Previous work has shown that the boron content of illite, corrected for grain size and composition effects (equivalent boron), is an index of paleosalinity. Equivalent boron for rocks containing clay mixtures usually can not be determined because precise mineralogical analysis of clays by X-ray methods is not possible. A method is described which enables a minimum illite content to be assigned to a mixture of clays. If this minimum content is high, the clay can be regarded as pure illite for purposes of computing equivalent boron, because the error involved is small.

A paleosalinity study is described from the lower part of the Yoredale Formation of northern England. The formation is a deltaic complex of Late Visean age, and was selected for study because its facies distribution is similar to subsurface distributions associated with stratigraphic hydrocarbon traps. The minimum possible illite content of clays from the Yoredale Formation generally exceeded 80 per cent and many exceeded 90 per cent. Thus, the clays could safely be assumed to be pure illite for purposes of computing equivalent boron.

The Yoredale Formation is a cyclic rock sequence; each cycle beginning with a marine limestone which passes up through shales into non-fossiliferous sandstones of assumed deltaic origin. Average paleosalinity, estimated from equivalent boron, was highest in marine limestones and lowest in sandstones. However, in coarse-grained porous sandstones, equivalent boron and presumably paleosalinity, were high. Some of these sandstones contained rare marine fossils and others could be shown to pass laterally into undoubted marine rocks over a short distance. These sandstones are interpreted as beach bar and shallow bay sediments, laid down at the seaward margin of the Yoredale delta. Thus, sandstone porosity in the Yoredale delta appears to be confined to marine sandstones, and increasing equi alent boron, the index of paleosalinity, is accompanied by increasing sandstone porosity. This suggests that exploration for porosity in or adjacent to subsurface deltaic complexes would be aided by a paleosalinity investigation similar to the one described in this paper.