| Site Description |
|---|
 | | Type section of the Ballintoy Chalk Member at Ballintoy Port, Co.Antrim. |
|
| Highlights: |
|---|
| The section contains excellent exposures of the Ulster White Limestone Formation and includes the type sections for the Boheeshane Chalk, Larry Bane Chalk and Ballintoy Chalk members. The section is representative of the Cretaceous succession of the Rathlin Basin. |
| Introduction: |
|---|
| The Mesozoic rocks from Dunnaglea to Larry Bane Bay outcrop on the north coast of Co. Antrim, 7km NW of Ballycastle. Portlock (1843) described ochre from Ballintoy and also gave a faunal list. The exposures include the cliff sections together with Larry Bane Quarry. The area was mapped and an explanatory memoir produced by Symes et al. (1888). More detailed work has been carried out in recent years by Fletcher (1977) and Wilson and Manning (1978). |
| Description: |
|---|
The Cretaceous rocks exposed between Dunnaglea and Larry Bane Bay include the Senonian members of the Ulster White Limestone Formation from the Cloghastucan Chalk Member to the Ballymagarry Chalk Member. West of Ballintoy Harbour in the Dunnaglea cliff section (Site a) there is a collapsed sea stack on the raised beach.
 | | Section of the Cloghastucan Chalk Member at Dunnaglea on the W side of Ballintoy Harbour, showing well the contact with the Galboly Chalk Member below and the Creggan Chalk Member above, and the Oweynamuck Flint Band. |
The stack is intensely wind weathered and the Galboly Chalk, Cloghastucan Chalk and Creggan Chalk members can be examined. The sedimentary structures and microfaunal content have been etched out and can thus be closely analysed. |
East of Ballintoy (Site b) is the type section of the Ballintoy Chalk Member and there are good exposures of the Altachuile Breccia. To the east of the Bendoo Plug (Site c) are the type sections of the Boheeshane Chalk Member and the Larry Bane Chalk Member. Immediately to the east of the plug on the rock platform are exposures of the Bendoo Pebble Bed at the top of the Creggan Chalk Member.
 | | Boheeshane Bay, showing platform exposures of the Bendoo Pebble Bed, with the far cliffs exposing members from the Cloghastucan Chalk to the base of the Ballintoy Chalk. |
The pebbles in this bed are predominantly glauconitised derived fossils, mainly rolled including hexactinellid sponges, echinoids and belemnites. |
| The Boheeshane Chalk Member reaches its maximum thickness of 24.19m in the type section in Boheeshane Bay. The base is defined by the Bendoo Pebble Bed and its upper limit is taken at the well developed erosion surface immediately under- lying the base of the Larry Bane Chalk Member. Three main beds can be differentiated. Bed A, 8.33m thick, is characterised by the development of small tabulate flints along incipient separation planes. This bed weathers into thin posts particularly near the base, where occasional green pebble horizons may occur. This bed comprises a lower flintless unit and an upper flinty unit. Bed B, 8.69m thick, comprises six main units and is marked by a central prominent nearly continuous flint band. Bed C, 7.17m thick, has very prominent flint bands. Gonioteuthis is found up to 3.66m below the upper limit of the Boheeshane Chalk Member. |
The most distinctive marker beds in the Rathlin Basin are the two limestone posts of the Larry Bane Chalk Member which are limited by prominent strong erosion surfaces. At the type section in Larry Bane Bay the member is 7.29m thick, with the lower Bed A 2.44m thick approximately one half the thickness of Bed B (4.85m thick).
