The drowned apostles: the longevity of sea stacks over eustatic cycles

Cliffed rocky coasts are erosional environments, the remnants of which can be preserved as sea stacks as the shoreline retreats. These sea stacks form spectacular landscapes, such as the iconic Twelve Apostles in Victoria, Australia. However, they are ephemeral features formed on a centennial scale, continually eroding and collapsing, meaning that coasts characterised by sea stacks often have fewer features than when first described. The question arises then as to the longevity of such features and whether they can be preserved over eustatic cycles. The modern Twelve Apostles, of which 8 are still standing, are comprised of the Miocene Port Campbell Limestone and reach 45 m above sea level. Recent multibeam sonar data show five features around 6 km offshore, in 40-50 m water depth that appear to be relict sea stacks. Based on the morphology and geology of both the modern and drowned Apostles, it is inferred that the drowned and modern stacks evolved in a similar manner. While the modern sea stacks have an average height of 45 m, the drowned stacks have an average height of 4 m, suggesting a much greater age and also the possibility of multiple exposures to subaerial processes. The drowned stacks lay 655 m seaward of a drowned cliff averaging 14 m high which likely represents a former interstadial shoreline. This is much greater than the 91 m average distance between stack and cliff for the Modern Apostles, which may imply a more prolonged period of erosion along the drowned coastline.