Bentonites as Tracers of Earliest Cretaceous Post-Breakup Volcanism off Northwestern Australia (Legs 122 and 123)

Abstract

Bentonites were discovered in Berriasian to Valanginian hemipelagic (shelfal) to eupelagic (deep-sea) sediments of the Wombat Plateau (Site 761), Argo Abyssal Plain (Sistes 261, 765), southern Exmouth Plateau (Site 763), and Gascoyne Abyssal Plain (Site 766). A volcaniclastic origin with trachyandesitic to rhyolitic ash as parent material is proved but the abundance of well-ordered montmorillonite, fresh to altered silicic glass shards, volcanogenic minerals and rock fragments, and by a vitroclastic ultra-fabric (smectitized glass shards). For the Argo Abyssal Plain, we can distinguish four types of bentonitic claystones of characteristic waxy appearance. The ash layers were progressively altered (smectitized) during diagenesis. Westward directed trade winds transported silicic ash from volcanic source areas to the Exmouth Plateau and, via turbidity currents, into the adjacent abyssal plains. The Wombat and Argo abyssal plain bentonites are interpreted, at least in parts, as proximal or distal ash turbidites. -from Authors