The geology of the Twelve Apostles in the Port Campbell Embayment in southeast Australia

The coastal strata of the Port Campbell region in southeast Australia host several 40–70 m-high limestone sea stacks called the ‘Twelve Apostles’ and over 40 km of near continuous sea cliffs. Despite their ‘iconic’ status, their geology is not well known. Mapping using photographic and digital imagery, with field mapping, stratigraphic and microfossil analyses, reveals their geology spans 15 million years of Earth history. The cliffs and sea stack preserve middle to late Miocene Heytesbury Group carbonates overlain by Pliocene to Quaternary ferruginous clay and calcareous siliciclastic units. The oldest unit east of the Twelve Apostles is the unconsolidated outer shelf middle Miocene Gellibrand Marl (15–14 Ma). The overlying shallow shelf Port Campbell Limestone is a brittle unit that forms most cliffs and sea stacks. Two Port Campbell Limestone units are present. A grey clay-rich unit in the lower half of cliffs and sea stacks deposited in deeper and shallower marine conditions from 14 to 11.6 Ma. Overlain by a yellow limestone unit in the upper half of the cliffs and stacks deposited in shallow shelf conditions from 11.6 to 8.6 Ma. Late Miocene uplift, accompanied by karstification and erosion, terminated limestone deposition after 8.6 Ma. This was accompanied by northwest–southeast compression causing 1–5° tilting, creating folds and thrust faults. Subsequently, early Pliocene (ca 5 Ma) Hanson Plains Sands were deposited in terrestrial to shallow marine conditions accompanied by ongoing Pliocene weathering, creating ferricrete laterite soils and the Hesse Clay ‘buckshot’ gravel. This is the youngest red brown unit at the top of cliff sections and sea stacks. In the last million years, coastal Bridgewater Formation siliciclastics were deposited as a series of dunes and soils during high sea-level periods. Holocene flooding since the Last Glacial Maximum 23 000–20 000 years ago created ‘modern’ estuaries, sea stacks and cliffs.