PETROFABRIC ANALYSIS OF SAMPLES FROM THE CYCLOTHEM SEQUENCES OF THE WESTERN IRISH NAMURIAN BASIN (WINB) DEFORMED WITHIN THE VARISCAN FORELAND FOLD THRUST BELT

Penetrative fabrics measured using traditional, normalized Fry techniques yielded little evidence for penetrative strain in samples collected within the classic cyclothem sequences of the Western Irish Namurian Basin (WINB). Indeed, even though strata are strongly deformed by kilometer-scale fault related folds associated with the Variscan (Hercynian) foreland fold thrust belt, grain-scale petrofabrics contain little to no discernable tectonic component. Five oriented samples were collected from the Ross, Tullig, and Kilkee Fms along a cross-strike transect through the fold-thrust belt from south (towards hinterland) to north (unreformed foreland). Samples range from sandy siltstone to very fine-grained sandstone, are weakly cemented with carbonate and/or quartz, and contain clay minerals, occasional mica aligned parallel to bedding, and rare carbonate and quartz grains. Cathodoluminescent investigation identified quartz overgrowth cements with the same orientations as crystal dust rims on several grains in the samples, but they do not obscure grain shapes. The EllispeFit computer program (Vollmer, 2014) was used to digitize the boundaries of 300 to 500 grains in photomicrographs of three mutually perpendicular thin sections prepared from each sample. EllispeFit compiled individual 2D fabric ellipses into a 3D ellipsoid for each sample. Measured fabrics are weakly elliptical (R values fall between 1.019 and 1.114). Oblate fabrics were measured in all but one sample and have shape parameters (K) ranging from 0.19 to 0.71. Oblate fabrics appear to be consistent with vertical flattening associated with deposition and compaction. The only prolate fabric (shape parameter K of 1.21) was measured in the southernmost sample, collected nearest to the Variscan hinterland. However, the long axis (X) of this fabric ellipsoid is near perpendicular to trend of fold axes within the fold-thrust belt and may represent a composite primary/tectonic fabric. The clay minerals in these rocks may provide a basis for further fabric analysis using the anisotropy of magnetic susceptibility (AMS). If successful, magnetic fabrics may be a more sensitive proxy for tectonic shortening directions within the WINB and provide an opportunity to examine the results of recent studies of regional macroscopic strain fabrics.