Borings not boring: Trypanites lcbnofacies from the Bay of Fundy, Nova Scotia
University of New Brunswick
Cobequid Bay represents the easternmost arm of the Bay of Fundy and suffers the most extreme tidal range in the Fundy system (up to 17m). Consequently, the intertidal zone is wide and unusually high (tidal) energy deposits are commonly exposed. A remarkable bored surface (Trypanites Ichnofacies) is present on the north margin of Cobequid Bay (Thomas' Cove) near Economy, Nova Scotia. The surface is the result of tidal ravinement and the subsequent colonization by boring and encrusting organisms. The borings are exclusively the work of the boring bivalves Petricola pholadiformis Lamarck and Zirfaea pilsbryi Lowe. Encrusting organisms include barnacles, mussels, kelp, algae, bryozoans, foraminifers, rare sponges and anemones. There are also several squatters present, such as amphipods, gastropods, various worms, and two other species of bivalve. Although the distribution of borings is patchy, the population density is highest in or near pools of standing water and at the bottom of rock-floored tidal channels. The largest continuously bored zone is 300m by 80m. Smaller patches are also common.Boring density locally exceeds 1250 borings per square meter, which suggests that Petricola and Zirfaea can be agents of extreme bioerosion in the area. Finally, animal relationships, discerned colonization patterns, and simple taphonomy show that the largest surface represents approximately 40 years of colonization; at least 15cm of erosion into the bedrock has occurred over this time span,and standing or flowing water is necessary for the activities of the animals. All of the above show that archetypal Trypanites surfaces probably represent dynamic biomes that have a discernible temporal significance and can be linked to sedimentary processes and, to a lesser degree, bathymetry.