During the Field School we will have a special opportunity of investigating the major Pofadder Shear Zone within the Namaqua-Natal Belt. During this investigation, you will have the opportunity of examining the transition from brittle-ductile shearing and also practice measuring kinematic indicators in the field.
The c. 1000 Ma NW-SE upper Greenschist-lower Amphibolite facies Pofadder Shear Zone is a c. 500 km long right-lateral strike-slip shear zone that extends across South Africa and Namibia with c. 30 km offset (Lambert, 2013). This feature is of especial interest because it has been exhumed to levels which highlight the brittle-ductile transition and highlights effects of a Paleo-Earthquake (Melosh et al., 2016).
The c. 1800 - 1000 Ma Namaqua-Natal Belt forms the western boundary of the Kalahari Shield and is associated with the formation of the Rodinia Supercontinent. During the field school we will be focusing on the Namaqualand sector of this Belt. The Namaqualand sector can be divided into four domains, each separated by major crustal shear zones:
Overview of the Namaqua-sector of the Namaqua-Natal Belt (Bilal et al., 2016)
The lithologies of these different domains can be summarised as follows:
Lithological subdivision across the various Namaqua-Natal domains (Edlington et al., 2006)
Deformation of the Namaqualand sector can is defined within four major events (Groenewald per coms):
D1: c. 1905-1885 Ma Orange River Orogeny, which formed the Richtersveld magmatic arc and intrusion of the Vioolsdrift Suite granitoids. Evidence of this deformation is only preserved in the low-grade Vioolsdrift Domain, within the Richtersveld. By the end of this field school, you will be experts on this geology, because it will form the basement geology throughout the Gariep Belt.
D2: c. 1220-1100 Ma Namaqua-Natal Orogeny, which is linked to continental collision during the main phase of Rodinia assembly, this event, together with the next formed most of the large thrust zones that separate the various domains. This event resulted in the formation of the gneissic and schistose fabric commonly observed along our Pofadder transect.
D3: c. 1100-1005 Ma Peak deformation during the Namaqua-Natal Orogeny. This event also resulted in the development of mega-scale refolding of D2-linked folds into upright and sheath folds.
D4: c. 1005-980 Ma development of dextral ductile shear zones, e.g. the Pofadder Shear Zone. This event can be further subdivided into six intermediate deformational phases (a-d):
D4a: Early stages of shear zone development with shear-linked reorientation and drag of earlier fabrics, with fabrics becoming steeper toward the core of the shear zone.
D4b: Formation of mylonitic and cataclasite units within the shear zone
D4c: Formation of the Pofadder very fine-grained ultramylonitic units, including the core, which furthermore crosscuts the D4b structures.
D4d: Brecciation of the ultramylonites and cataclasites, this represents a large-scale Paleoearthquake
D4e: Major steep faulting and the development of sub-vertical lineations
D4f: Late epidote-filled cross-cutting fractures
3 Kinematics
There are several features which may be used in a ductile shear zone to establish sense of movement, but firstly, an answer to the age old question: "which plane should I be looking at?"
Kinematic evidence of shearing can be seen at high-angles to the lineation (ETH, 2016)
Now that you know which plane to look at, there are a number of features which may assist you in establishing the sense of shearing; here are few listed below:
You may use S-C fabrics to establish the sense of shear (ETH, 2016)
You may also use C-C fabrics to establish the sense of shear (ETH, 2016)
And, the most common rotated porphyroclasts (ETH, 2016)
Here we continue further with the building of Gondwana and the formation of the Cape Fold Belt. Following the formation of Gondwana a period of regional subsidence presided and allowed for the formation of several basins of various sizes throughout Gondwana. These various basins were infilled from the late Carboniferous to early Jurassic, during a time that records a protracted period of environmentally-controlled sedimentation. The various sedimentary environments that existed during this period may be summarised below:
The various Karoo depositional basins across much of Gondwana formed in response to various tectonic settings, particularly, extensional/subsidence. The Main Karoo Basin of South Africa, which will be the focus during the Field School, likely formed as a foreland basin, developing in response to shallow-angle subduction along the southern margin of Gondwana. This subduction would further result in the formation of the Cape Fold Belt.
