Greensand at the Marshall Paraconformity

• Main Author: Dagg, Robert John
• Other Authors: Wilson, Gary
• Format: Thesis
• Language: English
• Published: University of Otago 2011
• Subjects: Marshall Paraconformity; Glaucony; Antarctic; Drake Passage; New Zealand; Southern Ocean; The Antarctic; Antarc*
• Online Access: http://hdl.handle.net/10523/1634

The Oligocene was a time of major tectonic development in the Southern Ocean. The final break-up of Gondwana was underway leading to the opening of the Drake Passage and Tasman Gateway allowing the development of the Antarctic Circumpolar Current. During the mid-Oligocene much of New Zealand was submerged and it was during this period the Marshall Paraconformity, a regional unconformity on the eastern margin of the South Island, was formed. The development of the Marshall Paraconformity is attributed to a number of causes including eustatic sea level fall, sub-aerial erosion, and/or a marine transgression cutting off sediment supply. In 2005, a 270 m long sedimentary core (TNW-1) passing through the Marshall Paraconformity was retrieved from the Tengawai River at the southern end of the Canterbury Basin. To determine the cause of the Marshall Paraconformity this thesis investigates the TNW-1 core between the depths of 170 m and 185 m, an interval which contains the Marshall Paraconformity (at 182.91m). After retrieval, the core was split, a detailed visual log made of the significant features and discrete samples taken at approximately 300 mm intervals. The discrete samples were subjected to non-destructive rock magnetic analysis including IRM, ARM, AMS and magnetic susceptibility. Following the rock magnetic analysis the samples were used to make polished sections for point counts, petrographic and microprobe analysis. The remaining sample was then chemically treated to remove the carbonate, oxides and opal and then used for grain size analysis. High-resolution magnetic susceptibility of the core was collected with the point count sensor on a Geotek MSCL. The core was divided into 4 lithostratigraphic subunits (LSU), three above and one below the paraconformity. The dominant mineral in the LSU above the paraconformity is dark green glaucony (DGG) with a light green glaucony (LGG) making up the remainder. The two types of glaucony are geochemically and petrographically distinct. Only DGG is present below the paraconformity and this cannot be geochemically distinguished from the DGG above the paraconformity. The LGG and DGG both exhibit a degree of fragmentation and many of the DGG are oxidised indicating the glaucony is allochthonous. Variation in the grain size distribution of the subunits was attributed to the development and waxing and waning of a marine current. The magnetic susceptibility of the interval responds to the variations in the percentage of glaucony in the sediment. Fourier and wavelet analyses of the interval show the presence of an orbital signal in the magnetic susceptibility above the paraconformity. The development of the paraconformity and the subsequent deposition of the overlying sediment with its orbital signal are attributed to the Tasman Current (TC) passing over the core location. The Marshall Paraconformity (MP) formed due to the TC flowing over the mostly submerged New Zealand landmass preventing the deposition of sediment between approximately 37.9 Ma and 25.8 Ma. The opening of the Drake Passage at approximately 25.2 Ma initiated development of the Antarctic Circumpolar Current (ACC). As the ACC strengthened the TC began to weaken, allowing deposition of the Kokoamu Greensand and Otekaike Limestone above the MP between 25.8 Ma and 25.2 Ma in the TNW-1 core in South Canterbury.