Cape Fold Belt Research Articles

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

The main Karoo Basin of South Africa is a Late Carboniferous–Middle Jurassic retroarc foreland fill, developed in front of the Cape Fold Belt (CFB) in relation to subduction of the palaeo‐Pacific plate underneath the Gondwana plate. The Karoo sedimentary fill corresponds to a first‐order sequence, with the basal and top contacts marking profound changes in the tectonic setting, i.e. from extensional to foreland and from foreland to extensional, respectively.Sedimentation within the Karoo Foreland Basin was closely controlled by orogenic cycles of loading and unloading in the CFB. During orogenic loading, episodes of subsidence and increase in accommodation adjacent to the orogen correlate to episodes of uplift and decrease in accommodation away from the thrust‐fold belt. During orogenic unloading the reverse occurred. As a consequence, the depocentre of the Karoo Basin alternated between the proximal region, during orogenic loading, and the distal region, during orogenic unloading. Orogenic loading dominated during the Late Carboniferous–Middle Triassic interval, leading to the accumulation of thick foredeep sequences with much thinner forebulge correlatives. The Late Triassic–Middle Jurassic interval was dominated by orogenic unloading, with deposition taking place in the distal region of the foreland system and coeval bypass and reworking of the older foredeep sequences.The out of phase history of base‐level changes generated contrasting stratigraphies between the proximal and distal regions of the foreland system separated by a stratigraphic hinge line. The patterns of hinge line migration show the flexural peripheral bulge advancing towards the craton during the Late Carboniferous–Permian interval in response to the progradation of the orogenic front. The orogenward migration of the foreland system recorded during the Triassic–Middle Jurassic may be attributed to piggyback thrusting accompanied by a retrogradation of the centre of weight within the orogenic belt during orogenic loading (Early Middle Triassic) or to the retrogradation of the orogenic load through the erosion of the orogenic front during times of orogenic unloading (Late Triassic–Middle Jurassic).

Late Precambrian (850-800 Ma) palaeomagnetic pole for the south Indian Shield from the Harohalli alkaline dykes: geotectonic implications for Gondwana reconstructions : Radhakrishna T. & Mathew J., Precambrian Research, 1996, 80/1–2 (77–87)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

Relationship between northwestern Tasmania and east Gondwanaland in the late Cambrian/early Ordovician: paleomagnetic evidence : Li Z.X., BAillie P.W. & Powell C.M., Tectonics, 1997, 16/1 (161–171)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

Alleghanian paleostress reconstruction in the northern Appalachians: intraplate deformation between Laurentia and Gondwana : Faure S., Tremblay A. & Angelier J., Geological Society of America Bulletin, 1996, 108/11 (1467–1480)