The deposition of the Karoo rocks began when the South Pole was located over Gondwana, covering much of the supercontinent with a large ice sheet. This allowed for the formation of glacier-lakes into which varved mudrock were deposited. Furthermore, as Gondwana continued to drift, much of the melt water derived from this glacier resulted in the deposition of the characteristic Dwyka diamictite. Several exotic clasts found within these till-deposits highlight the aerial extent reached by this ice sheet.
Much of the glacial melt water further contributed to the infilling of deep basins. These basins reached depths where reducing conditions prevailed. These were the conditions where much of the Ecca carbonaceous shale and sandstone were deposited. Within the Ecca, interlayered tuff and volcanic ash layers are also prominent, i.e. within the Collingham Formation.
It was at a time coeval to the deposition of the upper Ecca where the Cape Fold Belt mountains formed. Consequentially, sediment being derived from these high mountains resulted in the development of prograding deltas and fluvial environments that typify the upper Ecca. These comprise of many upward-fining turbidite sequences.
Continued basin infilling and progradation would eventually result in the formation of a dominant fluvial depositional environment and the consequential deposition of the Beaufort Group of sandstone and mudrock. The mudrock sequences were especially ideal in preserving much of the fauna that existed during this period, and on the field school, we should have plenty of opportunity to investigate this.
A climatic shift toward a semi-arid, desert-type environment allowed for the deposition of the clastic Stormberg Group. These depositional conditions are spectacularly preserved by the large dune-type deposits of the Clarens, which we will see in the Golden Gate National Park.
The deposition of the Karoo Supergroup was completed as Gondwana began to breakup and the Karoo Large Igneous Province was emplaced.
There is much contention regarding the style of formation for the Karoo. The Karoo is generally regarded as being a retro-arc foreland basin that formed behind a fold-thrust belt, i.e. forming due to the shallow-angle subduction below the southern margin of Gondwana. Much of the contention stems from the exact timing of the Cape Orogeny (i.e. c. 500-250 Ma) and the basement architecture. Much of the subsidence and faulting seen throughout the Karoo would have used and reactivated existing crustal discontinuities formed during the formation of Gondwana.
Geophysical information regarding the basement architecture and crustal discontinuities can be summarised as follows:
Basement architecture below South Africa (A)-magnetic; (B)-gravity (Tankard et al., 2009)
An overview of Karoo deposition, related to the tectonic history can be summarised as follows:
Overview of the tectonic evolution and depositional setting of the Cape and Karoo Basins (Tankard et al., 2009)
The Cape Fold Belt is a north-vergent fold and thrust belt with a significant strike-slip component. This affects much of the southern region of the Karoo. During the field school we will conduct a section across this southern region of the Karoo and we can expect the following:
Cross section across the Cape Fold Belt and into the Main Karoo Basin (Tankard et al., 2009)
4 Shale Gas
A study conducted by the US Energy Information Administration suggested that the Main Karoo Basin has copious quantities of shale gas (methane), up to c. 390 Tcf. While this figure is likely a gross miscalculation, more reasonable estimates suggest that the Lower Karoo rocks could have c. 10-20 Tcf of natural gas. Despite this large estimated reserve range difference, even the lesser estimate suggests that these reserves could represent a potential Game Changer for the South African Energy and Minerals landscape.
Overview of the lower Ecca shale gas-bearing units (Geel et al., 2013)
Chemostratigraphy across the shale gas-bearing units of the lower Ecca (Geel et al., 2013)
Factors potentially affecting the preservation and maturation of the shale gas reserves in the Karoo include the intrusion of the Karoo dolerite and the deformational extent of the Cape Fold Belt. These together result in the shale gas being overmatured and/or lost through degassing via brittle features. An ideal potential region can be shown as follows:
Recoverable shale gas from the Whitehill Formation (Cole, 2014)
There are still many factors that will be considered before any shale gas extraction occurs. However, the green light has been given for companies to continue with shale gas exploration. This is toward establishing exactly how much shale gas reserves are present in the Karoo. This remains a highly contentious issue that has implications toward the economy, energy production, climate change regulations and natural environment, all of which we will discuss in great detail.
