Accretionary lapilli-filled clastic dykes: a comparison of compaction strain estimates from dyke folding and lapilli shape factors

Abstract

Within pyroclastic and tuffaceous sediments of the late Archaean Fortescue Group, Western Australia, small-scale clastic dykes have been infilled by accretionary lapilli. This unusual occurrence allows quantification of compaction by two independent means using the folded nature of the dykes and the shape factors of the accretionary lapilli. The dyke-fold method consistently gives a much lower estimate of the compaction strain than the accretionary lapilli. Two dykes giving shortening of 20 and 2% by the dyke method both contain accretionary lapilli recording 40% compaction.