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Abstract -- The dolerite sill and ring-like structures of the Karoo Basin have been a matter of debate for a considerable period of time. However, the mechanism for their emplacement still remains an enigma. A review of the available literature shows that very little structural work has been carried out on these intrusions, which outcrop over two thirds of South Africa. A large quantity of geological information, maps, and field data have become available over the last 15 years. A comparative morpho-tectonic analysis of three sill-ring systems of the western Karoo, that is, Williston, Fraserburg, and Victoria West complexes, indicates that their shape is saucer-like with an inner sill at the bottom, an arcuate inclined sheet (the ring) on the periphery, and an outer sill on the rim. Many arcuate dykes are seen branching onto the ring structures. A mode of emplacement is proposed whereby dolerite dykes feed into the inclined sheets, which then propagate into an outer sill and thereafter into an inner sill.

Introduction and geological setting

The Jurassic Karoo volcanics (183 Ma; Duncan et al., 1997) intruded into the Carboniferous-Permian Karoo sediments during a period of extensive magmatic activity that took place over almost the entire southern African subcontinent during one of the phases in the break-up of the Gondwana (Bristow and Saggerson, 1983; Eales et al., 1984; Hunter and Reid, 1987). It produced one of the four major preserved continental flood basalts in the world (White, 1997). The igneous products of the Main Karoo Basin exhibit a much larger proportion of intrusions (dykes and sills) than extrusives (Lesotho basalts) (Figures 1 and 2). This extremely large volume of intrusive material now crops out over an area covering nearly two thirds of South Africa. In other parts of the world, extremely large-volume intrusive events are mostly expressed as giant radiating dolerite dyke swarms like the 1270 Ma Mackenzie or the 200 Ma Central Atlantic swarms (Ernst and Buchan, 1997a). However, no equivalent of the Karoo dyke and sill network has ever been described, except (to a much lesser extent) for the Proterozoic intrusions of the Guiana shield (Gibbs, 1987). Another important difference from other plateau basalts and giant dyke systems is the level of erosion that affected the Central Karoo basin, revealing the deeper portions of the intrusive system and a degree of tectonic complexity not encountered elsewhere.

Previous work on the central Karoo dolerites

The South African Karoo dolerites were first partially mapped by Rogers and Schwarz (1902), Rogers and Du Toit (1903), and Du Toit (1905). Follow-up research was mainly concerned with petrological investigations of the dolerite, which includes the pioneering work of Walker and Poldervaart (1941; 1942) and the more recent approach of Le Roex and Reid (1978), Marsh and Eales (1984), and Marsh et al. (1997) on the Lesotho lavas. The term Karoo dolerite includes a wide range of petrographical facies, extending from a leucogabbro to a dolerite-pegmatite, with the dolerite sensu stricto being the most abundant. The structural and tectonic aspects of the dolerite intrusives have not received much attention. Du Toit (1905; 1920) first described the behaviour of some of the inclined sheets and sills in the Eastern Cape. Scholtz (1936) described the geometry of a thick sill in East Griqualand. Walker and Poldervaart (1949) summarized various interesting but scattered observations made on the morphology of few dolerite intrusions. Wilke (1961), in his study of groundwater in the Fraserburg district, analysed and classified the intrusions according to their geometrical pattern. Reid et al. (1991) and Reid and Rex (1994) mapped the dolerite dykes of the western coast of South Africa. Hunter and Reid (1987) first attempted to integrate the central Karoo dolerite into the much wider African structural context. Different mechanisms of emplacement have been proposed for the ring-like structures (Du Toit, 1905; Lombaard, 1952; Meyboom and Wallace, 1978; Burger et al., 1981; Vivier et al., 1995) and are discussed below.

The exact geometry of these sills and ring complexes has not been fully documented and relevant structural observations are few compared to the extensive exposures available. During the last 15 years, more data have become available on the structure and tectonics of the western Karoo dolerites. New maps have been published by the Geological Survey, Landsat imagery became available, numerous boreholes were drilled by the Department of Water Affairs and Forestry, and the Water Research Commission funded extensive geohydrological investigations in the Karoo Basin where these intrusives play an important role in the occurrence of ground water.

