A sandstone diapir cutting the Amuri Limestone, North Canterbury, New Zealand

Abstract Well‐sorted sandstone intrudes the Amuri Limestone at Hurunui River Mouth, North Canterbury. Some intrusions pierced the Limestone and extruded onto the seafloor between Lower Oligocene (?Whaingaroan) and uppermost Oligocene to Lower Miocene (?Waitakian to Otaian) time. The source bed for the diapiric complex is an Eocene sandstone which conformably underlies the Amuri Limestone in this area. Flow structures, commonly denned by concentrations of brecciated limestone fragments, are present in many intrusions. A sharp, planar, undeformed but gently tilted unconformity truncates the diapiric complex and is marked by a concentration of shallow subhorizontal burrows. A mixture of reworked phosphatised limestone clasts, locally derived from a post‐Amuri disconformity, and unphosphatised limestone chips supplied by the sand diapir are present for several centimetres directly above the unconformity. Similar intrusive sands have been noted in the Amuri Limestone from North Canterbury to the Wairarapa, but most have not been studied in detail. Such widespread occurrence suggests that the cause of the intrusions was a tectonic event rather than localised slumping or excessive loading.