Earth Science Conservation Review
| The Cretaceous System in N. Ireland - Introduction | Antrim, Down, Londonderry, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Cretaceous (Campanian, Cenomanian, Coniacian, Maastrichtian, Santonian, Turonian, Upper Cretaceous) |
| Rock Name: | Ballintoy Chalk Member, Ballycastle Chalk Member, Ballymagarry Chalk Member, Belfast Marls Member, Boheeshane Chalk Member, Cloghastucan Chalk Member, Cloghfin Sponge Beds Member, Collinwell Sands Member, Creggan Chalk Member, Galboly Chalk Member, Garron |
| Rock Type: | Glauconitic sandstone, Greensand, Limestone, Sand, Sandstone |
| Interest | |
| Other interest: | Marine sediments |
Description:
INTRODUCTION
The Cretaceous System takes its name from creta, the Latin for chalk. The stages in the system are shown below:
-------------------------------------------- | | | Maastrichtian | | | |-----------------| | | | Campanian | | | |-----------------| | | | Santonian | | | UPPER |-----------------| | | | Coniacian | | | |-----------------| | | | Turonian | | | |-----------------| | | | Cenomanian | | |---------|-----------------| | | | Albian | | CRETACEOUS | |-----------------| | | | Aptian | | | |-----------------| | | | Barremian | | | LOWER |-----------------| | | | Hauterivian | | | |-----------------| | | | Valanginian | | | |-----------------| | | | Ryazanian | --------------------------------------------
CORRELATION OF CRETACEOUS STRATA
The Cretaceous system is divided into stages which are defined by their ammonite zones (Rawson et al., 1978). In N. Ireland, only Upper Cretaceous strata are known.
Since the papers of McGugan (1957) and Reid (1958a; 1958b) the White Limestone was divided into two separate faunal units: (i) beds earlier than the zone of Belemnitella mucronata, and (ii) the zone of Belemnitella mucronata (Hancock, 1961). The uppermost limit of Gonioteuthis is taken as the base of the Belemnitella mucronata Zone. In the Northern Irish succession this level falls within a bed marked by two prominent erosion surfaces. In the type section of the quadrata-mucronata zonal boundary succession, there is a prominent erosion surface 3.66m above the extinction level of Gonioteuthis, marking the base of the most easily recognised member of the Ulster White Limestone Formation, termed the Larry Bane Chalk Member by Fletcher (1977). His suggestion that successions be described as comprising pre- Larry Bane Chalk, Larry Bane Chalk or post-Larry Bane Chalk is followed here.
The oldest Ulster White Limestone deposits are of socialis Zone at White Park Bay, Carnlough and on Island Magee, and with succeeding testudinarius and pilula Zones form the lowest three members in the Larne-Lough Neagh Basin. The sequence is attenuated in the Larne area and the area may have been one of uplift during the Santonian.
In the Rathlin Basin these three Zones are present throughout but the succession is interrupted by a number of erosion surfaces and the upper part of the pilula Zone is missing. The Boheeshane Chalk Member of quadrata and lowest mucronata Zones is present in the main basins only and is overstepped on to the Dalradian ridge and the areas to the west by mucronata Chalk. The Garron Chalk is the earliest member in much of the area west of Belfast. The succeeding Portrush Chalk is the lowest bed in south Co. Londonderry. The uppermost Senonian chalk occurs extensively in the Rathlin Basin and locally in parts of the Lagan Valley and in the northern part of the Londonderry Shelf. These beds have been removed by erosion in the Larne-Lough Neagh Basin.
The Maastrichtian chalks occur mainly on the north coast.
DISTRIBUTION OF CRETACEOUS STRATA IN NORTHERN IRELAND
The outcrop of the Cretaceous rocks is largely restricted to the margins of the Antrim Plateau. During the Cretaceous the importance of basin and shelf development must be emphasised. A number of basins and more positive areas were defined by Fletcher (1977) and their use is continued here, although the names have been changed to reflect more closely local geographic regions.
The Hibernian Greensands Formation is thickest in the Larne-Lough Neagh Basin from Garron Point to Groganstown, near Belfast. It is also found on the northern part of the Londonderry Shelf.
The thickest and most complete successions of the Ulster White Limestone Formation occur in the Larne-Lough Neagh Basin and the Rathlin Basin. Towards the margins of the basins the successions are condensed and non-sequences develop, but taken as a whole the post-Santonian succession is one of the most complete in Northern Europe. The greatest known thickness is 91m in the Rathlin Basin though an anomalous 150m is recorded from an old borehole in Co. Tyrone.
