THE PERMIAN LOWER EVAPORITES OF FORDON IN YORKSHIRE

Abstract

Summary In the Fordon No. 1 borehole, sunk by the D’Arcy Exploration Company to a depth of 7559 feet near Scarborough, the Permian succession contains 1387 feet of evaporites. This paper describes and discusses the Lower Evaporites in detail. These are 1069 feet in thickness, and the petrography of each of the main ten subzones they contain is described. The major constituents are dolomite, anhydrite, polyhalite, kieserite and halite. Minor constituents include magnesite, glauberite, celestine, aphthitalite, pyrite, sulphoborite, sulphur and talc. Glauberite and sulphoborite are new to Britain. The origin of the rocks is discussed in relation to experimental work on salt systems. The ten subzones can be placed in three cycles of sedimentation, each ended by a relatively sudden increase in rate of subsidence, the middle cycle consisting of two sub-cycles. The upper and lower cycles are relatively simple, consisting mainly of halite and anhydrite, but the middle cycle is more complex, and contains rocks unlike any so far recorded from this country. On textural evidence it is concluded that, whereas gypsum was a primary mineral of the upper and lower cycles, the calcium sulphate of the middle cycle was deposited as anhydrite. The latter has been replaced by polyhalite on a large scale, and it is believed that this change was penecontemporaneous, and released enough calcium for the formation of a large amount of primary polyhalite. The later succession has therefore been considerably modified, and some of the polyhalite and kieserite-bearing subzones may represent the potash-free magnesium sulphate and kainite zones of the experimental succession. There is evidence of a considerable number of replacements. Most of the major ones are probably penecontemporaneous. Others are post-consolidation effects, probably due to rise of temperature and pressure during burial. Some, like those involving glauberite, have probably been effected by solutions rising from a lower cycle; others have been effected by downward or sideways moving solutions from the same or a higher cycle. Some replacements are later than movement and recrystallization of halite under pressure. A count of rhythmic layers in the middle cycle gives information on rates of deposition and suggests that this cycle may have been deposited in about 25000 years. The Fordon Lower Evaporite succession is compared with that in other parts of Yorkshire. The Fordon evidence favours Lotze’s correlation of the Lower Evaporites of Yorkshire with the Middle Zechstein of Germany.