Bedrock Geology UK North: Permian and Triassic - including the New Red Sandstone Supergroup
|This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK North and gives a brief explanation of their origins.
Author: P Stone (BGS); Contributor: A A Jackson (BGS)
299 to 200 million years ago
Permian and Triassic rocks have a wide distribution beneath the seas surrounding the northern parts of Britain and Ireland but on land are restricted mostly to coastal outcrops at the margins of the offshore basins. Indeed, much of the current coastline of Scotland and northern England is controlled by the location of the offshore Permo-Triassic basins and their marginal fault systems. It should be stressed that offshore Permo-Triassic sequences are much more extensive, thicker and more lithologically varied than the onshore remnants.
Both Permian and Triassic sedimentation took place within essentially the same basins, whose depositional areas extended with time under the influence of an east–west transtensional tectonic regime. Extension-related, alkaline magmatism resumed in the Scottish Midland Valley area for a few million years around 295 Ma, whilst in north-east England more protracted tholeiitic magmatism continued from the late Carboniferous into the early Permian.
Global sea level during Permo-Triassic times was relatively low, and the northern British region was located far from the contemporary coastline, within the interior of Pangaea and to the north of the Equator (Figure P785800e): about 10°N at the beginning of the Permian, drifting to about 30°N by the end of the Triassic. Thus the onshore sequences are largely the result of terrestrial sedimentation in an arid environment; these red-bed assemblages are combined as the New Red Sandstone Supergroup (PT). There was also much weathering and erosion of the newly created upland areas, with as much as 500 m of Carboniferous strata worn away in some places. Several late Permian marine transgressions are evident, particularly in the North Sea sequence and its onshore continuation in northern England, but more general marine transgression did not reach the region until late in the Triassic.
Permo-Triassic in Scotland
Scattered outcrops of Permian and Triassic strata are found around the northern Scottish coastline, for example along the south shore of the Moray Firth, in several small outliers along the north-west mainland coast and on several of the Hebridean islands; these are marginal to the much thicker sequences preserved in offshore basins. Red and yellow-brown sandstone predominates with some mudstone and pebble conglomerate. Much of the sandstone had an aeolian origin with some fluvial deposits probably showing the influence of seasonal flash floods. Around Stornoway, on Lewis, the sequence (which for many years was regarded as Torridonian) comprises conglomerate overlain by muddy debris-flow deposits. To the east, the Moray Firth sandstone sequence is notable for its fossil fauna of early reptiles and their footprints. In most cases the allocation of age is imprecise. The Stornoway and Moray Firth sequences appear to range from the Permian into the Triassic whereas most of the isolated outcrops on the Hebridean islands are thought to be largely Triassic.
More extensive Permian successions are present in several basins across central and southern Scotland, and are assigned to the Stewartry Group (P1). Red sandstone and sedimentary breccia are characteristic and in several of the basins, notably at Mauchline and Thornhill, there is a basal development of basaltic lava and volcaniclasic rock. The southern basins developed during east–west extension, as half-graben structures formed on either side of a more stable block underpinned by the Galloway granite plutons. Considerable thicknesses of sediment accumulated — up to about 1400 m in the Dumfries and Lochmaben basins, and as much as 2000 m at Stranraer — by mainly aeolian and fluvial processes. Around the Firth of Clyde, comparable (but not necessarily correlative) Permian red sandstone and breccia occur in a small coastal outlier at Ballantrae and form a more extensive sequence over 1200 m thick in southern Arran, where fluvial and alluvial fan environments were important. The Arran succession spans the Permo-Triassic boundary and continues upwards into Triassic red mudstone.
Permian of Northern England
In north-west England, the assemblage of red, aeolian sandstone and fluvial sandstone and breccia that unconformably overlies Carboniferous strata comprises the Appleby Group (P1). Its thickness varies considerably over short distances with representative strata known from the northern part of the Solway Basin, the Vale of Eden, west Cumbria, and the northern end of the Isle of Man. A particularly distinctive lithology is ‘brockram’ — a coarse, clast-supported breccia, derived locally from the immediately adjacent rocks and probably accumulating as fault-scarp talus (Plate P207609).
Across much of north-east England the unconformity above Carboniferous strata, which at the unconformity are typically reddened, is covered by up to about 70 m of soft and weakly consolidated, yellow aeolian sandstone that accumulated as dunes in an arid desert (Plate P548172). The yellow sandstone is assigned to the Rotliegendes Group (P1), which has a much thicker and more varied offshore development. Above the yellow sandstone, up to about 500 m of strata comprise four depositional cycles arising from marine inundation and the subsequent evaporation of the trapped sea water. This is the Zechstein Group (P2; P3) and, where complete, each cycle ranges from limestone at the base, through anhydrite and gypsum to chloride-bearing evaporites at the top; mudstone and fine-grained sandstone beds also occur intermittently. Much of the limestone is dolomitic (the whole assemblage has traditionally been known as the Magnesian Limestone) and in places contains distinctive concretionary lithologies (Plate P221294). Substantial parts of the succession (at least in its onshore outcrop) have been affected by collapse-brecciation caused by dissolution of evaporites. In the Vale of Eden, Solway Basin and west Cumbria, a broadly analogous sequence of dolomitic limestone, gypsum, mudstone and fine-grained sandstone makes up the Cumbrian Coast Group. It overlies the Appleby Group and, again, its principal development is offshore.