The Saldania Belt forms part of a system of Neoproterozoic mobile belts that surround and weld older cratons on the African continent. It is a poorly exposed, low-grade orogenic belt composed of a number of inliers unroofed in mega-anticlinal hinges of the Permo-Triassic Cape Fold Belt, along the southern tip of Africa. Deformed meta-volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep-rooted, sinistral/dextral strike–slip fault zones, which display evidence of prolonged reactivation.Sedimentary rocks and volcanics of the Boland terrane (Malmesbury Group), lower Kango Group and Gamtoos Group are considered to be distal facies of rift successions, overlying the mid-Proterozoic crystalline basement on the southwestern edge of the Kalahari Craton. These lithologies have been deposited in semi-pull-apart basins that formed in response to the breakup of Rodinia and progressive opening of the proto-Atlantic (Adamaster Ocean) from 780–750 Ma. Fragments of juvenile Pan-African crust with WPB–MORB characteristics occur in the Bridgetown Formation (Swartland terrane, Malmesbury inlier) and attest to ocean floor spreading in this belt. The thick turbidite successions of the Swartland and Tygerberg terranes (Malmesbury Group) reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean/continental margin basin marked by increasing sea levels and reduction of continental freeboard.Reversal of spreading and closing of the Adamastor Ocean, possibly initiated by the opening of Iapetus, occurred in the period 600–570 Ma. Although this movement vector caused sinistral transpressional reactivation of the Gariep and Saldanian margins, there is no proper collision orogen developed in the Saldania Belt. The Bridgetown Formation represents the suture between the southern, rifted margin succession of the Kalahari Craton and a poorly developed southern orogenic belt represented in part by the Swartland and Tygerberg terranes. This geosuture is similar to that of the South Atlantic, which formed in the analogous Gariep Belt between 575 and 543 Ma. Multiphase S-, I- and A-type granites of the Cape Granite Suite intruded the Saldania Belt in a pervasive transpressive regime between 550 and 510 Ma, comparing well with the timing of peak metamorphism/collision in the Gariep Belt (545 Ma). The absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the Saldania Belt seem to suggest that the Cape Granite Suite was generated in a strike–slip regime.A discrepancy of approximately 50 Ma between the age of the collision granites of the Dom Feliciano Belt and Cape Granite Suite suggests that they are genetically unrelated. Similarities in structural styles, magmatism and dated events between the Saldania–Ross–Delamarian chain of orogens, however, indicate a common history. The evolution of this chain and that of the Gariep–Damara–Dom Feliciano Belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of Laurentia from South America and opening of the Iapetus Ocean.Loading and depression of the western and southern Kalahari Craton margin by thrust stacks of the Late Neoproterozoic orogens led to the formation of syn- to post-orogenic peripheral foreland basins, such as those represented by the Nama and Vanrhynsdorp Groups, lowering of sea level and increased continental freeboard. Deposition of the unconformity based upper Kango Group, and possibly the molassic Franschhoek Formation, occurred in intra-orogen pull-apart basins, which possibly developed between 530 and 510 Ma in the Saldania Belt.

Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) revisited

As integral parts of du Toit’s (1927) “Samfrau Geosyncline”, the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) share similar paleoclimatic, paleogeographic, and paleotectonic aspects related to the Late Paleozoic tectono-magmatic activity along the Panthalassan continental margin of Gondwanaland. Late Carboniferou-earliest Permian glacial deposits were deposited in the Sauce Grande (Sauce Grande Formation) and Karoo (Dwyka Formation) basins and Falkland–Malvinas Islands (Lafonia Formation) during an initial (sag) phase of extension. The pre-breakup position of the Falkland (Malvinas) Islands on the easternmost part of the Karoo basin (immediately east of the coast of South Africa) is supported by recent paleomagnetic data, lithofacies associations, paleoice flow directions and age similarities between the Dwyka and the Lafonia glacial sequences. The desintegration of the Gondwanan Ice Sheet (GIS) triggered widespread transgressions, reflected in the stratigraphic record by the presence of inter-basinally correlatable, open marine, fine-grained deposits (Piedra Azul Formation in the Sauce Grande basin, Prince Albert Formation in the Karoo basin and Port Sussex Formation in the Falkland Islands) capping glacial marine sediments. These early postglacial transgressive deposits, characterised by fossils of the Eurydesma fauna and Glossopteris flora, represent the maximum flooding of the basins. Cratonward foreland subsidence was triggered by the San Rafael orogeny (ca. 270 Ma) in Argentina and propogated along the Gondwanan margin. This subsidence phase generated sufficient space to accommodate thick synorogenic sequences derived from the orogenic flanks of the Sauce Grande and Karoo basins. Compositionally, the initial extensional phase of these basins was characterized by quartz-rich, craton-derived detritus and was followed by a compressional (foreland) phase characterized by a paleocurrent reversal and dominance of arc/foldbelt-derived material. In the Sauce Grande basin, tuffs are interbedded in the upper half of the synorogenic, foldbelt-derived Tunas Formation (Early–early Late? Permian). Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late Early Permian (260 Ma) age. Silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) peaked between the late Early Permian and Late Permian. A link between these volcanics and the consanguineous airborne tuffs present in the Sauce Grande and Karoo basins is suggested on the basis of their similar compositions and ages.

