Palaeoenvironmental relationships of mid-cenozoic carbonate and volcanic rocks, Kakanui
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The rocks of the Eocene to Oligocene Waitaki region formed in a complex depositional environment, under the influence of changing sea levels, submarine volcanism, and carbonate production. This area was part of a marine basin, which developed while New Zealand was mostly submerged, and the main sediment sources were carbonate deposits, and volcanic material produced in a monogenetic volcanic field. The build-up of volcanic material and the formation of seamounts created a shallow zone offshore. The geology within this former volcanic field is characterised by inner – mid shelf fauna, while deeper-water assemblages occur closer to the palaeoshoreline. This investigation employs a multi-disciplinary approach, in order to understand the nature, timing and importance of the various factors which have influenced deposition within this unusual setting.
The study area of Kakanui is surrounded by good exposures of volcanic and carbonate rocks, formed within a changing depositional environment. In particular, the Kakanui River Section deposits show a number of lithological changes, which were driven by a number of interrelated factors. Sea level was relatively high at the base of the section, but fell by 20 - 30 m near to the top. This resulted in the development of an unconformity, and a shift to coarser grainsizes and shallower-water bioclasts. Volcanism also greatly influenced Kakanui River Section deposits. Nearby volcanoes provided a sudden influx of sediment, followed by a return to carbonate accumulation after the edifices degraded too much.
Geochemical fingerprinting of volcanic deposits around Kakanui revealed that a number of distinct sources were present. Older eruptions had a more felsic composition, but younger eruptions showed more enrichment in compatible elements. This shift is attributed to changing source magma geochemistry, combined with the influence of a new magma body.
This investigation used a combination of field mapping, microfossil investigation and geochemical analysis. These techniques complemented each other, and helped to build a more complete understanding of the palaeoenvironment at Kakanui.