Earth Science Conservation Review
| Carboniferous Biostratigraphy - Overview | Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Namurian, Stephanian, Tournasian, Visean, Westphalian) |
| Rock Type: | Limestone, Mudstone, Shale, Siltstone |
| Interest | |
| Fossil Groups: | Brachiopod, Conodont, Coral, Foraminifera, Goniatite, Plant, Trilobite |
| Other interest: | Marine sediments |
Description:
BIOSTRATIGRAPHY
In recent years it has become apparent that the most consistently reliable method for dating marine Carboniferous rocks lies with microfaunal studies, in particular of foraminifera. Unfortunately the distribution of foraminifera is facies controlled, to a degree, but is more often affected adversely by the nature of the sediment substrate and assemblages may thus be impoverished and inconclusive, for whatever reason.
In order to allow for this eventuality collections of macrofauna were also made during the course of this project wherever possible and when time permitted. Most Carboniferous fossil phyla are of some chronostratigraphical value in dating the rocks although some are more valuable than others. Although corals and brachiopods are strongly facies controlled, at certain horizons they are, nevertheless, valuable indicators of age and should not be discounted in any age assessment. They have the added benefit of being relatively common in many lithologies and it is often easy to assemble a representative collection in a short time. For this reason lists of the more important taxa of these phyla are always included in specific site descriptions.
In contrast fossil phyla that lived a planktonic existence, for example ammonoids (goniatites) and many species of trilobites are frequently restricted to a few lithologies. However because of their widespread geographical distribution they offer the best means of high resolution correlation which is often based on rapidly evolving species lineages or on rapidly changing assemblages. Specimens of these phyla are therefore rarely encountered in the majority of marine rock types and are difficult to collect.
BIOSTRATIGRAPHY OF THE BRITISH AND IRISH DINANTIAN
The following section is intended as a brief guide on the usefulness of various fossil phyla as a means of correlation in the Lower Carboniferous of Ireland. Much of this information was obtained from the paper (1990) by N. J. Riley whose expertise is acknowledged and whose contribution to our understanding of the Carboniferous in Northern Ireland, together with that of N. J. Turner, in the field of palynology, (both of the British Geological Survey) has been invaluable.
(i) Goniatites Goniatites characterize hemi-pelagic sequences which accumulated in deep water and contain a small amount of terrigenous material. They are rarely associated with coral/brachiopod faunas except in bioherms but may be common in peritidal settings (very shallow water) where conditions favoured local post-mortem accumulations such as along strand lines, in a beach environment. Goniatite zones are globally applicable, reflecting their largely pelagic lifestyle, and in the late Dinantian (Asbian-Brigantian Stages), as in the overlying Namurian, they provide the highest biostratigraphical resolution of any fossil group.
(ii) Conodonts Conodonts are present in nearly all Dinantian marine facies but particular taxa are facies selective. They do require digestion of considerable quantities of limestone (on average greater than 1kg) for a representative assemblage. Because conodonts were nekto-pelagic they have a global marine distribution rivalled only by the goniatites, in the Carboniferous and are therefore particularly significant in international correlation.
(iii) Coral/brachiopod zonation Coral/brachiopod assemblages dominate the Carboniferous Limestone macrofauna. Their main advantage is that they have been recorded where relevant in most stratigraphical studies. Specimens are usually easily seen in the field and are particularly suited to field mapping techniques. However, being large, non-mobile benthos they are very susceptible to facies controls and often occur, or are concentrated, at discrete horizons, being absent from much of the strata, that is known to lie within their zonal range and is apparently suited to their existence. The diachroneity of many fossil species of corals and brachiopods is borne out by comparison of their ranges with other biostrati- graphical schemes; in Ireland this has been admirably demonstrated in the Courceyan using conodonts as a control.
Similar problems also exist in the Viséan in Northern Ireland. Two significant examples include the age of the distinctive corals Dorlodotia pseudovermiculare and Lithostrotion portlocki. In England the former is restricted to the late Chadian but in N. Ireland it is known to occur there and to range upwards into the early Arundian. The latter species in England is apparently an Holkerian entry whereas in N. Ireland it does occur with typical Holkerian foraminifera but, somewhat unexpectedly, also is found in late Arundian strata. The upward stratigraphical extension, by later palaeontologists, of many of the coral taxa used by Vaughan (1905) as index species for his zonation of the Carboniferous Limestone of the Bristol area shows that their occurrences are linked strongly to lithology and are often the result of fortuitous, accidental preservation.
