Fault offsetting strata at Chancet Rocks, J.Thomson / GNS Science
Chancet Rocks reserve has two main limestone ridges, separated by a low area. The once horizontal rock layers here were laid down on the sea floor, and have been tilted up almost vertically by deformation of the crust. The outcrop has long been known for its vase-shaped “fossil sponges”, which are now recognised as a trace fossil, or burrow, called Paramoudra, created by an unknown animal. More recently, a very complete section through the Cretaceous-Paleogene boundary was identified and confirmed by the presence of the 'iridium anomaly' with 10’s of times the normal concentration of iridium (a very rare metal) in a thin clay layer. This layer has been recognised worldwide and marks the end of the Age of the Dinosaurs, caused by a devastating asteroid impact that struck part of where Mexico's Yucatan Peninsula is today. In these marine sediments Cretaceous microfossils become extinct at this layer, and earliest Paleogene ones occur immediately above it. In November, 2017, the Magnitude 7.8 Kaikoura Earthquake raised Chancet Rocks by about 2 metres, exposing large new areas of the tidal platform.