Abstract:
The volcano-sedimentary succession of the Sodium Group, in the south-western corner of the Kaapvaal
craton, rests unconformably on basement granites and is usually correlated with the middle Ventersdorp
Supergroup. This Neoarchean (∼2700 Ma) large igneous province (LIP) has been interpreted as deposited
in a tectonic escape basin scenario, following the collision of the Zimbabwe and Kaapvaal cratons, or
alternatively, in a mantle plume induced, cratonic rift basin. The basaltic-komatiitic to andesitic-dacitic
volcanic sequence and the accommodated sedimentary deposits developed in a graben to half-graben
setting within a cratonic, failed-rift system.
The Sodium Group sedimentary facies and geochemistry of volcanic rocks correlate well with the
Ventersdorp Supergroup. The major and trace element and REE geochemistry correspond with previously
published results (Crow and Condie, 1988; Grobler et al., 1989); and the Sr and Nd isotopy confirms crustal
contamination of the Sodium Group magmatic rocks. A preliminary weighted mean 207Pb/206Pb zircon
age of 2739 ± 39 Ma for a tuff in the upper Ongers River Formation is reported for the lower part of the
Sodium Group.
The Ongers River, T’Kuip and Omdraaivlei Formations reflect sedimentation and volcanism in a rift
induced graben setting with fast sedimentation from horst flanks and high burial rates in the graben
valleys, and subsequently, a less pronounced topography, post-graben environment. Accompanying
explosive phreatomagmatic and effusive, mafic to acidic, bimodal volcanism, with abundant pyroclastics,
delivered ample material for graben fill deposits. Repeated horst reactivation has episodically exposed
the basement to renewed erosion. Sedimentation along graben flanks comprised alluvial fans, mass flow
deposits, braided river and related modes of deposition. In distal, shallow lakes, volcaniclastic suspension
and current sedimentation, low-density turbidites and stromatolitic reefs were accumulated.
