Pebble Morphology of an Ancient Conglomerate: The Middelvlei Gold Placer, Witwatersrand, South Africa

Abstract

ABSTRACT A highly weathered outcrop afforded an opportunity to obtain pebbles for a morphometric clast study of a normally well indurated Early Proterozoic conglomerate, the Middelvlei gold placer. This quartz-pebble conglomerate occurs towards the base of the Central Rand Group of the Witwatersrand Supergroup, South Africa, and is a principal gold orebody mined in a goldfield known as the West Wits Line. The placer has been regarded as fluvial mainly because of a unimodal paleocurrent distribution, repeated fining-upward conglomerate-sandstone sequences, and the lenticular nature of its beds. More than 1,000 quartz pebbles from five separate conglomerate beds were measured. Pebble form was defined by maximum projection sphericity, coefficient of flatness and oblate-prolate index, all calculated from long, intermediate, and short pebble axes. A normal distribution was obtained for the first two form indices. For all five conglomerate beds, the means of all three indices are significantly greater than empirical lower limits determined by previous workers to distinguish fluvial from beach pebbles in modern gravels, confirming the fluvial origin of the beds. Although the range of pebble sizes in this study is relatively small, a significant decrease in sphericity with increasing size was found. Pebble roundness was determined for 444 undamaged pebbles by visual comparison with chart images of Krumbein (1941). Comparison of the overall mean of 0.59 with figures obtained in studies of modern rivers suggests relatively long transport distances, of the order of 160 km, for the placer.