PERMIAN EVAPORITES AND ASSOCIATED ROCKS IN TEXAS AND NEW MEXICO COMPARED WITH THOSE OF NORTHERN ENGLAND

Abstract

Summary Marine evaporites form an important part of the Permian succession of the United States and of England. This paper gives a review of their geological setting, petrology, and mineralogy. The American mid-continental Permian reaches its greatest thickness in Texas and New Mexico. In Leonard and Guadaloupe times several relatively deep basins, flanked by shallow-water shelf areas, were connected by channels to each other and to an ocean to the south. Reef and bank deposits accumulated at times on the margins of the basins, and separated clastic basinal deposits from back-reef limestone-evaporite-red bed facies on the shelves. There was a general south-westward shift of areas of evaporite deposition as time went on, corresponding to a general retreat of the epicontinental seas. These conditions culminated in the development of the Capitan reef which surrounded the Delaware basin in Upper Guadaloupe time. Later the area was uplifted and tilted, and the basinal evaporites of the Ochoa (more than 4,000 ft. in thickness) were deposited, filling the Delaware basin, overlapping its margin, and covering part of the shelf. The back-reef evaporites are mineralogically simple, and show a marked lateral zoning from less to more soluble salts in a landward direction. The Castile evaporites (Lower Ochoa) provide a striking example of relatively deep-water deposition in an almost isolated basin relatively free from influx of terrestrial water. The Salado and Rustler evaporites (Upper Ochoa), which are mineralogically complex, are shallow-water basinal types, and include important deposits of potassium salts. The British Permian evaporites, deposited in the Zechstein Sea, represent a small part of a large area of deposition. There is little evidence of back-reef deposition in England. The evaporites appear to be predominantly shallow-water basinal types like those of the Salado Formation of Texas and New Mexico. They include the products of three major cycles of desiccation, and it is suggested that these correspond to the upper three of the four major cycles of Hanover and Thuringia. The lowest cycle of this German succession may be represented in north-eastern England by replacement bodies of anhydrite in the Lower Magnesian Limestone. Lateral change of facies in the English evaporites is marked, and includes the passage of evaporites into red beds, and of more into less soluble salts, in a shoreward direction. Potassium salts are confined to the Whitby district, where the marine Permian reaches its greatest thickness. Halite, anhydrite, and the carbonates progressively die out as the shore-line is approached. Vertical zoning includes the upward passage of less to more soluble salts, and repetitions of sequence are numerous, but large-scale repetitions within each major cycle are much less abundant than in the Salado. The mineralogy of the two regions is compared and found to be generally similar. Striking differences include the restriction of rinneite to the English evaporites, and the virtual absence of langbeinite, a major constituent of some of the Salado potash zones, in England. The English potash field is notably richer in quartz and clay, and poorer in magnesium sulphates (except in the Lower Evaporite Bed) than the American. Some minor constituents found only in the Salado may be expected to be found as exploration proceeds in England. Widespread replacement and recrystallisation have modified the evaporites of both regions, and their present mineral composition differs a great deal from that of the original deposits.