REDDENED BEDS OF CARBONIFEROUS AGE IN NORTH-WEST ENGLAND AND THEIR ORIGIN

Abstract

Summary The vast quantities of iron, which are imprisoned in deltaic sediments of Carboniferous age, occur predominantly as ferrous iron. This ferrous iron is associated with carbonaceous matter and for the most part is found as siderite at two distinct horizons here termed the Upper and the Lower Ironestone. They lie respectively above and below the coal seam, and periodicity of deposition is made apparent by the repeated occurrence of this ferrous carbonate—carbonaceous matter association. It is considered that the carbonaceous matter has acted as a reducing agent thereby causing the precipitation of the iron in the ferrous state. When ferrous iron-bearing strata are subjected to oxidising conditions the iron reverts to the more normal ferric oxide and the strata become reddened. Such oxidising conditions existed on and beneath the pre-Permo-Triassic land surface. As proved in the field, and by numerous boreholes, a red zone of complete oxidation lies below the base of the Permo-Triassic rocks and this not infrequently is followed by alternating zones of partial oxidation and non-oxidation. The depth below the pre-Permo-Triassic land surface to which oxidising influences were operative on and to the west of the Pennines is of the order of 1,000 ft. with a maximum of 1,700 ft. It is considered that such deep penetration resulted from high relief in a semi-arid climate during the Carboniferous-Permian interval.