LITHOSTRATIGRAPHY OF THE CRETACEOUS
Whitehurst in 1786 equated the limestone with the Chalk of Southern England but the Cretaceous strata were first divided by Portlock (1843) into:-
Upper Chalk (White Limestone) Lower Chalk (Glauconitic Chalk) Greensands
Bryce in 1853 divided the greensands into three distinct units:-
3. Greyish white impure limestone pervaded with chlorite grains 2. Buff-coloured calcareous sandstone 1. Slightly cohering green sandy bed, a true greensand.
In 1865 Tate made the first major attempt to correlate the Cretaceous in Ireland with that in Britain. He detected a hiatus in the Irish sequence within the greensands and he stated that the Upper Cretaceous fell naturally into two lithological formations; Upper Chalk and underlying Greensands. The upper part of the greensand sequence he correlated with the Upper Chalk of England and for the sediment older in age he proposed the name `Hibernian Greensand'. In SE Antrim the greensands were readily separable into three distinct lithological units:-
Zone 3. Chloritic Sands and Sandstones Zone 2. Grey Marls and Yellow Sandstones with Chert Zone 1. Glauconitic Sands.
The upper limit of the Hibernian Greensand was drawn within Zone 3 at the incoming of an Upper Chalk fauna. Two further divisions were then made in the Upper Chalk:-
2. White Limestone or Hard Chalk 1. Chloritic Chalk or Basement-bed of the White Limestone.
The first examination of the whole Irish Cretaceous outcrop was undertaken by Hume (1897) who assigned a zonal classification to the lithological subdivisions of Tate. Further zonal classifications were undertaken by McGugan (1957) and Reid (1958b). A new revision was undertaken by Hancock (1961) who included `glauconitic chalk' in the Hibernian Greensand. Fletcher (1977) proposed that the name `Hibernian Greensands' be restricted to glauconitic sands and related clastic deposits and that `glauconitic chalk' be regarded as a local lithological variant of the more standard White Limestone units.
The formal lithostratigraphical classification of the Hibernian Greensands Formation was detailed by Griffith and Wilson (1982). The Ulster White Limestone was divided by Fletcher (1977) into fourteen members, of which eight are Campanian in age. The Cloghfin Sponge Beds Member was included as the basal member of the Ulster White Limestone Formation.
CRETACEOUS PALAEOGEOGRAPHY
The late Jurassic and early Cretaceous seem to have been times of erosion and of little or no deposition in Ireland. The extent of the Irish landmass is uncertain. In the early Cenomanian the sea transgressed the southern part of the area and deposited the Lower Cenomanian Greensands which were partly derived from granitic and metamorphic rocks to the north. The earliest deposits were the dark green glauconitic marls of the Belfast Marls Member which were followed by the yellow sandstones and mudstones of the Island Magee Siltstones Member. A sharp disconformity is followed by the local development of the Collinwell Sands Member. Following a period of non-deposition and widespread erosion, which removed the whole of the pre-Senonian sequence from many places, the Senonian Greensands, the Kilcoan Sands Member, were deposited in the Larne-Lough Neagh Basin. In the southern part of the outcrop this part of the succession begins with glauconitic sand and sandstone, soft and largely unbedded, with layers of Inoceramus fragments and reworked material from the Cenomanian Greensands. During the early- Senonian, the Rathlin Basin was scoured by northerly currents which transported only phosphatised fragments from the exposed Lower Lias floor. Chalk infillings in some of the fossils suggest that chalk sedimentation began earlier than the remaining Ulster White Limestone would indicate, but that the beds were removed by erosion.
Following a further period of temporary uplift, chalk deposition began. The resulting Cloghfin Sponge Beds Member rests disconformably on the Hibernian Greensands Formation.
Chalk sedimentation continued throughout the Senonian and early Maastrichtian and since great thicknesses of limestone were removed in the pre-basalt period of erosion it is quite possible that deposition was not terminated until very late in Cretaceous times. However chalk deposition was not constant over these stages and several breaks in the stratigraphical sequence indicate that conditions were not always tranquil.
