Geochemistry and minerals at DSDP Leg 81 Holes

The earliest, Phase 1, is marked by a sequence of tholeiitic basalts and hyaloclastites which form the dipping reflector sequence in Edoras Basin. Phase 2 is characterized by tuffs and lapilli tuffs of air-fall origin, ranging in composition from basic to intermediate.

They were generated by highly explosive igneous activity due to magma-water interaction, and terminate at the level of a major transgression. Subsequently, volcanism reverted to tholeiitic basalt type, producing the thin tuffs and minor basalt flows of Phase 3. Alteration of the volcanic glass and diagenesis of the tuffs and lapilli tuffs has been considerable in many cases, with a large number of diagenetic mineral phases observed, including smectite, celadonite, analcime, phillipsite, clinoptilolite, mordenite, and calcite.