SUBDIVIDING THE CAMBRIAN: A WORK IN PROGRESS

The international vocabulary for subdivisions of the Cambrian time-scale is not yet unified, and each major Cambrian region has at least one set of its own epochs and ages (and their nomenclatural twins, series and stages). There is good reason for this. Biogeographic differentiation was so strong that meaningful divisions of the biotic history of the shallow marine successions on the different Cambrian continents are largely distinct, making precise correlation of these widespread successions often difficult. Precision in intercontinental correlation is largely accomplished by using the agnostoid faunas of the slope facies on the Cambrian continents, and so far this works only for successions younger than the middle part of the Middle Cambrian. Both boundaries of the Middle Cambrian are inconsistent internationally, and the recent attempt to define the Cambrian/Ordovician boundary to satisfy graptolite specialists has only contributed further confusion. This talk will focus, therefore, on the historical development of time-scale nomenclature for series and stages (epochs and ages) of the Cambrian of Laurentia, begun by Walcott (1891) and most recently upgraded by Palmer (1998). Trilobite biostratigraphy provides the principal controls on this nomenclature. Because of the complexity of facies controls on trilobite distribution first pointed out by Robison (1976) and later elaborated by Westrop and Ludvigsen in the 1980’s, time-scale divisions at the level of zones on a continental scale need to be revised and updated and will not be discussed. Most zonal nomenclature currently applied is either very generalized, or only has local value.