The 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. (A sabkha is a wide area of coastal flats bordering the sea, the name coming from certain coastal areas of Arabia). 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 hyperslaine epeiric sea environment of the Mercia Mudstone group.
Read more about this topic: Geology Of Alderley Edge