Time and age in geology: The use of upper/lower, late/early in stratigraphic nomenclature

Abstract

Chronostratigraphic (time-rock) units (e.g. Cretaceous System, Eocene Series, Aptian Stage), divided into lower, (middle), and upper, are appropriate for attribution of age to rocks, and hence to graphic representations such as well logs, colummar sections, seismic sections, and stratigraphic correlation diagrams. Geochronological (time) units (e.g. Cretaceous Period, Eocene Epoch, Aptian Age), divided into early, (middle), and late, are appropriate for designating the time of occurrence of historical events, such as transgressions, periods of erosion and deposition, and episodes of folding or faulting. The dual system of nomenclature, if followed consistently, aids clarity in thought and expression of complex ideas, particularly in describing geological history.