An inorganic geochemical investigation of mudrocks and sandstone from the southern Bredasdorp Basin, off the south coast of South Africa was carried out to unravel the provenance, paleoweathering, and tectonic setting of the basin. Seventy-seven representative samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 underwent geochemical analysis involving major and trace elements. The major oxide compositions show that the sandstones could be classified as sub-arkose and sub-lithic arenite. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the Bredasdorp sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr, and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th∗10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Also, the closely similar compositions of the analysed samples and recent sedimentary rocks of the East African Rift System perhaps suggest a rifted basin tectonic setting for the Bredasdorp Basin. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations.
The geochemistry of clastic sedimentary rocks is a vital tool used in the study of provenance, paleoweathering conditions, and tectonic setting as well as to constrain the geodynamic development and composition of the upper continental crust (UCC) [1,2]. The mineralogical and chemical compositions of these rocks are the products of numerous variables that include provenance, weathering conditions, transport, diagenesis, climate, and tectonism . In geochemical studies, some designated trace elements such as La, Y, Sc, Cr, Th, Zr, Hf, Nb, and major oxides especially TiO2 are sensitive indicators of the source rocks, provenance, paleoweathering and paleoclimatic conditions, and tectonic setting [3,4]. This is due to their comparatively low mobility and insolubility during sedimentary processes [5,6,7]. Furthermore, the relative distribution of the immobile trace elements with varying concentrations in felsic and basic rocks has been employed to deduce the relative contribution of felsic and basic sources in shales from different tectonic environments . For example, trace elements like La and Th are enriched in felsic rocks, while Sc, Cr, and Co are more concentrated in basic rocks. Also, the relative reduction of oxides like Na2O and CaO, and enrichment of TiO2 and SiO2 give important clues for tectonic setting. Therefore, the distribution of these major oxides and trace elements in mudrocks and sandstones gives evidences or clues to the geological processes, provenance, and tectonic settings of their respective sources [6,7,8,9,10,11].
Sandstones have been widely investigated for tectonic provenance using the traditional classic petrographic methods. However, in some instances, sandstones with a significant amount of fine fraction may contain trace element ratios that could provide added information about the provenance. On the other hand, shales are the most abundant type of sediment in sedimentary basins worldwide and they are alleged to represent average crustal provenance composition much better than sandstones [12,13]. In geochemical provenance studies, fine-grained sedimentary rocks like shales are considered to be the most useful rocks due to their homogeneity before deposition, post-depositional impermeability, and higher abundance of trace elements [3,14,15,16]. Several researchers including Bhatia  and Roser and Korch  have highlighted that major element geochemistry of the clastic sedimentary rocks are very useful in discriminating between different tectonic settings. In fact, they proposed the use of bivariate diagram of K2O/Na2O versus SiO2 and calc-alkaline oxide (CaO–Na2O–K2O) ternary diagram to decipher tectonic setting of unknown basins. These diagrams are still widely used to unravel the tectonic setting of ancient basins. However, McLennan et al.  reported that the trace elements like La, Y, Sc, Cr, Th, Zr, Hf, and Nb, especially when used in combination with TiO2, are most appropriate for provenance and tectonic setting determination studies due to their fairly low mobility during sedimentary deposition. Major element analysis is also applicable in interpreting paleoweathering and tectonic setting [3,20,21]. Immobile elements like Al, Ti, and Zr are also helpful when estimating the nature of source rock [3,22]. 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 [23,24].
There have been quite a number of recent studies that investigated the elemental composition of mudrocks and sandstones with the aim of evaluating their provenance [6,11,25,26,27,28]. However, most of these studies widely used the traditional petrographic techniques in the calculation and interpretation of detrital modes of sandstones, and consequently, in the determination of their provenance and paleoweathering conditions [29,30]. In addition, the mineralogy of mudrocks, especially shale has been less considered or ignored due to the alteration of clay minerals during weathering and diagenesis . Consequently, up to date, little attention has been paid to the study of provenance, paleoweathering, and tectonic setting of the Bredasdorp Basin, despite the hydrocarbon potential of the basin. Also, there is no known documented or reported classification of the Bredasdorp sandstones, and previous geochemical studies have successfully used geochemistry to solve problems associated with sandstone petrology. Geochemical classification of sandstones is worth attempting due to the fact that modern analytical methods are generating extensive datasets on the composition of rocks. Hence, this study was aimed at evaluating the geochemistry of the Bredasdorp mudrocks and sandstones in order to provide information on the chemical classification, source rock characteristics, provenance, paleoweathering, and tectonic setting using their major and trace elements geochemistry.
