Subaqueous deposition of accretionary lapilli: Significance for palaeoenvironmental interpretations in Archaean greenstone belts

Abstract

Accretionary lapilli from phreatomagmatic eruptions are abundant in the late Archaean Fortescue Group of the Pilbara Region, Western Australia. Where the sequence is uncleaved and gently folded, well-preserved depositional structures include densely packed accretionary lapilli in trough cross-beds and sparse accretionary lapilli in trough and wedge to tabular cross-beds. Accretionary lapilli also occur in conglomerate layers and localised pebbly-channel-fills associated with granite and pegmatite clasts up to 10 cm in diameter. These lapilli are clearly robust being capable of withstanding fairly energetic aqueous transport in contrast to the opinion of many authors who consider that all accretionary lapilli would disintegrate if settled through extensive water columns or incorporated in turbidite deposits. Accretionary lapilli indicate subaerial or very shallow water eruption but their presence cannot be used alone to support very shallow water or subaerial deposition. Accretionary lapilli are found throughout many volcanic piles and are probably widespread in Archaean greenstone successions but their environmental significance has to be assessed carefully using a variety of associated features.