Storm-dominated shallow marine sediments of the Arisaig Group (Silurian–Devonian) of Nova Scotia

Abstract

Shallow marine deposits of the Arisaig Group (Silurian–Devonian) of Nova Scotia consist of shaly sediments with varying proportions of coarser interbeds. Brachiopods, graptolites, crinoids, corals, and bivalves are present in parts of the group.One end-member lithology consists of black, fissile shale with few fossils. This grades into grayer, less fissile, more fossiliferous shale with coarse siltstone interbeds up to 5 cm thick. Higher in the group, nearer nonmarine beds, the background sediments consist of mottled, nonfissile silty mudstone with many large in situ brachiopods and lenses of coarse siltstone. The interbeds are composed of coarse siltstone or fine sandstone and, in some cases, have lenses of coquina within them. They range from 5–50 cm in thickness, but in places have amalgamated by scouring and locally comprise almost 100% of the section. Most of the coarse interbeds have sharp bases with horizontal or low angle laminations, but some thicker beds show sets of low angle, curving laminations which truncate other sets, termed “hummocky cross-stratification," The top surfaces of these beds are covered by interference ripples and burrows.The coarse interbeds were laid down rapidly by fractional seaward-flowing bottom currents during storms. Some of the coarse beds were later modified by bioturbation during fair weather periods when settling of sediment from suspension and faunal activity were the dominant processes. The Arisaig Group represents the deposits of a storm-dominated shelf and illustrates the range in variation which can occur in the sediments of this type of environment.