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The stratigraphic relationship between the Waterberg and Soutpansberg Groups in Northern Province, South Africa: Evidence from the Blouberg area

Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby{at}postino.up.ac.za

The relative stratigraphic position of the mid-Proterozoic Waterberg and Soutpansberg Groups in Northern Province has long been a subject of debate. These two major Proterozoic groups are juxtaposed in the Blouberg area, although the nature of the contact between the two sedimentary units is generally regarded as being faulted along the southern strand of the Melinda Fault. The Blouberg area is also the type locality for yet another small mid-Proterozoic succession, that of the Blouberg Formation, which locally outcrops beneath both the Waterberg and Soutpansberg strata.

Existing maps of the Blouberg area show the Wyllie’s Poort Formation of the Soutpansberg Group to unconformably overlie rocks of the Blouberg Formation in the western foothills of Blouberg mountain. However, recent mapping suggests that these "Blouberg" strata themselves unconformably overlie the Blouberg Formation. This, therefore, raises important questions regarding the stratigraphic placement of these intermediate strata in the western foothills of Blouberg mountain, which outcrop unconformably above the Blouberg Formation and unconformably below the Wyllie’s Poort Formation. The intermediate strata are characterised by a thin basal conglomerate, consisting of cobbles of quartz, quartzite and B.I.F. This basal conglomerate grades vertically into trough cross-bedded sandstone and granulestone, which are characterised by heavy mineral concentrations on foresets.

Although this facies compares poorly with strata of the Mogalakwena Formation in adjacent areas, which are generally more conglomeratic, they are identical with strata from more distal outcrops of the Mogalakwena Formation further to the south-west Palaeocurrent directions recorded from the intermediate strata are unimodal towards the west-south-west, in common with those recorded from the Mogalakwena Formation. This suggests that the intermediate strata, unconformably overlain by the Wyllie’s Poort Formation of the Soutpansberg Group, can be correlated with strata of the Mogalakwena Formation. The Waterberg Group thus appears to pre-date the Soutpansberg rocks.

Additional evidence for the age relationships proposed here can be gained from an east-north-east-trending dyke swarm, which locally intrudes the basement, Blouberg and Waterberg rocks. This dyke swarm does not intrude Soutpansberg strata in adjacent areas. Patterns of spider diagrams of incompatible trace elements recorded from these dykes are very similar to those from the basaltic Sibasa Formation of the Soutpansberg Group. This suggests that the dyke swarm may have acted as feeders to the Sibasa lavas, and also suggests that the Soutpansberg Group post-dates the Waterberg Group.

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