2 Geological setting
The Bredasdorp Basin is the westernmost sub-basin of the greater Outeniqua Basin (Figure 1). The basin was developed along the South African continental margin, underneath the Indian Ocean due to extensional episodes during the initial stage of rifting in the Late Jurassic–Early Cretaceous . Sediments deposition in the Bredasdorp Basin was primarily controlled by the early continental rifting and tectonic development . The basin serves as a depocentre and was predominantly infilled with Late Jurassic and Early Cretaceous marine and continental sediments . The deposition of these syn-rift marine and continental sediment continued in the rift system until about 126 Ma, when most of the faulting stopped . So, the syn-rift deposition commenced and subsequently initiated a transitional phase and thereafter a drifting phase . The development of the Bredasdorp Basin is linked to the breakup of Gondwanaland and subsequent formation is associated with thermal subsidence in response to a single rifting event . The tectonic development of the Bredasdorp Basin is summarised in Table 1.
|1||Middle Jurassic–Valanginian (Basement–1At1)||Syn-rift I||Extension-instigated or driven subsidence and syn-rift basin fill. Isostatic uplift on both sides of the half-graben led to significant reduction in the erosion of syn-rift I sediments. Great or intensive marginal uplift and erosion of the northern part of the basin removed the whole syn-rift I succession in some places|
|2||Late Valanginian–Hauterivian (1At1–6At1)||Syn-rift II||Fast subsidence and extensive flooding. Continued or non-stop uplift caused additional truncation or reduction in structural highs. The deposition of deep water sediments happened within rift depocentres (southern sub-Basin and Arniston half-graben) led to the deposition of source rocks|
|3||Hauterivian–Aptian (6At1–13At1)||Transitional (Early drift)||Progradation enlargement of the shelf in the northern part over the Arniston half-graben, plus a sustained deepening of the southern sub-basin|
|4||Aptian–Maastrichtian (13At1–15At1)||Middle drift||Regional subsidence instigated by thermal cooling and sediment loading. Continued movement on the Arniston Fault|
|5||Paleocene–present day (15At1–seafloor)||Late drift||Oil-prone source in syn-rift depocentres and northern and central part of the Bredasdorp Basin proceed into the main phase of oil generation. Continued slight subsidence interrupted by the early tertiary alkaline intrusion activity over the central part of the basin. Late slanting of the basin together with uplift of the northern side led to the late removal or erosion of about 600 m succession in some places|
The Bredasdorp Basin is envisaged to host an Oxfordian (∼160 Ma) – recent stratigraphic column that overlies the rocks of the Cape Supergroup . The Late Jurassic–Early Cretaceous shallow marine and fluvial syn-rift deposits underlie Albian to recent marine sediments . The stratigraphic column shows the existence of a Middle Jurassic–Early Cretaceous syn-rift phase and it is subsequently overlain by the early Cretaceous to Tertiary post-rift phase . The syn-rift and drift (post-rift) sequences are separated by a regionally developed unconformity referred to as the 1At1, and it took place in the Lower Valanginian time  (Figure 2). The syn-rift sedimentation phases of the Bredasdorp Basin are subdivided into the syn-rift I and syn-rift II phases . According to ref. , the syn-rift I phase occurred in the middle Jurassic–late Valanginian (Basement up to 1At1), whereas the syn-rift II took place in the Late Valanginian–Hauterivian (1At1–6At1).
In the northern part of the basin, the basal syn-rift deposits that occurred during the Kimmeridgian–late Valanginian are divided into four intervals (from base to top), namely the Lower Fluvial (LF), Lower Shallow Marine (LSM), Upper Fluvial (UF), and Upper Shallow Marine (USM) intervals. The above-mentioned intervals underlie the late Valanginian 1At1 regional unconformity  (Figure 2). The LF interval consists of mudrocks, sandstones, and conglomerates signifying an early graben fill deposition in an alluvial fan and fluvial environments . On the other hand, the LSM interval points to the first marine invasion in the basin and it is made up of glauconitic sandstones, perhaps suggesting progradational anoxic marine deposits of the Kimmeridgian age [37,40]. The UF interval is made up of meandering fluvial and alluvial floodplain deposits, whereas the overlying USM interval is marked by the occurrence of massive glauconitic and fossil-rich sandstones of the Late Valanginian age. Broad et al.  reported that these massive glauconitic fossiliferous sandstones were laid down as transgressive beach facies along the northern and southern edges of the Bredasdorp Basin and it extends into the neighbouring sub-basins.
