NEW BIOSTRATIGRAPHICAL CONSTRAINTS ON THE LOWER CAMBRIAN RATCLIFFE BROOK FORMATION, IN AVALONIA OF SOUTHERN NEW BRUNSWICK, CANADA, FROM ORGANIC-WALLED MICROFOSSILS

Organic-walled microfossils have been recovered from two classical sections of the lower Cambrian Ratcliffe Brook Formation at Hanford Brook and Somerset Street in Avalonia of southern New Brunswick. The basal part of the Ratcliffe Brook Formation yields the polyhedral acritarch Octoedryxium truncatum. If early Cambrian, this represents one of the youngest records of this form and suggests that the Ratcliffe Brook Formation may extend into the Ediacaran. The three lowest Cambrian acritarch-based zones established in southeastern Poland and on the East European Platform are recognized for the first time in Avalonia. The distribution of acritarchs of the Asteridium tornatum-Comasphaeridium velvetum, Skiagia ornata-Fimbriaglomerella membranacea, and Heliosphaeridium dissimilare-Skiagia ciliosa zones shows that an ashbed previously dated at 531 Ma in the Somerset Street section predates, rather than postdates, an assemblage of small shelly fossils from a section at Hanford Brook that had been previously attributed to the Watsonella crosbyi Zone. The small shelly fossils at Hanford Brook 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 appearance of trilobites. The relationship of the 531 Ma ashbed to the Watsonella crosbyi Zone in Newfoundland is uncertain. However, on the basis of the available biostratigraphic data from New Brunswick and Newfoundland we find it likely that the 531 Ma ashbed provides an approximate age for the Watsonella crosbyi Zone.