Tuesday, 14 April 2020

A (Virtual) Geological Trip to the Karoo

A (Virtual) Geological Trip to the Karoo

Yet another correspondent forwarded me THIS LINK to a very interesting article mostly about Karoo dolerites and included three of her own photos. 

The Karoo is the "Fly Over" part of South Africa, between Cape Town and Johannesburg. Much of it is flat and boring but there are occasional spectacular, flat topped mountains and these are the result of intrusions of dolerite sills and dykes.

A particular emphasis of the article is the hope that dolerite intrusion has reduced the attractiveness of the area for companies looking to exploit shale gas. 

But my correspondents photos are the best for appreciating the dolerites of the Karoo.

Thaba Thabo, near Fouriesburg in the Freestate, near the Lesotho border.  A vertical dyke in sandstones has weathered out more than the baked margins - the hornfels.  Note that this is close to the contact with the overlying Drakensberg lavas, so it may have been a feeder dyke.
A thick dolerite body, which is climbing up section but nearer to a sill than a dyke., capping a hill near Harrismith in the North-east Freestate.

Top is a sill and another intrusion which not quite a dyke or a sill, which appears to have displaced the sandstone layers and bent them downwards, at the right.  There is a small dyklet feeding in to it along the fracture. On the road from Beaufort West to Calvinia in the Northern Cape.

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