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.
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.
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