The Cape Supergroup appears along the southern coast of South Africa and generally outcrops within the fold-thrust Cape Fold Belt mountains. The rocks of the Cape Supergroup were deposited into a large basin that extended across southern Gondwana. These rocks are easily correlated, e.g. in Argentina.
The rocks of the Cape Supergroup largely consist of sandstone/quartzite and shale/phyllite. These rocks are spectacularly exposed within the Cape Fold Belt, which will be the aim of our investigation here. The Cape Fold Belt highlights accretion along the southern margin of Gondwana with much of the deformational features preserved due to the reaction-style of the mostly rheologically-competent siliciclastic Cape Supergroup rocks.
2 Stratigraphy
The Cape Supergroup is subdivided into three Groups, from bottom to top, the largely quartzite of the Table Mountain Group, the interbedded mudstone, shale and sandstone of the Bokkeveld Group and the topmost interbedded sandstone and shale of the Witteberg Group. Folding and duplication by thrust repetition throughout the Cape Fold Belt has resulted in much of these rocks being tectonically thickened.
Extent of the Cape Supergroup and the correlated units in KZN (Shone and Booth, 2005)
The Table Mountain Group can be subdivided into, from bottom to top, the basal conglomerate layer of the Piekenierskloof Formation, the finely laminated siltstone and interbedded sandstone of the Graafwater Formation, the mostly quartz-arenite and quartzite of the Peninsula Formation, the glacially-derived Pakhuis diamictite, the black shale and interbedded mudrock, silstone and sandstone of the Cedarberg Formation and the uppermost sandstone/quartzite of the Nardouw Formation.
These formations contain abundant and varied sedimentary features, most notably planar and trough cross bedding features, which generally display a bimodal sedimentary transport direction, with a prevalence of south westerly flow direction. Within these units there are also an abundance of trace and body fossils, most notably within the finer Graafwater and Cedarberg Formations.
The Bokkeveld Group consists of interbedded dark mudrock, siltstone and fine-grained sandstone. The Bokkeveld rocks highlight at least five coarsening-up depositional cycles and also contain abundant crinoid, brachiopod, gastropod, bivalve and trilobite body fossils.
The Witteberg Group predominantly consists of interbedded sandstone and shale with lesser mudrock and siltstone also present.
Depositional settings are varied throughout the lithostragraphic profile of the Cape Supergroup, but there is a general consensus that the base of the Cape Supergroup represents coarse clastic sediment input from a high-energy erosional terrestrial setting.
Thereafter, deposition occurred within a shallow marine, tide dominated environment, e.g. as seen within the Graafwater Formation. The setting shifted sharply to a braided fluvial setting, of the Peninsula and then glaciation as seen by the Pakhuis. Melt waters derived from the glacier would have supplied much of the sediment for the Cedarberg deposition and finally, the Nardouw Formation probably represents another fluvial or marine setting.
3 The Cape Fold Belt
The c. 250 Ma Cape Fold Belt extends approximately 1200 km along the southern coast of South Africa and consists of an eastern and northern limb, joined within the syntaxis, where the two limbs merge in a region of high strain.
Location of the Cape Fold Belt in reference to Pangea (north) and Gondwana (south) at 270 Ma (Hansma et al., 2016)
Simplified map of the Cape Fold Belt, highlighting major faults (Hansma et al., 2016)
The style of deformation that resulted in the development of the Cape Fold Belt is hotly debated. This is largely due to the position of the Cape Fold Belt mountains almost 1000 km away from the African plate boundary. Far-field stresses have often been ascribed as a method for transmitting strain over such a great distance. Despite this, there is agreement that deformation ensued after shallow-angle subduction of the Paleo-pacific plate below the southern margin of Gondwana.
Several theories then postulate the mechanism for the Cape Fold Belt, and include; a compressional retro-arc foreland basin (e.g. Catuneanu et al., 1998); a transpressional retro-arc foreland basin (e.g. Johnson, 2000) or as the result of thin-skinned Jura style thrust propagation (e.g. Lindeque et al., 2011).
During the field school we will do at least two north-south traverses across the Cape Fold Belt, as well as east-west sections across the eastern limb, and a further north-south section along the western limb. In this time the participants will have an opportunity of investigating several examples of both ductile and brittle deformational features.
