Encrusting corals on a latest Ordovician to earliest Silurian rocky shore, southwest Hudson Bay, Manitoba, Canada

Abstract

Paleozoic tabulate corals are generally thought to have been free standing, a flattened disc-shaped to dome-shaped morphology providing a degree of stability in shallow-water, high-energy environments. The ability to encrust has previously been suggested by patterns of competitive overgrowth in certain species. Definite proof of encrustation by favositid corals is exhibited in an extraordinary example of an ancient rocky shore exposed for 350 m on Hudson Bay near Churchill, Manitoba. Carbonate strata attributed to the Upper Ordovician Port Nelson or Lower Silurian Severn River Formations locally transgress a massive Precambrian quartzite. An ancient shoreface is clearly marked by large, smoothly eroded boulders of the dark quartzite, commonly 2–10 m in diameter. The boulders are buried in coarse carbonate debris, but corals up to 20 cm in diameter are found cemented directly onto the surface of some boulders. Deep pitting of many boulders to a depth of 2–3 cm was contemporaneous and may have been promoted by unpreserved encrusters such as sponges or anemones.