Geochemistry and weathering history of the Balfour sandstone formation, Karoo basin, South Africa: Insight to provenance and tectonic setting

Highlights

• The paper summaries the results of geochemical analysis of Balfour Formation.

• Major elements and trace elements were analyzed to classify the sandstone.

• The provenance and tectonic setting along with weathering conditions in the source are deduced based on the geochemical analysis.

• The sediments are found to be chemically immature.

• The study concludes that sediments were derived from igneous, metamorphic and older sedimentary rocks with only minor mafic rocks.

Abstract

Geochemical analysis on sandstones from clastic sediments was carried out to understand the tectonic setting and subsequent post-depositional change in the Karoo basinal fill of the fluvial deposits of the Balfour Formation during the Late Permian to Early Triassic period. The major and trace element analysis reveal a relatively homogeneous provenance for the sandstones. The geochemical analysis shows that these rocks are first order mature sediments, derived from igneous and/or meta-igneous rocks of a felsic composition. The results show that the sandstone consists of SiO₂ (71.58 wt. %), followed by Al₂O₃ (14.48 wt. %), but with low contents of Fe₂O₃+MgO (4.09 wt. %) and TiO₂ (0.47 wt. %). These sandstones are classified as litharenites and arkoses based on the elements constituent ratio of various schemed adopted. The sandstone in the provenance discrimination diagram plots in the dissected and transitional arc fields suggesting an active margin and continental island arc provenance, preserving the signature of a recycled provenance. The Chemical Index of Alteration (CIA) ranging from 63.56 to 67.10% suggests recycling processes, and that the source area has undergone a moderate degree of chemical weathering. The geochemical characteristics of the sediment suggest the source area of uplifted terrane of folded and faulted strata with detritus of sedimentary and metasedimentary origin.

Introduction

The mineralogical compositions of sedimentary rocks do not reflect the mineralogy of the original source rocks due to post-depositional modification of the constituent grains. The geochemical composition of siliciclastic sedimentary rocks aids in the understanding of the setting and nature of source rocks. Many studies have shown that geochemical analysis of the siliciclastic rocks helps in the identification of their provenance and related depositional and post depositional processes (e.g., Banerjee and Banerjee, 2010; Jafarzadeh and Hosseini-Barzi, 2008; Liu et al., 2007; Rahmani and Suzuki, 2007; Cingolani et al., 2003; Raymond, 1995).

While the geochemical component/characteristics can be altered during weathering through oxidation (Taylor and McLennan, 1985), the bulk chemical composition is unaltered (McLennan et al., 1983). Geochemical analyses allow the clarification of the rocks composition not resolvable through petrographic analysis (Akarish and El-Gogary, 2008). Several studies have been carried out on various aspects of the Karoo basin sediments (e.g., Catuneanu and Elango (2001); Rubidge et al. (2000); Hiller and Stavrakis (1984); Haycock et al. (1997); Smith (1995); Visser and Dukas (1979); Kitching (1977); Tordiffe (1978); Keyser and Smith (1978)). None of those studies clarified the sources of the sediments using geochemical methods.

In this study, the geochemistry of selected representative sandstone samples from the Balfour Formation of the Karoo Supergroup was carried out using X-ray fluorescence spectrometry (XRF) analysis. This work contributes to the understanding of the depositional model of the Karoo Supergroup and the sedimentary environment along the southern margin of Gondwana.

The sediments studied are part of the southern Karoo Basin developed in response to the Late Palaeozoic to Early Mesozoic subduction episode of the palaeo-Pacific plate underneath the Gondwana plate (Catuneanu and Bowker, 2001; Catuneanu and Elango, 2001; Pysklywec and Mitrovica, 1999; Winter 1984; Lock, 1978). Fragments of the Gondwana foreland basin today are preserved across the world as the Parana Basin (South America), Karoo Basin (Southern Africa), Beacon Basin (Antarctica) and Bowen Basin (Australia) (Catuneanu, 2004; Catuneanu and Elango, 2001; Catuneanu et al., 1998). The Karoo Basin responded to eight tectonic events related to the Panthalassan (Palaeo-Pacific) plate beneath Gondwana from the time of deposition of the Dwyka through to the Elliot Formation (Table 1) (Andersson and Worden, 2006; Bamford, 2004; Catuneanu et al., 2002; Smith et al., 1993; Catuneanu et al., 1998; SACS, 1980). These tectonic events produced variation in the depositional sedimentary successions within the Karoo setting (Catuneanu et al., 1998) and suggest changes in climate. The fourth tectonic orogenic paroxysm event is assigned to Balfour sedimentation (Catuneanu et al., 1998) from which the study is based. The clastic sediments from the Balfour Formation developed during the Late Permian (Tatarian) to Early Triassic (Scythian) period and representing the upper part of the Adelaide Subgroup (Table 1) of the Beaufort Group in the southeastern part of the Karoo Supergroup along the southern margin of Gondwana continent in South Africa (Oghenekome et al., 2016; Catuneanu et al., 1998, 2002, 2005; Catuneanu and Bowker, 2001; Catuneanu and Elango, 2001 Rubidge, 2005; Pysklywec and Mitrovica, 1999; Johnson et al., 1996; Smith et al., 1993; Johnson, 1991; Tankard et al., 1982). The Balfour Formation comprises two finning upward megacycles of sedimentary deposits with a change in the sediment supply pattern from low-sinuosity to high-sinuosity river systems, which reflect both braided and meandering river deposits. Lithostratigraphically, it is subdivided into five members viz, the Oudeberg, Daggaboersnek, Barberskrans, Elandsberg, and Palingkloof Members (Table 1) (Oghenekome et al., 2016; Johnson et al., 2006; De-Kock and Kirschvink, 2003; Tordiffe et al., 1985; SACS, 1980). Sandstones from each Member were studied to understand the provenance of the Balfour Formation.