Die morfotektoniese ontwikkeling van die vallei van die Hartsrivier en die sedimentpetrologie van die sandgrond in die gebied van sy bo-lope

Abstract

The present drainage patterns of the Harts and Molopo Rivers have developed in consequence of crustal movements associated with the initiation of the Griqualand - Transvaal Axis of uplift towards the end of the Tertiary Period, The development of this axis along a northeast-southwest Precambrian lineament, which now forms the divide between the Harts River and the Skoonspruit drainage and Vaal River, caused disruption of the pale □ - Nossob, the pale - Dry Harts, and the pale - Harts Rivers. The disruption of these rivers, which were southward-draining tributaries of the Vaal River during the Tertiary, culminated in the evolution of the present physiographic features of the area lying to the north-west of the Vaal River between Klerksdorp and Barkly West, Tectonic movements also resulted in a significant reduction of the volume of water draining into the Vaal River from the north, This fact, coupled with the diastrophic steepening of the slope of the pre-Karroo surface on which the Vaal River was shifting towards the south-east, contributed to the formation of the "older" high- level gravel terraces on the right bank of the Vaal River. A complementary effect of the south-eastward tilt of the land on the southern side of the Griqualand - Transvaal Axis was the reduction of the gradients on the pediplaned surface which lay to the south-east of the Vaal River. This gave rise to the formation of numerous pans which now constitute the Pan Veld of the north-western Orange Free State, A sedimentary petrological investigation of the sandy soil in the catchment area of the upper Harts River, which is cultivated mainly for the production of maize in the districts of Lichtenburg and Delareyville, shows that it is a polygenetic type of soil, Both its grain-size characteristics and heavymineral composition point to the fact that it is a hybrid soil composed of different proportions of locally derived pedological material and a component of aeolian sand with a north-westerly provenance, As the aeolian sand is completely integrated with saprolite of the Tertiary erosion surface and predates the disruption of the Pliocene drainage, it is considered to be possibly middle to late Tertiary in age.