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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