One of the classic locations for the study of Triassic sandstones in the UK is at Alderley Edge in Cheshire. Numerous scientists from the early 19th century up to the present day have studied the area and it is a popular field site for universities around the UK. The sandstones also provide important insights into the nature of continental natural gas and petroleum reservoirs.
Nomenclature
The nomenclature of the English Triassic was rewritten in the 1980s and many of the previous names were changed. The classic terms 'Bunter' and 'Keuper' have now been abandoned, and the formations now recognized in the UK Triassic sequences, constitute three major stratigraphic units:
the Sherwood Sandstone Group (arenaceous siliciclastic) (equivalent to the Bunter and lower Keuper) the Mercia Mudstone Group (argillaceous, halitic) (equivalent to the Keuper Marl) the Penarth Group (not seen at Alderley). In order to maintain an understanding of earlier work, the older names with appropriate cross references are used in places.
History The geology of Alderley Edge has fascinated people from all walks of life, scientists, miners and tourists for hundreds of years. In 1811 Bakewell described it thus: "The hill is evidently of alluvial formation, being composed chiefly of gravel and soft white and reddish sandstone, – the white is intermixed with rounded quartz pebbles, the red with particle of mica. In some parts the red and white sandstone assume a nearly stratified appearance, in others the red stone intersects the white in very thin seams, branching in various directions. In the white sandstone are found various ores of lead as small portions of galena and in the same granular state intermixed with sandstone. In other places particles of blue and brown were collected in nodules of various sizes and imbedded along with pebbles in the sand rock like currants in a pudding." In 1882, Ormerod in his book The History of Cheshire describes it as follows: "Alderley Edge is an abrupt and elevated ridge, formerly the site of a beacon, which bears the appearance of having been detached by some great convulsion of nature from the range of the Macclesfield hills. Near the summit cobalt ore, lead and copper have been got in small quantities. The sides are varied with cultivated land, wood and rock; and the entire mass presents a striking object to all the surrounding district over which it commands a most extensive prospect."
Regional setting The Alderley sandstones are classic redbed deposits, a distinctive sedimentary facies traditionally associated with non-marine depositional environments such as alluvial floodplains and arid deserts. They form part of the Cheshire Basin and the Edge is one of the classic onshore localities in the UK for their study. They also provide important insights to the nature and evolution of deformation in continental clastic natural gas and petroleum reservoirs, such as those of the Rotliegend in the Southern North Sea Gas Basin and of the Sherwood Sandstone Group in the adjacent East Irish Sea Basin. Analogues for cemented cataclastic faults, which can compartmentalise reservoirs, are well displayed by the arrays of deformation bands within the Alderley outcrops.
Permo–Triassic horizon By the start of the Triassic period, around 252 million years ago, the Zechstein Sea of the Permian period had retreated and the climate had become a little wetter giving a gentle transition making the Permian-Triassic boundary uncertain in northern England as there are no fossil horizons or facies changes that make a definitive separation possible as there in continental Europe. The horizon however is characterised by a succession of red marls (calcareous mudstones) deposited on coastal flats, followed by the Sherwood Sandstone (formerly Bunter Sandstone). The 'British Isles' were not islands, but had an intra-continental position within Pangea. The area that now constitutes Great Britain was drifting northwards as Pangea rotated, was at a latitude of 10° – 20° north, equivalent to the latitude of the present day Sahara desert. Erosion of the then recently uplifted landmass formed Aeolian deposits in the southern and central parts of the country. This iron-rich silica sandstone was both oxidised and reworked to give it its red colouration and its name, "New Red Sandstone". British deposits of the era consist of these red beds, alluvial, fluvial and lake deposits, with some shallow-water marine and evaporite deposits. These Permo-Triassic outcrops can be seen on either side of the Pennines and in Devon. Within the main central England basin (Staffordshire and Cheshire), the deposits are dominated by pebbly sandstones and conglomerates (Chester Pebble Beds, Wilmslow and Wildmoor Sandstones), which have been interpreted as the deposits of a fluvial system running within well-confined channels. The Sherwood Sandstone Group comprises a series of conglomerates, coarse sandstones and mudstones. The Chester pebble beds to the south of Alderley represent material deposited in alluvial fan or braided river system. The finer sediments of the Wilmslow and Helsby Sandstone to the west of Alderley represent alluvial deposits of low sinuosity channels. The Alderley area represents the midpoint between the full braided river system and the lower energy area. Minor aeolian dunes and channel infill deposits in the Wilmslow Sandstones indicate an inter channel area or seasonal drying of some of the minor river channels.
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