Summary of geological history of SA
Bredasdorp Group
Most Kimberlites
Karoo Supergroup
Cape Supergroup
Cape Granite Suite
Malmesbury and Cango Caves Groups
Pilanesberg Complex (1400 Ma)
Bushveld Complex (2050 Ma)
Transvaal Supergoup
Witwatersrand Supergroup
Barberton greenstone belt
Swaziland ancient gneisses (3500 Ma)
Sand River
gneiss near
the Limpopo
River
3.5 billion year old pillow lavas in Barberton Group
Kaapvaal Craton
Fossil stomatolites provide earliest evidence for life on Earth.
Found in Barberton region of RSA in rocks 3.2Ga in age.
Oldest are 3.8 Ga in age.
Unchanged since Archaean –
Sharks Bay W Australia
Witwatersrand
Supergroup
3.0 to 2.7 Ga
World’s largest
gold deposit
Grains of gold in Witwatersrand conglomerate
Ventersdorp
Supergroup
2714 Ma
Basalt lavas
Base of the Ventersdorp
Klipriviersberg south of Johannesburg –
basalt lavas
Big Hole at
Kimberly
The solid
steep cliff
above the
water is
Ventersdorp
lava
Transvaal
Supergroup
2.65 – 2.05 Ga
Pretoria Group (shales, sandstones)
Chuniespoort Group (limestone,
dolomite)
Black Reef Formation (thin)
Modern Distribution Of Transvaal SG
Chuniespoort Group
• 2.4-2.6 billion yrs old, ~1 km thick unit of
carbonate rocks: limestones =>50%
CaCO3 (calcite) & dolomites = >50%
CaMg(CO3)2 (dolomite) (Malmani
Dolomite)
• Formed by colonial cyanobacteria as
stromatolites – their shape was
dependent on the water depth & energy
levels of the depositional environment
Fossil stromatolites in the carbonates
Unchanged since Archaean –
Sharks Bay W Australia
Formation of
dolomite
•
•
•
Chemical process in which part of the Ca is replaced
by Mg = CaMg(CO3)2
Usually occurs shortly after deposition and is related
to groundwater movement in nearshore areas
Dolomitization also leads to silica precipitation –
inorganic chert nodules are common – differential
weathering & elephant-skin
The Winter Park Sinkhole, Florida
SEFFNER, Florida (CNN) —
The ground just swallowed
him up.
A Florida man fell into a
sinkhole that opened
suddenly Thursday night
beneath the bedroom of his
suburban Tampa home,
calling out to his brother for
help as he fell, the brother
said Friday.
“I heard a loud crash, like a car coming through the house,”
Jeremy Bush told CNN affiliate WFTS. “I heard my brother
screaming and I ran back there and tried going inside his
room, but my old lady turned the light on and all I seen was
this big hole, a real big hole, and all I saw was his
mattress.”
Guatamala is the world
sinkhole capital
The transport of sediment (e.g.
sand) parallel to the shoreline,
mainly in the surf and swash
zone.
Maquoketa Caves, Iowa
The transport of sediment (e.g.
sand) parallel to the shoreline,
mainly in the surf and swash
zone.