A gradual downwarping of the basins took place to receive what might be in excess of 150m of chalk sediment. The more stable ancient underlying structures can be seen affecting the chalk deposition particularly near the Dalradian ridge which however was covered at all times by the Chalk sea. It seems likely that the Senonian transgression extended over much, if not all, of present-day Ireland and far beyond.
Late Cretaceous or early Palaeogene erosion has removed any trace of post-Maastrichtian beds if they ever existed.
The Cretaceous System takes its name from creta, the Latin for chalk. The stages in the system are shown below:
-------------------------------------------- | | | Maastrichtian | | | |-----------------| | | | Campanian | | | |-----------------| | | | Santonian | | | UPPER |-----------------| | | | Coniacian | | | |-----------------| | | | Turonian | | | |-----------------| | | | Cenomanian | | |---------|-----------------| | | | Albian | | CRETACEOUS | |-----------------| | | | Aptian | | | |-----------------| | | | Barremian | | | LOWER |-----------------| | | | Hauterivian | | | |-----------------| | | | Valanginian | | | |-----------------| | | | Ryazanian | --------------------------------------------
CORRELATION OF CRETACEOUS STRATA
The Cretaceous system is divided into stages which are defined by their ammonite zones (Rawson et al., 1978). In N. Ireland, only Upper Cretaceous strata are known.
Since the papers of McGugan (1957) and Reid (1958a; 1958b) the White Limestone was divided into two separate faunal units: (i) beds earlier than the zone of Belemnitella mucronata, and (ii) the zone of Belemnitella mucronata (Hancock, 1961). The uppermost limit of Gonioteuthis is taken as the base of the Belemnitella mucronata Zone. In the Northern Irish succession this level falls within a bed marked by two prominent erosion surfaces. In the type section of the quadrata-mucronata zonal boundary succession, there is a prominent erosion surface 3.66m above the extinction level of Gonioteuthis, marking the base of the most easily recognised member of the Ulster White Limestone Formation, termed the Larry Bane Chalk Member by Fletcher (1977). His suggestion that successions be described as comprising pre- Larry Bane Chalk, Larry Bane Chalk or post-Larry Bane Chalk is followed here.
The oldest Ulster White Limestone deposits are of socialis Zone at White Park Bay, Carnlough and on Island Magee, and with succeeding testudinarius and pilula Zones form the lowest three members in the Larne-Lough Neagh Basin. The sequence is attenuated in the Larne area and the area may have been one of uplift during the Santonian.
In the Rathlin Basin these three Zones are present throughout but the succession is interrupted by a number of erosion surfaces and the upper part of the pilula Zone is missing. The Boheeshane Chalk Member of quadrata and lowest mucronata Zones is present in the main basins only and is overstepped on to the Dalradian ridge and the areas to the west by mucronata Chalk. The Garron Chalk is the earliest member in much of the area west of Belfast. The succeeding Portrush Chalk is the lowest bed in south Co. Londonderry. The uppermost Senonian chalk occurs extensively in the Rathlin Basin and locally in parts of the Lagan Valley and in the northern part of the Londonderry Shelf. These beds have been removed by erosion in the Larne-Lough Neagh Basin.
The Maastrichtian chalks occur mainly on the north coast.
DISTRIBUTION OF CRETACEOUS STRATA IN NORTHERN IRELAND
The outcrop of the Cretaceous rocks is largely restricted to the margins of the Antrim Plateau. During the Cretaceous the importance of basin and shelf development must be emphasised. A number of basins and more positive areas were defined by Fletcher (1977) and their use is continued here, although the names have been changed to reflect more closely local geographic regions.
The Hibernian Greensands Formation is thickest in the Larne-Lough Neagh Basin from Garron Point to Groganstown, near Belfast. It is also found on the northern part of the Londonderry Shelf.
The thickest and most complete successions of the Ulster White Limestone Formation occur in the Larne-Lough Neagh Basin and the Rathlin Basin. Towards the margins of the basins the successions are condensed and non-sequences develop, but taken as a whole the post-Santonian succession is one of the most complete in Northern Europe. The greatest known thickness is 91m in the Rathlin Basin though an anomalous 150m is recorded from an old borehole in Co. Tyrone.
LITHOSTRATIGRAPHY OF THE CRETACEOUS
Whitehurst in 1786 equated the limestone with the Chalk of Southern England but the Cretaceous strata were first divided by Portlock (1843) into:-
Upper Chalk (White Limestone) Lower Chalk (Glauconitic Chalk) Greensands
Bryce in 1853 divided the greensands into three distinct units:-
3. Greyish white impure limestone pervaded with chlorite grains 2. Buff-coloured calcareous sandstone 1. Slightly cohering green sandy bed, a true greensand.