Regional structure of the western Karoo dolerites

The present study focuses on the Western Karoo, an area defined as stretching from Calvinia to Middelburg (Figures 1 and 2). The dolerite intrusions are so interconnected and anastomosed that it is nearly impossible to single out any particular intrusive or tectonic event. For instance, a dyke can act as a feeder to two different sills or an individual sill can be fed by many dykes of different orientations. This leads to the conclusion that during the intrusive phases, a very large number of fractures were simultaneously infilled with magma, and that the dolerite intrusive network acted as a shallow stockwork-like reservoir where the fractures were intruded by molten magma of different viscosities.

Dykes

The distribution of dolerite dykes in the western Karoo (Figure 1) was compiled from examination of aerial photography and satellite imagery, correlation with existing geological maps, and field verification. The remaining portions of the map, namely the eastern and northern Karoo, were compiled from existing 1:250 000 geological maps. The dykes of the western Karoo are confined to an east-west corridor, some 800 km in length and 250 km in width. Two basic types of tectonic patterns are visible:

1. An east-west dislocation zone extending from the Atlantic coast to Eastern Cape. This dislocation zone shows a right lateral shear pattern defined by several east-west-trending mega dykes (the strike-slip direction), Riedel shears (around N120), and P-type fractures (around N70) in accordance with other well-documented shear zones (Sylvester, 1988).
2. North-northwest-trending dyke swarms consisting of evenly spaced zones of major dyking and alternating with corridors of less fracturing. Many of these dykes show a regional curved trajectory from northwest in the south to north-south in the north. The north-northwest orientation is also a prominent trend in Namaqualand (De Beer, pers. comm., 1998) and is also found in the Cape Peninsula, although the age is much younger (130 Ma, Reid et al., 1991). Some of these dykes (like the Middelburg dyke) link onto the Eastern Cape east-west mega feeders.

The general Karoo dyke pattern over South Africa is similar to other giant dyke swarms described elsewhere, the geometry of which has been used to locate palaeo-plume centres (Ernst and Buchan, 1997a; 1997b). The sketch map of Figure 1 (inset a) shows that the South African dolerite megaswarms diverge from a point off the Eastern Cape coastline where the authors postulate a triple junction, and therefore a palaeo-plume. Studies done on other dyke megaswarms (Ernst, 1990; Halls and Bates, 1990) have shown that dyke propagation, and therefore magma flow, were probably laterally along strike and confined within a specific lithostratigraphic level. The right-lateral east-west shear zone is interpretated as a failed transformed fault between an active rift system in the east and a zone of rift pre-weakening in the west (Figure 1, inset b).

Sills and ring complexes

A map showing the distribution of dolerite sills and ring structures in the western Karoo (Figure 2) was compiled from geological maps published by the Geological Survey of South Africa. The sill and ring complexes often display a sub-circular basin-like shape. The rims of the ring are often topographically accentuated by preferential erosion of the sediments trapped within the structure, giving rise to the general appearance of a ring-like structure classically described for the first time by Du Toit (1905; 1920), in the vicinity of Queenstown.

On a regional scale, the western Karoo dolerite sills form extensive coalescing circular units. Each unit is composed of several sub-units of lesser size, which, in turn, are made of even smaller units and so forth, resulting in a `ring within a ring' or 'basin within a basin' pattern. The size of the ring complex appears to be related to stratigraphic level of intrusion, that is, the sills forming the larger structures have been intruded at the base of the Karoo sequence, while the smaller structures (<10 km in diameter) have been intruded into the upper part of the sequence. Lithology had a strong control on the emplacement of the sills. In an early interpretation of sill stratigraphy, Du Toit (1920) described the dolerite injections as ubiquitous, but pointed out the existence of preferential horizons of intrusion, such as the contacts between the Dwyka-Ecca Group, the Prince Albert-White Hill Formation, Upper Ecca-Lower Beaufort Group, and other lithological boundaries within the Beaufort Group. This was confirmed by later deep oil-exploration drilling in the Karoo Basin (Winter and Venter, 1970).

