The genesis of sub‐basaltic Silcretes from the Monaro, New South Wales

Abstract

Sub‐basaltic silcretes are widespread in the Monaro, New South Wales, and their origin has been hitherto linked with eruption of the overlying basalt. Investigation of silcretes from three localities in the Monaro at granite‐basalt, shale‐basalt, and basalt‐basalt contacts, has revealed a complex history of depositional events in these rocks, which cannot be related to a single event such as eruption of the overlying basalt lava flow. Major and trace element abundances suggest that the silcretes are formed from quartz sand with little admixture of feldspar and/or clays and that the overlying basalt has contributed nothing to their chemical composition. The evidence is consistent with low‐temperature precipitation of quartz. The significance of the silcrete‐basalt association is considered to be that the basalt acts as a cap rock, providing a confined aquifer within the sand deposit and a favourable environment for SiO₂ deposition.