Sheshymore Limestone Gallery
In the Sheshymora area of northwestern of County Clare, Ireland, the Carboniferous Brigantian
Slievenaglasha Formation crops out and is formed by 3 to 20m thick cyclic
units of crinoidal grainstones with coral thickets and chert
y limestones (Gillespie et al, 2001). The overlying Namurian Clare Shales lie unconformably over the Slievenaglasha Formation (Hodson, 1952). The Slievenaglasha Formation is exposed as pavements at Sheshymore. These provide a classic overview of exposures of a kharstified limestone surface. The margins of tabular blocks of limestone pavement, "clints", are marked by joints and fractures that exhibit evidence of dissolution, "grikes". This dissolution and widening of the fractures and joints is post glacial (McNamara, & Hennessy, 2010). Variscan folding initiated the fractures (Coller, 1984).
The Sheshmore pavements of have been described by Gillespie et al (2001) to have two prominent types of fracture: barren joints and mineralized veins. They explain how these two fracture sets have contrasting size and spacing characteristics. They show how the veins are clustered and have power-law size distributions, and explain how in contrast the joints are non-clustered
and do not exhibit power-law size distributions. Their paper reports on how using low level aerial photographs they created high-quality fracture maps that allowed relatively easy recognition and characterization of the fracture patterns.
Paraphrasing Gillespie et al (2001), they concluded that the Burren joints occur in strata bound fracture networks formed during uplift. They explain that joints have regular spacings, the scale of which is related to the thickness of the mechanical units. They recognized that they form well connected networks, with the lengths of the joints controlled by abutting relationships and providing a log-normal distribution.They attributed the different scaling properties of joints and veins to different overburden stresses at the time of formation. They proposed that the veins formed at greater depths than the uplift-related joints, under conditions that
allowed the propagation of fractures across bedding discontinuities and the consequent development of non-strata-bound scaling properties.
They record that modelling suggests the critical depth for vertical fracture propagation across bedding discontinuities in the Burren is 0.6 to .2.5 km, for stress ratios of 0.25±0.75, and for pore fluid pressures that are at, or in excess of, hydrostatic. They indicate these predictions are consistent with
the estimated depths of vein formation (1.25km) with evidence suggesting pore fluid pressures in excess of hydrostatic. The Burren study demonstrates that strata bound and non strata bound fractures, characterised by very different scaling properties, can be developed within the same lithological sequence but at different formation conditions.
Bibliography and References
Drew, D. (2001), Classic Landforms of the Burren Karst, Geographical Association in conjunction with the British Geomorphological Research Group. p. 52.
Drew, D.P. (1973) A preliminary study of the geomorphology of the Aillwee area, central Burren,Co. Clare: Proceedings of the University of Bristol Spelaeological Society 13, p. 227-244.
Drew, D.P., Burke, A.M., & Daly, D. (1996) Assessing the extent and degree of karstification in Ireland: Proceedings of International Conference on Karst Fractured aquifers - vulnerability and sustainability, Katowice-Ustron, Poland, p. 37-47.
Gillespie,P.A., Walsh, J.J., WattersonJ., Bonson, C.G., and Manzocchi, T. (2001), Scaling relationships of joint and vein arrays from The Burren, Co. Clare, Ireland, Journal of Structural Geology 23, 183-201.
Grahan, J.R. & Ryan, A. (2000) IAS DUBLIN 2000 Field Trip Guidebook. Department of Geology, Trinity College, Dublin. 152 pp.
McNamara, M. A and Hennessy, R. W., 2010, The Geology of the Burren region, Co. Clare, Ireland Burren Connect Project.175pp.
Simms, M. (2006) Exploring the limestone Landscapes of the Burren and the Gort Lowlands. Belfast. 64 pp.
Simms, M. J. (2005) Glacial and karst landscapes of the Gort lowlands and Burren, in Cocon, P. (Ed) The Quaternary of central western Ireland. Quaternary Research Association London Field Guide, p. 37-61.
Sweeting, M.M. (1955 ) The landforms of northwest County Clare, Ireland. The Institute of British Geographers 21, p.33-49.
Williams, W. (1966), Limestone pavements with special reference to Western Ireland. Transactions of the Institute of British Geographers 40, p. 155.
Tuesday, September 17, 2013