Stratigraphy, micropalaeontology and stable isotope analysis of Tertiary rocks in the North Canterbury region
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Stable isotope, stratigraphic and micropalaeontological studies provide an insight into the preservation of Tertiary global cooling episodes in shelf carbonate rocks from the Mandamus-Pahau District in North Canterbury. Local shelf carbonate isotope analysis (δ13C and δ18O) shows these shelf rocks have not retained the original sea-water isotopic signature but have been altered during burial diagenesis. The Palaeocene to Late Eocene Coal Creek Formation from this district is a glauconite-rich, inner shelf unit transitioning to a mid-outer shelf environment. Separation of this unit from the Mid-Late Oligocene Cookson Volcanic Group is marked by a ~3-4 Ma hiatus representative of the Marshall Paraconformity. The Marshall Paraconformity is inferred throughout the district due to a distinct break in style and sedimentation from the Late Eocene until the Mid-Late Oligocene. Cookson Volcanic Group tuffaceous beds and pillow basalts represent localised intra-plate volcanism that occurred prior to the deposition of the Late Oligocene Flaxdown Limestone. This Oligocene limestone represents a mid shelf, bryozoan and algal-rich grainstone. Widespread development of this carbonate unit illustrates the absence of tectonically derived sediments prior to the deposition of the Pahau Siltstone Member. The Early Miocene marks the initiation of a new tectonic regime in New Zealand, with the deposition of the inner shelf, quartz-rich Pahau Siltstone Member due to tectonic associated uplift and erosion. These shelf sediments proved inadequate as a record of global climate excursions, with grainsize and the dominance of burial diagenesis limiting their use in global isotope records. Shelf sediments seen in this study mostly consist of grainstones or sandstones with the coarse grainsize of these units allowing a high flow of pore fluids through sediments. These pore fluids occurred out of equilibrium with bioclasts within these Tertiary units, increasing the alteration potential of bioclasts. This thesis provides strong evidence for problematic global isotopic signatures in local shelf sediments due to grainsize and diagenesis, yet other studies have proved it is possible to reconstruct the global isotope curve from shelf sediment records. Accurate shelf records depend on proxies less prone to diagenetic alteration that act as a reliable gauge of local and global environmental change.