Terminal Proterozoic Time Scale: Constraints on Global Correlations and
Rates of Early Animal Evolution
GROTZINGER, JOHN P.
Over the past decade biostratigraphic, chemostratigraphic, and sequence stratigraphic data sets have converged to yield a functional chronostratigraphy for terminal Proterozoic time. Calibrated in absolute time by high-precision U-Pb zircon geochronology, this terminal Proterozoic chronostratigraphy has a resolution that is comparable to many Paleozoic systems. Given this new chronostratigraphy, it is now possible to undertake global correlation for the purpose of identifying major sea level events, times of source rock deposition (Siberia, Oman, China), and major evolutionary episodes, including the Cambrian "explosion." With regard to the latter, two distinct evolutionary pulses, represented by the Vendian Ediacaran fauna and Cambrian small shelly faunas, are generally thought to characterize the emergence of macroscopic animals at the end of the Proterozoic Eon. New data from Namibia indicate that most globally-distributed Ediacaran fossils are no older than 549 m.y. and some are as young as 543 m.y., essentially coincident with the Precambrian-Cambrian boundary. These data suggest that the most diverse assemblages of Ediacaran animals existed within 6 m.y. of the Precambrian-Cambrian boundary and that simple discoid animals may have appeared at least 50 m.y. earlier. U-Pb zircon data from lower Cambrian rocks located in northeast Siberia indicate that the Cambrian Period began approximately 543 m.y. ago, and that its oldest (Manykaian) stage lasted no less than 10 m.y. Other data indicate that the Tommotian and Atdabanian stages together lasted only 5--10 m.y. The resulting compression of Early Cambrian time accentuates the rapidity of both the faunal diversification and subsequent Cambrian turnover.
AAPG Search and Discovery Article #90938©1997-1998 AAPG Distinguished Lecturers