The syn-rift I succession is terminated by the 1At1 regional unconformity, separating deep-marine sediments from the underlying USM sediments. This 1At1 unconformity signifies the beginning of a renewed rifting (syn-rift II) phase initiated due to early or initial movement along the Agulhas–Falkland Fracture Zone at approximately 133 Ma (Valanginian–Hauterivian boundary) . The syn-rift II was later followed by the Transitional (early drift) phase, which occurred during Hauterivian–Early Aptian (6At1–13At1)  (Figure 2). The Transitional (early drift) phase was dominated by recurrent episodes of progradation and aggradation and it was mostly affected by tectonic events and eustatic sea-level changes [36,40]. The Transitional phase is considered the first deep water deposits in the Bredasdorp Basin and they were deposited due to major subsidence of the basin as well as an increase in water depth. On the other hand, the late drift phase trailed a major marine regression in the Bredasdorp Basin during the early Aptian . This regression event resulted in a major erosion which is marked by the 13At1 unconformity. The erosion period is followed by a marine transgression, which carried and deposited organic-rich mudstone, shale, or claystone in the basin under an anoxic condition . The onset of the late drift phase is manifested or noted by the 14At1 mid Albian unconformity (Figure 2), which marks the beginning of the active thermally instigated subsidence when the Columbine-Agulhas Arch was cleared by the trailing edge of the Falkland Plateau in the late Albian [40,41,42].
A total of seventy-seven representative mudrock and sandstones collected from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 (Figure 1) were studied under the petrographic microscope and analysed for the major oxides and trace element concentrations. X-ray fluorescence (XRF) analysis was performed at the geochemical laboratory in the Department of Geology and Mining, University of Limpopo, South Africa. The rock samples were neatly cleaned and crushed into smaller sizes using the Braun Chipmunk VD67 jaw crusher. Thereafter, the Herzog milling machine was used to mill the crushed samples into powdery form of grain size less than 50 µm. Pressed pellets were used for the major oxides and trace elements analyses. The pressed pellets were analysed by PANalytical Zetium XRF spectrometer equipped with a 4 kW Rh tube. The milled samples were also dried at 100°C (Weight A) and heated at 1,000°C (Weight B) for a period of at least 3 h to oxidize S and Fe2+ in order to determine the loss of ignition (LOI). The mathematical expression for the percentage LOI is given as:
The Epsilon software program was used to quantify the oxides and trace elements in the mudrocks and sandstones. Discriminatory binary and ternary plots of the major oxides and trace elements were used for geochemical classification as well as the determination of provenance and tectonic settings. In addition, CIA, chemical index of weathering (CIW), plagioclase index of alteration (PIA), and ICV were calculated and bivariate diagram of ICV against CIA and ternary diagram of Al2O3–(CaO Na2O)–K2O were plotted to deduce the degree of weathering. In the CIA, CIW, PIA, and ICV formulas, CaO∗ is the amount of CaO added into the silicate fraction of the rocks. In this study, rectification for CaO from the carbonate contribution was not carried out because of the absence of CO2 value. Hence, the proposed method from ref.  was used to calculate the amount of CaO∗ from the silicate fraction. The method suggested that CaO values should be accepted only if CaO Na2O. However, when CaO Na2O, it was alleged that the concentration of CaO is the same as that of Na2O. This procedure measures the ratio of the secondary aluminous mineral to feldspar, and forms a basis for measuring the intensity of weathering.
