Silicic pyroclastic deposits at the base of the Paleogene Skye Lava Field: Evidence from An Carnach, Strathaird Peninsula

Abstract

Synopsis Silicic pyroclastic deposits, attributed to two discrete volcanic phases, are reported from a basal sequence of the Paleogene Skye Lava Field, NW Scotland. Phase 1 comprises relict silicic fall deposits that have been subsequently altered to a bed of K-bentonite. These fall deposits pre-date a thick ( c . 70 m) sequence of basaltic plateau lavas and a later underlying tholeiitic sill. The K-bentonite bed has a fabric that strongly resembles that of eutaxitic welded ignimbrite. Phase 2 comprises both basic and silicic ignimbrites that post-date the lowermost plateau lavas. The nature of these distinct ignimbrite lithofacies can be used to infer that eruption parameters, such as chemistry, intensity and flux output, underwent temporal variations. The recognition of these two phases of pyroclastic volcanism has major implications for the established sequence of the Skye Lava Field and gives important new insights into the nature of these Paleogene eruptions.