Bedrock Geology UK South: Triassic

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This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK South and gives a brief explanation of their origins.
Author: A A Jackson (BGS); Contributor: P Stone (BGS)

251 to 200 million years ago

A major fault separates the Malvern Hills (Neoproterozoic and Lower Palaeozoic rock) from the younger and lower lying Triassic of the Worcester Basin seen in the distance. P212668.

The outcrop of Triassic strata follows approximately that of the Permian in the north of England, but it forms a continuous arc around the southern end of the Pennines and extends southwards through Leicestershire, Staffordshire, Worcestershire, south Wales and into Somerset and Devon. It is particularly well exposed on the north coast of Somerset and on the south coast of Devon. The outcrop is wider, reflecting, in part, the gentler dip and the greater thickness, over 5000m in some places.

The Triassic Period was the beginning of the Mesozoic or ‘middle age’ of planet Earth. At that time, southern Britain was located 15º to 20º north of the Equator. The climate was strongly seasonal, perhaps monsoonal, in the early part of the period but later became extremely hot in the interior of the supercontinent of Pangaea.

Mid Triassic palaeogeographical sketch map. P785823.

Regional extension that was directed roughly east–west continued as the dominant tectonic process and persisted throughout the Mesozoic. Many faults, both major and minor, cut the Mesozoic strata. Some of these controlled subsidence of the major depocentres such as the Cheshire and Worcester (Plate P212668) basins and the thickness of preserved strata changes dramatically across them: many are reactivated basement structures.

The early to mid Triassic was dominated by the deposition of sand and gravel (Figure P785823) along the course of the rivers that drained Armorica and the northern part of the Variscan Mountains. The Variscan Mountains lay to the south and their denudation supplied debris to rivers flowing northwards across southern Britain into the East Irish Sea Basin or the Southern North Sea Basin. These deposits now form the Sherwood Sandstone Group (T1), which can be traced from the English Channel through the Worcester, Cheshire and the East Irish Sea basins, the East England Shelf and the North Sea Basin. The sediments are dominated by fluvial channel and sheet sands (Plate P535767) but also show a strong aeolian influence, and are a distinctive red colour. In the Cheshire Basin and in the Midlands, the lowest strata are conglomeratic or pebbly sandstone, which contains characteristic quartzite pebbles that persist in the soil on weathering (Kidderminster, Nottingham Castle Sandstone and Chester Pebble Beds formations). Equivalent strata in the south-west are the Budleigh Salterton Pebble Beds Formation. At Nottingham, the Permian–Triassic boundary is placed within the Lenton Sandstone Formation (P4) a fluvial–aeolian sandstone that underlies the pebbly Nottingham Castle Sandstone Formation. The sandstones are important aquifers onshore, and offshore act as the reservoir for the East Irish Sea gas field.

Cross-bedded, pale-coloured fluvial sandstone and brighter orange, poorly bedded aeolian sandstone of the Helsby Sandstone Formation (Sherwood Sandstone Group) at Hilbre Island in the Dee estuary, Mersyside. P535767.

During the mid Triassic, depositional areas were gradually increased, establishing continuity with the East England Shelf and the North Sea Basin. The Mercia Mudstone Group (T2) rests conformably on the Sherwood Sandstone but also oversteps it along the basin margins (Plate P667862). It is predominantly red-brown mudstone, deposited as loess or wind-blown dust derived by lateritic weathering of adjacent terrain. Some interbedded greenish grey mudstone, siltstone and sandstone, for example the Tarporley Siltstone and Sneinton formations, are probably water-laid deposits. The Arden Sandstone Formation (Plate P212583) is a varicoloured (green, brown, purplish) horizon that can be traced from Gloucestershire to Nottinghamshire, and separates the Sidmouth and Branscombe mudstone formations. Halite is widespread in some of the depositional basins, gypsum/anhydrite and dolomite less so. Halite was formed as sea water, which flooded the low-lying coastal areas from time to time, evaporated in coastal marine sabkhas to produce a near-pure salt or in places a chaotic mix of halite and mudstone. The main halites of west Lancashire and the Cheshire Basin are up to 400m thick; they have been mined extensively causing subsidence in places. The maximum thickness of the Mercia Mudstone Group, about 1340m, was proved in a borehole at Prees in the Cheshire Basin.

The youngest Triassic strata represent a transgression that re-established marine deposition over much of England and at least part of Wales. The Blue Anchor Formation (Plate P579282) at the top of the Mercia Mudstone Group marks the beginning of this process. It consists of greenish grey dolomitic mudstone with thin limestones, and in south Wales, where it is about 16m thick, it has been interpreted as a lacustrine facies. Around Cardiff, it interfingers with a marginal facies of dolomitised breccia and bedded shore-face conglomerate that rests on the remnants of Triassic wave-cut platforms carved in the underlying Carboniferous limestone. The Penarth Group (Plate P007705) overlies the Blue Anchor Formation and both units are well exposed along the coast of south Wales and north Somerset. The Penarth Group comprises dark grey mudstone with subordinate sandstone (Westbury Formation) overlain by pale greenish grey mudstone with subordinate limestone (Lilstock Formation), and is about 12m thick. These thin units can be traced across the UK and have been identified to the north of the Lake District, near Carlisle.

By the end of the Triassic Period, the landmasses that were to influence the sedimentation patterns of the Jurassic and Cretaceous periods had emerged—the Lake District, Wales, Cornubia and the Anglo-Brabant massif or London Platform. Another mass extinction appears to have affected life on Earth, this time wiping out perhaps 50 per cent of the species.