THE OLDEST CAMBRIAN TRILOBITES IN THE AVALON TERRANE, NEWFOUNDLAND: STRATIGRAPHIC, ENVIRONMENTAL AND PALEOECOLOGIC CONTEXT

Although known for more than a century, the Early Cambrian trilobite faunas of the Brigus Formation of the Avalon Terrane, Newfoundland, remain poorly documented. The formation comprises two depositional sequences that correspond to the St. Mary’s and overlying Jigging Cove Members. The base of the St. Mary’s Member is a diachronous unconformity with the sub-trilobitic Lower Cambrian and older rocks as low as those of the upper Precambrian Avalonian orogen. All previously described trilobites occur in the St. Mary’s Member, which is composed of dominantly red and purple siliciclastic mudstones and minor nodular to bedded carbonates that were deposited from near storm-wave base to peritidal settings. Trilobites are the dominant faunal elements of subtidal carbonates, with minor small shelly fossils and a near-absence of mollusks. The ellipsocephaloid Strenuella occurs in almost all collections, as does the eodiscoid Hebediscus. Other common components of the St. Mary’s fauna include Callavia, Acanthomicmacca and Serrodiscus. Onlap of the St. Mary’s Member and the appearance of its eodiscoids is roughly correlated with eustatic rise recorded on other continents (e.g., Sinsk event in Siberia, upper Issafen Formation in Morocco). The trilobite assemblage of the St. Mary’s Member, generally assigned to the Callavia broeggeri Zone, is largely replaced at the sequence boundary with the terminal Lower Cambrian Jigging Cove Member (lower dysaerobic green and higher, oxygenated purple and red siliciclastic mudstones with nodular carbonates. The Jigging Cove has a sparse fauna of undescribed ellipsocephaloids. Although the St Mary’s Member records shallow subtidal conditions over most of southeast Newfoundland, peritidal carbonates occur in it near the southern tip of the Burin Penninsula. Here, assemblages are dominated by condensed hyolith and small shelly fossil packstones, but trilobites are very rare. This suggests that the initial Cambrian diversification of trilobites in Avalonia was concentrated in offshore, rather than nearshore environments.