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 .