(iv) Flora The most important floral components in a stratigraphical sense are miospores which are represented by spores or pollen grains less than 0.2mm diameter. These have the advantage of wide dispersal and, in the fossil record, are unique in that they can be used routinely to correlate between non-marine and marine sequences. They are particularly abundant in coals, palaeosols (fossil soils) and fine-grained terrigenous clastic sedimentary rocks e.g. shales and mudstones and only small volumes of rock are required for processing. Limestone-dominated marine sequences tend to yield either sparse or unidentifiable miospore assemblages. However where it is possible to compare miospore zonations with marine faunas the relationship between them appears to be consistent at least within Britain and Ireland.
The base of successive zones is indicated by the first entry of distinctive miospore species that do not occur in underlying zones but whose range upwards may well extend through several later zones. In N. Ireland, perhaps the most useful miospore taxon is Lycospora pusilla, the index species for the Pu Zone. The early part of this zone only differs from the underlying CM Zone by the appearance of L. pusilla, which is taken to indicate a Viséan (at least early Chadian) age as against a CM Zone of Tournaisian age.
(v) Foraminifera All Dinantian (Lower Carboniferous) foraminifera were benthonic and it is the free living forms which are particularly useful stratigraphically. Foraminifera were abundant in shallow to moderate water depths and because of their small size were subjected to considerable post-mortem distribution, hence they are also found with goniatites in deep sea sediments, in limestone turbidites that have flowed downhill from shallow to deep water and in tempestites (storm deposits) in peritidal (extreme shallow water to intertidal) environments. They are therefore an important group in correlation between basin and shelf settings. Lower Carboniferous limestones are highly indurated and foraminifera are identified in random orientation from thin section of bulk rock.
Foraminiferal taxonomy, morphological terminology and Lower Carboniferous zonation was largely developed in Belgium during the 1960s and 1970s. The Belgian scheme is applied with considerable success to the British and Irish sequences but there are problems outstanding with the Chadian and Asbian stratotypes in relation to foraminifera.
(vi) Trilobites Trilobites are locally common in many Dinantian marine settings and, as in previous periods, they adapted to a variety of habitats giving rise to a diverse array of forms. However because the range of many species is severely limited they offer a further accurate aid in determining the age of the rocks.
CONCLUSIONS
Vaughan's (1905) coral/brachiopod zonation scheme (of the Bristol area) influenced Lower Carboniferous stratigraphy in Great Britain and Ireland for much of this century. The Dinantian stages proposed by George et al. (1976) replaced this scheme. Stratigraphers need routinely to use more than one biostrati- graphical technique in order to reduce the influence of facies effects in correlation, and in the majority of cases during this study of the Lower Carboniferous in Northern Ireland, this philosophy has been followed closely.
In recent years it has become apparent that the most consistently reliable method for dating marine Carboniferous rocks lies with microfaunal studies, in particular of foraminifera. Unfortunately the distribution of foraminifera is facies controlled, to a degree, but is more often affected adversely by the nature of the sediment substrate and assemblages may thus be impoverished and inconclusive, for whatever reason.
In order to allow for this eventuality collections of macrofauna were also made during the course of this project wherever possible and when time permitted. Most Carboniferous fossil phyla are of some chronostratigraphical value in dating the rocks although some are more valuable than others. Although corals and brachiopods are strongly facies controlled, at certain horizons they are, nevertheless, valuable indicators of age and should not be discounted in any age assessment. They have the added benefit of being relatively common in many lithologies and it is often easy to assemble a representative collection in a short time. For this reason lists of the more important taxa of these phyla are always included in specific site descriptions.
In contrast fossil phyla that lived a planktonic existence, for example ammonoids (goniatites) and many species of trilobites are frequently restricted to a few lithologies. However because of their widespread geographical distribution they offer the best means of high resolution correlation which is often based on rapidly evolving species lineages or on rapidly changing assemblages. Specimens of these phyla are therefore rarely encountered in the majority of marine rock types and are difficult to collect.
BIOSTRATIGRAPHY OF THE BRITISH AND IRISH DINANTIAN
The following section is intended as a brief guide on the usefulness of various fossil phyla as a means of correlation in the Lower Carboniferous of Ireland. Much of this information was obtained from the paper (1990) by N. J. Riley whose expertise is acknowledged and whose contribution to our understanding of the Carboniferous in Northern Ireland, together with that of N. J. Turner, in the field of palynology, (both of the British Geological Survey) has been invaluable.
(i) Goniatites Goniatites characterize hemi-pelagic sequences which accumulated in deep water and contain a small amount of terrigenous material. They are rarely associated with coral/brachiopod faunas except in bioherms but may be common in peritidal settings (very shallow water) where conditions favoured local post-mortem accumulations such as along strand lines, in a beach environment. Goniatite zones are globally applicable, reflecting their largely pelagic lifestyle, and in the late Dinantian (Asbian-Brigantian Stages), as in the overlying Namurian, they provide the highest biostratigraphical resolution of any fossil group.
