Continuous sedimentary gradients are only crudely expressed by standard facies and microfacies methods which are more appropriate to situations where changes occur in relatively discrete steps. In carbonate sediments and rocks, continuous gradients are often represented by the arrangement of component grain types in a relay, that is, a systematic shifting of the relative importance of the components. Subdivision of such relays into (micro)facies can only be arbitrary.
Correspondence analysis is shown to be useful for detecting, isolating and describing relays. Particular use is made of the arch effect in which samples and components from data sets with a strong unidimensional structure (a relay) plot in the form of an arch in the plane of the first two factor axes. A relay index, indicating the position of samples in the relay, can be extracted from the analysis and plotted on maps and stratigraphic logs to reveal details of the sedimentary gradient in areal and/or stratigraphic context.
Examples are given from: (i) Recent shallow-marine carbonate sediments from northern Norway, illustrating a relatively simple depositional setting where surface sediments are viewed in plan; and (ii) Lower Carboniferous carbonates of southwest England, representing a more complex regional study of a particular stratigraphic interval viewed in cross-section. In both examples the relays can be related to identifiable environmental gradients.