The Bredasdorp sandstones are mostly composed of quartz, feldspar, glauconite, and clay minerals. The quartz, feldspar, and glauconite are the framework minerals, while the clay minerals are mostly the matrix and sometimes act as cement. The observed cements in the sandstones are authigenic clay mineral cement, quartz cement, and occasional glauconite cements. Texturally, these sandstones are moderately sorted to moderately well-sorted and of fine to medium grain sizes (Figures 3 and 4). Petrographic observations revealed that borehole E-AH1 consists mostly of massive, well-sorted, fine- to medium-grained glauconitic sandstone and claystone with minor siltstone interbeds. The claystones are characterised by near-horizontal bedding and sediment injection features (sandstone into claystone) near the contacts with the sandstones. Borehole E-AJ1 is made up of claystones and siltstones with occasional interbedded sandstones. The sandstones in borehole E-AJ1 are generally massive (structureless), fine to medium grained, and contain abundant glauconite and are moderately well-sorted. A distinctive feature in borehole E-AJ1 is the occurrence of stylolites, which are seen as irregular, undulatory, and coarsely sutured horizontal features and less commonly as finely sutured regular vertical to sub-vertical fractures. Borehole E-BA1 comprises mostly of massive moderately well-sorted, fine- to medium-grained glauconitic sandstone with minor claystone and siltstone. The sandstone is slightly porous, light brownish-grey with very fine to medium, well-sorted subangular grains. In addition, it is non-calcareous, slightly glauconitic, and slightly pyritic and contains green lithic fragments. The sandstone in borehole E-BB1 is mostly well-sorted, fine grained, very lithic (metaquartzite clasts), glauconitic, and slightly shelly and carbonaceous. In borehole E-D3, the sandstones are massive, moderately sorted, medium grained, and slightly glauconitic. In general, the roundness of the grains in the studied sandstone samples vary from subangular to rounded and the sphericity ranges from low to high (mostly low; oblong grains). The grain packing is irregular, showing both fairly packed and tightly packed grains. However, in most cases, the grains are moderately tight. Furthermore, the grain contact patterns vary from point contact to sutured contact, but predominantly long and concavo-convex contacts. The sandstone grains are often cemented or supported by clay matrix; however, some are grain supported. The change in grain size in the vertical succession indicates both regressive and transgressive conditions, although the transgressive conditions tend to dominate. The stratigraphy of the studied boreholes is presented in the supplementary data (Figure A1).
4.2 Major oxides
The major oxide concentrations in the Bredasdorp mudrocks and sandstones are presented in supplementary data (Table S1). The stratigraphic descriptions of the boreholes are presented in the supplementary data. The major oxide compositions are relatively variable, but are still comparable with the average compositions of oxides reported in refs [44,45,46] (supplementary data; Table S2). The studied samples have high percentages of SiO2, varying between 51.70 and 81.58%. The concentration of Al2O3, CaO, Fe2O3, MgO, and K2O ranges from 2.82 to 29.01%, 0.80 to 7.68%, 0.27 to 5.75%, 0.39 to 5.68%, and 0.43 to 5.11%, respectively. The percentages of MnO, TiO2, Na2O, and P2O5 are generally low, ranging from 0.01 to 0.10%, 0.16 to 0.99%, 0.03 to 1.30%, and 0.11 to 1.37%, respectively. The SiO2 content in sandstones are higher than shales. In contrast, the concentration of Fe2O3, K2O, and TiO2 are higher in the mudrocks, depicting their association with clay-sized phases.
To compare the abundances of the oxides, the concentration of the major oxides in the studied samples were plotted against the concentration of Al2O3, as shown in Figures 5 and 6. The Al2O3 contents were used as a normalisation factor because of its immobile nature during weathering, diagenesis, and metamorphism . Furthermore, the average percentages of the Post-Archaean Australian Shale (PAAS) and UCC were taken from refs [3,48], respectively, and incorporated in the plots for comparison purposes. In the mudstone and shale samples, the concentration of SiO2, Fe2O3, MgO, and Na2O shows negative correlation with Al2O3, while the concentrations of TiO2, MnO, CaO, K2O, and P2O5 show no specific trend (Figure 5). On the other hand, all the major oxides in the sandstones show no particular trend or correlation with Al2O3 (Figure 6). The positive correlations of some of the oxides in the mudstones and shales suggest their association with micaceous/clay minerals. The concentration of oxides in the mudrocks and sandstones were also normalised with those of UCC and PAAS (Figure 7). Relative to UCC, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 in the mudrocks are 0.85, 1.05, 1.61, 0.83, 0.49, 0.65, 0.47, 0.24, 1.16, and 1.13, respectively. Relative to PAAS, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5 in the mudrocks are 0.91, 0.68, 1.32, 0.58, 0.44, 0.74, 1.30, 0.67, 0.88, and 1.00, respectively.
Relative to UCC, the mudrocks are low in MnO, CaO, Na2O, and very high in Al2O3. As reported in ref. , aluminium (Al) is simply absorbed on clays and concentrated in the finer, more weathered materials. In support of this, XRD analysis of the mudrocks revealed that they are dominated with the mineral kaolinite (Al2Si2O5(OH)4). The concentration of these oxides in the mudrocks are generally comparable with those of PAAS. On the other hand, relative to UCC, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 in the sandstones are 1.04, 0.69, 0.29, 0.41, 0.52, 0.56, 0.50, 0.19, 0.74, and 0.93, respectively. The average concentrations of Al2O3, Fe2O3, and Na2O are low as compared to the UCC values. The depletion of Na2O (<1%) in the Bredasdorp rocks can be linked to the relatively smaller amount of Na-rich plagioclase in them, which is consistent with the petrographic results. The concentration of K2O and Na2O, and their ratios (K2O/Na2O 1) are also consistent with the petrographic observations, which revealed that K-feldspar dominates over plagioclase (albite) feldspar. The enrichment of K2O relates to the presence of illite as common clay mineral in the shales and sandstones. In addition, the enrichment of CaO can be attributed to the presence of diagenetic calcite cement, which is also consistent with the petrographic results. In comparison with PAAS, the average concentrations of SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 in the sandstones are 1.11, 0.54, 0.24, 0.29, 0.51, 0.63, 1.40, 0.53, 0.56, and 0.70, respectively. Again, the average concentrations of Al2O3 and Fe2O3 are low as compared to PAAS.
