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Page 2/3 Results At an early stage of the survey it already became clear that the sand and gravel filled depression in the south western part of the farm, which had been found by the test boreholes (AAB56 and AB6 drilled to depths of 32 and 30 metres respectively), consistently gave lower than average magnetic field readings. The readings were even lower than those encountered on in situ Dwyka Tillite which is exposed at a depth of only 4 metres below the surrounding fields in the bottom of a stream or donga at position N5E2, borehole B4 situated near the stream found Dwyka at a depth of 5 metres. The stream enters Muirton from the west, and runs across the farm to join the Vaal River east of N5E7. A single line survey was conducted upstream along this streambed together with Mr Gadney Fisher and Mr Coen Visser (a western extension between grid lines N5 and N6). Magnetic measurements were taken at a total of 39 stations; the values showing very little variation along this line, ranging from 33 to 59 nanoteslas. Exposures of Black Reef Quartzite of the Transvaal Supergroup were found approximately 400 metres west of the Muirton fence at the end of this traverse line. Taking the shallow bedrock exposures found in the vicinity of traverse line N5 into account, the presence of a depression which reaches depths in excess of 30 metres only 300 metres to the south is remarkable, and bears testimony to an amazing amount of riverbed erosion into Dwyka Tillite (or some other softmaterial) by an ancient bend of the Vaal River where its progress was restricted by an upwarped section of Ventersdorp Lava which now forms an outcrop on the hillside to the southwest, and forms the steep sided ravine through which the Vaal River flows on the farm Mosesberg. The low magnetic signature is probably caused by one of the following:
Whereas shallower gravels will produce clearly increased magnetic values especially if they contain banded ironstone pebbles, thickly sand-covered gravels in such an overdeepened trench as we have here give the opposite effect. The presence of power lines in the area of utmost interest is extremely unfortunate! The eastern half of the farm, by contrast, gave a most interesting pattern of increased magnetic field readings which were found on all the traverse lines. These increases are demonstrated by the magnetic contour lines which have been drawn according to the values that were measured along the lines and entered on the grid. Of particular and vital interest is the elongate pattern of magnetic values in excess of the 200 nt contour (i.e. >28100 nanoteslas). This zone of consistently high magnetic readings extending in a northeast-southwest direction merits some attention. They could be an indication of a relatively shallow gravel-filled channel running more or less parallel to the Vaal River, or they could be an indication of an as yet undiscovered Kimberlite Blow or Pipe with a fissure extension both to the northeast and to the southwest. The pattern of boreholes drilled on a 200 metre grid by Mr Gadney Fisher seems to have overstepped this high magnetic anomaly. Boreholes D6 and D7 came the closest to this zone, and registered Clay overlying Shale bedrock at a depth of 5 metres. It is also of interest to note that during 1995 a Mr. W. Visser excavated gravel and recovered a good quantity of diamonds in the area where borehole position D3 is situated (G. Fisher, pers. Comm.). |
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