Environmental gradients in carbonate sediments and rocks detected by correspondence analysis: examples from the Recent of Norway and the Dinantian of southwest England

Abstract

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.

On the provenance of flysch deposits in the External Hellenides of mainland Greece: results from heavy mineral studies

“…The closure effect of percentage data is reduced by the weighting procedure used. The computer programme for calculation was made available by A. Lees (see Hennebert & Lees, 1991).…”

Sedimentology of the upper Dinantian Charlestown Main Limestone: implications for the controls on cyclothem deposition, eastern Midland Valley of Scotland

“…Each sample or component can also be assigned a Relay Index (RI), its coordinate value on the first CA axis rescaled such that the range of all samples or components lies between 0 and 100. Hennebert and Lees (1991) illustrated how CA may be used to detect and analyse relays in carbonate rocks.…”

“…Correspondence analysis (CA) is an eigenvector method which enables a large mutlivariate cloud of points (representing both samples and componenets stored in a two-way contingency table) to be projected into a reduced space defined by the CA axes (Hennebert and Lees 1991). Moreover, CA calculates the variation (as a percentage) represented by the each CA axis.…”

Late Quaternary micropalaeontological record of a semi-enclosed marine basin, North Evoikos, central Aegean Sea

“…In order to detect environmentally important trends in foraminiferal assemblage data, we performed Correspondence Analysis, an eigenvector method which allows the projection of a large cloud of points (samples, parameters or both) into a reduced space (defined by the factor axes) while conserving the major part of the structured, meaningful information (Hennebert and Lees, 1991). Correspondence analysis (CA) is very illustrative as depicts species and samples at the same time.…”