South Africa West Coast

Geological Features, St Helena Bay

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rock The purpose of this writing is to give a broad and simplified history of the geological evolution and aspects of the St Helena Bay, West Coast area.

The earth formed approximately 4650 million years ago by the accretion of cosmic dust which initially developed as a result of the Big Bang, about 10-15 billion years ago. At this stage the young earth was a dynamic hot planet with no continents or oceans, simply a molten surface. Rapid cooling of the planet began soon after and about 4000 million years ago the first continents had begun to stabilize and the early oceans and atmosphere were beginning to form.
Right: A rock weathered by the sea at the bottom - the same as "Voëlklip" in Stompneus Bay. The sea level was approx. 30m higher when this rock was eroded by the sea. Location: Langklip

The oldest known rocks on Earth are approx. 3800 million years old and the oldest rocks in South Africa are approx. 3500 million years old (Barbeton). The first continent in South Africa was called the Kaapvaal Craton. Since the Kaapvaal Craton was formed many other rocks formed in South Africa and onto the Kaapvaal Continent a few smaller continental blocks accreted to form another continent called Gondwanaland. There after a long history of rock assimilation and deposition took place to form South Africa as it is today.

graniteRight: Three different types of granite intruding each other
Geological activity in the South Western Coastal areas developed a chain of basins in response to the formation of the supercontinent Gondwana and into which the Malmesbury sediments were deposited between 750 and 600 million years ago. These sediments can still be seen today at St Helena Bay.

At approx. 550 million years ago, the Cape Granite Suite of granites intruded the Malmesbury sediments. Today, St Helena Bay is underlain by these granites which are called the Vredenburg Granite. These granites were formed while South America and South Africa collided during continental drift about 600 million years ago.

duiker eilandRight: Assimilation of Malmesbury Sediments near Duiker Eiland
The South American rocks were forced underneath the South African continent through subduction. The rocks were melted and produced the granites which then intruded the Malmesbury sediments. Part of the sediments were assimilated and these remnants or xenoliths can be seen in the rocks along the coast of St Helena Bay.

Different stages of assimilation of the Malmesbury rocks are to be seen at St Helena Bay, which is a geological proof that the granites were molten when they intruded. This is not always believed by all the geologists but the proof can be seen at St Helena Bay. Different stages of assimilation are also clearly visible.

graniteRight: Inclusion of a granite in the surrounding granite (Vredenburg Granite) at the Stompneus Bay quarry
Approximately five types of granite intrusions can be seen at St Helena Bay. Intrusive contacts are conspicious along the coast and the different granites can be identified at St Helena Bay. The different granites have different ages.

Other rocks and geological features that can be seen at St Helena Bay are limestones, coastal dunes, raised beaches, phosphates and dolerite dykes.

Mineral resources which were mined are gypsum, phosphate, building stone, kaolin and building sand. Many other interesting geological features surround St Helena Bay and can be seen in the vicinity of Vredenburg and Paternoster and the surrounding farms.
Peter Siegfried
Council for Geoscience

My appreciation to Peter Siegfried for this very interesting article and photographs

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