WHY IS THE CERES / TANQUA KAROO IMPORTANT FOR THE GEOLOGIST?
Our planet is alive and its heart beat can be noticed occasionally.
The surface skin (10-60 km thick) consists of floating continents and
oceans. Continents occasionally break apart, forming oceans, or weld
together, forming mountains and deep troughs. Most of those movements
result in volcanic eruptions and earthquakes. This brief introduction
will help to understand what we presently see in the Tanqua Karoo and
its surroundings.
Between 278 and 230 million years ago four major mountain building
periods occurred, resulting in the east-west oriented Swartberg with a
trough (Laingsburg Basin) directly north of it. That mountain building
also formed the Cederberg and the Hex Rivier/Baviaanskloof Mountains.
The Tanqua Basin was formed during that period by the movement of the
Cederberg and the Hex Rivier Mountains pushing the surface down and
starting a shallow 500-700 meter deep basin. All these movements are the
result of pushing and pulling of the southern margin of the
supercontinent, called Gondwana. That large super-continent consisted of
what are now known as southern Africa, southern South America, Falkland
Islands, East Antarctica, and several smaller pieces.
Movements of continents over the surface of the Earth result in
variations of the amount of heat from the sun onto the land, which
causes ice ages and warm periods with desert climates. Between about 360
and 286 million years ago the southern part of Gondwana became the
victim of a long ice age (Dwyka glaciation), covering from East
Antarctica, across southern Africa to southern South America. Melting of
the ice resulted in thick (800 meters) glacial Dwyka deposits and a sea
level rise of about 100-150 meters. That cold time was succeeded by the
Ecca period (a duration of about 28 million years) during which the
large Karoo Basin formed, stretching from east of Johannesburg to the
Karoopoort northeast of Ceres. Much of the Karoo Basin became filled
from the northeast with delta deposits in which major coal beds were
formed. Any large basin has an irregular floor, resulting in different
types of sub-basins and deposits. The location of the still submersed
Cederberg and Hex Rivier-Baviaanskloof Mountains formed a stiff block
against which the southwestern part of the Karoo Basin moved down for a
while. During that time of increasing water depth mainly clays were
deposited, followed by the Tanqua Karoo deepwater sand deposits. After
that the basin floor moved up, and first clays were deposited followed
by the 200 meter thick sand-rich deltas as we see in the Koedoes
Mountains. The delta deposits are overlain by river deposits and those
by other land deposits. A total of 7-8 km of rock covered the deepwater
sands. The weight of that cover is reason that any oil or gas that may
have been present in the deep water and delta sediments has been cooked
out or pressed upward into overlying sediments that were later removed
by erosion.
Although most of the sediment filling the Karoo Basin, came from the
northeast, the southwestern part received its sediment from the
south-south-west from the forerunners of the Andes, a distance of at
least 600-800 km away. During that long transport any coarse sediment
was left behind and only fine sand and finer material reached the Tanqua
Basin. Major deltas moved from the southwest to the northeast across
shallow sea bottoms. Gradually those delta and river deposits moved
north-north-east, covering the earlier deposited deepwater sands. Later
erosion removed all of that overburden, except what is left on the
Koedoes Mountains.
The Tanqua Karoo deepwater sands can be divided into five units (fans),
separated by thick shales (schalies). Their total thickness is about 250
meters. To the southeast of Bizansgat is a sixth fan which is separate
from the other five fans by a major fault. The five fans differ in
thickness (30-60 m), and each one gradually thins the further it flowed
into the basin. The middle one (Fan 3) crops out in the cliff along the
Ongeluks River. The direction of sediment transport was to the
north-north-east. At Vaalfontein it terminates. Fans 1 and 2, numbered
from older to younger, only reveal their last part of the fan. The other
parts were located south of Ongeluksrivier and have been eroded away.
Fan 4 came from the west which means that an entire delta, located a few
hundred kilometers to the west when the Ceder Mountains and Swartberg
were still under water, had shifted to the north. Fan 5 came again from
the south. Each fan moved further into the Tanqua Basin than the older
ones. The fine-grained sand of a fan was transported from the shallow
sea bottom (shelf) via the slope to the flat floor of the basin by
turbulent flows (turbidity currents) that were strong enough to keep
most of the sand in motion. As soon as a turbidity current reached the
basin floor the initial fallout of sand began.
