Elsevier

Marine and Petroleum Geology

Volume 26, Issue 8, September 2009, Pages 1379-1412
Marine and Petroleum Geology

Tectonic evolution of the Cape and Karoo basins of South Africa

https://doi.org/10.1016/j.marpetgeo.2009.01.022Get rights and content

Abstract

The Cape and Karoo basins formed within the continental interior of Gondwana. Subsidence resulted from the vertical motion of rigid basement blocks and intervening crustal faults. Each basin episode records a three-stage evolution consisting of crustal uplift, fault-controlled subsidence, and long periods of regional subsidence largely unaccompanied by faulting or erosional truncation. The large-scale episodes of subsidence were probably the result of lithospheric deflection due to subduction-driven mantle flow. The early Paleozoic Cape basin records the combined effects of a north-dipping intra-crustal décollement (a late Neoproterozoic suture) and a right-stepping offset between thick Rio de la Plata craton and Namaqua basement. Following the Saldanian orogeny, a suite of small rift basins and their post-rift drape formed at this releasing stepover. Great thicknesses of quartz sandstone (Ordovician–Silurian) and mudstone (Devonian) accumulation are attributed to subsidence by rheological weakening and mantle flow. In contrast, the Karoo basin is a cratonic cover that mimics the underlying basement blocks. The Permian Ecca and lower Beaufort groups were deposited in a southward-deepening ramp syncline by extensional decoupling on the intra-crustal décollement. Reflection seismic and deep-burial diagenetic studies indicate that the Cape orogeny started in the Early Triassic. Deformation was partitioned into basement-involved strike-slip faults and thin-skinned thrusting. Uplift of the Namaqua basement resulted in erosion of the Beaufort cover. East of the Cape fold belt, contemporaneous subsidence and tilting of the Natal basement created a late Karoo transtensional foreland basin, the Stormberg depocentre. Early Jurassic tectonic resetting and continental flood basalts terminated the Karoo basin.

Introduction

The Karoo, a Khoikhoi (Hottentot) word meaning Thirstland, is the arid interior of South Africa, a broad plateau with the highest freeboard in the world. This plateau is supported by the Archean Kaapvaal craton and the surrounding Proterozoic basement, a suite of rigid basement blocks that controlled basin development throughout the Phanerozoic.

The Cape and Karoo basins represent two large-scale episodes of intermittent subsidence and sedimentation within the interior of Gondwana (Fig. 1, Fig. 2). The Early Ordovician to Early Carboniferous Cape basin (160 Ma) is believed to have developed by extensional processes (Cloetingh et al., 1992) in an episutural setting (Hälbich et al., 1993). However, located more than 1000 km from the paleo-Pacific plate, it was not a typical passive margin (e.g. Thamm and Johnson, 2006). The shallow marine and non-marine basin fill is notable for its great thicknesses of quartz sand accumulation and muddy deltaic deposits with a diverse marine invertebrate biota. After a 30 Ma hiatus, the Late Carboniferous to Jurassic Karoo basin (125 Ma) developed as a tabular cratonic cover. The Karoo is generally interpreted as a retro-arc foreland basin that developed behind an inferred magmatic arc and fold-thrust belt (e.g. Catuneanu et al., 1998, Johnson et al., 2006). But basement architecture, timing of the Cape orogeny, and stratigraphic relationships are atypical of a flexural foreland basin. An alternative explanation is that the long-wavelength component of subsidence resulted from lithospheric deflection due to mantle flow coupled to distant subduction (Pysklywec and Mitrovica, 1999). The terrestrial upper Karoo succession is acclaimed for its remarkable diversity of synapsid reptile fossils. Alex du Toit, in 1927, recognized the overarching importance of Karoo stratigraphy and paleontology in Gondwana-wide correlation.