4 Rodinia to Gondwana
After all our discussions about how the early continental crust had formed and major secular changes of the Earth's ocean and atmospheres, we now fast forward some three billions years and can now start thinking about supercontinental growth and destruction. Along the southern coastline of South Africa, and extending into our mapping region of the Gariep Belt, we will investigate two major supercontinental cycles, namely, from the supercontinent Rodinia to Gondwana.
The shift from Rodinia to Gondwana can be summarised in the following diagrams, after Johnson, 2014:
900 Ma Rodinia reconstruction
850-750 Ma primary phase of Rodinia dispersal
750-650 Ma secondary phase of Rodinia dispersal with block rotations
650-550 Ma primary phase of Gondwana formation
500 Ma final assembly of Gondwana
Finally, post 180 Ma breakup of Gondwana (Reeves, 2016)
Before we begin with the Transvaal Supergroup, we shall briefly summarise the geology we have seen (will see) thus far: We began with the c. 3.5 Ga Barberton Supergroup. This renowned geological terrane sheds light on the workings of a young Earth and the formation of early continental crust.
Thereafter, we visited the c. 2.9 Ga Pongola Supergroup. The Pongola may be correlated with the Witwatersrand Supergroup. These volcano-sedimentary sequences form some of the earliest sequences of their kind, deposited on a stable continental platform, i.e. the Kaapvaal Craton. Furthermore, here we should also highlight that the Witwatersrand rocks were deposited onto the c. 3.0 Ga Dominion Group lavas. The Dominion lavas, together with the Pongola and Witwatersrand rocks highlights an early possible continental rift environment.
Following the deposition of the Witwatersrand and Pongola was the outpouring of the c. 2.7 Ga Ventersdorp mafic and ultramafic lavas. And finally, deposited on top of the Ventersdorp lavas was the 2.5 Ga Transvaal Supergroup.
2 Introduction
The rocks of the Transvaal Supergroup are comparable in the eastern Transvaal Basin and western Griqualand West Basin. Here, we will be focusing on the Transvaal Basin and especially on the stromatolitic-bearing Malmani Subgroup. The Malmani is important because these represent the earliest and largest carbonate platform deposits on Earth. Moreover, the Transvaal rocks are also important because they mark a major transition from the Archean to the Paleoproterozoic furthering our discussions about the dynamic shift of the early Earth.
The deposition of the Transvaal rocks occurred at a time that saw a rapid accumulation in the growth of continental crust and major, and irreversible shift in chemistry of the Earth's atmosphere and oceans. Furthermore, rocks within this (and the Griqualand West Basin) host important mineral commodities, such as Gold, Iron and precious stones.
3 Stratigraphy
The Transvaal Supergroup is subdivided into the lowermost high-energy siliciclastic Black Reef formation, overlain by the chemical sediments of the Chuniespoort, which is further overlain by the largely siliciclastic Pretoria Group. The stratigraphy of these various rock types may be summarised in the following diagrams:
The lowermost rocks of the Transvaal Supergroup consists of the Black Reef Formation, which largely consists of conglomerates with interbedded sandstone and mudstone that was deposited in a braided fluvial to shallow marine lagoon/delta environment.
Unconformably overlying the Black Reef is the Chuniespoort Group, which may further be subdivided into seven formations. The lowermost sequence consists of the stromatolite-bearing Malmani Subgroup, which further consists of five formations. This includes from bottom to top, the Oaktree, Monte Christo, Lyttelton, Eccles and Frisco formations. These are distinguished from each other by their chemical, especially Mn-content and occurrence of interlayered chert. The Monte Christo and Eccles are chert rich, while the others are chert poor. The Malmani was deposited as a carbonate platform in a shallow marine setting, with interbedded shale that highlights proximity to the shoreline and terrestrial sediment input.
Overlying the Malmani is the Penge banded iron formation, which was probably deposited in a shallow marine setting, with the Ventersdorp lavas representing a likely Fe-source candidate. The topmost unit of the Chuniespoort is the dolomitic mudstone and interbedded quartzite of the Duitschland Formation.