4.3 Trace elements
The trace element concentrations in mudrocks and sandstones are presented in the supplementary data (Table S3a–b). The concentrations (in ppm) of these elements are fairly variable but still comparable with those average concentrations reported by refs [45,46,50,51] (supplementary data; Table S4). The concentration of large ion lithophile elements (LILE) like Rb, Ba, Sr, and Th in the studied samples are in range of 12.60–267.30, 55.20–1160.00, 29.80–324.70, and 4.50–16.50 ppm, respectively. The concentration of the high field strength elements (HFSE) such as Zr, Y, and Nb varies from 50.00 to 215.80 ppm, 1.00 to 127.70 ppm, and 2.70 to 40.80 ppm, respectively. Likewise, transition trace elements (TTE) like Sc, V, Cr, Ni, and Zn vary in the range of 1.80–4.90, 16.50–238.00, 247.70–592.80, 5.20–38.30, and 5.50–241.30 ppm, respectively. In general, sandstones have higher concentration of Ba, Th, Y, Nb, Sc, V, and Cr than the mudrocks, while the content of Rb, Sr, Zr, Ni, and Zn are higher in the mudrocks than the sandstones.
4.3.1 LILE: Rb, Ba, Sr, Th, and U
The concentration of the LILE in the mudrocks are generally comparable to those of UCC and PAAS. However, the concentrations of U and Cr are high, whereas Sr content is relatively low (Figure 8a and b). On the other hand, the concentration of Th in the sandstones is relative to UCC and PAAS. Although U content is comparable to UCC, it is high relative to PAAS. In addition, the concentrations of Rb, Ba, and Sr in the sandstones are generally low when compared to the UCC and PAAS values (Figure 8c and d). Th and Nb are positively correlated in the mudrocks and sandstones, perhaps suggesting that it may have been controlled by clays and/or other phases (i.e. Ti- and Nb-bearing phases) associated with clay minerals. Also, Rb has positive correlations with Ba in the mudrocks and sandstones, possibly indicating the same geochemical characteristics. These correlations show that their distributions are mostly controlled by illites with little contribution from other clay minerals.
4.3.2 HFSE: Zr, Hf, Y, and Nb
HFSE elements are often concentrated in felsic rocks than mafic rocks . The Zr, Hf, Nb, and Y contents in the mudrocks and sandstones are comparable to UCC and PAAS contents (Figure 8a and c), although Zr is relatively low, whereas Y is fairly high (Figure 8b and d).
4.3.3 TTE: Sc, V, Cr, Co, Cu, Ni, and Zn
The TTE in the Bredasdorp mudrocks and sandstones are depleted in comparison with the UCC and PAAS. In the mudrocks, V, Cu, and Zn are relatively comparable with UCC and PAAS, whereas the concentrations of Sc, Co, and Ni are low relative to UCC and PAAS (Figure 8). Furthermore, the concentration of Cr in the samples are generally high when compared to the UCC and PAAS.
5 Interpretations and discussion
5.1 Sandstone classification
The concentrations of silica, aluminium, potassium, sodium, and iron oxides have been used to classify sandstones [53,54]. The background geochemical classification diagrams of refs [53,54] were used to classify the Bredasdorp sandstones. Based on the Herron’s classification  scheme, the Bredasdorp sandstones are mainly sub-arkose and sub-lithic arenite (Figure 9). Similarly, the classification scheme in ref.  shows that the sandstones are sub-arkose and sub-lithic arenite, while the mudrocks are plotted in the field of shale (Figure 10). The geochemical classification of the sandstones is also in agreement or consistent with the petrographic classification of the sandstones.
