Pongola Supergroup

irekhodi lokuqala ngempilo yasendulo

Ngemuva kokwakheka, iKaapvaal Craton yaphakamiseka yavela emoyeni. Lokhu kwaholela ekuqhephukeni, ukuguguleka nokuhambiswa kwezicucu zaya zagcineka kobheseni abangajulanga. Kokubili iPongola Supergroup kanye nokufana nayo kodwa okugcwele igolide, amatshe ase-Witwatersrand agcineka kulabo bheseni bokuqala.

Ingxenye engaphansi yePongola Supergroup (Nsuze Group) yakhiwe ngokulandelana kwama-basalt, ama-sandstone kanye nama-limestone omncanyana. I-limestone equkethe ama-stromatolite agcineke kahle, kungama-fossil eminyaka eyizigidi eziyinkulungwane ezimbili namakhulu ayisishiyagalolunye obudala e-algae akwazi ukuphila olwandle olufudumele.

Ngaphezu kweNsuze kunohla lwamatshe akhiwe izicucu abizwa ngokuthi i-Mozaan Group, wona anothe ngegolide. Ukumbiwa okudala kwegolide kungabonakala ezindaweni zase-Danny Dalton, phakathi kwe-Vryheid ne-Melmoth. Imifantu yegolide iyambiwa futhi eduzane nomngcele waseSwazini.

Enyakatho neKZN naseSwazini, iPongola Supergroup yantshuntshwa yi-granite. Ngesikhathi lokhu kuphola, kwaguqula amatshe ayizungezile.

Some limestone units in the Nsuzi Group contain beautifully preserved stromatolites, fossils of 2900 million year old algal colonies which thrived in a shallow, warm ocean.

The photosynthesing bacteria (cyanobacteria) consumed carbon dioxide and caused the precipitation of calcium carbonate from the sea water, which stuck to the bacterial colonies. Gradually, layer upon layer of calcium carbonate was added, forming stromatolites. The shape (cones, domes ect.) assumed by stromatolites depends on water depth, tidal range, and wave and current activity.

Stromatolites
Stromatolites, White Umfolozi River (Photos: N. Hicks)
Stromatolite formation
This diagram shows the various forms that Stromatolites can assume.

Picture adapted from: The story of Earth & Life (T. McCarthy & B. Rubidge)

Banded iron formation
Banded iron formation, White Umfolozi river
Ropy lava, White Umfolozi river
Mudcracks, White Umfolozi river

Banded Iron Formation (sedimentary)

evidence of the ‘Great oxidation event’

Banded iron formations are distinctive units of sedimentary rock consisting of alternating layers of silver to black iron oxides in the form magnetite or hematite  and iron-poor chert, often red in colour.  The rock beds can be 50-600 m thick. Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria. The oxygen combined with the iron and sank to ocean floor as thin layers of insoluble iron oxide. The banding is thought to mark seasonal peaks in oxygen production.