UPPER DEVONIAN IRIDIUM ANOMALIES, CONODONT ZONATION AND THE FRASNIAN FAMENNIAN BOUNDARY IN THE CANNING BASIN, WESTERN-AUSTRALIA

Two iridium anomalies have been identified near the Frasnian-Famennian boundary (Upper Devonian) in the Canning Basin of Western Australia. Both anomalies are associated with the cyanobacterium Frutexites and are located in marginal slope facies. The first was identified in the Virgin Hills Formation on the west flank of McWhae Ridge near the southeastern end of the Devonian outcrop belt. This anomaly, initially identified as being from the Famennian Upper Palmatolepis triangularis Zone, is now known to be from the Early Palmatolepis crepida Zone, on the basis of the presence of Palmatolepis crepida in the bed. The second anomaly, found in drill core from the Napier Formation just south of the Napier Range, is older than the first and is from the Frasnian Montagne Noire Conodont Zone 12 or 13 of Klapper (1989)=the Palmatolepis rhenana Zone of Ziegler and Sandberg (1990). These two iridium anomalies are thus significantly below and above the Frasnian-Famennian boundary and are not associated with the extinction event in the Palmatolepis linguiformis Zone. In the Canning Basin the association of two iridium anomalies with beds containing abundant Frutexites microstromatolites indicates that the concentrations of iridium are probably associated with a process of organic concentration. There is no evidence that either example is directly associated with an impact event.