deposits up in a gentle two to five degree slope to the surface. The rocks were pushed with enough force to make them fold, and hot mineral rich fluids came up from deep below to fill the cracks. Uplifting continued for more than 250 million years, but as soon as the rocks were exposed, weathering began. Most of the higher levels are long gone, so what we see now are the lower, older and worn down remains of deposits that were once completely covered by Late Carboniferous rocks. In some places 30 million years of limestone dissolution has exposed the Old Red Sandstone from the earlier, Devonian, period. To the west, younger rocks have survived, and at the Cliffs of Moher, relatively robust siltstones and sandstones have acted as a protective cap for the softer shales below. Even so, those cliffs are being worn away, and two and a half million years ago they may have stood several kilometres to the west.
Layer upon layer of deposits built up over millions of years. The Cliffs of Moher, photograph, Tobias Halfrich.
Galway Granite Lower palaeozoic rocks Carboniferous Limestone Clare shales, siltstone or sandstone Old Red sandstone
Flat flags with perfectly preserved ripple marks. A cut through section from Galway Bay to the Cliffs of Moher showing the gentle slope and succession of rocks over the granite and Lower Palaeozoic basement. At Clare, geologists have an unusual opportunity to compare outcrops exposed on the high cliffs with deep cores taken from behind them. As Dr haughton commented, “Clare is a very important area because we have the very high large cliffs. These cliffs are pretty well much of the same magnitude as those you would see on seismic data.” however, exposures on the cliffs are weathered, particularly the mudstones and shales, but the cores, said Dr haughton, yield samples that are in pristine condition. In addition, drilling goes lower than the cliff exposure, so the cores enable geologists to build up a more complete picture. There is a lot of interest in the shale which were first encountered when the Ambassador Oil Company began exploratory drilling at Dunbeg in the 1960s. At the time, said Dr haughton, very little was known about the extent and where the shales occurred. It could
Deep Basins
ThE CLIffS along the coast of County Clare are of particular interest to geologists because they are like a slice through the edge of an ancient basin. As the geologist, Peter haughton explained at last year’s Geoconference, such basins are often associated with the presence of immense reservoirs of oil and gas. In Clare, he said, the Lower Carboniferous rocks, which are hard, stick out into the ocean where they fringe the shales that lie at the edge of an Upper Carboniferous basin, formed millions of years ago as sediments were being washed down from a great delta into the deeper sea. Dr haughton and his colleagues, Prof Patrick Shannon and PhD student, Colm Pearse from UCD have been studying this basin with support from industry partners and the Griffith Geoscience Programme.
The biggest task is to get down into the sedimentary rocks, and this is where oil company expertise from Statoil and SLP Consulting comes into play. “Statoil has funded the drilling of five drill holes to date,” he said. The cores from these drills provide the raw data on how the sediments built up and where they occur. It is also important to know where these sediments were coming from, and on this point another researcher, Shane Tyrrell, has been making a significant contribution. As reported on in Science Spin issues 22 and 44, Shane uses a mass spectrometer at UCD to examine sedimentary particles. The particular ratio of feldspar and lead isotopes indicates where these particles are likely to have come from, and, as Dr haughton commented, th e techniques developed for this project, apart from being of benefit in Clare can be applied anywhere, and not just in Ireland.
sCienCe spin issue 48 page 24