The Leitrim Group is a
lithostratigraphical term coined to refer to the succession of rock
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
which occur in
Northern Ireland
Northern Ireland ( ga, Tuaisceart Éireann ; sco, label= Ulster-Scots, Norlin Airlann) is a part of the United Kingdom, situated in the north-east of the island of Ireland, that is variously described as a country, province or region. Nort ...
within the
Visean and
Namurian
The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma (million years ago). It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is ...
stages
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Acting
* Stage (theatre), a space for the performance of theatrical productions
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* ''The Stage'', a weekly British theatre newspaper
* S ...
of the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
Period
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* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
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* Periodic sentence (or rhetorical period), a concept ...
. The group
disconformably overlies the Dartry Limestone of the
Tyrone Group
The Tyrone Group is a lithostratigraphical term coined to refer to a particular succession of rock strata which occur in Northern Ireland within the Visean Stage of the Carboniferous Period.
It comprises a series of limestones, shales and s ...
.
Stratigraphy
The group comprises a series of
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
s and
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
s which in stratigraphic order (youngest/uppermost at top) are:
Lackagh Sandstone Formation
The formation is 60-90m thick in the
Connaught Coalfield but only about 36m thick on
Cuilcagh Mountain
Cuilcagh () is a mountain on the border between County Fermanagh (in Northern Ireland) and County Cavan (in the Republic of Ireland). With a height of it is the highest point in both counties. It is also the 170th highest peak on the island of ...
where it forms the prominent cliff-edged summit surface as also at Belmore.
[''Marble Arch Caves Global Geopark; Exploring the landscape of Fermanagh and Cavan'' (map), 2015 GSNI ] It is unfossiliferous.
Gowlaun Shale Formation
Some 50-60m thickness of shales and mudstones of Arnsbergian age constitute this formation.
Briscloonagh Sandstone Formation
About 53m of Pendleian age sandstones with interlayered siltstones and some mudstone make up this formation.
Dergvone Shale Formation
Mostly dark coloured iron-rich shales of Pendleian age, it reaches up to 128m thick. From its top downwards, it is divided into the Lacoon Sandstone, Killooman Shale, Tonlegee Shale, Tullyclevaun Shale, Black Mountain Shales and Gubaveeny Shale members.
Carraun Shale Formation
From 50m to over 160m thick, this Brigantian age formation is largely shale but contains thin limestones, siltstones and sandstones, some of which are given their own names as
members
Member may refer to:
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e.g. the Doagh Limestone and Tawnyunshinagh Limestone members.
Bellavally Formation
This Asbian to Brigantian age formation varies from 45m to 80m thick and consists largely of sandstones and shales with lesser amounts of siltstone and limestone. From its top downwards, it is divided into the Corry, Sheena Shale, Glenkeel, Doobally Sandstone, Drummangarvagh, Lugasnaghta, Sraduffy, Larkfield and Tullyskeherny members.
Glenade Sandstone Formation
Referred to as the Yoredale Sandstone in the late nineteenth century, this Asbian age sequence varies from 4m thick in County Leitrim to as much as 350m in the north of its range. In places it sits directly on the Dartry Limestone where the Meenymore Formation rocks are missing due to the locally high relief of the underlying erosive surface. Animal fossils are almost wholly absent from this formation though fragments of ''Calamites'' and leaf remains are more common.
Meenymore Formation
The lowermost and hence oldest division of the Group is of Asbian age. It comprises mudstones and limestones and delta sandstones, two of which are mapped as separate ‘members’; the Glen and Quarry Sandstone, the former only being seen west of the Glen Syncline. The thickness of the formation varies from 18m at Doagh to 100m to the east of Cuilcagh Mountain. A depositional hiatus separates this formation from the underlying
Dartry Limestone
The Dartry Limestone is a geologic formation in Ireland. It preserves fossils dating back to the Carboniferous period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertai ...
.
References
*
{{Geology of Northern Ireland
Carboniferous System of Europe
Geology of Ireland
Geology of Northern Ireland
Geological groups of the United Kingdom