1. Geology of the Silurian Successions,
west of Louisburgh, County Mayo,
Ireland.
Ian Cronnolly

2. • Collected information created the evidence
that enabled to define a particular formation.
• A depositional environment could then be
suggested based on interpretations made.
• Objectives were to describe & map rock
formations in the locality.
• Lithology, bedding orientation and
sedimentary structures were all recorded.
Mapping Louisburgh.

3. Mapping Location
Louisburgh Town
N
Image source: Google Earth Satellite imagery.

4. Turloughbeg Strake Banded
Formation
• Dominantly composed of red siltstone interlaminated
with a medium light grey sandstones and greenish
grey siltstones.
• Evident sedimentary structures shows that formation
is younging in a south-easterly direction.
1m

5. Desiccation cracks in cross-section
Parallel & ripple cross laminations

6. Roonah Grey Sandstone
Formation
• Dominantly composed of medium light grey, fine
to medium grained sandstone.
1m
1m
• Heavy folding, faulting, soft sediment
deformation evident within this formation.

7. 1m
Soft sediment deformation Mudstone-sandstone facies
Soft & brittle deformation

8. Cleahy Siltstone Formation
• Composed of red siltstone interbedded with
light grey, fine grained calcareous sandstones.
• A Pollglass Member forms the top of this
formation and contains buff, red & light grey
sandstones and red siltstone beds.

9. Desiccation cracks
Pseudonodules in
calcareous sandstone

10. 5cm
Desiccation cracks
Truncated wave ripple laminations

11. Albristy Pebbly Arkose
Formation
• Consists of a buff pebbly sandstone interbedded
with minor red & medium light grey sandstones and
red siltstones.
• A gradual change is observed near the top of the
Cleahy Siltstone into a more dominant pebbly
sandstone formation.
1m
1m

12. Concretion
Convolute Laminations
Cross Stratification

13. Emlagh Formation
5m
• A metamorphic grade formation composed
mainly of light greenish grey phyllites and pelites.
• Outcrop brittle and cleaves easily in places.
• Foliations within outcrop were also observed.

14. Foliated outcrop
Deformation events
S1
S2

15. Stratigraphy column
Louisburgh-Clare Island Silurian Succession:
Croagh Patrick Silurian Succession:
Albristy Pebbly Arkose Formation.
Pollglass Member.
Cleahy Siltstone Formation.
Roonah Grey Sandstone Formation.
Turloughbeg Strake Banded Formation.
Emlagh Formation

16. 400 800m
N
inferred
inferred
Constructed map of the area.

17. Conclusion
A total of five formations were mapped over the designated area and concluded that
there were two main successions, one composed of sedimentary rock formations
mainly and the other of a metamorphic grade rock.
References
• Google Earth satellite imagery.
• PHILLIPS, W. E. A. 1974. The Stratigraphy, sedimentary environments & palaeogeography of the Silurian
strata of Clare Island, Co. Mayo, Ireland. Journal of the Geological Society, London, 130, 19-41.
• PHILLIPS, W. E. A., RICHARDS, R. B. & DEWEY, J. F. 1970. The lower Palaeozoic Rocks of the Louisburgh area,
Co. Mayo. Proceedings of the Royal Irish Academy, 70, 195-210.
Welcome any questions you may have!

18. Cleahy Siltstone
Formation
0.7m

19. Cleahy Siltstone
Formation
pinched out beds
& deformation
Albristy Pebbly
Arkose Formation

20. Turloughbeg Siltstone
Formation
Emlagh Schist
Formation