There is a substantial amount of geophysical and geological evidence that allows us to elaborate on these earlier basin interpretations. We will address the large-scale evolution of the Cape and Karoo basins, and show that the styles of subsidence, the Cape orogeny, and late Karoo magmatism were controlled by first-order basement faults and the motions of the basement blocks between them. The sedimentary fill is made up of several unconformity-bounded megasequences that show that each basin episode consists of a three-stage evolution, comprising crustal uplift, fault-controlled subsidence, and long periods of regional subsidence during which faulting was subordinate. This interpretation suggests that the component of regional subsidence was accommodated by vertical stresses of a ductile mantle, and that the stratigraphy is an intimate surface monitor of this behaviour.

This paper focuses on the three-dimensional basement architecture and the stratigraphic response to tectonism. We have mapped the structural framework of the basement from published geological maps and total field magnetic and Bouguer gravity data (Stettler et al., 1999, Venter et al., 1999), and have derived depths to Moho from teleseismic data (Nguuri et al., 2001, James et al., 2003). Kaapvaal basement structure is partly imaged in 500 km of 16 s seismic (de Wit and Tinker, 2004). Deep reflection seismic line SAGS-03-92 Beaufort images the southern Namaqua basement between Beaufort West and Droogekloof. This line was recorded 96-fold with a record length of 16 s TWT, conducted with a vibrator source and a 50 m geophone spacing, giving a 6–60 Hz sweep of frequencies. Finally, Soekor drilled 19 exploration wells to a maximum depth of 5560 m (Winter and Venter, 1970), but with an enormous well spacing (up to 250 km).

Section snippets

Tectonic framework

Gondwana was assembled in the late Neoproterozoic and Cambrian by Pan-African – Brasiliano orogenic belts that sutured the cratons of West Gondwana, and by collision with East Gondwana along the Mozambique belt during the late-stage East-African – Ross orogeny (Grunow et al., 1996, Unrug, 1997). This assembly includes the continental lithosphere of the Falkland Plateau which abutted South Africa along the Agulhas–Falkland transform (Richards et al., 1996, Jacobs et al., 1999), and crystalline

Early Paleozoic Cape basin: large extensional subsidence

The Cape Supergroup represents 160 myr of Ordovician through Early Carboniferous subsidence. The basin fill is about 8 km thick. Exploration wells and reflection seismic show that these sediments, including the Msikaba Formation of the Transkei, onlap the Namaqua basement (Fig. 2, Fig. 7). Onland the basin measures about 200,000 km2 to the subcrop edge, and is known to extend at least 100 km offshore. In this section we briefly discuss the episutural Saldanian geology and how the Cape basin was

Permian early Karoo basin: large-scale epeirogeny

The Karoo Supergroup spans 125 myr of latest Carboniferous – Early Jurassic time. It measures nearly 600,000 km2, and has a maximum thickness of ∼5500 m above the Namaqua basement block. The lower Karoo is separated from the upper Karoo by the Permian–Triassic boundary. Subsidence of the Permian lower Karoo basin pre-dated the Cape orogeny. Seismic stratigraphy (Fig. 5), the relationship of cleavage fabrics to diagenetic mineral assemblages (de Swardt and Rowsell, 1974), and provenance studies (

Cape orogeny and late Karoo foreland basin

The Cape orogeny dates to the Triassic uppermost Beaufort and Stormberg groups (∼250–215 Ma). This event is linked to an eastward shift of the locus of subsidence, deposition of coarse alluvial fans, and the appearance of the Lystrosaurus assemblage after the end-Permian crisis. In the southern Karoo, Beaufort seismic reflections at the leading edge of the Cape fold belt have gentle northward dips, parallel to the roof thrust of a tectonic wedge (Fig. 5, Fig. 17), thus directly demonstrating

Discussion

The Cape and Karoo basins formed within the continental interior more than 1000 km from the plate edge. Basin formation was controlled by the strength and anisotropy of rigid blocks of Precambrian basement and the crustal faults between them (Fig. 4, Fig. 6). The distributions and amplitudes of several of the basin-filling events, and the minor contribution of contemporaneous upper crustal brittle failure, require accommodation by ductile mantle flow. Upper mantle rather than lower crustal flow

Summary and conclusions

The Karoo basin is commonly interpreted as a retro-arc foreland basin, despite there being no geophysical evidence for the nearby magmatic arc and the Cape fold belt dating to late Karoo time. Neither does the basin fill display the onlapping characteristics of a flexural foreland basin. Another particularly weak part of the interpretation is that the lithosphere is not laterally uniform.

