Although on our doorstep, this topic has been sadly neglected by our club. Kit's talk was therefore particularly welcome.
The halite deposits which underlie the Cheshire Basin originated in the Triassic Period when the area was part of the vast supercontinent of Pangea, at a latitude of 20-30 degrees north. i.e. about that of the Sahara or the Persian Gulf.
Sea water comprises three and a half percent salt, together with other dissolved salts such as Carnallite (potassium magnesium chloride) Polyhalite hydrated potassium magnesium chloride, Sylvite (potassium Chloride), Sylvinite (sodium chloride with potassium chloride) , Gypsum, and carbonates.
The evaporation of about 100 m of seawater is required to produce just 5cm of deposits so simple evaporation of sea water is insufficient to produce such massive deposits so some replenishment mechanism is necessary.
In Cheshire, halite is dominant, with no calcium sulphate or potassium salts.
The preferred model for their formation is that of a "Barred Basin" in which access to sea water is limited and evaporation leads to a gradient of salinity in which carbonates, sulphates and chlorides are crystallised out in that order progressively at increasing distance from the "bar". This could occur in a situation like the Mediterranian, fed from Gibralter, or more interestingly, the Gulf of Garabogazköl in Turkmenistan. This is fed from the Caspian Sea and in 1980, it was felt that it was drying out the Caspian Sea so the inlet was blocked up. Unfortunately it triggered a problem with wind blown salt contaminating surrounding land and in 1990 the inlet was opened up again. To produce thick deposits, space must be created and a subsiding basin is required.
In the case of the Cheshire Basin, like the Zechstein Sea in the north of England, the flooding is associated with the opening of the North Atlantic and the formation of half grabens. 2000m of Jurassic and Cretaceous deposits followed before erosion brought the deposits to the near surface.
The halite beds form part of the Mercia Mudstones Group (Once called Keuper Marl) and comprise salt with associated muds and silts which give them their characteristic yellow, red and blue colouration.
In Cheshire salt was recovered from briney springs by the Romans by evaporation from lead pans, This developed in medieval times to shallow pumping from the "Wet rock head" (brecciated mudstone where meteoric water had dissolved salt) Round Winsford salt was recovered fron wells and open pans. Subsidence led to craters, meres and lakes.
In 1850-1919 the industry grew, based on wells and pans. Subsidence became a problem, often at a distance and in 1890 a levy was imposed on the industry to provide compensation. In addition in the 19th and 20th centuries, it was mined, leaving pillars to support the roof. This led to subsidence when mines were flooded including the "Great Subsidence" in 1881.
The largest workings are at Winsford where it is mined leaving pillars to support the roof.
The beds are not structureless with bedding planes showing up as laminae of muddy layers and interesting polygons are visible on a 4-14 metre scale with dish-like forms corresponding to their edges.
Winsford produces about a third of the total UK salt, used for treating roads in winter and as a beet fertiliser in East Anglia. Brines are also produced by controlled pumping for table salt and process salt for sodium hydroxide and chlorine production.
Spherical cavities produced by controlled brine extraction can be used as storage facilities for natural gas and have been applied for that purpose in East Yorkshire.
This overview of the geological and industrial aspects of our salt industry was much appreciated by members who resolved to try to include a field trip to the area in the future.