The major oxide concentrations in the mudrocks and sandstones have been used to decipher the provenance of the Bredasdorp rocks using the discriminant functions analysis proposed in ref. . The bivariate plot of the discriminant functions discriminates between quartzose sedimentary or recycled, felsic igneous, intermediate igneous, and mafic igneous provenance fields. The mudrock samples are plotted in both the quartzose sedimentary provenance and felsic igneous provenance fields (Figure 11a), whereas the sandstones fall within the quartzose sedimentary provenance field (Figure 11b).
The bivariate plot of TiO2 versus Zr shows that the mudrocks and sandstones are mostly from felsic igneous rocks with little contribution from intermediate igneous rocks (Figure 12). Also, the binary plot of TiO2 against Ni, La/Th versus Hf, and V–Ni–Th∗10 ternary diagram revealed that the source area for most of the Bredasdorp mudrocks and sandstones are predominantly acidic or felsic in nature (Figures 13–15). The concentration of HFSE (i.e. Hf) are higher in the sandstones than the shale samples. The low content of Hf and elemental ratios like La/Sc and Th/Sc indicate the presence of fractionated source rocks with lower compatible element contents and recycled sediments in the source area. Also, the variation in chemical composition, perhaps suggests changes in the supply of material and a fluctuation in physico-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 seem to have invaded the lattices of the silicates and clay minerals and structurally combined with them.
5.3 Tectonic setting of the source area
The chemical compositions of clastic rocks are greatly influenced by plate tectonic settings of their source area(s) and depositional basins [4,17]. Consequently, clastic rocks often have distinctive geochemical signatures or characteristics for a particular tectonic setting. As reported in ref. , tectonic setting discrimination ternary diagrams offer a steadfast outcome for clastic rocks that have not been sturdily affected by post-depositional weathering and metamorphism. The binary plots of major oxides and trace elements’ geochemistry have been widely used by several researchers to infer the tectonic setting of clastic rocks [4,17,18,58,59,60]. The major oxides and trace elements tectonic discriminate diagrams of refs [4,18,59] have been attempted in this study to deduce the tectonic setting for the Bredasdorp mudrocks and sandstones. These aforementioned discrimination diagrams divide tectonic settings into oceanic island arc (OIA), continental island arc (CIA), active continental margin (ACM), and passive continental margin (PM).
The binary plot of Log (K2O/Na2O) against SiO2 in the background tectonic setting discrimination diagram of ref.  shows that most of the Bredasdorp mudrocks are plotted in the ACM field, while a few samples fall in the PM field (Figure 16a). The opposite is observed for the Bredasdorp sandstones, where about 95% of the samples are plotted in the PM (Figure 16b). Also, the calc-alkaline (CaO–Na2O–K2O), Th–Sc–Zr/10, and La–Th–Sc ternary diagrams show that majority of the Bredasdorp mudrocks and sandstones are related to PM with few contributions from the ACM (Figures 17–19). These rocks also exhibit similar geochemical characteristics with recent sedimentary rocks of East African Rift System , which further support a rift basin tectonic setting for the Bredasdorp Basin. The PMs are basins on continental crust and basins associated with ocean floor spreading, failed rifts, and Atlantic-type continental margins. On the other hand, ACMs are subduction related basins, continental basins, and pull-apart basins associated with strike-slip fault zones.
5.4 Paleoweathering conditions
The degree or extent of chemical weathering of source rocks are mainly influenced by the composition of the source rock, duration of weathering, climatic conditions, and rates of tectonic uplift of source region . Several researchers including [3,63,64] have reported that about 75% of labile materials in the upper crust are made up of feldspars and volcanic glass. Chemical weathering of these materials would lead to the formation of clay minerals. Elements like Ca, Na, and K are mostly removed from source rocks during chemical weathering and the amount of these elements remaining in sediments derived from the rocks serve as pointers to the degree of chemical weathering . As documented by ref. , if clastic sedimentary rocks are free from alkali related post-depositional changes, then their alkali contents (K2O + Na2O) and K2O/Na2O ratios should be considered as good pointers to the degree of weathering of the source materials. A few indices of weathering have been proposed for the determination of the degree of source rock weathering. The indices of weathering/alteration include CIA, CIW, and PIA. These indices are based on the molecular percentages of the mobile and immobile element oxides like Na2O, CaO, K2O, and Al2O3.
The chemical composition of the products of chemical weathering in a basin is anticipated to unravel the mobility of the surviving elements during weathering . The CIA proposed in ref.  is the most widely used chemical index to determine the degree of source area weathering. Nesbitt and Young  expressed CIA as:
where is the content of CaO incorporated in silicate fraction.
