relative to PAAS content, the concentration of Y and Nb in the shales and sandstones are depleted, while Zr and Hf concentrations are relatively similar to PAAS contents (Figure 5b). Generally, Zr and Hf have high positive correlations and the ratio of Zr to Hf in the analysed samples range from approximately 21-43. This suggests that the elements are controlled by zircons, since the values are similar or nearly the same with those documented by [68] for zircon crystals. The average contents of Zr in the shales are lower than those in the sandstones, which perhaps indicate that the mineral zircon tend to be preferentially concentrated in finegrained sands. TTE in the Ecca sandstones and shales are depleted in comparison with UCC and PAAS (Figure 5a), except for Cu and Zn, which is relatively comparable with the UCC and PAAS in the shales. The TTE in the studied sandstones do not behave uniformly. Among TTE, Sc correlated positively with Ni which indicates that it is mainly concentrated in the phyllosilicates.

4.3 Provenance

The composition of major element or oxides in sandstones and shales has also been used to determine sedimentary provenance by the application of discriminant function analysis [69]. This discriminant function analysis distinguishes between four major provenance fields, namely, mafic igneous, intermediate igneous, felsic igneous and quartzose sedimentary or recycled. In Figure 6a, the sandstone samples plotted in the quartzose sedimentary provenance field. In contrast, the shale samples are scattered in both quartzose sedimentary provenance and intermediate igneous provenance field, but they are mostly within the quartzose sedimentary provenance field (Figure 6b). The binary plot of TiO2 versus Zr shows that all the shale and sandstone samples are from felsic igneous rocks (Figure 7).

Major element Discriminant Function diagram for sedimentary provenance (a) shale, (b) sandstones (after [68]). The discriminant functions are: Discriminant Function 1 = (−1.773 TiO2) + (0.607 Al2O3) + (0.760 Fe2O3) + (−1.500 MgO) + (0.616 CaO) + (0.509 Na2O) + (−1.224 K2O) + (−9.090); Discriminant Function 2 = (0.445 TiO2) + (0.070 Al2O3) + (−0.250 Fe2O3) + (−1.142 MgO) + (0.438 CaO) + (1.475 Na2O) + (−1.426 K2O) + (-6.861).
Figure 6

Major element Discriminant Function diagram for sedimentary provenance (a) shale, (b) sandstones (after [68]). The discriminant functions are: Discriminant Function 1 = (−1.773 TiO2) + (0.607 Al2O3) + (0.760 Fe2O3) + (−1.500 MgO) + (0.616 CaO) + (0.509 Na2O) + (−1.224 K2O) + (−9.090); Discriminant Function 2 = (0.445 TiO2) + (0.070 Al2O3) + (−0.250 Fe2O3) + (−1.142 MgO) + (0.438 CaO) + (1.475 Na2O) + (−1.426 K2O) + (-6.861).

TiO2–Zr plot of shales and sandstones samples from the Ecca Group (background field after [70]).
Figure 7

TiO2–Zr plot of shales and sandstones samples from the Ecca Group (background field after [70]).

Again, the TiO2-Ni diagram of [71] revealed that the source area for most of the samples are predominantly of acidic magmatic nature (Figures 8), despite the fact that a few samples plotted outside the field assigned for felsic source. The bivariate plot of La/Th against Hf (Figure 9) and ternary diagram of V-Ni-Th*10 (Figure 10) indicate that the studied sandstones and shales are derived from felsic source rocks.

TiO2 versus Ni bivariate plot for (a) sandstones, (b) shales from the Ecca Group (after [71]). The majority of the samples plot near the acidic source field.
Figure 8

TiO2 versus Ni bivariate plot for (a) sandstones, (b) shales from the Ecca Group (after [71]). The majority of the samples plot near the acidic source field.

Plot of Hf versus La/Th for the Ecca (a) shales, (b) sandstones (background field after [72]).
Figure 9

Plot of Hf versus La/Th for the Ecca (a) shales, (b) sandstones (background field after [72]).