Late Palaeozoic to Mesozoic structural evolution of the Falkland Islands: a displaced segment of the Cape Fold Belt

The Falkland Islands lie on a displaced crustal block presently forming part of the South American plate. The islands possess two roughly orthogonal structural grains, the relative chronology of which is here unequivocally established for the first time. D1 structures form a southerly verging, Permo-Triassic age fold belt, striking E–W to WNW–ESE, which represents a displaced segment of the Gondwanian orogenic belt. The strike swing from E–W to WNW–ESE is coincident with a decrease in deformation intensity toward the west. On West Falkland, D1 folds are superimposed by the early Mesozoic D2, NE–SW-trending, Hornby Mountains anticline, producing localized kilometre-scale Type I and III fold interference patterns. The Hornby Mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre-existing NE–SW basement fault. If the Falkland Islands are rotated 180°, the structure of the D1 fold belt, and tectono-sedimentary features of the Permian age Lafonian Supergroup, display a remarkable correlation with the structural style and tectonic evolution of the eastern Cape Fold Belt of South Africa. Our data, therefore support a 180° tectonic rotation of the Falkland Islands and reaffirm their pre-Gondwana break-up position adjacent to the southeast coast of South Africa. In such a reconstruction the Falkland Islands constrain the easterly extension and eventual lateral termination of the Cape Fold Belt.

Influence of Tectonics on Basin Shape and Deep-water Sedimentary fill Characteristics; Examples from the Karoo Basin, South Africa : ABSTRACT

ABSTRACT Local subbasins developed in the southwest corner of the Karoo Basin, South Africa, during the late Permian. The Tanqua and Laingsburg subbasins had near-contemporaneous formation and filling, and contain a number submarine fan systems. The five individual submarine fans in the broad, shallow Tanqua subbasin range from 150 to 450 km2 with lateral continuity of individual fans up to 34 km. With a sandstone/shale ratio of 75 - 90 percent, the arenaceous fans vary in thickness from 20 to 60 m. Intervening shales range from 20 to 75 m. The four submarine fans in the more typical foredeep style Laingsburg subbasin are thicker and hundreds of kilometers long. The history of the subbasins was heavily influenced by the events and associated tectonic structures of the Cape Fold Belt. Differential compression on the two branches of the Cape Fold Belt led to a basin floor high that effectively separated the transport to the two subbasins. Paleocurrent data indicates that this divided transport started at the early development of the subbasins. While the subbasin development is associated with the subduction zone on the southwest side of Gondwana, their fill indicate deposition during a tectonic quiescence. Petrographic analysis suggest a single source area located outside the adjacent fold-thrust belt (Cape Fold Belt). The separation of the subbasins makes it difficult to determine the timing of the deposition of the individual fan systems. No paleontological or palynological data could be obtained. From sedimentological and geochemical data, tentative timing of the deposition of the submarine fan systems can be postulated. Understanding the evolution of the subbasins and the conditions under which the submarine fans were deposited will lead to the determination of the influences on the formation of the fans and their resulting architecture.

Early Triassic palaeoenvironments in the eastern Karoo Foreland Basin, South Africa

The Beaufort Group sediments of the Karoo Supergroup in southern Karoo were deposited in a Permo-Triassic foreland basin associated with the development of the Cape Fold Belt. The eastern parts of this basin are often overlooked in palaeoenvironmental reconstructions. During the Early Triassic, deposition occurred in this region on a braided alluvial plain. Vegetation, although restricted to areas adjacent to water bodies, supported a healthy vertebrate population, notably Lystrosaurus and Thrinaxodon. Pulses of sedimentation produced interfingering of sandy Katberg Formation sediments and muddy Palingkloof Member sediments (Balfour Formation), as well as a lateral shifting of facies belts. The cause of these was episodes of tectonic activity associated with the development of the Cape Fold Belt. The source of the sediments was the Proterozoic basement of the Maurice Ewing Bank, which lay to the southeast of the area prior to Gondwana break-up. The exposure of the basement at this time is in marked contrast to the downward flexing of basement in areas further to the west. It defines the eastern side of the Karoo Basin and may account for the very thin sedimentary succession in Dronning Maud Land, Antarctica.

