Luke the Last Glacial Maximum (Clark et

Luke Bannister14320721Background and LiteratureThe glacial sediments found in the region around Shanganah and Shankill are a result of the British and Irish Ice sheet.  This Ice sheet covered the entirety of Britain and Ireland. It was formed about 32,000 years ago (Ballantyne and Cofaigh, 2017) and coincided roughly with the Last Glacial Maximum (Clark et al., 2009).  During its height the island of Ireland was covered entirely with ice and it extended beyond the current landmass towards the continental shelf (Ballantyne and Cofaigh, 2017).  The British and Irish Ice sheet reached its maximum extent 27,000 years ago (Clark et al., 2012) and subsequently began retreating.  By 15,000 years ago the majority of Ireland had deglaciated aside from a few local glaciations in mountainous regions (Charlesworth et al, 1973).  Although the Ice sheet was at its maximum extent at 27Ka, it reached its limits offshore at different times. (Ballantyne and Cofaigh, 2017).  In the west, the ice sheet reached as far as the porcupine shelf (Peters et al., 2015), however this only occurred around 25Ka.  In the south, it was initially theorised that the Irish Ice Sheet terminated at the South Irish End Moraine (SIEM) (Bowen et al., 2002).  This was a moraine that stretched from the Shannon estuary to the Wicklow mountains.  It is disputed that the ice sheet did not reach further south as there is a lack of evidence for this moraine in Carlow and Kilkenny.  Evidence for offshore moraines to the south of the country has also been found so it is likely that the Ice sheet did in fact extend south of the Irish shore. (Sejrup et al., 2005).  Differences in opinion on the extent of the ice margins are often a result of differing methods and interpretations of evidence.  Accurately establishing the real extent of an Ice sheet is fraught with uncertainty (Greenwood and Clark, 2009).Figure 1.1 Different Interpretations of the extent of the Irish Ice sheet (Barth et al., 2016) Fossils of species that tended to live in cold regions were also found in sediment cores taken far off the shore near the continental shelf. (Praeg et al., 2015)  The temperatures implied by these organisms indicate that the area must have been glaciated.  Bases on radiocarbon dating and comparing the ages of ice sheets in the region, it is likely that the Irish Ice Sheet reached its furthest southern extent between 24.3 and 23 Ka (Chiverrell et al., 2013)  In the East and Northeast the Irish Ice Sheet came into contact with ice from England, Scotland and Wales.During the Last Glacial Maximum the entirety of the island was covered with Ice, however the presence of erratics, tors and frost-shattered rocks suggest that areas such as the Wicklow mountains had their own independent ice caps which fed into the Irish Ice Sheet (Ballantyne, McCarroll and Stone, 2006).  Similar evidence is found both in the mountains of the southwest of Ireland (Ballantyne, McCarroll and Stone, 2011) and in the Connemara mountains (Ballantyne, Stone and McCarroll, 2008).  Periglacial trimlines are also found in the previously mentioned locations.  These range in altitude between roughly 400 to 800 metres, but are generally at an elevation of 700 metres.  This tells us that ice must have reached a thickness of roughly 700m across the island.  10Be dating of these trimlines has given very inconsistent dates, with samples being dated both long before and after the Last Glacial Maximum, however the wide range in dates can be explained by the complicated exposure history of the area and the fact that much of the bedrock has been eroded (Ballantyne and Cofaigh, 2017).  It is also possible that these trimlines do not mark the vertical limits of Irish Ice sheet and instead mark the transition from wet-based ice to cold based ice (Ballantyne, McCarroll and Stone, 2006, 2011; Ballantyne, Stone and McCarroll, 2008).Although the majority of ice in lowland areas had disappeared by 15Ka, the decline of the Irish Ice Sheet was not steady.  Between its maximum extent and eventual disappearance there were several standstills and readvances.  The only two readvances to have been properly dated are the Clogher head readvance at around 18.4Ka and the Killard Point readvance at 17.3-16.6Ka (McCabe, Knight and McCarron, 1998; McCabe, Clark and Clark, 2005; McCabe et al., 2007).  In addition to these readvances, Smaller glaciers persisted in upland regions around the island and these also underwent readvances.  Glaciers persisted in Loch Nahanagan in Wicklow (Gray and Coxon, 1991), and a readvance starting from this region would have had an impact on the sediments seen in the Shankill/Shanganah area.Fig 1.2 Aerial photograph of the area surveyed. Fig 1.3 Map of Shankill Beach MethodUpper Till Sampling and Grain Size AnalysisA sample weighing about two kilograms was taken from the upper till area of the sediment.  As the cliff was quite tall a specialised piece of equipment called a spade on the end of a very long stick was used to take the sample.  This allowed the team to stay a safe distance away from the cliffs.  Once an appropriate sample was taken it was placed in a plastic bag and brought back to the lab for analysis.Due to the large variation in particle sizes present in the sample, it was necessary to split the sample and analyse it using two methods.  