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THEME 6:

The Transvaal Supergroup

The Ventersdorp Supergroup represents rifting of the

Kaapvaal Craton. However the craton did not break-up,

and after a period of stability, renewed stretching and rifting

began at 2.65 Ga.

To the North the Thabazimbi Murchison Lineament, and

Sugarbush Lineament to the south formed the boundaries to the Wolkberg Group Basin

Protobasinal rocks: Wolkberg, Godwan, Bloempoort, Buffelsfontein groups

Angular unconformity

Black Reef Formation

Angular unconformity

Chuniespoort GroupDuitschland FmPenge FmMalmani Subgroup

Pretoria Group

Rooiberg GroupAngular unconformity

The Transvaal Supergroup

Once again, old sutures that initially formed during the assembly of the Kaapvaal

were reactivated, so formed the borders to sedimentary

basins

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Sand and mud washed into the Wolkberg trough. The

thick accumulations of sandstones have now been

metamorphosed to quartzite…a very resistant

rock

W

G

Bu

Bl

Outcrop localities of the Protobasinal rocksAfter rifting and deposition of

the protobasinal rocks, thermal subsidence took place (Steer’s head model). Almost entire Kaapvaal submerged

(except for Limpopo mountians in North).

The Transvaal Supergroup was deposited in this basin

Three sedimentary basins from this event are preserved on the

Kaapvaal Craton:The Transvaal Basin

The Griqualand West BasinThe Kanye basin (in Botswana)

The are filled with volcano-sedimentary strata of the

Transvaal Supergroup

Griqualand WestBasin

Tranvaal Basin

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During subsidence: Fluvial, then littoral then marine.Black Reef Formation =

Conglomerate, sandstone, mudstone

Salt casts (1cm across) in the Black Reef Fm,suggesting a dried-out mud flats on the coast

As the sea encroached over the Kaapvaal, there was a great deal of area within a shallow marine environment. A good habitat for Cyanobacteria (stromatolites).

Photosynthesis, removal of CO2 from sea water, production of O2,

precipitation of CaCO3 (limestone).

Calcium Carbonate deposits are covered by annual growth

layer of cyanobacteria.

In the Archaean and Shark Bay, no gastropods

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Requirements for stromatolite growth:

1. Shallow water (<100m)2. Low rate of clastic sediment

input3. No grazers

The Chuniespoort Group (and Ghaap Group in the GriqualandWest Basin) is thus made up of thick accumulations of Dolomite

(Mg replaces Ca)2.6-2.4 Ga

Chert (SiO2) also precipitated chemically amongst the dolomite

Dolomites therefore represent shallow water deposits. What

about deeper water environments?

Carbonate mud. Iron and manganese oxides in very

deep water

Fe and Mn released from fumaroles under the sea, and a product of weathering. They are soluble in water when in reduced form. However, O2 increasingly

present in sea water due to stromatolite photosynthesis.

=oxidised Fe and Mg (insoluble)=precipitation

Iron and Manganese layers interbedded with chert

preciptatesBanded Iron Formation (BIF)

Penge Formation (Transvaal).Asbesheuwels Subgroup

(GW)

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South Africa has large Iron and Manganese reserves (e.g. in the Northern Cape). Sishen

mine near KathuAlso Thabazimbi.

Whilst there is a large amount of dissolved (reduced) Fe and

Mn in the seawater, atmospheric oxygen cannot build up. Despite the high O2production by stromatolites,

the atmosphere remains without oxygen

BIF no longer appears in the stratigraphic record after about 2.0 Ga., as reservoir of Fe and Mn used up. Continental rocks change colour at this point in

time…..Oxygen in the atmosphere.

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Finally, sea began to retreat off the Craton at the end of

Chuniespoort / Ghaap times. Duitschland Fm (Tvl Basin): reworked Chuniepoort rocks

Koegas Subgroup (GW basin): Mixed clastic and

chemical sediments

Sea was retreating due to uplift: tectonism and erosion of the

Chuniespoort GroupMajor unconformity between

Chuniespoort Group and overlying Pretoria Group,

Koegas Subgroup and overlying Postmasburg Group

Deposition continued separately in the different basins. No longer a good correlation between the

different Transvaal basins. Pretoria Group (Tvl) and

Postmasburg Group (GW)

Pretoria Group (2350-2100 Ma) in Transvaal Basin

consists generally of Shale and Sandstone Formations, with some andesite. Up to

5km thick

The Pretoria Group was lain down due to two successive rifting events. During each

alluvial, volcanic and lacustrine rocks were

deposited, followed by thermal subsidence: deep marine

shales and coastal sandstones

Houtenbek, Steenkampsberg, Nederhorst, Lakenvlei, Vermont

Magaliesberg SstSilverton ShDaspoort Sst

Strubenkop Sh, Dwaalheuwel Sst, Hekpoort AndBoshoek Till

Timeball Hill ShRooihoogte Fm Sst

Low

er P

reto

ria G

roup

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Steeply dipping quartzite of the Magaliesberg Formation Magaliesberg Formation quartzite

The deep water sediments were lain down in an Epeiric

Sea.A shallow intracratonic

seaway (e.g. Black Sea)

The second rifting event was accompanied by a major andesitic

flood basalt. The Hekpoort Formation. Covered the craton.

Ongeluk Formation in GW basin.In Postmasburg Group, upper

Formations are chemical sediments (Hotazel and Mooidraai)

Also present within the Pretoria and Postmasburg

groups are diamictitesMakganyene Formation in GW

Boshoek Formation in Tvl

Diamictites contain at least two dominant

grain sizes. How can this happen? What does it tell us about the Pretoria Group

palaeoenvironment?

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Glacial palaeoenvironments have been demonstrated to

have occurred close to equatorial latitudes in the past, leading to the ‘snowball earth’

theory

Snowball earth accounts for the paired relationships of tillites and BIF even

after the 2.0 Ga ‘cut-off’.Oceans and atmosphere isolated due

to layer of ice. Photosynthesis reduced. Volcanic fumaroles emit Fe into sea water. No O2 in sea water.

CO2 from volcanoes builds up… greenhouse gas. Ice melts.

Atmospheric oxygen mixes with sea water….Fe precipitated

Summary ofTransvaal stratigraphyin the Transvaal basin

He uppermost group of the Transvaal is the Rooiberg

Group. At about 2053 Ma a mantle plume impinged under the middle of the Kaapvaal.

Partial melting of craton=granitic magma

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7km of acidic lavas (Rhyolite). Often called Rooiberg felsite (wrong name….felsite is an

intrusive rock)