 | | Larry Bane Bay, with vertical cliffs exposing members from the Boheeshane Chalk to the Ballintoy Chalk; higher members of the Ulster White Limestone Formation are exposed landward of the prominent fault plane. |
Very large belemnites are particularly prominent in this member and are notably numerous on the exposed bedding planes. |
| The Ballintoy Chalk Member conformably overlies the Larry Bane Chalk Member and is conformably succeeded by the Glenarm Chalk Member. It comprises two beds with a thickness ratio of approximately 3:1 and is delimited by conspicuous separation planes; it is terminated by the Altachuile Breccia. The type section is taken as the gently dipping rock platforms in Ballintoy Harbour. Two types of flint are present. The lower half of the member is characterised by numerous closely spaced small nodular flints which appear to be randomly scattered. The higher beds, at the top of Bed A and in Bed B, contain small, often discontinuous, tabular flints. A marked faunal change occurs in the lower part of Bed A which contains numerous large forms of Echinocorys ex gr. conica that are not represented in the higher horizons. The sediment of the Altachuile Breccia has a distinctly pelletal texture with lumps of indurated chalk present as discrete bodies within a matrix of hard wavy bedded limestone, and it contains small angular chips of flint scattered in a haphazard manner. |
| Importance: |
|---|
| Between Dunnaglea and Larry Bane Bay one of the most complete sequences of the Ulster White Limestone Formation is well exposed. The site includes the type sections for the Boheeshane Chalk, Larry Bane Chalk and Ballintoy Chalk members. The site is therefore of national interest and should be conserved. |
| Interpretation: |
|---|
| The Boheeshane Chalk Member occurs throughout the Rathlin Basin and in the Larne-Lough Neagh Basin. Over part of the Dalradian ridge, the Londonderry Shelf and the Lagan Valley it has been overlapped by younger members and is therefore not represented. Where, as a result of intraformational erosion and/or overlap in the marginal areas of depositional basins, the key Larry Bane Chalk Member is not preserved, the identification of the Boheeshane Chalk Member is possible only on palaeontological grounds. |
| The Larry Bane Chalk Member occurs throughout the Rathlin Basin and the Larne-Lough Neagh Basin. Over the Dalradian ridge, the Londonderry Shelf and the Lagan Valley it has been overlapped by younger members. The strongly marked erosion surfaces are emphasised by present day weathering. Sediments immediately above and below these levels show signs of considerable primary disturbance and, on the better weathered surfaces many fragmental limestone pellets are seen within the wavy bedded matrix. Shell debris is abundant and remani‚ shell and limestone fragments possibly indicate there are small breaks in the sequence at these points. |
| The Altachuile Breccia has been interpreted as resulting from the churning of partially consolidated bottom sediment during violent weather conditions such as a hurricane (Fletcher, 1977). The fragmentation of flint would suggest a synsedimentary origin with the flint fully formed in the sediment prior to disturbance of the latter by current action. Despite the disturbed nature of the sediment well preserved specimens of Offaster, which are not damaged, are present. Thus the sediment was again colonised after the turbulent event. |
| The Ballintoy Chalk Member is found in the Rathlin Basin and in the Larne-Lough Neagh Basin where sedimentation was influenced by the basement structures, the Carnlough and Larne axes but is absent from the Londonderry Shelf and the Lagan Valley. It forms the basal transgressive unit of the Ulster White Limestone Member over the crest of the Dalradian ridge. |
| Conclusions: |
|---|
| The Dunnaglea to Larry Bane Bay site provides the type sections for three of the Campanian members of the Ulster White Limestone Formation and excellent exposures of the oldest Campanian and Santonian members. The history of chalk sedimentation in the Rathlin Basin can be determined here. Also well exposed are the prominent marker bands, the Oweynamuck Flint Band, the Bendoo Pebble Bed, the Altachuile Breccia and the North Antrim Hardgrounds. The site should be conserved. |
| Notes: |
|---|
For general information on the Cretaceous System in N. Ireland see Key Site 1194 - The Cretaceous System in N. Ireland - Introduction. |
| For information on other Cretaceous sites see Key Site 1207 - White Rocks - Cretaceous, Key Site 1208 - White Park Bay, Key Site 1210 - Port Calliagh-Ballycastle, Key Site 1211 - Capecastle Quarry, Key Site 1212 - Garron Point-Cloghastucan, Key Site 1213 - Glenarm Quarry, Key Site 1214 - Creggan Quarry, Key Site 1215 - Portmuck, Island Magee, Key Site 1216 - White Head, Key Site 1217 - Clarehill Quarry, and Key Site 1218 - Kilcoan, Island Magee. |
| References |
|---|
| Fletcher, T. P. 1977: Lithostratigraphy of the Chalk (Ulster White Limestone Formation) in Northern Ireland. Report of the Institute of Geological Sciences, vol. 77/14 |
| Portlock, J.E. 1843: Report on the Geology of Londonderry and parts of Tyrone and Fermanagh. , pp.784 , Dublin |
| Symes, R.G., Egan, F.W. & McHenry, A. 1888: Explanatory Memoir to accompany Sheets 7 and 8. Memoirs of the Geological Survey of Ireland |
| Wilson, H. E. and Manning, P. I. 1978: Geology of the Causeway coast. Memoir for one-inch geological sheet 7. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast |