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

Ian Cronnolly

• 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.

Mapping Location

Louisburgh Town

N

Image source: Google Earth Satellite imagery.

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

Desiccation cracks in cross-section

Parallel & ripple cross laminations

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.

1m

Soft sediment deformation Mudstone-sandstone facies

Soft & brittle deformation

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.

Desiccation cracks Pseudonodules in calcareous sandstone

5cmDesiccation cracks

Truncated wave ripple laminations

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

Concretion

Convolute LaminationsCross Stratification

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.

Foliated outcrop

Deformation events

S1

S2

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

400 800m

N

inferred

inferred

Constructed map of the area.

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!

Cleahy Siltstone Formation

Doleritic Dyke0.7m

Cleahy Siltstone Formation

pinched out beds& deformation

Albristy PebblyArkose Formation

Turloughbeg SiltstoneFormation

Emlagh Schist Formation