In the latest issue of Down to Earth magazine (issue 92, August 2015, ISSN 0969-3408) we have part 3 on the Irish chalk formation near Killarney in Co Kerry. In this final episode of the story Peter Walsh discusses hypotheses to explain the formation and suggests the geological community is still nowhere near forming an adequate explanation.
What we have is a very big hole, that tapers inwards as it goes downwards, and it is filled in with chalk. The bedrock surrounding the hole is shale from a much earlier period in Earth's geology, and fragments of the shale have been found in the hole – flint has even formed around some of these shale shards. The flint is not in a bedding plane but is chaotic in location. Hence it is clear both chalk (probably as a sludge) and flint (in a semi liquid format so that it was able to envelope shale shards) entered the hole – and so the big question is, what caused the hole?
One view is that the shale behaved a bit like limestone in a karst formation, water and waves creating caves and holes. However, the Killarney hole is enormous when compared with karst formations in other parts of the world – and is therefore considered unlikely. The idea that the hole was formed by erosion (by any means) also requires a 'chalk transgression' – whatever that might be. A transgression of water on land is a flood – perhaps caused by rising sea levels or transiently by a tsunami wave. A chalk transgression suggests a flooding event – but that idea disputes the origin of chalk, said to be formed in shallow seas (being the bottom sediments). The fact that chalk is mostly pure and does not contain a lot beside planktonic shell remains is neither here nor there for the moment. The chalk at Killarney demands that this part of Ireland was submerged in the Dinosaur Age. The idea of a chalk transgression is therefore difficult to perceive – unless it was not submerged.
The presence of the Killarney chalk formation appears to defy mainstream geological theory – and its uniformitarian parameters. A later theory (1994) held that the shale and chalk juxtaposition came about as a result of tectonic slippage (and the rest of the chalk formation has long since eroded away, and Ice Age glaciation is blamed. However, the lack of chalk on the other side of the proposed fault line (which may not have been a fault line and is just conjecture) has not made this theory very popular. Lack of evidence is its downfall – but in theory it is attractive (although one would have thought a lot more geology layers were missing between the shale and the chalk).
There are few geological processes known that create depressions in hard rocks – on the land or on the sea bed. This seems to rule out non-catastrophic ideas such as karstatic subsidence (cave roofs collapsing), cryogenic processes, overdeepening by glaciation, sudden dewatering of soft sediment following earthquakes, halokinesis, anthropogenic activity, terrestrail landslips, faulting and sea floor spreading, aeolian ablation or cosmic impact (also ruled out). What then is left he asks – hoping readers might make an intuitive guess. On Iceland the author witnessed the evidence of geological violence, deep craters and odd formations all over the island. He asks could similar catastrophic violence have once taken place in the Killarney region of Ireland? Violence and catastrophe is indicated by the split nature of the shale – and its shattering into many shards, big and small. Was this caused by volcanism, or venting – or some other pressure on the rocks of Killarney, lifting them up in an explosion of some kind. At some point, probably at the same time, chalk sludge filled the cavity and encompassed the shards that had been in the process of falling back down – but where did the chalk come from?