V-Ni-Th*10 triangle diagram for the Ecca (a) sandstones, (b) shale samples (background field after [13]). Shaded area represents composition of the felsic, mafic, and ultramafic rocks.
Figure 10

V-Ni-Th*10 triangle diagram for the Ecca (a) sandstones, (b) shale samples (background field after [13]). Shaded area represents composition of the felsic, mafic, and ultramafic rocks.

4.4 Paleoweathering conditions

Intensity of chemical weathering of source rocks are mainly controlled by the composition of the source rock, duration of weathering, climatic conditions and rates of tectonic uplift of source region [9]. Several researchers like [4, 19, 73, 74] have documented that about 75% of labile materials in the upper crust are composed of feldspars and volcanic glass. Chemical weathering of these materials resulted in the formation of clay minerals. During chemical weathering, Ca, Na and K are largely removed from source rocks and the amount of these elements surviving in sediments derived from the rocks served as indicator of the intensity of chemical weathering [2]. According to [75], if siliciclastic sedimentary rocks are free from alkali related post-depositional modifications, then their alkali contents (k2O + Na2O) and k2O/Na2O ratios should be considered as reliable indicators of the intensity of source material weathering. In order to determine the degree of source rock weathering, a few indices of weathering have been proposed based on the molecular proportions of mobile and immobile element oxides (Na2O, CaO, k2O and Al2O3). Thus, the chemical composition of weathering products in a sedimentary basin is expected to reveal the mobility of various elements during weathering [76]. The indices ofweathering/alteration include chemical index of alteration (CIA), chemical index of weathering (CIW) and plagioclase index of alteration (PIA). Chemical index of alteration (CIA) proposed by [77] is the most widely used chemical index to determine the degree of source area weathering. [77] defined the CIA formula to evaluate the degree of chemical weathering as:

CIA=[Al2O3/(Al2O3+CaO+Na2O+K2O)]×100

Where CaO* is the content of CaO incorporated in silicate fraction.

The value of CIA gives a measure of the ratio of original/primary minerals and secondary products such as clay minerals. CIA values range from almost 50 in case of fresh rocks to 100 for completely weathered rocks. Thus, CIA values increase with increasing weathering intensity, reaching 100 when all the Ca, Na and K have been leached from weathering residue. The CIA values in the Ecca sandstone and shale samples range from 24.41 to 83.76 (averaging 66.30), and 53.77 to 78.28 (averaging 69.74), respectively. These CIA average values revealed relatively moderate to high degree of chemical weathering in the source area. In addition to CIA, chemical index of weathering (CIW) also provides information on the intensity of chemical weathering the sediments have undergone. In comparison to other weathering indices, the CIW is a superior method involving restricted number of components that are well-known with consistent geochemical behaviour during weathering. The CIW formula as expressed by [78] is shown below:

CIW=[Al2O3/(Al2O3+CaO+Na2O)]×100

The CIW values of the studied sandstone and shale samples range from 25.90 to 92.70 (averaging 75.20), and 76.23 to 96.25 (averaging 82.36) respectively. These CIW values point to moderate-high intensive chemical weathering. As documented by [74], source area weathering and elemental redistribution during diagenesis also can be assessed using the plagioclase index of alteration (PIA). PIA monitors and quantifies progressive weathering of feldspars to clay minerals [22, 74]. The maximum value of PIA is 100 for completely altered materials (i.e. kaolinite and gibbsite) and weathered plagioclase has PIA value of 50. [74] defined the PIA formula to evaluate the amount of chemical weathering as:

PIA=[(Al2O3K2O)/(Al2O3+CaO+Na2OK2O)]×100

The PIA values of the studied sandstone and shale samples range from 21.910 to 91.82 (averaging 72.23) and 54.92 to 95.14 (averaging 78.76), respectively. Again, The PIA values suggest moderate-high or intense destruction of feldspars during source weathering, transport, sedimentation, and diagenesis. During the initial stages of weathering, Ca is quickly leached than Na and K. With increasing weathering, the total alkali content (K2O + Na2O) decreases with increase in K-Na ratio (K2O/Na2O). This is due to destruction of feldspars among which plagioclase is more favourably removed than K-feldspars [19]. Feldspathic materials in the sandstones and shales were subjected to variable intensities of weathering during the different evolution stages. The individual bivariate plots of K2O/Na2O, K2O + Na2O, Na2O, K2O and CaO against PIA can be used to unravel the mobility of elements during the final stage of chemical weathering of previously altered feldspars. In the bivariate plot of K2O/Na2O against PIA (Figure 11a), the values of K2O/Na2O in the sandstone commonly increases with increasing value of PIA. On the contrary, in the plot of K2O + Na2O versus PIA (Figure 11b), the total content of alkalis in most of the samples decrease with increasing value of PIA. Figures 11 and 12 (c-e) shows the behaviour of Na, Ca and K during progressing weathering of feldspars in the sandstones and shales.

Bivariate diagrams depicting mobility of elements during weathering of feldspars in the sandstone samples from the Ecca Group. (a) (K2O/Na2O) wt. % versus PIA. (b) (K2O + Na2O) wt. % versus PIA. (c) Na2O wt. % versus PIA. (d) CaO wt. % versus PIA. (e) K2O wt. % versus PIA.
Figure 11

Bivariate diagrams depicting mobility of elements during weathering of feldspars in the sandstone samples from the Ecca Group. (a) (K2O/Na2O) wt. % versus PIA. (b) (K2O + Na2O) wt. % versus PIA. (c) Na2O wt. % versus PIA. (d) CaO wt. % versus PIA. (e) K2O wt. % versus PIA.

Bivariate diagrams depicting mobility of elements during weathering of feldspars in the shale samples from the Ecca Group. (a) (K2O/Na2O) wt. % versus PIA. (b) (K2O + Na2O) wt. % versus PIA. (c) Na2O wt. % versus PIA. (d) CaO wt. % versus PIA. (e) K2O wt. % versus PIA.
Figure 12

Bivariate diagrams depicting mobility of elements during weathering of feldspars in the shale samples from the Ecca Group. (a) (K2O/Na2O) wt. % versus PIA. (b) (K2O + Na2O) wt. % versus PIA. (c) Na2O wt. % versus PIA. (d) CaO wt. % versus PIA. (e) K2O wt. % versus PIA.

Ternary plot of A-CN-K proposed by [19] is another method that can be used to assess the composition of original source rock as well as the mobility of elements during the process of chemical weathering of source material and post-depositional chemical modifications. The ternary plot of Al2O3-(CaO+Na2O)-K2O (represented as A-CN-K) is useful for identifying compositional changes of shales and sandstones that are related to chemical weathering, diagenesis and source rock composition. Geochemical data of the sandstones and shales from the Ecca Group were plotted in an A-CN-K diagram (Figure 13). The arrows 1 to 5 in Figure 13 represent the weathering trends of gabbro, tonalite, granodiorite, adamellite and granite, respectively [79]. In the A-CN-K diagram (Figure 13), the sandstones and shales plotted above the line joining plagioclase and potash feldspar. The plots define a narrow linear trend which runs slightly at an angle parallel to the ACN edge. This is possibly due to the fact that the removal rate of Na and Ca from plagioclase is generally greater than the removal rates of K from microcline [19]. The plot trend towards illite on the A-K edge and does not show any inclination towards the K apex, thus indicating that the sandstones and shales are free from potash metasomatism during diagenesis. The trend line when extended backward intersects the plagioclase-potash feldspar join near arrow 3, which is the field of granodiorite (potential ultimate source). Linear weathering trend point to steady state of weathering conditions where material removal matches with production of weathering material [2].

A-CN-K ternary diagram of molecular proportions of Al2O3-(CaO+Na2O)-K2O for (a) sandstone, (b) shale samples from the Ecca Group (background field after [19]). The dotted arrow shows the actual weathering trend for the samples. The CIA scale shown at the left side is for comparison.
Figure 13

A-CN-K ternary diagram of molecular proportions of Al2O3-(CaO+Na2O)-K2O for (a) sandstone, (b) shale samples from the Ecca Group (background field after [19]). The dotted arrow shows the actual weathering trend for the samples. The CIA scale shown at the left side is for comparison.

