The 3D geometry of regional-scale dolerite saucer complexes and their feeders in the Secunda Complex, Karoo Basin André Coetzee a, ⁎, Alexander Kisters b a Geology Department, Sasol Mining (Pty) Ltd, Private Bag X1031, Secunda 2302, South Africa b Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Stellenbosch 7602, South Africa abstract article info Article history: Received 14 December 2015 Received in revised form 1 April 2016 Accepted 2 April 2016 Available online 6 April 2016 Dolerites in the Karoo Basin of South Africa commonly represent kilometre-scale, interconnected saucer-shaped structures that consist of inner sills, bounded by inclined sheets connected to stratigraphically higher outer sills. Based on information from over 3000 boreholes and mining operations extending over an area of ca. 500 km 2 and covering a N 3 km vertical section from Karoo strata into underlying basement rocks, this paper presents the re- sults of a 3D modelling exercise that describes the geometry and spatial relationships of a regional-scale saucer complex, locally referred to as the number 8 sill, from the Secunda (coal mine) Complex in the northern parts of the Karoo Basin. The composite number 8 sill complex consists of three main dolerite saucers (dolerites A to C). These dolerite saucers are hosted by the Karoo Supergroup and the connectivity and geometry of the saucers support a lateral, sill-feeding-sill relationship between dolerite saucers A, B and C. The saucers are underlain and fed by a shallowly-dipping sheet (dolerite D) in the basement rocks below the Karoo sequence. The 3D geometric strata model agrees well with experimental results of saucer formation from underlying feeders in sedimentary basins, but demonstrates a more intricate relationship where a single feeder can give rise to several split level saucers in one regionally extensive saucer complex. More localised dome- or ridge-shape protrusions are common in the flat lying sill parts of the regional-scale sau- cers. We suggest a mode of emplacement for these kilometre-scale dome- and ridge structures having formed as a result of lobate magma flow processes. Magma lobes, propagating in different directions ahead of the main magma sheet, undergo successive episodes of lobe arrest and inflation. The inflation of lobes initiates failure of the overlying strata and the formation of curved faults. Magma exploiting these faults transgresses the stratigra- phy and coalesces to form a ring-like inclined sheet that subsequently feeds a central flat lying roof at a higher stratigraphic level. On a regional scale, the kilometre-size saucer geometries reflect the lateral migration and transport of mafic magmas close to or at the level of the Karoo Supergroup, fed by only isolated feeders in the basement. On a more local scale, the complex internal geometries within saucers mainly reflect the flow pattern of the magmas and wall-rock accommodation structures. © 2016 Elsevier B.V. All rights reserved. Keywords: Saucer-shaped sills Secunda Complex Karoo Basin Sill-feeding-sill Dolerite 1. Introduction Karoo dolerites typically form laterally extensive, composite sill complexes that intruded across Southern Africa prior to the break-up of Gondwana c.180–183 Ma (Duncan and Marsh, 2006; Galerne et al., 2008). In most cases, these dolerite intrusions form networks of inter- connected saucer- or basin-shaped sheets that commonly consist of an inner sill, a steeply inclined sheet and a flat outer sill (Fig. 1) (Chevallier and Woodford, 1999; Galerne et al., 2008). The emplace- ment mechanisms and the controls on the emplacement level of the saucer-shaped intrusions are, however, still a contentious topic. Models for magma ascent and emplacement focus primarily on conditions that induce the dyke to sill transition in the upper crust. These models con- sider factors like abrupt changes in the stress field, discontinuities and rigidity contrasts (Pollard, 1973; Kavanagh et al., 2006), zones of equi- potential pressure (Bradley, 1965) and neutral buoyancy (Francis, 1982). Field observations from other sedimentary basins and analogue models propose sill formation to be primarily influenced by mechanical characteristics and anisotropies in the layered host-rock sequence (Chevallier and Woodford, 1999; Galland et al., 2009; Galerne et al., 2011; Muirhead et al., 2012). Rigidity contrasts between stiffer or com- petent units (high Young's modulus (E)) and softer units with a lower Young's modulus, in particular, provide preferential horizons for the deflection of feeder dykes into sills. This effect is magnified in the presence of well-developed bedding plane contacts that represent Journal of Volcanology and Geothermal Research 317 (2016) 66–79 ⁎ Corresponding author. E-mail addresses: acoetzee90@gmail.com, andre.coetzee2@sasol.com (A. Coetzee). http://dx.doi.org/10.1016/j.jvolgeores.2016.04.001 0377-0273/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores