By Joan and John Mylroie
Coastal carbonate rocks in Australia and New Zealand, with ages ranging from Oligocene to Pleistocene, contain numerous flank margin caves. In Australia, flank margin caves were found from near modern sea level up to elevations of 300 meters in eolianites of Pleistocene age and marine subtidal carbonates of Miocene age. Caves were explored in Victoria, southeastern Australia; Kangaroo Island, southern Australia; Rottnest Island, southwestern Australia; and Cape Range, northwestern Australia. The caves are small and entered primarily where cliff retreat has opened the cave chambers from the side. Because the caves form in the fresh-water lens at sea level, they have been used to determine past sea-level history as well as tectonic uplift rates and magnitudes. Some caves contain archeological items greater than 40,000 years in age.
In New Zealand, almost all known coastal outcrops of carbonate rocks were examined in the field on both North Island and South Island. The rocks ranged in age from Pliocene to Oligocene, all were marine subtidal carbonates that transitioned from eogenetic (porous and permeable rock masses) to teleogenetic (crystalline rock masses) in diagenetic maturity. The caves are small and on active coasts, with entrances formed by wave removal of rocky hillsides and cliffs. Some of the smaller caves in Napier, North Island, were uplifted and exposed by a 7.8 magnitude earthquake in 1931 that raised the coast 2.4 m. This vertical action pulled the caves up and out of the Holocene fresh-water lens, halting their growth and making them available for subaerial exploration. In Kaikoura and Punakaiki, South Island, the rocks are highly crystalline and altered by tectonics, the caves forming along closely spaced fractures and joints. Tide ranges in excess of 4 m on some coasts required careful planning to safely enter the caves.