The Council for Geoscience Field School provides an ideal opportunity for participants to experience just some of the special geological sites around South Africa!
Overview of the Geological Tour around South Africa
Day 1
Here we go! We begin our journey at the head office of the Council for Geoscience in Silverton, Pretoria. Our start is located within the central region of the Transvaal Basin. These rocks were deposited ca. 2.7 – 2.1 Ga within an extensional basin located atop an Archean basement of granite-gneiss and Witwatersrand rocks. Five distinct and unconformably bounded sequences are recognised. These include various basal clastic and fluvial sediments deposited along with volcanic lavas, i.e. Protobasinal rocks and the Black Reef. This is overlain by the Chuniespoort, most notably, the Malmani dolomite and chert. Continued extensional subsidence in the Transvaal Basin created a deep marine environment and the deposition of uppermost Chuniespoort of banded iron formation, followed by an extensive marine regressive sequence. This latter depositional phase defines the Pretoria Group rocks.
Continuing east along the N4 highway, we arrive in Nelspruit and cross the ca. 3.2 Ga Kaap Valley Tonalite. This pluton is of special interest as it forms an example of the earliest continental crust on Earth. This is the result of partial melting of subducted hydrated oceanic crust, highlighting the existence of early Earth tectonic processes.
From Nelspruit we continue south along the R40 into the ca. 3.5 – 3.2 Ga Barberton Greenstone Belt. The Barberton Greenstone Belt is one of the most well preserved fragments of the early Earth and holds many secrets of early life and geodynamic evolution. There are three distinct lithological zones. From bottom to top, these are: The shallow marine Onverwacht Group of ultramafic-mafic volcanic rocks with minor felsic volcanics, tuff and lesser sediments. The shallow to deep marine turbidite, shale, mudstone, interbedded chert and banded iron formation of the Fig Tree Group. And, finally the topmost Moodies Group. The latter was deposited in a shallow marine to fluvial environmental setting and consists of conglomerate, feldspathic quartzite, shale and lesser banded iron formation and some volcanic rocks. Terrane assembly ca. 3.2 Ga, along the Saddleback-Inyoka fault system sutured these zones together and formed the general NE structural trend.
Our route then continues and straddles the Swazi border through vast indigenous forest until we finally reach the N17 and follow it to our overnight destination of Ermelo.
Continuing east along the N4 highway, we arrive in Nelspruit and cross the ca. 3.2 Ga Kaap Valley Tonalite. This pluton is of special interest as it forms an example of the earliest continental crust on Earth. This is the result of partial melting of subducted hydrated oceanic crust, highlighting the existence of early Earth tectonic processes.
From Nelspruit we continue south along the R40 into the ca. 3.5 – 3.2 Ga Barberton Greenstone Belt. The Barberton Greenstone Belt is one of the most well preserved fragments of the early Earth and holds many secrets of early life and geodynamic evolution. There are three distinct lithological zones. From bottom to top, these are: The shallow marine Onverwacht Group of ultramafic-mafic volcanic rocks with minor felsic volcanics, tuff and lesser sediments. The shallow to deep marine turbidite, shale, mudstone, interbedded chert and banded iron formation of the Fig Tree Group. And, finally the topmost Moodies Group. The latter was deposited in a shallow marine to fluvial environmental setting and consists of conglomerate, feldspathic quartzite, shale and lesser banded iron formation and some volcanic rocks. Terrane assembly ca. 3.2 Ga, along the Saddleback-Inyoka fault system sutured these zones together and formed the general NE structural trend.
Our route then continues and straddles the Swazi border through vast indigenous forest until we finally reach the N17 and follow it to our overnight destination of Ermelo.
Day 2
We depart Ermelo nice and early (we hope) and continue east along the N2 toward Piet Retief. From here we head south toward Vryheid. Along the way we will encounter the Commondale Komatiites. The composition of these lavas implies a much higher melting point than what currently exists on Earth. This suggests that early Earth was much hotter, or perhaps had a sufficiently enough hydrous content. Further south, toward Paulpietersburg we cross the Pongola Basin and onto our final look at Archean granites. Heading further south and we enter the vast plains of the Karoo Supergroup.
