Title: Geological investigation of buried limestone valleys between Lough Corrib and Galway City
Researchers: Megan Dolan, Dr Bryan McCabe
Describe both the research purpose and the underlying need for this research. Include sufficient detail for the readers to get acquainted with the project without having to refer to your proposal This project was prompted by the discovery of a 109 m deep, sediment-filled karst solution feature during geotechnical investigation for the N6 Galway City Outer-bypass, led by Arup. The location of this feature (adjacent to Lackagh Quarry in Coolough, just north of Galway City) is the focus and study site for this project. Cores taken from this site yield unconsolidated materials including clay, sand, gravel, cobbles and boulders. Organic material is seen at several sections through the core. The purpose of this project is to better understand the nature of this geological feature and the impact of this feature to civil engineering work. These features are known to exist in Ireland however there has been no research on their implications to the development of our infrastructure. Until research is carried out these features will remain a source of uncertainty for engineers and construction workers and a threat to Irish development projects of all scales, particularly in Galway where we know them to exist and where one will be engaged with for the construction of a tunnel as part of the N6 Galway City Outer-bypass.
With the extent of karst conduits in the Galway region I hypothesize that the karst solution feature under investigation for this project is the result of one or more collapse events into a karst tunnel originally formed by a subsurface river (as seen near Gort at modern day). Eustatic and isostatic fluctuation possibly submerged and drained the tunnel repeatedly. It is likely that intense solution during the tropical climatic conditions of the Palaeogene and Neogene caused extensive solution of the overhead limestone causing the roof of the tunnel to become weak and eventually give way. Hence the feature could represent a collapse doline or a solution doline which has also experienced minor collapse episodes (limestone gravel suspended through core). I hypothesize that the collapse event took place prior to the Gortian Warm Stage (302,000-428,000) of the Quaternary and likely prior to the Quaternary. With three distinct and separate layers of boulder clay seen in core from the study site the lower infill sediment must date back to at least the Gortian, being the warm stage before the 3rd last glaciation. A preliminary inspection of pollen from organic horizons within the core suggests a Late–Tertiary or Early Pleistocene age but a thorough analysis is necessary to propose a confident age. The Galway limestone is heavily karstified, noticeable by the lack of surface streams and rivers, the abundance of turloughs and the presence of fresh water springs and swallow holes and this karstification is recorded in much literature. Another large karst depression (20 m deep) was found some 20km away from the project study site at Pollnahallia, north Galway, which hosts Neogene sediments (Coxon and Flegg, 1987). It is likely that other large karst depressions exist throughout the Galway limestone-bearing region. It seems reasonable that these features may be linked with certain limestone units (The Galway limestones are currently mapped as ‘undifferentiated Viséan limestones’). With boulder clay, sand, silty sand, sandy silt and laminated muds (both organic and inorganic) it is apparent that the infill sediment was deposited in a variety of environments seemingly inclusive of glacial, lacustrine and fluvial. This variety will prove the elucidation of sediment behaviour and its control on local hydrogeology difficult, even using sophisticated techniques in soil mechanics.
Aims: This project aims to resolve the formation, presence and implications of large-scale karst depressions across Galway. The project aims to determine the timing and nature of formation for this large karst depression whilst also exploring the extent of karstification throughout Galway limestones. Using a comprehensive desk-study, areas deemed as high-risk for potential large scale karst features will be delineated. The project also intends to recognise the geotechnical and hydrogeological implications of these features for consideration before future construction work within potential karst areas and particularly within the vicinity of high-risk areas outlined by the project.
Objectives: There are a number of objectives within this project to satisfy the three main criteria outlined within the project aims. 1) Formation – In order to understand how this large karst depression feature formed the local limestone in which the karst feature developed will be scrutinised in a petrographic study. Inferring the timing of formation is tricky however it is imperative to the overall understanding of the nature of karstification and so this project includes a pollen analysis of the infill which will allow for the deduction of palaeoenvironments and construction of pollen assemblages which will be used to correlate the deposit to other organic deposits found in Ireland and maybe elsewhere in Europe in an attempt to help date the sediment and thus help work out the age of the karst feature. 2) Presence – For the purpose of surmising the whereabouts of other large karst depressions the limestone in which the karst depression is located will be logged and where possible the limestones will be mapped on a more regional scale. A comprehensive desk study will compile all of the existing available geological, geotechnical and geophysical data for the Galway limestones, including GSI data, OSi data, environmental impact statements and geotechnical reports. This all-inclusive compilation will be used to help locate or predict the presence of other large scale karst features. 3) Implications – To gain an understanding of the geotechnical and hydrogeological implications of this kind of karst feature various geotechnical lab tests will be carried out on the infill sediment. Field investigation will explicate the impact of this feature on local groundwater.