Cambrian subdivisions and correlations: Introduction

Abstract

Efforts to define reference standards for the base of the Cambrian and its subdivisions have helped understanding of the successive stages of the Cambrian evolutionary radiation, have led to an impetus for rapid improvements in Cambrian geochronology, and have focused attention on sea-level history and global paleogeography during the latter phases of breakup of the Rodinia supercontinent. Over 10 years ago, a horizon at Fortune Head, southeastern Newfoundland, was proposed as a candidate for the Precambrian–Cambrian boundary global stratotype section and point (GSSP) (Narbonne et al. 1987), and joined the east Siberian and south Chinese Precambrian– Cambrian boundary candidates. Five years after an international field conference in southeastern Newfoundland (Landing et al. 1988), the Fortune Head candidate was ratified by the Cambrian Subcommission and International Stratigraphic Commission as the Precambrian–Cambrian boundary GSSP (Landing 1994). Current activity by the Cambrian Subcommission has switched to the definition of globally useful chronostratigraphic divisions of the Cambrian and to the investigation of potentially correlatable biotic and global events through the period. This effort has undergone several metamorphoses and has broadened the concerns of the working group during three successive international field conferences. The first field conference focused on the stratigraphy and zoneand stage-level divisions of the Lower–Middle Cambrian of southern Morocco (Geyer and Landing 1995). This conference was conducted for a “Lower Cambrian Stage Subdivision Working Group.” Its concern with defining global stage-level units was originally promoted by A.Yu. Rozanov and the previous officers of the Cambrian Subcommission. A new consensus developed on several themes during the Moroccan meeting. The first theme was that participants in the Lower Cambrian Stage Subdvision Working Group were interested in the upper limits of the Lower Cambrian and in the faunal and nonbiological events that accompanied the transition into the Middle Cambrian. Because working groups on Middle and Upper Cambrian chronostratigraphy were not active, this led to an upward encroachment of this working group’s concerns into the Middle Cambrian. The second theme that developed in Morocco featured several related concerns: (i) whether or not the very provincial marine faunal sequences on the numerous, widely separated Cambrian continents conveniently allow the development of a global chronostratigraphy at the level of the stage; and (ii) whether stage-level subdivisions are not best defined and limited to individual faunal provinces. This broadening of the scope of the Working Group’s stratigraphic interests and a concern with the definition of provincial stages to subdivide the evolutionary and immigration events within faunal provinces were evident in the second international field conference in Spain (Linan et al. 1996). As a result of these changes in emphasis, the Working Group’s second conference was held for a renamed “Cambrian Stage Subdivision Working Groups.” This conference, like the Moroccan meeting, dealt with Gondwanan faunal province sequences, but had a stronger Middle Cambrian focus because of the abundance of biostratigraphically useful Middle Cambrian trilobites. The six papers of this thematic section represent a further maturation of thought by the working group, and are one of the results of the 1997 field conference (Landing and Westrop 1998). This third field conference, hosted in southeastern Newfoundland and southern New Brunswick for a renamed “Cambrian Chronostratigraphy Working Group,” reviewed the earliest through terminal Cambrian cool-water marine biotic sequence, geochronology, and sequence stratigraphy and epeirogenic history of the Avalon continent (e.g., Landing 1996). As reviewed in this thematic section’s paper by Landing et al., the richly fossiliferous Cambrian of Avalon is unique among all known Cambrian successions in that it has numerous minor acidic volcanics that yield zircons suitable for U–Pb dating. In this paper, Landing et al. use high-resolution U–Pb dates from Avalon to demonstrate that the Early Cambrian is the longest part of the Cambrian and to affirm a tentative biostratigraphic correlation into the provincially distinct trilobite sequence of Morocco. The difficulties of conventional biostratigraphic correlations of the Cambrian are emphasized by Palmer’s and Geyer’s papers in the thematic section. Palmer’s proposal of internally consistent regional stages and series extends Ludvigsen and Westrop’s (1985) Laurentian chronostratigraphy down to the base of the Cambrian. In doing so, Palmer also emphasizes the importance of biomere events (i.e., trilobite extermination and replacement events) as the major biostratigraphic developments through the North American trilobite-bearing Cambrian. This proposal essentially regards the long time interval represented by the Lower Cambrian as a subsystem-level division of the period composed of a pretrilobitic and a trilobite-bearing Lower Cambrian series, somewhat comparable to developments worldwide (Landing 1992). Although a consensus should be developed at least for global series-level Cambrian divisions, no agreement exists. Geyer’s paper demonstrates the difficulties in defining the Lower–Middle Cambrian boundary even in the West Gondwanan faunal province. Long-standing problems in specieslevel identifications of paradoxidiid trilobites, the traditional guide for the Middle Cambrian in Avalon, Gondwana, and