Geology > Karoo Supergoup > Dolerite Intrusions

Dolerite Intrusions

Summarised from Hydrogeology of the Main Karoo Basin: Current Knowledge and Future Research Needs WRC Report Project K860 (2001).

Introduction

The Karoo dolerite, which includes a wide range of petrological facies, consists of an interconnected network of dykes and sills and it is nearly impossible to single out any particular intrusive or tectonic event. It would, however, appear that a very large number of fractures were intruded simultaneously by magma and that the dolerite intrusive network acted as a shallow stockwork-like reservoir.

Dolerite dykes, like many other magmatic intrusions, develop by rapid hydraulic fracturing via the propagation of a fluid-filled open fissure, resulting in a massive magmatic intrusion with a neat and transgressive contact with the country rock. This fracturing mechanism is in contrast to the slow mode of hydraulic fracturing responsible for breccia-intrusions (i.e. kimberlite). For the intrusion to develop the magma pressure at the tip of the fissure must overcome the tensile strength of the surrounding rock. Dykes can develop vertically upwards or laterally along-strike over very long distances, as long as the magma pressure at the tip of the fissure is maintained. The intrusion of dolerite and basaltic dykes are therefore never accompanied by brecciation, deformation or shearing of the host-rock, at least during their propagation.

The average thickness of Karoo dolerite dykes ranges between 2 and 10m. The country rock is often fractured during and after dyke emplacement. These fractures form a set of master joints parallel to its strike over a distance that does not vary greatly with the thickness of the dyke (between 5 and 15m).

One of the most prominent features of the present Karoo landscape is the large number of dolerite sills and ring-complexes. These structures often display a sub-circular saucer-like shape, the rims of which are commonly exposed as topographic highs and form ring-like outcrops. The Karoo dolerite sills and ring-complexes have the same geographical distribution as the dolerite dykes and they are by far the most common type of intrusion in the Karoo basin. The dolerite sills and dykes form a complex intrusive network that probably acted as a shallow magma storage system. The lithology of the country-rock strongly controlled the emplacement of the sills.

dyke_sill ring

Geohydrology

Dolerite dykes are vertical to sub-vertical discontinuities that, in general, represent thin, linear zones of relatively higher permeability which act as conduits for groundwater flow within the aquifer. They may also act as semi- to impermeable barriers to the movement of groundwater. The dykes are commonly expressed on the surface as a line of green bushes, which can be readily observed during the dry season. Dolerite dykes have always been and still are the preferred drilling target for groundwater in the Karoo. Both geohydrologists and groundwater-dowsers have sited many successful boreholes on these structures. There are a number of reasons why these features are preferred for groundwater exploration, namely:

•	there exists an apparent higher probability of drilling a wet borehole in or next to a dyke than in the host rock away from the dyke,

•	they are easily detected on remotely-sensed imagery, by relatively simple geophysical techniques (the magnetometer) and are clearly visible to both the skilled and unskilled eye in the field (i.e. if not outcropping, they are often conspicuous as lines of vegetation or animal-burrows).

•	their relatively simple and regular 3D geometry makes it easy to conceptualize and site an exploration borehole in the field, and

•	they are thus a very cost effective groundwater target.

The dolerite ring-complexes and associated sills have largely been over-looked by most geohydrologists and drillers, because of their size, thickness, rock hardness and structural complexity. Relatively few boreholes have been drilled by landowners on these structures, mainly because they commonly use water-dowsers to site their boreholes and they tend to site on the multitude of easier linear targets (dykes, fissures etc.).

Exploration drilling has shown that water-bearing open fractures develop at specific locations within the dolerite and surrounding host rock, i.e. at the junction between a feeder dyke / inclined sheet and a sill, in the sediment above an up-stepping sill or at the base of an inner-sill. In the first case fracturing is very localised, whilst in the second and third cases shear and ‘open’ fractures can extend some distance away from the dolerite contact into the country rock.