The base of a revised Middle Cambrian: are suitable concepts for a series boundary in reach?

Abstract

Defining the base of a series that replaces the traditional Middle Cambrian is among the difficult tasks of Cambrian stratigraphy. Non-traditional concepts (such as carbon isotope signatures), microplankton (such as acritarchs), and most invertebrate fossils (e.g., brachiopods) may act as auxiliary, tools for intercontinental correlation of regional calibration but are unable to provide the base for fine-scaled global correlation at present. As a result, the selection of a Global Stratigraphic Section and Point will have to root on trilobites which appear to be the only reliable index fossils to define such a GSSP. Five possible levels of correlation within the traditional Lower-Middle Cambrian boundary interval have been discussed: (1) the FAD ofOryctocephalus indicus; (2) the FAD ofOvatoryctocara gramulata and/orKiskinella cristata; (3) the FAD ofArthricocephalus chauveaui; (4) the base of theAcidiscus-Cephalopyge assemblage “zone”; and (5) the base of the STH “band”. The potentials of these levels are analyzed in this study. All of them suffer from certain deficiencies such as limited insight into the stratigraphic ranges of key species; problems of confident identification of the index species; or absence of key faunal elements on certain Cambrian continents; that make them invalid for high-precision correlation on a global scale. However, the combination of (1) through (4) promises an intercontinental correlation that can be used as a global framework, which will be sufficiently accurate to serve normal correlation purposes.