Durban's Geology
By Allan Jackson - June 2007

Some time ago, I got my hands on a leaflet all about the geology of the Durban area and how our coast was formed.

I soon came to the conclusion that my brain just didn't have the capacity to absorb what I was reading. My informant Wade Kidwell, in UKZN's Marine Geoscience Unit, arranged for me to chat to his colleague Hayley Cawthra, who laid things out for me in words of one syllable and gave me an invaluable article by John McCarthy to read.

It seems that, around 200 million years ago, there was a big continent called Gondwana which was subject to terrific forces that ripped the it apart. The various pieces of it became the southern continents, including Antarctica, Australia, South America, Africa, Madagascar and India, which drifted off in their own directions.

The really fascinating discovery, for me, was that the Falkland Islands started out in life adjacent to what became the coast of KZN. They were really one piece of land, with the Falklands plateau shearing away and moving off into the distance along the Agulhas/Falkland Fracture Zone. It is actually still moving away from us at a pretty nippy 6cm a year, thus steadily widening the Atlantic Ocean.

With the newly formed Indian Ocean now along the KZN coast, the next major event from 120-60 million years ago, was the Cretaceous marine transgression, when the sea level rose to as much as 400m above present levels. The sediment deposited by the sea during that period eventually turned to rock and, although much of it has been eroded on the surface, layers of it still underlie parts of Durban, including the city centre, Bluff and the beachfront.

The high sea levels of the Cretaceous gave way to the Tertiary Marine Regression when the sea level slowly began to fall due mainly to to cooling of the oceanic crust. Since the last million years or so sea levels have been up and down at least 5 times due to the Pleistocene ice ages.

During the height of an Ice age thick ice capes develop at the poles lowering sea levels. During the most recent Ice Age about 18 000 years ago, sea levels reached about 120m below current levels, meaning that Durban, if it had been around, would have been about 15km inland.

During this time, the features we know today as the Bluff and Berea, were formed from sand dunes which became dune, or aeolianite rock, through a process of lithification. This happened when rainwater percolated through the sand, dissolving calcium carbonate shell fragments which then dried into a type of cement and stuck the sand particles together. The red sand, found all over Durban, also known as Berea-type sand, is the result of the weathering and oxidation of dune rock.

Durban is pretty solidly grounded on a geological basement consisting of layers of rock which are known as the Karoo Supergroup. These rocks formed on the continent of Gondwana before it split up.

Around Durban one can find the Ecca Group, which consists of dark grey shales formed in an inland sea about 350 million years ago. Underlying the Ecca shales are tillite of the the Dwyka Group, which formed from rock material deposited by glaciers 300 million years ago. The tillite forms the huge cliffs on either side of the Umgeni River and is quarried around Durban for aggregate.

The Karoo Supergroup is cut by a number of faults. These include the Springfield fault, which runs slap-bang across Jan Smuts Avenue at 45th Cutting. It was apparently active during the breakup of Gondwana but the good news is that it, and the other faults in our area, are inactive. The area has been geologically stable for 30 million years, and thank goodness for that.

Home | Contents | Diary | Orders | Site Search | Contact Us