Abstract

The Douro Formation, one of the most widely recognised units in the Canadian Arctic Islands, consists predominantly of rubbly-weathering grey limestone with abundant smooth-shelled brachiopods. The relatively monotonous lithologic and paleontologic character of the formation hinders internal stratigraphy, and makes it difficult to trace individual beds for more than a few hundred metres. Nevertheless, consideration of the overall character of the Douro Formation over several tens of metres thickness permits recognition of more widely traceable units. Presence of argillaceous divisions and coral-rich zones provides two independent, supplementary criteria which can be used to correlate sections on southeastern Somerset Island. Coral-rich zones and associated reefs occur in similar stratigraphic positions on Somerset, Cornwallis and Devon Islands. Variation in total shale content between sections makes inter-island recognition of the argillaceous divisions more difficult, but on all three islands the lower part of the formation tends to be more argillaceous than the upper.

The Douro Formation was deposited on a subtidal shelf, probably in warm, turbid, predominantly tranquil waters. On southeastern Somerset Island, the subtidal Douro Formation lies conformably between two predominantly intertidal formations, and was deposited in two successive transgressive-regressive cycles. Sponge reefs characterised the two deepest parts of cycles, whereas oolites and oncolites formed rarely in the shallower parts of the cycles. The amount of argillaceous detritus in the rubbly limestones was largely independent of bathymetry, and was probably controlled by changes in the source area and in distribution patterns. The terrigenous content of the rubbly limestones increases regularly in a northeasterly direction.