Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa

A characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found. The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup. In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation. Palaeocurrents of the Middelvlei placer are towards the south-southeast in the western parts of the study area and towards the south-east in the east. Maximum pebble size, however, decreases towards the west-southwest, that is in a direction roughly perpendicular to the palaeocurrents. A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike. The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward-thinning wedge of underlying strata. The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.