A NEW, MAJOR SILURIAN REEF TRACT AND OVERVIEW OF REGIONAL SILURIAN REEF DEVELOPMENT, CANADIAN ARCTIC AND NORTH GREENLAND

A recently discovered Silurian reef tract in the high Canadian Arctic has an exposed length of more than 150 km and is locally more than 500 m thick. The reefs occur within a Silurian ramp and rimmed shelf-marg in sequence about 2 km thick. Conodont biostratigraphic data indicate that the reefal strata are late Llandovery to Ludlow in age, and reefs are composed largely of coral-microbial boundstone, The corals are ma inly digitate rugosans and large halysitids, associated with fewer str omatoporoids and lithistid sponges. However, most of the reefal strata consist of microbialite, which encrusts skeletal metazoans or is mass ive or thromboidal. Microbialite microstructure consists mainly of clo tted micrite or laminated micrite associated with common Renalcis, oth er calcimicrobes, and locally abundant early marine cements. Other ree f rock types include stromatactis-rich lime mudstone, cementstone, zeb roid stromatactis-bearing lime mud-stone, crinoidal grainstone, and, r arely, stromatoporoid boundstone. The reefs formed on the shelf margin and prograded basinward in three distinct phases during Wenlock and L udlow time. Underlying Llandovery, shelf-margin facies are highly dolo mitized and may represent an original stromatoporoid boundstone. Overl ying late Ludlow to Pridoli carbonates were deposited on a prograding carbonate ramp, dotted with small coral-stromatoporoid and other biost romes, Three periods of Silurian reef growth can be recognized in the Arctic: Llandovery, latest Llandovery-late Ludlow, and late Ludlow-Pri doli. The Llandovery phase was widespread, and large stromatoporoid-co ral reefs occupied a variety of depositional settings, including shelf -edge, intrashelf, drowned shelf, and slope. Late Llandovery to late L udlow reefs were also regionally extensive, but were predominantly cor al-microbial boundstone and locally grainstone-rich structures contain ing accessory metazoan (stromatoporoid-coral) framestone and boundston e. These formed high-relief structures with coarse allochthonous debri s aprons. Large stromatoporoid-coral reefs were also present, but thes e were confined to areas characterized by high rates of siliciclastic sedimentation or to intrashelf areas. Another regional change in reef composition occurred in the latest Ludlow. Initially, reefs were rare and dominated by lithistid sponges and microbialites, but in Pridoli t ime, they became widespread and were constructed by corals and stromat oporoids, as were their Llandovery ancestors. In addition, the Pridoli reefs did not attain the large sizes of reefs in the two older sequen ces. The three phases of reef development can be related to important, basinwide sequence boundaries. Microbialite-rich reefs have also been described from northwestern Russia, Alaska, and eastern Canada and ar e some of the largest reef structures known. Their large size reveals a need to reconsider popular reef evolutionary models, in which skelet al metazoans are regarded as the predominant constructors. Large reef size is implied in these evolutionary models, but, based on this and o ther reef studies, microbialite-rich reefs equal, and commonly surpass , the dimensions exhibited by coral-stromatoporoid reefs. Considering the microbialite-rich reefs, now known from both the Ordovician acid S ilurian, it is clear that benthic microbial communities played a very important role in reef construction through the lower Paleozoic.