Overlying the Chuniespoort are the largely volcano-sedimentary Pretoria Group, which is further subdivided into fourteen formations (which I will not write about here, but you may refer to the figures below for an overview).
Stratigraphic logs correlating the Transvaal and Griqualand West Basin (Frauenstein et al., 2009)
Lithostratigraphy correlated with tectonic setting and depositional environments of the Transvaal Basin (Catuneanu and Eriksson, 1999)
4 The Great Oxygenation Event
From our time in Barberton and Pongola, we would have discussed the presence of oxygen in the early Earth atmosphere. However, this concentration was not significant and the atmospheric and ocean chemistry often varied in composition from oxygen-rich to poor. However, the c. 2.33 Ga Great Oxygenation Event signals a time in the history of the Earth that saw an irreversible increase in the atmospheric oxygen concentration (i.e. Luo et al., 2016). This is especially important because this increase of the oxygen concentration heralds a shift and transition of the biological revolution on Earth.
This event occurred after large-scale photosynthesising cyanobacteria organisms consumed large quantities of delta-12 carbon from the largely reducing atmosphere and ocean. Complimentary to this was the release of a vast amount of oxygen that saturated the atmosphere and ocean, changing the chemical make-up completely.
The chemical reaction catalysed by the photosynthesising organism also resulted in an enrichment of delta-13 carbon, i.e. due to the drawdown of delta-12 carbon. This subsequently resulted in elevated d-12 carbon signatures seen in the Pretoria Group:
Location of the Great Oxygenation Event within the Pretoria Group (Luo et al., 2016)
5 Gold in the Malmani
In keeping with the theme of major mineralisation, the Malmani Subgroup has produced substantial quantities of gold since the early 1800's. Gold mining within the Malmani rocks was focused around the Sabie-Pilgrim's Rest region and largely consists of quartz-vein hosted lode deposits. During the field school, we will plan on visiting some abandoned mines in the Sabie region, however this will depend on safety. Many of these abandoned mines are currently being mined illegally, a risk to surrounding communities and also the miners themselves.
We will therefore have to discuss the current state of illegal mining in South Africa and how do we protect these miners and what are the possible mechanisms to remedy this situation?
The gold mineralisation in the Malmani is thought to have been sourced and/or remobilised by the Bushveld Complex. The intrusion of the Bushveld resulted in eastward directed thrusting. During the thrusting, remobilised gold-bearing fluids were concentrated along shallow-bedding-parallel detachment zones within the Malamani. Gold mineralisation is largely associated with iron-sulphides and as quartz-inclusions.
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.
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.
The c. 3.57-3.22 Ga Barberton Greenstone Belt is one of the most important geological terranes in the world. This is primarily because it is one of very few locations where a pristine fragment of the early Earth is preserved. The rocks found here hold abundant information about the geodynamic functionality and evolution of the early Earth, the formation of continental crust and even clues about the origins of life as we know it. The Council for Geoscience Field School will provide the participants an opportunity of visiting this famous geological terrane. Here we will investigate how fundamental geological field observations throughout the Barberton Greenstone Belt, together with advanced analytical techniques have shed light about the early Earth. We will also spend a few days mapping the lower part of the Onverwacht Group and look for evidence of early Archean geodynamic processes.
2 Geological Overview
The rocks of the Barberton Greenstone Belt comprises the oldest Supergroup in South Africa, namely, the Barberton Supergroup. This forms the eastern portion of the Kaapvaal Craton and appears as a NE trending belt of highly folded volcano-sedimentary rocks that has undergone greenschist facies metamorphism. The Barberton Greenstone Belt is further defined by several granitic and granite gneiss TTG (tonalite, trondhjemite and granodiorite) plutons that have intruded the belt at various locations and times throughout its evolution. The Barberton Greenstone Belt can be separated into a two geochemically distinctive terranes, i.e. a Northern and Southern terrane, which are separated by the Saddleback-Inyoka Fault.
