| Site Description |
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 | | Lower end of stream section in Curraghlane Townland, near Dungiven, Co.Londonderry, exposing red mudstones of the Sherwood Sandstone Group. |
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| Highlights: |
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| A well exposed stream section through lithologies representative of the Triassic Sherwood Sandstone Group. |
| Introduction: |
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| The exposure in Curraghlane townland occurs in the western escarpment of the Antrim plateau, ~4km east of Dungiven. An incised stream section exposes rock from C722 095 to C726 096. |
| The lithologies and sedimentary structures indicate a continental `red bed' sequence representative of the Triassic Sherwood Sandstone Group. The area was mapped during the original geological survey (GSI, 1884) and on the published map these sediments were labelled as Keuper Sandstone with marls. There is no modern literature specifically describing the Curraghlane section, and the rocks are exposed in an area currently under resurvey by the Geological Survey of Northern Ireland. |
| Description: |
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| The section, from the farm buildings (C722 095) to the road (C724 095), is composed of interbedded reddish brown calcareous mudstones (marls) and siltstones with subordinate fine to medium- grained, quartzose sandstones. Beds appear planar and are usually less than 20cm thick. Thicker mudstones (>1m) are also present through the section. Sandstone beds occasionally show low angle cross-stratification and contain scattered, well rounded quartz pebbles and angular intraclasts of mudstone. Sand grains are commonly very well rounded and have a high sphericity (`millet-seed' grains). Occasional beds, fractures, joints and irregular patches of the rock show a greenish grey coloration. |
| From the road (C724 095) to the top of the section, brick-red sandstone forms an increasing proportion of the sequence. Beds, up to 1m thick, are stacked, erosively based planar and trough cross-stratified. Grain size is medium to very coarse and a high proportion of grains are millet-seed. Numerous well rounded pebbles and angular intraclasts of mudstone are present on stratification surfaces. |
| Importance: |
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| Good quality sections through the Triassic Sherwood Sandstone Group are not common in this area. The rocks form extensive aquifers (e.g. Lagan Valley, Belfast) and potential hydrocarbon reservoirs (e.g. Rathlin Basin, Co. Antrim); it is therefore important to conserve them for geological study. |
| Interpretation: |
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| The sequence below C724 095 appears mainly to be a floodplain sequence, deposited under semi-arid conditions. The siltstones and fine sandstones were probably deposited from unconfined sheetflood processes. Thin mudstones represent late stage deposition from standing bodies of water; polygonal mudcracks formed as this material dried. The combination of processes explains the rhythmical nature of the sequence. The thicker mudstones may represent playa lake deposits. |
| The stacked sandstone beds within the section above C724 095 are interpreted as in-channel transverse bars, deposited within a high energy braided river. Coarse-grained, millet-seed beds, commonly containing pebble grade material are probably reworked aeolian deposits. Intraclasts represent transported mudcrack material. |
| The brick-red coloration is attributable to haematite rich clays; these coat sand grains and act as a weak cement. The presence of greenish grey beds, fracture and joint surfaces and irregular patches within the rocks indicates reduction of the haematite clays. This may in part be related to depositional processes, but has also occurred at depth as indicated by coloration associated with fault and joint surfaces. |
| Conclusions: |
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| Exposed at this site is a rare section through the Triassic Sherwood Sandstone Group. Its value lies in its similarity to aquifers and potential hydrocarbon reservoirs in Northern Ireland. It should therefore be conserved. |
| Notes: |
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For information on other Triassic sites see Key Site 1197 - Brown's Bay, Island Magee Key Site 1198 - Cloghfin Port, Island Magee Key Site 1199 - Collin Glen Key Site 1200 - Scrabo Hill - Triassic Key Site 1201 - Milltown Key Site 1202 - Drapersfield Quarry Key Site 1203 - Lissan Water. |
| For general information on the Triassic System in N. Ireland see Key Site 1192 - The Triassic System in N. Ireland - Introduction. |