EMPLACEMENT OF SAUCER-SHAPED SILL INTRUSIONS CONSTRAINED BY DETAILED FIELDWORK AND AMS ANALYSES OF THE GOLDEN VALLEY SILL COMPLEX, SOUTH AFRICA

Seismic reflection data show that saucer-shaped sill complexes represent a fundamental geometry in undeformed sedimentary basins. Exposed saucer-shaped sill complexes are common in the Karoo basin of South Africa. The remarkably well-exposed Golden Valley Sill forms a 10x20 km saucer-shaped basin that displays a horizontal inner sill continuing outward into steeply (30º) inclined sheets cross-cutting horizontally layered sedimentary strata and ending outwards into sub-horizontal outer sills. It is part of an interconnected 3D network of saucer-shaped sills, the Golden Valley Sill Complex, where dykes are of minor importance. Field investigations revealed that connected saucers are separated by chilled margins or a thin (<1m) sedimentary layer. Several types of macroscopic magma flow indicators have been observed, including ropy flow structures and elongated tubes up to 50 m wide. Fingers with diameters of 5-6 meters are found parallel to the tubes. These features are possibly related to thermal contraction of the crystallizing sill.

A total of 115 localities (6 specimens/site) have been sampled for anisotropy of magnetic susceptibility (AMS) analyses. The magnetic properties were defined by measuring hysteresis cycles and k-T curves on 40 specimens. The presence of large pseudo-single domain magnetite grains allows the correlation of the magnetic fabric with the petrofabric. The bulk susceptibility varies slightly with a value averaging 13.5 10-3. The magnetic lineations and foliations are well defined and behave consistently across the Golden Valley Sill. The magnetic lineations are generally parallel to the dip-direction of the inclined sheet and correlate well within 10 degrees of the magma flow direction as determined from the tubes and finger structures. The orientation of the magnetic foliation is almost parallel to the orientation of the sill/host rock contact further demonstrating that the magnetic fabrics are ‘normal' and can directly be used as a magma flow indicator.

In conclusion, AMS data and the observed magma flow geometries, derived from macroscopic flow indicators, correlate well and are used to constrain an emplacement model for the Golden Valley Sill Complex. This study further demonstrates that the Golden Valley could be regarded as a reference site for saucer-shaped sill complexes.