When driving along the N1 between Matjiesfontein and Laingsburg
you cannot miss the thin sedimentary band running parallel to the
road for many kilometres, standing above the surrounding
countryside like a low cream-coloured farm wall. It is very
prominent on the southern side of the N1 highway but if you look
carefully, it also repeats north of the N1 in places.
This whitish weathered deposit, often referred to as the
geological wonder of the Great Karoo, is known as the
“Matjiesfontein White Band Marker” and marks an
important geological boundary between the Dwyka layers below (the
Carboniferous period, starting 270 million years ago) and the Ecca
layers above (the Permian, starting 225 million years ago). Both
were named after local Karoo rivers. The bed consists of a very
hard sedimentary rock called chert. It has a high content of
microscopically small silica crystals, which give it a flinty
quality and made the chert ideal for stone tools, as the San
quickly discovered. Silica is a glassy mineral derived from fine
particles of volcanic ash.
This rock layer silently records a single brief, but immense,
underwater flood which occurred in the space of a few hours,
260-million years ago. At that time there were large volcanoes
active along the region where South America was splitting away from
South Africa, about 1000 km to the west. Then, as now, the
prevailing wind was from the northwest, and so volcanic ash spread
southeastward from the volcanoes extending over a region more than
1000 km away. The ash settled on the bed of a fairly deep inland
water body called the Ecca Sea, and gradually filtered to the
bottom to form discrete ash layers.Then a major earthquake,
probably associated with contemporaneous mountain-building to the
south, shook up and destabilised the thick underwater deposits of
mud and ash which had accumulated along the shallow edges of the
Ecca Sea. These slumped downslope, in the process mixing with
surrounding water to form a turbulent mass of soupy material, much
denser than the clearer waters above. This dense sediment-water
mixture, with a total volume exceeding 16 cubic kilometres, then
flowed under gravity along the sea bed into greater depths, finally
spreading for about 150 km across the almost level floor of the
Ecca Sea. The whole process probably lasted no more than a few
hours at most. This layer evidently dissolved, then precipitated as
a homogeneous layer, and is seen as the layer of very hard chert
(which has the same chemical composition as the ash). Subsequent
compaction of the sediment, combined with chemical alteration of
the silica-rich ash particles, cemented the deposit to form a
resistant bed of cherty rock. In time, this was gradually buried
beneath several kilometres of younger Karoo sediments and
lavas.
It was also the time of the Cape Fold Mountains being formed, of
which the nearby Swartberg range is part of, meaning that this
whole sedimentary package eventually got upended into the
(sub)vertical attitude we see today. This all happened deep
underground. Over the past 150-million years the overlaying
deposits have been removed by extensive erosion, revealing this
narrow white band of ancient rock.
Just below the chert layer (and therefore older) is a broad
(about 20 metres wide) layer of whitish deposit. This is weathered
material from the very dark rocks that underlie the mud deposits
described above. The blackness of these rocks is due to hydrocarbon
deposits – rather like oil shale, but not yet worth mining
for oil. These structures – hydrocarbon deposits (containing
rare fossils of some of the earliest terrestrial animals) and the
overlying sandstones – occur in a similar fashion on the east
coast of South America and are one of the reasons that the South
African geologist Alex du Toit was an early supporter of the theory
of continental drift early in the twentieth century.
Sources:
To claim “Found it” you must email me satisfactory
responses to the following:
Any logs not accompanied by an email will be deleted.
- Send me a picture of you/your party with your navigation device
taken with this feature in the background.
- How thick do you judge the chert layer is at this point?
- While driving this stretch of road between Matjiesfontein and
Laingsburg (east- or westbound), note your odometer readings
between the first and last occurrences of this feature. Tell me for
how many kilometres you saw it stretching?
- For a bonus; see if you can spot it north of the N1. Send me
the GPS coordinates of this observation and, if possible, a
picture.
Note: Do not post any spoiler
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