3D relationships between sills and their feeders: evidence from the Golden Valley Sill Complex (Karoo Basin) and experimental modelling

In this paper, we address sill emplacement mechanisms through the three-dimensional relationships between sills and their potential feeders (dykes or sills) in the well-exposed Golden Valley Sill Complex (GVSC), Karoo Basin, South Africa. New field observations combined with existing chemical analyses show that: 1) the contacts between sills in the GVSC are not sill-feeding-sill relationships, and 2) there are, however, close structural and geochemical relationships between one elliptical sill, the Golden Valley Sill (GVS), and a small dyke (d4). Such relationships suggest that GVS is fed by d4 and that the linear shape of d4 may have controlled the elliptical development of the GVS.To test this hypothesis, we present preliminary results of experimental modelling of sill emplacement, in which we vary the shape of the feeder. In the first experiment (E1) with a punctual feeder the sill develops a sub-circular geometry, whereas in the second experiment (E2) with a long linear feeder the sill develops an elliptical geometry. The geometrical relationships in E2 show that the elliptical shape of the sill is controlled by the linear shape and the length of the linear feeder. The experiment E2 presents strong similarities with the GVS–d4 relationships and thus supports the proposition that d4 is the feeder of the GVS. Our experimental results also indicate that the feeders of the other elliptical sills of the GVSC may be dykes.

Research Highlights
► Multidisciplinary study of the 3D relationships between sills and their feeders.
► Geology, geochemistry and analogue modelling of the Golden Valley Sill Complex.
► Emplacement of elliptical saucer-shaped sills controlled by their feeder dyke.