4.5 Climatic conditions and sediment maturity

The original character and maturity of sediments as well as the prevailed climatic conditions can be determined by calculating the index compositional variation (ICV) proposed by [58]. The ICV tends to be highest in minerals that are high in weathering intensity and decreases in more stable minerals (less weathered minerals). The ICV decreases further in the montmorillonite group clay minerals and lowest in the kaolinite group minerals [58]. In addition, more mature shale tends to have low ICV values (< 1.0).

ICV=(Fe2O3+K2O+Na2O+CaO+MgO+MnO)/Al2O3

As documented by [58], sandstones or shales with ICV > 1 are compositionally immature with the first cycle of sediments deposited in tectonically active settings. On the other hand, those with ICV < 1 are compositionally mature and were deposited in the tectonically quiescent or cratonic environment where sediment recycling was active. For the studied sandstones and shales, the ICV values range from 0.71 to 3.6 (averaging 1.20) and 0.41 to 1.05 (averaging 0.82).Based on the average ICV values, it can be inferred that the sandstones are compositionally immature whereas the shales are compositionally mature and deposited in the tectonically quiescent or cratonic environment. The K2O/Na2O ratios for the sandstones vary from 0.71 to 7.16 (averaging 1.61) and 1.04 to 8.29 (averaging 3.26) for the shale samples. These ratios revealed moderate to high maturity of the shales, which agrees with the ICV values [9]. The K2O/Na2O ratios are comparable to those of sediments from passive margins, which increase with maturity of rocks [24]. The binary plot of CIA against ICV for the studied samples (Figure 14) shows that most of the shales are geochemically mature and were derived from both weak and intensively weathered source rocks.

Binary plot of CIA against ICV for the Ecca shale and sandstone samples.
Figure 14

Binary plot of CIA against ICV for the Ecca shale and sandstone samples.

Alternatively, SiO2/Al2O3 ratios of siliciclastic rocks are sensitive to sediment recycling and weathering process and can serve as an indicator of sediment maturity. With increasing sediment maturity, quartz survives preferentially to feldspars, mafic minerals and lithics [25, 80]. The average SiO2/Al2O3 ratios in unaltered igneous rocks range from ~ 3.0 (basic rocks) to ~ 5.0 (acidic rocks). Values of SiO2/Al2O3 ratio > 5.0 in sandstones and shales point to progressive maturity [80]. The SiO2/Al2O3 ratios of the sandstones vary from 4.16 to 14.77 (averaging 9.45), while those of the shales range from 3.94 to 14.89 (averaging 5.50). The K2O/Na2O ratios of the sandstones range from 0.76 to 7.16 (averaging 1.61), while those of the shales vary from 1.04 to 8.29 (averaging 3.26). The low values of K2O/Na2O as well as the high values of SiO2/Al2 O3 indicate low to moderate sediment maturity. To constrain the climatic condition during sedimentation of siliciclastic sedimentary rocks, the proposed plot of SiO2 against (Al2O3 + K2O + Na2O) after [81] was used to classify the maturity of Ecca sandstones and shales as a function of climate. Figure 15 shows that the sandstones and shales mostly plot in the field of arid climate with few samples plotting in the humid climate field with varied maturity.

Chemical maturity of the Ecca sandstones and shales (background field after [81]).
Figure 15

Chemical maturity of the Ecca sandstones and shales (background field after [81]).

