En ligne depuis le 08/02/2005
A unique and spectacular Early Palaeozoic tropical archipelago is being unearthed near Churchill, Manitoba, Canada. Islands composed of Proterozoic quartzite are surrounded by shallow marine deposits of Late Ordovician and Early Silurian age. Field research since 1996 has revealed a total of 16 localities, representing shoreline and nearshore environments preserved both before and after the Late Ordovician mass extinction. The detailed data collected from surface outcrops were enhanced in 2003 by the extraction of drillcore from five sites, permitting three-dimensional interpretation.Upper Ordovician (Richmondian) carbonates and sandstones were deposited in a variety of settings around quartzite islands: muddy bay, boulder beach, shallow-water sands, and a cove with restricted water circulation. They contain diverse biotas: trilobites, corals, brachiopods, cephalopods, gastropods, and conodonts. A claystone of undetermined age overlies the Ordovician rocks. Above this are Lower Silurian (lower Llandovery) deposits, consisting of poorly fossiliferous muddy dolostones overlain by coral-rich dolostones and by strata containing monospecific assemblages of the brachiopod Virgiana decussata. The unusual depositional settings represented by these rocks provide an excellent opportunity to improve our understanding of the Late Ordovician extinction and of Early Palaeozoic shallow marine ecosystems.Data analysis and systematic description of the fossils are ongoing. Discoveries to date include: (1) records of environmental change through the extinction interval; (2) the largest known trilobite, Isotelus rex, and associated trace fossils; (3) unusual fossils including eurypterids and ophiuroids; and (4) diverse corals encrusting Ordovician shoreline boulders.
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