Reconsidering basin geometries of the West Coast: the influence of the Paparoa Core Complex on Oligocene Rift Systems

Oligocene and Miocene strata of the Nile Group are dominated by neritic cool‐water carbonates formed within the Paparoa Trough along the western flank of the Challenger Rift System. In outcrop west of the Paparoa Range, the Nile Group contains three formations, the Waitakere Limestone, dominated by photozoan floatstone and grainstone facies, the Tiropahi Limestone, containing both impure heterozoan packstones and wackestones, and the Potikohua Limestone, consisting entirely of skeletal heterozoan grainstones. These correlative formations represent lateral depositional facies interpreted to record a submarine depression bounded by a northern irregular palaeohigh and a southern open marine plateau. It is proposed that these variations are the product of reactivation of structures associated with the underlying Paparoa Core Complex superimposed on the NE–SW rift axis emphasized within current basin models. This interpretation is consistent with exposures elsewhere in the West Coast region and highlights the need for palaeogeographic models of South Island to take older, rift‐related basement structures into account.