Recent radiometric dating of some Cambrian rocks in southern Australia: relevance to the Cambrian time scale.

Abstract

The Cambrian time scale is in a state of flux due to a lack of reliable geochronological dates which can be tied to biostratigraphic scales. Most recent Australian dates relevant to the Cambrian geochronologic scale have been determined from the Early Cambrian successions of South Australia and the Middle and Late Cambrian sedimentary and volcanic rocks of western Tasmania. This work has been based on SHRIMP dates using zircon standard SL13. The reliability of these dates is questionable due to doubts as to the reliability of SL13. An analysis of the Lower Cambrian data from South Australia suggests that depending on the standard used the Botoman/Toyonian boundary may be younger than 532.8 ± 4 Ma or 522.8 ± 1.8 Ma. The base of the Botoman is older than 526 ± 4 Ma and may be several million years older, thus suggesting either that the Tommotian and Atdabanian are quite short or that the recently proposed figures of 534 Ma and 528 Ma for the base of the Tommotian and Atclabanian respectively need to be revised to older dates. The Middle Cambrian/Late Cambrian boundary is more likely to have a elate of 505 Ma than 498 Ma. The average time span for a trilobite zone (mainly agnostoid based) in the Middle and Late Cambrian is about 535,000 years. This average time span is such that the error bars of most geochronological dates encompass several trilobite zones.