Geophysical Research Abstracts, Vol. 5, 13219, 2003 c European Geophysical Society 2003
GEOCHRONOLOGICAL CONSTRAINTS ON TERMINAL NEOPROTEROZOIC EVENTS AND THE RISE OF METAZOANS S. Bowring (1), P. Myrow (2), E. Landing (3), J. Ramezani (1),and J. Grotzinger (1) (1)Massachusetts Institute of Technology, Cambridge MA,(2)Colorado College, Colorado Spring, CO,(3)New York State Museum, Albany, NY USA (
[email protected]/Fax: 617-253-6735)
A full understanding of Neoproterozoic history has been plagued by a lack of precise geochronological constraints. In particular the correlation and duration of global or "Snowball" glaciations has relied on physical stratigraphy and chemostratigraphy which has given rise to much debate about the number, magnitude, and duration of glacial deposits. New U-Pb constraints on the age of the Gaskiers Formation glacial deposits in Newfoundland, an age for the oldest Ediacaran fossils in the same area, and the age and significance of the Cambrian/Precambrian boundary in Oman have important implications for global correlation and the timing of the rise of Metazoans. In the central and eastern parts of the Avalon Peninsula, southeastern Newfoundland, the oldest rocks are arc-related tuffs, agglomerates, and flows of the Harbour Main Group (>1.5 km thick) that have published dates from 606-630 Ma. These are overlain by approximately 7.5 km of marine siliciclastic rocks of the Conception Group. Over 2,300 m of deep-water deposits of the Mall Bay and Drook formations are separated by the regionally extensive glacial diamictite of the Gaskiers Formation (up to 300 m-thick). This unit is often described as a Varanger-age glaciomarine deposit and is locally overlain by a thin cap carbonate bed with a highly negative C isotopic signature. Thin (1-15 cm) silicic ash beds are found interlayered with turbidites just below and above the glacial deposits and the glacial deposits locally contain volcanic bombs, pyroclastic debris, and lavas. U-Pb geochronology of zircons separated from ash-beds below, within, and above the glacial deposits indicates that they are ca 580 Ma. This is considerably younger than previous estimates and calls into question many
of the global correlations with similar rocks. An ash-bed within the overlying Drook formation is preserved in depositional contact with spectacular surfaces exposing Ediacaran fossils. Zircons separated from it yield an age of 575 Ma making these the oldest well-dated Metazoan fossils that occur within 5 Ma of the glacial deposits. Of particular interest is whether these fossils are correlative or younger than the fossil embryos of the Doushantuo of south China that also immediately overlie glacial deposits. Globally, Ediacaran fossils occur in abundance from 575 Ma to the NeoproterozoicCambrian transition, which is often marked by a distinctive negative excursion in the carbon isotopic composition of seawater. The disappearance of Ediacaran fossils across the boundary has been attributed to both lack of preservation and extinction. New geochronological and paleontological data from Oman are consistent with an extinction of Cloudina and Namacalathus at the Cambrian-Precambrian boundary ca 542-543 Ma and supports the idea of a global extinction event that set the scene for the Cambrian explosion.
