Carboniferous palaeontology, Midland Valley of Scotland

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Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.

Carboniferous palaeontology[edit]

Distribution of some Dinantian and Namurian fossils in the Midland Valley. P915538.
Distribution of stratigraphically important fossils in the Westphalian. P915539.

The abundant Scottish Carboniferous fauna and flora have been studied in detail for many years and the accumulated information is of prime importance in two respects. Firstly, it greatly assists comparison and correlation of stratal sequences in different parts of the region and many individual beds can be recognised by their faunal content. Secondly, the fossils present give an insight into the environment of deposition of the host rocks. The vertical ranges of selected species are shown on P915538 and P915539.

Revised lithostratigrapical nomenclature
Former name Current name
Coal Measures Scottish Upper Coal Measures Formation
Coal Measures Scottish Lower Coal Measures Formation
Passage Group Passage Formation
Upper Limestone Group Upper Limestone Formation
Limestone Coal Group Limestone Coal Formation
Lower Limestone Group Lower Limestone Formation
Calciferous Sandstone Measures Strathclyde Group

Classification and zonation[edit]

As a result of the cyclic nature of the sedimentation, each successive depositional facies persisted for a relatively short time. The fossil record shows a sequence of colonisations and extinctions of the various organisms suited to the prevalent facies over all or part of the region. Sediments deposited in marine environments form a relatively small part of the total succession and they tend to be concentrated in the Lower and Upper Limestone Groups. Any scheme of zonation based on marine organisms can only be of limited application and has to be complemented by schemes based on non-marine faunas and on microfloras.

Corals form a very small proportion of the marine faunas and are restricted in their lateral distribution so that a zonal scheme based on them is only of very general use. Goniatites are also confined to marine deposits and are seldom of common occurrence in the Scottish Carboniferous. In addition, the genera of goniatites which have proved to be most useful in zoning the upper Dinantian and Namurian of England are of extremely rare occurrence in Scotland. Despite these limitations studies on goniatites have shown that the base of the Namurian lies just below the Top Hosie Limestone and that the base of the Arnsbergian (E2) Stage of the Namurian is below the Orchard Limestone. In addition the presence of Anthracoceratites vanderbeckei in the Vanderbeckei (Queenslie) Marine Band indicates that the base of Westphalian B of the western European classification should be drawn at this horizon. Non-marine bivalves are found at some horizons throughout the Dinantian and Namurian strata. In the Westphalian, however, numerous species of several genera occur in profusion at many horizons. A zonation based on these forms in the English and Welsh coalfields is also applicable to the Scottish Coal Measures.

Plant miospores are exceedingly small and occur in myriads at many horizons in the succession. Early work to use them for zonal purposes was restricted to microfloras obtained from coals and was not entirely successful. Subsequent research resulted in a division of the Dinantian of the east of Scotland into five concurrent range zones based on miospores extracted from marine and non-marine mudstones and siltstones. This zonation has yet to be extended to cover the Namurian and Westphalian of the Midland Valley.

At present no single group of fossils can be used to subdivide the Scottish succession and the lithostratigraphical classification is the most suitable one for descriptive purposes.

Faunas[edit]

Calciferous Sandstone Measures[edit]

The strata of the Calciferous Sandstone Measures, the lower and major division of the Scottish Dinantian, are remarkable for the degree of lateral variation they show when traced across central Scotland. This phenomenon is reflected in the faunas except in the uppermost beds of the division. The fossils suggest that the major part of the sediments was deposited in non-marine conditions. The most commonly occurring forms are the calcareous-tubed worms Serpula and Spirorbis, the bivalves Paracarbonicola and Naiadites, mainly in the lower part succeeded by Curvirimula in the higher beds, and local accumulations of arthropods, particularly ostracods, and fish remains.

The first indication of the transgressing early Carboniferous sea is shown by the occurrence of the bivalve Modiolus latus in the basal cementstone facies in several areas. This species occurs in marine assemblages but was also able to thrive in conditions, perhaps brackish or hypersaline, which were unsuitable for other marine forms. The earliest varied marine assemblages are probably those associated with the limestone at Wormistone and Randerston in east Fife. Bryozoa, the gastropod Bellerophon randerstonensis and bivalves such as Schizodus pentlandicus are present. Other marine bands mainly yielding bivalves such as Leiopteria hendersoni, Sanguinolites clavatus and Schizodus pentlandicus occur in the lower part of the group in east Fife, Midlothian and East Lothian but they probably only represent minor, local marine incursions. The first major marine episode which can be correlated over a large part of the region is that recorded by the Macgregor Marine Bands. These comprise up to three separate bands which are found in east Fife and the Lothians. They contain a rich marine assemblage of bryozoa, brachiopods and molluscs with Punctospirifer scabricosta, Pteronites angustatus and Streblopteria redesdalensis the characteristic species. There is evidence that the sea invaded the region from an easterly direction. It is only in the upper part of the Calciferous Sandstone Measures that marine beds appear in the western part of the region in the Glasgow, Douglas and north Ayrshire areas. The horizons of the Hollybush and Blackbyre limestones of the Glasgow area can be traced with reasonable certainty across the region to East Lothian. These mark the first occasions when marine conditions prevailed across the region and rich faunas of corals, brachiopods and molluscs populated the sea bottom. The bivalve Actinopteria persulcata is commonly found in these beds and the Hollybush Limestone is characterised by an abundance of the productoid Semiplanus cf. latissimus.