4.6 Tectonic setting of the source area

Several researchers like [5, 24, 25] documented that the chemical compositions of siliciclastic sedimentary rocks are considerably controlled by plate tectonic settings of their provenances and depositional basins. Thus, siliciclastic rocks from different tectonic settings possess terrain-specific geochemical signatures. Tectonic setting discrimination diagrams give reliable results for siliciclastic rocks that have not been strongly affected by postdepositional weathering and metamorphism [8]. Bivariate plots of major and trace element geochemistry have been used by several researchers to determine the tectonic setting of sandstones and shales [5, 24, 25, 27, 30, 82]. Among the various tectonic setting discrimination diagrams, the major element-based discrimination diagrams of [5] and [24] are widely used. [24] divided series of tectonic plots to differentiate between four main tectonic settings, namely, oceanic island arc (OIA), continental island arc (CIA), active continental margin (ACM) and passive continental margin (PM). Chemical analyses data of the sandstones and shales have been plotted on 4 tectonic setting discrimination diagrams of [5, 24, 25, 27]. Bivariate plots of TiO2 versus (Fe2O3 + MgO) and Al2O3/SiO2 against Fe2O3 + MgO shows that most of the studied samples plotted in the passive margin, active continental margin and continental island arc fields (Figures 16a and 16b). In Figure 16b, only one sample plotted in the oceanic island arc field.The Bivariate plot of k2O/Na2O versus (Fe2O3 + MgO) and Al2O3/(CaO + Na2O) versus (Fe2O3+ MgO) revealed that the sandstones and shales are related to passive margin and active continental margin (Figures 16c and 16d).

Bivariate plots of (a) TiO2 (wt.%) versus (Fe2O3 + MgO) (wt.%), (b) Al2O3/SiO2 versus Fe2O3 + MgO (wt.%), (c) K2O/Na2O versus (Fe2O3+ MgO) (wt.%), (d) Al2O3/(CaO + Na2O) versus (Fe2O3+ MgO) (wt.%) of the Ecca rocks on the tectonic setting discrimination diagram of [24]. PM: Passive Margin, ACM: Active Continental Margin, CIA: Continental Island Arc, OIA: Oceanic Island Arc.
Figure 16

Bivariate plots of (a) TiO2 (wt.%) versus (Fe2O3 + MgO) (wt.%), (b) Al2O3/SiO2 versus Fe2O3 + MgO (wt.%), (c) K2O/Na2O versus (Fe2O3+ MgO) (wt.%), (d) Al2O3/(CaO + Na2O) versus (Fe2O3+ MgO) (wt.%) of the Ecca rocks on the tectonic setting discrimination diagram of [24]. PM: Passive Margin, ACM: Active Continental Margin, CIA: Continental Island Arc, OIA: Oceanic Island Arc.

Most of the shale samples represent the active continental margin and few samples fall in the passive continental margin field (Figure 17a). In addition, only one sample from the Prince Albert Formation plotted in the island arc field. Active continental margins are subduction related basins, continental basins and pull-apart basins associated with strike-slip fault zones. On the other hand, passive continental margins are basins on continental crust and basins associated with ocean floor spreading, failed rifts and Atlantic-type continental margins. Most of the sandstone samples plotted in the passive continental margins and two samples each falls in the island arc and active continental margin fields (Figure 17b). Similarly, more detailed results can be obtained using the calc-alkaline ternary diagram (CaO-Na2O-K2O). The calcalkaline ternary diagram depicted in Figure 18a shows that most sandstone samples are related to passive continental margin. Shale samples are also represented in both active continental margin and passive continental margin (Figure 18b). Th-Sc-Zr/10 tectonic discrimination diagram of [5] revealed that the source area for most of the Ecca samples is predominantly of passive continental margin (Figure 19). Several researchers like [5, 24, 25] have documented that immobile elements like La, Zr and Hf are enriched in the passive margin setting. La-Th-Sc tectonic discrimination diagram demonstrates the passive margin setting of the depositional basin for the Ecca sandstones and shales (Figure 20).

K2O/Na2O versus SiO2 tectonic-setting discrimination diagram for Ecca (a) shales, (b) sandstones (background field after [25]).
Figure 17

K2O/Na2O versus SiO2 tectonic-setting discrimination diagram for Ecca (a) shales, (b) sandstones (background field after [25]).

