Today’s episode is a response to a listener’s question about the close juxtaposition of glacial deposits and coal beds in Australia.
Despite the abundance of coal in the Carboniferous, especially in the northern hemisphere, and despite the changing climate that meant coal formation there largely ended with the end of the Carboniferous, there’s plenty of Permian coal too. Most of it is in the former Gondwana – Australia, South Africa, India, South America, and Antarctica, but there is a lot of Permian coal in Russia as well. In Gondwana, the coal is pretty closely associated with glacial deposits.
We might expect that glacial deposits and coal swamps would reflect two very different environments, but so far as I can tell, they pretty much co-existed in Permian time at least in quite a few places.
In Australia and elsewhere, the coal-bearing rocks and glacially deposited layers actually interfinger. So at best, we might have had some relatively rapid changes in climate to switch from glacial times to warmer, coal-swamp times, and from what I read there were at least 8 specific glacial periods in the late Carboniferous and early Permian.
But the alternative explanation, and from what I can gather it seems to be the preferred one, is that these areas were on the margin of the ice, and plant life actually thrived there. The keys to making coal are 1) lots of plants and 2) rapid burial of the plant matter so it does not have time to decay. Our typical vision of warm swampy areas with low oxygen to prevent decay is just one way to do that. A cold climate, with plants buried by glacial debris, would work just as well, if not better.
|Gondwana base map from Du Toit (1937, Our Wandering Continents). Blue line is highly generalized margin of glacial area; solid black are highly generalized coal deposits (based on Langford, 1992).|
I think that while Australia was certainly part of the glaciation in the southern polar part of Pangaea, it was probably far enough away from the pole (which was more or less in South Africa, but there are coal deposits there, too) that the climate might not have been like modern Antarctica, but perhaps more like modern Patagonia but with glaciers. So abundant plant life could have been growing, even thriving, near the glacial margin. A modern analogy would be the peat bogs of temperate and even arctic climates. When glaciers receded, forests and peat on the tundra would advance. When glaciers advanced, the deposits the glaciers carried would have buried the forests. This would be a good way to get the interfingering of glacial and coal deposits that we do observe.
This would not strictly be cyclothems, which represent rises and falls of sea level, alternately allowing swamps to form and then burying them in river sediment, but they would be cyclic nonetheless, like cyclothems. I do not know if the coal-glacial sediment packages follow the 8 known glacial periods or are (likely) something more complex, but if they do it would be on a periodicity of a few million years. Standard cyclothems can show alternations that may represent changes on scales of a few tens of thousands of years, or even fewer, as well as the longer periods of millions of years. If the coal results from glacier-margin plant life, as I infer it does, then the alternations would not reflect sea-level changes as cyclothems do, but more directly would reflect changes in position of the glaciers (together with the dumping of sediment to bury the forests or other vegetation).
—Richard I. Gibson
Permian of Australia
Permian coal in South Africa
Langford, 1992 - Gondwana’s Permian coal