New Zealand Journal of Geology & Geophysics abstracts
Review of unconformities in the late Eocene to early Miocene
successions of the South Island, New Zealand: ages, correlations, and
causes
Helen Lever
Institute of Petroleum Engineering
Heriot-Watt University
Edinburgh EH14 4AS, Scotland, UK
helen@verso.org
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.
Keywords Oligocene; Eocene;
unconformities;
stratigraphy; Marshall Paraconformity; Antarctic Circumpolar
Current
G06028; Online publication date 1 August 2007; Received 28 August
2006; accepted 28 June 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
245–261
0028–8306/07/5003–0245 © The Royal Society of New Zealand 2007
PDF file of entire paper: Print-quality
(2170K) | screen-quality (1609K)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