The entire complex of transport and deposition started with a delta that
followed the coast line across the shelf when sea level started to fall.
Once that lowering stopped sand piled up at the end of a channel. Rapid
deposition of that sand made it unstable which resulted in sliding away
(slumping). A large piece of the delta broke off and slid down across
the slope toward the basin. During that transport the sedimentary slump
carved out a canyon across the slope. As soon as that sediment mass
reached the basin it slowed down and it started to lose some sediment.
Initially a bundle of channel fills resulted (example: Fan 3 at
Ongeluksrivier) at the mouth of the canyon. It soon changed into a
channel with levees (examples: Kleine Rietfontein, along the Gemsbok
Rivier, Bloukop). Gradually those channels became shallower and the
levees could not constrain most of each turbidity current and the
deposition changed to oblong, rather thin sheet sands (examples:
Klipfontein, Vaalfontein). The above is a simplified version of the
reality.
When sea level started to rise again the coastlines moved back and sand
transport across the shelf stopped. Only muds could still be moved to
deep water. Those dark colored, very fine-grained sediments commonly are
darker than the sands because of the chopped-up plant fragments they
contain. Later the sands, when covered with younger material, became
sandstones, while the muds became shales. The plant fragments can become
coal under the weight of the younger sediments laying on top. Some plant
fragments and algae may transfer into oil or natural gas. Normally the
quantities are too low to even offset the costs of drilling. Deepwater
sands can be fantastic oil reservoirs, sometimes the sands are
disconnected and oil has no chance to flow from one layer to the next
one, and sometimes there is no oil or gas in them. Most of the organic
material that generates oil or gas comes from underlying formations that
once were shallow, such as lagoons and bays.
The shales have a possibility to become a source of natural gas. At this
time we only know it from the deepwater shales. They are constructed
from more than 75% dust-sized quartz. Once buried deep the very fine
grains may melt together, making them hard and brittle. Some folding of
the rocks will be sufficient to cause fractures. Newly formed gas will
move through those little fractures and concentrate in larger fractures,
or it moves into the pores of the overlying sandstone. These shales may
not be acceptable gas-containing rocks by themselves, but they can add
to the reservoir and thus increase the amount of available gas. Do not
forget that all the shales in the world make up 60% of all the
sediments, which makes it worthwhile to study them. Oil and gas are
non-renewable resources and we do not have sufficient other energy
sources to take over.
The big question the reader may have is, why is the Tanqua Karoo so
important to the geologist, geophysicist, and the petroleum engineers.
There are several types of deepwater fans, one major type is the
fine-grained fan. We normally find it in the oceans or in coastal areas
at great depth. However, the fine-grained fan is very seldom exposed in
outcrop, and when exposed (like the Laingsburg area) it normally becomes
intensely folded, and no detailed observations or in-depth
layer-by-layer correlations can be made to see if those layers have
sand-on-sand contacts that allows oil or gas to move upward. The Tanqua
Karoo is the only location in this world where the layers are basically
horizontal, making it possible to carry out detailed measurements and to
apply those observations and measurements to a buried example that may
be a reservoir. One should be aware of the fact that drilling and
recovery at sea, in water depths over 1000 m, can easily run into
several hundreds of millions of U.S. dollars, and often only one in four
wells will be economic. We should improve that number and thus prevent
the energy prices to increase steeply. The studies done so far have
provided unique information. The second step is to drill a number of
holes, retrieve a core, and deploy a string of well logging tools. That
project will deal with fresh rock, not weathered, and provides a better
three-dimensional distribution of information when added to the outcrop
studies. Students from Stellenbosch University will be involved.
Conducting such a program provides work for the local community and
business for the area. It will promote visits from others to the area to
study those rocks. The program can be used to make offshore drilling on
the west coast of South Africa, as well as international, more
attractive. It will help a new program from Stellenbosch University,
Cape Town University, and the University of the Western Cape, to provide
information so students can be hired by the oil- and gas industry
operating in South Africa as well as anywhere else. At the moment
students have to go to Europe or America to study this direction of
geology. Our program can be very helpful for South Africa and the
African Countries, as well as the entire world, to insure that oil and
natural gas will be longer available than is presently predicted.
Dr. Arnold H. Bouma
Endowed Professor of Sedimentary Geology
Department of Geology and Geophysics
Louisiana State University
Baton Rouge, Louisiana 70803, USA
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