The calculated indices of weathering (CIA, CIW, and PIS) are presented in the supplementary data (Table S5). The CIA is simply the ratio of primary minerals to secondary products (i.e. clay minerals). CIA values usually range from about 50 for unweathered rocks up to 100 in highly weathered rocks. Generally, CIA values increase with the increase in the degree of weathering, reaching 100 when all the Ca, Na, and K have been leached from weathering residue. The CIA values in the Bredasdorp mudrocks and sandstones vary in the range of 82.72–86.42, 30.71–87.20, 66.31–84.01, 31.22–85.52, and 30.98–77.89 in boreholes E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3, respectively. These average CIA values revealed relatively low to high degree of chemical weathering in the source area. In general, the CIA values in the mudrocks and sandstones vary from 73.50 to 90.16 and from 30.71 to 70.99, respectively. The high CIA values in the mudrocks possibly signifies the presence of clay minerals and low percentage of detrital feldspars. On the other hand, the low CIA values in the sandstones (<50%) suggest a low weathering condition in the source area and perhaps reflect cool conditions. The variations in CIA reveal changes in the properties of feldspar against aluminous clay minerals. These changes suggest that the studied samples were derived from source rocks that have been subjected to both chemical and physical weathering.
The CIW was also calculated to reveal the degree of chemical weathering that the Bredasdorp sediments have undergone. Just like the CIA, the CIW also accounts for the rate at which feldspars are converted to clay minerals. The formula for CIW is expressed in ref.  as:
The CIW values of the Bredasdorp mudrocks and sandstones are in the range of 93.69–94.82, 49.44–94.14, 72.32–93.97, 36.34–93.88, and 33.60–93.88 in boreholes E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3, respectively. Again, the obtained CIW values indicate low to high degree of chemical weathering. The source area weathering and elemental redistribution during diagenesis of the Bredasdorp rocks were assessed using the PIA. As reported in ref. , the PIA records and accounts for the rate at which feldspars are progressively changed or weathered to clay minerals. The maximum value of PIA for completely weathered materials (i.e. kaolinite and gibbsite) is 100. The formula for PIA is denoted in ref.  as:
The PIA values of the Bredasdorp mudrocks and sandstones are in the range of 92.86–94.26, 50.33–93.75, 69.56–93.16, 31.61–91.56, and 27.46–89.60 in boreholes E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3, respectively. These PIA values also signify low-high conversion rate of feldspars to clay minerals during source weathering, transport, redeposition, and diagenesis. During the preliminary stages of chemical weathering, Ca is easily leached than K and Na. As the weathering progresses, the total alkali ( ) content increases with the decrease in the K–Na ratio ( ), perhaps due to the destruction of feldspars; especially plagioclase ref. . The feldspathic materials in the Bredasdorp mudrocks and sandstones have undergone variable degrees of weathering during the different evolution phases. As reported in ref. , the binary plots of K2O/Na2O, K2O + Na2O, Na2O, K2O, and CaO against PIA can unravel the mobility of elements during the final phases of chemical weathering of previously altered feldspars. These plots were also attempted for the Bredasdorp samples and it is observed (in general) that as the K2O/Na2O increases, the PIA also increases, whereas as K2O + Na2O and CaO content increases, the PIA decreases (Figures 20–21). Generally, the bivariate plots show weak correlations which could be due to the presence of K-bearing minerals (i.e. muscovite and biotite) and retention of most of the mobilized K by aluminous material resulting in the formation of illite.
The Al2O3–(CaO + Na2O)–K2O (represented as A–CN–K) ternary diagram in ref.  is another useful approach for assessing the composition of original source rock and mobility of elements during chemical weathering of source material and post-depositional chemical modifications. The A–CN–K ternary plot of the Bredasdorp mudrocks and sandstones were plotted to unravel the compositional changes in the mudrocks and sandstones that are related to chemical weathering, diagenesis, and source rock composition. The Al2O3–(CaO + Na2O)–K2O composition of the Bredasdorp samples are plotted on the A–CN–K ternary diagram background fields of ref. .
The A–CN–K diagrams of the Bredasdorp samples show that about 100% of the mudrocks and 55% of the sandstones are plotted above the line joining plagioclase and K-feldspar (Figure 22). The weathering trendline of the Bredasdorp shales is closer to the A–K boundary, signifying that the silicates (i.e. feldspar) have experienced intense or high weathering resulting in the leaching of Ca and Na out of plagioclase. In addition, the trendline (red arrow in Figure 22) parallels with the A–K boundary and slightly extend towards the A apex, signifying leaching of K and enrichment of Al. This indicates that further weathering has led to the decomposition of K-bearing minerals (biotite, illite, and potassium feldspar). Thus, kaolinite dominates the secondary clay minerals. On the other hand, the weathering trendline of the Bredasdorp sandstones is relatively closer to the A–CN boundary, indicating that plagioclase are the first to be weathered, out of which Ca and Na leached rapidly, whereas K-feldspar is relatively stable. In the sandstones, illite and smectite are the main weathering products, whereas kaolinite did not dominate. Samples with CIA values below 50 are unweathered.
5.5 Sediment maturity and climatic conditions
The maturity of sediments and paleoclimatic conditions can be determined by computing the ICV suggested in ref. . The ICV values tend to be high in minerals that underwent intensive degree of weathering and low in less weathered or more stable minerals. As reported in ref. , ICV values generally decrease further in the montmorillonite group of clay minerals and are lowest in the kaolinite group minerals. Furthermore, mudrocks and sandstones that have ICV value of less than 1 are compositionally mature and are deposited in the tectonically quiescent or cratonic environment where sediment recycling is active. In contrast, mudrocks and sandstones that have ICV value of greater than 1 are compositionally immature with the first cycle of sediments deposited in tectonically active settings. The ICV formula is presented in ref.  as:
The ICV values for the Bredasdorp mudrocks vary from 0.23 to 0.61, averaging 0.43, whereas ICV values for the sandstones range from 0.71 to 3.75, averaging 2.82. In fact, only 4 out of the 26 sandstone samples have ICV of less than 1. Based on the average ICV values, it can be inferred that the shales are compositionally mature, while the sandstones are compositionally immature with the first cycle of sediments deposited in tectonically active settings. The bivariate plot of CIA versus ICV for the Bredasdorp rocks shows that the shales are geochemically mature and are derived from intensively weathered source rocks, whereas the sandstones are geochemically immature and derived from weak weathered source rocks (Figure 23). To constrain the climatic condition during sedimentation of clastic sedimentary rocks, the plot of SiO2 against ( ) proposed in ref.  was used to classify the maturity of Bredasdorp mudrocks and sandstones as a function of climate. The binary plot of SiO2 versus ( ) shows that the mudrocks are of arid climates, while the sandstones are of humid climate with varied maturities (Figure 24).
The provenance, tectonic setting, and paleoweathering conditions of the Cretaceous mudrocks and sandstones from the Bredasdorp Basin have been assessed using inorganic geochemical studies. On the basis of the major oxide compositions, the sandstones could be classified as sub-arkose and sub-lithic arenite. The major oxide and trace element concentrations in the mudrocks and sandstones show significant variation in composition across the samples. Higher concentrations of Al2O3, Fe2O3, K2O, Sc, V, Zr, Th, U, La, Ce, and Ni are observed in the mudrocks than the sandstones, whereas the sandstones are enriched with SiO2, Hf, Cr, and Zn than the mudrocks. The preferential enrichment of transitional elements like Sc, V, and Ni in the mudrocks is probably due to the surficial sorption. Furthermore, the low concentration of these elements, La/Sc ratio, and Th/Sc ratio suggest the presence of fractionated source rocks with lower compatible element contents and recycled sediments in the source area. Likewise, the discrimination diagrams based on major oxide geochemistry shows that the sediments are mainly of quartzose sedimentary provenance with little contribution from the intermediate igneous provenance, suggesting that they were mostly derived from a cratonic interior or recycled orogen. The tectonic setting discrimination diagrams support PM-ACM setting of the provenance. In addition, the closely similar geochemical compositions of the analysed samples and recent sedimentary rocks of the East African Rift System suggest a rifted basin tectonic setting for the Bredasdorp Basin. The CIA and PIA values, as well as the bivariate plots of ICV versus CIA, and SiO2 against total alkali (Al2O3 + K2O + Na2O) suggest that the source area of the Bredasdorp mudrocks and sandstones were subjected to low and intense weathering conditions under arid and humid climate, respectively.
The authors are grateful to the National Research Foundation-Southern African Systems Analysis Centre (NRF-SASAC) and the DSI-NRF Centre of Excellence (CoE) for Integrated Mineral and Energy Resource Analysis (CIMERA) for financial support. The Petroleum Agency of South Africa (PASA) and the Govan Mbeki Research and Development Centre (GMRDC) of the University of Fort Hare are appreciated for granting access to the cores and logistic supports, respectively.
Funding information: National Research Foundation-Southern African Systems Analysis Centre (NRF-SASAC) and the DSI-NRF Centre of Excellence (CoE) for Integrated Mineral and Energy Resource Analysis (CIMERA).
Conflict of interest: The authors declared that there is no 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.
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