It was determined that the more coarse elements of the sample would be examined via wet sieving and the finer elements would be processed with the laser granulometer.Before being split, the entire sample was dried in an oven at 100oC.  This was to remove moisture from the sample so that only weight from the particles themselves would be measured.  Once the sample was dried, a sub sample for use with the granulometer was taken using a riffle box.  The riffle box splits the sample in half.  To ensure that sampling was random it was decided that the box at the front of the riffle box would always be kept and the box at the rear would be removed from the process.  The sample was repeatedly divided in half until a subsample of roughly five grams was achieved.  This sample was then processed with the laser granulometer.All material not in the subsample was reamalgamated. To begin the wet sieving process.  The sample was weighed to two decimal places using a coarse balance.  The sample was then added to a bucket (that was cleaned appropriately) and covered with tap water.  To break the sediment down, 100ml of 50g/l sodium hexametaphosphate solution was added to the bucket.  The sample was stirred continuously until fully broken down.  Nine sieves ranging from 32mm down to 250µm were then selected.  They were stacked from highest mesh diameter down to lowest and rubber O-rings were placed between each one to ensure the sieves were sealed completely.  The sieve shaker was then arranged with the outlet tube facing the sink.  The stack of sieves was placed onto the sieve shaker base pan and sealed with an O-ring.  The sample was then poured from the bucket into the stack of sieves.  The outlet tub from the last sieve poured into a new bucket in the sink.  The first bucket was rinsed out with water to ensure all sediment went through the sieves.  The lid of the sieve shaker was firmly attached with twin nuts and fitted with a tube that supplied water from the tap.  The tap was then turned on and the sieve shaker turned switched on as the water began to flow through.  The amplitude of the shaker was set to 2 and it ran for roughly ten minutes until the water flowing out had turned clear.  When finished the sieves were separated out and placed into different trays.  These trays were again dried out in an oven at 100OC.  Once dried completely the contents of each sieve was weighed and recorded.As particles smaller than 250µm were lost in this process, the subsample was analysed using a Malvern Mastersizer 2000.  The results from these processes were then combined and entered into the Gradistat excel program.  All further particle analysis was done using this program.Fabric SamplingIn order to analyse the fabric of the material a series of strike and dip measurements were taken at various locations around the beach.  Roughly 50 measurements were taken at 8 different locations, along with some lithological data.  To measure the strike of a rock, the compass was placed with its edge against the rock, with the long axis of the compass parallel to the line of strike. The red needle was then aligned with the red arrow on the compass.  The direction of strike can then be read from the notch at either end of the compass, and either measurement taken will be correct. To measure the dip of the rock, the compass was set with an East-West orientation.  The compass was then held with its edge against the rock being measured.  The angle of dip could then be read from the black needle hanging vertically on the compass.  This was repeated for each rock at the location, and then the process was repeated at other locations.All strike and dip measurements taken were compiled using Microsoft excel. They were then made into rose diagrams using the Stereonet software and further analysed in this format.In addition to the fabric data and grain size information, a number of photographs were taken on site.  Sketches and sediment logs were also made.ResultsGrain Size Analysis Gravel Sand Mud Diagram Fabric AnalysisFor ease of analysis the fabric data taken at each location was processed using Stereonet software and represented with a rose diagram.Location IGR: O 26100 22703 Location IGR: O 26097 22735 Location IGR: O26067 23034 Location IGR: O 26054 23221 Location IGR: O 26099 22703 Location IGR: O 26098 22708 Location IGR: O 26026 22731 Location IGR: O 26060 23221 Clast lithology data showed that a large majority of the rocks in all locations were limestone, with some sandstones and mudstones also present along with minor amounts of chert, granite and quartzite.  The majority of rocks were sub-rounded or sub-angular.DiscussionA clear North-South trend is displayed in all eight of the rose diagrams, with some diagrams showing a very slight lean in the Northwest-Southeast direction.  A North-South trend is consistent with the direction of movement of the Irish Ice sheet in the Shankill region at the time. The grain size analysis provides valuable information when processed with the Gradistat program.  The proportions of various particle sizes from each sample are accurately displayed in such a way as to make them easily comparable.  Statistics such as the standard deviation of the particle size, the positive or negative skews in particle sizes or the kurtosis of each sample are valuable pieces of information and say much about conditions in which the sediment formed (Landim and Frakes, 1968).  