Stratigraphic and Geochronometric Framework. The time interval considered here is 1700–520 Ma; that is, the latest Paleoproterozoic Eon to the end of the Early Cambrian Period (Figure 1 and Table 1). U-Pb dates on accessory minerals in volcanic rocks of known relationship to fossiliferous strata are limited for this interval—but then, such data are also limited for younger Paleozoic fossils on which much greater paleobiological demands are placed. Quantitative analysis of the Paleozoic fossil record is possible because a well-defined chronostratigraphic, or relative, time-scale has been calibrated by radiometric data in a few key sections.
The Proterozoic-Cambrian time scale is developing along the same