Isotopic constrainst on the genesis and age of autochthonous glaucony in the Oligo-Miocene Torquay Group, south-eastern Australia

Abstract

The Oligo-Miocene Torquay Group at Bird Rock in south-eastern Australia comprises a sequence of fine-grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool-water, mid-shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb-Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution-crystallization reactions, which occurred during very early diagenesis in a closed or isoche..