Fossils help to solve a geological problem

FOSSILS HELP TO SOLVE

A GEOLOGICAL PROBLEM

portion of S ou th A frica. The oldest rocks of this sequence are those of the Dwyka Form ation w h ic h were deposited by glaciers d u rin g an ice age about 3 0 0 m illio n years ago, w h ile the youngest, the Clarens Form atio n, were deposited in a desert e n v iro n m e n t about 2 9 0 m illio n years ago.

Econom ically and strategically, however, the most im p o rta n t rocks of the Karoo Sequence are those of the Ecca G roup as all the coal w h ic h is mined in the Free State, Transvaal and Natal comes fro m these rocks. The Ecca rocks were deposited in a delta e n v iro n m e n t as large rivers met the central Karoo sea, and as there w a s plenty of w a te r in th is e nviro n m e n t, there w a s an abundance of plant g ro w th w h ic h e v entually led to the fo r m a tio n of coal beds.

Side view o f large D inocepha/ian s k u ll (scale — 1 cm). This was a p la n t-e a tin g a n im a l w h ic h was one o f the largest a n im a ls w h ic h live d in S o u th A fric a d u rin g B e a u fo rt times.

Im m ediately on top of the rocks of the Ecca Group, are the rocks of the B eaufort Group, w h ic h , as you can see fro m the map, cover most of S ou th A frica. A lth o u g h these rocks have not yet produced e con o m ic m in e ra ls 0in great q uantities, they are w o r ld -r e n o w n e d for the ir w e a lth of fossil reptiles.

A problem th a t has puzzled geologists for n um e ro u s years, is the exact position of the b ou n da ry between the Ecca and Beaufort Groups, and m u ch research has been done to try and solve th is problem.

B .S. R ubidge

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Side view o f s k u ll o f Eo d i c y n o d o n.

Research on fossils undertaken at the National M u s e u m , revealed a m ost in teresting pattern in the fossil content of the rocks of the co nta ct zone between the Ecca and B eaufort G roups in the south ern Karoo. Above a certain horizon, fossils of reptiles as w ell as well-preserved and complete plant fossils are found. However, b elow th is horizon, no reptiles but o nly fossils of finely gro u n d up plant fra g m e n ts have been found. This appears to indicate th a t the broken up plants were transported, w h ile the complete ones were preseved close to the position where they used to grow .

Closer research on the rocks themselves has revealed th a t at th is horizon the character of the rocks changes too. In fact those b elow the line have c h aracte ristics w h ic h indicate th a t they were deposited as sediments in a s h a llo w ''s ea ,” w h ile those above the horizon, were deposited on land.

This line, on the map alongside, represents the shoreline of the a ncient Karoo " s e a " in a portion of the south ern Karoo. It also illustrates as a very good divider between the rocks of the Ecca and B eaufort Groups, and m ay result in the Ecca- Beaufort c o nta ct in the southern Karoo being moved d o w n w a rd s .

The above is an illu s tra tio n of h o w geological problem s can be solved th r o u g h using fossils. N o s tril

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