Na2O-CaO-K2O ternary plot for (a) sandstone, (b) shale samples from the Ecca Group (background field after [27]). OIA = oceanic island arc, CIA = continental island arc, ACM = active continental margin, PM = passive continental margin.
Figure 18

Na2O-CaO-K2O ternary plot for (a) sandstone, (b) shale samples from the Ecca Group (background field after [27]). OIA = oceanic island arc, CIA = continental island arc, ACM = active continental margin, PM = passive continental margin.

Th-Sc-Zr/10 tectonic discrimination diagram for the Ecca (a) sandstones, (b) shales (background field after [5]). A = oceanic island arc B = continental island arc, C = active continental margin, D = passive continental margin.
Figure 19

Th-Sc-Zr/10 tectonic discrimination diagram for the Ecca (a) sandstones, (b) shales (background field after [5]). A = oceanic island arc B = continental island arc, C = active continental margin, D = passive continental margin.

La-Th-Sc tectonic discrimination diagram for the Ecca (a) sandstones, (b) shales (background field after [5]). A = passive and active continental margin, B = continental island arc, C = oceanic island arc.
Figure 20

La-Th-Sc tectonic discrimination diagram for the Ecca (a) sandstones, (b) shales (background field after [5]). A = passive and active continental margin, B = continental island arc, C = oceanic island arc.

5 Discussion and conclusion

Selected trace elements such as La, Ce, Nd, Y, Th, Zr, Hf, Nb, Sc, Co and Ti are useful in discriminating provenance and tectonic setting of sedimentary basins because of their relative immobility during sedimentary processes [5, 83]. These elements are present in very low concentrations in sea and river water, chiefly transported as particulate matter and reflect the signature of the parent material [5, 24, 83]. The trace element data of sandstones and shales from the Ecca Group are generally comparable with the trace element data of UCC and PAAS. In the studied samples, the concentration of high field strength elements like Hf and Zr are higher in the sandstones than the shale samples. The low content of these elements and elemental ratios like La/Sc and Th/Sc points to the presence of fractionated source rocks with lower compatible element contents and recycled sediments in the source area. The variation in chemical composition depicts change in the supply of material and a variation in physic-chemical environment of deposition. The pattern of geochemical behaviour of individual element shows that most of the trace elements that found their way into the ancient sediments seems to have invaded the lattices of the silicates and clay minerals and structurally combined with them.

The geochemical data of major and trace elements show that the studied sandstone and shales have the same source. Based on the discriminant function plots, it can be inferred that the sandstones and shales are mostly of quartzose sedimentary provenance, suggesting that they were derived from a cratonic interior or recycled orogen. The binary plots of TiO2 versus Ni suggest that most of the Ecca samples are sourced from an acidic magmatic nature (Figure 8). Furthermore, the binary plots of TiO2 versus Zr (Figure 7), La/Th against Hf (Figure 9) and the ternary diagrams of V-Ni-Th*10 (Figure 10) shows that the shale and sandstone samples were derived from felsic igneous rocks. The results of the trace elements correlate well with that of the major elements.

The CIA values (24.41-83.76%) in the Ecca rocks indicate low to high weathering degree of the source rocks. The relatively high CIA values (53.77 to 78.28%) in the shales probably reflect the presence of clay minerals and low percentage of detrital feldspars. Alternatively, the low CIA values in some of the shales (< 40%) point to a low weathering condition in the source area and perhaps reflect cool conditions. The variations in CIA reflect changes in the properties of feldspar versus aluminous clay minerals. The observed changes in the CIA values indicate that the sediments are moderately to highly weathered, which possibly suggests that the sediments were derived from source rocks that have been subjected to both chemical and physical weathering. The CIW values for the studied sandstones and shales range from 25.90 to 96.25%, suggesting moderate to high intensive chemical weathering. In the analysed samples, the CIW values are higher than those of CIA due to exclusion of K2O from the index. Based on the CIW values, the Ecca sandstones and shales are inferred to have undergone moderate to high chemical weathering. The PIA values of the studied samples range from 21.910 to 95.14%. Again, The PIA values suggest moderate to high or intense destruction of feldspars during source weathering, transport, sedimentation and diagenesis. During the initial stages of weathering, Ca is quickly leached than Na and K. With increasing weathering, the total alkali content (K2O + Na2O) decreases with increase in K-Na ratio (K2O/Na2O). This is due to destruction of feldspars among which plagioclase is more favourably removed than K-feldspars [2, 19].

