Science and Technology Education Centre
(STEC@UKZN)

Ecca group

drifting to warmer latitude

Ecca group map

As Gondwana moved north towards the equator, thick clay and silt beds were laid down in a large sea that occupied the Karoo basin. These sediments now form shales of the Pietermaritzburg Formation. The shales are easily weathered and often present slope stability problems.

Ecca shale
Ecca shale is typically dark-coloured as they are carbon-rich due to the high vegetation content of the original sediments. Fine bedding or laminations may also be noted, and the shales tend to easily crumbled.

Overlying the shale is a thick sequence dominated by light grey sandstones, called the Vryheid Formation. These sandstones were deposited along ancient sandy shorelines behind which lay vast swamplands with numberous Glossopteris  plants. Close examination of the sandstones reveal numerous fossilised burrows formed by ancient soft bodied animals. These structures are similar to those formed by sand-burrowing crustaceans today. 

Formation of coal
The northern coastline of the Karoo Sea was shallow, and large swampy deltas formed. The accumulation of plant material in these swamps resulted in the formation of peat. Gradually over time with increasing heat and pressure due to burial, the peat turned into coal.

Modified after: The story of Earth & Life (T. McCarthy & B. Rubidge)

Simplified map of active coal mines. (Courtesy Council for Geoscience)

When southern Gondwana emerged from below the ice, several tree-like plants had evolved. Glossopteris was one of the most dominant trees. These plants rapidly colonised the large deltas, where they grew in extensive swamps. In these swamps dead vegetation accumulated faster than it could decay. The burial of the vegetation in the swamps eventually formed coal which is  mined in northern KwaZulu-Natal.

Gondwana

Glossopteris leaves

near Cedara, KwaZulu-Natal. The original leaves have been oxidised and replaced by iron oxide. Move your mouse here for a detailed view. (Specimen: Geology Education Museum)

Coal (sedimentary)

Fossilised plant matter

Coal

Coal is economically important , as it is South Africa’s most abundant fossil fuel and source of energy. Coal is black to brown and a soft and light rock composed of the carbon remains of plants.  It is found as seams, or layers within shales and sandstones of the Ecca group.  The quality of South African coal is usually low with a low heat value and a high ash content. In KwaZulu-Natal, the seams are deeper and thinner than in other parts of the country, but of a higher quality.

Shale (sedimentary)

Lagoons, swamps and sandy shorelines

Shale is a fine-grained sedimentary rock formed by the consolidation of silt and clay. Ecca shales are typically dark-coloured as they are carbon-rich due to the high vegetation content of the original sediments. Fine bedding or laminations may also be noted, and the shales tend to be easily crumbled.