THE ADVENT OF THE CORAL REEF: ANIMAL ABUNDANCE IN UPPER ORDOVICIAN LOURDES FORMATION REEFS, WESTERN NEWFOUNDLAND

During the Middle to Late Ordovician, carbonate reefs experienced a drastic change. Corals and other metazoans began constructing reefs for the first time following the long-term microbial reef resurgence of the Cambro-Ordovician interval. The Lourdes Formation of western Newfoundland is a 75-m thick Upper Ordovician (late Whiterockian to early Mohawkian) mixed siliciclastic-carbonate unit exposed on the southwest coast of Long Point on the Port au Port Peninsula. In the Black Duck 3 Member of the Lourdes Formation, coral bioherms are well developed. The buildups measured in this work are up to 5 m-thick and ~5 m wide and occur through 16 m of stratigraphy. Here, we mapped and sampled 3 reef bioherms at 1 locality where the Black Duck 3 Member was well exposed. We measured and sampled each reef in detail, extracting 42 samples from the reef core, grainstone channels that form under and around the reefs, and the surrounding sediments. Each sample was thin sectioned and a subset of 26 well preserved samples were point counted. In general, fossil points accounted for 37.4% of all points counted from reef samples. The tabulate coral, Labyrinthites, makes up most of the skeletal material in the reef core (83% of fossil points). Crinoids make up only 7% of fossil points with ostracode, brachiopod, stromatoporoid, and bryozoan material making up 10% of the remaining fossil counts. Fossil material in the grainstone channels is composed of crinoids (66%), bryozoans (16%), and brachiopods (12%), with small amounts of ostracodes and trilobites (6% combined). Crinoids (48%) and brachiopods (26%) make up most of the skeletal material in the sediments that surround the reef, but in general, skeletal material was less abundant (11%) than in the reef. Our initial analysis of these bioherms confirms earlier work indicating corals are abundant in the reef core, likely acting as framework builders, and that crinoids are ubiquitous components of this reef ecosystem, probably serving as sediment bafflers. Additionally, we suggest that even at this early stage in coral bioherm development, an ecologically complex reef system developed in western Newfoundland where diverse skeletal material is much more prominent in these ecosystems than in any reefs that existed before this time.