The rocks of the Karoo Supergroup were deposited into numerous basins formed during tectonic processes defining the evolution of Gondwana; and a ca. 120 Ma geological record. For this trip, our interests lie with the Main Karoo Basin, which covers most of the country. Sedimentation of the Main Karoo Basin can be subdivided into five phases. These are: Glacial and the deposition of the Dwyka Group; Marine to coastal plains and the deposition of the Ecca Group; and fluvial to aeolian and the deposition of the Beaufort and Stormberg Groups. And finally, extensional tectonics and the outpouring of the Karoo Large Igneous Province.
As we head south we continue through the lower successions of the Ecca. This region is especially renowned for the vast coal deposits. Coal is the overwhelming fuel used for South Africa’s energy generation and is found largely within the Ecca; and also the Beaufort and Stormberg Groups. Depending on time, we will have numerous interesting sites to visit in the Karoo, including a Glacial Pavement developed on the ca. 2.9 Ga Mozaan Quartzites and several excellent stratigraphic unconformities and special fossil sites (ask me to tell you more about this while we’re in the field).
Heading even further south, we exit the Karoo Supergroup and enter the Natal Sector of the Namaqua-Natal Mobile Belt (NNMB). The NNMB is an orogenic suture that forms the basement underlying most of the Karoo. It represents the remnants of a collisional event defining the formation of Rodnia ca. 1250 – 950 Ma. The NNMB is exposed in two regions, namely, the Northern Cape and KZN. The Natal Sector comprises several distinct geological terrains that are thrust-bounded together. These are, from north to south; the Tugela greenschist ophiolite complex, Mzumbe upper-amphibolite facies granulites and Margate granulite facies rocks. A major feature of this region is the development of the Oribi Gorge Suite of granite and charnockite. These rocks are generally restricted to the Mzumbe and Margate terrains. We will encounter some of the granites as we head toward our overnight destination of Pietermartizburg.
The rocks of the Karoo Supergroup were deposited into numerous basins formed during tectonic processes defining the evolution of Gondwana; and a ca. 120 Ma geological record. For this trip, our interests lie with the Main Karoo Basin, which covers most of the country. Sedimentation of the Main Karoo Basin can be subdivided into five phases. These are: Glacial and the deposition of the Dwyka Group; Marine to coastal plains and the deposition of the Ecca Group; and fluvial to aeolian and the deposition of the Beaufort and Stormberg Groups. And finally, extensional tectonics and the outpouring of the Karoo Large Igneous Province.
As we head south we continue through the lower successions of the Ecca. This region is especially renowned for the vast coal deposits. Coal is the overwhelming fuel used for South Africa’s energy generation and is found largely within the Ecca; and also the Beaufort and Stormberg Groups. Depending on time, we will have numerous interesting sites to visit in the Karoo, including a Glacial Pavement developed on the ca. 2.9 Ga Mozaan Quartzites and several excellent stratigraphic unconformities and special fossil sites (ask me to tell you more about this while we’re in the field).
Heading even further south, we exit the Karoo Supergroup and enter the Natal Sector of the Namaqua-Natal Mobile Belt (NNMB). The NNMB is an orogenic suture that forms the basement underlying most of the Karoo. It represents the remnants of a collisional event defining the formation of Rodnia ca. 1250 – 950 Ma. The NNMB is exposed in two regions, namely, the Northern Cape and KZN. The Natal Sector comprises several distinct geological terrains that are thrust-bounded together. These are, from north to south; the Tugela greenschist ophiolite complex, Mzumbe upper-amphibolite facies granulites and Margate granulite facies rocks. A major feature of this region is the development of the Oribi Gorge Suite of granite and charnockite. These rocks are generally restricted to the Mzumbe and Margate terrains. We will encounter some of the granites as we head toward our overnight destination of Pietermartizburg.
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