Earth Science Conservation Review
| Carboniferous Subarea-11; Cranfield-Greencastle | Down |
| Site Type: | Coastal section |
| Site Status: | |
| Council area: | Newry & Mourne District Council |
| Grid Reference: | J26111050,J24031177 |
| Google maps: | 54.02881,-6.07564 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Visean) |
| Rock Name: | Carlingford Limestone Group |
| Rock Type: | Limestone, Oolitic limestone, Sand, Sandstone, Turbidite |
| Interest | |
| Fossil Groups: | Alga, Brachiopod, Coral, Foraminifera |
| Other interest: | cross-bedding, ripple marks, Marine sediments |
Introduction:
In the eastern half of N. Ireland the outcrop of Carboniferous rocks is restricted to four small outliers in Counties Antrim and Down. The outcrop in the Cranfield-Greencastle area of south Co. Down is the most southerly in N. Ireland and forms a small portion of a much larger outcrop that straddles the entrance to Carlingford Lough and extends along the north side of Dundalk Bay in the Republic of Ireland. This area has not yet been resurveyed by the Geological Survey of N. Ireland. The information contained in this report was gathered during a brief reconnaissance visit to the area. Nonetheless the new data contributes greatly to our understanding of the palaeogeographical development of N. Ireland during Lower Carboniferous times and this outcrop is an important addition to the network of Carboniferous ASSI in the Province. Apart from the name Carlingford Limestone Group (GSNI, 1978) no other subdivisions or formal lithostratigraphical names have been given to these strata but their mid-Arundian age is not in doubt and is clearly demonstrated by the abundant and diagnostic coral- brachiopod fauna.
Location: This small outcrop of Carboniferous rocks occurs at the southeasterly tip of Co. Down on the north side of the entrance to Carlingford Lough. With the exception of a few small inland exposures the best outcrops occur on the coast between Charleys Rock near the west end of Cranfield Bay and a small isolated knoll (H2475 1145) located 1.7km to the NW near Greencastle and 400m south of the ruins of Green Castle.
Bibliographic Review: The Carboniferous rocks of the Carlingford - south Down region were mapped by the Geological Survey of Ireland in 1870-73 and depicted on the One-Inch geological map (Sheet 71) of the Carlingford area in 1876. Since then the area has been largely ignored although Sevastopulo (in Holland, 1981) assigned a probable Chadian and Arundian age to the sequence of shallow water, shelf, sandy bioclastic limestones and 'birds-eye' micrites in the southern sector of the main outlier. The GSNI reviewed the geology of the Mourne Mountains (N. Ireland Special Sheet, 1:50,000 Series, 1978) and included a brief description of the Carlingford Limestone Group, on the north side of Carlingford Lough, in the geological notes. Amongst the macrofauna recorded from these rocks by the GSNI was the brachiopod Gigantoproductus sp. which would indicate a late-Vis‚an (Asbian-Brigantian Stages) age. The identification is now known to be incorrect and they are undoubtedly Arundian.
General Geology: The Carboniferous rocks of the Greencastle-Cranfield area are thought to rest unconformably on Lower Palaeozoic rocks of the Down-Longford Massif which crop out to the north of the basal unconformity. The position of this contact at the western end of the outlier near Greencastle is concealed by beach sand but occurs in an area some 500m wide, between adjacent outcrops of these rocks. The trace of this contact to the east of Greencastle is poorly constrained and is drawn at a point midway between opposing outcrops of the respective rock types. East of the disused airfield at Ameracam Lane, rock head is always covered by a very thick deposit of glacial till and fluvioglacial sand and gravel and the southeasterly trace of this contact, intersecting the coast some 1.3km NE of Cranfield Point, is wholly conjectural. In view of the orientation of Carboniferous strata in the western part of the outlier, and their proximity to the unfaulted (?) contact with the Lower Palaeozoic rocks at the base of this sequence, it would appear to be geometrically implausible for that contact to conform to the present configuration. A more likely scenario envisages the eastern segment of the sub- Carboniferous unconformity following the eastern edge of Cranfield Bay, only 5-600m east of the lowest strata exposed in the outlier, at Charleys Rock and having an identical orientation to the bedding at that exposure. Under these circumstances the Carboniferous strata in this part of south County Down may be in the order of 250-300m thick.