The frequencies of various particle sizes are useful in understanding the transport energies involved in their deposition.  A till or diamict that more frequently has larger particles was likely formed in an area with a higher transport energy.  The skew of a curve measures the length of the tails of each end.  A sample with a positive skew will have more fine material, whereas a negative skew tells us that there is more coarse material.  The skew also tells us that the sediment spent a disproportionate amount of time in an area with a certain transport energy, i.e. a negative skew tells us that the sediment was in an area of high transport energy for longer.  The kurtosis of a sample is a ratio between how well sorted the tails of a curve are and how well sorted its centre is.  Variations in kurtosis tell us how much fluctuations in transport energy impacted the tails of the sample relative to its centre.  Statistics such as the skew and kurtosis were generated by Gradistat but the data set is quite unwieldy and as such has not been included in this report.Fig 2.1 Photograph of Lower Diamicton Fig 2.2 Sediment log displaying both upper and lower diamictons The lower diamicton was an extremely poorly sorted gravelly mud.  There is a slight negative skew which implies that this sediment spent a slightly longer than average time in a high energy environment where coarser particles were more easily moved about.  As with the rest of the samples, it has a clear North-South fabric trend.  The sediment was also platykurtic, which implies that the medium of transport may have been more erratic than usual (Landim and Frakes, 1968).  The lower diamicton was highly over-consolidated, implying that it had undergone some form of stress, which was also demonstrated by the shear planes which can be seen in figure 2.1.Above the lower diamicton was a layer of “Very coarse silty sandy very coarse gravel”, labelled as Cobble Gravel in the particle size analysis chart.  This was a very poorly sorted sediment that had a positive skew, and was mesokurtic.  This sediment fits reasonably well with the criteria laid down in Landim and Frakes (1968) for an alluvial fan. It has a positive skew and large mean value.  It is mesokurtic rather than slightly leptokurtic however this is a minor difference.  The criteria do specify that the sediment should be more well sorted and this sample did not meet this, meaning that this analysis is inconclusive.The upper diamicton lies above the layer labelled cobble gravel.  It is similar to the lower diamicton in that it is extremely poorly sorted and is a coarse gravelly fine silt.  It differs in that there is no skew and it is mesokurtic.  The sediment is also less consolidated than the lower diamicton and there are no shear planes visible, suggesting that it has undergone much less stress than the lower diamicton.  The fabric of the upper diamicton also differs slightly.  It has a slight lean to Northwest-Southeast unlike the North-South of the lower diamicton.The results indicate that the lower diamicton was firstly deposited the initial advance of the Irish Ice sheet.  The fabric of the diamicton is consistent with the direction of ice flow at this time.  After the first diamicton was deposited and the area was briefly deglaciated, the sediment between the two diamictons was deposited.  The upper diamicton was then deposited during a glacial readvance.  It is possible that that this readvance was the Irish Ice sheet or ice advancing from the Wicklow mountain ice cap at the same time as the readvance.  Ice from the Wicklow mountains ice cap is consistent with the slight Northwest-Southeast fabric displayed by upper diamicton(Clerc et al., 2012), although this slight difference in fabric does not rule out the possibility that the readvance of the Irish Ice Sheet was responsible for the upper diamict, Due to similarities in the clast lithology data between the upper and lower diamicts, it appears that the upper diamict cannibalised material from the lower diamict.  The upper diamict also appears to be responsible for the overconsolidation of the first diamict.  The force of the glacial activity that led to the deposition of the upper diamict resulted in large amounts of shear stress being placed on the lower diamict (Rijsdijk, Warren and van der Meer, 2010) which is visible in the shear planes seen figure 2.1.Fig 2.3 Photograph of Upper Diamicton Fig 2.4 ConclusionGlacial sediments found in the Shankill region are a result of glacial activity from the Irish Ice Sheet.  Fabric and particle size data verify that that a lower diamict seen on Shankill beach is consistent with the initial advance of the Irish Ice Sheet.  After an interstitial period where more sediments were deposited a second glacial readvance led to the deposition of another diamict.  This readvance may have been the result of the Irish Ice Sheet advancing again but it is also possible that a readvance of ice from the Wicklow Mountain Ice Cap is responsible.  Slight differences in fabric are seen between the two diamicts and the second was formed from material cannibalised from the first.  The formation of the second diamict led to the lower diamict undergoing large amounts of shear stress.  This resulted in overconsolidation of the sediment and the presence of visible shear planes in the lower diamict.

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