(ii) Conodonts Conodonts are present in nearly all Dinantian marine facies but particular taxa are facies selective. They do require digestion of considerable quantities of limestone (on average greater than 1kg) for a representative assemblage. Because conodonts were nekto-pelagic they have a global marine distribution rivalled only by the goniatites, in the Carboniferous and are therefore particularly significant in international correlation.
(iii) Coral/brachiopod zonation Coral/brachiopod assemblages dominate the Carboniferous Limestone macrofauna. Their main advantage is that they have been recorded where relevant in most stratigraphical studies. Specimens are usually easily seen in the field and are particularly suited to field mapping techniques. However, being large, non-mobile benthos they are very susceptible to facies controls and often occur, or are concentrated, at discrete horizons, being absent from much of the strata, that is known to lie within their zonal range and is apparently suited to their existence. The diachroneity of many fossil species of corals and brachiopods is borne out by comparison of their ranges with other biostrati- graphical schemes; in Ireland this has been admirably demonstrated in the Courceyan using conodonts as a control.
Similar problems also exist in the Viséan in Northern Ireland. Two significant examples include the age of the distinctive corals Dorlodotia pseudovermiculare and Lithostrotion portlocki. In England the former is restricted to the late Chadian but in N. Ireland it is known to occur there and to range upwards into the early Arundian. The latter species in England is apparently an Holkerian entry whereas in N. Ireland it does occur with typical Holkerian foraminifera but, somewhat unexpectedly, also is found in late Arundian strata. The upward stratigraphical extension, by later palaeontologists, of many of the coral taxa used by Vaughan (1905) as index species for his zonation of the Carboniferous Limestone of the Bristol area shows that their occurrences are linked strongly to lithology and are often the result of fortuitous, accidental preservation.
(iv) Flora The most important floral components in a stratigraphical sense are miospores which are represented by spores or pollen grains less than 0.2mm diameter. These have the advantage of wide dispersal and, in the fossil record, are unique in that they can be used routinely to correlate between non-marine and marine sequences. They are particularly abundant in coals, palaeosols (fossil soils) and fine-grained terrigenous clastic sedimentary rocks e.g. shales and mudstones and only small volumes of rock are required for processing. Limestone-dominated marine sequences tend to yield either sparse or unidentifiable miospore assemblages. However where it is possible to compare miospore zonations with marine faunas the relationship between them appears to be consistent at least within Britain and Ireland.
The base of successive zones is indicated by the first entry of distinctive miospore species that do not occur in underlying zones but whose range upwards may well extend through several later zones. In N. Ireland, perhaps the most useful miospore taxon is Lycospora pusilla, the index species for the Pu Zone. The early part of this zone only differs from the underlying CM Zone by the appearance of L. pusilla, which is taken to indicate a Viséan (at least early Chadian) age as against a CM Zone of Tournaisian age.
(v) Foraminifera All Dinantian (Lower Carboniferous) foraminifera were benthonic and it is the free living forms which are particularly useful stratigraphically. Foraminifera were abundant in shallow to moderate water depths and because of their small size were subjected to considerable post-mortem distribution, hence they are also found with goniatites in deep sea sediments, in limestone turbidites that have flowed downhill from shallow to deep water and in tempestites (storm deposits) in peritidal (extreme shallow water to intertidal) environments. They are therefore an important group in correlation between basin and shelf settings. Lower Carboniferous limestones are highly indurated and foraminifera are identified in random orientation from thin section of bulk rock.
Foraminiferal taxonomy, morphological terminology and Lower Carboniferous zonation was largely developed in Belgium during the 1960s and 1970s. The Belgian scheme is applied with considerable success to the British and Irish sequences but there are problems outstanding with the Chadian and Asbian stratotypes in relation to foraminifera.
(vi) Trilobites Trilobites are locally common in many Dinantian marine settings and, as in previous periods, they adapted to a variety of habitats giving rise to a diverse array of forms. However because the range of many species is severely limited they offer a further accurate aid in determining the age of the rocks.
CONCLUSIONS
Vaughan's (1905) coral/brachiopod zonation scheme (of the Bristol area) influenced Lower Carboniferous stratigraphy in Great Britain and Ireland for much of this century. The Dinantian stages proposed by George et al. (1976) replaced this scheme. Stratigraphers need routinely to use more than one biostrati- graphical technique in order to reduce the influence of facies effects in correlation, and in the majority of cases during this study of the Lower Carboniferous in Northern Ireland, this philosophy has been followed closely.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Biostratigraphy - Overview, County Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1172. Accessed on 2024-12-26 |
| Previous Site | Next Site |