Morphotectonic analysis of selected sill-ring structures

Three medium-size ring complexes were investigated for the purpose of this study, namely the Williston, Fraserburg, and Victoria West structures [Figure 2(B)]. Each system displays a different degree of complexity. The following study combines analyses of cross-sections derived from detailed 1:50 000 scale geological maps, complementary aerial-photograph and Landsat image interpretation, aeromagnetic maps, fieldwork, and borehole information.

Williston sill-ring complex

Two 100-m-thick dolerite sills in the vicinity of Williston coalesce with one another. Each sill has a distinctive spectral response on the Landsat TM imagery due mainly to differing susceptibility to weathering (Figure 3), a distinctive magnetic signature, and a characteristic morphotectonics. The first one (Ghaapkop sill) corresponds to a flat-lying sill extending to the north and west of the town, and is characterized by a lightbrown/green colour on the satellite image, a weak magnetic signature, outcrops in the lower-lying areas, exhibits a 'smooth' relief, and is bounded by a poorly defined ring. The second structure, called the Williston sill, is mainly developed to the east and south of Williston and exhibits a more pronounced ring-shaped rim structure, is a dark-green colour on the satellite imagery, has a strong magnetic signature, and forms a prominent relief.

The Williston sill and ring complex forms a sub-circular basin some 40 km in diameter (Figure 4) and is composed of several coalescing arcuate structures. In detail, the rim of the basin has a complex geometry, with sharp angles and even concave outlines. This jagged shape often corresponds to the intersection of a single dyke or several dykes of differing orientations with the ring structure.

In cross-section, the Williston complex consists of a major flat-lying inner sill, which exploration drilling in the western portion of the structure showed to have a constant thickness of 100 m (Figure 4). In the eastern half of the complex, the sill thins to only 20 m. Few minor thinner sills also occur. The rim of the ring complex corresponds to an inclined sheet accompanied by severe up-turning of the sediments and thickening of the inner sill. The rim of the system is well exposed in the road cutting between Williston and Sutherland, where it shows an inward (north) dip of 50 Degrees. In the east, the inclined sheet dips at only 25 Degrees, as can be seen in the road cutting between Williston and Fraserburg. A small sill-ring structure is developed within the main ring structure, which caused uplifting of the Abrahamskraal Formation in the centre of the structure. The ring morphology is less conspicuous in the northern portion of the complex, but topographical and borehole information suggest the presence of a steeply dipping inclined sheet linking the inner sill (inside the ring) and an outer sill (outside the ring), one of which is situated at a higher elevation.

The inclined sheets and rims of these structures are heavily fractured. The rock is densely jointed and differential weathering results in the development of numerous hillocks. Observations made elsewhere show that lenses of sediments of different sizes are often trapped within these portions of the structure.

Fraserburg sill-ring complex

This structure is similar in size to that of the Williston complex (40 km diameter), but it only consists of arcuate dykes along its western rim and shallow inclined sheets and small sills along its eastern margin (Figure 5). It is poorly defined on the Landsat TM imagery and has a weak magnetic signature.

Aerial-photo and Landsat interpretation show that the arcuate dykes form an annular structure and are clearly generated from north-northwest-trending fissures, while dykes of other orientation trends played a lesser role. The arcuate dykes forming the rim of the structure and the north-northwesttrending dykes are relatively narrow, averaging 8 to 10 m in width. Their dip is often difficult to assess on the field, but is close to vertical.

No major thick sills are visible at surface. In cross-section, however, the eastern half of the complex displays a flat-basin morphology (Figure 5), which is also visible on Landsat image, that is attributed to a flat sill underlying the sediment. The sill outcrops on the eastern margin of the structure and can be clearly seen on the Fraserburg-Williston road, some 20 km from Fraserburg, where a steeply dipping sharp contact with the sediment is exposed. This sill has been removed by erosion along the western portion of the ring complex.

