Site of 1972 and 1988 drilling projects (courtesy National Cultural History Museum)

Drilling The Crater

Several shallow boreholes and wells had been sunk for industrial and testing purposes into the crater floor prior to 1972. In that year, the Pratley Mineral Exploration Company (Pty) Ltd sunk a borehole to a depth of 172 metres to explore the clay content of the crater floor. A rotary drill was used to sink this borehole near the centre of the crater floor. Core recovery was poor (only chips were recovered) and did not allow any firm conclusions regarding the origin of the crater.

In 1988/89 a second core was drilled under the supervision of Professor Tim Partridge of the University of the Witwatersrand. This time a combination of drilling techniques was used and the borehole was sunk, also near the centre of the crater, to a depth of 200 metres through the sediments accumulated in the crater since the time of its formation and into the subcrater basement, resulting in nearly complete recovery of a continuous drill core.

The reason for the drilling project was twofold:

• Professor Partridge, whose main interest was unravelling past climates, believed that a drill core through the whole sequence of crater sediments would allow much information to be obtained about the climatic conditions since that time. Understanding such change is imperative when trying to distinguish natural change from man-induced change, and for prediction of possible future climatic conditions and changes. Only very few sites, where complete and undisturbed sections are available for scientific study, are located in the Southern Hemisphere.

• It was hoped that mineralogical analysis of the drill core would provide irrefutable evidence of the cause of the cratering event.

Throughout the upper 90 metres of the drill core, predominantly dine-grained lacustrine sediments were penetrated. Below this zone lies some 60 metres of sandy material, which also contains a few granites boulders. Below 150 metres only fractured granite bedrock was encountered. At 200 metres below the lake surface, solid bedrock composed of Nebo Granite was encountered and drilling was finally abandoned.