Triassic of Northern England
Across west Cumbria and the Solway Basin the sequence continues upwards conformably from the Permian, Cumbrian Coast Group (P3), into the red sandstone of the Triassic Sherwood Sandstone Group (T1). This is a thick succession, up to about 800 m in north-west England but increasing offshore to over 1000 m and southwards to over 1500 m in the English Midlands. The lithology is typically fine-grained, reddish-brown sandstone (Plate P220632), which is interbedded with subordinate red mudstone that may show desiccation features. Deposition was largely effected by ephemeral streams and rivers on braided alluvial plains and playa mudflats.
Above the Sherwood Sandstone, the Triassic sequence continues in the central part of the Solway Basin where mudstone and gypsum of the Mercia Mudstone Group (T2) has been proved in boreholes. West of Carlisle the Penarth Group (T2) is also present as a mudstone and limestone succession (only about 15 m thick) containing a sparse bivalve fauna that records a major, late Triassic (Rhaetian) marine transgression. The Triassic, Mercia Mudstone Group sequence is also preserved farther south in Cumbria, on Walney Island, as approximately 500 m of mudstone and evaporite. The main onshore development of this group is farther south with almost 1500 m preserved in the Cheshire Basin and parts of the English Midlands. The group has been widely recognised in the offshore successions and, from these, links are possible with the Triassic mudstone sequences seen in Scotland: on Arran, on the south side of the Moray Firth and the muddy debris-flow deposits at Stornoway. The Penarth Group also has a wide distribution but is everywhere thin.
Permo-Triassic of Northern Ireland
The Red Arch Formation comprises about 440 m of red sandstone and conglomerate exposed on the east coast of County Antrim. It accumulated as an alluvial fan deposit and though traditionally regarded as a Devonian, ‘Old Red Sandstone’ lithology it is now thought to be possibly younger, perhaps early Permian in age. Elsewhere in Northern Ireland the Permian rocks preserved at outcrop are part of an attenuated sequence that is never more than a few tens of metres thick. In contrast, beneath the Palaeogene lavas of the Antrim plateau, the discovery in deep boreholes of a thick Permian succession that was partly volcanogenic demonstrated the existence of contemporaneous, discrete fault-bounded basins separated by areas of higher ground. Hence the basal, early Permian, clastic and volcanic assemblage of the Enler Group (P1) is highly variable. In the Belfast area and at Cookstown in County Tyrone it is restricted to a basal breccia about 7 m thick. However, in a deep borehole at Larne, on the east coast, the Enler Group consists of a thin conglomerate and sandstone sequence at the base, overlain by 550 m of pervasively altered basaltic, andesitic and trachytic lavas and tuffs. The upper part of the group as proved in the borehole consists of 400 m of sandstone and pebble beds. The early Permian volcanogenic succession of the Enler Group has much in common with the Stewartry Group of south-west Scotland.
Throughout Northern Ireland, the late Permian succession, the Belfast Group (P2), starts with a thin carbonate sequence, equivalent to the Magnesian Limestone. The thickness of the Belfast Group limestone ranges up to a maximum of 36 m and it is generally overlain by up to about 100 m of reddish calcareous mudstone (known traditionally as the Permian Marls). An exception was proved in a borehole at Larne where the Magnesian Limestone was succeeded by 113 m of halite. Overall, the late Permian succession resembles the succession of the Cumbrian Coast and Zechstein groups in northern England, and a similar hot, evaporitic depositional environment of reefs, sabkha flats, and ephemeral streams undoubtedly prevailed.
The Triassic sequence in Northern Ireland continues conformably upward from the Permian succession in the Belfast to Newtownards area, but is also present in outcrops to the west of Lough Neagh and at the margins of the Antrim lava plateau. Red, mainly fluvial sandstone of the Sherwood Sandstone Group (T1), which is up to 650 m thick, is succeeded by mudstone with evaporitic interbeds of halite, nodular anhydrite and gypsum. This mudstone-evaporite sequence, part of the Mercia Mudstone Group (T2), attains a total thickness of over 1000 m with some individual beds of halite locally up to 400 m thick. As in north-west England, the late Triassic (Rhaetian) Penarth Group (T2) is thin (up to about 25 m) and only rarely preserved. It rests conformably on the Mercia Mudstone Group, and comprises dark grey, fissile marine mudstone with abundant silty laminae and sporadic thin interbeds of fine-grained sandstone; in the upper part of the Penarth Group the mudstone is more calcareous and contains an abundant bivalve fauna.