Pretoria Group
• ~2.35 billion yrs ago : major angular
unconformity  tilting, weathering and
erosion of underlying BIFs and
dolomites
• New sequence of mudstones,
quartzites >> basalt = 5 km thick
• Depositional environment similar to
Chuniespoort Group: muddy tidal flat
and some (much less!) stromatolites
• Source area to the north (grain size
decreases towards south)
S
Magaliesberg Sandstones North of Pretoria
N
PRETORIA GROUP
Fault Block
Bushveld
Igneous
Complex
Hartebeespoort Dam
From: The Story of Earth and Life by McCarthy and Rubidge (2005) Struik
Photo: Grant Cawthorn
Blyde River Canyon – section through the Transvaal
Supergroup
Sishen Iron Ore Mine
Banded Iron Formation (BIF); oxidation of
Fe2+ in ocean water as O2 in atmosphere
increased, precipitated as hematite (Fe2O3)
Banded Iron Formations
Single celled –
complex cell
The Bushveld Complex (2059 Ma)
Bushveld
Gabbros
Sandsloot open cast PGE mine, 30 km N of Mokopane
The Bushveld contains 85% of the world’s PGE (Ru, Rh, Pd,
Os, Ir, Pt) as well as significant reserves of Cr, V, Fe, Sn
UG1 chromite
Merensky Reef, basal thin chromite and pyroxenite above a
norite footwall, Western Bushveld
Merensky Reef, basal thin chromite and pyroxenite above a
norite footwall, Western Bushveld (average of 10 ppm PGE)
The Vredefort
Dome
A 2.02 Ma
meteorite impact
UZ Main Magnetite layer, Magnet Heights, Eastern Bushveld
The Vredefort Dome is
the largest and oldest
(2.02 Ga) meteorite
impact structure on Earth
Bushveld Complex 2059 Ma
Witwatersrand Basin (31002700 Ma)
Vredefort impact
2020 Ma
Vredefort impact
Cape Town
Collar
Core
Geological map of the Vredefort Dome
Evidence for impact origin:
Vredefort is the type locality of
‘pseudotachylite’ (Shand 1916), and is one
of the two great pseudotachylite provinces
of the world (the other is the Sudbury impact
structure). Formed by frictional melting.
Shatter cones
PDF’s (Planar Deformation Features on a
microscopic scale in quartz)
High-pressure minerals (stishovite)
Shatter cone in Hospital
Hill quartzite (Lower
Wits) in northern collar
Two examples of PDF’s in quartz
from gneisses from the centre of
the structure
Photos from Gibson & Reimold, 2001, Memoir 92, Council for Geoscience
Collar rocks form a ring of hills
around core in N and W of dome
Looking W from near
Vredefort town
Class Test 2 has been marked and will be
given back in next weeks pracs
Everybody in GEO1008F gets a DP
Exam (3 hrs) will be at 12.30 pm on Tuesday
June 3rd in Beattie LT
Everybody needs to check that their class
marks have been entered correctly before
the exam. I will try and do this using the
Gradebook function in Vula
Main melt (pseudotachylite)
‘generation surface’
Some photos of the
granitic gneisses of
the core – Otavi
Quarry
The collar rocks
Quartzites dipping towards the core; direction of younging
away from the core – they are overturned!
Cross section through the Vredefort Dome
Tectonic domains of Southern Africa
1000-1100 Ma
Gneiss in
Namaqualand
High-T
metamorphic
rocks
(>700oC)
Folded Otavi Group, NW Namibia
Namaqua-Natal belt gneiss overlain by Nama
Group
Folded Nama Group dolomite, Orange River
Dolomite
quarry at
Robertson
Used to
make lime
TMG – Malmesbury
unconformity on Tafelberg Road
Granite
(540 Ma)
Saldanha
Gabbro
(540 Ma)
Yzerfontein
Layers in Peninsula
Granite at Llandudno
Table Mountain Group from top of
Matroosberg
N
C
Pak
P
Same sequence in the Koo Valley,
upside down
‘The Baths’ hot spring near Citrusdal
Nardouw SG
CF
shale
Peninsula Fm
Olifants River Valley syncline
Karoo
Supergroup
300-179 Ma
Laingsburg
Fish River Mouth
Laingsburg
Karoopoort (Tankwa Karoo)
Late Paleozoic Glacial Deposits
~350 Million Years Old
Coal deposits at Middelburg (Ecca
Group 250-280 Ma)
Clarens Formation - aeolian (wind
deposited) sandstone
Clarens Formation overlain
by basalt lava
Drakensberg Group (basalts and rhyolite 180 Ma)
Kimberlite
pipe
Most are
around 90
Ma in age
Mantle peridotite
xenolith
Roberts Victor Mine – one of many
abandoned diamond mines near Kimberley
Enon Group (140 Ma); sanstones
conglomerates. Red colour indicates
arid conditions
Robberg Fm. At Mossel Bay. Angular
unconformity between sandstones &
conglomerates and Tamble Mountain Group
(below)
Robberg Formation 130-110 Ma;
sandstone and conglomerate
High-energy environment
Bloukrans Bridge on the Garden Route
Movement of the
Cape Peninsula
shoreline
Changes in sea level
from the build up and
melting of ice sheets
Sandveld Group
limestone,
Langebaan Lagoon