Victoria West sill-ring complex

It is a typical 'basin within a basin' dolerite intrusive system consisting of many coalescing circular or arcuate structures of different sizes, attitudes, and stratigraphic levels of emplacement (Figure 6). All of these units are interconnected and belong to the same intrusive phase. The ring-like structures are well developed along on the western and southern margin of the complex, where the intrusives attain thickness in excess of 100 m. The shape and attitude of the inclined sheet linking the lower inner sill to the upper outer sill is clearly reflected in the geomorphology of the landscape (Figure 6, cross-section).

The best exposure is situated on the Victoria West-Loxton road where it crosses the rim of the ring structure, some 30 km from Victoria West. Here various morpho-structural features and their interrelation can be observed. The feeding dyke is 25 m thick and dips inwards at 85 Degrees E and it passes vertically into an inclined sheet dipping at 35 Degrees E, which forms a prominent topographic high. The inclined sheet bends sharply downward to coalesce with the lower inner sill, which then disappears under the sedimentary cover and is responsible for the morphology of the flat plain. The inclined sheet also bends abruptly to form an outer sill situated some 100 m above the inner sill. The Karoo sediments within the ring structure are thrusted and deformed in the vicinity of the feeding dyke. Clay and calcrete deposits are also well developed in this zone.

From aerial-photo and Landsat interpretation a series of north-northwest-trending dykes, which coalesce with the ring dykes, appear to be the main feeder systems in the south and western portions of the complex. The north-northwest-trending dykes can be traced further to the south in the vicinity of Beaufort West. The east-west-trending dykes interfere with the sill-ring complex, but do not seem to have played a major role in the feeding of the system.

Proposed morphotectonic model

No single sill-ring complex meaningfully displays a full 3-D picture of these structures. However, all of them do display a 'saucer-like' shape in outcrop. Comparative work on the three different systems exposed at different stratigraphic levels of erosion highlights the following five common morphological and structural features (Figure 7).

Feeder dykes

These are sub-vertical dykes (between 10 and 30 m thick) that branch into and commonly form the rim of the ring structure. Dykes of different orientations, propagating from different directions and crossing one another, are responsible for the sharp angles (sometimes concave) along the margins of the structure. On a regional scale, however, the north-northwesttrending intrusions are the major feeder conduits to the ring complexes. Feeder dykes are also more numerous where the ring structures are well developed.

Inclined sheet (rim of the ring)

The inclined sheet commonly dips at 60 Degrees or less and links the lower inner sill to an upper outer sill. It commonly forms a topographic high with a very jagged or irregular morphology. The rim of the ring structure is often asymmetrical and only well developed along one margin of the sill. The thickness of the sheet varies from 30 to 100 m. Analysis of cross-sections would indicate that the prominent inclined sheets are thicker than the corresponding inner sill.

Inner sill

A major flat-lying sill (60-100m thick), clearly reflected in the geomorphology of the area and forming the bottom of the 'saucer'.

Outer sill

A very extensive sill (up to 100 km), but often removed by erosion. It can be situated some 100 m above the inner sill.

Minor secondary ring structures

These features form smaller, secondary systems within the larger sill-ring complexes.

Discussion -- Mechanism of emplacement

Different morphotectonic models and mechanisms of emplacement have been proposed for the Karoo dolerite ring structures (Figure 8).

Du Toit (1905) and Rogers and Schwarz (1902) suggested an undulating sill forming a series of domes and basins for the Eastern Cape intrusions [Figure 8(A)]. Scholtz (1936) adopted a similar morphotectonic model for the Insizwa sheet. Meyboom and Wallace (1978) attempted to explain the emplacement mechanism for such a model by way of variations in a 'compensation surface' at relatively shallow depths in the crust. The compensation surface, theorized by Bradley (1965), is a surface where magma pressure would be equal to the lithostatic pressure. This 2-D model of an undulating sheet has severe geological and mechanical restrictions:

• the geology is over simplified and does not fit the 3-D geometry of the structure (saucer-like);
• 3-D domes do not exist, at least in the western Karoo, and what can be seen or interpreted from a 2-D crosssection is an 'arch effect' between two rings (with the top always eroded away);
• the sill and ring structures of the Karoo Basin are so interconnected and anastomosed that a multi-layered system of compensation surfaces would have developed, which is physically impossible;
• from the mechanical point of view there is no reason why a sill would propagate from a high-energy level (i.e. at the top of an undulation) to one of a lower level (i.e. at the bottom of an undulation); and
• some of the undulating sills described in the literature occur at the base of the Karoo sequence, that is, at the contact between the basement-Dwyka Group or between the Dwyka-Ecca Group, and all show an extremely low amplitude of undulation not easily discernible at a local scale (Du Toit, 1905; McLaren and Visser, 1978; unpubl. pers. obsns; see also 1:250 000 geological maps of South Africa).

Du Toit (1920) proposed more realistic geological cross-sections of these structures, showing flat-lying sills connected together by an inclined sheet of lesser thickness. He proposed a mechanism of intrusion by which lateral thrusts would operate along the inclined sheet as a result of uplift of the upper flat sill. Lombaard (1952) refers to these features as transgressive intrusions [Figure 8(B)] and proposed a cone-sheet model of emplacement because of their 3-D funnel-shaped geometry. This morphotectonic and mechanical model is more refined than the earlier model, but it still suffers from four major drawbacks, namely:

1. the inclined sheets are, in fact, very short and not comparable to cone sheets proposed by the authors or cone sheets developed on other volcanoes;
2. the emplacement of cone sheets requires a substantial source (magma chamber) with a strong capacity for uplift (updoming, stretching, and caldera formation);
3. the development of radial fracturing, which are the first fractures to develop during up-doming (absent in the Karoo); and
4. cone sheets rarely develop as a result of basaltic magmatism and are mainly associated with acidic volcanism (essentially trachytes).

Burger et al. (1981) and Vivier et al. (1995) adopted the laccolith emplacement model of Pollard and Johnson (1973) for similar Karoo dolerite intrusive complexes found in the Free State. This model assumes that the sill is thick enough in its centre to cause upwarping of the overlying sedimentary layers and create peripheral fracturing and dyking [Figure 8(C)]. Johnson and Pollard (1973) described three types of peripheral terminations in the diorite sill-laccoliths complexes of the Henry Mountains (Utah), that is, the blunt termination, vertical faulting, or peripheral dyking (upward inclined intrusion). The latter form of termination may be relevant to the Karoo dolerites. This phenomenon is, however, rare and will only develop where the thickness-half length ratio at the centre of the laccolith is great enough to provide the laccolith with sufficient leverage to lift the overlying sediments -- a 'bulge' shape and high viscosity only found in acidic magma. In fact, even under such favourable conditions, the peripheral dyke thins rapidly during upward propagation and the local tensional stress system does not generate conditions for further substantial intrusion (as it is the case for the Henry Mountains intrusives). Most of the Karoo dolerite sill-ring complexes differ substantially from this model:

• the sills do not show noticeable high thickness-half length ratios;
• the dolerite magma was not viscous;
• the ring dykes are often very thick, if not thicker than the sill itself; and
• a saucer-shape laccolith would require a central plug-like feeding intrusive, which has never been documented in the Karoo nor elsewhere.

Without rejecting this last model in its entirety and accepting that it could explain some of the dolerite structures, the authors propose a mechanism of emplacement where the ring dykes play a dominant role, which is more in accordance with our geological observations in the western Karoo [Figure 8(D)]. It has, however, not as yet been subjected to rigorous mechanical analysis and the following stages of development are empirical (Figure 9).

1. The inclined sheets form the 'back-bone' of the ring structure and are fed by regional vertical dykes which adopt a double curvature (along strike and in vertical section), leading to a 'trumpet'-shaped intrusion. The jagged outline of the margins of the ring structures are controlled by feeder dykes of various orientations, although north-northwest-orientated dykes are the most important.
2. The inclined sheet passes upwards into a flat-lying sill, uplifting the overlying sedimentary layers, and propagating laterally outwards to form the outer sill.
3. Uplift of the overlying sediments creates a 'drag' upon the upper contact or 'back' of the inclined sheet, as already suggested by Du Toit (1920). These forces of uplift and resistance create an upwarping of the sediment and formation of an open fracture adjacent to the sheet, but at a lower level.
4. Magma then intrudes into this opening and spreads inward at a lower elevation forming the inner sill.

