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chalk outlier

13 May 2015

Going back some weeks ago on chalk oddities, a chalk outlier near Killarney has caused geologists a few headaches. It is described as enigmatic and lies adjacent to the main road between Killarney and Trilee in SW Ireland – but a long way away from the chalk formations of southern England. It was exploited in the 19th century by lime burners but how did it get there, among a landscape of Upper Carboniferous shales and sandstones. One might predict no rational explanation was available – but that did not deter geologists. They came up with the idea the chalk was laid down in pulses – it came and went in stages.

Nice explanation. It is fairly obvious that even in southern England the chalk was not laid down as one whole dollop. Geologists divide it into distinct stages with a sequence of layers. It is suggested one such pulse, late Campanian in date (within the Cretaceous era) drowned the mid-Kerry landscape and left behind some chalk mud. Most of this has washed out at a later stage – leaving behind the outlier. This appeared to be a common sense solution to the mystery of the outlier as the chalk is undoubtedly there and it doesn't necessarily mean all of it has survived.

A residue remained to tantalise geologists and Peter Walsh decided to dig around and investigate. Why was it so closely attached to much older rocks. He discovered the chalk was not universally pure chalk within the outlier. A large portion of the outlier contains breccia – tiny shards and flakes of Clare shale. These go on to vary in size, the further he investigated, from the minute to large blocks of shale stone. The chalk also has flint nodules of different sizes but do not appear to be related to a bedding plane (as often seen in chalk geology elsewhere). Many of these flints actually encase pieces of shale – and this illustrates that flint was a silica fluid when laid down. In southern England it is commonly found encasing fossils such as sponges but in mid-Kerry it encased broken shale stones. It also shows quite clearly that the shale stone was introduced into the chalk while it was still a soft slurry. Slurry is a word bandied about by farmers but it sems very apt here.

The source rock, or shale formation, was fragmented suddenly and violently – and here the ogre of catastrophism creeps into the description – but there is an explanation as we shall see. For example, there is no trace of rounding of corners and edges of the shale in any part of the outlier. Neither is there any trace of bedding – or of sorting. For example, eskers sort out stones and pebbles by size – gravel in one part of the formation and bigger stones graded upwards as the esker is cut open. The relationship between the breccia and the chalk is chaotic – as expected in a sudden and violent origin. Hence, the process of producing the breccia points to a dramatic event.

The outlier is roughly triangular in shape and trenching and bore holes, supplemented by an electrical resistivity survey, has indicated it has a fairly straight face to the N, the SE and the SW, which may imply some kind of tectonic origin. Fault lines are straightish. The outlier is around 130m in length and nowhere does it exceed 150m in diameter. The depth, as far as could be ascertained, was over 120m. However, the base of the outlier was not found – and ever increasing amounts of shale may have petered out to a wholly broken shale stone bottom. Erosion of the top of the chalk is also an unknown.

We may conclude, he says, the outlier is contained within a steep sided closed depression and that the breccia formed (violently) around the same time the chalk was laid down (creating a mixed slurry). In other words it was washed into a ravine or a depression and this is why it survived – as further inroads of the sea were not capable of rinsing it out of its hidey hole. Another way of looking at it is that a heap of broken Clare shale was swamped by a mass of chalk mud (still soft) and collapsed into a deep cavity (described as a pipe) and the two were mixed together as they descended into the hidey hole. This allows the formation of broken shale rocks to occur prior to the laying down of the chalk mud (on a sea floor situation). This is perhaps indicated if the chalk mud failed to penetrate to the bottom of the hidey hole.

Another view might be that a powerful earthquake shattered the shale (providing the catastrophic element) and was closely followed by the chalk mud. The earthquake would have occurred at a fault margin (hence the straightness of the line of the formation is significant) but would require a slippage of 120m (or more) – which is quite remarkable by itself. It could then have easily involved a temporary transgression event – an intrusion by the sea.

One mystery not solved is the flint. Why should silica, as a viscuous fluid, be part of the earthquake, and transgression?

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