The Oligo-Miocene Torquay Group at Bird Rock, in southeastern Australia, comprises a sequence of fine-grained skeletal carbonates and argillaceous and glauconitic sandstones, which were deposited in a cool-water, mid-shelf environment. The Bird Rock glaucony consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. Petrographic and geochemical evidence indicates that the glaucony is autochthonous and comprises chemical components derived primarily from argillaceous matrix material; seawater is unlikely to be a significant source of ions. The glauconitization of the Bird Rock sediments occurred under sub-oxic partially reducing conditions, in the very shallow burial environment, and involved local iron redistribution. Sub-oxic conditions favour glauconitization because iron is stable in the soluble ferrous state and can be fixed in authigenic silicates due to the negligible concentrations of hydrogen sulphide. It is likely that localised acidic conditions were initiated during the glauconitization process; this acidity appears to have been buffered by the dissolution of bioclastic carbonate. The fact that glaucony predominantly occurs as bioclast infills and faecal pellet replacements implies that the physico-chemical conditions appropriate for glauconitization develop preferentially in such biogenic detritus. The development of appropriate micro-environments within such sediments probably relates to their physical confinement and=or high organic matter content. The Bird Rock glaucony developed during intervals of slow sedimentation and environmental quiescence associated with marine flooding events. These conditions facilitated glauconitization by allowing the favoured clay-rich sediments to accumulate and remain in the appropriate physico-chemical regime sufficiently long for the complex glauconitic structures to form.