The team completed the first section of the Field School
today as they travelled from the overnight stop of Laingsburg to Cape Town.
They will now swap the nomadic lifestyle of uncovering the geology of South
Africa for a short course on remote sensing, further preparation for looming
field work and last, but not least, a closer look at the fairest Cape’s
geology.
After having looked at the top of the Karoo on day 3 (e.g.
in the Golden Gate Highlands National Park), the team had an opportunity to now
investigate its lowermost successions. This was of major interest because some
of these units include the highly prospective black carbonaceous shale. After
scrambling to the top of one of the many hills around the Laingsburg region,
the team had a look at the Matjiesfontein Chert bed. This ca. 60 cm chert bed
forms a prominent marker horizon separating the White Hill from the overlying
Collingham Formation. Below the Whitehill we found the Prince Albert. Together
these three units share relatively high total organic carbon content, with the
lions share going to the White Hill. The team further considered various
aspects of structural geology at play, with the fissile black shale showing
peculiar pencil cleavage planes. The intersection of these planes results in
the shale breaking apart into needle-like splinters. Tiny crystal of the
mineral pyrite was common in most of the black shale. This signifies the effect
of sulphur-releasing bacteria assisting in deconstructing of organic material (i.e.
plant and animals reducing in anoxic deep marine environments). The release
sulphur would ultimately react with iron in the sediment and form pyrite.
After three days, the final stop within the Karoo Supergroup
was symbolically, to the bottom Dwyka Group. The team initially thought that it
looked very much like a volcanic rock of some sort, similar ideas shared with
some of the first geologists who had interpreted this rock many years ago. However,
the team would soon realise that the Dwyka was not volcanic, nor the result of
cataclysmic meteorite impact, but rather deposited by a glacier. Polar wander
theory would show that the South Pole was located near the location where the
Dwyka would have been deposited, with global-scale ice sheets advancing and
retreated, picking up foreign clasts from several thousand kilometres away; eventually
forming the famous Dwyka Tillite. This was also of particular interest because
in the Gariep the team will be seeing a similar tillite of different age.
Several ice-ages?
When one door closes, another opens, and behind the second
door was the Cape Supergroup. There was no respite for the team as they were
thrown into the deep end of classical structural geology. Driving south, along
the Seweweekspoort Pass, the team entered the Witteberg (i.e. the topmost of
the Cape Supergroup) and were once again totally awestruck by the level of
deformation shown. As we continued south, and west toward Cape Town we were
really treated to some of the best Alpine-style continental building demarking
thrusting and immense folding along the way. The Cape Fold Belt (CFB) was
formed as a consequence to the amalgamation of Gondwana. Of interest is the
fact that the CFB is located more than 1000 km away from the continental
margin, however strain was able to accumulate substantially enough to form the
exceptionally large mountains seen. How would this have happened?
Kogmanskloof, now geoheritaged in the Western Cape
Within the CFB, the team noted several hot springs aligned
major thrust faults. This provides evidence suggesting that this mountain belt
has high heat flows. Groundwater aquifers are recharged from the high mountains
and circulated by an intricate fault network. The groundwater is heated thanks
to the effect of the Cape Granite. Being relatively young ca. 550 million years
old, the granite provided a rich abundance of heat producing elements (i.e.
uranium, thorium and potassium). These elements would slowly undergo radiogenic
decay and emit heat. Heat, now enjoyed by thousands visiting the various spas
in the southern Cape!
The team will now focus on the mapping to come. Tomorrow
will be spent performing remote sensing and trying to delineate different
potential geological units in the Gariep Field area prior to arriving there on
Sunday.
Check out todays route here: Day 4
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