Geological overview of the Barberton Greenstone Belt, with simplified stratigraphic log (Hofmann, 2005)
3 Stratigraphy
The Barberton Supergroup is subdivided into three groups. This includes the lowermost c. 3.57-3.30 Ga Onverwacht Group of mafic and ultramafic volcanic rocks. Overlying the Onverwacht are the c. 3.55-3.25 Ga Fig Tree Group of chemical sedimentary rocks interlayered with mafic and ultramafic lavas. The topmost are the c. 3.23-3.22 Ga Moodies Group of mostly siliciclastic rocks.
3.1 The Onverwacht Group
The Onverwacht Group is a 8-10 km thick sequence of mafic/ultramafic mostly komatiitic basaltic rocks, interlayered with felsic tuff, chert and carbonate rocks. These rocks were probably formed in either a volcanic hot spot, or in an island arc setting. The Onverwacht is further subdivided into six formations. This includes, from bottom to top; the Sandspruit, Theespruit, Komati, Hooggenoeg, Kromberg and Mendon formations.
The Fig Tree Group attains a thickness of c. 3 km and predominantly consists of turbidite deposits, banded iron formation, stratiform barite and interlayered volcanoclastic rocks. The rocks of the Fig Tree Group can be further subdivided into two facies-type deposits separated across the Saddleback-Inyoka Fault. This includes a shallow-water and deep-water depositional facies, south and north of the Saddleback-Inyoka Fault, respectively.
The southern facies can be subdivided into a lower Mapepe Formation and an overlying Auber Villiers Formation. The Mapepe Formation consists of ferruginous shales, banded iron formation, sandstone and barite-bearing conglomerate. The Auber Villiers Formation consists of felsic volcanics interlayered with turbidites and chert-bearing conglomerate.
The northern facies can be subdivided into, from bottom to top; Sheba, Belvue Road uppermost Schoongezicht formations. The Sheba Formation consists of banded iron formation with interbedded turbidites. The Belvue Road Formation consists of shale, banded iron formation and interlayered felsic volcanic rocks. The Schoongezicht Formation comprises felsic volcanics rocks with interlayered turbidites and conglomerate.
Stratigraphy of the Fig Tree and Moodies Groups north in the northern region of the Barberton Greenstone Belt (Anhaeusser, 1974)
3.3 The Moodies Group
The Moodies Group attains a thickness of c. 3.7 km and consists of sandstone, siltstone and conglomerate that was probably deposited in a shallow marine setting, such as a braided fluvial-tidal system.
Generalised stratigraphy of the Moodies Group north of the Saddleback-Inyoka Fault (Heubeck et al., 2013 )
3.4 TTG rocks
The various TTG plutons that surround the Barberton Greenstone Belt can be summarised into five distinctive age groups:
3510-3502 Ma - e.g. Steynsdorp Pluton
3469-3437 Ma - e.g. Stolzburg, Theespruit, Doornhoek Plutons
3229 Ma - e.g. Kaap Valley Tonalite
3216 Ma - e.g. (undeformed) Dalmein Pluton
3105 Ma - e.g. (undeformed) Mpuluzi Batholith
4 Deformation
The structural evolution of the Barberton Greenstone Belt is contentious. On the basis of the existence of plate tectonics during the early Earth, a polyphase structural evolution can be summarised as follows (after De Ronde and De Wit, 1994):
D0 (3490-3450 Ma) - extensional tectonics and alteration processes near volcanic spreading centers.
D1 (3450-3416 Ma) - subduction-like processes with the emplacement of a possible ophiolite complex in an intraoceanic setting and intruded by TTG plutons.
D2 (3260-3225 Ma) - a second phase of subduction-related accretion of the northern and southern terranes and the formation the Saddleback-Inyoka Fault.
D3 (c. 3100 Ma) - transtensional and transpressional tectonics along this terrane boundary, which also resulted in the NE-trending expression of the Barberton Greenstone Belt and the formation of strike-parallel faults.
D4 (< 3100 Ma) - NW-directed extension and the formation of gold mineralisation-controlling normal faulting development.
