WHERE ON EARTH WAS LAURENTIA’S SOUTHERN MARGIN DURING THE NEOPROTEROZOIC AND EARLY PALEOZOIC?

Long neglected in paleogeographic reconstructions, the present southern margin of the Laurentian craton now provides some of the strongest clues to the configuration of the major continents during the critical Neoproterozoic to Early Paleozoic time interval. This encompasses the putative ‘Snowball Earth’, the Ediacaran fauna, the explosion of metazoan life into the Cambrian ‘greenhouse’ Earth, a major Ordovician extinction and the Hirnantian glaciation. In this presentation I will review these clues, together with the attending puzzles. Geochronological, geochemical and paleomagnetic data indicate that the Coats Land crustal block of Antarctica was part of southern Laurentia at c.1100 Ma. The presence of the Grenvillian-age Maud belt in Dronning Maud Land, Antarctica demonstrates that the Laurentian and Kalahari cratons collided c. 1000 Ma because it separates Laurentian Coats Land from the Grunehogna terrane that was part of the Kalahari craton until the opening of the Southwest Indian Ocean basin in Mesozoic times. The Mawson craton of East Antarctica and Australia collided with the Coats Land area of Laurentia during the Pan-African assembly of the Gondwanaland supercontinent that possibly included Laurentia briefly as the ephemeral southern supercontinent of Pannotia before the separation of Laurentia from the Coats Land block and Kalahari craton in the earliest Cambrian. Finally, the presence in present-day Northwest Argentina of the Cuyania terrane that originated in the Ouachita embayment of Laurentia following the deposition of Olenellid-bearing Cambrian-lower Ordovician limestones and ‘docked’ along the Iapetus margin of Gondanaland prior to the Hirnantian glaciations strongly suggests that the early Paleozoic Iapetus Ocean was narrow. This conclusion seems to be supported by the near synchroneity of Ordovician orogenesis from the British Isles to Argentina.