Preliminary analysis of penetrative strain fabrics from the Carboniferous Namurian Basin;

County Claire; Western Ireland. Lopez, Alexis R.1, Avalos, René M.1, Nittrouer, Jeff2 , Burmeister, Kurtis C.1

1. University of the Pacific, Stockton, Ca 2. Rice University, Houston, Tx

ABSTRACTPreliminary results of a three-dimensional normalized Fry analysis of grain scale strain suggest that strata

within Namurian Basin have accumulated very low magnitudes of penetrative strain. The five samples

analyzed were collected along a south (nearest to a collisional mountain belt) to north (towards the

undeformed interior) along the Loop Head Peninsula. All samples contain very low strain magnitudes (es

values of less than 0.2). The southernmost sample is from the Ross Formation at Killbaha Bay, and is the

only sample that contains a weak constrictional strain fabric likely produced by tectonic convergence.

Remaining samples were collected from the Ross (at Ross Bay and Bridge of Ross) the Tullig (at Killard

Bay), and Killkee Formations (at Quilty) have well developed flattening strain fabrics that are likely

associated with deposition and compaction. Analyses were conducted using three, mutually perpendiciular

petrographic thin sections prepared from orientated samples. Photomicrographs of these thin sections were

digitized using the EllipseFit software package (Vollmer, 2015). The EllipseFit program determines grain

centers and uses the distances between adjacent gains to calculate the ellipticity, orientation, and magnitude

of rock fabrics. The results of rock fabric analysis are best communicated with Flinn and Hsu diagrams,

which illustrate the degree of flattening, plane, or constrictional strain in each sample.

GEOLOGIC SETTINGSamples were collected from the Western Irish Namurian Basin (WINB) (Figure 1). This setting contains a

Late Carboniferous siliclastic sequence that was inverted during the Variscan Orogeny. Western Europe

tectonic setting during Late Carboniferous: West of the WINB is the orthogonal convergence, in the north

and east of England there is extension, and due west of Ireland there was a development of a large

carbonate ramp (Figure 2).

Units

The Ross Formation is distinguished by thick successions of turbidite deposits and reoccurring channel

infill. This was most likely caused by deposition on the proximal plane. The Tullig Formation is composed

of peat swamps, distal mouth-bar successions, and fluvio-deltaic channel sequences. The Kilkee sequence is

very similar to the Tullig Formation, with both being cycoltherm sequences (Figure 3).

Figure 1. A location map of Ireland displaying the locations in which the samples were collected. Included is the

orientation of each of the samples completed by 2D analysis of the parallel to bedding layer.

Figure 2. Cross section of the Western Irish Namurian Basin, showcasing the structural relationships between

Ross Formation, Tullig Cyclothem, and Kilkee Cyclothem. From Tanner, David C. et al, 2011.

Figure 3. Stratigraphic column of sediments in the Western Irish Namurian Basin. Samples A, B, and C were

collected from Ross Formation. Sample D was collected from Tullig formation, and sample E was collected from

Kilkee Cyclothem. Ross formation is a thick sheet-like sandstone, interbedded with fine layers of silty mudstones.

Tullig includes thick transgressive sedimentary package that marks the transition between fluvial deltaic channel

sandstone and a distill low grain mouthbar succession deltaic Cyclothem. Kilkee Formation is a deltaic cyclothem

similar to Tullig Formation. Modified from Tanner, David C. et al, 2011.

Figure 4. Sample A is from the Ross Formation located in Kilbaha

Bay. Sample B is also from the Ross formation in Ross Bay. Sample

C is part of the Ross Formation from Bridge of Ross. Sample D is

part of the Tullig Formation from Killard Bay. Sample E is a piece of

the Kilkee Formation from Quilty. Samples A, B, C, and D are all

litharenites. Sample 5 is a sublitharenite. Microscopic view of

litharenite samples A, B, C, D, and sublitharenite sample E.

Photomicrographs taken at x10 scale. Software by Vollmer, 2011-

2014.

Figure 5. Five samples

collected from the WINB were

each cut into three orientated

slices. The cuts were notched

to indicate the orientation of

the sample. The three cuts

were oriented along the X, Y,

and Z axes. Parallel to Strike

(PTS) is cut along the YZ axes.

In Dip Parallel (IDP) is cut

along the XZ axes. Parallel to

Bedding is cut along the XY

axes. Each individual cut

produced 2-D data. Once

analyzed, the information

from the three cuts was

compiled to produce 3-D data

to determine magnitude and

orientation of strain.

Figure 6.

Photomicrograph of sample A (IDP) showing the notch used to key orientation. Image was

uploaded into EllipseFit and grains were then traced to show preferred orientation of individual

grains. Software by Vollmer, 2011-2015.

Figure 8. Hsu diagram showing direction and

magnitude of strain in samples A, B, C, D, and E.

The V axis is the measure of direction of strain: -

1.0 through 0.0 is correlated to elongation,

whereas 0.0 through +1.0 is correlated to

flattening. 𝜀 describes the mean strain suffered by

the grains. Software by Vollmer, 2011-2015.

SUMMARY AND INTERPRETATIONSThe Western Irish Namurian Basin (WINB) contains the Ross Formation, Tullig Formation, and Kilkee Formation. This

area has undergone deformation by folding and faulting. Rock samples collected from this region would be expected to

have undergone a mentionable amount of strain. Using EllipseFit (Figure 6), ellipsoid data was compiled and

showcased a low magnitude of strain, contradicting the structural geology in the region. Flinn diagrams (Figure 7) of

samples B, C, D, and E produced an oblate ellipsoid; while sample A was found to be a prolate ellipsoid. Hsu diagrams

(Figure 8) were created and found that all samples were strained with very little magnitude. Hsu also showed that

samples B, C, D, and E were strained in favor of flattening, while sample A was strained in favor of elongation. Sample

A, collected in Killbaha Bay, was the most southern sample obtained. It is concluded that strain affected Killbaha Bay

differently. 2D analysis showed that grains were not trending parallel to strike, countering what is expected for fold

strain.

Figure 7. Flinn diagram showing strain direction of

samples A, B, C, D, and E. K=1 is the line of plane strain.

Above the line of plane strain represents prolate strain.

Below the line of plane strain represents oblate strain.

Software by Vollmer, 2011-2015.

REFERENCESBurmeister, Kurtis C., Nittrouer, Jeff, Phillips, Andrew C., and Bryk, Alexander B., 20, Nucleation of growth faults

along dewatering structures within the Namurian Tullig Formation; Killard Bay, Western Clare County, Ireland.

Tanner, D. C., Bense, F. A., & Ertl, G, 2011, Kinematic retro-modelling of a cross-section through a thrust-fold belt:

the Western Irish Namurian Basin, Geological Society, London, Special Publications, 61-76.

Vollmer, Frederick, 2011-2015, Ellipsefit Software.

ACKNOWLEDGEMENTS Dr. Jeff Nittrouer and Maya Stokes for logistical and scientific support. University of the Pacific Undergraduate

Research Fund and Pacific Fund for providing funding for this project. Dr. Frederick Vollmer for use of EllipseFit

software and insightful discussions.