Proud of our Karoo
The Karoo Basin has been the focus of
international attention over the past few years, but not just because of the heated
fracking debate. These rocks have a unique picture to offer the world. Around
the globe, other basins like the Karoo are the source of oil and gas that allow
your and my daily life to function. However, the internal structure of these
basins is often complicated by other factors, which in turn affects the extraction
of these natural resources. This is where our own Karoo basin comes in to play.
Around 300 million years ago a trip to South
America, the Antarctic or Australia would have involved an overland drive, as
all of these continents were merged into one big land mass. A large portion of
this area, including the majority of South Africa, was covered by a large
inland sea (so maybe a boat would have been more appropriate for the trip).
Over time the basin filled up with sediments that were carried in by glaciers
while the landmass was over the Polar Regions, and eventually rivers and
windblown sand as the continents drifted to hotter latitudes. All of this was
over a period of about a hundred and twenty million years.
After this the huge continental landmass
started to break up with the crust weakening under the stress. This paved the
way for magma from deep within the earth to travel up to the surface. Along the
way some of this magma got trapped between layers of these sediments and formed
what geologist call sills (horizontal magma tubes) and dykes (vertical magma
tubes). The magma that wasn’t trapped poured out onto the surface of the earth
forming a lava layer more than 1.8 km thick at a rapid rate of about 3 000
years (well, rapid in geological time).
The lavas used to cover the majority of South
Africa but due to uplift and erosion by rivers such as the Orange River, today
we see only a small remnant of these
lavas in Lesotho and elsewhere. A large portion of the sedimentary rocks in the
Karoo basin were also removed and in turn these dykes and sills were unearthed.
And it is these bodies that scientists from abroad have come to the Karoo to study.
The majority of these sills now form the tops of the flat-topped hills you see
when driving from Johannesburg to Durban (like the ones you always enjoy just
outside of Harrismith after your Wimpy breakfast pit-stop). These unique exposures
allow scientists to see with their own eyes the remnants of processes that are
otherwise taking place at great depths in the crust.
While
these on surface observations are indispensable, seismic data provide the best
picture of the rest of the basin that remains buried below our feet. Using this
method, energy as vibrations is transmitted into the earth. At certain layers within
the earth this energy bounces back to the surface. The time taken for this
energy to reach receivers at the earth's surface allows scientists to calculate
depths to these layers. Drilled boreholes in turn allow scientists to know
which layers will be strong reflectors, and to confirm seismic results. Soekor,
the forerunner to the Petroleum Agency of South Africa, collected over 13 000
line kilometres of these seismic data in the Karoo in the 1960s and 1970s. At
present these old, low resolution data are the only data scientists have to use,
as research organizations do not have the
finances or capacity to collect new data over the entire basin. The interest
from industry in the Karoo would provide an opportunity for higher resolution
data to be collected, and allow us to better understand the basin. This would provide
the country with a not-to-be-missed opportunity to quantify what shale gas resources
we have, if any.
These existing seismic data show us that these sills are
shaped like saucers, with diameters of between 5 and 30 km. While similar flat
sheets that form at greater depth (up to 5 km) can stretch for up to 150 km.
These sills have preferential been injected along certain layers, the organic-rich
shale layer in the Karoo sadly being one of them. Scientists have shown that
when the rising magmas
came into contact with these shale layers, large amounts of carbon-dioxide were
“burnt off” and often released explosively. This release of large volumes of
gas in turn led to 6⁰C global
warming and the extinction of marine species. This loss of gas will ultimately
affect whether the Karoo is found viable for shale gas exploration or not. If
too much gas has been released then fracking might not be profitable. On the
flip side, the magma might have heated up some of the shale layers, bringing
them to the correct maturity level for fracking. As this shale gas is 50%
cleaner than coal, it provides a stepping stone for South Africa towards
renewable energy…because let’s face it, we are all quite tired of load shedding
In addition, investigating the fracture
systems linked to these sills and dykes is of vital importance when searching
for ground water. In the Karoo this water is a lifeline for local communities,
and drilling borehole into these fractures can help with groundwater exploration.
These fracture systems will also need to be understood in order to avoid
groundwater contamination if fracking takes place. Companies are thankfully aware
of such complications and will steer clear of large concentrations of these sills
and dykes because of the added complications.
So our Karoo is famous for more than just
boiling hot days, wind mills and straight stretches of road. This massive basin
is helping to shed light on unanswered geological questions around the globe, another
reason to stamp it with the "Proudly South African" label.
For more information on the topic of fracking
check out Ivo Vegter’s book “Extreme Environment”, or the easy to read 2011 scientific article
by Prof. Maarten de Wit titled “The Great Shale Debate”.
- S.E. Scheiber-Enslin