Overwhelming mutually supportive evidence links the stratigraphy directly to the behaviour of rigid crustal

Acknowledgements

We remember with sadness Dairne Rowsell who died in 2007. Her diagenetic studies were of fundamental help to us.

This project has been an enormous undertaking, covering a long spell of basin evolution and many disciplines. We have checked as much of the data as we can, and greatly appreciate the help that we have received in doing so. In particular, we wish to thank Hugh Balkwill, Peter Booth, Dave Broad, Doug Cole, Bill Collins, Coenie de Beer, Erwin Ebner, Bob Ehrlich, Andrea Fildani, Miguel

References (235)

  • O. Catuneanu et al.

    Foredeep submarine fans and forebulge deltas: orogenic off-loading in the underfilled Karoo basin

    Journal of African Earth Sciences

    (2002)
  • O. Catuneanu et al.

    The Karoo basins of south-central Africa

    Journal of African Earth Sciences

    (2005)
  • L. Coney et al.

    Geochemical and mineralogical investigation of the Permian–Triassic boundary in the continental realm of the southern Karoo basin, South Africa

    Palaeoworld

    (2007)
  • L.C. da Silva et al.

    U–Pb SHRIMP and Sm–Nd age constraints on the timing and sources of the Pan-African Cape Granite Suite, South Africa

    Journal of African Earth Sciences

    (2000)
  • C.H. de Beer

    Fold interference from simultaneous shortening in different directions: the Cape fold belt syntaxis

    Journal of African Earth Sciences

    (1995)
  • M.O. de Kock et al.

    Paleomagnetic constraints on the Permian–Triassic boundary in terrestrial strata of the Karoo Supergroup, South Africa: implications for causes of the end-Permian extinction event

    Gondwana Research

    (2004)
  • B.M. Eglington

    Evolution of the Namaqua–Natal belt, southern Africa – a geochronological and isotope geochemical review

    Journal of African Earth Sciences

    (2006)
  • B.M. Eglington et al.

    Geochronological and isotope constraints on the Mesoproterozoic Namaqua–Natal belt: evidence from deep borehole intersections in South Africa

    Precambrian Research

    (2003)
  • K. Faure et al.

    Geochemical evidence for lacustrine microbial blooms in the vast Permian main Karoo, Paraná, Falkland Islands and Huab basins of southwestern Gondwana

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (1999)
  • E.C. Ferré et al.

    Preserved magnetic fabrics vs. annealed microstructures in the syntectonic recrystallised George granite, South Africa

    Journal of Structural Geology

    (2000)
  • P.G. Fölling et al.

    A novel approach to double-spike Pb–Pb dating of carbonate rocks: examples from Neoproterozoic sequences in southern Africa

    Chemical Geology

    (2000)
  • G.J.B. Germs

    The Neoproterozoic tectono-thermal evolution of the Gariep belt and its basement, Namibia/South Africa

    Precambrian Research

    (1995)
  • P.J. Hancox et al.

    Breakthroughs in the biodiversity, biogeography, biostratigraphy, and basin analysis of the Beaufort Group

    Journal of African Earth Sciences

    (2001)
  • C.A. Haycock et al.

    Early Triassic palaeoenvironments in the eastern Karoo foreland basin, South Africa

    Journal of African Earth Sciences

    (1997)
  • D.K. Hobday et al.

    Fluvial sedimentation and paleogeography of an early Paleozoic failed rift, southeastern margin of Africa

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (1979)
  • R.J. Aldridge et al.

