A paleomagnetic study of selected formations in the Waterberg Group, South Africa.

Abstract

A palaeomagnetic study of the Waterberg Group (Jones and McElhinny, 1967) did not find a consistent direction. However, a pattern was identified and interpreted in terms of apparent polar wander during the deposition and consolidation of the Waterberg sediments. These pole positions indicated that the Waterberg Group sedimentation commenced during emplacement of the Bushveld Complex, and intermittently continued through numerous tectonic events in the pre-existing Transvaal Basin to just before the Umkondo thermal event. The Swaershoek Formation as the basal unit of the Waterberg Group in the Nylstroom Protobasin has been tentatively correlated with the Wilge River Formation in the Middelburg Basin. A palaeomagnetic study has been conducted on the Swaershoek and Wilge River Formations in an attempt to redetermine the palaeomagnetic pole positions for these two Formations and to confirm the said geological correlation. A total of 49 sites across both basins were sampled, both within the sediments as well as in the associated diabase intrusions. Despite generally weak results, the calculated pole positions for the Swaershoek Formation (37.1° S, 335.9° E and A95:17.4°) and the Wilge River Formation (31.9° S, 332.7° E and A95: 20.5°) correlate very well, thereby confirming the association made between these two formations. The intrusive diabase in both basins was sampled to test for thermal overprinting of the calculated pole positions of the sediments. Although the calculated pole position for the diabase intrusion in the Nylstroom Protobasin (63.3° S, 233.2° E and A95: 14.2°). These two poles correlate very well with previous studies on post-Waterberg diabase (Jones and McElhinny, 1966) as well as on the Umkondo diabase (McElhinny and Opdyke, 1964). The pole positions from the Waterberg sediments and associated diabase are sufficiently displaced from each other to rule out any overprinting by these intrusions. Recent results from the Blouberg area (Bumby et al., 2001) indicate the Soutpansberg Group to be younger than the Waterberg Group. In an attempt to refine the Apparent Polar Wander Path (APWP) for the middle Proterozoic, another 6 sites from the Mogalakwena Formation (Waterberg Group) were sampled. Bumby et al. (2001) suggested that the Mogalakwena Formation pre-date the Wyllies Poort Formation (Soutpansberg Group). The calculated pole position for the Mogalakwena Formation was very weak, but correlated fairly well with a pole position (Group 2, McElhinny, 1968) from the Wyllies Poort Formation in the Soutpansberg Group. The location of the Mogalakwena Formation pole (36.1° S, 207.3° E and A95: 27.6°) on the APWP for Southern African confirms the Waterberg Group to be older than the Soutpansberg Group.