Paleogeography at the Precambrian-Cambrian transition

English Abstract:

The geologically abrupt appearance in the fossil record of almost all animal phyla is referred to as the Cambrian radiation or ‘explosion’ of life on Earth. Although now seen as somewhat more protracted than formerly recognized, it is still one of the most striking and critical events in the history of the biosphere. It accompanied initiation of the first of the two major marine transgressions of the Phanerozoic  Eon. Yet no major geographic or tectonic changes at that time have been identified. Almost all paleogeographic reconstructions for the early Cambrian feature a previously isolated Laurentia, the core of ancestral North America, separated from the newly amalgamated Gondwanaland supercontinent by the already established Iapetus Ocean. Yet geological evidence from five continents, integrated here, indicates that the present-day ‘southern cone’ of Laurentia was still attached to Gondwanaland into Cambrian times, and indeed geologic and paleomagnetic data suggest that a small fragment of southern Laurentia may still exist in Coats Land, East Antarctica. In the early Cambrian Laurentia was then isolated by the development of a major deep oceanic connection between the opening Iapetus Ocean basin and the already well-developed paleo-Pacific, as the marine transgression was initiated, major changes in ocean chemistry occurred, upwelling generated phosphorite deposits, and the number of fossilized metazoan phyla ‘exploded’ with morphologic disparity between Laurentia and Gondwanaland already established, the Olenellid trilobite fauna of Laurentia being separated from the Redlichiid fauna of Gondwana. The paleogeographic settings of the two great transgressions of the Phanerozoic, the Cambrian and Cretaceous, are remarkably similar. Both seem to have involved major comparatively rapid increase in ridge crest length within the ocean basins contemporaneous with the subduction of older oceanic crust elsewhere on the globe.