11th CGS Field School - Pongola

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

Field School 11 Itinerary - Pongola Supergroup

After our time in the Barberton Greenstone Belt we hope that our participants will have formulated some ideas about how the early-Earth functioned and how early continental crust formed. We now move on to the Pongola Supergroup, which provides an opportunity to investigate one of the earliest Archean volcano-sedimentary sequence deposited onto some stable continental margin. 

For a geological overview of the Pongola Supergroup click here. 

Overview of the various Pongola Supergroup geological stops

Stop 1

The first stop of the Pongola field trip will be to investigate the contact between the lowermost sequence of the Nsuze Group, i.e. the sandstone and volcanic rocks of the Matonga Formation and the pre-Pongola c. 3105 Ma TTG basement.

Stop 2

Thereafter, we will continue into the Archean TTG basement and investigate a Komatiitic remnant of the enigmatic, strongly mantle-depleted Commondale Greenstone Belt. Contact relationships of this greenstone sequence remains elusive.

Stop 3

The northern and southern domains of the Pongola Supergroup is largely separated by Karoo cover rocks. These particular lower Karoo rocks contain abundant coal reserves. We will therefore take some time to investigate some of these Karoo rocks near Vryheid.

Stop 4

Now, in the southern domain, we will investigate the Nsuze lavas, particularly the Agatha Formation, and discuss their features and implications on the development of this sequence.

Stop 5/6

Finally, we will investigate to contact relationship between the Nsuze and Mozaan Group. We will also discuss gold occurrences within the Mozaan and some of the mining history in this region. And, we will end this trip by looking at a major unconformity between the Pongola and Karoo Supergroups.

Field School 11 Field Guide - Pongola Supergroup

1 Introduction

Our time in the Barberton Greenstone Belt would have highlighted many discussion points regarding the functioning of the early Earth. During this time we would have especially focused on how early continental crust would have formed on Earth. In keeping with the topic of early Earth, we will now move onto the next major Supergroup of South African geology, namely the Pongola Supergroup.

The Pongola Supergroup represents one of the best preserved Archean volcano-sedimentary sequences in the world and may be correlated with the immensely gold-bearing Witwatersrand Supergroup. The Pongola is especially important because it also represents one of the earliest examples of a volcano-sedimentary sequence deposited on some kind of stable continental setting, i.e. the Kaapvaal Craton. Here, we are given an ideal opportunity to investigate a major early Earth geodynamic transition from the creation of continental crust, i.e. as seen in Barberton, to the deposition of rocks on that early crust. Deformation field evidence suggests the occurrence of potentially tectonically-relevant processes occurring at a time where much of the Earth had not produced any stabilised continental crust. 

2 Geological Overview

The Pongola Supergroup is generally separated into a relatively undeformed northern domain and a highly deformed southern domain. These rocks may be subdivided into the lower c. 2.9 Ga volcano-sedimentary Nsuze Group, which is overlain by the c. 2.8 Ga mostly siliciclastic Mozaan Group.

The c. 4-9 km thick Nsuze Group consists of basal quartzite and conglomerate and upper sequences of a large variety of mafic and felsic volcanic rocks. The siliciclastic rocks appear interlayered with the volcanic rocks, which further display a wide variety of features, including lapilli tuff beds, volcanic bombs and amygdaloidal lavas. 

Simplified geological overview of the Pongola Supergroup (Ossa et al., 2016)

The c. 5 km thick Mozaan Group predominantly consists of interlayered sandstone, mudstone, stromatolitic carbonates and banded iron formation. There are also lesser interlayered volcanic rocks near the upper part of the sequence. Sedimentary features are common and suggest deposition in a shallow-marine, wave-dominated environment.

The Mozaan Group can be further subdivided into six formations. These include, from bottom to top; the sandstone, shale and banded iron formation of the Sinqeni Formation; the mudstone, ferruginous mudstone, siltstone and sandstone of the Ntombe Formation; the mudstone, ferruginous mudstone, sandstone and banded iron formation of the Thalu Formation; the sandstone, silstone and pebble-conglomerate of the Hlashane Formation; the mudstone, ferruginous mudstone, diamictite and sandstone of the Odwaleni Formation; and the topmost sandsone, mudstone and volcanoclastics of the Nkoneni Formation. 

Detailed stratigraphic log across the Nsuze-Mozaan transition (Ossa et al., 2016)

3 Deposition of the Pongola Supergroup

The Pongola rocks were deposited on the stabilised Kaapvaal Craton in some kind of continental environment. Considering the occurrence of numerous banded iron formation and stromatolitic carbonate assemblages we may further suggest that deposition occurred in shallow marine, highly reducing environment, which saw a major shift in the chemical composition of the atmosphere (e.g.Beukes and Cairncross, 1991), leading up to, and following the formation of stromatolitic carbonates.

Furthermore, structural evidence (e.g. Gold and Von Veh, 1995) suggests the occurrence of at least one major extensional event that reactivated along pre-exisisting discontinuities during the late Archean. Compression is also noted and potentially linked to the intrusion of major granitic plutons. Once more, strain associated with the compressional regime was partitioned along pre-exisisting discontinuities. Considering these discontinuities and their link to apparent sediment transport directions, the Pongola is considered to have potentially been deposited into a continental rift environment.

Cross section across the Pongola Basin (Gold and Von Veh, 1995)

4 Gold in the Pongola Supergroup

We will also investige gold mineralisation within the Pongola Supergroup during our visit to this terrane. Considering the link and possible similarities in the depositional environments of the Pongola and Witwatersrand Basins, extensive gold exploration was performed throughout the Pongola region. However, gold grades and tonnage never met expectations and development remained low.

Gold mineralisation within the Pongola region manifests in two forms; placer, i.e. gold mineralisation concentrated during sedimentary transport, and lode, i.e. remobilisation of gold-bearing hydrothermal fluids usually into brittle structures, deposits. The placer gold deposits are especially notable in conglomerate layers within the Mozaan and Nsuze Groups.

During the field school we will visit the now abandoned Denny Dalton mine, which is formed in the Sinqeni Formation. The Denny Dalton Member is a clast-supported conglomerate layer that hosts an erratic occurrence of gold, iron-sulphides and uranium.

Snapshots - Day 2

Great news! The baton has been handed over, thus I have now much less work to do. From today our field school participants have held a mutiny to this blog and will start sharing their stories and what they've learnt during our trip. With that being said, I can now simply share some snapshots of the day!

Day 2 - AMAZING Geology

Nigel Hicks: Senior scientist and expert from the Pietermaritzburg regional office of the Council for Geoscience explaining the intricacies of Delta depositional environments. Note the expert drawing of the background Vryheid Shales trough cross beds - Ecca Group of the Karoo Supergroup.

Expert team work is important in these situations. 

Conglomerates of the Sinqeni Formation - 2.9 Ga Pongola Supergroup, with the notably erratic Mozaan Contact Reef, which was mined by the Denny Dalton family. Note the sulphides and uranium staining (yellow tinge).

The most AMAZING of all. Seems like the Dwyka does not only carry rocks, but also Baas Groenewald and Baas Mofokeng, who both exist ca. 390 Ma and 2900 Ma. Note the striations all over the quartzites.

RSA Geotour 2015: Day 1 - 2

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.

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.