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

The Cape Fold Belt syntaxis displays antitaxial bends on either side of a well-developed intermediate fold trend. Folds of the NW-trending and E-trending branches merge relatively smoothly with the NE-trending syntaxial folds. The general pattern of fold coalescence, arcuations and the presence of many smaller folds of divergent trends show strong similarities with patterns generated in experimental models simulating the shortening of a multilayer simultaneously in two perpendicular directions and at unlike magnitudes. Interference between structures formed in response to different shortening directions led to a distinct regional variation of fold trends in the Witteberg Group of the Ceres arc. The special fold overprinting relationships at fold intersections in this area are interpreted in terms of bulk constrictional deformation. The structure of the syntaxis is probably the combined effect of the superpositioning of two roughly coeval, non-coaxial fold belts upon a much larger nonrotational arc, initially formed by the molding of the orogen around the irregular shape of the craton.

Deep Water Deposits of the Tanqua and Laingsburg Subbasins, Southwest Karoo Basin, South Africa: Analog for the Gulf of Mexico: ABSTRACT

ABSTRACT The Tanqua and Laingsburg subbasins in South Africa had near-contemporaneous formation and filling and contain Permian-age basin-floor and slope fans that display characteristics similar to deposits in the northern Gulf of Mexico. Outcrop area for each subbasin is about 650 km2 and individual fans range from 150 to 450 km2 with lateral continuity of individual fans up to 34 km. With a sandstone/shale ratio of 75-90%, the arenaceous fans vary in thickness from 20 to 60 m with interlying shales ranging from 20 to 75m in thickness. Both subbasins were influenced in their formation and in the architecture of their deposits by structures and events associated with the Cape Fold Belt. Having undergone little tectonic tilting, the highly continuous outcrops in the Tanqua subbasin expose two fans continuing from mid-fan channel complexes with associated levee-overbank deposits to sheet-like outer fan lobes, while three other fans expose outer fan depositional lobes. Paleocurrent directions for four of the fans are from SSW to S and a direction of W to WNW for the fifth. These fans most likely had a,single point source which migrated over the time of basin fill. Unrestricted deposition suggests an open basin depositional setting. The Laingsburg subbasin was strongly influenced by the tectonism associated with the Cape Fold Belt. Deposition occurred in a deeper and narrower basin and the deposits, except for the overlying deltaics cannot be correlated with those of the Tanqua subbasin. The two subbasins, while associated with an active margin, were likely filled at slightly different times. Both had a distant source area which led to deposits exhibiting characteristics of a passive margin depositional environment. Understanding the evolution of the subbasins and the tectonic conditions under which the submarine fans were deposited leads to the determination of the mechanisms that influenced the formation of the fans and their resulting architecture. These fans permit detailed studies on their architecture necessary to 1) increase our understanding of fine-grained, low sandstone/shale ratio fans, 2) determine influences of paleostructures and tectonics on basin fill, 3) carry out detailed reservoir simulation programs, and 4) make predictive models of deep-water sands in the northern Gulf of Mexico. End_of_Record - Last_Page 646-------

A structural sequence in low strain sedimentary rocks in the Beaufort Group of the south-western Karoo and its bearing on syngenetic and epigenetic mineralization

Several sets of planar and linear structures are described from the very gently folded Karoo strata of the northern-most tectonic zone of the Permo-Triassic Cape Fold Belt. The superposition of these structures reveals a time sequence. Horizontal compression occurred before compaction was complete. After compaction further horizontal compression developed a sequence of structures that are readily interpreted in terms of visco-elastic strain in multilayers. This analysis reveals that a set of mainly antithetic kinks in the form of monoclines developed out of earlier incipient sinusoidal macrofolds during the final stages of horizontal N-S compression. The model is in agreement with evidence for synsedimentary folding that controlled the sites for erosion channels as well as the direction of sedimentary transport. Exploration efforts for syngenetic and/or fluid controlled epigenetic mineralization in permeable strata of this fluviatile sedimentary sequence should be directed along certain structural lines.

