Review of unconformities in the late Eocene to early Miocene successions of the South Island, New Zealand: Ages, correlations, and causes

Abstract

Unconformities are physical surfaces dividing strata of different ages. However, the term is used also to mean the time that is not represented by sediments, or missing time. While unconformities are created by geological events that either prevent deposition or remove deposited sediment, a single unconformity surface could represent multiple events, and if the last event is of sufficient magnitude, the unconformity's appearance will represent the processes occurring only during that event. It should be of no surprise, therefore, that unravelling the history of successions containing multiple unconformities is difficult, and that is especially the case where dating is poor and where unconformity surfaces are hard to correlate. An example of the difficulty presented by unconformable successions can be seen in the Oligocene stratigraphy of New Zealand, where the Marshall Paraconformity has been the focus of controversial interpretation, correlation, and dating. The succession was originally described as conformable, as the unconformities generally (but not always) occur between units orientated parallel to each other. The first location (in Mid Canterbury) where a significant unconformity was identified by biostratigraphy placed the age of the unconformity between early Oligocene sediments and late Oligocene sediments. Other unconformities identified around the South Island in a similar stratigraphic position were correlated with this unconformity. Later dating of a newly designated type section placed the Marshall Paraconformity within the Whaingaroan Stage, in the early Oligocene. Although many biostratigraphic ages for unconformities in South Island successions are not precise, the available data show a distinct range of unconformity ages in different basins. In general, Oligocene deposits are condensed, but there appears to be no one time in the Oligocene when unconformities developed everywhere. Unconformities in the Oligocene could be caused by global sea‐level falls, relative sea‐level high‐stand, local faulting and volcanic activity, and oceanic current activity. These causes could have formed both local and regionally extensive unconformities. In successions with only one unconformity surface, that surface may represent the occurrence of multiple unconformity‐causing processes, further complicating the unravelling of Oligocene geologic history in New Zealand.