Resolving cryptic stratigraphy in the Ecca Group, Karoo Basin, South Africa: Elucidating the Ripon and Laingsburg depocenters
Graphical abstract
Introduction
The 700 000 km2 Karoo Basin, preserves one of the most complete stratigraphic records, spanning the Carboniferous to Early Jurassic. The Karoo, a basin tectonically developed pre-rift/drift phase of Gondwana, is correlative to the Paraná (South America), Bowen (Australia), and Beacon (Antarctica) basins (Catuneanu et al., 2005). Palaeogeographically, the Karoo would have lied on the very southern margin of the amalgamated Gondwana supercontinent. The Karoo is bounded by the Cape Fold Belt to the south and west, while the northern margin of the basin thins out over the Kaapvaal Craton (Cole, 1992; Catuneanu et al., 2002). Broad scale studies by Kingsley (1981), Johnson (1991), Wickens (1994), Sixsmith et al. (2004), Hodgson et al. (2006), Van der Merwe et al. (2009) and Flint et al. (2011) focused on elucidating palaeoenvironmental context and developmental history for the Ecca Group along the southern margin of the basin. Currently, the most widely accepted morphology for the Karoo Basin is a retro-arc foreland basin, and its basinal history preserves the under-filled to over-filled phases (Dickenson, 1974; Visser, 1993; Catuneanu et al., 2002; Catuneanu, 2004; Johnson et al., 2006; Fildani et al., 2009; Tankard et al., 2009). Sedimentary units typically are thickest in the south (~10.0 km) and thin trending north by northeast (~1000.0 m) (Veevers et al., 1994; Pysklywec and Mitrovica, 1999). Yet, regional correlation becomes problematic due to limited surface outcrop and irregular facies occurrences, resulting in unresolved cryptic sedimentological and stratigraphic relationships and interpretations.
Three subbasins have been identified in the southwestern Karoo Basin namely the Tanqua subbasin (west of the Cape Syntaxis), the Laingsburg subbasin (east of the Cape Syntaxis) and the Grahamstown/Southern subbasin (Eastern Cape) (Kingsley, 1981; Wickens, 1994; Catuneanu et al., 2002). This study will be referring to these subbasins as depocenters, where the Grahamstown subbasin is referred to as the Ripon depocenter. Both the Tanqua and Laingsburg depocenters have been extensively studied and detailed stratigraphic correlations have been achieved (Wickens, 1994; Johnson et al., 2001; Catuneanu et al., 2002; Grecula et al., 2003; Andersson et al., 2004; Figueiredo et al., 2010; Spychala et al., 2015). Conversely, little research has been undertaken on the Ripon depocenter and its lateral stratigraphic relationship with the southwestern Laingsburg depocenter (Kingsley, 1981; Johnson, 1991). The 1:250 000 geological maps of Oudtshoorn 3322 (1979) and Ladismith 3320 (1991), published by the Council for Geoscience of South Africa, amplifies this stratigraphic uncertainty with individual lithological units mapped as different formations across the two geological maps; i.e the same lithological unit is mapped as the Laingsburg Formation on the Ladismith 3320 map and conversely the Ripon Formation on the Oudtshoorn 3322 map. A few studies have attempted to correlate the Laingsburg and Ripon depocenters (Catuneanu et al., 2002; Van der Merwe et al., 2009; McKay et al., 2016), but this problem lacks resolution.
This study sought to provide clarity to localized cryptic stratigraphy as a result of the interfingering relationship between the two separate depocenters. Situated between the town of Prince Albert (Western Cape, South Africa) and extends eastwards along strike of the Cape Fold Belt to a few km's east of the N12 towards the town of Willowmore (Eastern Cape, South Africa), this study elucidated crucial sedimentological and stratigraphic context between the Vischkuil/Laingsburg formations (Laingsburg depocenter) and the Ripon Formation (Ripon depocenter) (Fig. 1). The study area was designated to cover a large extent of the lateral morphology between the Vischkuil/Laingsburg formations and the Ripon Formation. Ultimately, this study provides a deeper understanding of the southern Karoo Basin development and temporal evolution of the Laingsburg and Ripon depocenters, focusing on contextualizing deep marine to slope sedimentation and resolving the stratigraphic discordance occurring locally within the basin.
Section snippets
Sedimentology
The southern Ecca Group presents a unique Permian regressive succession from basin floor to shelf sedimentation. The Ecca Group represents the under-filled phase from a deep marine to deltaic environment and most of the Lower Ecca formations (Prince Albert, Whitehill and Collingham) extend across the entire southwestern Karoo Basin. Deposition of the Ecca Group in the southern depocenters of the Main Karoo Basin represents a prograding basin-fill with influences of eustatic sea-level rise and
Sedimentology
Facies descriptions and stratigraphic relationships were obtained in-field from which a set of seven stratigraphic profiles were drawn up. The study area encompasses folded strata caused by the Cape Fold Belt tectonism and were corrected for during the construction of stratigraphic profiles. Distinct bounding surfaces and marker beds were walked out to determine lateral extent and facies changes. The marker horizons were defined by laterally extensive (greater than 50.0 km) mudrock units,
Facies analysis
This study identified twelve genetically related and repeating facies (Table 4), which were then grouped into seven facies associations (Table 5). Minor variations do occur within a single facies association, yet overall similarities remain within all facies association.
Basin development
This multifaceted study sought to better contextualize the mechanisms of sediment emplacement and resolve cryptic stratigraphic relationships in the Lower Ecca, southern Karoo Basin. Thus far, a detailed facies analysis of the Vischkuil, Laingsburg and Ripon formations agrees with previous studies that the Laingsburg (Cole, 1992; Viljoen and Wickens, 1992; Wickens, 1994; Van der Merwe et al., 2009) and Ripon formations (Kingsley, 1977; Johnson, 1991) were deposited within basin-floor to slope
Conclusion
Palaeoenvironmental and basin reconstruction indicates a progressive change from deep marine to base of slope deposition within the regional setting of the basin as well as lateral variations in basin-floor evolution, indicating earlier development of submarine fan complexes to the east (Ripon depocenter) and progressively younger development to the west (Laingsburg depocenter). Depositional systems along the southern margin of the Karoo Basin, north of the Cape Fold Belt, are much more
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge Prof. B. Rubidge, the DST-NRF (African Origins Platform), South Africa, as well as Prof. G.Henry DSI-NRF (CIMERA), South Africa for project providing. A special acknowledgement goes to Dr. De.V.Wickens for project guidance, along with Stellenbosch University and CAF for both academic and analytical support throughout this work. Lastly, we wish to thank the editorial staff along reviewer 1, their comments and suggestions greatly strengthened this manuscript from
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