ABSTRACT

Dolomitic nodules were dredged from modern muds of 50 to 70 m water depths, from Loch Sunart on the west coast of Scotland. The nodules which are characterized by a central open passage which may branch and show a swelling at the branching points are believed to have formed around the burrows of thalassinidean crustaceans. After concretion formation the lithified burrows were exhumed and both their external and internal surfaces colonised by encrusters and borers.

The concretions are analogous to Jurassic concretions described from Poland, and are the first such structures to be described from the Recent.

Concretion formation is thought to be due to enhancement of carbonate precipitation by sulphate reducing bacteria concomitant with decay of organic material producing increased alkalinity. From an initial CaCO₃ phase, addition of equal amounts of the ubiquitous Mg²⁺ ion plus CO₃^2- from further sulphate reduction by bacterial action gives rise to the formation of dolomite (48 mole % MgCO₃).