Oh, Id love to address that. I did my thesis on parts of the Karoo Supergroup and Im still working on those areas. Thats my area of expertise.
Strata in the Main Karoo Basin, consisting of mostly glacial formations at the bottom, followed by coal seams amongst sandstones, shales and mudstones, followed by sandstone, shales and mustones, followed by Aeolian deposits, followed by interbedded aeolian and basaltic deposits in the upper parts. It records an almost continuous sequence of marine glacial to terrestrial deposition from the Late Carbonifeous to the Early Jurassic, a period of about a hundred million years. The Supergroup is up to 8 km thick in places. The coal seams occur in the Ecca Group (the second lowest group of strata), which dates to the middle Permian period.
The number of 800 000 fossils was given by someone who found a number of fossils at a spot, multiplied that number by the area of the Main Karoo Basin and declared 800 000 fossils in the Karoo. The aerial extent of the main basin of the Karoo Supergroup is around 50% of the surface of the country (thus Main Karoo Basin area is around 610 518.5 square kilometers, depending on how you model the Karoo Sequence underneath Quaternary and Tertiary deposits). The number of fossils is looked at with great questions surrounding it, for a variety of reasons, one of them is that because we simply dont know how many fossils there are, seeing that we dont know exactly know how many fossils we have underground. Another one is that, from the fossils we find, only a small percentage is found in fossil graveyards. Most of them are found as lonely fossils dispersed amongst the rocks. We also have an excellent record of transitional fossils in the Karoo Sequence, from reptile to mammals. Fossils we find in the upper parts of the Permian, are reptiles. No mammals there. Some fossils show transitional features in rocks from the late Permian to very early Triassic. Then we find mammals in the Triassic. A great sequence of transitional fossils. Not buried in fossil graveyards at all, but dispersed randomly in a definite sequence in the strata from bottom to top.
Can I answer? Ive been working on strata in the Karoo Supergroup for more than 20 years? Theres no evidence for a large, big, fossil bone yard in the Karoo Supergroup. Fossils appear to be dispersed in the Supergroup, with fossils also found in aeolian deposits. A great source for information: The Stratigraphy of South Africa, Handbook 8, part 1, Lithostratigraphy of the Republic of South Africa, South West Africa/Namibia and the Republics of Bophuthatswana, Transkei and Venda; published by the Geological Survey of South Africa, in 1980. All the information you need to start a basic study of the Karoo fossils!