Geology of Cape Town

[1][2]: Ch2 The late-Precambrian age Malmesbury Group is the oldest rock formation in the area, consisting of alternating layers of dark grey fine-grained greywacke, sandstone and slate, seen along the rocky Sea Point and Bloubergstrand shorelines, and from the Strand to Gordon's Bay.

The sequence was subsequently metamorphosed by heat and pressure and folded tightly in a NW direction during the Saldanian orogeny so that the rock layers are now almost vertical.

[3] Most of the exposed shoreline Malmesbury rocks are steeply dipped, and weathered to form sharp edged ridges where more resistant layers stand out among the softer strata.

The Cape Granite suite is three huge batholiths that were intruded into the Malmesbury Group about 630 million years ago as molten rock and crystallised deep in the earth, but have since then been exposed by prolonged erosion.

The characteristic spheroidal shapes of granite boulders are a result of preferential weathering along intersecting fractures and are well displayed around Llandudno and Simonstown.

Close up, the granite is a coarse-grained rock consisting of large (2–5 cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica, and containing inclusions of dark Malmesbury hornfels near the contact zone.

[1] The contact zone where the Malmesbury Group was intruded by molten granite can be seen at Sea Point and was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S.

Large feldspar crystals occur in both the granite and dark hornfels layers Though initially intruded at great depth, prolonged erosion eventually exposed the granite at the surface and it and what remains of the similarly eroded Malmesbury group now form a basement upon which younger sedimentary rocks of the Table Mountain Group were deposited.

The colour is generally pale to medium grey, and the surface is typically fairly rough, with clearly visible crystals, and no layered structure.

Table Mountain group sandstones were deposited on the eroded surface of granite and Malmesbury series basement, in the stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 450 million years ago.

[1] The basal Graafwater Formation (300-450m thick) consists of interlayered pale brown sandstone, laminated pink siltstone and dark maroon coloured shale.

[3] The Peninsula Formation (800-1500m thick) consisting of hard, light grey medium to coarse grained pebbly quartz sandstone, dominates the steep mountain cliffs.

Current bedding and pebble layers suggest that it was originally deposited as migrating sand bars in broad river channels.

[3] The Pakhuis Formation tillite (a lithified glacial outwash gravel) occurs on the highest point of Table Mountain, at Maclear's Beacon and on parts of the Hottentots-Holland range.

It contains clusters of angular boulders and pebbles and was deposited at a time when the Gondwana continent, of which Africa was a part, was situated close to the south pole.

[3] The formation of the Cape Fold Belt is the result of a collision of tectonic plates that ended over 200 million years ago The accumulated strata of the Cape Supergroup and the older granites and Malmesbury group were raised and deformed by the pressure of the South American, Antarctic and African continental plates slowly moving together.

These more easily eroded zones are marked by ravines; cross-cutting faults separate multiple peaks of the Twelve Apostles.

Some fault zones of crushed rock (breccia) are re-cemented by dark brown coloured iron and manganese oxide minerals.

There are small areas of shelly gravel and mud near Gordon's Bay between the rocky shoreline reef and the sand bottom.

[3] On the False Bay coast just below the old lighthouse at Cape Point, a basic agglomerate plug intrudes course grained porphyritic granite.

It consists of a heterogeneous mass of dark green to grey basic rock with red-brown specks that enclose randomly oriented clasts of variable size up to 300 mm.

Geological map of the Cape Peninsula and False Bay
Geological section of the Cape Peninsula and False Bay
Possible tectonic setting during deposition of the Malmesbury group. (after Compton 2004)
Granitic magma intruded into the Malmesbury rocks as the ancient Adamastor ocean closed up and the South American and Antarctic continents collided with Africa. The collision resulted in uplift and deformation of the Malmesbury sediments. Later erosion exposed the granite at the surface. (after Compton 2004)
Deposition of the Cape Supergroup on the flat eroded surface of granite-intruded Malmesbury rocks. Alternating deposition of sandstone and shale reflects changes in relative sea level along an overall subsiding, passive continental margin. (after Compton 2004)
The Cape Fold belt formed during a major mountain-building episode as the continents collided to form Pangaea. Volcanic ash deposits in the Karoo basin indicate the presence of active subduction zones to the west and south as the South American and Antarctic plates crumple the Cape Supergroup and its underlying strata into the Cape Fold Belt Mountains. (after Compton 2004)
Possible evolution of the Cape Town landscape: The nearly horizontal Table Mountain sandstones represent the trough and the steeply dipping sandstones of the same formations of the Hottentots Holland Mountains to the east the limb of a large fold that has since eroded away to expose the underlying shale. Thus the modern landscape may represent an inverted version of an earlier landscape (dashed lines) characterised by a large mountain where the Cape Flats are now. (after Compton 2004)
The Cape Peninsula shoreline in the case of (left) a 25m higher sea level that occurred around 5 and 1.5 million years ago, and (right) a 125m lower sea level at the time of maximum ice build-up during cold periods – the most recent being 20 000 years ago (after John S. Compton 2004)