Westphalian, Midland Valley of Scotland
From: Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.
Westphalian, introduction
The Westphalian succession in the Midland Valley consists of the Lower, Middle and Upper Coal Measures. During the Lower and Middle Coal Measures sedimentary conditions were similar to those in the Limestone Coal Group of the Namurian Series, but the Upper Coal Measures are more arenaceous and show evidence of increasing aridity in the upper part. The rocks are mudstones, sandstones and siltstones with seams of coal and seatclay and rarely of ironstone deposited in cyclic sequence. A few thin marine bands occur which are important for correlation. Coal seams are well developed in the Lower and Middle Coal Measures and up to 20 seams have been worked extensively. The sediments in the Upper Coal Measures are secondarily reddened and coal seams are mostly destroyed by oxidation.
The strata were deposited in a fluvio-deltaic environment in which the marine influence was restricted to a few short-lived intervals. Although subsidence was considerable in order to receive the large amount of sediment brought into the area, sedimentation kept pace to the extent that the water depth was never great and the depositional surface was always close to sea level. However, the degree of subsidence varied greatly from place to place with the contemporaneous formation of local basins and differential subsidence across lines of faulting.
The rock sequence consists of the repetition, in whole or in part, of a sedimentary cycle which commonly has a mudstone at the base and a root-bed and coal at the top. Beds of sandstone, siltstone and mudstone may appear in the cycle in a number of combinations below the root bed. Marine horizons are rare, but strata with non-marine bivalves or ‘mussels’ are not uncommon. The cycles range in thickness up to about 30 m but average 10 m. There is considerable variety, within these limits, between one cycle and another and laterally within a single cycle.
Apart from fragmentary plant material, the most abundant fossils present are the non-marine bivalves (mussels) which in conjunction with the marine bands, form the basis of correlation both within and between coalfields. The Vanderbeckei and Aegiranum marine bands contain species unique to the individual horizons and to the equivalent bands elsewhere in western Europe. This suggests that the depositional facies was uniform over a very large area in Westphalian times. The presence of these bands in Scotland allows the recognition of Westphalian A, B and C of the western European classification (P915556).
In the Upper Coal Measures evidence of marine conditions is scarce and confined to the lowermost parts of the sequence. Mussels become scarce compared with their occurrence in the Middle Coal Measures.
The base of the Lower Coal Measures is drawn at a locally convenient horizon in each coalfield area since the Subcrenatum Marine Band, which marks the base of the Westphalian, has not been recorded in Scotland. Its horizon probably lies within the upper part of the Passage Group. The base of the Middle Coal Measures is defined as the base of the Vanderbeckei (Queenslie) Marine Band and the base of Upper Coal Measures is drawn at the top of the Aegiranum (Skipsey’s) Marine Band.
The distribution of the Westphalian outcrops is shown on P915536 which shows that the sequence is now only preserved in a few separate areas. The outcrops shown offshore in the Firth of Clyde and the Firth of Forth are known from geophysical data and a few boreholes. Knowledge of the succession in these areas is meagre. The generalised successions in the various coalfields are shown on P915537 together with the stratigraphical classification and the names of some of the principal coals.
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