The Falkland Islands: A key element in Gondwana paleogeography

Stratigraphic and sedimentological fieldwork on the Falkland Islands confirms that they were originally part of South Africa. East and West Falkland have contrasting geological and deformation histories and are separated by the Falkland Sound fault, which marks a significant deformation edge to the Cape Fold Belt. The degree of structural deformation and metamorphism in West Falkland is substantially less than that of East Falkland, adjustments to differential shortening between the two islands being reflected by structures developed along the Falkland Sound fault. Prior to fragmentation of Gondwana, the Falkland Platform and Maurice Ewing Bank were positioned in the Natal Valley, adjacent to the east coast of South Africa. This predrift reconstruction shows that the Falkland Islands have subsequently been rotated by 180° relative to South Africa and complements recent paleomagnetic data. There was a complete overlap of the Maurice Ewing Bank and Falkland Platform to the exclusion of the Falkland Plateau Basin which is a small oceanic basin formed during the Jurassic fragmentation of Gondwana. Extension in this basin was accommodated by movements along the Aghulas Falkland Fracture Zone and the Gastre Fault System. Predrift restoration of these faults shows that Patagonia was positioned closer to both Africa and Antarctica. The Falkland Plateau Basin was an integral component of the West Gondwana Jurassic microplate system and provides an invaluable insight into the evolution of its western margin. At some time prior to the main opening of the South Atlantic, the Falkland Platform was rotated clockwise by 120° against the Agulhas Falkland Fracture Zone, such that the Falkland Islands were translated some 300 km to the south. The driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the Falkland Plateau Basin as the Gastre Fault System, within continental crust, became unable to accommodate the continuous dextral shear in the Agulhas Falkland Fracture Zone. The Falkland Islands represent an outboard extension of the Eastern Cape and are significant for understanding the paleogeography of both the Cape and Karoo supergroups. East Falkland is closely similar to South Africa in its geology, while West Falkland shows significant contrasts with both the Devonian and Permian successions being more proximal in character but still demonstrate the eastward continuation of the Cape and Karoo basins.

BASIN EVOLUTION AND PROSPECTIVITY OF THE ARGENTINE CONTINENTAL MARGIN

Nine Mesozoic‐Tertiary basins of different orientations lie along the Argentine continental margin, over a distance exceeding 2,000 km: these are the Salado, Colorado, Valdes, Rawson, San Jorge offshore, North Malvinas (San Julian), West Malvinas and Magallanes (Austral) Basins on the Continental Shelf, together with the Continental Slope. These basins formed following the latest Jurassic — Early Cretaceous extension that accompanied the onset of South Atlantic rifting. Strain was modified by earlier basement fabrics, with consequent transtension. The Cape Fold Belt (Permo‐Triassic) provided a NW‐SE grain to pre‐Mesozoic cover off NE Argentina. In the central‐southern sector, several phases of oblique NEwards Pacific subduction and terrane accretion during the Permo‐Triassic formed back‐arc basins and volcanic belts, producing a more variable fabric orientated close to NNW‐SSE. Atlantic basin fill, of Lower Cretaceous‐Tertiary clastics, was deposited as a result of rift‐shoulder erosion and Atlantic flooding, following eastward tilting. Basin fill thickness is typically 2–4 km, but locally exceeds 6km. The hydrocarbon potential of these basins hinges as much upon the preservation of source rocks within the pre‐rift succession as it does on that of those within the base‐rift succession, and subsequent Atlantic anoxic events.

The Karoo Basin of South Africa: type basin for the coal-bearing deposits of southern Africa

