RING STRUCTURES MADE BY SHORE ICE IN A MUDDY TIDAL FLAT, ST-LAWRENCE ESTUARY, CANADA

Large-size ring sedimentary structures occurring in a muddy tidal flat, on the south shore of the St. Lawrence estuary, are reported for the first time. They range from 15 to 50 m in diameter and occur as isolated features or in groups of 3 to 5. In plan they form well-defined circles scattered throughout the higher mud flat. Some structures are characterized by an inner ring and by a chaotic surface of alternating micro-ridges and depressions. The outer surrounding mud ridge is a few decimeters high and wide (25-60 cm). Ring structures occur at the emplacement of icefoot pustules and are related to vertical pressures exerted on the substrate by ice blocks moved up and down by tide motion. Icefoot pustules form every winter but not necessarily at the same location. It is likely that the related ring structures can be fossilized and preserved in the sedimentary sequence. Consequently, they can be of assistance in palaeoenvironmental interpretation.