An important question, however, remains -- why would a linear and vertical feeder dyke gently curve both along its strike and in depth to form a trumpet-like shape'? The north northwest-trending dolerite dykes in the western Karoo, as mentioned earlier, could be the extension of the arm of a triple junction located off the eastern coast, and could have propagated laterally along strike from that triple junction (Figure 1). They also feed magma into the fissures of the east-west right-lateral shear zone. The curvature along strike could result from the interaction between these two structures, that is, the laterally fed north-northwest-trending dykes and dextral east-west shear zone. The curvature of the dykes with depth may be explained if the dykes are strata-bound, that is, if sills within a specific stratigraphic level are fed by dykes propagating within a similar level. For example, the sills intruded into the contact of the Ecca and Beaufort Group sediments, which includes the Williston complex, were fed by dykes propagating within the Ecca Group. Similarly, the sills intruded at the contact of the Carnarvon-Abrahamskraal Formations, which includes the Victoria West complex, were fed by dykes propagating within the Carnarvon sediments.

The authors' mechanical interpretation does not explain all the structural observations made in the western Karoo. There are rings or portions of ring structures that do not seem to be linked to any feeder-dyke system, although their shape is often controlled by the regional dyke pattern. They are quite aware that other intrusive mechanisms (Burger et al., 1981) could be involved to explained variations in the dyke and sill habits of the western Karoo.

Aknowledgements

The authors would like to express their gratitude to the Council for Geoscience, Department of Water Affairs and Forestry, and the Water Research Commission for their support.

S: Figure 1 Dolerite dykes of the main Karoo Basin. Inset a: simplied structural map showing the relation between the east-west right-lateral shear zone and the north-northwest-trending dykes and the position of a postulate triple junction off the east coast. Inset b: geodynamic interpretation of the western Karoo dolerite structural set-up.

S: Figure 2 Dolerite sills of the western Karoo compiled from published 1:250 000 geological maps (Geological Survey of South Africa. A) Area stretching from Calvinia to Queenstown. Note the mega basin unit (200 km across) outlined by the sills in the South. B) The study area and the 'basin-within-a-basin structure'. For description of Williston, Fraserburg, and Victoria West sills see text.

Figure 3 Landsat image of Williston dolerite sill and ring.

Figure 4 Williston sill and ring map and cross-sections.

Figure 5 Fraserburg ring map and section. The saucer-like sill morphology is preserved in the east and eroded in the west where only the roots of the structure (the ring dykes) outcrop.

Figure 6 Victoria West sill and ring. Note the 'ring-within-ring' structure.

S: Figure 7 Morpho-tectonic model of Karoo dolerite sill and ring systems. A) Schematic map, B) 3-D model.

S: Figure 8 The different modes of emplacement of the sills and rings. A) Undulating sill of Du Toit (1905), B) the transgressive intrusion of Lombaard (1952), C) the laccolith of Burger et al. (1981), D) the ring dyke feeder (this study).

S: Figure 9 The different steps of emplacement: A) curvature along strike and dip of a regional dyke adopting a 'trumpet-like shape', B) flattening and thickening of inclined sheet and propagation of outer sill, C) gap opening at the base of the inclined sheet as a result of sediment up-drag, D) propagation of inner sill.

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Editorial handling: S. McCourt.

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By L. Chevallier, Council for Geoscience, P.O. Box 572, Bellville, 7535 Republic of South Africa; E-mail: luc@geobell.org.za and A. Woodford, Directorate of Geohydrology, Department of Water Affairs and Forestry, Private Bag X 16, Salamhof, 7532 Republic of South Africa; E-mail: awoody@iafrica.com; Present Address: Toens and Partners Consulting, 9 Lester road, Wynberg, 7800 Republic of South Africa.