New biostratigraphical constraints on the lower Cambrian Ratcliffe Brook Formation, southern New Brunswick, Canada, from organic-walled microfossils

摘要

Organic-walled microfossils from the Ratcliffe Brook Formation of southern New Brunswick enable recognition of the three lowest Cambrian acritarch-based zones established in southeastern Poland and on the East European Platform. This is the first record of these zones in Avalonia. The distribution of acritarchs of the Asteridium tornatum-Comasphaeridium velvetum, Skiagia ornata-Fimbriaglomerella membranacea, and Heliosphaeridium dissimilare-Skiagia ciliosa zones leads us to propose a revised correlation of sections of the Ratcliffe Brook Formation, in which an ash bed dated at 531 Ma now predates rather than postdates an assemblage of small shelly fossils that had been previously attributed to the Watsonella crosbyi Zone. The small shelly fossils are here suggested to belong to a younger biozone as they occur with acritarchs of the Skiagia ornata- Fimbriaglomerella membranacea Zone, the base of which is currently thought to approximate in time to the global appearance of trilobites. The occurrence of acritarchs of the Helio-sphaeridium dissimilare-Skiagia ciliosa Zone close to the top of the Ratcliffe Brook Formation suggests that the upper part of this formation and the overlying Glen Falls Formation are younger than previously thought and that their correlation with strata in Newfoundland is in need of revision. Biostratigraphic constraints are currently insufficient to position the 531 Ma ash bed with precision with respect to the Watsonella crosbyi Zone in Newfoundland, but it is likely that the date is within this zone.