Abstract
M.Sc.
The sedimentary succession of the Paleoproterozoic Pretoria Group is very important for
understanding Earth’s ancient history. It represents a time of extreme environmental changes on
Earth, from global ice-ages to hot-houses. However, the genetic stratigraphy of the succession is
poorly understood so that the stratigraphic relationships between the events remain uncertain.
This dissertation provides a genetic stratigraphic model of the succession by utilising an
integrated sedimentological and geochemical approach which culminates in a new sequence
stratigraphic subdivision of the Pretoria Group. The study focuses on the Potchefstroom area in
the western part of the Transvaal depository.
The Pretoria Group commences with the Rooihoogte Formation which overlies the Chuniespoort
Group with erosional contact. New stratigraphic data indicates that the Rooihoogte Formation is
a correlative of the Duitschland Formation in the eastern Transvaal. The succession was
deposited in a foreland basin. An important new finding is that a diamictite at the base of the
formation contains striated and bull-nosed pebbles and is of glacial origin.
The discordantly overlying Timeball Hill Formation is composed of a coarsening upward
carbonaceous shale – hematite oolite-bearing quartzite unit overlain by a second carbonaceous
shale, capped by a second glacial diamictite (the well known Rietfonteindam diamictite). The
oolitic ironstones in the quartzites suggest that they formed in a warm oxidizing environment.
The shales display mature chemical indices of alteration which supports this theory. ä13Corganic
values increase from –35‰ to –24‰ from the bottom to the top of the Timeball Hill Formation
indicating net carbon burial, which translate to a decrease in atmospheric CO2 and colder
climates as deposition evolved. In turn this can be linked to the presence of the glacial
Rietfonteindam diamictite in the upper part of the Timeball Hill Formation.
The Rietfonteindam diamictite is overlain by conglomerate, quartzite and shale of the Boshoek
Formation, which were deposited as an upwards fining transgressive sedimentary unit following
on post-glacial eustatic sea-level rise. It is in turn overlain by the 2.22Ga. Hekpoort basalt. This
basalt is metasomatically altered, but has remained virtually unaffected by regional
metamorphism, as shown by detailed SEM petrographic analyses. Excellent examples of zeolite-
filled amygdales are preserved in the lavas.
The Hekpoort lavas are overlain by fluvial red beds of the Dwaalheuwel Formation. A lateritic
paleosol (Hekpoort paleosol) is developed below the red bed succession. The red beds are
overlain with sharp gradational contact by the carbonaceous shelf mudstone of the Strubenskop
Formation which grades up into the shallow marine Daspoort quartzite. The Silverton
Formation, mainly composed of carbonaceous shale, overlies the Daspoort Formation with
sharp gradational contact and grades upwards into shallow marine Magaliesberg quartzite.
ä13Corganic values decrease from –25‰ to –29‰, from middle to top of the Silverton Formation,
most probably indicating carbon input into the atmosphere and therefore rising atmospheric
temperature. The Machadodorp lava, which was previously thought to be restricted to the
eastern part of the Transvaal basin, was found to be present in the Potchefstroom area as well.
Five unconformity-bounded sequences are present in the succession. Estimates are that they were
deposited in time intervals of 60m.y. each.