Temperate skeletal carbonate sediments on Scott shelf, northwestren Vancouver Island, Canada

Abstract

Scott shelf ( 2000 km2), centred at Scott Islands, northwestern Vancouver Island, is mainly shallower than 150 m, topographically diversified, and floored by bedrock outcrops and terrigenous lithic gravels and sands left stranded following the post-glacial rise in sea level (13,000 yrs B.P.). It lies in a zone of vigorous wind-wave currents and strong tidal flows, but is largely starved of modern terrigenous sediment. As a consequence it is slowly accumulating a thin, discontinuous blanket of clean, skeletal carbonate sands and gravels, admixed to varying degrees with the underlying terrigenous deposits. Principal skeletal contributors are infaunal bivalves (on coarse sandy and gravelly substrates), barnacles (on low-amplitude gravelly ridges), bryozoans (on bedrock outcrops and boulders) and benthic foraminifera (on fine sands in deep (> 100 m) waters south of the Scott Islands). Living carbonate benthos are scattered and generally sparse, occupying specific ecologic niches, and the shelf-wide rate of carbonate production is low. Skeletons are fragmented, transported and mixed during storms and are concentrated within bedrock hollows and crevices, and shallow depressions between gravel ridges. Where infaunal bivalves are abundant the carbonates are dominated by aragonite, but otherwise the skeletal hashes are predominantly calcitic. Many grains, and especially aragonitic ones, are corroded and weakened by epilithic and endolithic bioerosion, and probably also by marine and dissolution. The most corroded shells have ages of only about 1000 yrs so that their preservation potential is low. The character of Scott shelf skeletal carbonate deposits reflects their temperate latitude, cold-water heritage.