    The Soom Shale

  • J.M. Anderson et al.

    The Palaeoflora of Southern Africa: Molteno Formation (Triassic)

    (1983)
  • R. Armstrong et al.

    Cape Town's Table Mountain reveals rapid Pan-African uplift of its basement rocks

    Journal of African Earth Sciences

    (1998)
  • W. Barnett et al.

    Stratigraphy of the upper Neoproterozoic Kango and lower Palaeozoic Table Mountain groups of the Cape fold belt revisited

    South African Journal of Geology

    (1997)
  • R.W. Belcher et al.

    Lithostratigraphic correlations in the western branch of the Pan-African Saldania belt, South Africa: the Malmesbury Group revisited

    South African Journal of Geology

    (2003)
  • K.T. Biddle et al.

    Plateau de las Malvinas

  • H.J. Blignault

    Ice sheet deformation in the Table Mountain Group, western Cape

    Annals of the University of Stellenbosch, Series A1 (Geology)

    (1981)
  • P.W.K. Booth

    The relationship between folding and thrusting in the Floriskraal formation (upper Witteberg Group), Steytlerville, eastern Cape

    South African Journal of Geology

    (1996)
  • E.M. Bordy et al.

    Basin development during the deposition of the Elliot formation (Late Triassic–Early Jurassic), Karoo Supergroup, South Africa

    South African Journal of Geology

    (2004)
  • E.M. Bordy et al.

    Provenance study of the Late Triassic–Early Jurassic Elliot formation, main Karoo basin, south Africa

    South African Journal of Geology

    (2004)
  • E.M. Bordy et al.

    The control of the Molteno and Elliot formations through the main Karoo basin, South Africa: a second-order sequence boundary

    South African Journal of Geology

    (2005)
  • A.H. Bouma et al.

    Permian passive margin submarine fan complex, Karoo basin, South Africa: possible model to Gulf of Mexico

    Transactions of the Gulf Coast Association of Geological Societies

    (1991)
  • D.S. Broad et al.

    South Africa offers exploratory potential in variety of basins

    Oil and Gas Journal

    (December 1993)
  • D.S. Broad et al.

    Offshore mesozoic basins

  • C.A.M. Broquet

    Trace Fossils and Ichnosedimentary Facies from the Lower Palaeozoic Peninsula Formation, Cape Peninsula, South Africa

    (1990)
  • A.B. Cadle et al.

    Coal Exploration, Economics and Assessment. V. Basement Geology, Regional Sedimentary Patterns and Exploration Targets

    (1982)
  • B. Cairncross

    Anastomosing river deposits: palaeoenvironmental control on coal quality and distribution, northern Karoo basin

    Transactions of the Geological Society of South Africa

    (1980)
  • B. Cairncross et al.

    Palaeoecology of the Triassic Molteno formation, Karoo basin, south Africa – sedimentological and palaeontological evidence

    South African Journal of Geology

    (1995)
  • B. Cairncross et al.

    The bivalve Megadesmus from Permian Volksrust Shale formation (Karoo Supergroup), northeastern Karoo basin, South Africa: implications for Late Permian basin development

    South African Journal of Geology

    (2005)
  • J.N. Carney et al.

    The geology of Botswana

    Geological Survey Department, Botswana, Bulletin

    (1994)
  • O. Catuneanu et al.

    Reciprocal flexural behaviour and contrasting stratigraphies: a new basin development model for the Karoo retroarc foreland system, South Africa

    Basin Research

    (1998)
  • P.A. Cawood

    Terra Australis orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic

    Earth-Science Reviews

    (2005)
  • N. Christie-Blick et al.

    Deformation and basin formation along strike-slip faults

  • S. Cloetingh et al.

    Subsidence history analysis and forward modelling of the Cape and Karoo supergroups

  • L.R.M. Cocks et al.

    The first lower Palaeozoic fauna proved from South Africa

    Quarterly Journal of the Geological Society (London)

    (1970)
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