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.
Lower Triassic
The Sherwood Sandstone Group can be broadly divided into an upper (previously Keuper) and a lower (previously Bunter) unit, at the level of a widely recognized intra-Sherwood Sandstone disconformity perhaps within the uppermost Lower Triassic This disconformity separates two quite distinct environmental systems. This disconformity is widely assumed to be equivalent to the Hardegsen disconformity of the central European/Southern North Sea Basin, although the age of this disconformity is not constrained as it is in Europe by biostratigraphical indicators
Below the intra-Sherwood Sandstone disconformity these deposits are dominated by evidence of a major braided river system, this river system responsible for transporting the sand and gravel to the Alderley area was named by Wills (1970) the Budleighensis River, after Budleigh Salterton in Devon, where its existence was first established.
Within the western onshore basins there are several clues indicating that the river flowed in a northerly direction. Within the Alderley conglomerates, there are well rounded liver coloured quartzite pebbles from a source found only in the Variscan mountains of Brittany, France. Some pebbles contain microfossils of marine animals (Lingula lesueuri and Orthis budleighensis) which can be traced to the same source area in the Armorican Massif.
Pebbles of Carboniferous limestone and reworked earlier Triassic sandstones indicate their source as being from the South Derbyshire area and the Clent Formation south of Birmingham respectively. There is further evidence from the distribution of the pebbles themselves, the average size of the stones on the southern margin of the basin is noticeably larger than that to the north, which is consistent with a flow of water from the south.
The river system flowed northwards through the Wessex Basin, the Worcester Basin, the various midland basins and on into the East Irish Sea Basin just to the north west of the Alderley area. A second branch probably flowed eastwards into the southern North Sea. This Budleighensis river system is evident by a series of sandstones with generally northwards directed palaeocurrents, which can be traced along a south to north line along the central parts of Britain.
Indications are that it probably had large seasonal changes in discharge, evident by cross-bedded sandstones deposited at stages of lower flow, although whilst the flow was seasonal, it is perhaps doubtful if this system was ephemeral in nature for there is relatively little evidence of any large-scale aeolian sandstones in the basinal settings of the Midlands. It is clear that more locally sourced material such as carboniferous limestone and reworked earlier Triassic sandstone from the Clent Formation just south of Birmingham were also important components of the river systems as they flowed northwards giving some indication of the relief of the basin margins.
Towards the end of the Triassic, the sea level once again rose and periodic flooding caused by high spring tides and strong on-shore winds led to the formation of on shore saline lagoons or sabkha environments. Intense evaporation from these lagoons resulted in the precipitation of a carbonate-sulphate complex and the thick halite beds as seen to the south west of Alderley in Northwich where the salt is mined commercially. It was in this type of environment that the Mercia Mudstone Group (formerly Keuper Marl) was deposited.
The sequence of formations in the Sherwood and Mercia mudstone groups in this region illustrates clearly the upward transition from continental fluvial to deltaic and littoral marine and ultimately to the hypersaline lake epeiric sea environment of the Mercia Mudstone group.
Alderley Sandstones
The upper division (ex-Keuper) follows the lower (ex-Bunter) unconformably. The upper is very thick and in Cheshire reaches a thickness of about 1250 m, locally conglomeritic, but consists mainly of fine water lain sandstones.
Interest in the area took off with vigour in the mid-19th century with the first real efforts made to quantify the strata of the country using the new mapping methods that geologists like William Smith had pioneered. The economical value of the area in those times lead to a detailed examination of Alderley. Boreholes starting to be used for geological logging and research whereas previously they had been used simply for wells with no real interest taken in the actual geological make up of the rock below. In 1894, a borehole was sunk to 280 ft below surface and the strata recorded in a scientific manner. This borehole, the Isaac Massey (NGR SK 84237819) was the first serious attempt to understand the geology of the area. The area has since been subject of much more detailed research and still provides many clues to basin formation and mineralization processes. The original borehole was crude by today’s standards but it gave an overview of the strata below the Alderley area.
Now the Wilmslow Formation (part of the Sherwood Sandstone Group), the lower Triassic was previously a threefold division, the Lower Mottled Sandstone, The Pebble Beds and the Upper Mottled Sandstone (only the later is seen at Alderley), which merged into the Conglomerates of the upper Triassic. The Upper Mottled Sandstone – now the lower Wilmslow Member – is a medium- to coarse-grained friable false-bedded sandstone with abrupt colour changes from bright red to white. It is composed mainly of rounded grains but sub-angular grains also occur. Normally the rock lacks coherence and weathers into sand down to 6 m and even unweathered rock can be crushed to sand easily. However, near faults, ghost crystals of baryte make the sandstone harder and resistant.
Lower sandstones
The lower sandstone (Upper Mottled Sandstone) has a full thickness of 305 m generally a reddy brown colour with some white patches, which give it its name, these patches are often associated with organic nuclei. The sandstone has two distinct types, a bright dark red known as the Moulding Sand and mottled paler sandstone. The Moulding Sand is so called after its use in the foundry industry for making the casting moulds. Resting on top is a conglomerate, which attains thickness of 30 m; it is the Engine Vein Conglomerate. These conglomerates are made up of angular grained sands which were laid down in cyclic sequences. This sequence is an upward fining sequence of three or four cycles.