THEME 6: The Transvaal Supergroup

Transcription

1 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. The Transvaal Supergroup To the North the Thabazimbi Murchison Lineament, and Sugarbush Lineament to the south formed the boundaries to the Wolkberg Group Basin Duitschland Fm Penge Fm Malmani Subgroup Rooiberg Group Angular unconformity Pretoria Group Angular unconformity Chuniespoort Group Black Reef Formation Angular unconformity Protobasinal rocks: Wolkberg, Godwan, Bloempoort, Buffelsfontein groups Once again, old sutures that initially formed during the assembly of the Kaapvaal were reactivated, so formed the borders to sedimentary basins 1

2 Sand and mud washed into the Wolkberg trough. The thick accumulations of sandstones have now been metamorphosed to quartzite a very resistant rock Outcrop localities of the Protobasinal rocks W Bu Bl G After 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 Basin The Kanye basin (in Botswana) The are filled with volcanosedimentary strata of the Transvaal Supergroup Tranvaal Basin Griqualand West Basin 2

3 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 CO 2 from sea water, production of O 2, precipitation of CaCO 3 (limestone). Calcium Carbonate deposits are covered by annual growth layer of cyanobacteria. In the Archaean and Shark Bay, no gastropods 3

4 Requirements for stromatolite growth: 1. Shallow water (<100m) 2. Low rate of clastic sediment input 3. No grazers The Chuniespoort Group (and Ghaap Group in the Griqualand West Basin) is thus made up of thick accumulations of Dolomite (Mg replaces Ca) Ga Chert (SiO 2 ) 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, O 2 increasingly present in sea water due to stromatolite photosynthesis. =oxidised Fe and Mg (insoluble)=precipitation Iron and Manganese layers interbedded with chert preciptates Banded Iron Formation (BIF) Penge Formation (Transvaal). Asbesheuwels Subgroup (GW) 4

5 South Africa has large Iron and Manganese reserves (e.g. in the Northern Cape). Sishen mine near Kathu Also Thabazimbi. Whilst there is a large amount of dissolved (reduced) Fe and Mn in the seawater, atmospheric oxygen cannot build up. Despite the high O 2 production 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. 5

6 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 Group Major 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 ( 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 Lower Pretoria Group Houtenbek, Steenkampsberg, Nederhorst, Lakenvlei, Vermont Magaliesberg Sst Silverton Sh Daspoort Sst Strubenkop Sh, Dwaalheuwel Sst, Hekpoort And Boshoek Till Timeball Hill Sh Rooihoogte Fm Sst 6

7 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 diamictites Makganyene 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? 7

8 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 O 2 in sea water. CO 2 from volcanoes builds up greenhouse gas. Ice melts. Atmospheric oxygen mixes with sea water.fe precipitated Summary of Transvaal stratigraphy in 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 8

9 7km of acidic lavas (Rhyolite). Often called Rooiberg felsite (wrong name.felsite is an intrusive rock) 9