Introduction

In any attempt to construct a set of paleogeographic reconstructions, it is necessary to make sense out of a variety of sometimes conflicting sets of geological, paleontological, and geophysical constraints. Because the applicability of biostratigraphic correlation diminishes with increasing geologic age (Chapter 10), it is necessary that reconstructions of Late Proterozoic paleogeography (as well as those of the earliest Paleozoic) be anchored well within the Paleozoic, and only then be extended as far as feasible into the Precambrian. However, few reconstructions of Cambrian paleogeography have reconciled in a satisfactory manner both the pronounced lithologic and faunal variations that are known to occur with the often sparse paleomagnetic constraints (Jell 1974; Shergold 1988; Courjault-Radé 1987; 1988). Thus, it is necessary first to produce consistent tectonic and paleontological models for Cambrian time before attempting an extension into the Precambrian.

Temporal correlation is the most severe problem encountered in dealing with the Proterozoic time. Paleomagnetic data from rocks of this age are of highly variable quality, and are plagued with large uncertainties in the age of various components. After careful examination of the existing paleomagnetic data, we have chosen to limit our attempts at a detailed reconstruction to the Vendian, from the approximate time of the last glacial episode of the Neo-Proterozoic Cryogenian System (see Table 1.1.1) through to the Cambrian-Ordovician boundary.