GEOLOGY OF SOUTHERN AFRICA
Summarized notes : A guide to provide you with direction and a bibliography for further reading.
CONTENT: 1. PRINCIPLES OF STRATIGRAPHY
2. STRATIGRAPHIC NOMENCLATURE
3. THE STRATIGRAPHY OF SOUTHERN AFRICA: CRATONS, LIMPOPO BELT
4.GREENSTONE BELTS
5.IGNEOUS INTRUSIONS WITHIN GREEN STONE BELTS
Reference Material:
Hunter, D.R, Johnson, M.R., Anhaeusser, C.R., Thomas, R.J. (2006). Introduction. In: Johnson,
M.R., Anhaeusser, C.R. and Thomas, R.J. (Eds.) (2006). The Geology of South Africa. Geological
Society of South Africa, Johannesburg / Council for Geoscience, Pretoria.
McCarthy, T and Rubidge, B. (2005). The Story of Earth and Life. Struik Nature, Cape Town.
De Wit, M.J. and Ashwal, L.D. (1995). Greenstone belts: what are they? S. Afr. J. Geol., 98. 505-
520.
De Wit, M.J., Jones, M.G., and Buchanan, D.L. (1992). The geology and evolution of the
Pietersburg greenstone belt, South Africa. Precambrian Res., 55, 111-122.
1
,PART A
PRINCIPLES OF STRATIGRAPHY
In sedimentary (and pyroclastic) rocks, individual rock layers are called strata and layering is known
as stratification. However, the word “stratigraphy” is very occasionally and informally also used for
layered igneous rocks such as the Bushveld Igneous Complex.
Stratum – singular
Strata – plural
Sedimentary strata.
Bedding Plane
The smallest division in a sedimentary rock and refers to the surface which separates one stratum
from another.
2
, Unconformity
Gaps in the geological record which indicate erosion or non-deposition (a hiatus). It represents a gap
in geological time.
Siccar Point in Scotland. A classic unconformity first described by James Hutton in 1788. (Jones CC
General Geology)
Principle of Uniformitarianism
This states that “the present is the key to the past”. In other words, the same natural laws which
operate in the universe today have always operated the same in the past. Geologists now realise
that this is not entirely true. The early atmosphere, for example, had quite a different composition to
the atmosphere today, for instance there was no free oxygen in the early atmosphere, and this
allowed the deposition of gold deposits in Archaean times. This could not happen today because the
gold would be oxidised. Oxygen levels started to build up only after photosynthetic plants appeared.
Principle of Superposition
This is the fundamental concept of stratigraphy, which states that the youngest layer of rocks is at
the top and the oldest at the bottom. This assumption can be overturned (literally) by tectonism. In
this case, various “way up criteria”, usually sedimentary structures, can be used to establish the
correct way up. For instance, worm burrows and other trace fossils go down, not up. Rock fragments
from older strata may be incorporated in younger strata, but clearly not the other way around.
3
Summarized notes : A guide to provide you with direction and a bibliography for further reading.
CONTENT: 1. PRINCIPLES OF STRATIGRAPHY
2. STRATIGRAPHIC NOMENCLATURE
3. THE STRATIGRAPHY OF SOUTHERN AFRICA: CRATONS, LIMPOPO BELT
4.GREENSTONE BELTS
5.IGNEOUS INTRUSIONS WITHIN GREEN STONE BELTS
Reference Material:
Hunter, D.R, Johnson, M.R., Anhaeusser, C.R., Thomas, R.J. (2006). Introduction. In: Johnson,
M.R., Anhaeusser, C.R. and Thomas, R.J. (Eds.) (2006). The Geology of South Africa. Geological
Society of South Africa, Johannesburg / Council for Geoscience, Pretoria.
McCarthy, T and Rubidge, B. (2005). The Story of Earth and Life. Struik Nature, Cape Town.
De Wit, M.J. and Ashwal, L.D. (1995). Greenstone belts: what are they? S. Afr. J. Geol., 98. 505-
520.
De Wit, M.J., Jones, M.G., and Buchanan, D.L. (1992). The geology and evolution of the
Pietersburg greenstone belt, South Africa. Precambrian Res., 55, 111-122.
1
,PART A
PRINCIPLES OF STRATIGRAPHY
In sedimentary (and pyroclastic) rocks, individual rock layers are called strata and layering is known
as stratification. However, the word “stratigraphy” is very occasionally and informally also used for
layered igneous rocks such as the Bushveld Igneous Complex.
Stratum – singular
Strata – plural
Sedimentary strata.
Bedding Plane
The smallest division in a sedimentary rock and refers to the surface which separates one stratum
from another.
2
, Unconformity
Gaps in the geological record which indicate erosion or non-deposition (a hiatus). It represents a gap
in geological time.
Siccar Point in Scotland. A classic unconformity first described by James Hutton in 1788. (Jones CC
General Geology)
Principle of Uniformitarianism
This states that “the present is the key to the past”. In other words, the same natural laws which
operate in the universe today have always operated the same in the past. Geologists now realise
that this is not entirely true. The early atmosphere, for example, had quite a different composition to
the atmosphere today, for instance there was no free oxygen in the early atmosphere, and this
allowed the deposition of gold deposits in Archaean times. This could not happen today because the
gold would be oxidised. Oxygen levels started to build up only after photosynthetic plants appeared.
Principle of Superposition
This is the fundamental concept of stratigraphy, which states that the youngest layer of rocks is at
the top and the oldest at the bottom. This assumption can be overturned (literally) by tectonism. In
this case, various “way up criteria”, usually sedimentary structures, can be used to establish the
correct way up. For instance, worm burrows and other trace fossils go down, not up. Rock fragments
from older strata may be incorporated in younger strata, but clearly not the other way around.
3