The coal-bearing sediments and coal seams of the Karoo Basin, Southern Africa are described and discussed. The Karoo Basin is bounded on its southern margin by the Cape Fold Belt, onlaps onto the Kaapvaal Craton in the north and is classified as a foreland basin. Coal seams are present within the Early Permian Vryheid Formation and the Triassic Molteno Formation. The peats of the Vryheid Formation accumulated within swamps in a cool temperate climatic regime. Lower and upper delta plain, back-barrier and fluvial environments were associated with peat formation. Thick, laterally extensive coal seams have preferentially accumulated in fluvial environments. The coals are in general inertinite-rich and high in ash. However, increasing vitrinite and decreasing ash contents within seams occur from west to east across the coalfields. The Triassic Molteno coal seams accumulated with aerially restricted swamps in fluvial environments. These Molteno coals are thin, laterally impersistent, vitrinite-rich and shaly, and formed under a warm temperate climatic regime. Palaeoclimate, depositional systems, differential subsidence and basin tectonics influence to varying degrees, the maceral content, thickness and lateral extent of coal seams. However, the geographic position of peat-forming swamps within a foreland basin, coupled with basin tectonics and differential subsidence are envisaged as the primary controls on coal parameters. The Permian coals are situated in proximal positions on the passive margin of the foreland basin. Here, subsidence was limited which enhanced oxidation of organic matter and hence the formation of inertinitic coals. The coals in this tectonic setting are thick and laterally extensive. The Triassci coals are situated within the tectonically active foreland basin margin. Rapid subsidence and sedimentation rates occurred during peat formation which resulted in the preservation of thin, laterally impersistent, high ash, vitrinite-rich, shaly coals.

The tectonic evolution of southern Africa: an overview

This paper provides an overview of recent ideas concerning the tectonic framework and evolution of the southern African subcontinent from the Early Archaean to the present. Mid-Archaean crustal growth of ancient continental fragments resulted from episodic terrane accretion, a process that culminated in the juxtaposition of the Kaapvaal and Zimbabwe granitoid-greenstone terranes during the Limpopo orogeny at ∼2.7 Ga. Whilst intracratonic sedimentary basins developed on this stable cratonic block from the late Archaean to Mid-Protorozoic (3.0–2.4 Ga), crustal growth occurred during the successive Eburnian (∼2.0–1.7 Ga), Kibaran (∼1.2−1.0 Ga) and Pan-African (∼500 Ma) global accretionary events. The Palaeozoic evolution of southern Africa following the assembly of the Gondwana supercontinent is dominated by the development of the Cape and Karoo basins and their subsequent inversion to produce the Cape Fold Belt (∼280−230 Ma). Mesozoic fragmentation of Gondwana (∼180−130 Ma) was associated with continental rift volcanism and fault reactivation. Minor tectonic activity continues to the present day.

The vegetation of the Haasvlakte, Houhoek, South Africa

The results of an analysis of the vegetation found on the Haasvlakte, located on the edge of the Elgin Basin in sandstone mountains of the Cape Fold Belt, are presented. Forty-five releves were distributed through physiographic/physiognomic units identified on colour aerial photographs. Phytosociological data analysis, using the T winspan and P ctables programs, resulted in eight Cape Fynbos shrubland communities being distinguished. These communities identified are related to environmental attributes. The vegetation is broadly correlated to soil complexes although the boundaries between units do not concur precisely. A preliminary check-list of 375 species (170 genera in 53 families) is included. Nine of these species are known to be rare-and-endangered. The largest family is the Restionaceae containing 36 species (13 genera).

A Passive Margin-type Submarine Fan Complex, Permian Ecca Group, South Africa

A submarine fan complex, comprising five arenaceous fan systems separated by basinal shale units, occurs in the southwestern part of the intracratonic Karoo basin in South Africa. Although basin development is related to a subduction zone bordering the palaeo-Pacific ocean to the south of Gondwanaland and the evolution of the Cape Fold Belt, the entire Lower Permian Ecca Group basin-fill succession reflects depositional characteristics of a passive-margin setting. The submarine fan complex, 250 m thick, originated from sediments supplied by Mississippi-type deltas dominating the Ecca coastline. The fine grain-size and low sand/shale ratio of the submarine fan and deltaic deposits reflect the maturity of the ancient river systems. Outcrops of the fan complex are well exposed and cover an area of 650 km{sup 2}. The strata are not affected by folding, and deep erosion allows three-dimensional viewing of mid-fan to outer-fan deposits. Features of interest include stacked lobe deposits displayed along 2.5 km of a 60 m high cliff section, and a transverse cliff section through channel-fill deposits 500 m wide. Paleocurrent directions reveal that each sequence had its own main source area located to the northwest and south of its present geographic location. The cyclic nature of the fanmore » complex is attributed to relative sea-level changes; deposition took place on the basin floor in water depths that do not exceed 500 m. Shoaling of the basin to wave base depths is reflected in the pro-delta and delta front deposits overlying the uppermost fan sequence. Major factors in controlling direction of fan progradation were delta switching and basin floor topography.« less

