Permian and Triassic, 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.
Permian and Triassic, introduction
Rocks ascribed to the Permian and Triassic consist mainly of red sandstones and mudstones with basalt lavas at the base and they contain little fossil evidence of their age. They are known collectively as the New Red Sandstone and the term is applied to rocks younger than Carboniferous but older than Jurassic. They have a very restricted outcrop in the Midland Valley. The Permian is present in the Mauchline Basin and in Arran (outside the present region), and also offshore in the Firth of Clyde and the Forth Approaches. Triassic rocks occur only in south Arran and in the Firth of Clyde. The outcrops in the Midland Valley and neighbouring offshore areas are shown in P915540. The offshore outcrops are known from geophysical data, some drill samples and by extrapolation from onshore sections in the Mauchline Basin and on Arran.
A climatic change occurred during the late Carboniferous from humid to semi-arid or arid conditions and the change in climate is reflected in the sedimentary sequence. The Upper Coal Measures are fluviodeltaic and fluviatile deposits which show signs of increasing aridity and the Lower Permian sediments are aeolian desert sandstone. There is no apparent angular discordance at the base of the Permian in the Mauchline Basin but there is, however, a distinct lithological break and an unconformity underlies the Permian rocks on Arran. The reddening of the Upper Coal Measures presumably took place in late Carboniferous or Permian times.
The fossil evidence for the age of the oldest New Red Sandstone rocks is weak but it indicates a probable Lower Permian (Autunian) age on the basis of some plant material found in sediments between lava flows near the base of the volcanic succession in the Mauchline Basin (Wagner, 1983). Radiometric dates of 275 to 272 Ma have been obtained by the potassium/argon whole rock method for the Permian lavas near Mauchline.
The rocks of the Mauchline Basin are subdivided into two groups: the Mauchline Volcanic Group at the base is overlain by the Mauchline Sandstone.
|Brick-red sandstone characterised by the presence of wind-rounded grains, with cross-bedding of dune type||450 + m|
|Mauchline Volcanic Group|
|Basaltic lava flows, usually thin, intercalated with beds of agglomerate, tuff, desert sandstone and mudstone||90 to 235|
|Basal tuffs and sediments||3 to 80 m.|
The base of the Permian is placed at the first appearance in the succession of either bright red sandstone with wind-rounded grains or a sediment containing lava fragments. Most commonly the base is a red sandstone with wind-rounded grains, but where the basal beds are thickest they consist of water-lain siltstone and sandstone with lenses of basaltic detritus. Nodular calcareous concretions also occur, which resemble the cornstones of the Upper Devonian, and may be analogous to the caliche formed in soils in present-day arid regions.
Sediments intercalated with the lavas consist of sandstones with pebbly bands of volcanic detritus and thin argillaceous beds. Pyroclastic rocks have a matrix of mud and contain wind-rounded grains of sand.
Both plant remains and the fine-grained, water-lain sediments which occur interbedded with the lavas indicate that the aridity was tempered, at least occasionally, by rainfall.
The Mauchline Sandstone is brick-red to orange-coloured and is the result of the lithification of sand-dunes. The large-scale cross-bedding of the dunes can be seen in the cliffs along the River Ayr near Mauchline. The sandstone is highly quartzose, almost devoid of mica and many of the larger quartz grains are rounded and polished by wind action.
The sandstone was formerly quarried for building stone at Ballochmyle, near Mauchline and was used in many buildings and monuments in the west of Scotland. The quarry has since been used for dumping colliery waste and the face which once displayed fine examples of dune-bedding (P000072) can no longer be seen.
In the Firth of Clyde the area shown as Permian in P915540 is assumed to contain sediments similar to the Mauchline Sandstone. This interpretation is based on geophysical data and a few drill samples.
The Permian rocks in the west of Scotland are terrestrial deposits probably of Lower Permian age and equivalent to the Rotliegendes of the European succession. In the Forth Approaches the Rotliegendes appears to be absent and Upper Permian marine deposits of the Zechstein Sea are deposited on Devonian and Carboniferous rocks. The outcrop is shown on P915540 and is based on geophysical data and borehole sampling. The rock samples include gypsum, anhydrite, dolomite, limestone and marl.
Palaeogeography and deposition
In Permian times the region lay within a large continental land mass about 8° north of the equator (see P915541). The climate was arid to semi-arid and aeolian sediments indicate that the prevailing wind came from the east.
Deposition of the Lower Permian rocks was by both aeolian and flash flood processes. The Upper Permian in the Forth Approaches on the other hand indicates evaporitic deposition from the hypersaline Zechstein Sea.
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