Week 10: Flashcards
What is Stratigraphy?
The study of stratifies rocks (sediments and volcanics) in their sequence of time. The character and correlation of the beds.
A way of recognising sequences to predict positions of units
What are the types of rock depositions.
Supergroups - Rocks deposited during a single event.
Supgroup
Formation
Bed
What can you tell from a stratigraphic column?
Oldest at base.
Thickness same as height often.
Why must we know stratigraphy?
Roads often follow geological features. They give you a barrior or better material to build on.
Oldest geological feature? 3100 My.
Kaapvaal Craton, our basement rock. Contain mostly granite, quartz rich rock.
What else is sprinkled into the Kaapvaal Craton other than granite?
Greenstone belts.
What are the big geotechnical problem with the Kaapvaal craton?
Deep collapsible sands (very soft) on old erosion surfaces of granites.
Core stones associated with granites, can cause big problems.
Expansive clays associated with greenstones.
Highly variable conditions associated with greenstones.
Summarise the events.
3100 My Craton stabilised.
3074 My Rifting, thinning of the crust and sedimentation.
2970 My Witwatersrand Supergroup accumulates from
What is more stable the craton or much newer rocks in the Cape?
Older Craton is much more stable.
Younger Cape experiences much more tectonic action.
What are geotechnical problems relating to the Witwatersrand supergroup?
Subsidence, sinkholes, acid mine drainage and seismic events. Collapsible sands on the Northern side.
Deep, compressible soils in the graben (Joburg). Required putting in deep basements in order to pile solid foundation deep down, up to 60m.
Hard quartzite ridges, so no access.
Explain the Ventersdorp Supergroup.
When the Zimbabwe Craton collided with the Kaapvaal Craton causing lava to flow over an extremely large area. Andesitic and basaltic lava. Its non quartzite so wears away more easily.
3 main areas:
Semi arid conditions - No problems.
Sub-humid condition in the south of Joburg - highly expansive clay soils.
Sub-humid moist - Deep, highly compressible soils.
What should you beware of with quartzites?
Sulphides.
Economic value with Witwatersrand?
Gold and Uranium source.
Transvaal Supergroup.
Crust stabilised after Ventersdorp Supergroup volcanism and mountains created.
2650 My rifting of Kaapvaal Craton creating a trough which filled with sediment. Many shallow seas.
Quartzite then Dolomite then banded ironstones and manganese deposits and finally quartzite and shales with some andesite.
Economic value of transvaal supergroup?
Early gold mines in Black Reef quartzites. Dolomite makes a good aggregate and buffer in acidic environments. Binds well with cement. Slate used in floors and roofs when metamorphosed. Silica is extracted from quartzite. Largest source of manganese in the world. And iron.
What are some problems with dolomite?
Sinkholes and subsidence. (Has killed people). Still some problems with mining with some problems. Limited expansive clays. Collapsible sands on the North side of the ridges.
Bushveld Complex?
Largest Layered Igneous Complex in the World. When a Rhyolite erupted across the centre of the Kaapvaal Craton. Then basalt magma intruded below the rhyolite. Layered complex is formed. One of the rare formations where the older rock is at the top. Sitting on top of the Transvaal Supergroup.
Economic value of Bushveld complex?
Platinum, chrome and rhodium minerals. Marble. A good aggregate from some norites. Black granite from norite and gabbro.
Engineering problems with the Bushvelt complex?
Expansive clays, cause the most damage to structures. Associated with mafic rocks (gabbro, norite and anorthosite) called black turf.
Ultra basic rocks disintegrate very rapidly (pyroxenites).
Summarise events from 3500 My to 1500 My.
Formation of 3 continents. 1600 to 1400 My subduction zone forming an island arc belt. 700 My fragmentation occurred and deposition formed. Rifting caused seas to open up.
Malmesbury group and related granite intrusions.
600 My old. Incredibly complex structure with strong deformation and folding. Low grade metamorphism. Intruded by Cape Granite - resulting in mineral changes. Largely quartzites, arkoses, slates and conglomerates with some phyllites and limestones.
Characteristics of Mamesbury? NB to know for A2.
Slope stability - Failure along steep bedding/cleavage and highly jointed. Smooth bedding planes. Alkali aggregate reaction - when it reacts with concrete and forms alkali silicate gel and causes the concrete to pop. From the stress. No great economic value.
Cape Granite Suite characteristics? NB.
Intruded into the Malmesbury group. Forms dome shaped bare rock hills. Good aggregate source, beware of xenocrysts. Soil depth depends on climate and topography, shallow in North and deep in South, (Kaolin mining). Core stone development. Dispersive nature of soils.
Summary of events 500 My ago to 310 My ago?
500 My supercontinent formed, Gondwana.
450 My rifting began, crust thinned and sedimentation of the cape supergroup started.
310 My subduction zone developed along the southern margin of Gondwana.
