Earth Science Conservation Review
| Carboniferous Subarea-8; Newtownstewart Outlier | Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Omagh District Council, Strabane District Council |
| Grid Reference: | H4987 |
| Google maps: | 54.72906,-7.23941 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Tournasian) |
| Rock Name: | Omagh Sandstone Group |
| Rock Type: | Crinoidal limestone, Limestone, Mudstone, Sand, Sandstone, Shale |
| Interest | |
| Fossil Groups: | Miospore |
| Other interest: | Deltaic sediments, Lacustrine sediments, Marine sediments |
Description:
INTRODUCTION
The Geological Survey of Northern Ireland have not remapped the area that includes the Carboniferous rocks of the Newtownstewart Outlier. However new information on the age of these beds and, to a lesser extent, on the variation in lithologies present at some localities was gathered during a reconnaissance survey of the area. Historically these strata were assigned to the Lower and Upper divisions of the Lower Carboniferous Sandstone by the Geological Survey of Ireland. Since then the area was briefly referred to in an unpublished Ph.D thesis and by Sheridan (1972) who recognised similarities with the rocks of the Omagh Sandstone Group (Simpson, 1955) in the Omagh area but saw no strong reason for assigning the entire outlier to that group. For the purposes of this review the rocks are referred to the Omagh Sandstone Group but it has not been possible or considered reasonable to differentiate individual formations or members on the basis of a rapid inspection. The stream section chosen as the representative site for these rocks contains only a portion of the lithologies represented in the outlier but is located just above the basal unconformity with the metamorphic basement and has yielded miospore assemblages that accurately define the age of the rocks.
The interpretation of the palaeogeography of the landscape in late Tournaisian time in N. Ireland is discussed in Key Site 1173 and will only be briefly referred to in this report.
I - OMAGH SANDSTONE GROUP
The term Omagh Sandstone Group was introduced by Simpson (1955) for a suite of sedimentary rocks located stratigraphically at the base of the Lower Carboniferous succession in, what he referred to as, the Omagh Syncline. His estimated thickness of 600m was excessive and he failed to recognise representatives of this group which crop out at the SW end of the Lack Inlier, resting unconformably on the Dalradian. The group was composed of, ".... quartzose conglomerate, red grits and sandstones with occasional beds of grey shale and muddy limestone" (p.393). In its stratotype area, north of Omagh, this description is largely correct. However, in other parts of the group outcrop, e.g., on the north and SW sides of the Lack Inlier, the sequence commences in thin, coarse-grained clastics but quickly passes up into shales, mudstones and fine-grained sandstones with abundant examples of laminated stromatolitic limestones, serpulid limestones and crinoidal bioclastic limestones. These lithologies were clearly deposited initially in a deltaic or lacustrine environment but were rapidly overwhelmed by encroaching marine conditions. Thus the Omagh Sandstone Group, in the Kesh-Omagh area is a highly variable sequence.
The succession of lithologies in the Newtownstewart Outlier is dissimilar to Simpson's description of the Omagh Sandstone Group but in many respects conforms precisely to the composition of the group as recognised, by the GSNI, in parts of the Kesh-Omagh area described above.
The provisional assignment of the rocks in the Newtownstewart Outlier to the Omagh Sandstone Group is supported further by the common occurrence of CM Zone miospore assemblages in both areas.
For site specific information see; Key Site 283 - Middletown Burn, Culvacullion Townland.
The Geological Survey of Northern Ireland have not remapped the area that includes the Carboniferous rocks of the Newtownstewart Outlier. However new information on the age of these beds and, to a lesser extent, on the variation in lithologies present at some localities was gathered during a reconnaissance survey of the area. Historically these strata were assigned to the Lower and Upper divisions of the Lower Carboniferous Sandstone by the Geological Survey of Ireland. Since then the area was briefly referred to in an unpublished Ph.D thesis and by Sheridan (1972) who recognised similarities with the rocks of the Omagh Sandstone Group (Simpson, 1955) in the Omagh area but saw no strong reason for assigning the entire outlier to that group. For the purposes of this review the rocks are referred to the Omagh Sandstone Group but it has not been possible or considered reasonable to differentiate individual formations or members on the basis of a rapid inspection. The stream section chosen as the representative site for these rocks contains only a portion of the lithologies represented in the outlier but is located just above the basal unconformity with the metamorphic basement and has yielded miospore assemblages that accurately define the age of the rocks.
The interpretation of the palaeogeography of the landscape in late Tournaisian time in N. Ireland is discussed in Key Site 1173 and will only be briefly referred to in this report.
I - OMAGH SANDSTONE GROUP
The term Omagh Sandstone Group was introduced by Simpson (1955) for a suite of sedimentary rocks located stratigraphically at the base of the Lower Carboniferous succession in, what he referred to as, the Omagh Syncline. His estimated thickness of 600m was excessive and he failed to recognise representatives of this group which crop out at the SW end of the Lack Inlier, resting unconformably on the Dalradian. The group was composed of, ".... quartzose conglomerate, red grits and sandstones with occasional beds of grey shale and muddy limestone" (p.393). In its stratotype area, north of Omagh, this description is largely correct. However, in other parts of the group outcrop, e.g., on the north and SW sides of the Lack Inlier, the sequence commences in thin, coarse-grained clastics but quickly passes up into shales, mudstones and fine-grained sandstones with abundant examples of laminated stromatolitic limestones, serpulid limestones and crinoidal bioclastic limestones. These lithologies were clearly deposited initially in a deltaic or lacustrine environment but were rapidly overwhelmed by encroaching marine conditions. Thus the Omagh Sandstone Group, in the Kesh-Omagh area is a highly variable sequence.
The succession of lithologies in the Newtownstewart Outlier is dissimilar to Simpson's description of the Omagh Sandstone Group but in many respects conforms precisely to the composition of the group as recognised, by the GSNI, in parts of the Kesh-Omagh area described above.
The provisional assignment of the rocks in the Newtownstewart Outlier to the Omagh Sandstone Group is supported further by the common occurrence of CM Zone miospore assemblages in both areas.
For site specific information see; Key Site 283 - Middletown Burn, Culvacullion Townland.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-8; Newtownstewart Outlier, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1181. Accessed on 2024-12-26 |
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