Following the recent excellent days in the Barberton Greenstone Belt, our team continued discussing early Earth dynamics when they visited the Pongola Supergroup. The rocks of Pongola form some of the earliest volcano-sedimentary units that formed atop some kind of stable continental platform that was likely undergoing a certain degree of extensional tectonic movement.
The existence of ultramafic greenstone fragments within the c. 3105 Ma Pre-Pongola basement granitoids and the relationship with the overlying Pongola rocks makes this another important location to study the dynamics of the Archean.
c. 3105 Ma Pre-Pongola basement rocks (with ant for scale)
The day began with the team passing the Commondale lavas (though I missed the turn); and then investigating the Pre-Pongola basement rocks. These granitoids form some of the earliest stable continental crust. Luckily, by now the team are really proficient at describing and characterising plutonic rocks and very quickly were able to classify these as "granitoids".
The weather changed rapidly resulting in us being constantly rained upon, however that didn't stop us from looking at the Pongola Nsuze and Mozaan Groups near the White Mfolozi River valler. We started with ultramafic and felsic lavas in the Nsuze and then quartzites, banded iron formation, and then gold-bearing conglomerates of the Mozaan.
These rocks display an exceptionally well-preserved range of magma compositions and chemically varied sedimentary rocks. The occurrence of stromatolites and banded iron formation speaks of the changes in the atmospheric conditions at this time too.
TL: Ultramafic lavas of the Nsuze Group; TR: Felsic pyroclastic lavas, with bombs of the Nsuze Group; BL: Banded iron formation of the Mozaan Group; BR: Denny Dalton conglomerates within the Mozaan Group.
We also had some time to look at the very classical Vryheid delta sequence. This forms part of the lower Karoo (Ecca Group) and displays excellent sedimentary features related to its deposition within a barrier-delta system. Fine coal seams related the extensive coal mining occurring in this region.
L: Deltaic sequence of the coal-bearing Vryheid Formation; R: Coal seam within the Vryheid Formation
How do I get down?!
Vryheid Formation, with outgoing full and returning empty coal trains, looking south!
2700 Ma unconformity - Dwyka tillite and underlying striated Mozaan Quartzite
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