THE SALISBURY TUFF, DOMINICA, LESSER ANTILLES

The island of Dominica, located in the central part of the Lesser Antilles island arc, has witnessed, probably in the last 100,000 years, three Plinian eruptions. One of these, from Morne Diablotins, generated extensive ignimbrites and associated pumiceous surges that extend as three flow fans: the Pointe Ronde and Grand Savane fans on the west coast and the Londonderry fan on the east coast. The largest of these is the Grande Savane fan, the submarine extension of which extends well into the Caribbean Sea. Stratigraphical studies show a basal pumice fall horizon (20 cm) overlying an older block and ash flow deposit. Above the fall occurs a welded ignimbrite that extends from a poorly welded basal zone (40 cm) through a densely welded, columnar jointed central zone (800 cm) to a poorly welded upper zone (400 cm). This sequence is directly overlain by a thin phreato-magmatic fall deposit containing accretionary lapilli. This fall deposit is, in turn, overlain by a sequence of pumiceous surge deposits. Within the welded section of the ignimbrite, fiamme can reach up to 15 cm long with aspect ratios of up to 1:7. Results from detailed granulometric studies and a model of formation of this densely welded, relatively thin ignimbrite, will be presented.