ON THE UTILITY OF SEDIMENTARY ROCKS AND STRATIGRAPHIC SEQUENCES TO CONSTRAIN PRECAMBRIAN SUPERCONTINENTS: APPROACHES, SUCCESSES AND LIMITATIONS

The utility of sedimentary rocks and stratigraphic sequences to constrain the configuration of supercontinents hinges on the identification of events in the sedimentary record that were of global significance and distribution. The most familiar of these are glacial deposits that appear to have been preserved on several continents at approximately the same time, giving rise to the Snowball-Earth hypothesis. Limitations to this hypothesis have been presented, including evidence for an active hydrological cycle during the snowball and concerns about the synchroneity of the various Neoproterozoic glaciations. There is little doubt that interiors of supercontinents were occupied by vast sedimentary basins (epeiric seas) that probably were connected to an exterior ocean by intracontinental seaways during periods of marine transgression. These basins were composed of numerous sub-basins and embayments that were isolated during periods of marine regression. The stratigraphy of these basins records multiple cycles of basin freshening (transgression) and restriction (regression) that can be correlated between occurrences on now widely separated continents. Supercontinent break-up is recorded by penecontemporaneous regional unconformities and associated rift deposits, including flood basalts, located at the tops of these basins. Such sequence stratigraphic correlations are being augmented by isotopic studies of carbonates that are interpreted to indicate secular variations in the composition of basinal waters. As well, chronostratigraphy, aided by new methods for directly dating sedimentary sequences, is steadily improving our means for matching sedimentary sequences between supercontinent building blocks. Detrital zircon geochronology has been used to identify unique provenance signatures and age profile “bar codes” for particular regions, which can then be matched to establish proximity. Although each of these approaches presents a means of establishing a connection, their main limitation is that they do not provide a way to position continents relative to each other. Therefore, such studies need to be accompanied by magnetostratigraphic analysis and other methods that can help to locate piercing points between formerly contiguous continents.