In 1865 Tate made the first major attempt to correlate the Cretaceous in Ireland with that in Britain. He detected a hiatus in the Irish sequence within the greensands and he stated that the Upper Cretaceous fell naturally into two lithological formations; Upper Chalk and underlying Greensands. The upper part of the greensand sequence he correlated with the Upper Chalk of England and for the sediment older in age he proposed the name `Hibernian Greensand'. In SE Antrim the greensands were readily separable into three distinct lithological units:-
Zone 3. Chloritic Sands and Sandstones Zone 2. Grey Marls and Yellow Sandstones with Chert Zone 1. Glauconitic Sands.
The upper limit of the Hibernian Greensand was drawn within Zone 3 at the incoming of an Upper Chalk fauna. Two further divisions were then made in the Upper Chalk:-
2. White Limestone or Hard Chalk 1. Chloritic Chalk or Basement-bed of the White Limestone.
The first examination of the whole Irish Cretaceous outcrop was undertaken by Hume (1897) who assigned a zonal classification to the lithological subdivisions of Tate. Further zonal classifications were undertaken by McGugan (1957) and Reid (1958b). A new revision was undertaken by Hancock (1961) who included `glauconitic chalk' in the Hibernian Greensand. Fletcher (1977) proposed that the name `Hibernian Greensands' be restricted to glauconitic sands and related clastic deposits and that `glauconitic chalk' be regarded as a local lithological variant of the more standard White Limestone units.
The formal lithostratigraphical classification of the Hibernian Greensands Formation was detailed by Griffith and Wilson (1982). The Ulster White Limestone was divided by Fletcher (1977) into fourteen members, of which eight are Campanian in age. The Cloghfin Sponge Beds Member was included as the basal member of the Ulster White Limestone Formation.
CRETACEOUS PALAEOGEOGRAPHY
The late Jurassic and early Cretaceous seem to have been times of erosion and of little or no deposition in Ireland. The extent of the Irish landmass is uncertain. In the early Cenomanian the sea transgressed the southern part of the area and deposited the Lower Cenomanian Greensands which were partly derived from granitic and metamorphic rocks to the north. The earliest deposits were the dark green glauconitic marls of the Belfast Marls Member which were followed by the yellow sandstones and mudstones of the Island Magee Siltstones Member. A sharp disconformity is followed by the local development of the Collinwell Sands Member. Following a period of non-deposition and widespread erosion, which removed the whole of the pre-Senonian sequence from many places, the Senonian Greensands, the Kilcoan Sands Member, were deposited in the Larne-Lough Neagh Basin. In the southern part of the outcrop this part of the succession begins with glauconitic sand and sandstone, soft and largely unbedded, with layers of Inoceramus fragments and reworked material from the Cenomanian Greensands. During the early- Senonian, the Rathlin Basin was scoured by northerly currents which transported only phosphatised fragments from the exposed Lower Lias floor. Chalk infillings in some of the fossils suggest that chalk sedimentation began earlier than the remaining Ulster White Limestone would indicate, but that the beds were removed by erosion.
Following a further period of temporary uplift, chalk deposition began. The resulting Cloghfin Sponge Beds Member rests disconformably on the Hibernian Greensands Formation.
Chalk sedimentation continued throughout the Senonian and early Maastrichtian and since great thicknesses of limestone were removed in the pre-basalt period of erosion it is quite possible that deposition was not terminated until very late in Cretaceous times. However chalk deposition was not constant over these stages and several breaks in the stratigraphical sequence indicate that conditions were not always tranquil.
A gradual downwarping of the basins took place to receive what might be in excess of 150m of chalk sediment. The more stable ancient underlying structures can be seen affecting the chalk deposition particularly near the Dalradian ridge which however was covered at all times by the Chalk sea. It seems likely that the Senonian transgression extended over much, if not all, of present-day Ireland and far beyond.
Late Cretaceous or early Palaeogene erosion has removed any trace of post-Maastrichtian beds if they ever existed.
| Enlander, I., Dempster, M. & Doughty, P., 2024. The Cretaceous System in N. Ireland - Introduction, County Antrim, Down, Londonderry, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1194. Accessed on 2024-12-26 |
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