Open Geosci. 2017; 9:340–360 Research Article Open Access Christopher Baiyegunhi*, Kuiwu Liu, and Oswald Gwavava 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 https://doi.org/10.1515/geo-2017-0028 Keywords: Geochemistry, provenance, weathering, tec- Received November 16, 2016; accepted April 11, 2017 tonic setting, Ecca Group Abstract: Geochemical compositions of twenty-four sand- stone and shale samples from the Ecca Group were anal- ysed to decipher their provenance, paleoweathering con- 1 Introduction ditions and tectonic setting. The shales have high Fe2O3, K2O, TiO2, Ce, Cu, Ga, La, Nb, Nd, Rb, Sc, Sr, Th and Y Clastic sedimentary rocks have vital information about the content more than the sandstones, whereas, sandstones composition, tectonic setting and evolution of continental are higher in SiO2, Hf and Zr than the shales. The posi- crust, mainly when the traditional petrographic methods tive correlations of Al2O3 with other elements as well as are unclear. Nonetheless, their chemical and mineralogi- the abundance of Ba, Ce, Th, Rb, Zn and Zr suggest that cal composition can be influenced by factors like source these elements are primarily controlled by the dominant rock characteristics, weathering, sorting processes during clay minerals. Tectonic discrimination diagrams revealed transportation, sedimentation and diagenetic processes to that the sandstones and shales are mostly of quartzose an extent [1–3]. Trace elements such as La, Y, Sc, Cr, Th, Zr, sedimentary provenance, suggesting that they were de- Hf, Nb and rare earth elements (REE) are thought to be use- rived from a cratonic interior or recycled orogen. The bi- ful indicators of provenance, geological processes and tec- nary plots of TiO2versus Ni, TiO2 against Zr and La/Th ver- tonic setting due to their relatively low mobility and insolu- sus Hf as well as the ternary diagrams of V-Ni-Th*10 in- bility during sedimentary processes [4–8]. Hence, the geo- dicate that the shales and sandstones were derived from chemistry of clastic sediments (i.e. sandstone and shale) felsic igneous rocks. A-CN-K (Al2O3-CaO-K2O) ternary dia- reflects a combination of provenance, chemical weather- gram and indices of weathering (CIA, CIW and PIS) suggest ing, hydraulic sorting, and abrasion [2, 4, 8–11]. that the granitic source rocks underwent moderate to high In geochemical provenance studies, fine grained sed- degree of chemical weathering. The CIA values range be- imentary rocks like shales are considered to be the most tween 24.41% and 83.76%, indicating low to high weather- useful rock because of their homogeneity before deposi- ing conditions. The CIW values for the studied sandstones tion, post-depositional impermeability and higher abun- and shales range from 25.90 to 96.25%, suggesting mod- dance of trace elements [4, 10, 12, 13]. Some relatively im- erate to high intensive chemical weathering. ICV values mobile elements like Sc, Th, Zr, Hf and rare earth ele- for the sandstones and shales vary from 0.71 to 3.6 (aver- ments (REE) show very low concentrations in natural wa- aging 1.20) and 0.41 to 1.05 (averaging 0.82), respectively. ters and are transported almost quantitatively throughout The K2O/Na2O ratios for the studied samples vary from the sedimentary process from parent rocks to clastic sed- 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 *Corresponding Author: Christopher Baiyegunhi: Department of and intensively weathered source rocks. The tectonic set- Geology, Faculty of Science and Agriculture, University of Fort Hare, ting discrimination diagrams support passive-active con- Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa, tinental margin setting of the provenance. E-mail: [email protected], [email protected] Kuiwu Liu, Oswald Gwavava: Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa Open Access. © 2017 Christopher Baiyegunhi et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Geochemistry of sandstones and shales from the Ecca Group Ë 341 iments [4, 14]. The relative distribution or enrichment of and sandstones in the study area in order to provide infor- these immobile elements in felsic and basic rocks have mation on the source rock characteristics, provenance, pa- been used to infer the relative contribution of felsic and leoweathering and tectonic setting using their major, trace basic sources in shales from different tectonic environ- and rare earth elements geochemistry. ments [15]. For example, La and Th are enriched in felsic rocks, whereas Sc, Cr, and Co are more concentrated in ba- sic rocks relative to felsic rocks. These elements are rela- 2 General geology and stratigraphy tively immobile during weathering [6, 16–18]. Hence, the ratios of La or Th to Co, Sc, or Cr are sensitive indicators of The word “Karoo” was derived from the Main Karoo Basin source rock compositions. Similarly, felsic igneous rocks of South Africa to describe sedimentary fill of all basins contain negative Eu anomalies (Eu/Eu* from chondrite- of akin age across Gondwana. The Main Karoo Basin of normalized plots of the REE), whereas basic igneous rocks South Africa is bordered in the southern part by a fold have little or no Eu anomalies, and the size of the negative belt (Cape Fold Belt), while the northern part is held anomalies in the provenance seems to be preserved in fine- by Achaean Kaapvaal Craton [33]. It is a unique type of grained sediment [18]. Furthermore, some major elements basin of all the Karoo basins in southern Africa because such as alkali and alkali earth elements, which are water it contains the thickest and stratigraphically most com- mobile elements and very sensitive to climatic change, can plete mega-sequence of several depositories of the Permo- be used as a proxy of paleoclimate evolution [19, 20]. Geo- Carboniferous to Jurassic age sediments in southwestern chemical data on sandstones of unmetamorphosed sedi- Gondwana continent [33]. In addition, it serves as a datum mentary sequences deposited in epicratonic or intracra- for classifying Karoo basins in central and southern Africa. tonic basins also give important clues on paleoweathering The bulk of the Karoo strata occur in the Main Karoo Basin, conditions, variations in provenance composition and tec- with maximum preserved thickness adjacent to the Cape tonic settings [21–23]. Fold Belt in excess of 6 km [34]. The basin covers up to Several researchers such as [24] and [25] have pro- 700, 000 km2 and represents about 110 Ma of sedimenta- posed K O/Na O versus SiO tectonic setting discrimina- 2 2 2 tion spanning from 290 Ma to 180 Ma, and has its rocks cov- tion diagrams for sedimentary rocks in order to identify ering almost half of the area of South Africa [35]. The sed- tectonic setting of unknown basins. These diagrams are imentary succession reflects changing environments from still commonly used to deduce the tectonic setting of an- glacial to marine, deltaic, fluvial and finally aeolian [36]. cient basins. However, more detailed results can be ob- The Karoo Supergroup is believed to have originated tained using the calc-alkaline oxide ternary diagram (CaO- from the Gondwana Supercontinent [33]. The several litho- Na O-K O) of [26] and modified by [27]. [19, 28] docu- 2 2 spheric plates that separated to form the current Southern mented that the index of compositional variability (ICV), Hemisphere continents and India was once together as the K O/Al O ratio, chemical index of alteration (CIA) and 2 2 3 Gondwana supercontinent. The southern African remnant Al O -(CaO+Na O)-K O (A-CN-K) ternary plots are useful 2 3 2 2 of these continents contains the Karoo basins that include geochemical parameters for the study of provenance and the Main Karoo Basin and Great Kalahari Basin (Kalahari maturity of the rocks. In addition, [25, 29, 30] used al- Karoo, Aranos, and Mid-Zambezi Basins), as well as other kali metal oxides to reveal information about the prove- smaller basins in South Africa, Namibia, Zimbabwe and nance of clastic sediments. Recent geochemical investiga- Mozambique. The Karoo Supergroup has evolved from two tions on sandstones and shales have focused on decipher- distinct tectonic regimes sourced from the southern and ing the provenance and tectonic evolution of sedimentary the northern margin of Gondwana [37]. The southern tec- basins [22, 31, 32]. However, little attention has been paid tonic regimes are believed to be related to processes of sub- to the study of provenance in this region, despite the fact duction and orogenesis along the Panthalassan (paleo- that the targeted carbonaceous shales for shale gas explo- Pacific) margin of Gondwana, this resulted in the forma- ration in the region is hosted in the Ecca Group. Further- tion of a retro-arc foreland system known as the “Main more, it is also important in understanding the tectonic Karoo Basin” in association with the primary subsidence evolution of the southeastern Karoo basin. To date, the mechanism represented by flexural and dynamic load- geochemistry of fine-grained rocks of the Ecca Group inthe ing. The northern event was associated with extensional Eastern Cape Province of South Africa has not been stud- stresses that propagated southwards into the supercon- ied in detail to determine their source rock characteristics, tinent from the divergent Tethyan margin of Gondwana. provenance and tectonic setting. This study was therefore The Karoo Supergroup is a 12 km thick sequence of sedi- aimed at evaluating the geochemistry of the Ecca shales 342 Ë C.