Distribution of mafic sills in the Transvaal Supergroup, northeastern South Africa

Abstract

Systematic measured profiles on a regional scale are used to document the distribution of noritic and amphibolitic sills (787 in total) in the largely sedimentary latest Archaean–early Proterozoic succession of the Transvaal Supergroup in northeastern South Africa. An aggregate thickness of over 2.2 km of sills intruded this 12 km thick succession. The proportion of sill to sill plus sedimentary rock increases upward (average 16%), towards the overlying mafic phase of the Bushveld Complex. The average thickness of sills is 34 m, but increases in the same sense. The emplacement level of the Bushveld Complex transgresses across some 5 km of the stratigraphy of the upper formations of the Transvaal Supergroup. A node of maximum intensity of noritic sill intrusion is developed in these uppermost strata adjacent to the location where the transgressive relationship is most pronounced. The structural styles of the Transvaal Supergroup strata influence intensity of sill intrusion. Other factors being equal, maximum intrusion intensity is associated with areas of shallow homoclinal dip (<20°) and minimum intensity with areas of steeper dip and folding. A lithological control on sill emplacement is also documented, with quartzite- and shale-dominated formations being more heavily intruded than dolomite and basaltic lava. Supplementary material: A spreadsheet including all 440 formation traverses including grid references, formation thicknesses, and number and thicknesses of every sill therein is available at http://www.geolsoc.org.uk/SUP18807 .

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

“…Due to paucity of outcrop, we could not yet identify large potholes at Turfspruit, but cm-sized potholes are abundant in drill core and in Shaft #1 exposures (Online resource 2). Synmagmatic doming of the floor rocks to the Bushveld is equally to be expected, in view of the abundant Bushveld aged sills below the complex (Sharpe 1981;Button and Cawthorn 2015) that likely caused localised partial melting of the sedimentary rocks. Examples of such domes have been documented by Sharpe and Chadwick (1982), Uken and Watkeys (1997) and Scoon (2002).…”

Lateral Magma Flow in Mafic Sill‐complexes

“…The role of dikes in sedimentary basins may therefore be underestimated. However, it is important to note that the majority of sills within a sill complex likely have thicknesses below the limit of detectability in seismic data, implying that the volume of sills may be underestimated (Button and Cawthorn, 2015;Schofield et al, 2015).…”

“…1A; e.g., Curtis, 1968;Cartwright and Hansen, 2006;Gudmundsson, 2006;Aoki et al, 2013;Aizawa et al, 2014;Muirhead et al, 2014;Tibaldi, 2015) or giant radiating dike swarms, which can facilitate lateral magma flow over hundreds to thousands of kilometers (e.g., Ernst and Baragar, 1992;Ernst et al, 1995;Macdonald et al, 2010). Recent field-, seismic reflection-, and modeling-based research further indicate that plumbing systems at shallow levels (typically <3 km depth), particularly those hosted in sedimentary basins, may instead be characterized by extensive sill complexes (e.g., Chevallier and Woodford, 1999;Smallwood and Maresh, 2002;Thomson and Hutton, 2004;Planke et al, 2005;Cartwright and Hansen, 2006;Kavanagh et al, 2006Kavanagh et al, , 2015Lee et al, 2006;Leuthold et al, 2012;Leat, 2008;Menand, 2008;Polteau et al, 2008a;Cukur et al, 2010;Bunger and Cruden, 2011;Gudmundsson and Løtveit, 2012;Muirhead et al, 2012;Svensen et al, 2012Svensen et al, , 2015Jackson et al, 2013;Magee et al, 2014;Zhao et al, 2014;Button and Cawthorn, 2015;Schofield et al, 2015). Sill complexes imaged in seismic data and observed in the field extend laterally for tens to thousands of kilometers and are dominated by an interconnected network of mafic, relatively thin (typically <100 m thick) sills, which frequently exhibit saucer-shaped morphologies, and inclined sheets with only a small proportion of dikes (e.g., …”

“…2). We highlight that (1) sill complexes can facili tate lateral magma flow (e.g., Leat, 2008), with little or no accompanying surface deformation (Jackson et al, 2013;Magee et al, 2013a; , 2014); (2) the majority of sill complexes occur in sedimentary basins and their emplacement is primarily related to the intrusion of magma along mechanical discontinuities such as bedding planes and/or compliant lithologies (e.g., Mudge, 1968;Gretener, 1969;Einsele et al, 1980;Gudmundsson, 1986Gudmundsson, , 1990Gudmundsson, , 2011Annen et al, 2006;Kavanagh et al, 2006;Menand, 2008;Thomson and Schofield, 2008;Schofield et al, 2010Schofield et al, , 2012aMagee et al, 2013a;Barnett and Gudmundsson, 2014;Button and Cawthorn, 2015;Kavanagh et al, 2015;Tibaldi, 2015); (3) faults can limit the lateral extent of sill complexes and provide important magma flow pathways (Delaney et al, 1986;Bedard et al, 2012;Magee et al, 2013c); (4) magma rheology and chemistry appear to influence intrusion network structure (Cruden and McCaffrey, 2001;Bunger and Cruden, 2011); and (5) eruption sites may be laterally offset from the melt source and shallow-level magma reservoirs (Magee et al, 2013b). In contrast to dike-fed volcanoes, the distributions of which are commonly used to map zones of Magee et al | Lateral magma flow in mafic sill complexes GEOSPHERE | Volume 12 | Number 3 lower crustal and/or mantle melting or magma storage (e.g., Cashman and Sparks, 2013;Annen et al, 2015), the dispersal of eruption sites associated with a sill complex may not be a reliable indicator of magma generation zones.…”

Lateral magma flow in mafic sill complexes

“…1A; e.g., Curtis, 1968;Cartwright and Hansen, 2006;Gudmundsson, 2006;Aoki et al, 2013;Aizawa et al, 2014;Muirhead et al, 2014;Tibaldi, 2015) or giant radiating dike swarms, which can facilitate lateral magma flow over hundreds to thousands of kilometers (e.g., Ernst and Baragar, 1992;Ernst et al, 1995;Macdonald et al, 2010). Recent field-, seismic reflection-, and modeling-based research further indicate that plumbing systems at shallow levels (typically <3 km depth), particularly those hosted in sedimentary basins, may instead be characterized by extensive sill complexes (e.g., Chevallier and Woodford, 1999;Smallwood and Maresh, 2002;Thomson and Hutton, 2004;Planke et al, 2005;Cartwright and Hansen, 2006;Kavanagh et al, 2006Kavanagh et al, , 2015Lee et al, 2006;Leuthold et al, 2012;Leat, 2008;Menand, 2008;Polteau et al, 2008a;Cukur et al, 2010;Bunger and Cruden, 2011;Gudmundsson and Løtveit, 2012;Muirhead et al, 2012;Svensen et al, 2012Svensen et al, , 2015Jackson et al, 2013;Magee et al, 2014;Zhao et al, 2014;Button and Cawthorn, 2015;Schofield et al, 2015). Sill complexes imaged in seismic data and observed in the field extend laterally for tens to thousands of kilometers and are dominated by an interconnected network of mafic, relatively thin (typically <100 m thick) sills, which frequently exhibit saucer-shaped morphologies, and inclined sheets with only a small proportion of dikes (e.g.,…”