- © 2006 Geological Society of South Africa
The ~2.67 to ~2.46 Ga lower Transvaal Supergroup, South Africa, consists of a mixed siliciclastic-carbonate ramp that grades upward into an extensive carbonate platform, overlain by deep subtidal banded iron-formation. It is composed of 14 third-order sequences that develop from a mixed siliciclastic-carbonate ramp to a steepened margin followed by a rimmed margin that separated lagoonal environments from the open ocean. Drowning of the platform coincided with deposition of banded iron-formation across the Kaapvaal Craton. The geometry and stacking of these sequences are consistent with more recent patterns of carbonate accumulation, demonstrating that Neoarchean carbonate accumulation responded to subsidence, sea level change, and carbonate production similarly to Proterozoic and Phanerozoic platforms. The similarity of carbonate platform geometry through time, even with significant changes in dominant biota, demonstrates that rimmed margins are localized primarily by physiochemical conditions rather than growth dynamics of specific organisms.
Stratigraphic patterns during deposition of the Schmidtsdrift and Campbellrand-Malmani subgroups are most consistent with variable thinning of the Kaapvaal Craton during extrusion of the ~2.7 Ga Ventersdorp lavas. Although depositional patterns are consistent with rifting of the western margin of the Kaapvaal Craton during this time, a rift-to-drift transition is not required to explain subsidence. Heating and thinning during Ventersdorp time can produce the observed thermal subsidence from ~2.7 to ~2.45 Ga if the thermal diffusivity of the craton was moderately low.