The bivariate plots of Na2O versus PIA, K2O against PIA and CaO versus PIA shows weak correlation which could be attributed to the presence K-bearing minerals (i.e. muscovite and biotite) as well as the retention of most of the mobilized K by aluminous material resulting in the formation of illite. The above described and documented imprints of progressing chemical weathering of detrital feldspars perhaps give an impression (may be misleading) that major event of chemical weathering of detrital feldspars has taken place essentially in the terminal basin prior to the lithification of the detritus and during diagenesis. The observed systematic depletion of Na2O content with increasing degree of chemical weathering of the feldspars (Figures 11c and 12c) tend to support above assumption. However, it is known that detrital feldspar grains can survive more than one sedimentary cycle. The A-CN-K diagram of the sandstone and shale samples show that paleoclimate of the source area was relatively warm, which caused chemical weathering of source rocks reducing some initial feldspar in source rocks. Fairly long distance of transport, (i.e., perhaps hundreds of kilometres) is also an important process responsible for further reduction of feldspars. Diagenetic alteration of feldspar was the most important factor reducing feldspar in the sandstone.

The ICV values for the Ecca sandstones and shales range from 0.41 to 3.6, while k2O/Na2O ratios vary from 0.71 to 8.29. The K2O/Na2O ratios are comparable to those of sediments from passive margins, which increase with maturity of rocks (Bhatia, 1983). The binary plot of CIA against ICV (Figure 14) shows that most of the shales are geochemically mature and were derived from both weak and intensively weathered source rocks. The bivariate plot of k2O/Na2O versus (Fe2O3+ MgO), Al2O3/CaO + Na2O) against (Fe2O3+ MgO) and log (K2O/Na2O) versus SiO2 revealed that the sandstones and shales are generally related to passive margin and active continental margin. The Na2O-CaO-K2O ternary plot after [27] suggests passive continental margin and active continental margin provenance for the Ecca shales and sandstones. Triangular Th-Sc-Zr/10 tectonic discrimination diagram of [5] revealed that the source area for most of the Ecca samples are predominantly of passive continental margin with a minor contribution from continental island arc and active continental margin sources (Figure 19). The La-Th-Sc ternary plot also suggests passive and active margin settings for the Ecca samples. The similarity of the La/Th versus Hf diagram (Figure 9) and the La-Th-Sc tectonic discrimination diagram (Figure 20) is a clear indication of a passive-active margin depositional basin. The data from these immobile trace elements correlate well with Figure 17 (K2O/Na2O versus SiO2) as well as Figure 18 (Na2O-CaO-K2O).

In summary, the study of paleoweathering conditions based on chemical index of alteration (CIA), plagioclase index of alteration (PIA) and A-CN-K (Al2O3-CaO+Na2O-K2O) indicate that perhaps chemical weathering in the source area and recycling processes have been more significant in the shales and sandstones. The CIA values indicate low to high weathering conditions of the samples and the paleoclimate of the source area was probably warm. The tectonic setting discrimination diagrams support passiveactive continental margin setting of the provenance.

Acknowledgement

Thanks to National Research Foundation (NRF) of South Africa (UID: 101980) and Govan Mbeki Research and Development Centre (GMRDC) of the University of Fort Hare for financial support. The authors thank the editors and anonymous reviewers for their constructive comments, which really help in improving the manuscript.

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About the article


Received: 2016-11-16

Accepted: 2017-04-11

Published Online: 2017-08-16


Declaration of interestThe authors declared that there is no any actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations regarding the publication of this original manuscript.


Citation Information: Open Geosciences, Volume 9, Issue 1, Pages 340–360, ISSN (Online) 2391-5447, DOI: https://doi.org/10.1515/geo-2017-0028.

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© 2017 Christopher Baiyegunhi et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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