FORTRAN program for lateral tracing of time-stratigraphic units based on faunal assemblage zones

Abstract

INTRODUCTION Many methods are available for multivariate analysis of faunal zones (see Brower, Millendorf, and Dyman, 1978 for discussion). Especially popular are the techniques of numerical taxonomy, such as cluster analysis, factor analysis, principal-components analysis and principal coordinates. Generally in such analyses all samples from all stratigraphic sections are analyzed simultaneously. However, this approach is reasonable only when the purpose of the analysis is to depict ecological zones or biofacies. If, on the other hand, the results are to portray time-stratigraphic correlations, an alternative approach is recommended; namely, lateral tracing. At a given time of deposition, environmental conditions and thus faunal composition change in any particular geographic area. This variation generally is continuous, with faunal elements gradually being replaced by others both laterally (geographically) and vertically (stratigraphically). In the assemblage zone methods mentioned (e.g. the usual practice of Hazel, 1970, 1977; Brower, Millendorf, and Dyman, 1978), all samples from within the study area generally are analyzed together. This approach ignores the effects of faunal gradation within an isochronous unit with respect to geographic position. Thus, if the faunal composition of such an isochronous unit changes across the study area, samples taken from distant points in the unit might be dissimilar enough not to cluster. Simply, the greater the lateral variation and the larger the distance between them, the less similar are two isochronous samples. This difficulty can be ameliorated by utilizing a priori knowledge of the geographic distribution of the stratigraphic sections under study. Lateral tracing uses this knowledge which allows gradation of stratal character without affecting stratigraphic resolution. This is accomplished simply by performing the analysis in a stepwise fashion between pairs of adjacent stratigraphic sections. The lateral tracing algorithm was proposed originally by Sneath (1975a, 1975b) with an intended use for lithostratigraphy utilizing petrological and geochemical data. A