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.

High-level stratigraphic scheme for New Zealand rocks

“…In other places however, such as in Taranaki Basin and adjacent basins to the east, peak marine inundation occurred significantly later during the latest Oligocene (Kamp et al 2014;Strogen et al 2014). Moreover, the Marshall Unconformity is not recognised everywhere, has different ages and potentially different origins and occurs variably within or beneath the overall Oligocene carbonate-dominated succession (King et al 1999;Lever 2007). All things considered, our preference in this paper is not to use the Marshall Unconformity sensu stricto as a defining boundary between supergroups, as it is only one of many local unconformities developed in this period (see also McMillan & Wilson 1997).…”

The effect of volcanism on cool-water carbonate facies during maximum inundation of Zealandia in the Waitaki–Oamaru region

“…These two formations have been previously interpreted to represent the maximum level of marine transgression in the Waitaki-Oamaru area (Carter & Landis 1972;Lewis & Belliss 1984;Carter 1985Carter , 1988Jenkins 1987;Fulthorpe et al 1996;Lever 2007).…”

“…2), often referred to as the Marshall Paraconformity although it is really an obvious unconformity (e.g. Carter & Landis 1972;Carter & Carter 1982;Lewis & Belliss 1984;Carter 1985;Gage 1988;Lewis 1992;Fulthorpe et al 1996;McMillan & Wilson 1997;Carter et al 1999;Lever 2007;Fordyce et al 2009). This is overlain by the mid to Late Oligocene Kokoamu Greensand (Kekenodon Group), a calcareous greensand that grades into the Late Oligocene-earliest Miocene Otekaike Limestone (Kekenodon Group), a fossiliferous glauconitic limestone (Gage 1957(Gage , 1988 Figure 1 Geological map of the study area within the Waitaki-Oamaru region showing the locations of stratigraphic columns and outcrops used for this study.…”

“…The period of maximum flooding is generally regarded as in the Waitakian (latest Oligocene to earliest Miocene) and its extent has implications for the development of New Zealand biota (Suggate et al 1978;Kamp & Nelson 1987;McMillan & Wilson 1997;Campbell & Landis 2001;Lee et al 2001;Gibbs 2006;Fordyce et al 2009). Several significant unconformities developed during the Oligocene, reflective of subaerial exposure or major submarine current development (Carter & Landis 1972;Carter & Carter 1982;Lewis & Belliss 1984;Carter 1985;Kamp & Nelson 1987;Gage 1988;Lewis 1992;Fulthorpe et al 1996;McMillan & Wilson 1997;Carter et al 1999;Lever 2007;Fordyce et al 2009). The development of the Pacific-Australian plate boundary and the instigation of the Alpine Fault in the Miocene saw a steady increase of terrigenous sediment once again, and the onset of regional regression (Bradshaw 1989;Lemarche et al 1997;Sutherland 1999;Fukuda et al 2012).…”

Expedition summary

“…New Zealand lay directly in the path of this developing current system. The paraconformity has also been studied in onshore outcrops (e.g., Findlay, 1980;Field and Browne, 1989;Carter and Landis, 1972;Lever, 2007) and dated by biostratigraphy and more recently by strontium isotopes at a South Canterbury outcrop. The hiatus has been dated at 32.4-29 Ma (Fulthorpe et al, 1996).…”