A sedimentological study of the conglomerates in the Elsburg stage on the Welkom and Western holdings gold mines

[摘要] A sedimentological description of the layers of conglomeratein the Elsburg Stage of the Witwatersrand System, as disclosedby 23 boreholes and underground development on the Welkom andWestern Holdings Gold Mines in the Orange Free State Goldfield,is given.The palaeocurrents in the Elsburg Stage as inferred frompebble-size distribution and preferred pebble orientation, werefrom two different directions. On Western Holdings Limited andthe Welkom Mine area west of the Arrarat Fault, the layers ofconglomerate were derived from the north-west. On the WelkomMine area east of the Arrarat Fault, the direction of transport wasfrom the south-west.These two different palaeocurrent directions can be explainedby lateral movement along the Arrarat Fault. Thus, the easternblock of the lease area of the Welkom Gold Mining Company, whichwas formerly situated on the north-eastern flank of a delta fan,was displaced along the strike of the Arrarat Fault for a distanceof approximately 6 000 m towards the south, while the westernblock (Welkom No.3 Shaft and Western Holdings Limited), wasdisplaced towards the north.A detailed analysis regarding the composition, roundness,shape, orientation and sorting of the pebbles in the layers ofconglomerate was undertaken. The sorting and roundness of thepebbles increase towards the centre of the basin, while themajority are orientated with their long axes parallel to thepalaeocurrent direction. The percentage of non-durabIe pebbletypes also decreases in a down-current direction, while thedurable types remain constant. The pebbles are mostlyspheroidal, although certain types show a strong tendency todisc shaped. These features indicate that the layers ofconglomerate were probably deposited on an alluvial fanbordering an inland lake or sea.

[发布机构] University of the Free State