Paleoenvironmental and tectonic changes across the Cretaceous/Tertiary boundary at Tora, southeast Wairarapa, New Zealand: A link between Marlborough and Hawke's Bay

Abstract

The Late Cretaceous‐Paleocene succession exposed on the Tora coast, near the southeastern tip of the North Island, is distinguished by an unusual lithofacies of the Whangai Formation, and by an apparently unique formation, Manurewa Formation, which spans the Cretaceous/Tertiary (K/T) boundary. The Late Cretaceous siliceous Whangai Formation at Tora includes zones of slumps and olistostromes, containing megaclasts of limestone up to 3 m long. The olistostromal deposits suggest steep submarine topography with a high rate of erosion, and imply tectonic activity. The common occurrence of hummocky cross‐stratification suggests deposition in shelf depths above storm wave base. The sharply overlying Manurewa Formation is interpreted as the infill of a major shallow channel complex, perhaps >9 km wide and spanning the K/T boundary in time. The older of two channelled units is of latest Cretaceous (latest Haumurian/late Maastrichtian) age, and consists of bioturbated alternating thin sandstone and mudstone with thin conglomerate lenses and limestone beds. It is likely to have been deposited in a low‐energy environment, probably deeper than that of the Whangai. The younger channel system, of early Paleocene (early Teurian) age, erodes into the older in the northeast, and into the underlying Whangai Formation in the southwest. Basal deposits consist predominantly of medium to coarse, thick‐bedded, glauconitic sandstone, with local low‐angle cross‐stratification and microflora typical of low salinity conditions, suggesting deposition in shallow shelf depths. These deposits contain olistrostromes with megaclasts up to 1 m long of limestone and rarer dark grey siltstone or very fine sandstone clasts typical of Whangai Formation. The inclusion of megaclasts of Whangai Formation indicates that local emergence and erosion of older strata was occurring. Deposits grade upward into well‐sorted bioturbated sandstones of the Awhea Formation, with prominent low‐angle cross‐stratification, interpreted as very shallow marine, probably nearshore deposits. The channel system represented by the Manurewa Formation records an initial relative sea‐level rise, followed by an abrupt sea‐level fall at, or close to, the K/T boundary. New Zealand was in a passive margin tectonic setting at the time, but the widespread presence of olistostromes, some including clasts derived from older strata, suggest that local tectonic activity and uplift was occurring. The effects may have been enhanced by a climatic shift in storm tracks and intensity in the latest Cretaceous, which is supported by the evidence of strong wave activity. By contrast, to the south in Marlborough, the K/T boundary succession is commonly characterised by an apparently conformable lithologic change from limestone to chert, although with local hiatus. To the north, in southern Hawke's Bay, the coeval succession is characterised by a disconformity separating greensand from underlying light grey, slightly calcareous mudstone of the Whangai Formation. The Tora sequence may provide the link between two distinctly different lithologic successions.