Lower Limestone Group[edit]

The Lower Limestone Group is the upper division of the Dinantian succession in the Midland Valley and it marks the acme of marine conditions in the Scottish Carboniferous. The limestones and mudstones in the lower part contain rich faunas of corals, bryozoa, brachiopods, molluscs, crinoids and trilobites, many species being confined to the Group. The most distinctive fauna is that of the Neilson Shell Bed which is correlated from north Ayrshire to East Lothian and Fife. Some of the species such as the brachiopod Tornquistia youngi and the bivalve Posidonia corrugata gigantea appear to be confined to this horizon. The Hosie Limestones and their equivalents in the upper part of the Group are characterised by an abundance of the brachiopods Eomarginifera, Pleuropugnoides, Productus and Schizophoria and the bivalves Posidonia corrugata and Sanguinolites costellatus. Curvirimula is the predominant non-marine bivalve in the Group.

Limestone Coal Group[edit]

The Limestone Coal Group is the lowest division of the Scottish Namurian succession. The prevailing conditions of deposition were non-marine as is evidenced by the number of coals developed. Only two major marine bands are present over the whole region and these are the Johnstone Shell Bed and the Black Metals Marine Band. Both marine bands are in the lower half of the Group and the faunas are mainly of brachiopod and mollusc species which were present in the region in Lower Limestone Group times. The Group contains numerous bands yielding Lingula only, some of which occur over a large area. These are interpreted as partially developed marine incursions in which only Lingula was able to thrive in the environment. Non-marine bivalves are represented by Curvirimula, Naiadites and Paracarbonicola. Fish remains are reasonably common at some horizons and occurrences of amphibia have been recorded from the Group.

Upper Limestone Group[edit]

Following the predominant coal-swamp conditions of the Limestone Coal Group the Upper Limestone Group marks the return of several widespread marine episodes with limestone deposition and rich faunas of corals, bryozoa, brachiopods, molluscs and trilobites.

There are consistent differences in the faunas of the Group between those in the Kincardine Basin and Central Coalfield to the west and those of east Fife and Midlothian to the east. An outstanding example of this is the abundance of latissimoid productoids in the Index Limestone in the western area and their rarity at this level in the east whereas in the Orchard Limestone the reverse situation exists. Such differences persist between the faunas of the two areas throughout the Group except in the topmost bed, the Castlecary Limestone. It seems probable that a feature associated with the Burntisland Anticline affected the depositional conditions on either side of the structure which resulted in different facies in the two areas.

Some species are confined to one horizon such as the productoid Antiquatonia costata being found only in the Orchard Limestone. The most noteworthy example of the lateral spread of a species is that of the marine bivalve Edmondia punctatella just below the Calmy Limestone. It occurs in profusion in a thin band which can be traced from north Ayrshire to west Fife and as far south as Douglas. It is also present in east Fife and Midlothian but is relatively scarce there.

The non-marine bivalves present in the Group are Curvirimula and Naiadites.

Passage Group[edit]

The fossils in the Passage Group record a gradual withdrawal of the sea from the region. There are several marine bands in the lower part of the Group which contain rich faunas of brachiopods and molluscs. The best developed limestone is in No. 2 Marine Band (Roman Cement) which is packed with orthotetoids and Schizophoria. Little other than Lingula is present in the few fossiliferous bands in the upper part of the Group. The only notable occurrence of non-marine bivalves is of abundant Curvirimula in a carbonaceous shale at the base of the Group in the immediate roof of the Castlecary Limestone. Carboniferous palaeontology 97

Coal Measures[edit]

During Westphalian times when the Coal Measures were deposited, nonmarine conditions prevailed for most of the period. Only on two occasions did the sea become established over the whole region. These are marked by the Vanderbeckei (Queenslie) Marine Band and the Aegiranum (Skipsey’s) Marine Band. The first named divides the Lower from the Middle Coal Measures and the second is at the base of the Upper Coal Measures. Both bands contain varied marine faunas including brachiopods and molluscs but the Vanderbeckei Marine Band yields only Lingula over much of the region and productoids, pectinoids and goniatites are confined to relatively small areas.

Marine bands are also present locally near the base of the Lower Coal Measures and above and below the Aegiranum Band. They are impersistent in lateral development and normally only contain Lingula but marine molluscs are present at some localities.

It is the non-marine bivalves, however, which are the principal fossils in the Coal Measures (P915539). At numerous horizons, normally in the mudstone roof of a coal, they occur in vast numbers and some of these ‘musselbands’ can be traced over large areas. A succession of species of Carbonicola are the principal forms in the Lower Coal Measures and these are followed in the Middle Coal Measures by species of Anthracosia. A marked diminution of these faunas takes place at the Aegiranum Marine Band. Of the six genera present in the Lower and Middle Coal Measures only Anthraconaia and Naiadites persist into the Upper Coal Measures and are relatively rare in these beds. In the upper part of the Upper Coal Measures the only fossils found are restricted to scarce plant remains and the worm Spirorbis.

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