Just over 6km to the north of these Carboniferous rocks is the southern margin of the Palaeocene Mourne Mountains Granites that are intruded into, and thermally metamorphose, the country rock of Lower Palaeozoic greywackes and slates. Between 7-8km west of Greencastle on the west side of Carlingford Lough is the somewhat smaller intrusive and extrusive Carlingford Complex which intrudes both Lower Palaeozoic and Carboniferous rocks, the latter in particular, in places, being intensely recrystallised and altered to calc-silicate minerals. Thermal alteration of the Carboniferous rocks in the Greencastle-Cranfield area is however minimal and is confined to slight recrystallisation of the carbonate matrix and occasional remobilisation of late stage calcite cements where they infill cavities in fossil remains, particularly in colonial corals such as Siphonodendron and Syringopora. It is also possible that some of these effects may have been caused by the later intrusion of basic dykes, several of which traverse this outlier.
Location: This small outcrop of Carboniferous rocks occurs at the southeasterly tip of Co. Down on the north side of the entrance to Carlingford Lough. With the exception of a few small inland exposures the best outcrops occur on the coast between Charleys Rock near the west end of Cranfield Bay and a small isolated knoll (H2475 1145) located 1.7km to the NW near Greencastle and 400m south of the ruins of Green Castle.
Bibliographic Review: The Carboniferous rocks of the Carlingford - south Down region were mapped by the Geological Survey of Ireland in 1870-73 and depicted on the One-Inch geological map (Sheet 71) of the Carlingford area in 1876. Since then the area has been largely ignored although Sevastopulo (in Holland, 1981) assigned a probable Chadian and Arundian age to the sequence of shallow water, shelf, sandy bioclastic limestones and 'birds-eye' micrites in the southern sector of the main outlier. The GSNI reviewed the geology of the Mourne Mountains (N. Ireland Special Sheet, 1:50,000 Series, 1978) and included a brief description of the Carlingford Limestone Group, on the north side of Carlingford Lough, in the geological notes. Amongst the macrofauna recorded from these rocks by the GSNI was the brachiopod Gigantoproductus sp. which would indicate a late-Vis‚an (Asbian-Brigantian Stages) age. The identification is now known to be incorrect and they are undoubtedly Arundian.
General Geology: The Carboniferous rocks of the Greencastle-Cranfield area are thought to rest unconformably on Lower Palaeozoic rocks of the Down-Longford Massif which crop out to the north of the basal unconformity. The position of this contact at the western end of the outlier near Greencastle is concealed by beach sand but occurs in an area some 500m wide, between adjacent outcrops of these rocks. The trace of this contact to the east of Greencastle is poorly constrained and is drawn at a point midway between opposing outcrops of the respective rock types. East of the disused airfield at Ameracam Lane, rock head is always covered by a very thick deposit of glacial till and fluvioglacial sand and gravel and the southeasterly trace of this contact, intersecting the coast some 1.3km NE of Cranfield Point, is wholly conjectural. In view of the orientation of Carboniferous strata in the western part of the outlier, and their proximity to the unfaulted (?) contact with the Lower Palaeozoic rocks at the base of this sequence, it would appear to be geometrically implausible for that contact to conform to the present configuration. A more likely scenario envisages the eastern segment of the sub- Carboniferous unconformity following the eastern edge of Cranfield Bay, only 5-600m east of the lowest strata exposed in the outlier, at Charleys Rock and having an identical orientation to the bedding at that exposure. Under these circumstances the Carboniferous strata in this part of south County Down may be in the order of 250-300m thick.
Just over 6km to the north of these Carboniferous rocks is the southern margin of the Palaeocene Mourne Mountains Granites that are intruded into, and thermally metamorphose, the country rock of Lower Palaeozoic greywackes and slates. Between 7-8km west of Greencastle on the west side of Carlingford Lough is the somewhat smaller intrusive and extrusive Carlingford Complex which intrudes both Lower Palaeozoic and Carboniferous rocks, the latter in particular, in places, being intensely recrystallised and altered to calc-silicate minerals. Thermal alteration of the Carboniferous rocks in the Greencastle-Cranfield area is however minimal and is confined to slight recrystallisation of the carbonate matrix and occasional remobilisation of late stage calcite cements where they infill cavities in fossil remains, particularly in colonial corals such as Siphonodendron and Syringopora. It is also possible that some of these effects may have been caused by the later intrusion of basic dykes, several of which traverse this outlier.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-11; Cranfield-Greencastle, County Down, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1184. Accessed on 2024-12-26 |
| Previous Site | Next Site |