Magnetic Polarity Stratigraphy of the Pliocene Hamilton and Forsyth's Bank Local Sections, Hamilton (Victoria), Australia

The Hamilton Local Fauna has the combined assets of being a large and well studied mammalian fauna and being directly overlain by a$^{40}K-^{40}Ar$dated basalt. The overlying basalt ($4.46 \pm 0.01 Ma$) gives it the best constrained age of any fauna on the Australian continent and makes it an important Pliocene locality. This study provides a basal absolute age for the Hamilton Local Fauna through the use of magnetic polarity stratigraphy. The fauna occurs within a zone of normal magnetic polarity correlated to the Gilbert Chron at 4.40-4.47 Ma. Therefore, the age of the Hamilton Local Fauna is constrained to fall within a range of$4.46 \pm 0.01$and 4.47 Ma. A section that contains the nearby Forsyth's Bank mammal fauna was also studied. A characteristic magnetic component could not be isolated in samples taken from this section. The age of this locality remains constrained by the Kalimnan Stage marine invertebrate fauna that it contains and by the overlying basalt dated at$4.46 \pm 0.01 Ma$.