A Back-Projection Method for Imaging Large-Scale Lateral Variations of Lg Coda Q with Application to Continental Africa

SUMMARY A new method is developed which uses back-projection tomography to regionalize large-scale lateral variations of coda Q for Lg waves which have traversed long continental paths. Successful use of this method requires precise and stable single-trace measurements of Lg coda Q (Xie & Nuttli 1988). the method converges rapidly and requires minimal computer storage. It also allows quantitative estimation of resolution and error in imaging lateral variations of coda Q. the spatial resolution of this method is limited by the uneven spatial coverage of the data base, by our limited knowledge of Lg coda generation, and by the trade-off between the stability and the spatial resolution of the coda Q inversion. the method is applied to a large set of digital Lg coda data from Africa, where large-scale lateral variations of Lg coda Q are found to correlate well with major tectonic features. Most of Africa is stable, and like other stable regions, has relatively high coda Q values. the lowest values of coda Q are associated with the African rift system. Other regions of low-Q values include the Atlas mountains and Cape Fold Belt, regions of Mesozoic and younger deformation. the lateral variation of frequency dependence of Lg coda Q correlates, in most regions, with that of Q at 1 Hz. Our analysis of the spatial resolution of this method indicates that the resolving power of Lg coda Q imaging is comparable to that of velocity tomography using long-period surface waves. Using empirical approaches, we estimate that uncertainties in Lg coda Q and its power-law frequency dependence parameter are less than about 60 and about 0.2, respectively, for most of Africa.

Paleomagnetism of the western Cape Fold belt, South Africa, and its bearing on the Paleozoic apparent polar wander path for Gondwana

In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28°N, 14°E ( k = 25, α 95 = 8.8°, N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but can help to constrain the apparent polar wander path for Gondwana. Our results give only paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60–70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval.

African anorogenic alkaline magmatism and mineralization — a discussion with reference to the Niger-Nigerian Province

The close of the Pan-African was characterized by a change from subduction and arc related magmatism to post tectonic alkaline magmatism in Saudi Arabia and West Africa. For the remainder of the Phanerozoic, folding and tectonism was more restricted in Africa than on any other continent and was limited to the Caledonian, Hercynian and Alpine orogenies in the extreme north, and the Cape fold belt at the southern tip of the continent, with an equivalent orogenic belt on the extreme east of the Arabian peninsula. Thus Phanerozoic magmatism was dominantly anorogenic in nature and alkaline in character throughout the African Plate. Such magmatism was initiated by reactivation of deep-seated lineaments with the peak of activity during the Mesozoic, which can be correlated with fragmentation of Gondwanaland. The Phanerozoic alkaline magmatism is characterised by small centres of subvolanic to plutonic nature often in the form of ring complexes. These can be subdivided into two separate associations consisting of Provinces dominated by centres of oversaturated magmatism such as Nigeria and Sudan. There are also provinces with undersaturated complexes and carbonatites with oversaturated magmatism such as in Namibia, Angola, and the East African rift system. Despite the contrasting nature of undersaturated and oversaturated magmatism many comparisons can be drawn between carbonatites and granitic complexes. Such comparisons include the source of the magmas, the mechanisms of emplacement, the cause and effect of alkali metasomatism and processes of hydrothermal alteration, the trace element geochemistry, the abundance of zircon, uranium, thorium and complex Ti-Zr silicates, the importance of columbite, pyrochlore, fluorine, and rare earth enrichment in mineralization, and in their current and future economic potential. The common feature is the development and reactivation of deep-seated lineaments which were not only controlled the emplacement of anorogenic complexes but also acted as channelways for hydrothermal mineralizing fluids. The development of a fluid phase during crystallisation occurred in both undersaturated and oversaturated alkaline rocks. Subsequent hydrothermal modifications, subsolidus reactions, geochemical variations and mineralization were dependent upon whether this fluid phase was retained or expelled.