Another deformation model considers little to no plate tectonics at play, and rather the effect of rising and falling of mantle plumes and convective overturn (i.e. Van Kranendonk et al., 2009). These features are especially notable when considering the overall pattern across the Barberton Greenstone Belt, which highlights a dome and keel structure:
Dome and keel morphology of the Barberton Greenstone Belt, surrounding TTG plutons (Lana et al., 2010)
5 Gold mineralisation in Barberton
The Barberton Greenstone Belt is an important gold producing terrane in South Africa, with more than 350 operational gold mines. Gold mineralisation in Barberton is structurally controlled and is associated to the later deformational event, i.e. D4. Gold mineralisation was likely formed during a major protracted hydrothermal event as remobilised gold-bearing fluids flowed through permeable brittle fractures, i.e. extensional features. Two kinds of gold mineralisation is noted, namely, mineralisation associated to iron-sulphides and gold within quartz.
Overview of large gold mineralisation locations throughout the Barberton Greenstone Belt (Altigani et al., 2016)
6 A model for the formation of early continental crust
A model describing the formation of Archean continental crust is provided by De Wit, 1998, and can by summarised as followed, starting with some important facts about Archean crust:
1. Archean Cratons, such as the Kaapvaal Craton have very deep subcontinental mantle keels, which reach depths of almost 400 km and were formed by c. 3.5-3.1 Ga. Trace element analysis on eclogite inclusions suggest that these keels may represent hydrothermally altered Archean oceanic crust (i.e. basaltic crust).
2. Deep seismic analysis suggest the presence of mantle discontinuities, which may represent crustal-scale shear zones. These shear zones apparently separate different crustal domains that were amalgamated together.
3. The Archean crust highlights low mantle heat flow patterns. This is probably due to low heat production and insulating effects.
Combining these factors suggests that the early Archean crust formed by tectonic stacking of hydrothermally altered oceanic crust. The model proposed by De Wit, 1998 suggests that this process formed as follows:
During Hadean times, there was dry recycling of ocean crust, where seawater levels remained low and the oceanic spreading centers were above sea level. Continued mantle degassing from c. 4.5 Ga onward, together with meteorite bombardment, would have seen the sea level rise enough to drown these oceanic spreading centers c. 4.0 Ga,
Thereafter, subduction of oceanic crust would have undergone adequate hydrothermal alteration, which allowed for the formation of more buoyant oceanic-lithosphere. This lighter and more buoyant material would resist subduction, thus forming duplex structures, i.e. stacks of oceanic crust. As this stacking process continued and the lower parts of the hydrothermally altered mafic rocks were buried deeper, these rocks underwent dehydration melting, with the products of melting representing the early TTG rocks. The TTG plutonic emplacement would have been facilitated along extensional shear zones.
Over time, the gradual burial and partial melting of TTG rocks would have resulted in the formation of younger GGM rocks. During this evolution, the slab-pull effect became more prevalent and plate tectonic processes began to function more rapidly. This would eventually result in these early Archean fragments colliding and amalgamating to form larger continental masses.
Geodynamic model for the creation of early Archean crust (Polat, 2012)
7 Conditions on the early Earth
A model for the conditions of the early Earth is provided by Lowe and Tice, 2007, and can be summarised as followed:
The Earth was covered by large oceans and had few continental masses. Meteorite impacts were common and there would have been lots of tsunamis. There was a high concentration of volcanic activity and therefore a large outpouring of mantle gas. Due to a high concentration of these greenhouse gasses (i.e. carbon dioxide and methane), the temperature on Earth was substantially hotter than today, probably c. 70 degrees Celsius. In these conditions cyanobacteria would have struggled to survive and thus the Earth would likely have been dominated by thermophile bacteria. These conditions were however suitable for the formation of early Algoma-type banded iron formation.
Early Earth might have looked something like this (without the humans and drama)
As continental crust continued to grow, rates of weathering increased. This resulted in a drawdown of the carbon dioxide and eventually methane. This eventually resulted in global cooling and even glaciation c. 3.0-2.9 Ga.
The global cooling and relatively stable conditions on the earth Earth, in addition to the changes to the chemical atmospheric conditions allowed for the growth of stromatolitic-producing cyanobacteria and oxygentation of the Archean atmosphere.
This process of an increase in greenhouse gas, growth of continental crust and eventual drawdown of greenhouse gas seen c. 3.5-2.9 Ga appears cyclic throughout the Earth history and appears again c. 2.75-2.2 Ga and 1.0-0.5 Ga.
