Chapter 18 Statistical Analysis of High-Resolution Heavy Mineral Stratigraphic Data from the Ordovician of Western Ireland and its Tectonic Consequences

High-resolution heavy mineral data from the Ordovician of the South Mayo Trough, western Ireland is subject to a detailed statistical analysis. The aim of this study is to develop new techniques for treating such data thereby providing an independent and objective way to help elucidate the stratigraphy and tectonic history of the region. A new method is presented for plotting normalised scores to emphasise stratigraphic trends of heavy minerals and specific heavy mineral varieties, which are shown to be largely non-normally distributed, with significant variation between formations. A method is developed for testing significance of variation within a formation using non-parametric tests. Techniques are introduced for comparing and correlating formations on the basis of heavy mineral data even when sample sizes are small. The likely causes of correlations between heavy mineral distributions and zircon varieties are discussed, and principal components analysis is used to identify major source regions throughout the life of this basin. This analysis shows that there is a statistically significant change with early deposits dominated by arc-accretionary prism detritus, whereas juvenile metamorphic detritus becomes increasing more important up-section. The South Mayo Trough remained below sea-level during this progressive change in provenance. The generally accepted model for the evolution of this basin is that it represents a fore-arc basin that collided with the Laurentian margin during the Grampian orogeny. The problem of how a hanging wall basin remained below sea-level during and after arc-continent collision is discussed and explained.