Volcanic and sedimentary geology of the basaltic Karaka Volcano, South Auckland
Hansen, M. K. I. (2020). Volcanic and sedimentary geology of the basaltic Karaka Volcano, South Auckland (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13572
Permanent Research Commons link: https://hdl.handle.net/10289/13572
Intraplate monogenetic volcanic fields are a common volcanic feature of the upper western North Island. There are four geographically related fields, approximately 38 km apart which young northwards from Raglan to Auckland. The South Auckland Volcanic Field (SAVF; 1.56–0.51 Ma) is older and more degraded than the Auckland Volcanic Field (AVF; active since 250 ka) to the north. Located between the two fields and overlying the Late Pliocene to Mid-Pleistocene pumiceous fluvial deposits of the Puketoka Formation, is the newly discovered Karaka Volcano. Although residents had referred to a volcanic landform in the area, it was in 2018 that evidence for a volcano based on geomorphology, a magnetic anomaly, weathered surficial deposits and water bore data was reported. This thesis will present the recent findings of the volcano-sedimentary geology of the Karaka volcano. Evidence is based on stratigraphic analysis of two new drill cores, a complementary resistivity survey, petrography, scanning electron microscopy and geochemical investigations. One drill core intersected an upper 6 m blanket of Hamilton Ash Formation and a lower 7 m succession of interbedded, laminated dark brown organic-rich silts, and distal tephras. The lower succession is consistent with lake sediments associated with a crater lake within the geomorphic tuff ring. The second drill core, on the central high point of the volcanic landform, intersected an upper 6 m of brown and red clay. Beneath this, is a lower 9.5 m succession of volcanic ash, coarsening downwards, to a basaltic lapilli ash deposit with a lithic-rich matrix. This represents a phreatomagmatic phase in the upper part of the volcanic sequence. The resistivity survey connects the two drill holes giving an indication of extent and layering of the geology beneath the surface. No recognised volcanoes are within 5 km of this site, so it is unlikely that this pyroclastic deposit came from a distal vent, despite its isolated location outside of the predefined margins of the South Auckland and Auckland volcanic fields. Basalt lapilli within the pyroclastic succession are porphyritic with a trachytic to glassy groundmass with predominant phenocrysts of olivine, augite, opaque minerals and plagioclase. Geochemical analysis was conducted on whole rock samples of basalt lapilli by X-ray fluorescence spectroscopy (XRF) and on pyroxene, olivine, plagioclase and zeolites by electron probe microanalysis (EPMA). This data identified Karaka Basalt as a member of the Group B basalts of the SAVF. Lithics were fragments of the underlying sandstone and siltstone of the Waitemata Group. The surrounding matrix comprised small basalt fragments, lithics, quartz and plagioclase. Glass compositions of tephra by EPMA provide information that has been used to produce a tentative minimum age on the crater lake and by proxy the volcano itself. Tephra TW gives a possible correlation to the Kidnappers Ignimbrite which has been dated previously at ~0.99 Ma. The age of this tephra along with belonging to the SAVF, and a covering of Kauroa and Hamilton Ashes, gives the Karaka Volcano an approximate age range of 1.56 – 0.99 Ma.
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