Tlhomamo Ogone Blom
221053119
19 August 2023
Field report 2
This field trip was taken on the 16th of August 2023. My classmates and I, together
with our lecturer and two demonstrators, visited many localities, exploring rocks of the
Transvaal Supergroup, along the R512 to Hartebeespoort Dam
The first rock encountered at our first stop were granitoids of the Johannesburg Dome
that resembled sandstones. The Johannesburg Dome is Paleoarchean-Mesoarchean
in age and includes granitic rocks, gneisses and greenstones (Anhaeusser, 2015).
Therefore, these granites form part of the Archean basement. The granites were
coarse, moderately weathered and the presence of multiple quartz veins is evidence
that they underwent some level of deformation, likely shearing.
These were overlain by conglomerates, of which we saw several scattered boulders.
The conglomerates, as seen in Figure 1 below, were polymictic as the pebbles were
made of both K-Feldspar and quartz and the matrix, in the seen boulders, was sand
sized. They were also moderately weathered. The conglomerates varied, in that some
boulders were clast supported (a higher percentage of clasts compared to matrix), and
some were matrix supported (the pebble sized clasts, ranging from 5mm-50mm were
minimal, compared to the matrix). The quartz pebbles were a milky-dark grey colour,
and this indicates that they are vein quartz, likely deposited by precipitation.
Figure 1: Moderately weathered polymictic conglomerate boulder.
Along the traverse, just a distance from the scattered conglomerate boulders, we
encountered sandstones. These were moderately weathered, with the weathered
surface a distinct red, almost orange colour.
The sandstone outcrop was relatively continuous, forming a very small hill. The bottom
of this sandstone outcrop had asymmetrical mega ripples, similar to dunes, as seen in
Figure 2 below.
Figure 2: mega ripples at the bottom of a sandstone outcrop.
These mega ripples are likely current ripples. Despite them being asymmetrical, they
had a lee and stoss side which is characteristic of current ripples, and evidence that
they formed near the coastline. The lee side was towards the East, and therefore the
deduction was made that the water current flowed from the East to the West.
The mega ripples were a distinct black colour, as seen in figure 2, and this is because
of thin mud drapes that are unfortunately too small to be seen on the figure. These
mud drapes formed when the sandstone was covered my mud. The mud then,
gradually, fossilised and stuck to the mega ripple.
The sandstone outcrop was also populated by quartz veins, as seen in Figure 3 below.
These veins were all oriented in different directions.
Figure 3: quartz veins on a sandstone outcrop
The sandstone outcrop underwent folding, together with the mud drapes. The quartz
veins were parallel to the limbs of the fold, and all pointed towards the fold axis. Some
parts of the outcrop had echelon veins/ gashes (Figure 4) that were be cut off, likely
due to the deformation that occurred. These serve as evidence of shear faulting having
occurred.
Figure 4: Tension gashes on a sandstone outcrop.
The sandstone beds were dipping to the North, at about 50°.
The next rock encountered on the traverse, as we were heading back to the cars, was
highly weathered slate, seen in Figure 5 below.
Figure 5: highly weathered slate.
The slate had detrital mica grains that were fine grained and aligned in a manner that
gave the rock a slaty cleavage, along which the rock broke into slabs. The presence
of the detrital mica grains is what enabled us to determine that it was indeed a slate,
and not a shale, that had undergone low grade metamorphism, likely in the greenschist
facies.
The rock possesses both foliation and bedding and on a specific rock sample, there
was a lineation that clearly separated the bedding and foliation. In comparison to the
bedding, the foliation was a bit shallower. The slate outcrop preserved primary
structures in the form of planar laminations.
Between the sandstone and slate outcrop, there was a definite change in depositional
environment. Where the sandstone was deposited in ahigh energy, the protolith to the
slate, likely a shale, was deposited in a low energy environment, owing to sea rise.
This must have been below the wave base, so at approximately 50 m or more.
The wave base is defined as “the downward unit to which waves can move bottom
particles, the depth where wave motion can no longer be appreciated” (Fairbridge and
Dietz, 1968).
We then encountered chemical sedimentary rocks, specifically dolostone(dolomite)
that contained stromatolites, seen in Figure 6 below.
Figure 6: dolostone with visible stromatolites.
The stromatolite domes varied in size, from large ones to small ones. The larger ones
are evidence of deeper water. This is because for stromatolites to thrive, they need
low siliciclastic input, enough sunlight to penetrate. The big domes could be evidence
of them adapting and “making themselves bigger” so that they could get enough
sunlight in order to thrive and not die out. The ones with bigger domes could have
been formed in a similar water depth to the slate, just with lower siliciclastic input,
The idea that these were deposited in similar environment to the protolith of the slate
is further supported by the fact that along the traverse, there were alternating
packages of both the slate and dolostones.
These are a part of the Oaktree Formation, that is a part of the Malmani Subgroup.
We then made a road stop, where next to the road, we encountered dolomites again.
However, these were lighter than the ones that have been described above, and they
contained ooids (+/- 1mm) that formed oolites on the surface of the rock. The colour
contrast is because the latter weren’t as iron rich, as the dolomites described from
Figure 6 above. The stromatolite domes were also larger, and the rock outcrop had
wavy internal laminations.
Another distinguishing factor was that the stromatolites here could be used to
determine younging direction, as the beds were turned. The younging direction (right
way up) was to the North-East. The outcrop also underwent post depositional
silicification, possibly during diagenesis, leading to chert layers being deposited in
between the laminations. These dolomites are part of the Monte Christo Formation.
The next rock encountered, after a short drive, was sandstones of the Pretoria Group,
particularly the Timeball Hill Formation.
The sandstones were relatively fine grained, and moderately weathered. The
weathered surface had a red-purple colour while the fresh surface was a blue-dark
grey colour. This rock outcrop contained current ripples, that were symmetrical with
most of them having linear crests. The rocks were also internally laminated.
Even though rocks of this outcrop possessed the distinct red colour, they would not be
red beds as this is a term used to describe sedimentary rocks that contain ferric oxides,
which didn’t seem to be present in this outcrop.
Our last stop of the day brought us to the Magaliesburg quartzites, specifically the
Magaliesburg Formation, that stratigraphically, are below the Bushveld Igneous
Complex. The quartzites lie near the top of the Pretoria Group of rocks. Between the
quartzites and the Igneous Complex, there is an intrusive contact. As a result of the
intrusive contact, we deducted that the protolith was a sandstone that due to contact
metamorphism, that occurred approximately 2.1 billion years ago, recrystallised into
the now visible quartzites.
The quartzite outcrop was laterally continuous, with symmetrical ripples that continue
extensively. From this, the deduction was made that the depositional environment for
these quartzites was likely an intertidal environment (tidal flats). The outcrop was
coarse grained, with grains ranging from 10-15mm.
REFERENCES:
Anhaeusser, C.R. (2015) ‘Metasomatized and hybrid rocks associated with a
Palaeoarchaean layered ultramafic intrusion on the Johannesburg Dome, South
Africa’, Journal of African Earth Sciences, 102, pp. 203–217.
doi:10.1016/j.jafrearsci.2014.10.012.
Dietz, R.S. and Fairbridge, R.W. (1968) ‘Wave base’, Encyclopedia of Earth Science,
pp. 1224–1228. doi:10.1007/3-540-31060-6_404.